Programmed cell death

This paper reviews data on programmed cell death (apoptosis) in animals and plants. Necrosis is a pathological scenario of cell death, which entails an inflammatory response in animal tissues. Apoptosis results in the disintegration of animal/plant cells into membrane vesicles enclosing the intracellular content, which are thereupon engulfed by adjacent or specialized cells (phagocytes) in animals. Plants lack such specialized cells, and plant cell walls prevent phagocytosis. The paper considers the main molecular mechanisms of apoptosis in animals and the pathways of activation of caspases, evolutionarily conserved cysteine proteases. A self-contained section concerns itself with the process of programmed cell death (PCD) in microorganisms including: 1) cell death in the myxomycete Dictyostelium discoideum and the parasitic flagellate Trypanosoma cruzi; 2) PCD in genetically manipulated yeast expressing the proapoptotic Bax and Bak proteins; 3) the death of a part of a prokaryotic cell population upon the depletion of nutrient resources or under stress; 4) the elimination of cells after a loss of a plasmid encoding a stable cytotoxic agent in combination with an unstable antidote; and 5) PCD in phage-infected bacterial cells.     

Role of Polyamines in Apoptosis and Other Recent Advances in Plant Polyamines

Numerous citations in the literature indicate that polyamines are intensively studied in plants. Polyamines are implicated in many functions of the plant cell. Excellent recent reviews cover much of this original research. This article is focused primarily on literature that relates research on the role of polyamines in apoptosis and programmed cell death in both plants and animals. Apoptosis and programmed cell death are considerably better studied in animal systems, and this review demonstrates that the role of polyamines in these processes in plant systems are remarkably congruent with what is known in animal systems. In addition, key recent research reports are reviewed that describe the functional analysis of key polyamine biosynthesis genes in plants in relation to responses to environmental stress signals. Molecular analysis is providing strong evidence for the polyamine biosynthetic pathways to play major roles in ameliorating plant responses to abiotic stresses.     

动物细胞培养过程中的细胞凋亡

细胞培养过程中的细胞凋亡是细胞受环境因素的影响而发生的现象。随着对细胞凋亡的分子生物学和细胞生物学了解的深入,显示了有效地控制动物细胞培养中细胞凋亡的巨大潜力。包括采用ＤＮＡ重组技术把抗细胞凋亡的基因导入细胞和在培基中加入具有抗细胞凋亡的生存因子或化合物等手段已用于控制细胞培养过程中的细胞凋亡。这些技术将大大延长细胞达到饱和密度后的培养时间,提高细胞培养系统的生产效率。     

Programmed cell death during plant growth and development

This review describes programmed cell death as it signifies the terminal differentiation of cells in anthers, xylem, the suspensor and senescing leaves and petals. Also described are cell suicide programs initiated by stress (e.g., hypoxia-induced aerenchyma formation) and those that depend on communication between neighboring cells, as observed for incompatible pollen tubes, the suspensor and synergids in some species. Although certain elements of apoptosis are detectable during some plant programmed cell death processes, the participation of autolytic and perhaps autophagic mechanisms of cell killing during aerenchyma formation, tracheary element differentiation, suspensor degeneration and senescence support the conclusion that nonapoptotic programmed cell death pathways are essential to normal plant growth and development. Heterophagic elimination of dead cells, a prominent feature of animal apoptosis, is not evident in plants. Rather autolysis and autophagy appear to govern the elimination of cells during plant cell suicide.     

Extracellular ATP: a potential regulator of plant cell death

Adenosine 5′‐triphosphate (ATP) has been regarded as an intracellular energy currency molecule for many years. In recent decades, it has been determined that ATP is released into the extracellular milieu by animal, plant and microbial cells. In animal cells, this extracellular ATP (eATP) functions as a signalling compound to mediate many cellular processes through its interaction with membrane‐associated receptor proteins. It has also been reported that eATP is a signalling molecule required for the regulation of plant growth, development and responses to environmental stimuli. Recently, the first plant receptor for eATP was identified in Arabidopsis thaliana. Interestingly, some studies have shown that eATP is of particular importance in the control of plant cell death. In this review article, we summarize and discuss the theoretical and experimental advances that have been made with regard to the roles and mechanisms of eATP in plant cell death. We also make an attempt to address some speculative aspects to help develop and expand future research in this area.     

Apoptosis and multistage carcinogenesis in rat liver

Apoptosis is a type of active cell death. It is involved in the homeostasis of cell number in tissues and is controlled by the growth regulatory network in the organism. It is also involved in the active removal of damaged cells. We have studied the role of apoptosis in cancer pre-stages and overt cancer in vivo, using rat liver as our main model system. Quantitative determination of apoptosis in histological specimens revealed that the rate of apoptosis tends to increase from normal to (pre)neoplastic to malignant cells. Thereby active cell death largely counterbalances the increasing replicative activity in developing malignancy. Tumor promoters shift the balance in favor of cell replication, whereas promoter withdrawal, fasting or TGF-β1 favor apoptosis (anti-promotion). Preneoplastic cells are more susceptible than normal liver cells to stimulation of both cell replication or cell death. Consequentially (pre)neoplastic tissue may preferentially grow or die during the appropriate treatment. Regimens that favor apoptosis and lower cell replication are shown to result in the elimination of preneoplastic cell clones from the liver (anti-initiation) and to reduce the cancer risk of the animal.     

Endonucleases and their involvement in plant apoptosis

This review considers modern data about the set, nature, specificity of action, and other properties of plant endonucleases involved in various forms of programmed cell death (PCD) in various plant tissues (organs). Apoptosis is an obligatory component of plant development; plant development is impossible without apoptosis. In dependence on the conditions of plant growth, this process can be induced by various biotic and abiotic factors, including stressors. Endonucleases accomplishing apoptotic degradation of nuclear material in the plant cell play one of the main roles in PCD. Plant endonucleases belong to at least two classes: (1) Ca²⁺- and Mg²⁺-dependent and (2) Zn²⁺-dependent nucleases. The set and activities of endonucleases change with plant age and during apoptosis in a tissue-specific manner. Apoptosis is accompanied by the induction of specific endonucleases hydrolyzing DNA in chromatin with the formation firstly of large domains and then internucleosomal DNA fragments; the products produced are of about 140 nucleotides in length with their subsequent degradation to low-molecular-weight oligonucleotides and mononucleotides. About 30 enzymes are involved in apoptotic DNA degradation. Histone H1 modulates endonuclease activity; separate (sub)fractions of this nuclear protein can stimulate or inhibit corresponding plant endonucleases. In the nucleus and cytoplasm of the plant cells, Ca²⁺/Mg²⁺-dependent endonucleases recognizing substrate DNA methylation status were revealed and described for the first time; their action resembles that of bacterial restrictases, which activity is modulated by the donor of methyl groups, S-adenosylmethionine. This indicates that higher eukaryotes (higher plants) might possess the system of restriction-modification to some degree analogous to that of prokaryotes.     

Endoreduplication and activation of the anaphase-promoting complex during symbiotic cell development

Postembryonic development of plant organs requires a constant interplay between the cell cycle and the developmental programs. Upon endo- and exogenous signals, plant cells can enter, exit or modify the cell cycle. Alteration of mitotic cycles to endoreduplication cycles, where the genome is duplicated without mitosis, is common in plants and may play a role in cell differentiation. The switch from the mitotic to endocycles is regulated by Ccs52A, a plant orthologue of the yeast and animal Cdhl proteins, acting as substrate-specific activator of the anaphase-promoting complex E3 ubiquitin ligase. Here, several aspects of endoreduplication are discussed with special attention on nitrogen-fixing nodule development where endoreduplication is an integral part of symbiotic cell differentiation.     

动物细胞全能性的研究

在植物细胞全能性研究的基础上引出动物细胞全能性这一热点研究课题.介绍了动物细胞全能性的表现,分析了细胞全能性表现程度差异的原因,最后对动物细胞全能性的广泛应用及存在的问题进行探讨,并对细胞命运及其调控进行了展望.     

Autophagy and cell-death proteases in plants: two wheels of a funeral cart

Apoptosis is an evolutionarily young cell-death strategy evolved to disassemble animal cells through the action of the caspase family of proteases and phagocytic clearance. This strategy does not work in plants, which instead feature a phylogenetically older autophagic programmed cell death (PCD), as a bona fide type of cellular suicide. Recent work has begun to address the mechanistic roles for autophagic and proteolytic components, as well as their possible cooperation in plant PCD. A recent study has shown autophagosomal localization of a key cell-death proteolytic activity at the early stage of plant PCD. Here we focus on the relationship between autophagic and proteoloytic components in plant PCD at the cellular and organismal levels.     

Cells en route to apoptosis are characterized by the upregulation of c-fos,c-myc,c-jun,cdc2, and RB phosphorylation,resembling events of early cell-cycle traverse

Apoptosis is an important mechanism enabling the selection of the non-self-reactive T cell repertoire and for maintaining homeostasis in the immune system after it has expanded to combat infections. Highly activated, proliferating T cells become susceptible to apoptosis driven by a number of stimuli, and T cells activated during a viral infection become susceptible to “activation induced cell death” after repeated stimulation through the T cell receptor (TcR). This is a major mechanism for the immune deficiencies observed during many viral infections. During infections with a high antigen load this can lead to a selective deletion of virus-specific cytotoxic T lymphocytes (CTL) and to the establishment of persistent infection. More commonly, the CTL control the infection first, and high levels of apoptosis in the expanded lymphocyte population occur after antigen and growth factors become limiting. This cell death does not seem to depend on TcR specificity, as the residual population contains a remarkably stable population of memory CTL precursors that approximate the frequency per CD8 cell of that seen during the peak of the acute infection. Subsequent infections with heterologous viruses result in an expansion and then an apoptotic elimination of T cells, with the consequence being a reduction in precursor CTL specific for the first virus. Thus, apoptosis shapes the quality and quantity of T cell memory. © 1995 Wiley-Liss, Inc.     

Can plant oncogenes inhibit programmed cell death? The rolB oncogene reduces apoptosis-like symptoms in transformed plant cells

The rolB oncogene was previously identified as an important player in ROS metabolism in transformed plant cells. Numerous reports indicate a crucial role for animal oncogenes in apoptotic cell death. Whether plant oncogenes such as rolB can induce programmed cell death (PCD) in transformed plant cells is of particular importance. In this investigation, we used a single-cell assay based on confocal microscopy and fluorescent dyes capable of discriminating between apoptotic and necrotic cells. Our results indicate that the expression of rolB in plant cells was sufficient to decrease the proportion of apoptotic cells in steady-state conditions and diminish the rate of apoptotic cells during induced PCD. These data suggest that plant oncogenes, like animal oncogenes, may be involved in the processes mediating PCD.     

What happened to plant caspases?

The extent of conservation in the programmed cell death pathways that are activated in species belonging to different kingdoms is not clear. Caspases are key components of animal apoptosis; caspase activities are detected in both animal and plant cells. Yet, while animals have caspase genes, plants do not have orthologous sequences in their genomes. It is 10 years since the first caspase activity was reported in plants, and there are now at least eight caspase activities that have been measured in plant extracts using caspase substrates. Various caspase inhibitors can block many forms of plant programmed cell death, suggesting that caspase-like activities are required for completion of the process. Since plant metacaspases do not have caspase activities, a major challenge is to identify the plant proteases that are responsible for the caspase-like activities and to understand how they relate, if at all, to animal caspases. The protease vacuolar processing enzyme, a legumain, is responsible for the cleavage of caspase-1 synthetic substrate in plant extracts. Saspase, a serine protease, cleaves caspase-8 and some caspase-6 synthetic substrates. Possible scenarios that could explain why plants have caspase activities without caspases are discussed.     

Cell cycle regulation in the course of nodule organogenesis in Medicago

The molecular mechanisms of de novo meristem formation, cell differentiation and the integration of the cell cycle machinery into appropriate stages of the developmental programmes are still largely unknown in plants. Legume root nodules, which house nitrogen-fixing rhizobia, are unique plant organs and their development may serve as a model for organogenetic processes in plants. Nodules form and are essential for the plant only under limitation of combined nitrogen in the soil. Moreover, their development is triggered by external mitogenic signals produced by their symbiotic partners, the rhizobia. These signals, the lipochitooligosaccharide Nod factors, act as host-specific morphogens and induce the re-entry of root cortical cells into mitotic cycles. Maintenance of cell division activity leads to the formation of a persistent nodule meristem from which cells exit continuously and enter the nodule differentiation programme, involving multiple cycles of endoreduplication and enlargement of nuclear and cell volumes. While the small diploid 2C cells remain uninfected, the large polyploid cells can be invaded and, after completing the differentiation programme, host the nitrogen-fixing bacteroids. This review summarizes the present knowledge on cell cycle reactivation and meristem formation in response to Nod factors and reports on a novel plant cell cycle regulator that can switch mitotic cycles to differentiation programmes.     

The hypersensitive response and the induction of cell death in plants

The hypersensitive response, or HR, is a form of cell death often associated with plant resistance to pathogen infection. Reactive oxygen intermediates and ion fluxes are proximal responses probably required for the HR. Apoptosis as defined in animal systems is, thus far, not a strict paradigm for the HR. The diversity observed in plant cell death morphologies suggests that there may be multiple pathways through which the HR can be triggered. Signals from pathogens appear to interfere with these pathways. HR may play in plants the same role as certain programmed cell deaths in animals with respect to restricting pathogen growth. In addition, the HR could regulate the defense responses of the plant in both local and distant tissues.     

动植物细胞或组织生物反应器悬浮培养过程中的通气方式

通气在动植物细胞或组织生物反应器培养过程中起着至关重要的作用,而同时通气过程所产生的机械损伤力亦可对细胞造成直接的伤害,因此,通气方式是动植物细胞或组织生物反应器培养过程设计与工程放大的关键技术之一。本文综述了动植物细胞或组织生物反应器悬浮培养过程中三种主要通气(异养培养时又称供氧)方式的结构特点,及其对气液传质、生物量、代谢产物量和细胞损伤的影响,以及改进的新型通气方式和几种通气方式的融合并用。     

Expression of animal CED-9 anti-apoptotic gene in tobacco modifies plasma membrane ion fluxes in response to salinity and oxidative stress

Apoptosis, one form of programmed cell death (PCD), plays an important role in mediating plant adaptive responses to the environment. Recent studies suggest that expression of animal anti-apoptotic genes in transgenic plants may significantly improve a plant’s ability to tolerate a variety of biotic and abiotic stresses. The underlying cellular mechanisms of this process remain unexplored. In this study, we investigated specific ion flux “signatures” in Nicotiana benthamiana plants transiently expressing CED-9 anti-apoptotic gene and undergoing salt- and oxidative stresses. Using a range of electrophysiological techniques, we show that expression of CED-9 increased plant salt and oxidative stress tolerance by altering K⁺ and H⁺ flux patterns across the plasma membrane. Our data shows that PVX/CED-9 plants are capable of preventing stress-induced K⁺ efflux from mesophyll cells, so maintaining intracellular K⁺ homeostasis. We attribute these effects to the ability of CED-9 to control at least two types of K⁺-permeable channels; outward-rectifying depolarization-activating K⁺ channels (KOR) and non-selective cation channels (NSCC). A possible scenario linking CED-9 expression and ionic relations in plant cell is suggested. To the best of our knowledge, this study is the first to link “ion flux signatures” and mechanisms involved in regulation of PCD in plants.     

ER stress is involved in B cell antigen receptor ligation-induced apoptosis

Apoptosis of B cells upon ligation of the B cell antigen receptor (BCR) plays a role in elimination of self-reactive B cells. Previously, BCR ligation was shown to induce expression of the molecules involved in the unfolded protein response (UPR). However, the role of the UPR in BCR-mediated apoptosis is poorly understood. Here, we demonstrate that activation of various UPR molecules are induced when BCR ligation induces apoptosis in the B cell line WEHI-231 and mouse spleen B cells. BCR ligation-induced UPR is attenuated by survival signaling through CD40 in these cells. When overexpression of BiP suppresses the UPR in WEHI-231 cells, activation of p38 MAPK is blocked and apoptosis is reduced. Moreover, the p38 MAPK inhibitor SB203580 reduces BCR ligation-induced apoptosis. These results suggest that the UPR is involved in BCR ligation-induced apoptosis and that p38 MAPK is crucial for apoptosis during the UPR in B cells.     

鸡胚三类组织体外培养细胞的生物学特性分析

采用组织培养的方法获取鸡胚不同组织细胞,利用M199培养基进行原代、传代培养,经形态学观察、生长曲线绘制、分裂指数测定等进行生物学特性分析。实验表明,鸡胚不同组织细胞具有不同的生物学特性,从形态结构到生长周期都有明显差异。获得的躯体来源细胞、心来源细胞为成纤维型,肺来源细胞为上皮型;其中,躯体来源细胞生长能力最强,心来源细胞次之,肺来源细胞最慢,躯体来源细胞倍增时间最短;核型分析表明,该实验体外培养的鸡胚细胞染色体数目为78条。上述结果可为今后鸡胚不同组织细胞的深入研究提供实验材料和依据。