Extracellular proteases and neuronal cell death.

Neuronal cell death occurs during development of the central nervous system as well as in pathological situations such as acute injury and progressive degenerative diseases. For instance, granule cells in the developing cerebellum and neuronal precursor cells in the cortex undergo programmed cell death, or apoptosis. There is currently strong debate conceming the mechanism of death in many degenerative events such as ischemia, blunt head trauma, excitotoxicity and neurodegenerative diseases, i.e. Alzheimer's disease. Neurons can die a necrotic death when the initial insult is too great; apoptosis requires "planning." For example, the cell death seen in the core of an ischemic infarct is necrotic, while in the surrounding penumbra region the death is probably apoptotic. Regardless of the degenerative pathway, damaged or dead neurons are a hallmark of many diseases including Alzheimer's, Parkinson's, glaucoma, ischemia and multiple sclerosis. Molecules such as cytokines, chemokines, reactive nitrogen/oxygen species, and proteases play an important role in promoting and/or mediating neurodegeneration. Proteases have been implicated in both physiological and pathological events, suggesting their intervention in key points when things go awry. In this review we will summarize recent findings linking extracellular proteases with neuronal cell death in both human diseases and their animal models.     

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.     

Baculovirus p35 prevents developmentally programmed cell death and rescues a ced-9 mutant in the nematode Caenorhabditis elegans.

Programmed cell death, or apoptosis, occurs throughout the course of normal development in most animals and can also be elicited by a number of stimuli such as growth factor deprivation and viral infection. Certain morphological and biochemical characteristics of programmed cell death are similar among different tissues and species. During development of the nematode Caenorhabditis elegans, a single genetic pathway promotes the death of selected cells in a lineally fixed pattern. This pathway appears to be conserved among animal species. The baculovirus p35-encoding gene (p35) is an inhibitor of virus-induced apoptosis in insect cells. Here we demonstrate that expression of p35 in C. elegans prevents death of cells normally programmed to die. This suppression of developmentally programmed cell death results in appearance of extra surviving cells. Expression of p35 can rescue the embryonic lethality of a mutation in ced-9, an endogenous gene homologous to the mammalian apoptotic suppressor bcl-2, whose absence leads to ectopic cell deaths. These results support the hypothesis that viral infection can activate the same cell death pathway as is used during normal development and suggest that baculovirus p35 may act downstream or independently of ced-9 in this pathway.     

细胞凋亡中的钙离子调控

凋亡是细胞的一种生理性、主动性的自杀行为,它使机体能够有效清除多余或病态的细胞。作为细胞内普遍存在的第二信使,Ca2 在信号转导过程中发挥重要作用。它能够将细胞感受的刺激转化为其在不同细胞组分间的分布差异及自身浓度的振荡,这种在细胞内和细胞间的波动协调了细胞生命活动的各个方面。以往的研究认为细胞内Ca2 浓度的升高是凋亡进行到后期的结果,而最近的研究发现Ca2 也可以在凋亡通路的各个层次,通过不同的方式精细调控凋亡的进程,这构成了凋亡中复杂的钙调控网络。现对钙离子和线粒体凋亡途径中分子间的复杂联系以及钙调控细胞凋亡研究的最新进展进行综述。     

Programmed Cell Death in Floral Organs: How and Why do Flowers Die?

BACKGROUND: Flowers have a species-specific, limited life span with an irreversible programme of senescence, which is largely independent of environmental factors, unlike leaf senescence, which is much more closely linked with external stimuli. TIMING: Life span of the whole flower is regulated for ecological and energetic reasons, but the death of individual tissues and cells within the flower is co-ordinated at many levels to ensure correct timing. Some floral cells die selectively during organ development, whereas others are retained until the whole organ dies. TRIGGERS: Pollination is an important floral cell death trigger in many species, and its effects are mediated by the plant growth regulator (PGR) ethylene. In some species ethylene is a major regulator of floral senescence, but in others it plays a very minor role and the co-ordinating signals involved remain elusive. Other PGRs such as cytokinin and brassinosteroids are also important but their role is understood only in some specific systems. MECHANISMS: In two floral cell types (the tapetum and the pollen-tube) there is strong evidence for apoptotic-type cell death, similar to that in animal cells. However, in petals there is stronger evidence for an autophagous type of cell death involving endoplasmic reticulum-derived vesicles and the vacuole. Proteases are important, and homologues to animal caspases, key regulators of animal cell death, exist in plants. However, their role is not yet clear. COMPARISON WITH OTHER ORGANS: There are similarities to cell death in other plant organs, and many of the same genes are up-regulated in both leaf and petal senescence; however, there are also important differences for example in the role of PGRs. CONCLUSIONS: Understanding gene regulation may help to understand cell death in floral organs better, but alone it cannot provide all the answers.     

Flexibility exists in the region of [A6-A11, A7-B7] disulfide bonds during insulin precursor folding

Programmed cell death or apoptosis is the regulatory mechanism for removing unneeded cells during animal development and in tissue homeostasis. Perturbation of the cell death mechanisms leads to various disorders, including neurodegenerative diseases, immunodeficiency diseases, and tumors. c-Jun N-terminal kinase (JNK) has crucial roles in the regulation of cell death in response to many stimuli. Since JNK is highly conserved from yeast to mammals, genetic studies using model animals are helpful in understanding the principal cell death mechanisms regulated by JNK. For example, loss-of-function studies using the targeted disruption of murine genes have established the genetic framework of the mechanisms of the cell death induced by UV radiation. Also, in Drosophila, many cell death-related genes have been identified by genetics. Genetic studies of JNK-dependent cell death mechanisms should shed light on the regulation of both physiological and pathological cell death.     

线虫发育模式的分子生物学研究概况

线虫是研究动物发育的理想模式系统.由其细胞谱系研究深入到细胞间相互作用及信号传递的研究、细胞程序性死亡的研究,乃至多基因相互作用调控发育的研究,这些都是今后这一领域的研究热点.     

Role of autophagy in Caenorhabditis elegans

Autophagy is an evolutionarily conserved intracellular catabolic system. During Caenorhabditis elegans development, autophagy plays an important role in many physiological processes, including survival under starvation conditions, modulation of life span, and regulation of necrotic cell death caused by toxic ion-channel variants. Recently, it has been demonstrated that during embryogenesis, basal levels of autophagy selectively remove a group of proteins in somatic cells, including the aggregate-prone components of germline P granules. Degradation of these protein aggregates provides a genetic model to identify essential autophagy components and also to elucidate how the autophagic machinery selectively recognizes and degrades specific targets during animal development.     

Programmed cell death in cereal aleurone

Progress in understanding programmed cell death (PCD) in the cereal aleurone is described. Cereal aleurone cells are specialized endosperm cells that function to synthesize and secrete hydrolytic enzymes that break down reserves in the starchy endosperm. Unlike the cells of the starchy endosperm, aleurone cells are viable in mature grain but undergo PCD when germination is triggered or when isolated aleurone layers or protoplasts are incubated in gibberellic acid (GA). Abscisic acid (ABA) slows down the process of aleurone cell death and isolated aleurone protoplasts can be kept alive in media containing ABA for up to 6 months. Cell death in barley aleurone occurs only after cells become highly vacuolated and is manifested in an abrupt loss of plasma membrane integrity. Aleurone cell death does not follow the apoptotic pathway found in many animal cells. The hallmarks of apoptosis, including internucleosomal DNA cleavage, plasma membrane and nuclear blebbing and formation of apoptotic bodies, are not observed in dying aleurone cells. PCD in barley aleurone cells is accompanied by the accumulation of a spectrum of nuclease and protease activities and the loss of organelles as a result of cellular autolysis.     

Cell death and the developing enteric nervous system

This review discusses current knowledge about cell death in the developing enteric nervous system (ENS). It also includes findings about the molecular mechanisms by which such death is mediated. Additional consideration is given to trophic factors that contribute to survival of the precursors and neurons and glia of the ENS, as well to genes that, when mutated or deleted, trigger their death. Although further confirmation is needed, present observations support the view that enteric neural crest-derived precursor cells en route to the gut undergo substantial levels of apoptotic death, but that once these cells colonize the gut, there is relatively little death of precursor cells or of neurons and glia during the fetal period. There are also indications that normal neuron loss occurs in the ENS, but at times beyond the perinatal stage. Taken together, these findings suggest that ENS development is similar is some ways, but different in others from extra-enteric areas of the vertebrate central and peripheral nervous systems, in which large-scale apoptotic death of precursor neurons and glia occurs during the fetal and perinatal periods. Potential reasons for these differences are discussed such as a fetal enteric microenvironment that is especially rich in trophic support. In addition to the cell death that occurs during normal ENS development, this review discusses mechanisms of experimentally-induced ENS cell death, such as those that are associated with defective glial cell-line derived neurotrophic factor/Ret signaling, which are an animal model of human congenital megacolon (aganglionosis; Hirschsprung’s disease). Such considerations underscore the importance of understanding cell death in the developing ENS, not just from a curiosity-driven point of view, but also because the pathophysiology behind many disorders of human gastrointestinal function may originate in abnormalities of the mechanisms that govern cell survival and death during ENS development.     

Uncommitted Xenopus blastula cells can be directed to uniform muscle gene expression by gradient interpretation and a community effect

The animal cap cells of Xenopus blastulae behave as multi-potent stem cells in so far as they can differentiate along many unrelated pathways according to the kind and amount of signal factor that they experience. At first, animal cap cells activate early zygotic genes across a broad range of TGFbeta concentrations; soon after this, they activate later genes more intensely and over a narrow concentration range. Here we show that uncommitted blastula cells can be directed, by the sequential influence of a particular concentration of a TGFbeta morphogen and an FGF-mediated community effect, to form a homogeneous single cell type. As a result of these two signalling processes, an entire population of animal cap cells can be converted, in the absence of other signals, to a uniform population of one tissue type. Mesoderm cells that experience a particular concentration of activin increase their XMyoD expression by 10-fold and become distinct from neighbouring cells that received lower or higher concentrations of activin. The signalling processes that we employ here may be important in normal development and useful in guiding stem cell differentiation.     

Programmed cell death and its clinical implications

Cell death is a highly regulated process that is ubiquitous in all eukaryotes. Programmed cell death (PCD) is an integral part of both animal and plant development. Studies on apoptosis, the well characterized form of programmed cell death led to the identification of a central tripartite death switch i.e. apoptosome consisting of Apaf-1, Apaf-2 and Apaf-3. The caspases, a family of cysteine-dependent aspartate directed-proteases, constitute the central executioners of apoptosis. Much of the attention on programmed cell death is focused on caspases, however, cell death can still occur even when the caspase cascade is blocked, revealing the existence of nonapoptotic alternative pathway(s) of cell death. The mitochondrial release of cytochrome C following a PCD inducing stimulus in both plants and animals suggests the evolutionary conservation of death pathways. Dysregulation of apoptosis may be related to the development of several disease states as well as ageing. Excessive apoptosis is associated with neurodegenerative disorders, AIDS etc., whereas deficient apoptosis is associated with cancer, auto-immunity, viral infections etc. Understanding the regulation of programmed cell death would throw light in designing drugs and gene therapies that can target specific molecules in the apoptotic pathway opening the vistas for new therapeutic endeavors in many areas of medicine.     

Sphingolipids in apoptosis, survival and regeneration in the nervous system

Simple sphingolipids such as ceramide, sphingosine and sphingosine 1-phosphate are key regulators of diverse cellular functions. Their roles in the nervous system are supported by extensive evidence derived primarily from studies in cultured cells. More recently animal studies and studies with human samples have revealed the importance of ceramide and its metabolites in the development and progression of neurodegenerative disorders. The roles of sphingolipids in neurons and glial cells are complex, cell dependent, and many times contradictory. In this review I will summarize the effects elicited by ceramide and ceramide metabolites in cells of the nervous system, in particular those effects related to cell survival and death, emphasizing the molecular mechanisms involved. I also discuss recent evidence for the implication of sphingolipids in the development and progression of certain dementias.     

Sphingolipids in apoptosis, survival and regeneration in the nervous system

Simple sphingolipids such as ceramide, sphingosine and sphingosine 1-phosphate are key regulators of diverse cellular functions. Their roles in the nervous system are supported by extensive evidence derived primarily from studies in cultured cells. More recently animal studies and studies with human samples have revealed the importance of ceramide and its metabolites in the development and progression of neurodegenerative disorders. The roles of sphingolipids in neurons and glial cells are complex, cell dependent, and many times contradictory. In this review I will summarize the effects elicited by ceramide and ceramide metabolites in cells of the nervous system, in particular those effects related to cell survival and death, emphasizing the molecular mechanisms involved. I also discuss recent evidence for the implication of sphingolipids in the development and progression of certain dementias.     

Inherited retinal degenerations: therapeutic prospects

Retinitis pigmentosa (RP) is a heterogeneous group of inherited retinal degenerative diseases, characterized by the progressive death of rod and cone photoreceptors. A tremendous genetic heterogeneity is associated with the RP phenotype. Most mutations affect rods selectively and, through an unknown pathway, cause the rod cells to die by apoptosis. Cones, on the other hand, are seldom directly affected by the identified mutations, and yet, in many cases, they degenerate secondarily to rods, which accounts for loss of central vision and complete blindness. Many animal models of RP are available and have led to a better understanding of the disease and to the development of therapeutic strategies aimed at curing the specific genetic disorder (gene therapy), slowing down or even stopping the process of photoreceptor degeneration (growth factors or calcium blockers applications, vitamin supplementation), preserving the cones implicated in the central visual function (identification of endogenous cone viability factors) or even replacing the lost cells (transplantation, use of stem or precursor cells). Still, many obstacles will need to be overcome before most of these strategies can be applied to humans. In this review, we describe the different therapeutic strategies being studied worldwide and report the latest results in this field.     

Metabolic engineering of apoptosis in cultured animal cells: implications for the biotechnology industry

Animal cells have been widely used to obtain a wide range of products for human and animal healthcare applications. However, the extreme sensitivity of these cells in respect to changes experienced in their environment is evidenced by the activation of a gene-encoded program known as apoptosis, resulting in their death and destruction. From the bioprocess angle, losses in cell viability bring lower productivities and higher risks of product degradation. Consequently, many research efforts have been devoted to the development of apoptosis protective mechanisms, including the metabolic engineering of apoptosis pathways, that has proven effective in diminishing programmed cell death in a variety of biotechnological relevant cell lines. This review is focused especially in the encouraging initial results obtained with the over-expression of cloned anti-apoptosis genes, from both endogenous and viral origin interfering at mitochondrial and initiator caspases levels.     

Pathways to photoreceptor cell death in inherited retinal degenerations.

The mutations that cause many forms of inherited retinal degenerations have been identified, yet the mechanisms by which these mutations lead to death of photoreceptor cells of the retina are not completely understood. Investigations of the pathways from mutation to retinal degeneration have focused on spontaneous and engineered animal models of disease. Based on the studies performed to date, four major categories of degeneration mechanism can be identified. These include disruption of photoreceptor outer segment morphogenesis, metabolic overload, dysfunction of retinal pigment epithelial cells, and chronic activation of phototransduction. Future investigations will likely identify additional mechanisms of photoreceptor damage. This review will summarize what has been learned from studying animal models of non-syndromic inherited retinal degenerations.     

NF-kappaB protects lung epithelium against hyperoxia-induced nonapoptotic cell death-oncosis

Prolonged exposure to hyperoxia induces pulmonary epithelial cell death and acute lung injury. Although both apoptotic and nonapoptotic morphologies are observed in hyperoxic animal lungs, nonapoptotic cell death had only been recorded in transformed lung epithelium cultured in hyperoxia. To test whether the nonapoptotic characteristics in hyperoxic animal lungs are direct effects of hyperoxia, the mode of cell death was determined both morphologically and biochemically in human primary lung epithelium exposed to 95% O(2). In contrast to characteristics observed in apoptotic cells, hyperoxia induced swelling of nuclei and an increase in cell size, with no evidence for any augmentation in the levels of either caspase-3 activity or annexin V incorporation. These data suggest that hyperoxia can directly induce nonapoptotic cell death in primary lung epithelium. Although hyperoxia-induced nonapoptotic cell death was associated with NF-kappaB activation, it is unknown whether NF-kappaB activation plays any causal role in nonapoptotic cell death. This study shows that inhibition of NF-kappaB activation can accelerate hyperoxia-induced epithelial cell death in both primary and transformed lung epithelium. Corresponding to the reduced cell survival in hyperoxia, the levels of MnSOD were also low in NF-kappaB-deficient cells. These results demonstrate that NF-kappaB protects lung epithelial cells from hyperoxia-induced nonapoptotic cell death.