Current Pharmaceutical Design - Volume 14, Issue 2, 2008

Extensive studies have been devoted to mechanisms leading to the activation of cell death. Three major types of cell death are distinguished based on different criteria: apoptosis, oncosis and autophagic cell death. All these processes are well conserved in life evolution as highlighted in the reviews here gathered. Samara and Tavernarakis review the current understanding of cell death pathways in Caenorhabditis elegans, focusing on autophagy, the main cellular process for bulk protein and organelle recycling, in nematode cell death. These studies reveal that autophagic mechanisms have a prominent role in both apoptosis and necrosis. Tettamanti et al. summarize recent findings on the role of autophagy in two different invertebrate taxa, Platyhelminthes and Insects, focusing attention on two complex events occurring in those systems, namely planarian regeneration and insect metamorphosis. Both represent good models in which to investigate the process of autophagy and its relationship with other programmed cell death mechanisms. Malagoli highlights the findings on stress-induced cell death in a new in vitro invertebrate model, i.e. the IPLB-LdFB insect cell line derived from the larval fat body of the lepidopteron Lymantria dispar. Apoptotic, oncotic and autophagic cell death have been described in these cells as a consequence of oxidative stress or ATP deprivation, and similarities between IPLB-LdFB and mammalian apoptotic pathways have been demonstrated. Terahara and Takahashi focus on immunological roles and molecular mechanisms of apoptosis related to functions of hemocytes in molluscan species living in an environment that changes incessantly according to microorganisms, industrial pollutants, temperature, and salinity. Such environmental factors might directly or indirectly induce apoptosis in molluscan cells. Ballarin et al. reported in the the ascidian Botryllus schlosseri natural apoptosis can be studied in different phases of colony life. From these results B. schlosseri is proposed as a new invertebrate species alternative to Drosophila and Caenorhabditis for the study of apoptosis. Dos Santos et al. in their article review structural and functional data on the most important apoptosis-related molecules, namely deathreceptor, Bcl-2 and caspase families, and mechanisms. The data point to the existence in fish of apoptotic pathways equivalent to those of mammals. Silva et al. advance the knowledge about fish on the role of apoptosis in viral infections and of apoptosis and necrosis in bacterial infections. The use of fish for research on apoptosis-related issues relevant for human physiology and pathology and for the design of apoptosismodulating drugs will continue to increase. Penaloza et al. discuss the types and distributions of cell death in developing mammalian embryos as well as the gene products that may regulate the process. Several types of cell death, as identified by their morphological and biochemical features, can be seen in embryos, tissues associated with pregnancy, and in adult organisms. Cell deaths help sculpt the embryo from the grossest to the finest details of its development. Developmental abnormalities can be traced to aberrant patterns of cell death. References [1] Samara C, Tavernarakis N. Autophagy and cell death in Caenorhabditis elegans. Curr Pham Des 2008; 14(2): 97-115. [2] Tettamanti G, Salo E, Gonzales-Estevez C, Felix DA, Grimaldi A, de Eguileor M. Autophagy in invertebrates: insights into development, regeneration and body remodeling. Curr Pham Des 2008; 14(2): 116-125. [3] Malagoli D. Cell death in the IPLB-LdFB insect cell line: Facts and implications. Curr Pham Des 2008; 14(2): 126-130. [4] Terahara K, Takahashi KG. Mechanisms and immunological roles of apoptosis in molluscs. Curr Pham Des 2008; 14(2): 131-137. [5] Ballarin L, Burighel P, Cima F. A tale of death and life: Natural apoptosis in the colonial ascidian Botryllus schlosseri (Urochordata, Ascidiacea). Curr Pham Des 2008; 14(2): 138-147. [6] dos Santos NMS, do Vale A, Reis MIR, Silva MT. Fish and apoptosis: Molecules and pathways. Curr Pham Des 2008; 14(2): 148- 169. [7] Silva MT, do Vale A, dos Santos NMS. Fish and apoptosis: Studies in disease and pharmaceutical design. Curr Pham Des 2008; 14(2): 170-183. [8] Penaloza C, Orlanski S, Ye Y, Entezari-Zaher T, Javdan M, Zakeri Z. Cell death in mammalian development. Curr Pham Des 2008; 14(2): 184-196.

Cell death is a major component of developmental programs. Controlled killing of specific cells at appropriate time points is required for normal growth and shaping of organisms. However, cellular demolition can also result in a variety of pathologies that are frequently fatal, when implemented inappropriately. Delineation of cell death mechanisms has been greatly facilitated by the use of simple model organisms such as the nematode worm Caenorhabditis elegans. Research in C. elegans has proven instrumental for the elucidation of the molecular mechanisms underlying both apoptotic and necrotic cell death. Here, we introduce the C. elegans model and review the current understanding of cell death pathways in this organism. We further focus on recent studies implicating autophagy, the main cellular process for bulk protein and organelle recycling, in nematode cell death. These studies reveal that autophagic mechanisms have a prominent role in both apoptosis and necrosis. We survey the relevant findings in C. elegans and also consider the contribution of autophagy in cell death in other experimental systems. Comparative analysis suggests that the involvement of autophagy in cell death is evolutionary conserved in metazoans. Thus, interfering with the autophagic process may facilitate therapeutic intervention in human pathologies where aberrant cell death is a contributing factor.

Autophagy is a process in which eukaryotic cells sequester and degrade cytoplasm and organelles via the lysosomal pathway. This process allows turnover of intracellular organelles, participates in the maintenance of cellular homeostasis and prevents accumulation of defective cellular structures. Increased autophagy is normally induced by environmental cues such as starvation and hormones, while excessive levels of autophagy can lead to autophagic programmed cell death (PCD), with features that differ from those of the apoptotic PCD process. Since autophagic PCD plays a key role in development, morphogenesis and regeneration in several animal taxa, identification of evolutionarily conserved components of the autophagic machinery is a basic starting point in order to unravel the role of autophagy under both physiological and pathological conditions. Here we summarize recent findings on the role of autophagy in two different invertebrate taxa, Platyhelminthes and Insects, focusing attention on two complex events occurring in those systems, namely planarian regeneration and insect metamorphosis. Both represent good models in which to investigate the process of autophagy and its relationship with other PCD mechanisms.

The present review summarizes findings on stress-induced cell death in the IPLB-LdFB insect cell line derived from the larval fat body of the lepidopteron Lymantria dispar. Apoptotic, oncotic and autophagic cell death have been described in these cells as a consequence of oxidative stress or ATP deprivation, and similarities between IPLB-LdFB and mammalian apoptotic pathways have been highlighted. Furthermore, starting from observations in the IPLB-LdFB cells, a link has been surmised between relevance of autophagic cell death and developmental processes in the metazoan taxa.

Molluscan defense mechanisms are regulated to innate immunity, which is largely dependent on cellular components such as hemocytes possessing phagocytic and bactericidal activities. Among immune responses, apoptosis is an indispensable process because it enables the adequate clearance of damaged, senescent and infected cells without inflammation. Available information related to the molecular mechanisms of apoptosis has been accumulated for many molluscan species during the last decade. Almost all molluscan species live in an environment that changes incessantly according to microorganisms, industrial pollutants, temperature, and salinity. Such environmental factors might directly or indirectly induce apoptosis in molluscan cells. One type of apoptotic agent, reactive oxygen intermediates (ROIs), which are produced by a stress signal or phagocytosis, triggers apoptotic cell death in molluscan hemocytes. Dysfunction of ROI-mediated hemocytic apoptosis putatively causes disease morbidity and/or mortality when molluscan organisms are infected by pathogens. Furthermore, integrins have attracted attention for their unique functions because integrins regulate the phagocytic ability of molluscan hemocytes and induce hemocytic apoptosis. That process might be the result of ROI-generation. In this review, we summarize the roles and molecular mechanisms of apoptosis related to immunological functions of molluscan hemocytes.

The colonial ascidian Botryllus schlosseri forms new zooids by blastogenesis, through the formation of palleal buds which progressively grow and mature until adults are formed. At a temperature of 19°C, adult zooids remain active for about one week; then they contract, close their siphons and are gradually resorbed, being replaced by buds which reach functional maturity, open their siphons and begin their filtering activity as adult zooids. This recurrent generation change, known as take-over, is characterised by the occurrence of diffuse programmed cell death by apoptosis. Immediately before the take-over, an increase in the expression of molecules recognised by anti-Bax antibodies and a parallel decrease in the expression of molecules immunopositive to anti-Bcl-2 antibodies were observed in zooid tissues, suggesting a mitochondrion-dependent apoptotic pathway. During the take-over, circulating phagocytes infiltrate the zooid tissues and engulf apoptotic cells; in addition, the frequency of haemocytes showing nuclear condensation and annexin-V labelling significantly increases. Previous experiments showed the involvement of phosphatidylserine and CD36 in the recognition of effete cell. The resorption of old zooids is closely related to the rejuvenation of the colony occurring at the take-over. The death of adult zooids puts a quantity of material at the colony disposal. This material is represented by senescent cells, which, once ingested and digested by phagocytes, can be recycled and used to sustain the burden of blastogenesis: this involves a cross-talk between old tissues, phagocytes and developing buds. Therefore, B. schlosseri can be considered a new and promising model organism for the study of natural apoptosis.

Apoptosis is a genetically controlled and evolutionarily conserved form of active cell death, albeit with an increase in complexity with continuing development. A high conservation at the functional and molecular level has been described between the players of the apoptotic machinery in invertebrates (Caenorhabditis elegans and Drosophila) and mammals. However, fish represent an excellent and advantageous model for the study of vertebrate development and disease, bridging the gap between the C. elegans/Drosophila and mouse/human models. Moreover, contrary to C. elegans and Drosophila, fish can be used for studying the development and function of vertebrate-specific organs and have a fully developed immune system similar to that of mammals. Last but not less important, both the environment and human health will obviously gain by using the knowledge generated through the use of fish models, for developing better prophylactic and therapeutic measures with impact on the aquaculture industry. In the present article, structural and functional data on the most important apoptosis related molecules, namely death-receptor, Bcl-2 and caspase families, and mechanisms are reviewed. The data point to the existence in fish of apoptotic pathways equivalent to those of mammals, making fish useful animal models for studying apoptosis, which may have great applicability for the advance of the knowledge on the role of apoptotic cell death in human apoptosisrelated disorders as well as in pharmaceutical design.

The relevance of fish research has been rising due to the expansion of aquaculture and to the increasing use of fish as replacements for mammals in the study of human physiological and pathological issues. Fish have much smaller genomes compared to mammals, and zebrafish, fugu, medaka and spotted green puffer fish have the sequence of their genomes completed or near completion. Fish have several of the virtues of Drosophila melanogaster and Caenorhabditis elegans for apoptosis research, but offer additional advantages because they are vertebrates and have a developed immune system and apoptotic pathways similar to those of mammals. Many phenotypes in the zebrafish resemble human diseases and this fish has been increasingly used in pharmaceutical design of apoptosis modulating drugs. The roles of microRNAs, bcl-2, p53, insulin-like growth factor-binding protein-3, and cellular apoptosis susceptibility (CAS) and c-Myc genes (involved in the interaction apoptosis/cancer), and Aβ peptides, presenilin enhancer 2, cyclin-dependent kinase 5 and tau (factors with relevant roles in apoptosis-associated human neurodegenerative disorders), have also been successfully investigated in fish models. Results of research with fish that have advanced the knowledge on the participation of apoptosis in viral infections and of apoptosis and secondary necrosis in bacterial infections are also reviewed. It is expectable that the use of fish for research on apoptosisrelated issues relevant for human physiology and pathology and for the design of apoptosis-modulating drugs will continue to increase.

During embryogenesis there is an exquisite orchestration of cellular division, movement, differentiation, and death. Cell death is one of the most important aspects of organization of the developing embryo, as alteration in timing, level, or pattern of cell death can lead to developmental anomalies. Cell death shapes the embryo and defines the eventual functions of the organs. Cells die using different paths; understanding which path a dying cell takes helps us define the signals that regulate the fate of the cell. Our understanding of cell death in development stems from a number of observations indicating genetic regulation of the death process. With today's increased knowledge of the pathways of cell death and the identification of the genes whose products regulate the pathways we know that, although elimination of some of these gene products has no developmental phenotype, alteration of several others has profound effects. In this review we discuss the types and distributions of cell death seen in developing mammalian embryos as well as the gene products that may regulate the process.