How to kill a mocking bug?

All metazoans have evolved means to protect themselves from threats present in the environment: injuries, viruses, fungi, bacteria and other parasites. Insect protection includes innate physical barriers and both cellular and humoral responses. The insect innate immune response, best characterized in Drosophila melanogaster, is a rapid broad response, triggered by pathogen-associated molecular patterns (PAMPs) recognition, which produces a limited range of effectors that does not alter upon continued pathogen exposure and lacks immunological memory. The Drosophila response, particularly its humoral response, has been investigated by both low and high-throughput methods. Three signalling pathways conserved between insects and mammals have been implicated in this response: Toll (equivalent to mammalian TLR), Imd (equivalent to TNFalpha) and Hop (equivalent to JAK/STAT). This review provides an entry point to the insect immune system literature outlining the main themes in D. melanogaster bacterial pathogen detection and humoral and cellular immune responses. The Drosophila immune response is compared with other insects and the mammalian immune system.     

Does autophagy have a license to kill mammalian cells?

Macroautophagy is an evolutionarily conserved vacuolar, self-digesting mechanism for cellular components, which end up in the lysosomal compartment. In mammalian cells, macroautophagy is cytoprotective, and protects the cells against the accumulation of damaged organelles or protein aggregates, the loss of interaction with the extracellular matrix, and the toxicity of cancer therapies. During periods of nutrient starvation, stimulating macroautophagy provides the fuel required to maintain an active metabolism and the production of ATP. Macroautophagy can inhibit the induction of several forms of cell death, such as apoptosis and necrosis. However, it can also be part of the cascades of events that lead to cell death, either by collaborating with other cell death mechanisms or by causing cell death on its own. Loss of the regulation of bulk macroautophagy can prime self-destruction by cells, and some forms of selective autophagy and non-canonical forms of macroautophagy have been shown to be associated with cell demise. There is now mounting evidence that autophagy and apoptosis share several common regulatory elements that are crucial in any attempt to understand the dual role of autophagy in cell survival and cell death.     

Yeast as a model to study apoptosis?

Programmed cell death (PCD) serves as a major mechanism for the precise regulation of cell numbers, and as a defense mechanism to remove unwanted and potentially dangerous cells. Despite the striking heterogeneity of cell death induction pathways, the execution of the death program is often associated with characteristic morphological and biochemical changes termed apoptosis. Although for a long time the absence of mitochondrial changes was considered as a hallmark of apoptosis, mitochondria appear today as the central executioner of programmed cell death. This crucial position of mitochondria in programmed cell death control is not due to a simple loss of function (deficit in energy supplying), but rather to an active process in the regulation of effector mechanisms.The large diversity of regulators of apoptosis in mammals and their numerous interactions complicate the analysis of their individual functions. Yeast, eukaryotic but unicellular organism, lack the main regulators of apoptosis (caspases, Bcl-2 family members, . . .) found in mammals. This absence render them a powerful tool for heterologous expression, functional studies, and even cloning of new regulators of apoptosis.Great advances have thus been made in our understanding of the molecular mechanisms of Bcl-2 family members interactions with themselves and other cellular proteins, specially thanks to the two hybrid system and the easy manipulation of yeast (molecular biology and genetics).This review will focus on the use of yeast as a tool to identify new regulators and study function of mammalian apoptosis regulators.     

Does yeast shmooing mean a commitment to apoptosis?

Treatment of yeast Saccharomyces cerevisiae with alpha-pheromone has been reported to lead to massive apoptosis of cells finding no conjugation partner [Severin FF, Hyman AA. Pheromone induces programmed cell death in S. cerevisiae. Curr Biol 2002;12:R233-5]. We report here that this effect is not common in yeast. Using different yeast strains, we demonstrate that identical treatment results in a low mortality even after prolonged treatment with the pheromone. These findings are followed by a general discussion of the biological relevance of apoptosis in yeast.     

Granzymes in cytolytic lymphocytes--to kill a killer?

Granzymes (gzm) are major components of the granules of cytolytic lymphocytes, natural killer and cytotoxic T cells. Their generally accepted mode of action consists of their directed secretion towards a virus-infected or neoplastic target cell and perforin-dependent delivery to the target cell cytosol, where they engage in various actions resulting in target cell apoptosis. Here, based on observations of infection of gzmAxB(-/-) mice with ectromelia virus, mousepox, we propose an additional--and distinct--function for gzmA and B. In this model, gzm constitute one of the first lines of defence of immune cells against virus infection of immune cells themselves. Accordingly, endogenous gzm interfere with viral replication in cytolytic lymphocytes either directly, as a result of their proteolytic activity, leading to destruction of viral proteins, or indirectly, via: (i) processes akin to the caspase cascade when acting as effector molecules in the induction of target cell apoptosis; or (ii) their capacity to induce early inflammatory mediators. We discuss the predictions of the model in the light of available data.     

P53 licensed to kill? Operating the assassin

The p53 protein is a key player in the cellular response to stress. Proper regulation of p53 is imperative for the suppression of tumor development. This regulation is largely governed by its master inhibitor, Mdm2, which both blocks p53 activities and promotes its destabilization. This tight regulation of p53 by Mdm2 must be interrupted under stress conditions in order for p53 to be stabilized in an active form. A combined action of partner proteins and modifying enzymes is essential for the relief of p53 from Mdm2. The recent revelation of p53 association with the PML-nuclear bodies provides one explanation of how this regulatory network is coordinated within the nucleus in response to certain stress conditions. Thus, it is not only the nature of the p53 regulatory complex but also the spatial and temporal context of this association that governs the output inhibitory signals mediated by p53.     

Preventing retinal apoptosis — Is there a common therapeutic theme?

There is an urgent need for therapies for retinal diseases; retinitis pigmentosa sufferers have no treatment options available and those targeted at other retinopathies have shown limited effectiveness. The process of programmed cell death or apoptosis although complex, remains a possible target for the treatment of retinal diseases. Having identified apoptosis in the vertebrate retina in populations of immature neurons as an essential part of development it was proposed that re-activation of these developmental cell death pathways might provide insight into the death mechanisms operating in retinal diseases. However, the discovery that numerous factors initiate and mediate the apoptotic cascade in mature photoreceptors has resulted in a relatively untargeted approach to examining and arresting apoptosis in the retina. In the last 5 years, mouse models have been treated with a diverse range of drugs or factors including anti-oxidants, growth factors, steroid hormones, calcium/calpain inhibitors and tetracycline antibiotics. Therefore to draw a unifying theme from these broad research areas is challenging. However, this review focusses on two targets which are currently under investigation, reactive oxygen species and mammalian target of rapamycin, drawing together the common themes of these research areas.     

Sensitization of T cells to apoptosis—A role for ROS?

T cell homeostasis is achieved by balancing the production and proliferation of T cells with their apoptotic cell death. Activation of naïve T cells by antigen in the context of MHC results in the massive expansion of antigen-specific T cells and the production of reactive oxygen species (ROS). Following expansion, the majority of the T cells die via apoptosis, while a small number of them survive and differentiate into memory T cells. This cell fate decision is crucial to our understanding of how autoimmunity is avoided and how immunity is maintained. It has become increasingly clear that ROS can affect this cell fate decision by sensitizing T cells to apoptosis. Interestingly, ROS have effects on both intrinsic and extrinsic apoptosis pathways through modulation of expression of the major molecules in these pathways, Bcl-2 and FasL. In this review, we will focus on the pro-apoptotic effects of ROS and mechanisms by which they regulate the death of T cells.     

The actin cytoskeleton: a key regulator of apoptosis and ageing?

Evidence from many organisms has shown that the accumulation of reactive oxygen species (ROS) has a detrimental effect on cell well-being. High levels of ROS have been linked to programmed cell death pathways and to ageing. Recent reports have implicated changes to the dynamics of the actin cytoskeleton in the release of ROS from mitochondria and subsequent cell death.     

“Licensed to kill”: Tyrosine dephosphorylation and Bak activation

The genomes of multicellular organisms are under constant assault from a host of environmental agents. The efficient elimination of cells harboring damage is essential to avoid the accumulation of deleterious changes that may promote tumorigenesis. Consequently, a complex and elaborate series of damage responses have evolved to either ensure that correct repair of the DNA has been carried out, or alternatively, to initiate programmes that result in the ablation of the damaged cell. Apoptosis is recognized as both a fast an efficient way of disposing of damaged or unwanted cells before they accumulate changes that may result in the acquisition of neoplastic autonomy. The mitochondrial apoptotic pathway relies upon two effector proteins of the Bcl2 family, Bax and Bak, that when activated form pores in the outer mitochondrial membrane that release cytochrome c and other apoptogenic factors. We have recently shown that the initiation of Bak activation is controlled by dephosphorylation. In particular, we found that a specific tyrosine dephosphorylation was required for Bak activation to proceed and that tyrosine phosphatases may serve to integrate apoptotic signals that culminate in Bak dephosphorylation. Here, we discuss these findings and present additional data underlining the importance of dephosphorylation in the Bak activation process and how the modulation of Bak phosphorylation status may be modified to enhance cell killing.Key words: apoptosis, mitochondria, phosphatase, BH3, p53, Bak, signaling, phosphorylation     

HIV-1 Tat, apoptosis and the mitochondria: a tubulin link?

A recent in silico search for coding sequences of retroviral origin present in the human genome has unraveled two new envelope genes that add to the 16 genes previously identified. A systematic search among the latter for a fusogenic activity had led to the identification of two bona fide genes, named syncytin-1 and syncytin-2, most probably co-opted by primate genomes for a placental function related to the formation of the syncytiotrophoblast by cell-cell fusion. Here, we show that one of the newly identified envelope gene, named env P(b), is fusogenic in an ex vivo assay, but that its expression – as quantified by real-time RT-PCR on a large panel of human tissues – is ubiquitous, albeit with a rather low value in most tissues. Conversely, the second envelope gene, named env V, discloses a placenta-specific expression, but is not fusogenic in any of the cells tested. Altogether, these results suggest that at least one of these env genes may play a role in placentation, but most probably through a process different from that of the two previously identified syncytins.