Caspase-mediated activation of Caenorhabditis elegans CED-8 promotes apoptosis and phosphatidylserine externalization

Chrysin-organogermanium (IV) complex induced Colo205 cell apoptosis-associated mitochondrial function and anti-angiogenesis

Colorectal cancer, a kind of malignant cancer, has more than 1 million new patients and results in 0.5 million deaths every year globally based on the estimation of Globocan in 2008. One of the most important issues against colon cancer is tumor metastasis. Anti-angiogenesis, a form of targeted therapy uses drugs or other substances to prevent the new blood vessel formation, which is critical for tumor metastasis. In our previous studies, we have demonstrated a simple method to synthesize Chry-Ge complex through the reaction between chrysin and triphenylgermanium bromide. In this work, we investigated the mechanism of Chry-Ge induced Colo205 cell apoptosis. We found that Chry-Ge could induce apoptosis in Colo205 cells in mitochondrial-dependent pathway, cause the reorganization of cytoskeleton and induce the damage of nucleus in Colo205 cells. Besides, Chry-Ge was also found to induce membrane ultrastructural changes in Colo205 cells by AFM. Further, we found that Chry-Ge can inhibit tube formation of human umbilical vascular endothelial cell in vitro. Chry-Ge was also tested in vivo in the chicken chorioallantoic membrane (CAM) assay and found to inhibit bFGF-treated CAMs development. These results suggested that Chry-Ge could induce Colo205 cell apoptosis by mitochondrial pathway and anti-angiogenesis, highlighting the use of organic germanium agents for the treatment of colorectal cancer.

Necrotic Cells Actively Attract Phagocytes through the Collaborative Action of Two Distinct PS-Exposure Mechanisms

Necrosis, a kind of cell death closely associated with pathogenesis and genetic programs, is distinct from apoptosis in both morphology and mechanism. Like apoptotic cells, necrotic cells are swiftly removed from animal bodies to prevent harmful inflammatory and autoimmune responses. In the nematode Caenorhabditis elegans, gain-of-function mutations in certain ion channel subunits result in the excitotoxic necrosis of six touch neurons and their subsequent engulfment and degradation inside engulfing cells. How necrotic cells are recognized by engulfing cells is unclear. Phosphatidylserine (PS) is an important apoptotic-cell surface signal that attracts engulfing cells. Here we observed PS exposure on the surface of necrotic touch neurons. In addition, the phagocytic receptor CED-1 clusters around necrotic cells and promotes their engulfment. The extracellular domain of CED-1 associates with PS in vitro. We further identified a necrotic cell-specific function of CED-7, a member of the ATP-binding cassette (ABC) transporter family, in promoting PS exposure. In addition to CED-7, anoctamin homolog-1 (ANOH-1), the C. elegans homolog of the mammalian Ca(2+)-dependent phospholipid scramblase TMEM16F, plays an independent role in promoting PS exposure on necrotic cells. The combined activities from CED-7 and ANOH-1 ensure efficient exposure of PS on necrotic cells to attract their phagocytes. In addition, CED-8, the C. elegans homolog of mammalian Xk-related protein 8 also makes a contribution to necrotic cell-removal at the first larval stage. Our work indicates that cells killed by different mechanisms (necrosis or apoptosis) expose a common "eat me" signal to attract their phagocytic receptor(s); furthermore, unlike what was previously believed, necrotic cells actively present PS on their outer surfaces through at least two distinct molecular mechanisms rather than leaking out PS passively.

Substrate trajectory through phospholipid-transporting P4-ATPases

A difference in the lipid composition between the two leaflets of the same membrane is a relatively simple instance of lipid compositional heterogeneity. The large activation energy barrier for transbilayer movement for some (but not all) membrane lipids creates a regime governed by active transport processes. An early step in eukaryote evolution was the development of a capacity for generating transbilayer compositional heterogeneity far from equilibrium by directly tapping energy from the ATP pool. The mechanism of the P-type ATPases that create lipid asymmetry is well understood in terms of ATP hydrolysis, but the trajectory taken by the phospholipid substrate through the enzyme is a matter of current active research. There are currently three different models for this trajectory, all with support by mutation/activity measurements and analogies with known atomic structures.

The role of the effector caspases drICE and dcp-1 for cell death and corpse clearance in the developing optic lobe in Drosophila

In the developing Drosophila optic lobe, cell death occurs via apoptosis and in a distinctive spatio-temporal pattern of dying cell clusters. We analyzed the role of effector caspases drICE and dcp-1 in optic lobe cell death and subsequent corpse clearance using mutants. Neurons in many clusters required either drICE or dcp-1 and each one is sufficient. This suggests that drICE and dcp-1 function in cell death redundantly. However, dying neurons in a few clusters strictly required drICE but not dcp-1, but required drICE and dcp-1 when drICE activity was reduced via hypomorphic mutation. In addition, analysis of the mutants suggests an important role of effecter caspases in corpse clearance. In both null and hypomorphic drICE mutants, greater number of TUNEL-positive cells were observed than in wild type, and many TUNEL-positive cells remained until later stages. Lysotracker staining showed that there was a defect in corpse clearance in these mutants. All the results suggested that drICE plays an important role in activating corpse clearance in dying cells, and that an additional function of effector caspases is required for the activation of corpse clearance as well as that for carrying out cell death.

Live to die another way: modes of programmed cell death and the signals emanating from dying cells

All life ends in death, but perhaps one of life's grander ironies is that it also depends on death. Cell-intrinsic suicide pathways, termed programmed cell death (PCD), are crucial for animal development, tissue homeostasis and pathogenesis. Originally, PCD was almost synonymous with apoptosis; recently, however, alternative mechanisms of PCD have been reported. Here, we provide an overview of several distinct PCD mechanisms, namely apoptosis, autophagy and necroptosis. In addition, we discuss the complex signals that emanate from dying cells, which can either trigger regeneration or instruct additional killing. Further advances in understanding the physiological roles of the various mechanisms of cell death and their associated signals will be important to selectively manipulate PCD for therapeutic purposes.

CED-3 caspase acts with miRNAs to regulate non-apoptotic gene expression dynamics for robust development in C. elegans

Genetic redundancy and pleiotropism have limited the discovery of functions associated with miRNAs and other regulatory mechanisms. To overcome this, we performed an enhancer screen for developmental defects caused by compromising both global miRISC function and individual genes in Caenorhabditis elegans. Among 126 interactors with miRNAs, we surprisingly found the CED-3 caspase that has only been well studied for its role in promoting apoptosis, mostly through protein activation. We provide evidence for a non-apoptotic function of CED-3 caspase that regulates multiple developmental events through proteolytic inactivation. Specifically, LIN-14, LIN-28, and DISL-2 proteins are known miRNA targets, key regulators of developmental timing, and/or stem cell pluripotency factors involved in miRNA processing. We show CED-3 cleaves these proteins in vitro. We also show CED-3 down-regulates LIN-28 in vivo, possibly rendering it more susceptible to proteasomal degradation. This mechanism may critically contribute to the robustness of gene expression dynamics governing proper developmental control.

DOI:http://dx.doi.org/10.7554/eLife.04265.001

For an organism to develop from a single cell into a collection of many different, specialized cells, different genes must be switched on or off at particular times. However, some of these genes involved in development are ‘redundant’ and carry out the same or similar tasks. This acts like a backup system, so if one of the genes is unable to complete a task, the others can compensate and the organism will still develop correctly.

To produce a protein from a gene, the DNA sequence that makes up the gene is used as a template to create another molecule called messenger RNA. Genes can also be ‘silenced’—prevented from making proteins—by small molecules called microRNAs, which bind to messenger RNA molecules and mark them for destruction. MicroRNA molecules therefore play an important role in controlling development. However, as many microRNA molecules often work together, and as many genes are redundant, it can be difficult to discover the effects of specific microRNAs. It is also difficult to discover whether any other mechanisms work alongside the microRNAs to control development.

Weaver, Zabinsky et al. used mutant forms of the nematode worm Caenorhabditis elegans, in which microRNA gene regulation did not work correctly, to investigate the mechanisms that work alongside microRNAs to control development. Genes in these worms were silenced; those silenced genes that caused additional developmental defects were considered likely to work ‘redundantly’ in the same role as a microRNA molecule. This revealed over one hundred genes that were previously unknown to work with microRNA molecules.

Weaver, Zabinsky et al. focused on one of these genes, called ced-3. The CED-3 protein produced from this gene is known to execute programmed cell death, a carefully controlled process also known as apoptosis, but was not known to have other developmental functions. However, the worms with mutant forms of the ced-3 gene already have problems performing apoptosis but are otherwise relatively normal, so Weaver, Zabinsky et al. reasoned that the CED-3 protein must also have another role in development.

Further investigation revealed that ced-3 mutations most severely disrupt development when they are combined with mutations in one particular family of microRNAs. These microRNAs are particularly important for controlling both when cells specialize into a particular type of cell, and the timing of when certain stages of development happen. Experiments using purified proteins showed that CED-3 breaks down three proteins that are produced from genes controlled by this family of microRNA molecules, and one of these proteins was also broken down by CED-3 in experiments with mutant worms. Weaver, Zabinsky et al. therefore propose that CED-3 is part of a semi-redundant system that ensures the proteins are produced at the right level and at the right time even if the microRNAs insufficiently regulate them. This finding demonstrated both a specific role and specific targets for the CED-3 protein during development, entirely distinct from its role in apoptosis.

Although Weaver, Zabinsky et al. have identified a large number of genes that work alongside microRNAs to control development, these are only the genes that cause obvious developmental defects in healthy worms. Further experiments using similar techniques performed on worms under stress may reveal yet more such genes.

DOI:http://dx.doi.org/10.7554/eLife.04265.002

Caspase-activated phosphoinositide binding by CNT-1 promotes apoptosis by inhibiting the AKT pathway

Inactivation of cell survival factors is a crucial step in apoptosis. The phosphoinositide 3 kinase (PI3K) and AKT signaling pathway promotes cell growth, proliferation and survival and its deregulation causes cancer. How this pathway is suppressed to promote apoptosis is poorly understood. Here we report the identification of a CED-3 caspase substrate in C. elegans, CNT-1, that upon cleavage by CED-3 during apoptosis activates an N-terminal phosphoinositide-binding fragment (tCNT-1), which translocates from cytoplasm to plasma membrane to block AKT binding to phosphatidylinositol (3,4,5)-triphosphate (PIP₃), thereby disabling AKT activation and its pro-survival activity. Our findings reveal a new mechanism that negatively regulates AKT cell signaling to promote apoptosis and that may restrict cell growth and proliferation in normal cells.

Exposure of phosphatidylserine by Xk-related protein family members during apoptosis

Apoptotic cells expose phosphatidylserine (PtdSer) on their surface as an "eat me" signal. Mammalian Xk-related (Xkr) protein 8, which is predicted to contain six transmembrane regions, and its Caenorhabditis elegans homolog CED-8 promote apoptotic PtdSer exposure. The mouse and human Xkr families consist of eight and nine members, respectively. Here, we found that mouse Xkr family members, with the exception of Xkr2, are localized to the plasma membrane. When Xkr8-deficient cells, which do not expose PtdSer during apoptosis, were transformed by Xkr family members, the transformants expressing Xkr4, Xkr8, or Xkr9 responded to apoptotic stimuli by exposing cell surface PtdSer and were efficiently engulfed by macrophages. Like Xkr8, Xkr4 and Xkr9 were found to possess a caspase recognition site in the C-terminal region and to require its direct cleavage by caspases for their function. Site-directed mutagenesis of the amino acid residues conserved among CED-8, Xkr4, Xkr8, and Xkr9 identified several essential residues in the second transmembrane and second cytoplasmic regions. Real time PCR analysis indicated that unlike Xkr8, which is ubiquitously expressed, Xkr4 and Xkr9 expression is tissue-specific.

Exploring developmental and physiological functions of fatty acid and lipid variants through worm and fly genetics

Lipids are more than biomolecules for energy storage and membrane structure. With ample structural variation, lipids critically participate in nearly all aspects of cellular function. Lipid homeostasis and metabolism are closely related to major human diseases and health problems. However, lipid functional studies have been significantly underdeveloped, partly because of the difficulty in applying genetics and common molecular approaches to tackle the complexity associated with lipid biosynthesis, metabolism, and function. In the past decade, a number of laboratories began to analyze the roles of lipid metabolism in development and other physiological functions using animal models and combining genetics, genomics, and biochemical approaches. These pioneering efforts have not only provided valuable insights regarding lipid functions in vivo but have also established feasible methodology for future studies. Here, we review a subset of these studies using Caenorhabditis elegans and Drosophila melanogaster.

Delivery of proteins to mammalian cells via gold nanoparticle mediated laser transfection

Nanoparticle laser interactions are in widespread use in cell manipulation. In particular, molecular medicine needs techniques for the directed delivery of molecules into mammalian cells. Proteins are the final mediator of most cellular cascades. However, despite several methodical approaches, the efficient delivery of proteins to cells remains challenging. This paper presents a new protein transfection technique via laser scanning of cells previously incubated with gold nanoparticles. The laser-induced plasmonic effects on the gold nanoparticles cause a transient permeabilization of the cellular membrane, allowing proteins to enter the cell. Applying this technique, it was possible to deliver green fluorescent protein into mammalian cells with an efficiency of 43%, maintaining a high level of cell viability. Furthermore, a functional delivery of Caspase 3, an apoptosis mediating protein, was demonstrated and evaluated in several cellular assays. Compared to conventional protein transfection techniques such as microinjection, the methodical approach presented here enables high-throughput transfection of about 10 000 cells per second. Moreover, a well-defined point in time of delivery is guaranteed by gold nanoparticle mediated laser transfection, allowing the detailed temporal analysis of cellular pathways and protein trafficking.

Does antigen masking by ubiquitin chains protect from the development of autoimmune diseases?

Autoimmune diseases are characterized by the production of antibodies against self-antigens and generally arise from a failure of central or peripheral tolerance. However, these diseases may develop when newly appearing antigens are not recognized as self by the immune system. The mechanism by which some antigens are “invisible” to the immune system is not completely understood. Apoptotic and complement system defects or autophagy imbalance can generate this antigenic autoreactivity. Under particular circumstances, cellular debris containing autoreactive antigens can be recognized by innate immune receptors or other sensors and can eventually lead to autoimmunity. Ubiquitination may be one of the mechanisms protecting autoreactive antigens from the immune system that, if disrupted, can lead to autoimmunity. Ubiquitination is an essential post-translational modification used by cells to target proteins for degradation or to regulate other intracellular processes. The level of ubiquitination is regulated during T cell tolerance and apoptosis and E3 ligases have emerged as a crucial signaling pathway for the regulation of T cell tolerance toward self-antigens. I propose here that an unrecognized role of ubiquitin and ubiquitin-like proteins could be to render intracellular or foreign antigens (present in cellular debris resulting from apoptosis, complement system, or autophagy defects) invisible to the immune system in order to prevent the development of autoimmunity.

The lysosomal cathepsin protease CPL-1 plays a leading role in phagosomal degradation of apoptotic cells in Caenorhabditis elegans

In Caenorhabditis elegans, the lysosomal cathepsin protease CPL-1 is indispensable for clearance of apoptotic cells by playing a leading role in destruction of cell corpses in phagolysosomes.

During programmed cell death, the clearance of apoptotic cells is achieved by their phagocytosis and delivery to lysosomes for destruction in engulfing cells. However, the role of lysosomal proteases in cell corpse destruction is not understood. Here we report the identification of the lysosomal cathepsin CPL-1 as an indispensable protease for apoptotic cell removal in Caenorhabditis elegans. We find that loss of cpl-1 function leads to strong accumulation of germ cell corpses, which results from a failure in degradation rather than engulfment. CPL-1 is expressed in a variety of cell types, including engulfment cells, and its mutation does not affect the maturation of cell corpse–containing phagosomes, including phagosomal recruitment of maturation effectors and phagosome acidification. Of importance, we find that phagosomal recruitment and incorporation of CPL-1 occurs before digestion of cell corpses, which depends on factors required for phagolysosome formation. Using RNA interference, we further examine the role of other candidate lysosomal proteases in cell corpse clearance but find that they do not obviously affect this process. Collectively, these findings establish CPL-1 as the leading lysosomal protease required for elimination of apoptotic cells in C. elegans.