From autophagic to necrotic cell death in Dictyostelium

Group-selection via aggregative propagule-formation enables cooperative multicellularity in an individual based, spatial model

The emergence of multicellularity is one of the major transitions in evolution that happened multiple times independently. During aggregative multicellularity, genetically potentially unrelated lineages cooperate to form transient multicellular groups. Unlike clonal multicellularity, aggregative multicellular organisms do not rely on kin selection instead other mechanisms maintain cooperation against cheater phenotypes that benefit from cooperators but do not contribute to groups. Spatiality with limited diffusion can facilitate group selection, as interactions among individuals are restricted to local neighbourhoods only. Selection for larger size (e.g. avoiding predation) may facilitate the emergence of aggregation, though it is unknown, whether and how much role such selection played during the evolution of aggregative multicellularity. We have investigated the effect of spatiality and the necessity of predation on the stability of aggregative multicellularity via individual-based modelling on the ecological timescale. We have examined whether aggregation facilitates the survival of cooperators in a temporally heterogeneous environment against cheaters, where only a subset of the population is allowed to periodically colonize a new, resource-rich habitat. Cooperators constitutively produce adhesive molecules to promote aggregation and propagule-formation while cheaters spare this expense to grow faster but cannot aggregate on their own, hence depending on cooperators for long-term survival. We have compared different population-level reproduction modes with and without individual selection (predation) to evaluate the different hypotheses. In a temporally homogeneous environment without propagule-based colonization, cheaters always win. Predation can benefit cooperators, but it is not enough to maintain the necessary cooperator amount in successive dispersals, either randomly or by fragmentation. Aggregation-based propagation however can ensure the adequate ratio of cooperators-to-cheaters in the propagule and is sufficient to do so even without predation. Spatiality combined with temporal heterogeneity helps cooperators via group selection, thus facilitating aggregative multicellularity. External stress selecting for larger size (e.g. predation) may facilitate aggregation, however, according to our results, it is neither necessary nor sufficient for aggregative multicellularity to be maintained when there is effective group-selection.

Therapeutic potential of a dual mTORC1/2 inhibitor for the prevention of posterior capsule opacification: An in vitro study

Mammalian target of rapamycin (mTOR) serves a central role in regulating cell growth and survival, and has been demonstrated to be involved in the pathological progression of posterior capsule opacification (PCO). In the present study, the potency of PP242, a novel dual inhibitor of mTOR complex 1/2 (mTORC1/2), in the suppression of the growth of human lens epithelial cells (HLECs) was investigated. Using a Cell Counting Kit‑8 and a wound healing assay, it was demonstrated that PP242 inhibited the proliferation and migration of HLECs. In addition, western blot analysis indicated that PP242 completely inhibited mTORC1 and mTORC2 downstream signaling activities, whereas rapamycin only partially inhibited mTORC1 activity within LECs. Furthermore, PP242 treatment led to an upregulation of the expression levels of p53 and B cell lymphoma‑2 (Bcl‑2)‑associated X and downregulation of Bcl‑2. In addition, flow cytometric analysis demonstrated that PP242 induced the cell cycle arrest at the G0/G1 phase, which may have caused apoptosis and induced autophagy within the LECs. The results of the present study suggested that administration of PP242 may potentially offer a novel therapeutic approach for the prevention of PCO.

Chronic mTOR activation promotes cell survival in Merkel cell carcinoma

Merkel cell carcinoma (MCC) is an aggressive skin cancer with rising incidence. In this study, we demonstrate that mTOR activation and suppressed autophagy is common in MCCs. mTOR inhibition in two primary human MCC cell lines induces autophagy and cell death that is independent of caspase activation but can be attenuated by autophagy inhibition. This is the first study to evaluate mTOR and autophagy in MCC. Our data suggests a potential role of autophagic cell death upon mTOR inhibition and thus uncovers a previously underappreciated role of mTOR signaling and cell survival, and merits further studies for potential therapeutic targets.

Killing a cancer: what are the alternatives?

Evading programmed cell death is one of the hallmarks of cancer. Conversely, inducing cell death by pharmacological means is the basis of almost every non-invasive cancer therapy. Research over the past decade has greatly increased our understanding of non-apoptotic programmed cell death events, such as lysosomal-mediated cell death, necroptosis and cell death with autophagy. It is becoming clear that an intricate effector network connects many of these classical and non-classical death pathways. In this Review, we discuss converging and diverging features of these pathways, as well as attempts to exploit this newly gained knowledge pharmacologically to provide therapeutics for cancer.

Evolution and diversity of dictyostelid social amoebae

Dictyostelid social amoebae are a large and ancient group of soil microbes with an unusual multicellular stage in their life cycle. Taxonomically, they belong to the eukaryotic supergroup Amoebozoa, the sister group to Opisthokonta (animals + fungi). Roughly half of the ~150 known dictyostelid species were discovered during the last five years and probably many more remain to be found. The traditional classification system of Dictyostelia was completely overturned by cladistic analyses and molecular phylogenies of the past six years. As a result, it now appears that, instead of three major divisions there are eight, none of which correspond to traditional higher-level taxa. In addition to the widely studied Dictyostelium discoideum, there are now efforts to develop model organisms and complete genome sequences for each major group. Thus Dictyostelia is becoming an excellent model for both practical, medically related research and for studying basic principles in cell-cell communication and developmental evolution. In this review we summarize the latest information about their life cycle, taxonomy, evolutionary history, genome projects and practical importance.

Autophagic cell death: the story of a misnomer

Dying cells often display a large-scale accumulation of autophagosomes and hence adopt a morphology called autophagic cell death. In many cases, it is agreed that this autophagic cell death is cell death with autophagy rather than cell death by autophagy. Here, we evaluate the accumulating body of literature that argues that cell death occurs by autophagy. We also list the caveats that must be considered when deciding whether or not autophagy is an important effector mechanism of cell death.

The Legionella pneumophila replication vacuole: making a cosy niche inside host cells

The pathogenesis of Legionella pneumophila is derived from its growth within lung macrophages after aerosols are inhaled from contaminated water sources. Interest in this bacterium stems from its ability to manipulate host cell vesicular-trafficking pathways and establish a membrane-bound replication vacuole, making it a model for intravacuolar pathogens. Establishment of the replication compartment requires a specialized translocation system that transports a large cadre of protein substrates across the vacuolar membrane. These substrates regulate vesicle traffic and survival pathways in the host cell. This Review focuses on the strategies that L. pneumophila uses to establish intracellular growth and evaluates why this microorganism has accumulated an unprecedented number of translocated substrates that are targeted at host cells.

Autophagic cell death: the story of a misnomer

Dying cells often display a large-scale accumulation of autophagosomes and hence adopt a morphology called autophagic cell death. In many cases, it is agreed that this autophagic cell death is cell death with autophagy rather than cell death by autophagy. Here, we evaluate the accumulating body of literature that argues that cell death occurs by autophagy. We also list the caveats that must be considered when deciding whether or not autophagy is an important effector mechanism of cell death.

Caspase-dependent and -independent lipotoxic cell-death pathways in fission yeast

Understanding the mechanisms underlying lipid-induced cell death has significant implications in both cell biology and human diseases. Previously, we showed that fission-yeast Schizosaccharomyces pombe cells deficient in triacylglycerol synthesis display apoptotic markers upon entry into stationary phase. Here, we characterize the sequential molecular events that take place at the onset of cell death in S. pombe, including a surge of diacylglycerol, post-mitotic arrest, alterations in mitochondrial activities and in intracellular redox balance, chromatin condensation, nuclear-envelope fragmentation, and eventually plasma-membrane permeabilization. Our results demonstrated active roles of mitochondria and reactive oxygen species in cell death, and identified novel cell-death regulators--including metacaspase Pca1, BH3-domain protein Rad9, and diacylglycerol-binding proteins Pck1 and Bzz1. Most importantly, we show that, under different conditions and stimuli, failure to maintain intracellular-lipid homeostasis can lead to cell death with different phenotypic manifestations, genetic criteria and cellular mechanisms, pointing to the existence of multiple lipotoxic pathways in this organism. Our study represents the first in-depth analysis of cell-death pathways in S. pombe.

Programmed cell death in protists

Programmed cell death in protists does not seem to make sense at first sight. However, apoptotic markers in unicellular organisms have been observed in all but one of the six/eight major groups of eukaryotes suggesting an ancient evolutionary origin of this regulated process. This review summarizes the available data on apoptotic markers in non-opisthokonts and elucidates potential functions and evolution of programmed cell death. A newly discovered family of caspase-like proteases, the metacaspases, is considered to exert the function of caspases in unicellular organisms. Important results on metacaspases, however, showed that they cannot be always correlated to the measured proteolytic activity during protist cell death. Thus, a major challenge for apoptosis research in a variety of protists remains the identification of the molecular cell death machinery.