Eaten to death

Autophagy in Drosophila and Zebrafish

Autophagy is a highly conserved cellular process that delivers cellular contents to the lysosome for degradation. It not only serves as a bulk degradation system for various cytoplasmic components but also functions selectively to clear damaged organelles, aggregated proteins, and invading pathogens (Feng et al., Cell Res 24:24-41, 2014; Galluzzi et al., EMBO J 36:1811-36, 2017; Klionsky et al., Autophagy 12:1-222, 2016). The malfunction of autophagy leads to multiple developmental defects and diseases (Mizushima et al., Nature 451:1069-75, 2008). Drosophila and zebrafish are higher metazoan model systems with sophisticated genetic tools readily available, which make it possible to dissect the autophagic processes and to understand the physiological functions of autophagy (Lorincz et al., Cells 6:22, 2017a; Mathai et al., Cells 6:21, 2017; Zhang and Baehrecke, Trends Cell Biol 25:376-87, 2015). In this chapter, we will discuss recent progress that has been made in the autophagic field by using these animal models. We will focus on the protein machineries required for autophagosome formation and maturation as well as the physiological roles of autophagy in both Drosophila and zebrafish.

The Distribution of Genes Associated With Regulated Cell Death Is Decoupled From the Mitochondrial Phenotypes Within Unicellular Eukaryotic Hosts

Genetically regulated cell death (RCD) occurs in all domains of life. In eukaryotes, the evolutionary origin of the mitochondrion and of certain forms of RCD, in particular apoptosis, are thought to coincide, suggesting a central general role for mitochondria in cellular suicide. We tested this mitochondrial centrality hypothesis across a dataset of 67 species of protists, presenting 5 classes of mitochondrial phenotypes, including functional mitochondria, metabolically diversified mitochondria, functionally reduced mitochondria (Mitochondrion Related Organelle or MRO) and even complete absence of mitochondria. We investigated the distribution of genes associated with various forms of RCD. No homologs for described mammalian regulators of regulated necrosis could be identified in our set of 67 unicellular taxa. Protists with MRO and the secondarily a mitochondriate Monocercomonoides exilis display heterogeneous reductions of apoptosis gene sets with respect to typical mitochondriate protists. Remarkably, despite the total lack of mitochondria in M. exilis, apoptosis-associated genes could still be identified. These same species of protists with MRO and M. exilis harbored non-reduced autophagic cell death gene sets. Moreover, transiently multicellular protist taxa appeared enriched in apoptotic and autophagy associated genes compared to free-living protists. This analysis suggests that genes associated with apoptosis in animals and the presence of the mitochondria are significant yet non-essential biological components for RCD in protists. More generally, our results support the hypothesis of a selection for RCD, including both apoptosis and autophagy, as a developmental mechanism linked to multicellularity.

Autophagy in Plant Immunity

The highly conserved catabolic process of autophagy delivers unwanted proteins or damaged organelles to vacuoles for degradation and recycling. This is essential for the regulation of cellular homeostasis, stress adaptation, and programmed cell death in eukaryotes. In particular, emerging evidence indicates that autophagy plays a multifunctional regulatory role in plant innate immunity during plant-pathogen interactions. In this review, we highlight existing knowledge regarding the involvement of autophagy in plant immunity, mechanisms functioning in the induction of autophagy upon pathogen infection, and possible directions for future research.

In the Midst of Life-Cell Death: What Is It, What Is It Good for, and How to Study It

Cell death, one of the most fundamental biological processes, has not made it into the public consciousness in the same way that genetic inheritance, cell division, or DNA replication has. Everyone knows they get their genes from their parents, but few would be aware that even before they were born a lot of essential cell death has shaped their development. The greater population, for the most part, is blissfully unaware that every day millions of their own cells die in a programmed way and that this is essential for normal human physiology-their well-being, in fact. Nowhere is the burial liturgy, "In the midst of life we are in death," more apt. Despite this public underappreciation, cell death research is a major industry. A search in PubMed for "apoptosis," a special form of cell death that is caused by caspases, returns approximately 280,000 hits. The intense research interest arises from the realization that abnormal cell death responses play an important role in two of the biggest killers in the western world: cancer and cardio/cerebrovascular disease. Furthermore, the manner in which cells die can also influence the development of autoimmune and autoinflammatory diseases. It is therefore of paramount importance to ensure that experiments accurately quantitate and correctly identify cell death in all its guises. That is the goal of this protocol collection.

Autophagy and its implication in human oral diseases

Macroautophagy/autophagy is a conserved lysosomal degradation process essential for cell physiology and human health. By regulating apoptosis, inflammation, pathogen clearance, immune response and other cellular processes, autophagy acts as a modulator of pathogenesis and is a potential therapeutic target in diverse diseases. With regard to oral disease, autophagy can be problematic either when it is activated or impaired, because this process is involved in diverse functions, depending on the specific disease and its level of progression. In particular, activated autophagy functions as a cytoprotective mechanism under environmental stress conditions, which regulates tumor growth and mediates resistance to anticancer treatment in established tumors. During infections and inflammation, activated autophagy selectively delivers microbial antigens to the immune systems, and is therefore connected to the elimination of intracellular pathogens. Impaired autophagy contributes to oxidative stress, genomic instability, chronic tissue damage, inflammation and tumorigenesis, and is involved in aberrant bacterial clearance and immune priming. Hence, substantial progress in the study of autophagy provides new insights into the pathogenesis of oral diseases. This review outlines the mechanisms of autophagy, and highlights the emerging roles of this process in oral cancer, periapical lesions, periodontal diseases, and oral candidiasis.

The cell's dilemma, or the story of cell death: an entertainment in three acts

Cells. They assemble, thrive, and cooperate to compose an organism, simple or complex. And like any living thing, they die. They die by catastrophe, they become sabotaged by condition, or they remove themselves on command from within or without. Each small life is followed by a death, to the benefit or the harm of the whole. Our story, here, is not of how each quietus occurs, but instead, of our ongoing effort to understand these tiny demises, to manipulate them, and to some day control them.

Cannabinoid-induced autophagy: Protective or death role?

Autophagy, the "self-digestion" mechanism of the cells, is an evolutionary conserved catabolic process that targets portions of cytoplasm, damaged organelles and proteins for lysosomal degradation, which plays a crucial role in development and disease. Cannabinoids are active compounds of Cannabis sativa and the most prevalent psychoactive substance is Δ(9)-tetrahydrocannabinol (THC). Cannabinoid compounds can be divided in three types: the plant-derived natural products (phytocannabinoids), the cannabinoids produced endogenously (endocannabinoids) and the synthesized compounds (synthetic cannabinoids). Various studies reported a cannabinoid-induced autophagy mechanism in cancer and non-cancer cells. In this review we focus on the recent advances in the cannabinoid-induced autophagy and highlight the molecular mechanisms involved in these processes.

Tribenuron-Methyl Induces Male Sterility through Anther-Specific Inhibition of Acetolactate Synthase Leading to Autophagic Cell Death

Tribenuron-methyl (TM) is a powerful sulfonylurea herbicide that inhibits branched-chain amino acid (BCAA) biosynthesis by targeting the catalytic subunit (CSR1) of acetolactate synthase (ALS). Selective induction of male sterility by foliar spraying of TM at low doses has been widely used for hybrid seed production in rapeseed (Brassica napus); however, the underlying mechanism remains unknown. Here, we report greater TM accumulation and subsequent stronger ALS inhibition and BCAA starvation in anthers than in leaves and stems after TM application. Constitutive or anther-specific expression of csr1-1D (a CSR1 mutant) eliminated anther-selective ALS inhibition and reversed the TM-induced male sterile phenotype in both rapeseed and Arabidopsis. The results of TM daub-stem experiments, combined with the observations of little TM accumulation in anthers and reversion of TM-induced male sterility by targeted expression of the TM metabolism gene Bel in either the mesophyll or phloem, suggested that foliar-sprayed TM was polar-transported to anthers mainly through the mesophyll and phloem. Microscopy and immunoblotting revealed that autophagy, a bulk degradation process induced during cell death, was elevated in TM-induced male sterile anthers and by anther-specific knockdown of ALS. Moreover, TM-induced pollen abortion was significantly inhibited by the autophagy inhibitor 3-MA. These data suggested that TM was polar-transported to anthers, resulting in BCAA starvation via anther-specific ALS inhibition and, ultimately, autophagic cell death in anthers.

Current questions and possible controversies in autophagy

Interest in autophagy has exploded over the last decade, with publications highlighting crosstalk with several other cellular processes including secretion, endocytosis, and cell suicide pathways including apoptosis. Autophagy proteins have also been implicated in other cellular processes independently of their roles in autophagy, creating complexities in the interpretation of autophagy (Atg) mutant gene data. Interestingly, this self-eating process is a survival mechanism that can also promote cell death, but when and how autophagy may ‘switch’ its function is still under debate. Indeed, there are currently many models of how autophagy actually influences cell death. In this review, we highlight some outstanding questions and possible controversies in the autophagy field.

Recent insights into cell death and autophagy

Macroautophagy (hereafter autophagy) is an evolutionarily-ancient mechanism by which cellular material is delivered to lysosomes for degradation. Autophagy and cell death are intimately linked. For example, both processes often use the same molecular machinery and recent work suggests that autophagy has great influence over a cell's decision to live or die. However, this decision-making is complicated by the fact that the role of autophagy in determining whether a cell should live or die goes both ways: autophagy inhibition can result in more or less cell death depending on the death stimulus, cell type or context. Autophagy may also differentially affect different types of cell death. In the present review, we discuss the recent literature that helps make sense of this apparently inconsistent role of autophagy in influencing a cell to live or die.

The End of the Beginning: Cell Death in the Germline

Programmed cell death occurs in the germline of many organisms, both as an essential part of development and throughout adult life. Germline cell death can be apoptotic or nonapoptotic, depending on the stimulus or stage of development. Here, we focus on the Drosophila ovary, which is a powerful model for studying diverse types of cell death. In Drosophila, the death of primordial germ cells occurs normally during embryonic development, and germline nurse cells are programmed to die during oocyte development in adult flies. Cell death of previtellogenic egg chambers in adults can also be induced by starvation or other environmental cues. Mid-oogenesis seems to be particularly sensitive to such cues and has been proposed to serve as a checkpoint to avoid the energetically expensive cost of egg production. After the germline dies in mid-oogenesis, the remnants are engulfed by an epithelial layer of follicle cells; thus, the fly ovary also serves as a highly tractable model for engulfment by epithelial cells. These examples of cell death in the fly ovary share many similarities to the types of cell death seen in the mammalian germline. Recent progress in elucidating the molecular mechanisms of cell death in the germline is discussed.

Inhibition of the Na+/H+ antiporter induces cell death in TF-1 erythroleukemia cells stimulated by the stem cell factor

Leukemia cells produce acidic metabolites due to their high metabolic condition. An alkaline pHi (intracellular pH) shift, caused by activation of the Na+/H+ exchange, is an important event in the mechanism of growth factor activity. However, the role of the Na(+)/H(+) exchanger in the survival of erythroleukemia TF-1 cells has not yet been studied in detail. The aim of this study was to identify the effects of 5-(N-ethyl-N-isopropyl) amiloride (EIPA), a highly specific blocker of the Na(+)/H(+) exchanger, on the survival of SCF-dependent TF-1 cells. The effects of EIPA on survival and mitochondrial membrane potential were studied when exposing wild type TF-1 cells and TF-1 cells expressing bcl-2 to EIPA for 48h. Ectopic expression of the bcl-2 gene maintained a mildly alkaline pH and prevented the simultaneous appearance of apoptosis and autophagy (typically displayed by TF-1 cells) in the presence of EIPA. Consistent with Stem Cell Factor (SCF) function, we found that this molecule rescued TF-1 cells during autophagy but not apoptosis, allowing these cells to subsequently respond to GM-CSF. Serum deprivation or SCF withdrawal induced cell death at 36h in TF-1 and TF-1 neo cells, whereas TF-1/bcl-2 cells tended to undergo apoptosis and show acidic vacuoles after 96h, pointing to a transient anti-apoptotic effect. The present study shows the suppressive effect of EIPA on the proliferation of leukemia cell line stimulated with SCF, apparently by decreasing the mitochondria membrane potential and averting alkalinization. Through the constitutive expression of bcl-2, TF-1 cells were survival factor independent. Proliferation in these cells was not affected by EIPA at the concentrations used against parental TF-1 cells, indicating that the inhibitory effect in SCF-stimulated cells can be attributed to specific blocking of the Na(+)/H(+) exchanger.

Live and Let Die: Roles of Autophagy in Cadmium Nephrotoxicity

The transition metal ion cadmium (Cd²⁺) is a significant environmental contaminant. With a biological half-life of ~20 years, Cd²⁺ accumulates in the kidney cortex, where it particularly damages proximal tubule (PT) cells and can result in renal fibrosis, failure, or cancer. Because death represents a powerful means by which cells avoid malignant transformation, it is crucial to clearly identify and understand the pathways that determine cell fate in chronic Cd²⁺ nephrotoxicity. When cells are subjected to stress, they make a decision to adapt and survive, or—depending on the magnitude and duration of stress—to die by several modes of death (programmed cell death), including autophagic cell death (ACD). Autophagy is part of a larger system of intracellular protein degradation and represents the channel by which organelles and long-lived proteins are delivered to the lysosome for degradation. Basal autophagy levels in all eukaryotic cells serve as a dynamic physiological recycling system, but they can also be induced by intra- or extracellular stress and pathological processes, such as endoplasmic reticulum (ER) stress. In a context-dependent manner, autophagy can either be protective and hence contribute to survival, or promote death by non-apoptotic or apoptotic pathways. So far, the role of autophagy in Cd²⁺-induced nephrotoxicity has remained unsettled due to contradictory results. In this review, we critically survey the current literature on autophagy in Cd²⁺-induced nephrotoxicity in light of our own ongoing studies. Data obtained in kidney cells illustrate a dual and complex function of autophagy in a stimulus- and time-dependent manner that possibly reflects distinct outcomes in vitro and in vivo. A better understanding of the context-specific regulation of cell fate by autophagy may ultimately contribute to the development of preventive and novel therapeutic strategies for acute and chronic Cd²⁺ nephrotoxicity.