An early history of T cell-mediated cytotoxicity.

After 60 years of intense fundamental research into T cell-mediated cytotoxicity, we have gained a detailed knowledge of the cells involved, specific recognition mechanisms and post-recognition perforin-granzyme-based and FAS-based molecular mechanisms. What could not be anticipated at the outset was how discovery of the mechanisms regulating the activation and function of cytotoxic T cells would lead to new developments in cancer immunotherapy. Given the profound recent interest in therapeutic manipulation of cytotoxic T cell responses, it is an opportune time to look back on the early history of the field. This Timeline describes how the early findings occurred and eventually led to current therapeutic applications.

Conserved nucleolar stress at the onset of cell death.

Cell death pervasiveness among multicellular eukaryotes suggested that some core steps of cell death may be conserved. This could be addressed by comparing the course of cell death in organisms belonging to distinct eukaryotic kingdoms. A search for early cell death events in a protist revealed nucleolar disorganization similar to the nucleolar stress often reported in dying animal cells. This indicated a conserved role for the nucleolus at the onset of eukaryotic cell death and leads one to consider the course of cell death as a succession of unequally conserved modules.

Autophagy in Dictyostelium: Mechanisms, regulation and disease in a simple biomedical model.

Autophagy is a fast-moving field with an enormous impact on human health and disease. Understanding the complexity of the mechanism and regulation of this process often benefits from the use of simple experimental models such as the social amoeba Dictyostelium discoideum. Since the publication of the first review describing the potential of D. discoideum in autophagy, significant advances have been made that demonstrate both the experimental advantages and interest in using this model. Since our previous review, research in D. discoideum has shed light on the mechanisms that regulate autophagosome formation and contributed significantly to the study of autophagy-related pathologies. Here, we review these advances, as well as the current techniques to monitor autophagy in D. discoideum. The comprehensive bioinformatics search of autophagic proteins that was a substantial part of the previous review has not been revisited here except for those aspects that challenged previous predictions such as the composition of the Atg1 complex. In recent years our understanding of, and ability to investigate, autophagy in D. discoideum has evolved significantly and will surely enable and accelerate future research using this model.

c-di-GMP induction of Dictyostelium cell death requires the polyketide DIF-1.

Cell death in the model organism Dictyostelium, as studied in monolayers in vitro, can be induced by the polyketide DIF-1 or by the cyclical dinucleotide c-di-GMP. c-di-GMP, a universal bacterial second messenger, can trigger innate immunity in bacterially infected animal cells and is involved in developmental cell death in Dictyostelium. We show here that c-di-GMP was not sufficient to induce cell death in Dictyostelium cell monolayers. Unexpectedly, it also required the DIF-1 polyketide. The latter could be exogenous, as revealed by a telling synergy between c-di-GMP and DIF-1. The required DIF-1 polyketide could also be endogenous, as shown by the inability of c-di-GMP to induce cell death in Dictyostelium HMX44A cells and DH1 cells upon pharmacological or genetic inhibition of DIF-1 biosynthesis. In these cases, c-di-GMP-induced cell death was rescued by complementation with exogenous DIF-1. Taken together, these results demonstrated that c-di-GMP could trigger cell death in Dictyostelium only in the presence of the DIF-1 polyketide or its metabolites. This identified another element of control to this cell death and perhaps also to c-di-GMP effects in other situations and organisms.

Atg1 allows second-signaled autophagic cell death in Dictyostelium.

We investigated the role of Atg1 in autophagic cell death (ACD) in a Dictyostelium monolayer model. The model is especially propitious, not only because of genetic tractability and absence of apoptosis machinery, but also because induction of ACD requires two successive exogenous signals, first the combination of starvation and cAMP, second the differentiation factor DIF-1. This enables one to analyze separately first-signal-induced autophagy and subsequent second-signal-induced ACD. We used mutants of atg1, a gene that plays an essential role in the initiation of autophagy. Upon starvation/cAMP, in contrast to parental cells, atg1 mutant cells showed irreversible lesions, clearly establishing a protective role for Atg1. Upon subsequent exposure to DIF-1 or to more ACD-specific second signals, starved parental cells progressed to ACD, but starved atg1 mutant cells did not, showing that Atg1 was required for ACD. Thus, in the same cells Atg1 was required in two apparently opposite ways, upon first-signaling for cell survival and upon second-signaling for ACD. Our findings strongly suggest that Atg1, thus presumably autophagy, protects the cells from starvation-induced cell death, allowing subsequent induction of ACD by the second signal. ACD is therefore not only "with" autophagy (since it showed signs of autophagy throughout), but is also "allowed by" autophagy. This does not exclude a role for autophagy also after second signaling. These results may account for discrepancies reported in the literature, encourage searches for second signals in different developmental models of ACD, and incite caution in autophagy-related therapeutic attempts.

A second signal for autophagic cell death?

Dictyostelium cells in monolayers in vitro lend themselves well to a study of autophagic cell death (ACD). There is no apoptosis machinery in the protist Dictyostelium, no caspase nor Bcl-2 family members (except a paracaspase whose inactivation does not alter cell death), thus there is no apoptosis that could interfere with, or substitute for, nonapoptotic cell death. Also, Dictyostelium, a eukaryote, has a haploid genome, which facilitates random insertional mutagenesis.

Autophagy in Dictyostelium: genes and pathways, cell death and infection.

The use of simple organisms to understand the molecular and cellular function of complex processes is instrumental for the rapid development of biomedical research. A remarkable example has been the discovery in S. cerevisiae of a group of proteins involved in the pathways of autophagy. Orthologues of these proteins have been identified in humans and experimental model organisms. Interestingly, some mammalian autophagy proteins do not seem to have homologues in yeast but are present in Dictyostelium, a social amoeba with two distinctive life phases, a unicellular stage in nutrient-rich conditions that differentiates upon starvation into a multicellular stage that depends on autophagy. This review focuses on the identification and annotation of the putative Dictyostelium autophagy genes and on the role of autophagy in development, cell death and infection by bacterial pathogens.

Autophagic cell death in Dictyostelium requires the receptor histidine kinase DhkM.

Dictyostelium constitutes a genetically tractable model for the analysis of autophagic cell death (ACD). During ACD, Dictyostelium cells first transform into paddle cells and then become round, synthesize cellulose, vacuolize, and die. Through random insertional mutagenesis, we identified the receptor histidine kinase DhkM as being essential for ACD. Surprisingly, different DhkM mutants showed distinct nonvacuolizing ACD phenotypes. One class of mutants arrested ACD at the paddle cell stage, perhaps through a dominant-negative effect. Other mutants, however, progressed further in the ACD program. They underwent rounding and cellulose synthesis but stopped before vacuolization. Moreover, they underwent clonogenic but not morphological cell death. Exogenous 8-bromo-cAMP restored vacuolization and death. A role for a membrane receptor at a late stage of the ACD pathway is puzzling, raising questions as to which ligand it is a receptor for and which moieties it phosphorylates. Together, DhkM is the most downstream-known molecule required for this model ACD, and its distinct mutants genetically separate previously undissociated late cell death events.

Autophagic cell death: analysis in Dictyostelium.

Autophagic cell death (ACD) can be operationally described as cell death with an autophagic component. While most molecular bases of this autophagic component are known, in ACD the mechanism of cell death proper is not well defined, in particular because in animal cells there is poor experimental distinction between what triggers autophagy and what triggers ACD. Perhaps as a consequence, it is often thought that in animal cells a little autophagy is protective while a lot is destructive and leads to ACD, thus that the shift from autophagy to ACD is quantitative. The aim of this article is to review current knowledge on ACD in Dictyostelium, a very favorable model, with emphasis on (1) the qualitative, not quantitative nature of the shift from autophagy to ACD, in contrast to the above, and (2) random or targeted mutations of in particular the following genes: iplA (IP3R), TalB (talinB), DcsA (cellulose synthase), GbfA, ugpB, glcS (glycogen synthase) and atg1. These mutations allowed the genetic dissection of ACD features, dissociating in particular vacuolisation from cell death.

Necrotic cell death: From reversible mitochondrial uncoupling to irreversible lysosomal permeabilization.

Dictyostelium atg1- mutant cells provide an experimentally and genetically favorable model to study necrotic cell death (NCD) with no interference from apoptosis or autophagy. In such cells subjected to starvation and cAMP, induction by the differentiation-inducing factor DIF or by classical uncouplers led within minutes to mitochondrial uncoupling, which causally initiated NCD. We now report that (1) in this model, NCD included a mitochondrial-lysosomal cascade of events, (2) mitochondrial uncoupling and therefore initial stages of death showed reversibility for a surprisingly long time, (3) subsequent lysosomal permeabilization could be demonstrated using Lysosensor blue, acridin orange, Texas red-dextran and cathepsin B substrate, (4) this lysosomal permeabilization was irreversible, and (5) the presence of the uncoupler was required to maintain mitochondrial lesions but also to induce lysosomal lesions, suggesting that signaling from mitochondria to lysosomes must be sustained by the continuous presence of the uncoupler. These results further characterized the NCD pathway in this priviledged model, contributed to a definition of NCD at the lysosomal level, and suggested that in mammalian NCD even late reversibility attempts by removal of the inducer may be of therapeutic interest.

Marked mitochondrial alterations upon starvation without cell death, caspases or Bcl-2 family members.

Dictyostelium HMX44A cells can withstand starvation under monolayer conditions for a few days without dying. They die only when the differentiation factor DIF-1 is exogenously added. Still, when HMX44A were subjected to starvation without addition of DIF-1 they showed, by electron microscopy and electron tomography, gross mitochondrial lesions including marked cristae alterations with frequent "holes" probably originating from dilated cristae. Since these cells did not die as shown for instance by FACS analysis, these results showed unexpected resilience of cells bearing markedly altered mitochondria, and thus showed that apparently destructive mitochondrial alterations may not lead to cell death. Also, these marked mitochondrial lesions could not be caused by caspases or bcl-2 family members, which these cells do not encode.

Analysis of autophagic and necrotic cell death in Dictyostelium.

Non-apoptotic cell death types can be conveniently studied in Dictyostelium discoideum, an exceptionally favorable model not only because of its well-known genetic and experimental advantages, but also because in Dictyostelium there is no apoptosis machinery that could interfere with non-apoptotic cell death. We show here how to conveniently demonstrate, assess, and study these non-apoptotic cell death types. These can be generated by use of modifications of the monolayer technique of Rob Kay et al., and either wild-type HMX44A Dictyostelium cells, leading to autophagic cell death, or the corresponding atg1- autophagy gene mutant cells, leading to necrotic cell death. Methods to follow these non-apoptotic cell death types qualitatively and quantitatively will be reported.

A UDP-glucose derivative is required for vacuolar autophagic cell death.

Autophagic cell death in Dictyostelium can be dissociated into a starvation-induced sensitization stage and a death induction stage. A UDP-glucose pyrophosphorylase (ugpB) mutant and a glycogen synthase (glcS) mutant shared the same abnormal phenotype. In vitro, upon starvation alone mutant cells showed altered contorted morphology, indicating that the mutations affected the pre-death sensitization stage. Upon induction of cell death, most of these mutant cells underwent death without vacuolization, distinct from either autophagic or necrotic cell death. Autophagy itself was not grossly altered as shown by conventional and electron microscopy. Exogenous glycogen or maltose could complement both ugpB(-) and glcS(-) mutations, leading back to autophagic cell death. The glcS(-) mutation could also be complemented by 2-deoxyglucose that cannot undergo glycolysis. In agreement with the in vitro data, upon development glcS(-) stalk cells died but most were not vacuolated. We conclude that a UDP-glucose derivative (such as glycogen or maltose) plays an essential energy-independent role in autophagic cell death.

A specific pathway inducing autophagic cell death is marked by an IP3R mutation.

Three main advantages make Dictyostelium a very favorable model to study the induction of autophagic cell death in vitro. First, its small, sequenced and haploid genome facilitates genetic approaches. Second, the Dictyostelium genome does not encode the two main molecular families involved in apoptosis (caspases and bcl-2 family), which therefore cannot interfere in this case with autophagic cell death. Third, induction of autophagic cell death follows in this case a two-step process, namely starvation-induced sensitization leading to autophagy but not to death, followed by a DIF-1-induced pathway leading to cell death proper. The latter, DIF-1-induced pathway is defined experimentally, through sequential additions, and most important also genetically, through random mutagenesis leading in particular to the preparation and study of an iplA mutant. The iplA gene encodes the IP3 Receptor, and its mutation leads to the absence of vacuolization and of death when autophagic cell death is triggered. Further study of the DIF-1 pathway should shed additional light on the induction of autophagic cell death (as opposed to that of just autophagy) in Dictyostelium and by extension perhaps in other organisms.

The inositol 1,4,5-trisphosphate receptor is required to signal autophagic cell death.

The signaling pathways governing pathophysiologically important autophagic (ACD) and necrotic (NCD) cell death are not entirely known. In the Dictyostelium eukaryote model, which benefits from both unique analytical and genetic advantages and absence of potentially interfering apoptotic machinery, the differentiation factor DIF leads from starvation-induced autophagy to ACD, or, if atg1 is inactivated, to NCD. Here, through random insertional mutagenesis, we found that inactivation of the iplA gene, the only gene encoding an inositol 1,4,5-trisphosphate receptor (IP3R) in this organism, prevented ACD. The IP3R is a ligand-gated channel governing Ca(2+) efflux from endoplasmic reticulum stores to the cytosol. Accordingly, Ca(2+)-related drugs also affected DIF signaling leading to ACD. Thus, in this system, a main pathway signaling ACD requires IP3R and further Ca(2+)-dependent steps. This is one of the first insights in the molecular understanding of a signaling pathway leading to autophagic cell death.

Autophagy and autophagic cell death in Dictyostelium.

Autophagic cell death can be conveniently studied in Dictyostelium discoideum, an exceptionally favorable model not only because of its well-known genetic and experimental advantages but also because in Dictyostelium there is no apoptosis machinery that could interfere with nonapoptotic cell death. Moreover, autophagic cell death in Dictyostelium can be dissociated into a starvation-induced sensitization stage, during which autophagy is induced, and a death induction stage. We show here how to demonstrate, assess and analyze this autophagic cell death. This can be studied in vivo during the development of Dictyostelium, and in vitro, using modifications of the monolayer technique of Rob Kay et al. Methods to follow this autophagic cell death qualitatively and quantitatively are reported.

Autophagic or necrotic cell death in the absence of caspase and bcl-2 family members.

How is one to investigate autophagic or necrotic cell death in the absence of interference from the apoptosis machinery? In the protist Dictyostelium, a model for the study of these two cell death types, we previously showed that autophagic cell death does not require paracaspase, the only caspase family member in this organism. In this report, we prepared two distinct paracaspase- atg1- double mutants, and we used them to demonstrate that paracaspase is not required for necrotic cell death either. Also, in silico investigation showed that the genome of Dictyostelium harbored no detectable member of the bcl-2 family and no single BH3 domain-bearing molecules. Altogether, in this model system both autophagic and necrotic cell death could occur, and could be investigated, with no interference from the two main molecular families involved in apoptosis, the caspase and the bcl-2 families.

Cell death by necrosis: towards a molecular definition.

Necrosis has been defined as a type of cell death that lacks the features of apoptosis and autophagy, and is usually considered to be uncontrolled. Recent research suggests, however, that its occurrence and course might be tightly regulated. After signaling- or damage-induced lesions, necrosis can include signs of controlled processes such as mitochondrial dysfunction, enhanced generation of reactive oxygen species, ATP depletion, proteolysis by calpains and cathepsins, and early plasma membrane rupture. In addition, the inhibition of specific proteins involved in regulating apoptosis or autophagy can change the type of cell death to necrosis. Because necrosis is prominent in ischemia, trauma and possibly some forms of neurodegeneration, further biochemical comprehension and molecular definition of this process could have important clinical implications.

From autophagic to necrotic cell death in Dictyostelium.

Among unusual models to study cell death mechanisms, the protist Dictyostelium is remarkable because of its strategic phylogenetic position, with early emergence among eukaryotes and unicellular/multicellular transition, and its very favorable experimental and genetic flexibility. Dictyostelium shows developmental vacuolar cell death, and in vitro monolayer approaches revealed both an autophagic vacuolar and a necrotic type of cell death. These are described in some detail, as well as implications and future prospects.