The origin of programmed cell death

Apoptotic proteins Reaper and Grim induce stable inactivation in voltage-gated K+ channels

Drosophila genes reaper, grim, and head-involution-defective (hid) induce apoptosis in several cellular contexts. N-terminal sequences of these proteins are highly conserved and are similar to N-terminal inactivation domains of voltage-gated potassium (K+) channels. Synthetic Reaper and Grim N terminus peptides induced fast inactivation of Shaker-type K+ channels when applied to the cytoplasmic side of the channel that was qualitatively similar to the inactivation produced by other K+ channel inactivation particles. Mutations that reduce the apoptotic activity of Reaper also reduced the synthetic peptide's ability to induce channel inactivation, indicating that K+ channel inactivation correlated with apoptotic activity. Coexpression of Reaper RNA or direct injection of full length Reaper protein caused near irreversible block of the K+ channels. These results suggest that Reaper and Grim may participate in initiating apoptosis by stably blocking K+ channels.

The hypersensitive response. A programmed cell death associated with plant resistance

In plants, the hypersensitive response (HR) is defined as a rapid cell death occurring in response to pathogen attack, and is closely related to active resistance. Initiation of the HR process begins with the recognition of the pathogen by the plant, which is mediated mainly by the pathogen avirulence genes and the plant resistance genes. Then, complex signal transduction pathways intervene, involving changes in protein phosphorylation, production of reactive oxygen species and modification of ion fluxes. Components required for the regulation of the HR cell death are now being identified genetically by the isolation of mutants, in contrast to those involved in the execution of the cell death programme, which are still largely unknown. Further genetic and biochemical analyses will undoubtedly answer the question as to whether this form of programmed cell death (PCD) can be compared with other forms of PCD in plants and with apoptosis in animals.

Mitochondria, glutamate neurotoxicity and the death cascade

This review focuses on two questions: the role of mitochondria in excitotoxic neuronal death and the connection of mitochondria with the apoptotic death cascade. The goal is to highlight the regulatory role of mitochondrial channels on the mitochondrial membrane potential, Deltapsi, and their involvement in determining neuronal survival or death. A hypothesis is developed centered on the notion that protein-protein interactions between members of the Bcl-2 family of death suppressor and promoter proteins lead to the selective elimination of depolarizing currents that, in turn, collapse Deltapsi and set in motion the irreversible pathway of cell death. The model considers the remarkable propensity of Bcl-2 family proteins to dimerize or oligomerize and thereby restrict the localization of partner molecules to mitochondrial membrane contact sites. The fundamental principle invoked here is that through a concerted set of protein-protein interactions, information is exchanged by specific heterodimers, one of the partners acting as a toxic protein and the second as its antidote. The review concludes with the elaboration of a speculative model about cellular mechanisms for the prevention of cell destruction as triggered by extracellular signals which may be conserved in its molecular design from bacteria to eukaryotes.

The entericidin locus of Escherichia coli and its implications for programmed bacterial cell death

Antidote/toxin gene pairs known as "addiction modules" can maintain plasmids in bacterial populations by means of post-segregational killing. However, several chromosome-encoded addiction modules may provide an entirely distinct function in the programmed cell death of moribund subpopulations under starvation conditions. We now report a novel chromosomal bacteriolytic module of Escherichia coli called the entericidin locus, which is activated in stationary phase under high osmolarity conditions by sigmaS and simultaneously repressed by the osmoregulatory EnvZ/OmpR signal transduction pathway. The entericidin locus encodes tandem paralogous genes (ecnAB) and directs the synthesis of two small cell-envelope lipoproteins. An attenuator precedes ecnA and an ompR-sensitive sigmaS promoter governs expression of ecnB. The entericidin A lipoprotein is an antidote to the bacteriolytic lipoprotein entericidin B. The entericidins are predicted to adopt amphipathic alpha-helical structures and to reciprocally modulate membrane stability. The entericidin locus is not present on any known plasmids, but is conserved in the homologous region of the Citrobacter freundii chromosome. Although the cloned C. freundii entericidin locus is expressed in E. coli independently of ompR, it carries an additional ompR-like gene called ecnR. The organization of the entericidin locus as a chromosomal antidote/toxin gene pair, which is regulated by both positive and negative osmotic signals during starvation, is consistent with an emerging paradigm of programmed bacterial cell death.

Osmotin, a plant antifungal protein, subverts signal transduction to enhance fungal cell susceptibility

The plant pathogenesis-related protein osmotin is an antifungal cytotoxic agent that causes rapid cell death in the yeast S. cerevisiae. We show here that osmotin uses a signal transduction pathway to weaken defensive cell wall barriers and increase its cytotoxic efficacy. The pathway activated by osmotin includes the regulatory elements of the mating pheromone response STE4, STE18, STE20, STE5, STE11, STE7, FUS3, KSS1, and STE12. Neither the pheromone receptor nor its associated G protein alpha subunit GPA1 are required for osmotin action. However, mutation of SST2, a negative regulator of G alpha proteins, resulted in supersensitivity to osmotin. Phosphorylation of STE7 was rapidly stimulated by osmotin preceding any changes in cell vitality or morphology. These results demonstrate that osmotin subverts target cell signal transduction as part of its mechanism of action.

The Mitochondrial F0F1-ATPase proton pump is required for function of the proapoptotic protein Bax in yeast and mammalian cells

The proapoptotic mammalian protein Bax associates with mitochondrial membranes and confers a lethal phenotype when expressed in yeast. By generating Bax-resistant mutant yeast and using classical complementation cloning methods, subunits of the mitochondrial F0F1-ATPase proton pump were determined to be critical for Bax-mediated killing in S. cerevisiae. A pharmacological inhibitor of the proton pump, oligomycin, also partially abrogated the cytotoxic actions of Bax in yeast. In mammalian cells, oligomycin also inhibited Bax-induced apoptosis and activation of cell death proteases. The findings imply that an intact F0F1-ATPase in the inner membrane of mitochondria is necessary for optimal function of Bax in both yeast and mammalian cells.

Biochemical and molecular mechanisms regulating apoptosis

In eukaryotes, the regulation of tissue cell numbers is a critical homeostatic objective that is achieved through tight control of apoptosis, mitosis and differentiation. While much is known about the genetic regulation of cell growth and differentiation, the molecular basis of apoptosis is less well understood. Genes involved in both cell proliferation and apoptosis reflect the role of some stimuli in both of these processes, the cell response depending on the overall cellular milieu. Recent research has given fascinating insights into the complex genetic and molecular mechanisms regulating apoptosis. A picture is emerging of the initiation in certain cells, after an apoptotic trigger, of sequential gene expression and specific signal transduction cascades that guide cells along the cell death pathway. Changes in gene expression precede the better known biochemical and morphological changes of apoptosis. It seems possible that, as a result of increased understanding of the cellular events preceding cell death, apoptosis may become more amenable to manipulation by appropriate drug- and gene-based therapies.

Programmed cell death in plant disease: the purpose and promise of cellular suicide

The interaction of pathogens with plants leads to a disruption in cellular homeostasis, often leading to cell death, in both compatible and incompatible relationships. The mechanistic basis of this cellular disruption and consequent death is complex and poorly characterized, but it is established that host responses to pathogens are dependent on gene expression, involve signal transduction, and require energy. Recent data suggest that in animals, a genetically regulated, signal transduction-dependent programmed cell death process, commonly referred to as apoptosis, is conserved over a wide range of phyla. The basic function of apoptosis is to direct the selective elimination of certain cells during development, but it also is a master template that is involved in host responses to many pathogens. Programmed cell death in plants, while widely observed, has not been studied extensively at either the biochemical or genetic level. Current data suggest that activation or suppression of programmed cell death may underlie diseases in plants as it does in animals. This review describes some of the fundamental characteristics of apoptosis in animals and points to a number of connections to programmed cell death in plants that may lead to both a better understanding of disease processes and novel strategies for engineering disease resistance in plants.

The evolution of virus-induced apoptosis

Viruses from several different families are able to exploit their host's cell death programmes so as to maximize viral fitness. Consideration of the evolution of such strategies has lead to the suggestion that the virus should inhibit apoptosis, in order to prolong the life of the cell and thereby maximize the number of progeny virions. The host, on the other hand, should stimulate apoptosis thereby inhibiting viral growth and blocking viral spread. For example, the function of the latent membrane protein I (LMPI) of the Epstein-Barr virus and the bcl-2 homologue gene A179L of African swine fever virus is to inhibit apoptosis. However, in other cases it is the virus that stimulates cell death or the host that benefits from inhibiting apoptosis, such as in fatal alphavirus encephalitis. This has been explained by assuming that virus-induced apoptosis in non-regenerating cells would be detrimental to the host. We present a mathematical framework for understanding virus-induced apoptosis which accounts for these two opposite solutions to virus infection with respect to the mode of virus replication and the life cycle of the target cell.

Identification of functional surfaces of the zinc binding domains of intracellular receptors

Transcriptional regulatory factor complexes assemble on genomic response elements to control gene expression. To gain insights on the surfaces that determine this assembly in the zinc binding domains from intracellular receptors, we systematically analyzed the variations in sequence and function of those domains in the context of their invariant fold. Taking the intracellular receptor superfamily as a whole revealed a hierarchy of amino acid residues along the DNA interface that correlated with response element binding specificity. When only steroid receptors were considered, two additional sites appeared: the known dimer interface, and a novel putative interface suitably located to contact regulatory factors bound to the free face of palindromic response elements commonly used by steroid receptors. Surprisingly, retinoic acid receptors, not known to bind palindromic response elements, contain both of these surfaces, implying that they may dimerize at palindromic elements under some circumstances. This work extends Evolutionary Trace analysis of functional surfaces to protein-DNA interactions, suggests how coordinated exchange of trace residues may predictably switch binding specificity, and demonstrates how to detect functional surfaces that are not apparent from sequence comparison alone.

Signaling in unicellular eukaryotes

Aspects of intercellular and intracellular signaling systems in cell survival, proliferation, differentiation, chemosensory behavior, and programmed cell death in free-living unicellular eukaryotes have been reviewed. Comparisons have been made with both bacteria and metazoa. The central organisms were flagellates (Trypanosoma, Leishmania, and Crithidia), slime molds (Dictyostelium), yeast cells (Saccharomyces cerevisiae), and ciliates (Paramecium, Euplotes, and Tetrahymena). There are two novel aspects in this review. First, cellular responses are viewed in an evolutionary perspective, rather than from the more prevailing one, in which the unicellular eukaryotes are seen by the mammalian organisms. Second, results obtained with cell cultures in minimal, chemically defined nutrient media at low cell densities where intercellular signaling is strongly reduced are discussed. These results shed light on control mechanisms and their cooperation inside the living cell. Intracellular systems have many common features in unicellular and multicellular organisms.

Origin of eukaryotic programmed cell death: a consequence of aerobic metabolism?

A marked feature of eukaryotic programmed cell death is an early drop in mitochondrial transmembrane potential. This results from the opening of permeability transition pores, which are composed of adenine nucleotide translocators and mitochondrial porins. The latter share striking similarities with bacterial porins, including down-regulation of their pore size by purine nucleotides), suggesting a common origin. The porins of some invasive bacteria play a crucial role during their accommodation inside the host cell and this coexistence resembles the endosymbiotic origin of mitochondria. The above observations suggest that early in eukaryotic evolution, former invaders may have used porin-type channels to enter their host and to induce its death when the levels of its cytoplasmic purine nucleotides were dropped. The appearance of adenosine nucleotide translocators in the primitive eukaryotes, which permitted usage of the oxidative metabolism of the invaders, provided the basis for the permeability transition phenomena, now linked to the apoptotic process. Bcl-2-type molecules, being able to modulate the permeability transition pores by interaction with adenosine nucleotide translocators, may have played an essential role in conferring a means of controlling apoptosis.

Molecular and cellular prospects for repair, augmentation, and replacement of the failing heart

Molecular and cellular biology offer the promise of new approaches to the treatment of heart failure. This article discusses the basic science background, the current state of investigation, and the potential for therapeutic application of these new sciences. It also emphasizes the limitations and unknowns in this frontier. Three approaches are presented: First, increasing the number of myocytes in the heart, previously held to be untenable because postnatal cardiomyocytes do not divide, may be possible by regulating the cell cycle to reinduce cardiac growth. Also, nonmyocytes extant in the heart may be coaxed into differentiating into cardiomyocytes, or exogenous muscle cells may be grafted into the myocardium. Second, cardiac function may be augmented by molecular therapies that increase contractile protein function or regulate beta-adrenergic receptors or Ca++ channels. Third, improved prospects for transplantation of the failed heart may occur by genetic modification of a xenograft donor heart that reduces the chance of immune rejection by the human recipient. The formulation for the successful application of any of these therapies depends on not only the creativity of scientists but also the wisdom of physicians.

Human Bcl-2 reverses survival defects in yeast lacking superoxide dismutase and delays death of wild-type yeast

We expressed the human anti-apoptotic protein, Bcl-2, in Saccharomyces cerevisiae to investigate its effects on antioxidant protection and stationary phase survival. Yeast lacking copper-zinc superoxide dismutase (sod1Delta) show a profound defect in entry into and survival during stationary phase even under conditions optimal for survival of wild-type strains (incubation in water after stationary phase is reached). Expression of Bcl-2 in the sod1Delta strain caused a large improvement in viability at entry into stationary phase, as well as increased resistance to 100% oxygen and increased catalase activity. In addition, Bcl-2 expression reduced mutation frequency in both wild-type and sod1Delta strains. In another set of experiments, wild-type yeast incubated in expired minimal medium instead of water lost viability quickly; expression of Bcl-2 significantly delayed this stationary phase death. Our results demonstrate that Bcl-2 has activities in yeast that are similar to activities it is known to possess in mammalian cells: (a) stimulation of antioxidant protection and (b) delay of processes leading to cell death.

Facing death in the fly: genetic analysis of apoptosis in Drosophila

Apoptosis, a gene-directed form of cell death, occurs normally during development and plays a major role in many diseases, including cancer and neurodegenerative disorders. Molecular genetic studies in Drosophila have revealed the existence of three novel apoptotic activators, reaper, head involution defective and grim. Additionally, Drosophila homologs of evolutionarily conserved IAPs (inhibitor of apoptosis proteins) and CED-3/ICE-like proteases have been identified and characterized. Through the combined use of genetic, molecular, biochemical and cell biological techniques in Drosophila it should now be possible to elucidate the precise mechanism by which apoptosis occurs, and how the death program is activated in response to many distinct death-inducing signals.

Human Bak induces cell death in Schizosaccharomyces pombe with morphological changes similar to those with apoptosis in mammalian cells

Apoptosis as a form of programmed cell death (PCD) in multicellular organisms is a well-established genetically controlled process that leads to elimination of unnecessary or damaged cells. Recently, PCD has also been described for unicellular organisms as a process for the socially advantageous regulation of cell survival. The human Bcl-2 family member Bak induces apoptosis in mammalian cells which is counteracted by the Bcl-x(L) protein. We show that Bak also kills the unicellular fission yeast Schizosaccharomyces pombe and that this is inhibited by coexpression of human Bcl-x(L). Moreover, the same critical BH3 domain of Bak that is required for induction of apoptosis in mammalian cells is also required for inducing death in yeast. This suggests that Bak kills mammalian and yeast cells by similar mechanisms. The phenotype of the Bak-induced death in yeast involves condensation and fragmentation of the chromatin as well as dissolution of the nuclear envelope, all of which are features of mammalian apoptosis. These data suggest that the evolutionarily conserved metazoan PCD pathway is also present in unicellular yeast.

Kinase cascades regulating entry into apoptosis

All cells are constantly exposed to conflicting environment cues that signal cell survival or cell death. Survival signals are delivered by autocrine or paracrine factors that actively suppress a default death pathway. In addition to survival factor withdrawal, cell death can be triggered by environmental stresses such as heat, UV light, and hyperosmolarity or by dedicated death receptors (e.g., FAS/APO-1 and tumor necrosis factor [TNF] receptors) that are counterparts of growth factor or survival receptors at the cell surface. One of the ways that cells integrate conflicting exogenous stimuli is by phosphorylation (or dephosphorylation) of cellular constituents by interacting cascades of serine/threonine and tyrosine protein kinases (and phosphatases). Survival factors (e.g., growth factors and mitogens) activate receptor tyrosine kinases and selected mitogen-activated, cyclin-dependent, lipid-activated, nucleic acid-dependent, and cyclic AMP-dependent kinases to promote cell survival and proliferation, whereas environmental stress (or death factors such as FAS/APO-1 ligand and TNF-alpha) activates different members of these kinase families to inhibit cell growth and, under some circumstances, promote apoptotic cell death. Because individual kinase cascades can interact with one another, they are able to integrate conflicting exogenous stimuli and provide a link between cell surface receptors and the biochemical pathways leading to cell proliferation or cell death.

Bax- and Bak-induced cell death in the fission yeast Schizosaccharomyces pombe

The effects of the expression of the human Bcl-2 family proteins Bax, Bak, Bcl-2, and Bcl-XL were examined in the fission yeast Schizosaccharomyces pombe and compared with Bax-induced cell death in mammalian cells. Expression of the proapoptotic proteins Bax and Bak conferred a lethal phenotype in this yeast, which was strongly suppressed by coexpression of the anti-apoptotic protein Bcl-XL. Bcl-2 also partially abrogated Bax-mediated cytotoxicity in S. pombe, whereas a mutant of Bcl-2 (Gly145Ala) that fails to heterodimerize with Bax or block apoptosis in mammalian cells was inactive. However, other features distinguished Bax- and Bak-induced death in S. pombe from animal cell apoptosis. Electron microscopic analysis of S. pombe cells dying in response to Bax or Bak expression demonstrated massive cytosolic vacuolization and multifocal nuclear chromatin condensation, thus distinguishing this form of cell death from the classical morphological features of apoptosis seen in animal cells. Unlike Bax-induced apoptosis in 293 cells that led to the induction of interleukin-1 beta-converting enzyme (ICE)/CED-3-like protease activity, Bax- and Bak-induced cell death in S. pombe was accompanied neither by internucleosomal DNA fragmentation nor by activation of proteases with specificities similar to the ICE/CED-3 family. In addition, the baculovirus protease inhibitor p35, which is a potent inhibitor of ICE/CED-3 family proteases and a blocker of apoptosis in animal cells, failed to prevent cell death induction by Bax or Bak in fission yeast, whereas p35 inhibited Bax-induced cell death in mammalian cells. Taken together, these findings suggest that Bcl-2 family proteins may retain an evolutionarily conserved ability to regulate cell survival and death but also indicate differences in the downstream events that are activated by overexpression of Bax or Bak in divergent cell types.

DCP-1, a Drosophila cell death protease essential for development

Apoptosis, a form of cellular suicide, involves the activation of CED-3-related cysteine proteases (caspases). The regulation of caspases by apoptotic signals and the precise mechanism by which they kill the cell remain unknown. In Drosophila, different death-inducing stimuli induce the expression of the apoptotic activator reaper. Cell killing by reaper and two genetically linked apoptotic activators, hid and grim, requires caspase activity. A Drosophila caspase, named Drosophila caspase-1 (DCP-1), was identified and found to be structurally and biochemically similar to Caenorhabditis elegans CED-3. Loss of zygotic DCP-1 function in Drosophila caused larval lethality and melanotic tumors, showing that this gene is essential for normal development.

Mitochondria and programmed cell death: back to the future

Programmed cell death, or apoptosis, has in the past few years undoubtedly become one of the most intensively investigated biological processes. However, fundamental questions concerning the molecular and biochemical mechanisms remain to be elucidated. The central question concerns the biochemical steps shared by the numerous death induction pathways elicited by different stimuli. Heterogeneous death signals precede a common effector phase during which cells pass a threshold of 'no return' and are engaged in a degradation phase where they acquire the typical onset of late apoptosis. Alterations in mitochondrial permeability transition linked to membrane potential disruption precede nuclear and plasma membrane changes. In vitro induction of permeability transition in isolated mitochondria provokes the release of a protein factor capable of inducing nuclear chromatin condensation and fragmentation. This permeability transition is regulated by multiple endogenous effectors, including members of the bcl-2 gene family. Inhibition of these effects prevents apoptosis.