Critical function of endogenous XIAP in regulating caspase activation during sympathetic neuronal apoptosis

mRNA for N-Bak, a neuron-specific BH3-only splice isoform of Bak, escapes nonsense-mediated decay and is translationally repressed in the neurons

mRNA for neuronal Bak (N-Bak), a splice variant of pro-apoptotic Bcl-2 family member Bak is expressed in the neurons. Surprisingly the endogeneous N-Bak protein cannot be demonstrated in the neurons, although the antibodies recognize N-Bak protein from in vitro translation or transiently transfected cells. As N-Bak mRNA contains premature termination codon (PTC) at 89 nucleotides upstream from the last exon–exon junction, it could be degraded by nonsense-mediated decay (NMD) during the pioneer round of translation thus explaining the absence of the protein. We show here that the endogeneous neuronal N-Bak mRNA is not the NMD substrate, as it is not accumulating by cycloheximide treatment, it has a long lifetime, and even prevention of PTC by interfering with the alternative splicing did not lead to translation of the Bak mRNA. N-Bak protein is also not revealed by proteasome inhibitors. Our data suggest strong translational arrest of N-Bak mRNA in the neurons. We show that this arrest is partially mediated by 5′-untranslated region of Bak mRNA and it is not released during mitochondrial apoptosis.

ARTS, the unusual septin: structural and functional aspects

The human Septin 4 gene (Sept4) encodes two major protein isoforms; Sept4_i1 (H5/PNUTL2) and Sept4_i2/ARTS. Septins have been traditionally studied for their role in cytokinesis and their filament-forming abilities, but subsequently have been implicated in diverse functions, including membrane dynamics, cytoskeletal reorganization, vesicle trafficking, and tumorigenesis. ARTS is localized at mitochondria and promotes programmed cell death (apoptosis). These features distinguish ARTS from any other known human septin family member. This review compares the structural and functional properties of ARTS with other septins. In addition, it describes how a combination of two distinct promoters, differential splicing, and intron retention leads to the generation of two different Sept4 variants with diverse biological activity.

Promotion of Caspase Activation by Caspase-9-mediated Feedback Amplification of Mitochondrial Damage

Mitochondrial disruption during apoptosis results in the activation of caspase-9 and a downstream caspase cascade. Triggering this caspase cascade leads to the cleavage of anti-apoptotic Bcl-2 family proteins, resulting in feedback amplification of mitochondrial disruption. However, whether such a feedback loop plays an important role in the promotion of caspase activation and execution of apoptosis has not been well established. We observed that mutated Bcl-2 or Bcl-xL that are resistant to cleavage by caspases inhibited caspase-9-induced caspase activation in human H9 T cells. The release of Smac after the activation of caspase-9 was also inhibited by cleavage-resistant Bcl-2 or Bcl-xL. Consistently, caspase-9-deficient cells were defective in the release of Smac after induction of apoptosis. Moreover, addition of a Smac mimetic overcame the inhibitory effects of cleavage-resistant Bcl-2/Bcl-xL, and restored caspase-9-mediated cell death. Our data suggest that caspase-9-induced feedback disruption of mitochondria plays an important role in promoting the activation of caspases, while a defect in this process can be overcome by promoting Smac functions.

ARTS binds to a distinct domain in XIAP-BIR3 and promotes apoptosis by a mechanism that is different from other IAP-antagonists

ARTS (Sept4_i2), is a pro-apoptotic protein localized at the mitochondria of living cells. In response to apoptotic signals, ARTS rapidly translocates to the cytosol where it binds and antagonizes XIAP to promote caspase activation. However, the mechanism of interaction between these two proteins and how it is regulated remained to be explored. In this study, we show that ARTS and XIAP bind directly to each other, as recombinant ARTS and XIAP proteins co-immunoprecipitate together. We also show that over expression of ARTS alone is sufficient to induce a strong down-regulation of XIAP protein levels and that this reduction occurs through the ubiquitin proteasome system (UPS). Using various deletion and mutation constructs of XIAP we show that ARTS specifically binds to the BIR3 domain in XIAP. Moreover, we found that ARTS binds to different sequences in BIR3 than other IAP antagonists such as SMAC/Diablo. Computational analysis comparing the location of the putative ARTS interface in BIR3 with the known interfaces of SMAC/Diablo and caspase 9 support our results indicating that ARTS interacts with residues in BIR3 that are different from those involved in binding SMAC/Diablo and caspase 9. We therefore suggest that ARTS binds and antagonizes XIAP in a way which is distinct from other IAP-antagonists to promote apoptosis.

The IAP-antagonist ARTS initiates caspase activation upstream of cytochrome C and SMAC/Diablo

ARTS (Sept4_i2) is a pro-apoptotic tumor suppressor protein that functions as an antagonist of X-linked IAP (XIAP) to promote apoptosis. It is generally thought that mitochondrial outer membrane permeabilization (MOMP) occurs before activation of caspases and is required for it. Here, we show that ARTS initiates caspase activation upstream of MOMP. In living cells, ARTS is localized to the mitochondrial outer membrane. In response to apoptotic signals, ARTS translocates rapidly to the cytosol in a caspase-independent manner, where it binds XIAP and promotes caspase activation. This translocation precedes the release of cytochrome C and SMAC/Diablo, and ARTS function is required for the normal timing of MOMP. We also show that ARTS-induced caspase activation leads to cleavage of the pro-apoptotic Bcl-2 family protein Bid, known to promote MOMP. We propose that translocation of ARTS initiates a first wave of caspase activation that can promote MOMP. This leads to the subsequent release of additional mitochondrial factors, including cytochrome C and SMAC/Diablo, which then amplifies the caspase cascade and causes apoptosis.

Activation of apoptosis by cytoplasmic microinjection of cytochrome c

Apoptosis, or programmed cell death, is a conserved and highly regulated pathway by which cells die¹. Apoptosis can be triggered when cells encounter a wide range of cytotoxic stresses. These insults initiate signaling cascades that ultimately cause the release of cytochrome c from the mitochondrial intermembrane space to the cytoplasm². The release of cytochrome c from mitochondria is a key event that triggers the rapid activation of caspases, the key cellular proteases which ultimately execute cell death³⁻⁴. The pathway of apoptosis is regulated at points upstream and downstream of cytochrome c release from mitochondria⁵. In order to study the post-mitochondrial regulation of caspase activation, many investigators have turned to direct cytoplasmic microinjection of holocytochrome c (heme-attached) protein into cells⁶⁻⁹. Cytochrome c is normally localized to the mitochondria where attachment of a heme group is necessary to enable it to activate apoptosis¹⁰⁻¹¹. Therefore, to directly activate caspases, it is necessary to inject the holocytochrome c protein instead of its cDNA, because while the expression of cytochrome c from cDNA constructs will result in mitochondrial targeting and heme attachment, it will be sequestered from cytosolic caspases. Thus, the direct cytosolic microinjection of purified heme-attached cytochrome c protein is a useful tool to mimic mitochondrial cytochrome c release and apoptosis without the use of toxic insults which cause cellular and mitochondrial damage. In this article, we describe a method for the microinjection of cytochrome c protein into cells, using mouse embryonic fibroblasts (MEFs) and primary sympathetic neurons as examples. While this protocol focuses on the injection of cytochrome c for investigations of apoptosis, the techniques shown here can also be easily adapted for microinjection of other proteins of interest.

Dimerization of Smac is crucial for its mitochondrial retention by XIAP subsequent to mitochondrial outer membrane permeabilization

Following the apoptotic permeabilization of the outer mitochondrial membrane, the inter-membrane space protein second mitochondria-derived activator of caspases (Smac) is released into the cytosol. Smac efficiently promotes apoptosis by antagonizing x-linked inhibitor of apoptosis protein (XIAP), an inhibitor of caspases-9, -3, and -7, via a short NH(2)-terminal inhibitor of apoptosis protein (IAP) binding motif (AVPI). Native Smac dimerizes to form a highly stable and inflexible elongated arch, however, a functional role for this outstretched structure so far remained unknown. Using time-lapse single-cell imaging of DLD-1 and HCT-116 colon cancer cells, we here demonstrate that upon mitochondrial outer membrane permeabilization physiological expression levels of XIAP are sufficient to selectively prolong the release of dimeric but not monomeric Smac. Elevating the expression of XIAP further extended the release duration of dimeric Smac and resulted in the mitochondrial retention of a significant proportion of the Smac pool. In contrast, monomeric Smac was always fully released and the release kinetics were not affected by altered XIAP expression. Our findings therefore indicate that the dimerization of Smac is critical for the XIAP-mediated retention of Smac at or inside the mitochondria. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

miR-29b is activated during neuronal maturation and targets BH3-only genes to restrict apoptosis

The execution of apoptosis is critical for proper development of the nervous system. However, it is equally important that neurons strictly inhibit apoptosis after development to ensure their survival throughout the lifetime of the organism. Here we show that a microRNA, miR-29b, is markedly induced with neuronal maturation and functions as a novel inhibitor of neuronal apoptosis. The prosurvival function of miR-29b is mediated by targeting genes in the proapoptotic BH3-only family. Our results identify a unique strategy evolved by maturing neurons that uses a single microRNA to inhibit the multiple, redundant BH3-only proteins that are key initiators of apoptosis.

Transnitrosylation of XIAP regulates caspase-dependent neuronal cell death

X-linked inhibitor of apoptosis (XIAP) is a potent antagonist of caspase apoptotic activity. XIAP also functions as an E3 ubiquitin ligase, targeting caspases for degradation. However, molecular pathways controlling XIAP activities remain unclear. Here, we report that nitric oxide (NO) reacts with XIAP by S-nitrosylating its RING domain (forming SNO-XIAP), thereby inhibiting E3 ligase and antiapoptotic activity. NO-mediated neurotoxicity and caspase activation have been linked to several neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's diseases. We find significant SNO-XIAP formation in brains of patients with these diseases, implicating this reaction in the etiology of neuronal damage. Conversely, S-nitrosylation of caspases is known to inhibit apoptotic activity. Unexpectedly, we find that SNO-caspase transnitrosylates (transfers its NO group) to XIAP, forming SNO-XIAP, and thus promotes cell injury and death. These findings provide insights into the regulation of caspase activation in neurodegenerative disorders mediated, at least in part, by nitrosative stress.

Mitochondria and cell death: outer membrane permeabilization and beyond

Mitochondrial outer membrane permeabilization (MOMP) is often required for activation of the caspase proteases that cause apoptotic cell death. Various intermembrane space (IMS) proteins, such as cytochrome c, promote caspase activation following their mitochondrial release. As a consequence, mitochondrial outer membrane integrity is highly controlled, primarily through interactions between pro- and anti-apoptotic members of the B cell lymphoma 2 (BCL-2) protein family. Following MOMP by pro-apoptotic BCL-2-associated X protein (BAX) or BCL-2 antagonist or killer (BAK), additional regulatory mechanisms govern the mitochondrial release of IMS proteins and caspase activity. MOMP typically leads to cell death irrespective of caspase activity by causing a progressive decline in mitochondrial function, although cells can survive this under certain circumstances, which may have pathophysiological consequences.

XIAP impairs Smac release from the mitochondria during apoptosis

X-linked inhibitor of apoptosis protein (XIAP) is a potent inhibitor of caspases 3, 7 and 9, and mitochondrial Smac (second mitochondria-derived activator of caspase) release during apoptosis inhibits the activity of XIAP. In this study we show that cytosolic XIAP also feeds back to mitochondria to impair Smac release. We constructed a fluorescent XIAP-fusion protein by labelling NH₂- and COOH-termini with Cerulean fluorescent protein (C-XIAP-C). Immunoprecipitation confirmed that C-XIAP-C retained the ability to interact with Smac and impaired extrinsically and intrinsically activated apoptosis in response to tumour necrosis factor-related apoptosis-inducing ligand/cycloheximide and staurosporine. In C-XIAP-C-expressing cells, cytochrome c release from mitochondria proceeded normally, whereas Smac release was significantly prolonged and incomplete. In addition, physiological expression of native XIAP prolonged or limited Smac release in HCT-116 colon cancer cells and primary mouse cortical neurons. The Smac-binding capacity of XIAP, but not caspase inhibition, was central for mitochondrial Smac retention, as evidenced in experiments using XIAP mutants that cannot bind to Smac or effector caspases. Similarly, the release of a Smac mutant that cannot bind to XIAP was not impaired by C-XIAP-C expression. Full Smac release could however be provoked by rapid cytosolic C-XIAP-C depletion upon digitonin-induced plasma membrane permeabilization. Our findings suggest that although mitochondria may already contain pores sufficient for cytochrome c release, elevated amounts of XIAP can selectively impair and limit the release of Smac.

AMP kinase-mediated activation of the BH3-only protein Bim couples energy depletion to stress-induced apoptosis

Disturbances in cellular ion gradients by excitotoxicity promote apoptosis through activation of the Bcl-2 family member Bim.

Excitotoxicity after glutamate receptor overactivation induces disturbances in cellular ion gradients, resulting in necrosis or apoptosis. Excitotoxic necrosis is triggered by rapid, irreversible ATP depletion, whereas the ability to recover cellular bioenergetics is suggested to be necessary for the activation of excitotoxic apoptosis. In this study, we demonstrate that even a transient decrease in cellular bioenergetics and an associated activation of adenosine monophosphate–activated protein kinase (AMPK) is necessary for the activation of excitotoxic apoptosis. We show that the Bcl-2 homology domain 3 (BH3)–only protein Bim, a proapoptotic Bcl-2 family member, is activated in multiple excitotoxicity paradigms, mediates excitotoxic apoptosis, and inhibits delayed Ca²⁺ deregulation, mitochondrial depolarization, and apoptosis-inducing factor translocation. We demonstrate that bim activation required the activation of AMPK and that prolonged AMPK activation is sufficient to induce bim gene expression and to trigger a bim-dependent cell death. Collectively, our data demonstrate that AMPK activation and the BH3-only protein Bim couple transient energy depletion to stress-induced neuronal apoptosis.

Suppression of the intrinsic apoptosis pathway by synaptic activity

Synaptic activity promotes resistance to diverse apoptotic insults, the mechanism behind which is incompletely understood. We show here that a coordinated downregulation of core components of the intrinsic apoptosis pathway by neuronal activity forms a key part of the underlying mechanism. Activity-dependent protection against apoptotic insults is associated with inhibition of cytochrome c release in most but not all neurons, indicative of anti-apoptotic signaling both upstream and downstream of this step. We find that enhanced firing activity suppresses expression of the proapoptotic BH3-only member gene Puma in a NMDA receptor-dependent, p53-independent manner. Puma expression is sufficient to induce cytochrome c loss and neuronal apoptosis. Puma deficiency protects neurons against apoptosis and also occludes the protective effect of synaptic activity, while blockade of physiological NMDA receptor activity in the developing mouse brain induces neuronal apoptosis that is preceded by upregulation of Puma. However, enhanced activity can also confer resistance to Puma-induced apoptosis, acting downstream of cytochrome c release. This mechanism is mediated by transcriptional suppression of apoptosome components Apaf-1 and procaspase-9, and limiting caspase-9 activity, since overexpression of procaspase-9 accelerates the rate of apoptosis in active neurons back to control levels. Synaptic activity does not exert further significant anti-apoptotic effects downstream of caspase-9 activation, since an inducible form of caspase-9 overrides the protective effect of synaptic activity, despite activity-induced transcriptional suppression of caspase-3. Thus, suppression of apoptotic gene expression may synergize with other activity-dependent events such as enhancement of antioxidant defenses to promote neuronal survival.

Orexins/hypocretins and orexin receptors in apoptosis: a mini-review

An unexpected and fascinating aspect of the neuropeptides orexins has recently emerged when it was shown that orexins acting at orexin receptors OX1R or OX2R induce dramatic apoptosis resulting in massive reduction in cell growth in various cancer cell lines. This mini-review will provide the reader with recent findings related to the proapoptotic actions of orexins and the entirely novel mechanism whereby the seven membrane-spanning G-protein-coupled receptor (GPCR) OX1R triggers apoptosis. Recent data show that orexins induce tyrosine phosphorylation of the tyrosine-based motifs - immunoreceptor tyrosine-based inhibitory motif and immunoreceptor tyrosine-based switch motif - in OX1R. These phosphorylations result in the recruitment and activation of the phosphotyrosine phosphatase SHP-2 and subsequent cytochrome c-mediated mitochondrial apoptosis. Finally, this mini-review will also speculate on: (1) the potential importance of tyrosine-based motifs in the large family of GPCRs; (2) the interest of orexin receptors as therapeutic targets in cancer therapy; (3) the possible role of orexin receptor-mediated apoptosis in physiology and pathophysiology in the brain (neurodevelopment, neurodegenerative diseases) and in the periphery.

Mitochondrial damage: a target for new therapeutic horizons

Traumatic brain injury (TBI) represents a leading cause of death and morbidity, as well as a considerable social and economical burden in western countries, and has thus emerged as a formidable therapeutic challenge. Yet despite tremendous efforts enlightening the mechanisms of neuronal death, hopes for the "magic bullet" have been repeatedly deceived, and TBI management has remained focused on the control of increased intracranial pressure. Indeed, impairment of cerebral metabolism is traditionally attributed to impaired oxygen delivery mediated by reduced cerebral perfusion in the swollen cerebral parenchyma. Although intuitively appealing, this hypothesis is not entirely supported by physiological facts and does not take into consideration mitochondrial dysfunction that has been repeatedly reported in both human and animal TBI. Although the nature and origin of the events leading to mitochondrial damage may be different, most share a permeabilization of mitochondrial membrane, which therefore may represent a logical target for new therapeutic strategies. Therefore, the proteins mediating these events may represent promising targets for new TBI therapies. Furthermore, mimicking anti-apoptotic proteins, such as Bcl-2 or XIAP, or inhibiting mitochondrial pro-apoptotic proteins, such as Smac/DIABLO, Omi/HTRA2, and ARTS (septin 4 isoform 2) may represent useful novel therapeutic strategies. This review focuses on mechanisms of the mitochondrial membrane permeabilization and its consequences and discusses the current and possible future therapeutic implications of this key event of neuronal death.

Inhibition of Akt sensitises neuroblastoma cells to gold(III) porphyrin 1a, a novel antitumour drug induced apoptosis and growth inhibition

BACKGROUND

Gold(III) porphyrin 1a is a new class of anticancer drug, which inhibits cell proliferation of wide range of human cancer cell lines and induces apoptosis in human nasopharyngeal carcinoma cells. However, the underlying signalling mechanism by which gold(III) porphyrin 1a modifies the intracellular apoptosis pathways in tumour cells has not been explained in detail in neuroblastoma cells.

METHODS

Cell proliferation and apoptosis were determined by measuring 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and Annexin V binding, respectively. Western blot assay was used to detect proteins involved in apoptotic and Akt pathways. In vivo tumour growth was assessed by inoculating tumour cells to nude mice subcutaneously, and gold(III) porphyrin 1a was administrated intravenously.

RESULTS

This study assessed the antitumour effect and mechanism of gold(III) porphyrin 1a on neuroblastoma in vitro and in vivo. Gold(III) porphyrin 1a displayed a growth inhibition and induction of apoptosis in neuroblastoma cells effectively in vitro, which was accompanied with release of cytochrome c and Smac/DIABLO and caspases activation. Further studies indicated that gold(III) porphyrin 1a inhibited X-linked inhibitor of apoptosis (XIAP). However, we found that gold(III) porphyrin 1a can induce a survival signal, Akt activation within minutes and could last for at least 24 h. To further confirm association between activation of Akt and the effectiveness of gold(III) porphyrin 1a, neuroblastoma cells were treated with API-2, an Akt-specific inhibitor. API-2 sensitised cells to gold(III) porphyrin 1a-induced apoptosis and growth inhibition.

CONCLUSION

These results suggested that Akt may be considered as a molecular 'brake' that neuroblastoma cells rely on to slow down gold(III) porphyrin 1a-induced apoptosis and antiproliferation. Gold(III) porphyrin 1a is a mitochondrial apoptotic stimulus but also activates Akt, suggesting an involvement of Akt in mediating the effectiveness to growth inhibition and apoptosis by gold(III) porphyrin 1a and that inhibition of Akt can enhance the anticancer activity of gold(III) porphyrin 1a in neuroblastoma.

Lack of X-linked inhibitor of apoptosis protein leads to increased apoptosis and tissue loss following neonatal brain injury

Neurological deficits caused by H-I (hypoxia-ischaemia) to the perinatal brain are often severely debilitating and lead to motor impairment, intellectual disability and seizures. Perinatal brain injury is distinct from adult brain injury in that the developing brain is undergoing the normal process of neuronal elimination by apoptotic cell death and thus the apoptotic machinery is more easily engaged and activated in response to injury. Thus cell death in response to neonatal H-I brain injury is partially due to mitochondrial dysfunction and activation of the apoptosome and caspase 3. An important regulator of the apoptotic response following mitochondrial dysfunction is XIAP (X-linked inhibitor of apoptosis protein). XIAP inhibits apoptosis at the level of caspase 9 and caspase 3 activation, and lack of XIAP in vitro has been shown to lead to increased apoptotic cell death. In the present study we show that mice lacking the gene encoding the XIAP protein have an exacerbated response to neonatal H-I injury as measured by tissue loss at 7 days following the injury. In addition, when the XIAP-deficient mice were studied at 24 h post-H-I we found that the increase in injury correlates with an increased apoptotic response in the XIAP-deficient mice and also with brain imaging changes in T2-weighted magnetic resonance imaging and apparent diffusion coefficient that correspond to the location of apoptotic cell death. These results identify a critical role of XIAP in regulating neuronal apoptosis in vivo and demonstrate the enhanced vulnerability of neurons to injury in the absence of XIAP in the developing brain.

Skeletal muscle differentiation evokes endogenous XIAP to restrict the apoptotic pathway

Myotube apoptosis occurs normally during muscle development and aging but it can lead to destruction of skeletal muscle in neuromuscular diseases. Therefore, understanding how myotube apoptosis is regulated is important for developing novel strategies for treatment of muscle loss. We investigated the regulation of apoptosis in skeletal muscle and report a striking increase in resistance to apoptosis following differentiation. We find mitotic C2C12 cells (myoblast-like cells) are sensitive to cytosolic cytochrome c microinjection. However, differentiated C2C12 cells (myotube-like cells) and primary myotubes are markedly resistant. This resistance is due to endogenous X-linked inhibitor of apoptotic protein (XIAP). Importantly, the selective difference in the ability of XIAP to block myotube but not myoblast apoptosis is not due to a change in XIAP but rather a decrease in Apaf-1 expression. This decrease in Apaf-1 links XIAP to caspase activation and death. Our findings suggest that in order for myotubes to die, they may degrade XIAP, functionally inactivate XIAP or upregulate Apaf-1. Importantly, we identify a role for endogenous Smac in overcoming XIAP to allow myotube death. However, in postmitotic cardiomyocytes, where XIAP also restricts apoptosis, endogenous Smac was not capable of overcoming XIAP to cause death. These results show that as skeletal muscle differentiate, they become resistant to apoptosis because of the ability of XIAP to regulate caspase activation. The increased restriction of apoptosis in myotubes is presumably important to ensure the long term survival of these postmitotic cells as they play a vital role in the physiology of organisms.

The mitochondrial death pathway: a promising therapeutic target in diseases

The mitochondrial pathway to apoptosis is a major pathway of physiological cell death in vertebrates. The mitochondrial cell death pathway commences when apoptogenic molecules present between the outer and inner mitochondrial membranes are released into the cytosol by mitochondrial outer membrane permeabilization (MOMP). BCL-2 family members are the sentinels of MOMP in the mitochondrial apoptotic pathway; the pro-apoptotic B cell lymphoma (BCL)-2 proteins, BCL-2 associated x protein and BCL-2 antagonist killer 1 induce MOMP whereas the anti-apoptotic BCL-2 proteins, BCL-2, BCL-x_l and myeloid cell leukaemia 1 prevent MOMP from occurring. The release of pro-apoptotic factors such as cytochrome c from mitochondria leads to formation of a multimeric complex known as the apoptosome and initiates caspase activation cascades. These pathways are important for normal cellular homeostasis and play key roles in the pathogenesis of many diseases. In this review, we will provide a brief overview of the mitochondrial death pathway and focus on a selection of diseases whose pathogenesis involves the mitochondrial death pathway and we will examine the various pharmacological approaches that target this pathway.

Noncytolytic clearance of sindbis virus infection from neurons by gamma interferon is dependent on Jak/STAT signaling

The alphavirus Sindbis virus (SINV) causes encephalomyelitis in mice by infecting neurons of the brain and spinal cord. The outcome is age dependent. Young animals develop fatal disease, while older animals recover from infection. Recovery requires noncytolytic clearance of SINV from neurons, and gamma interferon (IFN-gamma) is an important contributor to clearance in vivo. IFN-gamma-dependent clearance has been studied using immortalized CSM14.1 rat neuronal cells that can be differentiated in vitro. Previous studies have shown that differentiated, but not undifferentiated, cells develop prolonged SINV replication and respond to IFN-gamma treatment with noncytolytic clearance of virus preceded by suppression of genomic viral RNA synthesis and reactivation of cellular protein synthesis. To determine the signaling mechanisms responsible for clearance, the responses of SINV-infected differentiated neurons to IFN-gamma were examined. IFN-gamma treatment of SINV-infected differentiated CSM14.1 cells, AP-7 olfactory neuronal cells, and primary dorsal root ganglia neurons triggered prolonged Stat-1 Tyr(701) phosphorylation, Stat-1 Ser(727) phosphorylation, and transient Stat-5 phosphorylation. Inhibition of Jak kinase activity with Jak inhibitor I completely reversed the neuroprotective and antiviral activities of IFN-gamma in differentiated cells. We conclude that activation of the Jak/Stat pathway is the primary mechanism for IFN-gamma-mediated clearance of SINV infection from mature neurons.

Caspase-independent cell death: leaving the set without the final cut

Apoptosis is dependent upon caspase activation leading to substrate cleavage and, ultimately, cell death. Although required for the apoptotic phenotype, it has become apparent that cells frequently die even when caspase function is blocked. This process, termed caspase-independent cell death (CICD), occurs in response to most intrinsic apoptotic cues, provided that mitochondrial outer membrane permeabilization has occurred. Death receptor ligation can also trigger a form of CICD termed necroptosis. In this review, we will examine the molecular mechanisms governing CICD, highlight recent findings demonstrating recovery from conditions of CICD and discuss potential pathophysiological functions of these processes.

Glucose metabolism inhibits apoptosis in neurons and cancer cells by redox inactivation of cytochrome c

Neurons and cancer cells utilize glucose extensively, yet the precise advantage of this adaptation remains elusive. These two seemingly disparate cell types also exhibit an increased regulation of the apoptotic pathway, which allows for their long term survival1. Here we show that both neurons and cancer cells strictly inhibit cytochrome c-mediated apoptosis by a mechanism dependent on glucose metabolism. We report that the proapoptotic activity of cytochrome c is influenced by its redox state and that increases in Reactive Oxygen Species (ROS) following an apoptotic insult lead to the oxidation and activation of cytochrome c. In healthy neurons and cancer cells, however, cytochrome c is reduced and held inactive by intracellular glutathione (GSH) generated as a result of glucose metabolism by the pentose phosphate pathway. These results uncover a striking similarity in apoptosis regulation between neurons and cancer cells and provide insight into an adaptive advantage offered by the Warburg effect for cancer cell evasion of apoptosis and for long-term neuronal survival.

Regulation of apoptosis by XIAP ubiquitin-ligase activity

Inhibitor of Apoptosis Proteins (IAPs) can bind to and inhibit caspases, the key executioners of apoptosis. Because IAPs are frequently overexpressed in human tumors, they have become major pharmacological targets for developing new cancer therapeutics. However, the precise physiological function of individual mammalian IAPs and their role as E3 ubiquitin-ligases in situ remain largely obscure. Here, we investigated the function of XIAP ubiquitin-ligase activity by inactivating the RING motif via gene targeting in the mouse. Removing the RING stabilized XIAP in apoptotic thymocytes, demonstrating that XIAP ubiquitin-ligase activity is a major determinant of XIAP protein stability. Surprisingly, the increased amounts of "XIAP-BIR-only" protein did not lead to attenuated but rather increased caspase activity and apoptosis. DeltaRING embryonic stem cells and fibroblasts had elevated caspase-3 enzyme activity, and XIAP DeltaRING embryonic fibroblasts were strongly sensitized to TNF-alpha-induced apoptosis. Similar results were obtained with XIAP deficient mice. Furthermore, deletion of the RING also improved the survival of mice in the Emu-Myc lymphoma model. This demonstrates a physiological requirement of XIAP ubiquitin-ligase activity for the inhibition of caspases and for tumor suppression in vivo.

Intracellular signaling dynamics during apoptosis execution in the presence or absence of X-linked-inhibitor-of-apoptosis-protein

X-linked-inhibitor-of-apoptosis-protein (XIAP) is the most potent intracellular inhibitor of caspases-9, -3 and -7. While highly elevated XIAP levels reduce the apoptotic response to various stimuli, the potency of physiological XIAP expression to control caspase activation and the consequences of XIAP deficiency on apoptosis execution remain controversial. We therefore analyzed parental and XIAP-deficient DLD-1 and HCT-116 colon cancer cells by employing fluorescence-based single-cell imaging of mitochondrial permeabilisation and effector caspase activation. Our results demonstrate that physiological XIAP expression maintains a transient "off"-state for effector caspase activation following mitochondrial permeabilisation. Loss of XIAP expression instead accelerated the caspase activation response, but did not enhance the measured caspase activity. Apoptosis execution kinetics were independent of activating the intrinsic or extrinsic pathway by either staurosporine or TRAIL, and corresponded to computational systems analyses of caspase activation dynamics. We confirmed a protective role of XIAP upstream of mitochondrial permeabilisation during TRAIL-induced apoptosis, however, once mitochondria permeabilised ultimately no cell could escape effector caspase activation, regardless of potential cell-to-cell variability within the populations or the presence of XIAP. Our study provides comprehensive kinetic and mechanistic insight into the rapid molecular dynamics during apoptosis execution in the presence or absence of physiological XIAP expression.

Analysis of Ret knockin mice reveals a critical role for IKKs, but not PI 3-K, in neurotrophic factor-induced survival of sympathetic neurons

We analyzed the survival responses and downstream signaling elicited by GDNF on sympathetic neurons from different Ret knockin mice. Lack of tyrosine 1062, a multidocking site in Ret, completely prevented GDNF-mediated survival. Importantly, lack of tyrosine 981, although abrogating Akt phosphorylation, had no effect on neuronal survival, indicating that the PI 3-K/Akt pathway is not necessary for survival of sympathetic neurons. In contrast, silencing of B-Raf completely prevented not only GDNF-mediated but also NGF-mediated cell survival, independently of MEK-1/2. We identified IKKs as the main effectors of the protective effects of B-Raf. First, B-Raf interacted with and activated IKKs. Second, knockdown of IKKs reversed the protection afforded by a constitutively active form of B-Raf. Third, knockdown of IKKs prevented both NGF- and GDNF-mediated survival. In conclusion, our data delineate a novel survival pathway for sympathetic neurons linking B-Raf to IKKs, independently of both PI 3-K and MEK-1/2 pathways.

IAPs: what's in a name?

Originally described in insect viruses, cellular proteins with Baculoviral IAP repeat (BIR) motifs have been thought to function primarily as inhibitors of apoptosis. The subsequent finding that a subset of IAPs that contain a RING domain have ubiquitin protein ligase (E3) activity implied the presence of other functions. It is now known that IAPs are involved in mitotic chromosome segregation, cellular morphogenesis, copper homeostasis, and intracellular signaling. Here, we review the current understanding of the roles of IAPs in apoptotic and nonapoptotic processes and explore the notion that the latter represents the primary physiologic activities of IAPs.

The mitochondrial serine protease HtrA2/Omi: an overview

The HtrA family refers to a group of related oligomeric serine proteases that combine a trypsin-like protease domain with at least one PDZ interaction domain. Mammals encode four HtrA proteases, named HtrA1-4. The protease activity of the HtrA member HtrA2/Omi is required for mitochondrial homeostasis in mice and humans and inactivating mutations associated with neurodegenerative disorders such as Parkinson's disease. Moreover, HtrA2/Omi is released in the cytosol, where it contributes to apoptosis through both caspase-dependent and -independent pathways. Here, we review the current knowledge of HtrA2/Omi biology and discuss the signaling pathways that underlie its mitochondrial and apoptotic functions from an evolutionary perspective.

Chromatin modification of Apaf-1 restricts the apoptotic pathway in mature neurons

Although apoptosis has been extensively studied in developing neurons, the dynamic changes in this pathway after neuronal maturation remain largely unexplored. We show that as neurons mature, cytochrome c– mediated apoptosis progresses from inhibitor of apoptosis protein–dependent to –independent regulation because of a complete loss of Apaf-1 expression. However, after DNA damage, mature neurons resynthesize Apaf-1 through the cell cycle–related E2F1 pathway and restore their apoptotic potential. Surprisingly, we find that E2F1 is sufficient to induce Apaf-1 expression in developing but not mature neurons. Rather, Apaf-1 up-regulation in mature neurons requires both chromatin derepression and E2F1 transcriptional activity. This differential capacity of E2F1 to induce Apaf-1 transcription is because of the association of the Apaf-1 promoter with active chromatin in developing neurons and repressed chromatin in mature neurons. These data specifically illustrate how the apoptotic pathway in mature neurons becomes increasingly restricted by a novel mechanism involving the regulation of chromatin structure.

The inhibitors of apoptosis (IAPs) as cancer targets

Apoptosis has been accepted as a fundamental component in the pathogenesis of cancer, in addition to other human diseases including neurodegeneration, coronary disease and diabetes. The origin of cancer involves deregulated cellular proliferation and the suppression of apoptotic processes, ultimately leading to tumor establishment and growth. Several lines of evidence point toward the IAP family of proteins playing a role in oncogenesis, via their effective suppression of apoptosis. The central mechanisms of IAP apoptotic suppression appear to be through direct caspase and pro-caspase inhibition (primarily caspase 3 and 7) and modulation of, and by, the transcription factor NF-kappaB. Thus, when the IAPs are over-expressed or over-active, as is the case in many cancers, cells are no longer able to die in a physiologically programmed fashion and become increasingly resistant to standard chemo- and radiation therapies. To date several approaches have been taken to target and eliminate IAP function in an attempt to re-establish sensitivity, reduce toxicity, and improve efficacy of cancer treatment. In this review, we address IAP proteins as therapeutic targets for the treatment of cancer and emphasize the importance of novel therapeutic approaches for cancer therapy. Novel targets of IAP function are being identified and include gene therapy strategies and small molecule inhibitors that are based on endogenous IAP antagonists. As well, molecular mechanistic approaches, such as RNAi to deplete IAP expression, are in development.

Bile salts induce resistance to apoptosis through NF-kappaB-mediated XIAP expression

Apoptosis plays a critical role in intestinal mucosal homeostasis. We previously showed that the bile salt taurodeoxycholate has a beneficial effect on the intestinal mucosa through an increase in resistance to apoptosis mediated by nuclear factor (NF)-kappaB. The current study further characterizes the effect of bile salts on intestinal epithelial cell susceptibility to apoptosis and determines if the X-linked inhibitor of apoptosis protein (XIAP) regulates bile salt-induced resistance to apoptosis. Exposure of normal intestinal epithelial cells (IEC-6) to the conjugated bile salts taurodeoxycholate (TDCA) and taurochenodeoxycholate (TCDCA) resulted in an increase in resistance to tumor necrosis factor (TNF)-alpha and cycloheximide (CHX)-induced apoptosis, and NF-kappaB activation. Treatment with TDCA and TCDCA resulted in an increase in XIAP expression. Specific inhibition of NF-kappaB by infection with an adenoviral vector that expresses the IkappaBalpha super-repressor (IkappaBSR) prevented the induction of XIAP expression and the bile salt-mediated resistance to apoptosis. Treatment with the specific XIAP inhibitor Smac also overcame this increase in enterocyte resistance to apoptosis. Bile salts inhibited formation of the active caspase-3 from its precursor procaspase-3. Smac prevented the inhibitory effect of bile salts on caspase-3 activation. These results indicate that bile salts increase intestinal epithelial cell resistance to apoptosis through NF-kappaB-mediated XIAP expression. Bile salt-induced XIAP mediates resistance to TNF-alpha/CHX-induced apoptosis, at least partially, through inhibition of caspase-3 activity. These data support an important beneficial role of bile salts in regulation of mucosal integrity. Decreased enterocyte exposure to luminal bile salts, as occurs during starvation and parenteral nutrition, may have a detrimental effect on mucosal integrity.

Rho kinase in the regulation of cell death and survival

Rho kinase (ROCK) belongs to a family of serine/threonine kinases that are activated via interaction with Rho GTPases. ROCK is involved in a wide range of fundamental cellular functions, such as contraction, adhesion, migration, and proliferation. Recent studies have shown that ROCK plays an important role in the regulation of apoptosis in various cell types and animal disease models. Two ROCK isoforms, ROCK1 and ROCK2, are assumed to be function redundant, this based largely on kinase construct overexpression and chemical inhibitors (Y27632 and fasudil) which inhibit both ROCK1 and ROCK2. Gene targeting and RNA interference approaches allow further dissection of distinct cellular, physiological, and patho-physiological functions of the two ROCK isoforms. This review, based on recent molecular, cellular, and animal studies, focuses on the current understanding of ROCK signaling in the regulation of apoptosis and highlights new findings from recently generated ROCK-deficient mice.

Calcium blocks formation of apoptosome by preventing nucleotide exchange in Apaf-1

Apaf-1 plays an essential role in apoptosis. In the presence of cytochrome c and dATP, Apaf-1 assembles into an oligomeric apoptosome, which is responsible for the activation of procaspase-9 and the maintenance of the enzymatic activity of the processed caspase-9. Regulation of apoptosome assembly by other cellular factors is poorly understood. Here we report that physiological concentrations of calcium ion negatively affect the assembly of apoptosome by inhibiting nucleotide exchange in the monomeric, autoinhibited Apaf-1 protein. Consequently, calcium blocks the ability of Apaf-1 to activate caspase-9. These observations suggest an important role of calcium homeostasis on the Apaf-1-dependent apoptotic pathway.

Endoplasmic reticulum stress-induced apoptosis requires bax for commitment and Apaf-1 for execution in primary neurons

Apoptosis triggered by endoplasmic reticulum (ER) stress is associated with various pathophysiological conditions including neurodegenerative diseases and ischemia. However, the mechanism by which ER stress induces neuronal apoptosis remains controversial. Here we identify the pathway of apoptosis carried out in sympathetic neurons triggered to die by ER stress-inducing agent tunicamycin. We find that ER stress induces a neuronal apoptotic pathway which upregulates BH3-only genes DP5 and Puma. Importantly, we show that ER stress commits neurons to die before cytochrome c release and this commitment requires Bax activation and c-jun N-terminal kinase signaling. Furthermore, ER stress engages the mitochondrial pathway of death as neurons release cytochrome c and Apaf-1 deficiency is sufficient to block apoptosis. Our findings identify a critical function of Bax in committing neurons to ER stress-induced apoptosis and clarify the importance of the apoptosome as the non-redundant caspase activation pathway to execute neuronal apoptosis in response to ER stress.

Essential postmitochondrial function of p53 uncovered in DNA damage-induced apoptosis in neurons

In postmitotic sympathetic neurons, unlike most mitotic cells, death by apoptosis requires not only the release of cytochrome c from the mitochondria, but also an additional step to relieve X-linked inhibitor of apoptosis protein (XIAP)'s inhibition of caspases. Here, we examined the mechanism by which XIAP is inactivated following DNA damage and found that it is achieved by a mechanism completely different from that following apoptosis by nerve growth factor (NGF) deprivation. NGF deprivation relieves XIAP by selectively degrading it, whereas DNA damage overcomes XIAP via a p53-mediated induction of Apaf-1. Unlike wild-type neurons, p53-deficient neurons fail to overcome XIAP and remain resistant to cytochrome c after DNA damage. Restoring Apaf-1 induction in p53-deficient neurons is sufficient to overcome XIAP and sensitize cells to cytochrome c. Although a role for p53 in apoptosis upstream of cytochrome c release has been well established, this study uncovers an additional, essential role for p53 in regulating caspase activation downstream of mitochondria following DNA damage in neurons.

The apoptosis inhibitor ARC undergoes ubiquitin-proteasomal-mediated degradation in response to death stimuli: identification of a degradation-resistant mutant

Efficient induction of apoptosis requires not only the activation of death-promoting proteins but also the inactivation of inhibitors of cell death. ARC (apoptosis repressor with caspase recruitment domain) is an endogenous inhibitor of apoptosis that antagonizes both central apoptosis pathways. Despite its potent inhibition of cell death, cells that express abundant ARC eventually succumb. A possible explanation is that ARC protein levels decrease dramatically in response to death stimuli. The mechanisms that mediate decreases in ARC protein levels during apoptosis and whether these decreases initiate the subsequent cell death are not known. Here we show that endogenous ARC protein levels decrease in response to death stimuli in a variety of cell contexts as well as in a model of myocardial ischemia-reperfusion in intact mice. Decreases in ARC protein levels are not explained by alterations in the abundance of ARC transcripts. Rather, pulse-chase experiments show that decreases in steady state ARC protein levels during apoptosis result from marked destabilization of ARC protein. ARC protein destabilization, in turn, is mediated by the ubiquitin-proteasomal pathway, as mutation of ARC ubiquitin acceptor residues stabilizes ARC protein and preserves its steady state levels during apoptosis. In addition, this degradation-resistant ARC mutant exhibits improved cytoprotection. We conclude that decreases in ARC protein levels in response to death stimuli are mediated by increased ARC protein degradation via the ubiquitin-proteasomal pathway. Moreover, these data demonstrate that decreases in ARC protein levels are a trigger, and not merely a consequence, of the ensuing cell death.

Systems analysis of effector caspase activation and its control by X-linked inhibitor of apoptosis protein

Activation of effector caspases is a final step during apoptosis. Single-cell imaging studies have demonstrated that this process may occur as a rapid, all-or-none response, triggering a complete substrate cleavage within 15 min. Based on biochemical data from HeLa cells, we have developed a computational model of apoptosome-dependent caspase activation that was sufficient to remodel the rapid kinetics of effector caspase activation observed in vivo. Sensitivity analyses predicted a critical role for caspase-3-dependent feedback signalling and the X-linked-inhibitor-of-apoptosis-protein (XIAP), but a less prominent role for the XIAP antagonist Smac. Single-cell experiments employing a caspase fluorescence resonance energy transfer substrate verified these model predictions qualitatively and quantitatively. XIAP was predicted to control this all-or-none response, with concentrations as high as 0.15 microM enabling, but concentrations >0.30 microM significantly blocking substrate cleavage. Overexpression of XIAP within these threshold concentrations produced cells showing slow effector caspase activation and submaximal substrate cleavage. Our study supports the hypothesis that high levels of XIAP control caspase activation and substrate cleavage, and may promote apoptosis resistance and sublethal caspase activation in vivo.

Mathematical modeling identifies inhibitors of apoptosis as mediators of positive feedback and bistability

The intrinsic, or mitochondrial, pathway of caspase activation is essential for apoptosis induction by various stimuli including cytotoxic stress. It depends on the cellular context, whether cytochrome c released from mitochondria induces caspase activation gradually or in an all-or-none fashion, and whether caspase activation irreversibly commits cells to apoptosis. By analyzing a quantitative kinetic model, we show that inhibition of caspase-3 (Casp3) and Casp9 by inhibitors of apoptosis (IAPs) results in an implicit positive feedback, since cleaved Casp3 augments its own activation by sequestering IAPs away from Casp9. We demonstrate that this positive feedback brings about bistability (i.e., all-or-none behaviour), and that it cooperates with Casp3-mediated feedback cleavage of Casp9 to generate irreversibility in caspase activation. Our calculations also unravel how cell-specific protein expression brings about the observed qualitative differences in caspase activation (gradual versus all-or-none and reversible versus irreversible). Finally, known regulators of the pathway are shown to efficiently shift the apoptotic threshold stimulus, suggesting that the bistable caspase cascade computes multiple inputs into an all-or-none caspase output. As cellular inhibitory proteins (e.g., IAPs) frequently inhibit consecutive intermediates in cellular signaling cascades (e.g., Casp3 and Casp9), the feedback mechanism described in this paper is likely to be a widespread principle on how cells achieve ultrasensitivity, bistability, and irreversibility.

Apoptosome dependent caspase-3 activation pathway is non-redundant and necessary for apoptosis in sympathetic neurons

Although sympathetic neurons are a well-studied model for neuronal apoptosis, the role of the apoptosome in activating caspases in these neurons remains debated. We find that the ability of sympathetic neurons to undergo apoptosis in response to nerve growth factor (NGF) deprivation is completely dependent on having an intact apoptosome pathway. Genetic deletion of Apaf-1, caspase-9, or caspase-3 prevents apoptosis after NGF deprivation, and importantly, allows these neurons to recover and survive long-term following readdition of NGF. The inability of caspase-3 deficient sympathetic neurons to undergo apoptosis is particularly striking, as apoptosis in dermal fibroblasts and cortical neurons proceeds even in the absence of caspase-3. Our results show that in contrast to dermal fibroblasts and cortical neurons, sympathetic neurons express no detectable levels of caspase-7. The strict requirement for an intact apoptosome, coupled with a lack of effector caspase redundancy, provides sympathetic neurons with a markedly increased control over their apoptotic pathway.

The apoptosome: physiological, developmental, and pathological modes of regulation

Apoptosis, a form of programmed cell death, is executed by a family of zymogenic proteases known as caspases, which cleave an array of intracellular substrates in the dying cell. Many proapoptotic stimuli trigger cytochrome c release from mitochondria, promoting the formation of a complex between Apaf-1 and caspase-9 in a caspase-activating structure known as the apoptosome. In this review, we describe knockout and knockin studies of apoptosome components, elegant structural and biochemical experiments, and analyses of the apoptosome in various cancers and other disease states, all of which have provided new insight into this critical locus of apoptotic control.

Autophagic stress in neuronal injury and disease

Autophagy is the regulated process by which cytoplasmic organelles and long-lived proteins are delivered for lysosomal degradation. Increased numbers of autophagosomes and autolysosomes often represent prominent ultrastructural features of degenerating or dying neurons. This morphology is characteristic not only of neurons undergoing pathologic degeneration, but also during developmental programmed cell death of some neuronal populations. In recent years, a growing number of reports highlight potentially important roles for autophagy-related processes in relation to protein aggregation, regulated cell death pathways, and neurodegeneration. While starvation-induced autophagy involves nonselective bulk degradation of cytoplasm, mechanisms that regulate selective targeting of damaged organelles form an emerging area. As the study of autophagy evolves from physiologic homeostasis to pathologic situations, consideration of terminology and definitions becomes important. Increased autophagic vacuoles do not necessarily correlate with increased autophagic activity or flux. Instead, the striking accumulation of autophagic vacuoles in dying or degenerating neurons likely reflects an imbalance between the rates of autophagic sequestration and completion of the degradative process. In other words, these cells can be thought of as undergoing "autophagic stress." The concept of autophagic stress may reconcile apparently conflicting roles of autophagy-related processes in adaptive, homeostatic responses and in pathways of neurodegeneration and cell death.

Relative timing of redistribution of cytochrome c and Smac/DIABLO from mitochondria during apoptosis assessed by double immunocytochemistry on mammalian cells

Redistribution of cytochrome c and Smac/DIABLO from mitochondria occurs during apoptosis, although the relative timing of their release is not well characterized. Double immunocytochemistry was utilized here to study quantitatively the patterns of release of cytochrome c and Smac/DIABLO from mitochondria in single cells. Human osteosarcoma cells and murine embryonic cortical neurons were analyzed during apoptosis induced by staurosporine. In osteosarcoma cells treated with staurosporine for 24 h, a substantial proportion of cells (36%) released cytochrome c from the mitochondria before Smac/DIABLO. In contrast, these proteins were released mostly concordantly in neurons; only a minority of cells (< or = 15%) released cytochrome c without Smac/DIABLO (or vice versa) from mitochondria. Patterns of release in either cell type were unaltered by addition of the caspase inhibitor, zVAD-fmk. The double immunocytochemistry procedure facilitated clear definition of the temporal release of cytochrome c and Smac/DIABLO from mitochondria in intact apoptotic cells, enabling us to demonstrate for the first time that their mutual redistribution during apoptosis varies between different cell types.

Enhanced Sensitivity to Cytochrome c–Induced Apoptosis Mediated by PHAPI in Breast Cancer Cells

Apoptotic signaling defects both promote tumorigenesis and confound chemotherapy. Typically, chemotherapeutics stimulate cytochrome c release to the cytoplasm, thereby activating the apoptosome. Although cancer cells can be refractory to cytochrome c release, many malignant cells also exhibit defects in cytochrome c-induced apoptosome activation, further promoting chemotherapeutic resistance. We have found that breast cancer cells display an unusual sensitivity to cytochrome c-induced apoptosis when compared with their normal counterparts. This sensitivity, not observed in other cancers, resulted from enhanced recruitment of caspase-9 to the Apaf-1 caspase recruitment domain. Augmented caspase activation was mediated by PHAPI, which is overexpressed in breast cancers. Furthermore, cytochrome c microinjection into mammary epithelial cells preferentially killed malignant cells, suggesting that this phenomenon might be exploited for chemotherapeutic purposes.

Targeting endogenous inhibitors of apoptosis for treatment of cancer, stroke and multiple sclerosis

The inhibitor of apoptosis (IAP) genes have emerged as probably the most important intrinsic regulators of apoptosis. The members of the IAP family are highly conserved in evolutionarily distant species and perform the critical role of binding to and inhibiting distinct caspases. This inhibition is mediated by discrete baculoviral IAP repeat domains that, in a domain-specific manner, inhibit either the initiator or executioner caspases. As such the function of IAPs lies at the very centre of virtually all apoptotic pathways. Since many, if not most, human pathologies involve aberrant apoptosis, the modulation of IAP levels or their activity offers huge therapeutic potential for treatment of various disorders. Indeed, available data suggest that the therapeutic downregulation of IAPs by antisense targeting or their adenovirally-mediated overexpression, can in fact be used to successfully modulate cell death.

Reduced Apaf-1 levels in cardiomyocytes engage strict regulation of apoptosis by endogenous XIAP

Overexpression studies have identified X-linked inhibitor of apoptosis protein (XIAP) as a potent inhibitor of caspases. However, the exact function of endogenous XIAP in regulating mammalian apoptosis is less clear. Endogenous XIAP strictly regulates cytochrome c-dependent caspase activation in sympathetic neurons but not in many mitotic cells. We report that postmitotic cardiomyocytes, unlike fibroblasts, are remarkably resistant to cytosolic microinjection of cytochrome c. The cardiomyocyte resistance to cytochrome c is mediated by endogenous XIAP, as XIAP-deficient cardiomyocytes die rapidly with cytosolic cytochrome c alone. Importantly, we found that cardiomyocytes, like neurons, have markedly reduced Apaf-1 levels and that this decrease in Apaf-1 is directly linked to the tight regulation of caspase activation by XIAP. These data identify an important function of XIAP in cardiomyocytes and point to a striking similarity in the regulation of apoptosis in postmitotic cells.

Reawakening the cellular death program in neoplasia through the therapeutic blockade of IAP function

Recent studies have shown that members of the inhibitor of apoptosis (IAP) protein family are highly expressed in several classes of cancer. The primary implication of these findings is that the elevated expression of IAPs is not coincidental but actually participates in oncogenesis by helping to allow the malignant cell to avoid apoptotic cell death. This concept, together with the discovery of several IAP-regulatory proteins that use a conserved mode of action, has stimulated a major effort by many research groups to devise IAP-targeting strategies as a means of developing novel antineoplastic drugs. In this Review, we consider the evidence both for and against the IAPs being valid therapeutic targets, and we describe the types of strategies being used to neutralize their functions.

Apoptosis in Drosophila: neither fish nor fowl (nor man, nor worm)

Studies in a wide variety of organisms have produced a general model for the induction of apoptosis in which multiple signaling pathways lead ultimately to activation of the caspase family of proteases. Once activated, these enzymes cleave key cellular substrates to promote the orderly dismantling of dying cells. A broad similarity exists in the cell death pathways operating in different organisms and there is a clear evolutionary conservation of apoptotic regulators such as caspases, Bcl-2 family members, inhibitor of apoptosis (IAP) proteins, IAP antagonists and caspase activators. Despite this, studies in Caenorhabditis elegans, Drosophila and vertebrates have revealed some apparent differences both in the way apoptosis is regulated and in the way individual molecules contribute to the propagation of the death signal. For example, whereas cytochrome c released from mitochondria clearly promotes caspase activation in vertebrates, there is no documented role for cytochrome c in C. elegans apoptosis and its role in Drosophila is highly controversial. In addition, the apoptotic potency of IAP antagonists appears to be greater in Drosophila than in vertebrates, indicating that IAPs may be of different relative importance in different organisms. Thus, although Drosophila, worms and humans share a host of apoptotic regulators, the way in which they function may not be identical.

Apoptosis inducing factor mediates caspase-independent 1-methyl-4-phenylpyridinium toxicity in dopaminergic cells

Parkinson's disease is a debilitating neurodegenerative disease characterized by loss of midbrain dopaminergic neurons. These neurons are particularly sensitive to the neurotoxin 1-methyl-4-phenylpyridinium (MPP+), the active metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which causes parkinsonian syndromes in humans, monkeys and rodents. Although apoptotic cell death has been implicated in MPTP/MPP+ toxicity, several recent studies have challenged the role of caspase-dependent apoptosis in dopaminergic neurons. Using the midbrain-derived MN9D dopaminergic cell line, we found that MPP+ treatment resulted in an active form of cell death that could not be prevented by caspase inhibitors or over-expression of a dominant negative inhibitor of apoptotic protease activating factor 1/caspase-9. Apoptosis inducing factor (AIF) is a mitochondrial protein that may mediate caspase-independent forms of regulated cell death following its translocation to the nucleus. We found that MPP+ treatment elicited nuclear translocation of AIF accompanied by large-scale DNA fragmentation. To establish the role of AIF in MPP+ toxicity, we constructed a DNA vector encoding a short hairpin sequence targeted against AIF. Reduction of AIF expression by RNA interference inhibited large-scale DNA fragmentation and conferred significant protection against MPP+ toxicity. Studies of primary mouse midbrain cultures further supported a role for AIF in caspase-independent cell death in MPP+-treated dopaminergic neurons.

Apoptosis is secondary to non-apoptotic axonal degeneration in neurons exposed to Abeta in distal axons

The goal of this study was to assess if neurons exposed to amyloid-beta peptide (Abeta) exclusively in distal axons, undergo apoptosis. This is relevant to the loss of cholinergic neurons in Alzheimer's disease. Using a three-compartmented culture system for rat sympathetic neurons, we demonstrate that exposure of axons to Abeta1-42 activates an independent destruction program in axons, which leads to nuclear apoptosis. Abeta-induced axonal degeneration does not involve local caspase activation, but causes caspase activation in cell bodies. Accordingly, inhibition of caspase activation blocks Abeta-induced apoptosis but not axonal degeneration. In agreement with previous suggestions that disruption of nerve growth factor (NGF)-mediated signaling might contribute to the loss of cholinergic neurons, we found that provision of NGF to cell bodies protects sympathetic neurons from Abeta-induced apoptosis. However, our data indicate that Abeta-induced axonal degeneration follows a mechanism different than that activated by NGF withdrawal. Only Abeta-induced axonal degeneration is prevented by the calpain inhibitor calpastatin and is insensitive to the inhibitor of the ubiquitin-proteasome system MG132. Importantly, inhibition of Abeta-induced axonal degeneration by calpastatin prevents nuclear apoptosis.