Alternating metabolic pathways in NGF-deprived sympathetic neurons affect caspase-independent death

Oxidative phosphorylation mediated pathogenesis of Parkinson's disease and its implication via Akt signaling

Substantia Nigra Pars-compacta (SNpc), in the basal ganglion region, is a primary source of dopamine release. These dopaminergic neurons require more energy than other neurons, as they are highly arborized and redundant. Neurons meet most of their energy demand (∼90%) from mitochondria. Oxidative phosphorylation (OxPhos) is the primary pathway for energy production. Many genes involved in Parkinson's disease (PD) have been associated with OxPhos, especially complex I. Abrogation in complex I leads to reduced ATP formation in these neurons, succumbing to death by inducing apoptosis. This review discusses the interconnection between complex I-associated PD genes and specific mitochondrial metabolic factors (MMFs) of OxPhos. Interestingly, all the complex I-associated PD genes discussed here have been linked to the Akt signaling pathway; thus, neuron survival is promoted and smooth mitochondrial function is ensured. Any changes in these genes disrupt the Akt pathway, which hampers the opening of the permeability transition pore (PTP) via GSK3β dephosphorylation; promotes destabilization of OxPhos; and triggers the release of pro-apoptotic factors.

NGF-Dependent and BDNF-Dependent DRG Sensory Neurons Deploy Distinct Degenerative Signaling Mechanisms

The nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are trophic factors required by distinct population of sensory neurons during development of the nervous system. Neurons that fail to receive appropriate trophic support are lost during this period of naturally occurring cell death. In the last decade, our understanding of the signaling pathways regulating neuronal death following NGF deprivation has advanced substantially. However, the signaling mechanisms promoting BDNF deprivation-induced sensory neuron degeneration are largely unknown. Using a well-established in vitro culture model of dorsal root ganglion (DRG), we have examined degeneration mechanisms triggered on BDNF withdrawal in sensory neurons. Our results indicate differences and similarities between the molecular signaling pathways behind NGF and BDNF deprivation-induced death. For instance, we observed that the inhibition of Trk receptors (K252a), PKC (Gö6976), protein translation (cycloheximide; CHX), or caspases (zVAD-fmk) provides protection from NGF deprivation-induced death but not from degeneration evoked by BDNF-withdrawal. Interestingly, degeneration of BDNF-dependent sensory neurons requires BAX and appears to rely on reactive oxygen species (ROS) generation rather than caspases to induce degeneration. These results highlight the complexity and divergence of mechanisms regulating developmental sensory neuron death.

Topoisomerase IIα is essential for maintenance of mitotic chromosome structure

Topoisomerase IIα (TOP2A) is a core component of mitotic chromosomes and important for establishing mitotic chromosome condensation. The primary roles of TOP2A in mitosis have been difficult to decipher due to its multiple functions across the cell cycle. To more precisely understand the role of TOP2A in mitosis, we used the auxin-inducible degron (AID) system to rapidly degrade the protein at different stages of the human cell cycle. Removal of TOP2A prior to mitosis does not affect prophase timing or the initiation of chromosome condensation. Instead, it prevents chromatin condensation in prometaphase, extends the length of prometaphase, and ultimately causes cells to exit mitosis without chromosome segregation occurring. Surprisingly, we find that removal of TOP2A from cells arrested in prometaphase or metaphase cause dramatic loss of compacted mitotic chromosome structure and conclude that TOP2A is crucial for maintenance of mitotic chromosomes. Treatments with drugs used to poison/inhibit TOP2A function, such as etoposide and ICRF-193, do not phenocopy the effects on chromosome structure of TOP2A degradation by AID. Our data point to a role for TOP2A as a structural chromosome maintenance enzyme locking in condensation states once sufficient compaction is achieved.

Apoptotic cell death regulation in neurons

Apoptosis plays a major role in shaping the developing nervous system during embryogenesis as neuronal precursors differentiate to become post-mitotic neurons. However, once neurons are incorporated into functional circuits and become mature, they greatly restrict their capacity to die via apoptosis, thus allowing the mature nervous system to persist in a healthy and functional state throughout life. This robust restriction of the apoptotic pathway during neuronal differentiation and maturation is defined by multiple unique mechanisms that function to more precisely control and restrict the intrinsic apoptotic pathway. However, while these mechanisms are necessary for neuronal survival, mature neurons are still capable of activating the apoptotic pathway in certain pathological contexts. In this review, we highlight key mechanisms governing the survival of post-mitotic neurons, while also detailing the physiological and pathological contexts in which neurons are capable of overcoming this high apoptotic threshold.

Phosphatidylserine is a marker for axonal debris engulfment but its exposure can be decoupled from degeneration

Apoptotic cells expose Phosphatidylserine (PS), that serves as an “eat me” signal for engulfing cells. Previous studies have shown that PS also marks degenerating axonsduring developmental pruning or in response to insults (Wallerian degeneration), but the pathways that control PS exposure on degenerating axons are largely unknown. Here, we used a series of in vitro assays to systematically explore the regulation of PS exposure during axonal degeneration. Our results show that PS exposure is regulated by the upstream activators of axonal pruning and Wallerian degeneration. However, our investigation of signaling further downstream revealed divergence between axon degeneration and PS exposure. Importantly, elevation of the axonal energetic status hindered PS exposure, while inhibition of mitochondrial activity caused PS exposure, without degeneration. Overall, our results suggest that the levels of PS on the outer axonal membrane can be dissociated from the degeneration process and that the axonal energetic status plays a key role in the regulation of PS exposure.

Bax and caspases regulate increased production of mitochondria-derived reactive species in neuronal apoptosis: LACK of A role for depletion of cytochrome c from the mitochondrial electron transport chain

A Bax-dependent increase of reactive oxygen species (ROS) and other reactive species (RS) occurs after withdrawing NGF from mouse sympathetic neurons in cell culture. Possible mechanisms underlying the increased ROS/RS are leakage of electrons from the mitochondrial electron transport chain secondary to caspase cleavage of respiratory complexes or leakage secondary to depletion of cytochrome c from the chain. We previously demonstrated that deletion of Bax or caspase 3 from these cells reduces ROS/RS production to near baseline levels indicating a central role for both Bax and caspase 3 in generating the ROS/RS. Here we depleted cytochrome c to a similar level in neurons from wild type and bax hemizygous or knockout mice by NGF withdrawal or treatment with H₂O₂. Death was prevented with a caspase inhibitor that caused a partial reduction of ROS/RS levels but did not completely prevent the ROS/RS increase. ROS/RS was highest in bax wild-type cells, lowest in bax knockout cells, and at an intermediate level in the bax hemizygous cells. These and our previous findings indicate that Bax and caspase 3 are necessary for the increased ROS/RS after withdrawing NGF from these cells and that little or none of the increased ROS/RS are secondary to a depletion of cytochrome c from the electron transport chain.

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Bax lies upstream of all increased production of ROS by mitochondria in NGF-deprived sympathetic neurons.

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The broad-spectrum caspase inhibitor BAF blocks some but not all increased ROS after NGF withdrawal.

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Caspase 3 deletion blocks almost all of the increased ROS after NGF withdrawal suggesting that BAF does not block all caspase activity or that it has non-specific pro-oxidant effects.

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Depletion of cytochrome c from the electron transport chain contributes little or nothing to increased ROS after NGF withdrawal.

HSP70 mediates survival in apoptotic cells-Boolean network prediction and experimental validation

Neuronal stress or injury results in the activation of proteins, which regulate the balance between survival and apoptosis. However, the complex mechanism of cell signaling involving cell death and survival, activated in response to cellular stress is not yet completely understood. To bring more clarity about these mechanisms, a Boolean network was constructed that represented the apoptotic pathway in neuronal cells. FasL and neurotrophic growth factor (NGF) were considered as inputs in the absence and presence of heat shock proteins known to shift the balance toward survival by rescuing pro-apoptotic cells. The probabilities of survival, DNA repair and apoptosis as cellular fates, in the presence of either the growth factor or FasL, revealed a survival bias encoded in the network. Boolean predictions tested by measuring the mRNA level of caspase-3, caspase-8, and BAX in neuronal Neuro2a (N2a) cell line with NGF and FasL as external input, showed positive correlation with the observed experimental results for survival and apoptotic states. It was observed that HSP70 contributed more toward rescuing cells from apoptosis in comparison to HSP27, HSP40, and HSP90. Overexpression of HSP70 in N2a transfected cells showed reversal of cellular fate from FasL-induced apoptosis to survival. Further, the pro-survival role of the proteins BCL2, IAP, cFLIP, and NFκB determined by vertex perturbation analysis was experimentally validated through protein inhibition experiments using EM20-25, Embelin and Wedelolactone, which resulted in 1.27-, 1.26-, and 1.46-fold increase in apoptosis of N2a cells. The existence of a one-to-one correspondence between cellular fates and attractor states shows that Boolean networks may be employed with confidence in qualitative analytical studies of biological networks.

Mitochondria-derived reactive oxygen species mediate caspase-dependent and -independent neuronal deaths

Mitochondrial dysfunction and oxidative stress are implicated in many neurodegenerative diseases. Mitochondria-targeted drugs that effectively decrease oxidative stress, protect mitochondrial energetics, and prevent neuronal loss may therefore lend therapeutic benefit to these currently incurable diseases. To investigate the efficacy of such drugs, we examined the effects of mitochondria-targeted antioxidants MitoQ10 and MitoE2 on neuronal death induced by neurotrophin deficiency. Our results indicate that MitoQ10 blocked apoptosis by preventing increased mitochondria-derived reactive oxygen species (ROS) and subsequent cytochrome c release, caspase activation, and mitochondrial damage in nerve growth factor (NGF)-deprived sympathetic neurons, while MitoE2 was largely ineffective. In this paradigm, the most proximal point of divergence was the ability of MitoQ10 to scavenge mitochondrial superoxide (O2(-)). MitoQ10 also prevented caspase-independent neuronal death in these cells demonstrating that the mitochondrial redox state significantly influences both apoptotic and nonapoptotic pathways leading to neuronal death. We suggest that mitochondria-targeted antioxidants may provide tools for delineating the role and significance of mitochondrial ROS in neuronal death and provide a new therapeutic approach for neurodegenerative conditions involving trophic factor deficits and multiple modes of cell death.

The E3 ligase PARC mediates the degradation of cytosolic cytochrome c to promote survival in neurons and cancer cells

The ability to withstand mitochondrial damage is especially critical for the survival of postmitotic cells, such as neurons. Likewise, cancer cells can also survive mitochondrial stress. We found that cytochrome c (Cyt c), which induces apoptosis upon its release from damaged mitochondria, is targeted for proteasome-mediated degradation in mouse neurons, cardiomyocytes, and myotubes and in human glioma and neuroblastoma cells, but not in proliferating human fibroblasts. In mouse neurons, apoptotic protease-activating factor 1 (Apaf-1) prevented the proteasome-dependent degradation of Cyt c in response to induced mitochondrial stress. An RNA interference screen in U-87 MG glioma cells identified p53-associated Parkin-like cytoplasmic protein (PARC, also known as CUL9) as an E3 ligase that targets Cyt c for degradation. The abundance of PARC positively correlated with differentiation in mouse neurons, and overexpression of PARC reduced the abundance of mitochondrially-released cytosolic Cyt c in various cancer cell lines and in mouse embryonic fibroblasts. Conversely, neurons from Parc-deficient mice had increased sensitivity to mitochondrial damage, and neuroblastoma or glioma cells in which PARC or ubiquitin was knocked down had increased abundance of mitochondrially-released cytosolic Cyt c and decreased viability in response to stress. These findings suggest that PARC-mediated ubiquitination and degradation of Cyt c is a strategy engaged by both neurons and cancer cells to prevent apoptosis during conditions of mitochondrial stress.

Fetal alcohol spectrum disorder: pathogenesis and mechanisms

This chapter provides an overview of animal model-based studies that have generated information critical to our understanding of the pathogenesis and mechanisms underlying alcohol-induced birth defects, in particular those involving the brain. Focus is placed on the developing organism itself, rather than the mother, placenta, or other extraembryonic tissues. Components of the cascades of alcohol-induced damage that are considered herein are excessive cell death, changes in the cell cycle and proliferation, cell migration, cell morphogenesis, and gene expression as well as free radical damage and interference with cell signaling. The roles played by one or more of these various factors in the genesis of structural and functional birth defects are dependent upon alcohol exposure patterns and dosage, the involved tissue, and the prenatal stage(s) at the time of exposure. Technologic advances and rapidly increasing knowledge in the fields of genetics, cell, developmental, and neurobiology are critical to accurately piecing together experimental evidence in refining our understanding of the genesis of alcohol-induced birth defects, to the planning and execution of future studies, and to applying the knowledge gained to diminish the severity or occurrence of fetal alcohol spectrum disorder.

Nmnat delays axonal degeneration caused by mitochondrial and oxidative stress

Axonal degeneration is a prominent feature of many neurological disorders that are associated with mitochondrial dysfunction, including Parkinson's disease, motor neuron disease, and inherited peripheral neuropathies. Studies of the Wld(s) mutant mouse, which undergoes delayed Wallerian degeneration in response to axonal injury, suggest that axonal degeneration is an active process. Wld(s) mice also have slower axonal degeneration and disease progression in numerous models of neurodegenerative disease. The Wld(s) mutation results in the production of a chimeric protein that contains the full-length coding sequence of nicotinamide mononucleotide adenylyltransferase 1 (Nmnat1), which alone is sufficient for axonal protection in vitro. To test the effects of increased Nmnat expression on axonal degeneration induced by mitochondrial dysfunction, we examined dorsal root ganglion (DRG) neurons treated with rotenone. Rotenone induced profound axonal degeneration in DRG neurons; however, this degeneration was delayed by expression of Nmnat. Nmnat-mediated protection resulted in decreased axonal accumulation and sensitivity to reactive oxygen species (ROS) but did not affect the change in the rate of rotenone-induced loss in neuronal ATP. Nmnat also prevented axonal degeneration caused by exposure to exogenous oxidants and reduced the level of axonal ROS after treatment with vincristine, further supporting the idea that Nmnat promotes axonal protection by mitigating the effects of ROS.

Reactive oxygen species induced by proteasome inhibition in neuronal cells mediate mitochondrial dysfunction and a caspase-independent cell death

While increasing evidence shows that proteasome inhibition triggers oxidative damage, mitochondrial dysfunction and death in neuronal cells, the regulatory relationship among these events is unclear. Using mouse neuronal cells we show that the cytotoxicity induced by mild (0.25 microM) and potent (5.0 microM) doses of the proteasome inhibitor, N-Benzyloxycarbonyl-Ile-Glu (O-t-butyl)-Ala-leucinal, (PSI) involved a dose-dependent increase in caspase activation, overproduction of reactive oxygen species (ROS) and a mitochondrial dysfunction manifested by the translocation of the proapoptotic protein, Bax, from the cytoplasm to the mitochondria, membrane depolarization and the release of cytochrome c and the apoptosis inducing factor (AIF) from mitochondria to the cytoplasm and nucleus, respectively. Whereas caspase or Bax inhibition failed to prevent mitochondrial membrane depolarization and neuronal cell death, pretreatments with the antioxidant N-acetyl-L-cysteine (NAC) or overexpression of the antiapoptotic protein Bcl-xL abrogated these events in cells exposed to mild levels of PSI. These findings implicated ROS as a mediator of PSI-induced cytotoxicity. However, depletions in glutathione and Bcl-xL with potent proteasome inhibition exacerbated this response whereupon survival required the cooperative protection of NAC with Bcl-xL overexpression. Collectively, ROS induced by proteasome inhibition mediates a mitochondrial dysfunction in neuronal cells that culminates in death through caspase- and Bax-independent mechanisms.

Axon & dendrite degeneration: its mechanisms and protective experimental paradigms

Accumulating evidence suggests that axon and dendrite (or neurite) degeneration both in vivo and in vitro requires self-destructive programs independent of cell death programs to segregate neurite degeneration from cell soma demise. This review will deal with the mechanisms of neurite degeneration caused by several experimental paradigms including trophic factor deprivation and Wallerian degeneration as well as those under pathological conditions. The involvement of autophagy and mitochondrial dysfunction is emphasized in these mechanisms. The mechanisms through which protective agents including the Wld(s) protein rescue neurites from degeneration or fail to do so will be discussed.

Mitochondrial membrane permeabilization in cell death

Irrespective of the morphological features of end-stage cell death (that may be apoptotic, necrotic, autophagic, or mitotic), mitochondrial membrane permeabilization (MMP) is frequently the decisive event that delimits the frontier between survival and death. Thus mitochondrial membranes constitute the battleground on which opposing signals combat to seal the cell's fate. Local players that determine the propensity to MMP include the pro- and antiapoptotic members of the Bcl-2 family, proteins from the mitochondrialpermeability transition pore complex, as well as a plethora of interacting partners including mitochondrial lipids. Intermediate metabolites, redox processes, sphingolipids, ion gradients, transcription factors, as well as kinases and phosphatases link lethal and vital signals emanating from distinct subcellular compartments to mitochondria. Thus mitochondria integrate a variety of proapoptotic signals. Once MMP has been induced, it causes the release of catabolic hydrolases and activators of such enzymes (including those of caspases) from mitochondria. These catabolic enzymes as well as the cessation of the bioenergetic and redox functions of mitochondria finally lead to cell death, meaning that mitochondria coordinate the late stage of cellular demise. Pathological cell death induced by ischemia/reperfusion, intoxication with xenobiotics, neurodegenerative diseases, or viral infection also relies on MMP as a critical event. The inhibition of MMP constitutes an important strategy for the pharmaceutical prevention of unwarranted cell death. Conversely, induction of MMP in tumor cells constitutes the goal of anticancer chemotherapy.

Bax affects production of reactive oxygen by the mitochondria of non-apoptotic neurons

Depriving sympathetic neurons in cell culture of nerve growth factor (NGF) causes their apoptotic death. Bax-induced release of cytochrome c from mitochondria and the subsequent activation of cytosolic caspases are central to this death. A Bax-dependent increase of mitochondrial-derived reactive oxygen species (ROS) that is an important component of the apoptotic cascade in these cells begins soon after NGF withdrawal. Here we report that Bax can also influence mitochondrial production of ROS in non-apoptotic sympathetic neurons. We determined ROS levels by using confocal microscopy to monitor changes in the fluorescence intensity of a redox-sensitive dye loaded into single cells. ROS levels were similar in NGF-replete bax wild-type neurons and neurons from which bax had been deleted. To enhance any effects that Bax might have on ROS levels in NGF-replete cells we exposed cultures to the ATP synthase inhibitor, oligomycin. This treatment hyperpolarizes mitochondrial membrane potential (DeltaPsi(m)), an event that can favor increased ROS production. NGF-replete neurons from mice in which bax had been deleted had much higher levels of mitochondrial-derived ROS when treated with oligomycin than did bax wild-type cells. Oligomycin treatment also caused greater hyperpolarization of DeltaPsi(m) in bax-deleted cells than in wild-type cells. These findings indicate that Bax can affect mitochondrial ROS production in non-apoptotic neurons and may do so by altering DeltaPsi(m).

Neuronal life-and-death signaling, apoptosis, and neurodegenerative disorders

When subjected to excessive oxidative stress, neurons may respond adaptively to overcome the stress, or they may activate a programmed cell death pathway called apoptosis. Apoptosis is characterized by alterations in mitochondria and the endoplasmic reticulum and activation of cysteine proteases called caspases. Increasing evidence suggests that apoptotic biochemical cascades are involved in the dysfunction and death of neurons in neurodegenerative disorders such as Alzheimer's, Parkinson, and Huntington's diseases. Studies of normal aging, of genetic mutations that cause disease, and of environmental factors that affect disease risk are revealing cellular and molecular alterations that may cause excessive oxidative stress and trigger neuronal apoptosis. Accumulation of self-aggregating proteins such as amyloid beta-peptide, tau, alpha-synuclein, and huntingtin may be involved in apoptosis both upstream and downstream of oxidative stress. Membrane-associated oxidative stress resulting in perturbed lipid metabolism and disruption of cellular calcium homeostasis may trigger apoptosis in several different neurodegenerative disorders. Counteracting neurodegenerative processes are an array of mechanisms including neurotrophic factor signaling, antioxidant enzymes, protein chaperones, antiapoptotic proteins, and ionostatic systems. Emerging findings suggest that the resistance of neurons to death during aging can be enhanced by modifications of diet and lifestyle.

Purkinje cell death: differences between developmental cell death and neurodegenerative death in mutant mice

This review is devoted to Purkinje cell death occurring during development and in spontaneous cerebellar mutations of the mouse. We first present evidence in favor of an apoptotic developmental Purkinje cell death. Then, the different types of Purkinje cell degeneration occurring in mutant mice primarily affecting this neuronal population (nervous, purkinje cell degeneration, Lurcher, toppler, and woozy) are described and discussed. In addition, we show, by reporting new data, that cell death in tambaleante mutant mice can be related to autophagy. Last, we discuss the fact that the cell death pathways in mutant mice are more complex than the three types of developmental death generally described (apoptosis, autophagy, necrosis), since they share often characteristics of more than one type of these developmental cell deaths, particularly autophagy and apoptosis.

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 alpha-synuclein mutation E46K promotes aggregation in cultured cells

Parkinson's disease (PD) is characterized by the polymerization of wild-type (WT) or mutant alpha-synuclein (AS) into aggregates and fibrils, which are observed as Lewy bodies (LBs) and Lewy neurites (LNs) in PD patients. However, inability to demonstrate aggregation in many cell culture systems is a major drawback for effective in vitro modeling of AS aggregation. Utilizing PCR-based cloning approach, we generated A30P, A53T, and the recently reported E46K encoding mutation in the KTKEGV repeat region of AS gene. While cloning E46K mutant, a glycine deletion mutation (E46KDeltaG) adjacent to the intended lysine mutation was serendipitously generated. Expression of mutant constructs and green fluorescent protein (GFP)-tagged mutant constructs in catecholaminergic SH-SY5Y (5Y) cells revealed 40% of AS-E46KDeltaG and 18% of AS-E46K transfected cells formed aggregates as compared to 12% in AS-A53T, 6% in AS-WT, and 2% in AS-A30P transfected cells. Western blot analysis demonstrated the formation of high molecular weight AS aggregates. Electron microscopic analysis of 5Y cells expressing the E46K and E46KDeltaG mutants demonstrated two distinct kinds of inclusions: Type I, which showed dense granular profile; and Type II, which were largely membranous vacuolar inclusions without granular material. These two inclusions are reminiscent of Lewy bodies and pale bodies observed in PD postmortem brain samples. Our results demonstrate that mutations in 4th KTKEGV repeat lead to higher propensity of aggregation of AS compared to other mutants.

Fluorescence based oxygen uptake analysis in the study of metabolic responses to apoptosis induction

Mitochondrial activity has been shown to be centrally involved in the progression of apoptosis. The electron transport chain is a major player in this process and oxygen uptake analysis provides detailed information on its activity. Here we examined the ability of a fluorescence based oxygen uptake assay to inform on cellular responses to apoptosis induction. HL60 cells treated with camptothecin and UV light were used as a model and the ability of the assay to detect dose and time dependent decreases in respiratory activity analysed. The data obtained were compared to more specific markers of apoptosis including annexin V binding, and caspase-3 activity. Reductions in oxygen uptake rates were seen at lower doses than increases in annexin V binding or mitochondrial membrane potential depolarisation. These reductions were observed earlier than detectable caspase-3 activity and were unaffected by pre-treatment with the caspase-3 inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoro-methylketone (zVADfmk).

The apoptosis/autophagy paradox: autophagic vacuolization before apoptotic death

Autophagic cell death is morphologically characterized by an accumulation of autophagic vacuoles. Here, we show that inactivation of LAMP2 by RNA interference or by homologous recombination leads to autophagic vacuolization in nutrient-depleted cells. Cells that lack LAMP2 expression showed an enhanced accumulation of vacuoles carrying the marker LC3, yet a decreased colocalization of LC3 and lysosomes, suggesting that the fusion between autophagic vacuoles and lysosomes was inhibited. While a fraction of mitochondria from starved LAMP2-expressing cells colocalized with lysosomal markers, within autophagolysosomes, no such colocalization was found on removal of LAMP2 from the experimental system. Of note, LAMP1 depletion had no such effects and did not aggravate the phenotype induced by LAMP2-specific small interfering RNA. Serum and amino acid-starved LAMP2-negative cells exhibited an accumulation of autophagic vacuoles and then succumbed to cell death with hallmarks of apoptosis such as loss of the mitochondrial transmembrane potential, caspase activation and chromatin condensation. While caspase inhibition retarded cell death, it had no protective effect on mitochondria. Stabilization of mitochondria by overexpression of Bcl-2 or the mitochondrion-targeted cytomegalovirus protein vMIA, however, blocked all signs of apoptosis. Neither caspase inhibition nor mitochondrial stabilization antagonized autophagic vacuolization in LAMP2-deficient cells. Altogether, these data indicate that accumulation of autophagic vacuoles can precede apoptotic cell death. These findings argue against the clear-cut distinction between type 1 (apoptotic) and type 2 (autophagic) cell death.

Role of caspase-3 in ethanol-induced developmental neurodegeneration

Acute, transient exposure to ethanol causes a widespread pattern of caspase-3 activation and neuroapoptosis in the developing rodent brain. To determine whether caspase-3 activation is an essential step in ethanol-induced developmental neuroapoptosis, we treated homozygous caspase-3 knockout mice or wild-type mice on postnatal day 7 with an apoptosis-inducing dose of ethanol and examined the brains at appropriate survival times for evidence of apoptotic neurodegeneration. In caspase-3 knockout mice, the cell death process evolved more slowly than in wild-type mice, and morphological changes observed were not those typically associated with apoptosis. However, neuronal cell counts performed 2 weeks post-treatment revealed that the extent of neuron loss was similar in wild-type and caspase-3-deficient mice. We conclude that absence of functional caspase-3 alters the time course and morphological characteristics of the neurodegenerative process but does not prevent ethanol-induced neuron death.

Glucose metabolism and Alzheimer's disease

The brain is the organ with the highest basal rate of glucose consumption. Most of the energy generated by the oxidation of glucose is used for the work necessary to maintain the ionic balances associated with synaptic transmission. When the nervous system is subjected to the oxidative stress of age-associated disease, there is a redistribution of glucose breakdown to pathways that more efficiently produce molecules involved in antioxidant metabolism. This shift is at least in part mediated by the transcription factor HIF-1. The clinical implications of this change in glucose metabolism are discussed in the context of aging and Alzheimer's disease.

Caspase-dependent and -independent neuronal death: two distinct pathways to neuronal injury

Caspases are cysteine proteases that mediate apoptotic death in a variety of cellular systems, including neurons. Caspases are activated through extrinsic or intrinsic pathways. The latter is used by most neurons in most situations. In this pathway, release of mitochondrial cytochrome c into the cytoplasm induces formation of the apoptosome, which leads to the activation of caspase 9 and subsequently other caspases. Recent data demonstrate that when caspase activation is inhibited at or downstream of the apoptosome, neurons undergo a delayed, caspase-independent death. Furthermore, there are instances, most notably following excitotoxic injury and calcium overload, in which the direct cell death pathway elicited differs from classical apoptosis. The molecular and biochemical features of such caspase-independent, nonapoptotic forms of neuronal death are just beginning to be elucidated, but alterations at the level of the mitochondria and noncaspase proteases play significant roles. Mitochondrial alterations in caspase-independent death may include energy depletion, generation of free radicals, opening of the permeability transition pore, and release of cytotoxic proteins, such as apoptosis-inducing factor. The particular mechanisms employed can be context dependent. In disease states, in which a combination of apoptotic and nonapoptotic death occurs, therapeutic strategies need to take into account both caspase-dependent and -independent pathways.

Apoptosome inactivation rescues proneural and neural cells from neurodegeneration

Deficiency of the apoptosome component Apaf1 leads to accumulation of supernumerary brain cells in mouse embryos. We observed that neural precursor cells (NPCs) in Apaf1(-/-) embryos escape programmed cell death, proliferate and retain their potential to differentiate. To evaluate the circumstances of Apaf1(-/-) NPC survival and investigate their fate under neurodegenerative conditions, we established cell lines of embryonic origin (ETNA). We found that Apaf1(-/-) NPCs resist common apoptotic stimuli and neurodegenerative inducers such as amyloid-beta peptide (typical of Alzheimer's disease) and mutant G93A superoxide dismutase 1 (typical of familial amyotrophic lateral sclerosis). Similar results were obtained in Apaf1(-/-) primary cells. When death is prevented by Apaf1 deficiency, cytochrome c is released from mitochondria and rapidly degraded by the proteasome, but mitochondria remain intact. Under these conditions, neither activation by cleavage of initiator caspases nor release of alternative apoptotic inducers from mitochondria takes place. In addition, NPCs can still differentiate, as revealed by neurite outgrowth and expression of differentiation markers. Our findings imply that the mitochondrion/apoptosome pathway is the main route of proneural and neural cells to death and that its inhibition prevents them from dismantling in neurodegenerative conditions. Indeed, the ETNA cell model is ideally suited for exploring the potential of novel cell therapies for the treatment of human neurodegenerations.

Men are but worms: neuronal cell death in C elegans and vertebrates

Awarding the 2002 Nobel Prize in Physiology or Medicine to Sydney Brenner, H Robert Horvitz, and John E Sulston for 'their discoveries concerning the genetic regulation of organ development and programmed cell death (PCD)' highlights the significant contribution that the study of experimental organisms, such as the nematode Caenorhabditis elegans, has made to our understanding of human physiology and pathophysiology. Their studies of lineage determination in worms established the 'central dogma' of apoptosis: The BH3-only protein EGL-1 is induced in cells destined to die, interacts with the BCL-2-like inhibitor CED-9, displacing the adaptor CED-4, which then promotes activation of the caspase CED-3. The vast majority of cells undergoing PCD during development in C. elegans, as in vertebrates, are neurons. Accordingly, the genetic regulation of apoptosis is strikingly similar in nematode and vertebrate neurons. This review summarizes these similarities - and the important differences - in the molecular mechanisms responsible for neuronal PCD in C. elegans and vertebrates, and examines the implications that our understanding of physiological neuronal apoptosis may have for the diagnosis and treatment of acute and chronic human neurodegenerative disorders.

The pathophysiology of mitochondrial cell death

In the mitochondrial pathway of apoptosis, caspase activation is closely linked to mitochondrial outer membrane permeabilization (MOMP). Numerous pro-apoptotic signal-transducing molecules and pathological stimuli converge on mitochondria to induce MOMP. The local regulation and execution of MOMP involve proteins from the Bcl-2 family, mitochondrial lipids, proteins that regulate bioenergetic metabolite flux, and putative components of the permeability transition pore. MOMP is lethal because it results in the release of caspase-activating molecules and caspase-independent death effectors, metabolic failure in the mitochondria, or both. Drugs designed to suppress excessive MOMP may avoid pathological cell death, and the therapeutic induction of MOMP may restore apoptosis in cancer cells in which it is disabled. The general rules governing the pathophysiology of MOMP and controversial issues regarding its regulation are discussed.

Role of AIF in caspase-dependent and caspase-independent cell death

The major challenge in treating cancer is that many tumor cells carry mutations in key apoptotic genes such as p53, Bcl family proteins or those affecting caspase signaling. Such defects render treatment with traditional chemotherapeutic agents ineffective. Many studies have demonstrated the importance of caspase-independent cell death pathways in injury, degenerative diseases and tumor tissue. It is now recognized that in addition to their critical role in the production of cellular energy, mitochondria are also the source of key proapoptotic molecules involved in caspase activation. More recently, it has been discovered that in response to apoptotic stimuli, mitochondria can also release caspase-independent cell death effectors such as AIF and Endonuclease G. In this review, we examine the role of Bcl family proteins and poly(ADP-ribose) polymerase-1 signaling in the regulation of these apoptotic pathways and address the ongoing controversies in this field. Continued study of the mechanisms of apoptosis including caspase-independent death processes are likely to reveal novel therapeutic targets for the treatment of diverse human pathologies including cancer, neurodegenerative diseases and acute injuries such as stroke or myocardial infarction.

B-cell homeostasis: digital survival or analog growth?

Maintenance of B-lymphocyte homeostasis requires balanced cell production, death, and proliferation. To coordinate these processes, B cells are dependent on cell extrinsic signals. In lymphocyte development, precursor cells are dependent on Fms-like tyrosine kinase ligand 3 (Flt3L), and pre-B cells are dependent on the cytokine interleukin-7. Transitional B cells require B-lymphocyte stimulator (BLyS) for survival. Mature B cells require B-cell receptor (BCR) signals and also remain sensitive to their microenvironment. An emerging model suggests that extrinsic signals do not regulate B-cell survival through a digital mechanism where cells are simply instructed to survive or die. Instead, availability and competition for extrinsic signals regulates cellular physiology and metabolism in an analog fashion that then influences cell commitment to apoptosis or proliferation. Decreases in cellular metabolism may sensitize cells to activation and action of the pro-apoptotic Bcl-2 family members, Bak and Bax, and promote apoptosis. In contrast, increases in metabolism may predispose cells to proliferate. Analog control of cell physiology can, thus, be integrated with other inputs by individual cells to produce a fate decision for survival, proliferation, or apoptosis and prevent diseases of cell death, such as immunodeficiency, and cell activation and proliferation, such as autoimmunity or cancer.