The role of mitochondria in apoptosis*

The mitochondrial pathways of apoptosis

Apoptosis is a process of programmed cell death that serves as a major mechanism for the precise regulation of cell numbers, and as a defense mechanism to remove unwanted and potentially dangerous cells. Studies in nematode, Drosophila and mammals have shown that, although regulation of the cell death machinery is somehow different from one species to another, it is controlled by homologous proteins and involves mitochondria. In mammals, activation of caspases (cysteine proteases that are the main executioners of apoptosis) is under the tight control of the Bcl-2 family proteins, named in reference to the first discovered mammalian cell death regulator. These proteins mainly act by regulating the release of caspases activators from mitochondria. Although for a long time the absence of mitochondrial changes was considered as a hallmark of apoptosis, mitochondria appear today as the central executioner of apoptosis. In this chapter, we present the current view on the mitochondrial pathway of apoptosis with a particular attention to new aspects of the regulation of the Bcl-2 proteins family control of mitochondrial membrane permeabilization: the mechanisms implicated in their mitochondrial targeting and activation during apoptosis, the function(s) of the oncosuppressive protein p53 at the mitochondria and the role of the processes of mitochondrial fusion and fission.

Mitochondrial effects of plant-made compounds

SIGNIFICANCE

Plants produce many small molecules with biomedical potential. Their absorption from foods, metabolism, their effects on physiological and pathological processes, and the mechanisms of action are intensely investigated. Many are known to affect multiple cellular functions. Mitochondria are coming to be recognized as a major target for these compounds, especially redox-active ones, but the mechanisms involved still need clarification. At the same time, frontline research is uncovering the importance of processes involving these organelles for the cell and for an array of physiological and pathological processes. We review the major functions and possible dysfunctions of mitochondria, identify signaling pathways through which plant-derived molecules have an impact, and show how this may be relevant for major pathologies.

RECENT ADVANCES

Antioxidant, protective effects may arise as a reaction to a low-level pro-oxidant activity, largely taking place at mitochondria. Some plant-derived molecules can activate AMP-dependent kinase, with a consequent upregulation of mitochondrial biogenesis and a potential favorable impact on aging, pathologies like diabetes and neurodegeneration, and on ischemic damage.

CRITICAL ISSUES

The extrapolation of in vitro results and the verification of paradigms in vivo is a key issue for current research on both plant-derived compounds and mitochondria. The low bioavailability of many of these molecules poses a problem for both the study of their activities and their utilization.

FUTURE DIRECTIONS

The further clarification of the role of mitochondria in the activities of plant dietary compounds and their metabolites, mitochondrial targeting, the development of analogs and pro-drugs are all topics for promising research.

Mitochondrial disulfide relay: redox-regulated protein import into the intermembrane space

99% of all mitochondrial proteins are synthesized in the cytosol, from where they are imported into mitochondria. In contrast to matrix proteins, many proteins of the intermembrane space (IMS) lack presequences and are imported in an oxidation-driven reaction by the mitochondrial disulfide relay. Incoming polypeptides are recognized and oxidized by the IMS-located receptor Mia40. Reoxidation of Mia40 is facilitated by the sulfhydryl oxidase Erv1 and the respiratory chain. Although structurally unrelated, the mitochondrial disulfide relay functionally resembles the Dsb (disufide bond) system of the bacterial periplasm, the compartment from which the IMS was derived 2 billion years ago.

Glutaredoxin 2 knockout increases sensitivity to oxidative stress in mouse lens epithelial cells

Glutaredoxin belongs to the oxidoreductase family, with cytosolic glutaredoxin 1 (Grx1) and mitochondrial glutaredoxin 2 (Grx2) isoforms. Of the two isozymes, the function of Grx2 is not well understood. This paper describes the effects of Grx2 deletion on cellular function using primary lens epithelial cell cultures isolated from Grx2 gene knockout (KO) and wild-type (WT) mice. We found that both cell types showed similar growth patterns and morphology and comparable mitochondrial glutathione pool and complex I activity. Cells with deleted Grx2 did not show affected Grx1 or thioredoxin expression but exhibited high sensitivity to oxidative stress. Under treatment with H(2)O(2), the KO cells showed less viability, higher membrane leakage, enhanced ATP loss and complex I inactivation, and weakened ability to detoxify H(2)O(2) in comparison with the WT cells. The KO cells had higher glutathionylation in the mitochondrial proteins, particularly the 75-kDa subunit of complex I. Recombinant Grx2 deglutathionylated complex I and restored most of its activity. We conclude that Grx2 has a function that protects cells against H(2)O(2)-induced injury via its peroxidase and dethiolase activities; particularly, Grx2 prevents complex I inactivation and preserves mitochondrial function.

Functional redundancy between DNA ligases I and III in DNA replication in vertebrate cells

In eukaryotes, the three families of ATP-dependent DNA ligases are associated with specific functions in DNA metabolism. DNA ligase I (LigI) catalyzes Okazaki-fragment ligation at the replication fork and nucleotide excision repair (NER). DNA ligase IV (LigIV) mediates repair of DNA double strand breaks (DSB) via the canonical non-homologous end-joining (NHEJ) pathway. The evolutionary younger DNA ligase III (LigIII) is restricted to higher eukaryotes and has been associated with base excision (BER) and single strand break repair (SSBR). Here, using conditional knockout strategies for LIG3 and concomitant inactivation of the LIG1 and LIG4 genes, we show that in DT40 cells LigIII efficiently supports semi-conservative DNA replication. Our observations demonstrate a high functional versatility for the evolutionary new LigIII in DNA replication and mitochondrial metabolism, and suggest the presence of an alternative pathway for Okazaki fragment ligation.

Listeria infection modulates mitochondrial dynamics

Mitochondria are highly dynamic organelles that are central to several cellular processes, the most prominent being energy production. Several reports have shown that pathogens target mitochondria in various ways to interfere with apoptosis, but to our knowledge only one study has specifically addressed the effects of infection on mitochondrial dynamics. We focused on this aspect during infection with the intracellular pathogen L. monocytogenes and could recently show that this bacterium profoundly alters mitochondrial dynamics, causing transient fragmentation of the mitochondrial network. This mitochondrial fragmentation occurs early during infection and is specific to pathogenic L. monocytogenes, as it is not observed with other intracellular pathogens. The relevance of mitochondrial dynamics for L. monocytogenes infection is highlighted by the finding that siRNA-mediated inhibition of mitochondrial fusion or fission decreases or increases the efficiency of L. monocytogenes infection, respectively. The main bacterial factor responsible for mitochondrial network disruption was identified as the secreted pore-forming toxin listeriolysin O, which also appeared to impair mitochondrial function. Our work suggests that in order to establish an efficient infection, L. monocytogenes interferes with cellular physiology at early timepoints by transient disruption of mitochondrial dynamics and function.

Effect of RNA interference of BID and BAX mRNAs on apoptosis in granulosa cell-derived KGN cells

In mitochondrion-dependent type II apoptosis, BH3-interacting domain death agonist (BID) and BCL-2-associated X protein (BAX) promote death ligand and receptor-mediated cell death. In porcine ovaries, the levels of BID and BAX increase in follicular granulosa cells during atresia. In the present study, to confirm the pro-apoptotic activity of BID and BAX in granulosa cells, we examined the effect of RNA interference of BID or BAX on apoptosis using a human ovarian granulosa tumor cell line, KGN. By reverse transcription polymerase chain reaction (RT-PCR) and Western blotting, expression of BID and BAX was detected in KGN cells. Then, we suppressed BID and BAX mRNA expression in KGN cells using small interfering RNA (siRNA). When BID or BAX was suppressed, a significant decrease in the apoptotic cell rate was noted. In granulosa-derived cells, BID and BAX showed pro-apoptotic activity. These results suggest that BID and BAX act as signal-transducing factors in mitochondrion-dependent type II apoptosis.

Insights into the structural stability of Bax from molecular dynamics simulations at high temperatures

Bax is a member of the Bcl-2 protein family that participates in mitochondrion-mediated apoptosis. In the early stages of the apoptotic pathway, this protein migrates from the cytosol to the outer mitochondrial membrane, where it is inserted and usually oligomerizes, making cytochrome c-compatible pores. Although several cellular and structural studies have been reported, a description of the stability of Bax at the molecular level remains elusive. This article reports molecular dynamics simulations of monomeric Bax at 300, 400, and 500 K, focusing on the most relevant structural changes and relating them to biological experimental results. Bax gradually loses its α-helices when it is submitted to high temperatures, yet it maintains its globular conformation. The resistance of Bax to adopt an extended conformation could be due to several interactions that were found to be responsible for maintaining the structural stability of this protein. Among these interactions, we found salt bridges, hydrophobic interactions, and hydrogen bonds. Remarkably, salt bridges were the most relevant to prevent the elongation of the structure. In addition, the analysis of our results suggests which conformational movements are implicated in the activation/oligomerization of Bax. This atomistic description might have important implications for understanding the functionality and stability of Bax in vitro as well as within the cellular environment.

Properties of Ca(2+) transport in mitochondria of Drosophila melanogaster

We have studied the pathways for Ca(2+) transport in mitochondria of the fruit fly Drosophila melanogaster. We demonstrate the presence of ruthenium red (RR)-sensitive Ca(2+) uptake, of RR-insensitive Ca(2+) release, and of Na(+)-stimulated Ca(2+) release in energized mitochondria, which match well characterized Ca(2+) transport pathways of mammalian mitochondria. Following larger matrix Ca(2+) loading Drosophila mitochondria underwent spontaneous RR-insensitive Ca(2+) release, an event that in mammals is due to opening of the permeability transition pore (PTP). Like the PTP of mammals, Drosophila Ca(2+)-induced Ca(2+) release could be triggered by uncoupler, diamide, and N-ethylmaleimide, indicating the existence of regulatory voltage- and redox-sensitive sites and was inhibited by tetracaine. Unlike PTP-mediated Ca(2+) release in mammals, however, it was (i) insensitive to cyclosporin A, ubiquinone 0, and ADP; (ii) inhibited by P(i), as is the PTP of yeast mitochondria; and (iii) not accompanied by matrix swelling and cytochrome c release even in KCl-based medium. We conclude that Drosophila mitochondria possess a selective Ca(2+) release channel with features intermediate between the PTP of yeast and mammals.

Classes of programmed cell death in plants, compared to those in animals

Relatively little is known about programmed cell death (PCD) in plants. It is nonetheless suggested here that tonoplast rupture and the subsequent rapid destruction of the cytoplasm can distinguish two large PCD classes. One class, which is here called 'autolytic', shows this feature, whilst the second class (called 'non-autolytic') can include tonoplast rupture but does not show the rapid cytoplasm clearance. Examples of the 'autolytic' PCD class mainly occur during normal plant development and after mild abiotic stress. The 'non-autolytic' PCD class is mainly found during PCD that is due to plant-pathogen interactions. Three categories of PCD are currently recognized in animals: apoptosis, autophagy, and necrosis. An attempt is made to reconcile the recognized plant PCD classes with these groups. Apoptosis is apparently absent in plants. Autophagic PCD in animals is defined as being accompanied by an increase in the number of autophagosomes, autolysosomes, and small lytic vacuoles produced by autolysosomes. When very strictly adhering to this definition, there is no (proof for) autophagic PCD in plants. Upon a slightly more lenient definition, however, the 'autolytic' class of plant PCD can be merged with the autophagic PCD type in animal cells. The 'non-autolytic' class of plant PCD, as defined here, can be merged with necrotic PCD in animals.

Bcl-xL regulates metabolic efficiency of neurons through interaction with the mitochondrial F1FO ATP synthase

Anti-apoptotic Bcl2 family proteins such as Bcl-x_L protect cells from death by sequestering apoptotic molecules, but also contribute to normal neuronal function. We find in hippocampal neurons that Bcl-x_L enhances the efficiency of energy metabolism. Our evidence suggests that Bcl-x_L interacts directly with the beta subunit of the F₁F_O ATP synthase, decreasing an ion leak within the F₁F_O ATPase complex and thereby increasing net transport of H⁺ by F₁F_O during F₁F_O ATPase activity. By patch clamping submitochondrial vesicles enriched in F₁F_O ATP synthase complexes, we find that, in the presence of ATP, pharmacological or genetic inhibition of Bcl-x_L increases the membrane leak conductance. In addition, recombinant Bcl-x_L protein directly increases ATPase activity of purified synthase complexes, while inhibition of endogenous Bcl-x_L decreases F₁F_O enzymatic activity. Our findings suggest that increased mitochondrial efficiency contributes to the enhanced synaptic efficacy found in Bcl-x_L expressing neurons.

Reactive oxygen species and p38 MAPK regulate Bax translocation and calcium redistribution in salubrinal-induced apoptosis of EBV-transformed B cells

Salubrinal is a specific eIF2α phosphatase inhibitor that inhibits ER stress-mediated apoptosis. However, maintaining hyper-phosphorylated eIF2α state with high doses of salubrinal treatment promotes apoptosis in some cancer cells. In this report, we found that salubrinal induced apoptosis of EBV-transformed B cells. Notably, salubrinal induced ROS generation and p38 MPAK activation, which then induced expression of FasL. Moreover, salubrinal subsequently led to activation of caspases, calcium redistribution, Bax translocation, cytochrome c release, and apoptosis. These findings suggest that salubrinal may be a novel therapeutic approach for EBV-associated malignant diseases.

Caspase dependent programmed cell death in developing embryos: a potential target for therapeutic intervention against pathogenic nematodes

Background

Successful embryogenesis is a critical rate limiting step for the survival and transmission of parasitic worms as well as pathology mediated by them. Hence, blockage of this important process through therapeutic induction of apoptosis in their embryonic stages offers promise for developing effective anti-parasitic measures against these extra cellular parasites. However, unlike in the case of protozoan parasites, induction of apoptosis as a therapeutic approach is yet to be explored against metazoan helminth parasites.

Methodology/Principal Findings

For the first time, here we developed and evaluated flow cytometry based assays to assess several conserved features of apoptosis in developing embryos of a pathogenic filarial nematode Setaria digitata, in-vitro as well as ex-vivo. We validated programmed cell death in developing embryos by using immuno-fluorescence microscopy and scoring expression profile of nematode specific proteins related to apoptosis [e.g. CED-3, CED-4 and CED-9]. Mechanistically, apoptotic death of embryonic stages was found to be a caspase dependent phenomenon mediated primarily through induction of intracellular ROS. The apoptogenicity of some pharmacological compounds viz. DEC, Chloroquine, Primaquine and Curcumin were also evaluated. Curcumin was found to be the most effective pharmacological agent followed by Primaquine while Chloroquine displayed minimal effect and DEC had no demonstrable effect. Further, demonstration of induction of apoptosis in embryonic stages by lipid peroxidation products [molecules commonly associated with inflammatory responses in filarial disease] and demonstration of in-situ apoptosis of developing embryos in adult parasites in a natural bovine model of filariasis have offered a framework to understand anti-fecundity host immunity operational against parasitic helminths.

Conclusions/Significance

Our observations have revealed for the first time, that induction of apoptosis in developing embryos can be a potential approach for therapeutic intervention against pathogenic nematodes and flow cytometry can be used to address different issues of biological importance during embryogenesis of parasitic worms.

Pathogenic nematodes currently infect billions of people around the world and pose serious challenges to the economic welfare and public health in most developing countries. At present, limitations of existing therapies warrant identification of new anti-parasitic drugs/drug targets to effectively treat and control neglected tropical diseases [NTD] caused by nematode pathogens. The current gold standard for measuring/screening drug effectiveness against most helminth parasites is in-vitro assessment of motility of parasites/larvae and larval development assays which fails to provide any conclusive idea about the precise mechanism of death of parasitic worms or their larval stages. Given the huge load of parasites or their larval stages in an infected host, a compound which shows promise in in-vitro/motility screening assays but induces necrotic death in parasites/larvae will be of limited use, as it may elicit severe inflammatory response in infected hosts. In this context, the present study, which demonstrates induction of apoptotic death in developing embryos of a pathogenic nematode as a potential drug target for the first time, and provides scope for high throughput screening of pharmacological agents for their apoptogenicity against nematode embryos, is a step forward to develop novel anti-parasitic measures to challenge NTD caused by nematode pathogens.

Mitochondria and antiviral innate immunity

Mitochondria, dynamic organelles that undergo continuous cycles of fusion and fission, are the powerhouses of eukaryotic cells. Recent research indicates that mitochondria also act as platforms for antiviral immunity in vertebrates. Mitochondrial-mediated antiviral immunity depends on activation of the retinoic acid-inducible gene I (RIG-I)-like receptors signal transduction pathway and the participation of the mitochondrial outer membrane adaptor protein "mitochondrial antiviral signaling (MAVS)". Here we discuss recent findings that suggest how mitochondria contribute to antiviral innate immunity.

Apoptosis: why and how does it occur in biology?

The literature on apoptosis has grown tremendously in recent years, and the mechanisms that are involved in this programmed cell death pathway have been enlightened. It is now known that apoptosis takes place starting from early development to adult stage for the homeostasis of multicellular organisms, during disease development and in response to different stimuli in many different systems. In this review, we attempted to summarize the current knowledge on the circumstances and the mechanisms that lead to induction of apoptosis, while going over the molecular details of the modulator and mediators of apoptosis as well as drawing the lines between programmed and non-programmed cell death pathways. The review will particularly focus on Bcl-2 family proteins, the role of different caspases in the process of apoptosis, and their inhibitors as well as the importance of apoptosis during different disease states. Understanding the molecular mechanisms involved in apoptosis better will make a big impact on human diseases, particularly cancer, and its management in the clinics.

Pancreatic β-cell death in response to pro-inflammatory cytokines is distinct from genuine apoptosis

A reduction in functional β-cell mass leads to both major forms of diabetes; pro-inflammatory cytokines, such as interleukin-1beta (IL-1β) and gamma-interferon (γ-IFN), activate signaling pathways that direct pancreatic β-cell death and dysfunction. However, the molecular mechanism of β-cell death in this context is not well understood. In this report, we tested the hypothesis that individual cellular death pathways display characteristic phenotypes that allow them to be distinguished by the precise biochemical and metabolic responses that occur during stimulus-specific initiation. Using 832/13 and INS-1E rat insulinoma cells and isolated rat islets, we provide evidence that apoptosis is unlikely to be the primary pathway underlying β-cell death in response to IL-1β+γ-IFN. This conclusion was reached via the experimental results of several different interdisciplinary strategies, which included: 1) tandem mass spectrometry to delineate the metabolic differences between IL-1β+γ-IFN exposure versus apoptotic induction by camptothecin and 2) pharmacological and molecular interference with either NF-κB activity or apoptosome formation. These approaches provided clear distinctions in cell death pathways initiated by pro-inflammatory cytokines and bona fide inducers of apoptosis. Collectively, the results reported herein demonstrate that pancreatic β-cells undergo apoptosis in response to camptothecin or staurosporine, but not pro-inflammatory cytokines.

The apoptotic regulator Nrz controls cytoskeletal dynamics via the regulation of Ca2+ trafficking in the zebrafish blastula

Bcl-2 family members are key regulators of apoptosis. Their involvement in other cellular processes has been so far overlooked. We have studied the role of the Bcl-2 homolog Nrz in the developing zebrafish. Nrz was found to be localized to the yolk syncytial layer, a region containing numerous mitochondria and ER membranes. Nrz knockdown resulted in developmental arrest before gastrulation, due to free Ca(2+) increase in the yolk cell, activating myosin light chain kinase, which led to premature contraction of actin-myosin cables in the margin and separation of the blastomeres from the yolk cell. In the yolk syncytial layer, Nrz appears to prevent the release of Ca(2+) from the endoplasmic reticulum by directly interacting with the IP3R1 Ca(2+) channel. Thus, the Bcl-2 family may participate in early development, not only by controlling apoptosis but also by acting on cytoskeletal dynamics and cell movements via Ca(2+) fluxes inside the embryo.

Cytotoxicity and mitochondrial damage caused by silica nanoparticles

Amorphous silica nanoparticles are widely applied in many fields. But the adverse effects of silica nanoparticle exposure were unclear. The present study investigated the cytotoxicity and mitochondrial damage of silica nanoparticles exposure in hepatocellular carcinoma cell line (HepG2). The cells were treated with 43 nm non-modified amorphous silica nanoparticles which dispersed in serum-free DMEM at concentrations of 0, 25, 50, 100 and 200 μg/mL for 3 and 24 h. The results showed that the silica nanoparticles could lead to increasing cellular reactive oxygen species (ROS) production for 3 and 24 h exposure. Moreover, the oxidative stress induced by the particles could play an important role of the mitochondrial membrane damage and the cell apoptosis. It indicated that apoptosis through mitochondrial pathway mediated by oxidative stress was a potential mechanism of cytotoxicity induced by silica nanoparticles. The particles could enter the cells through different pathways and dispersed in cytoplasm and deposited inside mitochondria. Mitochondria were the major organelles for the cytotoxicity of silica nanoparticles exposure. Mitochondrial damage was related to the oxidative stress and the direct injurious effect of nanoparticles. It can be considered as the potential mechanism for the cytotoxic effects of amorphous silica nanoparticles.

P43-dependent mitochondrial activity regulates myoblast differentiation and slow myosin isoform expression by control of Calcineurin expression

We have previously shown that mitochondrial protein synthesis regulates myoblast differentiation, partly through the control of c-Myc expression, a cellular oncogene regulating myogenin expression and myoblast withdrawal from the cell cycle. In this study we provide evidence of the involvement of Calcineurin in this regulation. In C2C12 myoblasts, inhibition of mitochondrial protein synthesis by chloramphenicol decreases Calcineurin expression. Conversely, stimulation of this process by overexpressing the T3 mitochondrial receptor (p43) increases Calcineurin expression. Moreover, expression of a constitutively active Calcineurin (ΔCN) stimulates myoblast differentiation, whereas a Calcineurin antisense has the opposite effect. Lastly, ΔCN expression or stimulation of mitochondrial protein synthesis specifically increases slow myosin heavy chain expression. In conclusion, these data clearly suggest that, partly via Calcineurin expression, mitochondrial protein synthesis is involved in muscle development through the control of myoblast differentiation and probably the acquisition of the contractile and metabolic phenotype of muscle fibres.

Trans-plasma membrane electron transport in mammals: functional significance in health and disease

Trans-plasma membrane electron transport (t-PMET) has been established since the 1960s, but it has only been subject to more intensive research in the last decade. The discovery and characterization at the molecular level of its novel components has increased our understanding of how t-PMET regulates distinct cellular functions. This review will give an update on t-PMET, with particular emphasis on how its malfunction relates to some diseases, such as cancer, abnormal cell death, cardiovascular diseases, aging, obesity, neurodegenerative diseases, pulmonary fibrosis, asthma, and genetically linked pathologies. Understanding these relationships may provide novel therapeutic approaches for pathologies associated with unbalanced redox state.

Bax: Addressed to kill

The pro-apoptototic protein Bax (Bcl-2 Associated protein X) plays a central role in the mitochondria-dependent apoptotic pathway. In healthy mammalian cells, Bax is essentially cytosolic and inactive. Following a death signal, the protein is translocated to the outer mitochondrial membrane, where it promotes a permeabilization that favors the release of different apoptogenic factors, such as cytochrome c. The regulation of Bax translocation is associated to conformational changes that are under the control of different factors. The evidences showing the involvement of different Bax domains in its mitochondrial localization are presented. The interactions between Bax and its different partners are described in relation to their ability to promote (or prevent) Bax conformational changes leading to mitochondrial addressing and to the acquisition of the capacity to permeabilize the outer mitochondrial membrane.

Mitochondrial division: molecular machinery and physiological functions

Mitochondrial division has emerged as a key mechanism for this essential organelle to maintain its structural integrity, intracellular distribution, and functional competence. An evolutionarily conserved dynamin-related GTPase, Dnm1p/Drp1, interacts with other proteins to form the core machinery involved in mitochondrial division. We summarize recent progress in understanding how the division machinery assembles onto mitochondria and how mitochondrial division contributes to cellular physiology and human diseases.

Mitochondrial DNA toxicity compromises mitochondrial dynamics and induces hippocampal antioxidant defenses

Mitochondria are highly dynamic organelles that can be actively transported within the cell to satisfy local requirements. They are vital for providing cellular energy, but are also an important endogenous source of reactive oxygen species. The distribution of mitochondria is particularly important for neurons because of the morphological complexity of these cells, and because neural processing is metabolically expensive. Defects in mitochondrial distribution, observed in several neurodegenerative diseases, can result in synaptic dysfunction. We have generated transgenic mice expressing an enzyme in forebrain neurons that causes mitochondrial DNA (mtDNA) damage in the form of abasic-sites, creating mtDNA toxicity. Here, we report that mitochondrial distribution is disturbed in hippocampal neurons of these mice. Moreover, mtDNA copy number and mitochondrial transcription are reduced, and oxidative stress is increased. There is also a loss of receptors at excitatory glutamatergic synapses in the dentate gyrus, and the size of the postsynaptic density in this region is abnormal. We speculate that the loss of synaptic mitochondria caused by accumulation in the neuronal cell body contributes to the observed synaptic abnormalities, as well as the overall loss of mtDNA and diminished mitochondrial transcription. Collectively, these changes lead to mitochondria with reduced function and increased oxidative stress.

Drosophila melanogaster: a new model to study cisplatin-induced neurotoxicity

Platinum-based compounds are widely used and effective chemotherapeutic agents; however, sensory peripheral neuropathy is a dose-limiting and long term side effect for 20-30% of patients. A critical question is whether the mechanisms of cell death underlying clinical efficacy can be separated from the effects on neurons in order to develop strategies that prevent platinum-induced neuropathy. In rodent dorsal root ganglion neurons (DRG), cisplatin has been shown to bind and damage neuronal DNA, inducing apoptosis; however genetic manipulation in order to study mechanisms of this phenomenon in the rodent model system is costly and time-consuming. Drosophila melanogaster are commonly used to study neurological disorders, have DNA damage-apoptosis mechanisms homologous to mammalian systems, and have readily-available, inexpensive tools for rapid genetic manipulation. We therefore sought to develop adult Drosophila as a new model to study cisplatin-induced neurotoxicity. Adult Drosophila were exposed to 10, 25, 50, 100, 200 and 400 μg/ml cisplatin for 3 days and observed for fly survival and geotactic climbing behavior, cisplatin-DNA binding and cellular apoptosis. On day 3, 50 μg/ml cisplatin reduced the number of flies able to climb above 2 cm to 43% while fly survival was maintained at 92%. 100% lethality was observed at 400 μg/ml cisplatin. Whole fly platinum-genomic DNA adducts were measured and found to be comparable to adduct levels previously measured in rat DRG neurons. Brain, ovaries, kidney and heart harvested from cisplatin treated flies were stained for active caspase 3. Apoptosis was found in ovaries and brain but not in heart and kidney. Brain apoptosis was confirmed by transmission electron microscopy. Expression of the anti-apoptotic baculoviral protein, p35, in neurons using the GAL4-UAS system prevented cisplatin-induced apoptosis in the brain and restored climbing behavior. In conclusion, cisplatin-induced behavioral and apoptotic changes in Drosophila resemble those seen in mammals. Furthermore, the use of lethality and climbing assays combined with powerful gene manipulation, make Drosophila a suitable model to study mechanisms of cisplatin neurotoxicity.

Mitochondrial dysfunction and oxidative stress contribute to the pathogenesis of spinocerebellar ataxia type 12 (SCA12)

Spinal cerebellar ataxia type 12 (SCA12) has been attributed to the elevated expression of ppp2r2b. To better elucidate the pathomechanism of the neuronal disorder and to search for a pharmacological treatment, Drosophila models of SCA12 were generated by overexpression of a human ppp2r2b and its Drosophila homolog tws. Ectopic expression of ppp2r2b or tws caused various pathological features, including neurodegeneration, apoptosis, and shortened life span. More detailed analysis revealed that elevated ppp2r2b and tws induced fission of mitochondria accompanied by increases in cytosolic reactive oxygen species (ROS), cytochrome c, and caspase 3 activity. Transmission electron microscopy revealed that fragmented mitochondria with disrupted cristae were engulfed by autophagosomes in photoreceptor neurons of flies overexpressing tws. Additionally, transgenic flies were more susceptible to oxidative injury induced by paraquat. By contrast, ectopic Drosophila Sod2 expression and antioxidant treatment reduced ROS and caspase 3 activity and extended the life span of the SCA12 fly model. In summary, our study demonstrates that oxidative stress induced by mitochondrial dysfunction plays a causal role in SCA12, and reduction of ROS is a potential therapeutic intervention for this neuropathy.

Mitochondrial medicine: to a new era of gene therapy for mitochondrial DNA mutations

Mitochondrial disorders can no longer be ignored in most medical disciplines. Such disorders include specific and widespread organ involvement, with tissue degeneration or tumor formation. Primary or secondary actors, mitochondrial dysfunctions also play a role in the aging process. Despite progresses made in identification of their molecular bases, nearly everything remains to be done as regards therapy. Research dealing with mitochondrial physiology and pathology has >20 years of history around the world. We are involved, as are many other laboratories, in the challenge of finding ways to fight these diseases. However, our main limitation is the scarcety of animal models required for both understanding the molecular mechanisms underlying the diseases and evaluating therapeutic strategies. This is especially true for diseases due to mutations in mitochondrial DNA (mtDNA), since an authentic genetic model of mtDNA mutations is technically a very difficult task due to both the inability of manipulating the mitochondrial genome of living mammalian cells and to its multicopy nature. This has led researchers in the field to consider the prospect of gene therapy approaches that can roughly be divided into three groups: (1) import of wild-type copies or relevant sections of DNA or RNA into mitochondria, (2) manipulation of mitochondrial genetic content, and (3) rescue of a defect by expression of an engineered gene product from the nucleus (allotopic or xenotropic expression). We briefly introduce these concepts and indicate where promising progress has been made in the last decade.

Signaling cell death from the endoplasmic reticulum stress response

Inability to meet protein folding demands within the endoplasmic reticulum (ER) activates the unfolded protein response (UPR), a signaling pathway with both adaptive and apoptotic outputs. While some secretory cell types have a remarkable ability to increase protein folding capacity, their upper limits can be reached when pathological conditions overwhelm the fidelity and/or output of the secretory pathway. Irremediable 'ER stress' induces apoptosis and contributes to cell loss in several common human diseases, including type 2 diabetes and neurodegeneration. Researchers have begun to elucidate the molecular switches that determine when ER stress is too great to repair and the signals that are then sent from the UPR to execute the cell.

Bid and Bax are involved in granulosa cell apoptosis during follicular atresia in porcine ovaries

More than 99% of follicles undergo "atresia" during follicular development and growth. Follicular atresia is predominantly regulated by granulosa cell apoptosis. However, the intracellular signaling pathway of apoptosis in granulosa cells has not been revealed. In the present study, we examined changes in the expression of BH3-interacting domain death agonist (Bid) and Bcl-2-associated X protein (Bax), which are considered to promote the cell death ligand/receptor-mediated process in mitochondrion-dependent type II apoptosis, in porcine granulosa cells during atresia. Levels of mRNA and protein of Bid and Bax were determined by the reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting techniques, respectively. Levels of Bid and Bax mRNA and protein were markedly increased in granulosa cells of early atretic follicles compared with those of healthy follicles. In situ hybridization and immunohistochemical staining revealed that mRNA and protein of Bid and Bax were present in the granulosa cells, though only traces were found in healthy follicles; however, strong staining was noted in atretic follicles. These results indicate that Bid and Bax appear to be signal transduction factors in granulosa cells during follicular atresia and appear to play proapoptotic roles and confirm that the porcine granulosa cell is a mitochondrion-dependent type II apoptotic cell.

The therapeutic potential of mitochondrial channels in cancer, ischemia-reperfusion injury, and neurodegeneration

Mitochondria communicate with the rest of the cell through channels located in their inner and outer membranes. Most of the time, the message is encoded by the flow of anions and cations e.g., through VDAC and PTP, respectively. However, proteins are also both imported and exported across the mitochondrial membranes e.g., through TOM and MAC, respectively. Transport through mitochondrial channels is exquisitely regulated and controls a myriad of processes; from energy production to cell death. Here, we examine the role of some of the mitochondrial channels involved in neurodegeneration, ischemia-reperfusion injury and cancer in the context of their potential as therapeutic targets.

EndoG links Bnip3-induced mitochondrial damage and caspase-independent DNA fragmentation in ischemic cardiomyocytes

Mitochondrial dysfunction, caspase activation and caspase-dependent DNA fragmentation are involved in cell damage in many tissues. However, differentiated cardiomyocytes repress the expression of the canonical apoptotic pathway and their death during ischemia is caspase-independent. The atypical BH3-only protein Bnip3 is involved in the process leading to caspase-independent DNA fragmentation in cardiomyocytes. However, the pathway by which DNA degradation ensues following Bnip3 activation is not resolved. To identify the mechanism involved, we analyzed the interdependence of Bnip3, Nix and EndoG in mitochondrial damage and DNA fragmentation during experimental ischemia in neonatal rat ventricular cardiomyocytes. Our results show that the expression of EndoG and Bnip3 increases in the heart throughout development, while the caspase-dependent machinery is silenced. TUNEL-positive DNA damage, which depends on caspase activity in other cells, is caspase-independent in ischemic cardiomyocytes and ischemia-induced DNA high and low molecular weight fragmentation is blocked by repressing EndoG expression. Ischemia-induced EndoG translocation and DNA degradation are prevented by silencing the expression of Bnip3, but not Nix, or by overexpressing Bcl-x_L. These data establish a link between Bnip3 and EndoG-dependent, TUNEL-positive, DNA fragmentation in ischemic cardiomyocytes in the absence of caspases, defining an alternative cell death pathway in postmitotic cells.

BID is cleaved by caspase-8 within a native complex on the mitochondrial membrane

Caspase-8 stably inserts into the mitochondrial outer membrane during extrinsic apoptosis. Inhibition of caspase-8 enrichment on the mitochondria impairs caspase-8 activation and prevents apoptosis. However, the function of active caspase-8 on the mitochondrial membrane remains unknown. In this study, we have identified a native complex containing caspase-8 and BID on the mitochondrial membrane, and showed that death receptor activation by Fas or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induced the cleavage of BID (tBID formation) within this complex. tBID then shifted to separate mitochondria-associated complexes that contained other BCL-2 family members, such as BAK and BCL-X(L). We report that cells stabilize active caspase-8 on the mitochondria in order to specifically target mitochondria-associated BID, and that BID cleavage on the mitochondria is essential for caspase-8-induced cytochrome c release. Our findings indicate that during extrinsic apoptosis, caspase-8 can specifically target BID where it is mostly needed, on the surface of mitochondria.

Apoptosis and aging: increased resistance to apoptosis enhances the aging process

Apoptosis is a vital component in the evolutionarily conserved host defense system. Apoptosis is the guardian of tissue integrity by removing unfit and injured cells without evoking inflammation. However, apoptosis seems to be a double-edged sword since during low-level chronic stress, such as in aging, increased resistance to apoptosis can lead to the survival of functionally deficient, post-mitotic cells with damaged housekeeping functions. Senescent cells are remarkably resistant to apoptosis, and several studies indicate that host defense mechanisms can enhance anti-apoptotic signaling, which subsequently induces a senescent, pro-inflammatory phenotype during the aging process. At the molecular level, age-related resistance to apoptosis involves (1) functional deficiency in p53 network, (2) increased activity in the NF-κB-IAP/JNK axis, and (3) changes in molecular chaperones, microRNAs, and epigenetic regulation. We will discuss the molecular basis of age-related resistance to apoptosis and emphasize that increased resistance could enhance the aging process.

BAD contributes to RAF-mediated proliferation and cooperates with B-RAF-V600E in cancer signaling

BAD (Bcl-2 antagonist of cell death) belongs to the proapoptotic BH3-only subfamily of Bcl-2 proteins. Physiological activity of BAD is highly controlled by phosphorylation. To further analyze the regulation of BAD function, we investigated the role of recently identified phosphorylation sites on BAD-mediated apoptosis. We found that in contrast to the N-terminal phosphorylation sites, the serines 124 and 134 act in an antiapoptotic manner because the replacement by alanine led to enhanced cell death. Our results further indicate that RAF kinases represent, besides PAK1, BAD serine 134 phosphorylating kinases. Importantly, in the presence of wild type BAD, co-expression of survival kinases, such as RAF and PAK1, leads to a strongly increased proliferation, whereas substitution of serine 134 by alanine abolishes this process. Furthermore, we identified BAD serine 134 to be strongly involved in survival signaling of B-RAF-V600E-containing tumor cells and found that phosphorylation of BAD at this residue is critical for efficient proliferation in these cells. Collectively, our findings provide new insights into the regulation of BAD function by phosphorylation and its role in cancer signaling.

Mitochondrial membrane potential is required for MAVS-mediated antiviral signaling

Mitochondria, dynamic organelles that undergo cycles of fusion and fission, are the powerhouses of eukaryotic cells and are also involved in cellular innate antiviral immunity in mammals. Mitochondrial antiviral immunity depends on activation of the cytoplasmic retinoic acid-inducible gene I (RIG-I)-like receptor (RLR) signaling pathway and the participation of a mitochondrial outer membrane adaptor protein called MAVS (mitochondrial antiviral signaling). We found that cells that lack the ability to undergo mitochondrial fusion as a result of targeted deletion of both mitofusin 1 (Mfn1) and mitofusin 2 (Mfn2) exhibited impaired induction of interferons and proinflammatory cytokines in response to viral infection, resulting in increased viral replication. In contrast, cells with null mutations in either Mfn1 or Mfn2 retained their RLR-induced antiviral responses. We also found that a reduced mitochondrial membrane potential (ΔΨ(m)) correlated with the reduced antiviral response. The dissipation in ΔΨ(m) did not affect the activation of the transcription factor interferon regulatory factor 3 downstream of MAVS, which suggests that ΔΨ(m) and MAVS are coupled at the same stage in the RLR signaling pathway. Our results provide evidence that the physiological function of mitochondria plays a key role in innate antiviral immunity.

Bcl-2 proteins and autophagy regulate mitochondrial dynamics during programmed cell death in the Drosophila ovary

The Bcl-2 family has been shown to regulate mitochondrial dynamics during cell death in mammals and C. elegans, but evidence for this in Drosophila has been elusive. Here, we investigate the regulation of mitochondrial dynamics during germline cell death in the Drosophila melanogaster ovary. We find that mitochondria undergo a series of events during the progression of cell death, with remodeling, cluster formation and uptake of clusters by somatic follicle cells. These mitochondrial dynamics are dependent on caspases, the Bcl-2 family, the mitochondrial fission and fusion machinery, and the autophagy machinery. Furthermore, Bcl-2 family mutants show a striking defect in cell death in the ovary. These data indicate that a mitochondrial pathway is a major mechanism for activation of cell death in Drosophila oogenesis.

The Differential Effect of Toxoplasma Gondii Infection on the Stability of BCL2-Family Members Involves Multiple Activities

The regulation of mitochondrial permeability, a key event in the initiation of apoptosis is governed by the opposing actions of the pro- and anti-apoptotic members of the BCL2-family of proteins. The BCL2-family can be classified further based on the number of BCL-homology (BH) domains they encode. Pathogen mediated modulation of BCL2-family members play a significant role in their ability to affect the apoptotic pathways in the infected host cell. The protozoan parasite Toxoplasma gondii establishes a profound blockade of apoptosis noted by a requirement for host NFκB activity and correlating with the selective degradation of pro-apoptotic BCL2-family members. In this study, we explore the potential activities associated with the inherent stability of the anti-apoptotic BCL2 as well as the selective degradation of the pro-apoptotic proteins BAX, BAD, and BID. We find that multiple activities govern the relative stability of BCL2-family members suggesting a complex and balanced network of stability-enhancing and–destabilizing activities are perturbed by parasite infection. The data leave open the possibility for both parasite induced host activities as well as the direct consequence of parasite effectors in governing the relative levels of BCL2-proteins in the course of infection.

Metergoline-induced cell death in Candida krusei

Metergoline possesses potent antifungal activity against Candida krusei, a notorious yeast species that is inherently resistant to the common antifungal agents. In an attempt to elucidate the action mechanisms of metergoline, the present study was designed to investigate its effects on a number of classical markers of apoptosis in C. krusei. The results showed that transient exposure (2h) to metergoline led to a massive intracellular accumulation of reactive oxygen species (ROS) and depolarization of mitochondrial membrane potential in a concentration-dependent fashion. Analyses of the treated fungal cells after prolonged incubation (12h) with metergoline by flow cytometry and fluorescence microscopy clearly demonstrated phosphatidylserine externalization, the presence of terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling-positive cells and fungal cells undergoing necrosis. Taken together, our data provide evidence that metergoline elicited cell death process in C. krusei through elevation of the intracellular ROS level and perturbation of mitochondrial homeostasis, followed by damage of nucleus and eventual cell demise.

Role and treatment of mitochondrial DNA-related mitochondrial dysfunction in sporadic neurodegenerative diseases

Several sporadic neurodegenerative diseases display phenomena that directly or indirectly relate to mitochondrial function. Data suggesting altered mitochondrial function in these diseases could arise from mitochondrial DNA (mtDNA) are reviewed. Approaches for manipulating mitochondrial function and minimizing the downstream consequences of mitochondrial dysfunction are discussed.

The interferon stimulated gene 54 promotes apoptosis

The ability of interferons (IFNs) to inhibit viral replication and cellular proliferation is well established, but the specific contribution of each IFN-stimulated gene (ISG) to these biological responses remains to be completely understood. In this report we demonstrate that ISG54, also known as IFN-induced protein with tetratricopeptide repeats 2 (IFIT2), is a mediator of apoptosis. Expression of ISG54, independent of IFN stimulation, elicits apoptotic cell death. Cell death and apoptosis were quantified by propidium iodide uptake and annexin-V staining, respectively. The activation of caspase-3, a key mediator of the execution phase of apoptosis, was clearly apparent in cells expressing ISG54. The anti-apoptotic B cell lymphoma-xl (Bcl-xl) protein inhibited the apoptotic effects of ISG54 as did the anti-apoptotic adenoviral E1B-19K protein. In addition, ISG54 was not able to promote cell death in the absence of pro-apoptotic Bcl family members, Bax and Bak. Analyses of binding partners of ISG54 revealed association with two homologous proteins, ISG56/IFIT1 and ISG60/IFIT3. In addition, ISG60 binding negatively regulates the apoptotic effects of ISG54. The results reveal a previously unidentified role of ISG54 in the induction of apoptosis via a mitochondrial pathway and shed new light on the mechanism by which IFN elicits anti-viral and anti-cancer effects.

Nuclear permeable ruthenium(II) β-carboline complexes induce autophagy to antagonize mitochondrial-mediated apoptosis

The role of autophagy in cancer development and response to cancer therapy has been a subject of debate. Here we demonstrate that a series of ruthenium(II) complexes containing a β-carboline alkaloid as ligand can simultaneously induce autophagy and apoptosis in tumor cells. These Ru(II) complexes are nuclear permeable and highly active against a panel of human cancer cell lines, with complex 3 displaying activities greater than those of cisplatin. The antiproliferative potentialities of 1-3 are in accordance with their relative lipophilicities, cell membrane penetration abilities, and in vitro DNA binding affinities. Complexes 1-3 trigger release of reactive oxygen species (ROS) and attenuation of ROS by scavengers reduced the sub-G1 population, suggesting ROS-dependent apoptosis. Inhibition of ROS generation also reduces autophagy, indicating that ROS triggers autophagy. Further studies show that suppression of autophagy using pharmacological inhibitors (3-methyladenine and chloroquine) enhances apoptotic cell death.

Fragmented mitochondria are sensitized to Bax insertion and activation during apoptosis

Recent studies have shown mitochondrial fragmentation during cell stress and have suggested a role for the morphological change in mitochondrial injury and ensuing apoptosis. However, the underlying mechanism remains elusive. Here we demonstrate that mitochondrial fragmentation facilitates Bax insertion and activation in mitochondria, resulting in the release of apoptogenic factors. In HeLa cells, overexpression of mitofusins attenuated mitochondrial fragmentation during cisplatin- and azide-induced cell injury, which was accompanied by less apoptosis and less cytochrome c release from mitochondria. Similar effects were shown by inhibiting the mitochondrial fission protein Drp1 with a dominant negative mutant (dn-Drp1). Mitofusins and dn-Drp1 did not seem to significantly affect Bax translocation/accumulation to mitochondria; however, they blocked Bax insertion and activation in mitochondrial membrane. Consistently, in rat kidney proximal tubular cells, small interfering RNA knockdown of Drp1 prevented mitochondrial fragmentation during azide-induced ATP depletion, which was accompanied by less Bax activation, insertion, and oligomerization in mitochondria. These cells released less cytochrome c and AIF from mitochondria and showed significantly lower apoptosis. Finally, mitofusin-null mouse embryonic fibroblasts (MEF) had fragmented mitochondria. These MEFs were more sensitive to cisplatin-induced Bax activation, release of cytochrome c, and apoptosis. Together, this study provides further support for a role of mitochondrial fragmentation in mitochondrial injury and apoptosis. Mechanistically, mitochondrial fragmentation may sensitize the cells to Bax insertion and activation in mitochondria, facilitating the release of apoptogenic factors and consequent apoptosis.