A complex II defect affects mitochondrial structure, leading to ced-3- and ced-4-dependent apoptosis and aging

Knockdown of mitochondrial heat shock protein 70 promotes progeria-like phenotypes in caenorhabditis elegans

Mitochondrial heat shock protein 70 (mthsp70) functions as a mitochondrial import motor and is essential in mitochondrial biogenesis and energy generation in eukaryotic cells. HSP-6 (hsp70F) is a nematode orthologue of mthsp70. Knockdown of HSP-6 by RNA interference in young adult nematodes caused a reduction in the levels of ATP-2, HSP-60 and CLK-1, leading to abnormal mitochondrial morphology and lower ATP levels. As a result, RNA interference-treated worms had lower motility, defects in oogenesis, earlier accumulation of autofluorescent material, and a shorter life span. These are the major phenotypes observed during the aging of worms, suggesting that the reduction of HSP-6 causes early aging or progeria-like phenotypes. The amount of HSP-6 became dramatically reduced at the expected mean life span in not only wild-type but also in long and short life span mutant worms (wild-type, daf-2, and daf-16). Mitochondrial HSP-60 and ATP-2 were also reduced following the reduction of HSP-6 during aging. These results suggest that the reduction of HSP-6 causes defects in mitochondrial function at the final stage of aging, leading to mortality.

The role of Sdh4p Tyr-89 in ubiquinone reduction by the Saccharomyces cerevisiae succinate dehydrogenase

Succinate dehydrogenase (complex II or succinate:ubiquinone oxidoreductase) is a tetrameric, membrane-bound enzyme that catalyzes the oxidation of succinate and the reduction of ubiquinone in the mitochondrial respiratory chain. Two electrons from succinate are transferred one at a time through a flavin cofactor and a chain of iron-sulfur clusters to reduce ubiquinone to an ubisemiquinone intermediate and to ubiquinol. Residues that form the proximal quinone-binding site (Q(P)) must recognize ubiquinone, stabilize the ubisemiquinone intermediate, and protonate the ubiquinone to ubiquinol, while minimizing the production of reactive oxygen species. We have investigated the role of the yeast Sdh4p Tyr-89, which forms a hydrogen bond with ubiquinone in the Q(P) site. This tyrosine residue is conserved in all succinate:ubiquinone oxidoreductases studied to date. In the human SDH, mutation of this tyrosine to cysteine results in paraganglioma, tumors of the parasympathetic ganglia in the head and neck. We demonstrate that Tyr-89 is essential for ubiquinone reductase activity and that mutation of Tyr-89 to other residues does not increase the production of reactive oxygen species. Our results support a role for Tyr-89 in the protonation of ubiquinone and argue that the generation of reactive oxygen species is not causative of tumor formation.

Pax2/5/8 proteins promote cell survival inC. elegans

Programmed cell death, or apoptosis, plays an important role during normal development, and is disrupted in a range of disease states. Although the key molecular events that occur during apoptosis are well characterized, less is known about the regulatory inputs that influence whether a cell will live or die. Work in mouse and human cells has shown that Pax transcription factors can influence cell death and promote cell survival, but the mechanism for their activity is not clear. Here, we show that two Pax2/5/8-related genes (egl-38 and pax-2) influence both somatic and germline cell death in C. elegans. Using genetic and molecular experiments, we show that the Pax proteins act as transcriptional regulators of ced-9, the C. elegans bcl-2 gene. These results identify a mechanism for Pax2/5/8-mediated regulation of cell death, and underscore the importance of transcriptional regulation of core apoptotic pathway genes in influencing cell survival.

The role of the electron transport gene SDHC on lifespan and cancer

Much attention has been focused on the hypothesis that oxidative damage contributes to cellular and organismal aging. A mev-1 mutation in the cytochrome b large subunit (SDHC) of complex II results in superoxide anion (O(2)(-)) overproduction and therefore leads to apoptosis and precocious aging in the nematode Caenorhabditis elegans. To extend these data, a transgenic mouse cell line was constructed with a homologous mutation to mev-1. Many of the mutant nematode phenotypes (e.g., increased superoxide anion production, apoptosis) were recapitulated in the mouse. In addition, a significant fraction of the cells that survived apoptosis were transformed. These data support the notion that oxidative stress from mitochondria play an important role of both apoptosis, which leads to precocious aging, and cancer.

Long-lived C. elegans mitochondrial mutants as a model for human mitochondrial-associated diseases

Mitochondria play a pivotal role in the life of cells, controlling diverse processes ranging from energy production to the regulation of cell death. In humans, numerous pathological conditions have been linked to mitochondrial dysfunction. Cancer, diabetes, obesity, neurodegeneration, cardiomyopathy and even aging are all associated with mitochondrial dysfunction. Over 400 mutations in mitochondrial DNA result directly in pathology and many more disorders associated with mitochondrial dysfunction arise from mutations in nuclear DNA. It is counter-intuitive then, that a class of mitochondrially defective mutants in the nematode Caenorhabditis elegans, the so called Mit (Mitochondrial) mutants, in fact live longer than wild-type animals. In this review, we will reconcile this paradox and provide support for the idea that the Mit mutants are in fact an excellent model for studying human mitochondrial associated diseases (HMADs). In the context of the 'Mitochondrial Threshold Effect Theory', we propose that the kinds of processes induced to counteract mitochondrial mutations in the Mit mutants (and which mediate their life extension), are very likely the same ones activated in many HMADs to delay disease appearance. The identification of such compensatory pathways opens a window of possibility for future preventative therapies for many HMADs. They may also provide a way of potentially extending human life span.

Modulation of mitochondrial metabolic function by phorbol 12-myristate 13-acetate through increased mitochondrial translocation of protein kinase Calpha in C2C12 myocytes

Protein kinase C (PKC) agonists including phorbol 12-myristate 13-acetate (PMA) not only induce the redistribution of cytosolic PKC to various subcellular compartments but also activate the kinase domain of the protein. In the present study we have investigated the nature of mitochondrial PKC pool and its effects on mitochondrial function in cells treated with PMA. Treatment of C2C12 myoblasts, C6 glioma and COS7 cells with PMA resulted in a dramatic redistribution of intracellular PKCalpha pool, with large fraction of the protein pool sequestered in the mitochondrial compartment. We also observed mitochondrial PKCdelta accumulation in a cell restricted manner. The intramitochondrial localization was ascertained by using a combination of protection against protease treatment of isolated mitochondria and immunofluorescence microscopy. PMA-induced mitochondrial localization of PKCalpha was accompanied by increased mitochondrial PKC activity, altered cell morphology, disruption of mitochondrial membrane potential, decreased complex I and pyruvate dehydrogenase activities, and increased mitochondrial ROS production. All of these changes could be retarded by treatment with PKC inhibitors. These results show a direct role for PMA-mediated PKCalpha translocation to mitochondria in inducing mitochondrial toxicity.

Age-related changes of mitochondrial structure and function in Caenorhabditis elegans

A number of observations have been made to examine the role that mitochrondrial energetics and superoxide anion production play in the aging of wild-type Caenorhabditis elegans. Ultrastructural analyses reveal the presence of swollen mitochondria, presumably produced by fusion events. Two key mitochondrial functions - the activity of two electron transport chain complexes and oxygen consumption - decreased as animals aged. Carbonylated proteins, one byproduct of oxidative stress, accumulated in mitochondria much more than in the cytoplasm. This is consistent with the notion that mitochondria are the primary source of endogenous reactive oxygen species. However, the level of mitochondrially generated superoxide anion did not change significantly during aging, suggesting that the accumulation of oxidative damage is not due to excessive production of superoxide anion in geriatric animals. In concert, these data support the notion that the mitochondrial function is an important aging determinant in wild-type C. elegans.

The role of stress in ageing: research on the nematode, Caenorhabditis elegans

The nematode Caenorhabditis elegans has proven a robust genetic model for studies of aging and the roles of stress. In this review we focus on the genetics of select long-lived and short-lived mutants of C. elegans that have proven useful in revealing the relationships that exist between oxidative stress and life span. The former are known to be controlled by an insulin/insulin-like signaling pathway, while the latter are affected by mitochondrial functions.

3-nitropropionic acid is a suicide inhibitor of mitochondrial respiration that, upon oxidation by complex II, forms a covalent adduct with a catalytic base arginine in the active site of the enzyme

We report three new structures of mitochondrial respiratory Complex II (succinate ubiquinone oxidoreductase, E.C. 1.3.5.1) at up to 2.1 A resolution, with various inhibitors. The structures define the conformation of the bound inhibitors and suggest the residues involved in substrate binding and catalysis at the dicarboxylate site. In particular they support the role of Arg(297) as a general base catalyst accepting a proton in the dehydrogenation of succinate. The dicarboxylate ligand in oxaloacetate-containing crystals appears to be the same as that reported for Shewanella flavocytochrome c treated with fumarate. The plant and fungal toxin 3-nitropropionic acid, an irreversible inactivator of succinate dehydrogenase, forms a covalent adduct with the side chain of Arg(297). The modification eliminates a trypsin cleavage site in the flavoprotein, and tandem mass spectroscopic analysis of the new fragment shows the mass of Arg(297) to be increased by 83 Da and to have the potential of losing 44 Da, consistent with decarboxylation, during fragmentation.

Examination of the requirement for ucp-4, a putative homolog of mammalian uncoupling proteins, for stress tolerance and longevity in C. elegans

Reactive oxygen species (ROS) are generated by mitochondrial respiration and can react with and damage cellular components. According to the free radical theory of aging, oxidative damage from mitochondrial ROS is a major cause of cellular decline during aging. Mitochondrial uncoupling proteins (UCPs) uncouple ATP production from electron transport and can be stimulated by free radicals, suggesting UCPs may perform a cytoprotective function. The nematode, Caenorhabditis elegans, contains one UCP-like protein, encoded by the ucp-4 gene. We have investigated the genetic requirement for ucp-4 in normal aging and stress resistance. Consistent with the hypothesis that ucp-4 encodes a putative uncoupling protein, animals lacking ucp-4 function contained elevated ATP levels. However, the absence of ucp-4 function did not affect adult lifespan or survival in the presence of thermal or oxidative stress. Together, these results demonstrate that ucp-4 is a negative regulator of ATP production in C. elegans, but is not required for normal lifespan.

Secrets of the lac operon. Glucose hysteresis as a mechanism in dietary restriction, aging and disease

Elevated blood glucose associated with diabetes produces progressive and apparently irreversible damage to many cell types. Conversely, reduction of glucose extends life span in yeast, and dietary restriction reduces blood glucose. Therefore it has been hypothesized that cumulative toxic effects of glucose drive at least some aspects of the aging process and, conversely, that protective effects of dietary restriction are mediated by a reduction in exposure to glucose. The mechanisms mediating cumulative toxic effects of glucose are suggested by two general principles of metabolic processes, illustrated by the lac operon but also observed with glucose-induced gene expression. First, metabolites induce the machinery of their own metabolism. Second, induction of gene expression by metabolites can entail a form of molecular memory called hysteresis. When applied to glucose-regulated gene expression, these two principles suggest a mechanism whereby repetitive exposure to postprandial excursions of glucose leads to an age-related increase in glycolytic capacity (and reduction in beta-oxidation of free fatty acids), which in turn leads to an increased generation of oxidative damage and a decreased capacity to respond to oxidative damage, independent of metabolic rate. According to this mechanism, dietary restriction increases life span and reduces pathology by reducing exposure to glucose and therefore delaying the development of glucose-induced glycolytic capacity.

Bax-like protein Drob-1 protects neurons from expanded polyglutamine-induced toxicity in Drosophila

Bcl-2 family proteins regulate cell death through the mitochondrial apoptotic pathway. Here, we show that the Drosophila Bax-like Bcl-2 family protein Drob-1 maintains mitochondrial function to protect cells from neurodegeneration. A pan-neuronal knockdown of Drob-1 results in lower locomotor activity and a shorter lifespan in adult flies. Either the RNAi-mediated downregulation of Drob-1 or overexpression of Drob-1 antagonist Buffy strongly enhances the polyglutamine-induced accumulation of ubiquitinated proteins and subsequent neurodegeneration. Furthermore, ectopic expression of Drob-1 suppresses the neurodegeneration and premature death of flies caused by expanded polyglutamine. Drob-1 knockdown decreases cellular ATP levels, and enhances respiratory inhibitor-induced mitochondrial defects such as loss of membrane potential (Deltapsim), morphological abnormalities, and reductions in activities of complex I+III and complex II+III, as well as cell death. Taken together, these results suggest that Drob-1 is essential for neuronal cell function, and that Drob-1 protects neurons from expanded polyglutamine-mediated neurodegeneration through the regulation of mitochondrial homeostasis.

A Mutation in the SDHC Gene of Complex II Increases Oxidative Stress, Resulting in Apoptosis and Tumorigenesis

Intracellular oxidative stress from mitochondria is thought to be important in carcinogenesis and tumorigenesis, but direct experimental proof is limited. In this study, a transgenic mouse cell line (SDHC E69) with a mutated SDHC gene (a subunit of complex II in the electron transport chain) was constructed to test this question. The SDHC E69 cells overproduced superoxide anion (O2−) from mitochondria, had elevated cytoplasmic carbonyl proteins and 8-OH-deoxyguanine in their DNA as well as significantly higher mutation frequencies than wild type. There were many apoptotic cells in this cell line, as predicted by the observed increase in caspase 3 activity, decrease in mitochondrial membrane potential, and structural changes in their mitochondria. In addition, some cells that escaped from apoptosis underwent transformation, as evidenced by the fact that SDHC E69 cells caused benign tumors when injected under the epithelium of nude mice. These results underscore the notion that mitochondrially generated oxidative stress can contribute to nuclear DNA damage, mutagenesis, and ultimately, tumorigenesis.

Mechanisms of the interaction of nitroxyl with mitochondria

It is now thought that NO* (nitric oxide) and its redox congeners may play a role in the physiological regulation of mitochondrial function. The inhibition of cytochrome c oxidase by NO* is characterized as being reversible and oxygen dependent. In contrast, peroxynitrite, the product of the reaction of NO* with superoxide, irreversibly inhibits several of the respiratory complexes. However, little is known about the effects of HNO (nitroxyl) on mitochondrial function. This is especially important, since HNO has been shown to be more cytotoxic than NO*, may potentially be generated in vivo, and elicits biological responses with some of the characteristics of NO and peroxynitrite. In the present study, we present evidence that isolated mitochondria, in the absence or presence of substrate, convert HNO into NO* by a process that is dependent on mitochondrial concentration as well as the concentration of the HNO donor Angeli's salt. In addition, HNO is able to inhibit mitochondrial respiration through the inhibition of complexes I and II, most probably via modification of specific cysteine residues in the proteins. Using a proteomics approach, extensive modification of mitochondrial protein thiols was demonstrated. From these data it is evident that HNO interacts with mitochondria through mechanisms distinct from those of either NO* or peroxynitrite, including the generation of NO*, the modification of thiols and the inhibition of complexes I and II.

Mitochondrial oxidative stress can lead to nuclear hypermutability

Reactive oxygen species (ROS) are generated in mitochondria and are thought to be important in aging, carcinogenesis, and the development of other pathologies. We now provide direct experimental evidence linking mitochondrial ROS generation to the induction of nuclear DNA damage and subsequent mutagenesis of a chromosomal gene. Specifically, we demonstrate that the mev-1 mutant of Caenorhabditis elegans has elevated levels of oxidative damage in its chromosomal DNA. This mutant was shown previously to overproduce ROS in its mitochondria. We also show that mutation frequencies were higher in the mev-1 mutant under hypoxia than in the wild type strain. By extension, these data imply that mitochondrially derived ROS mutate other genes, including tumor suppressor genes and oncogenes. We propose that this three-step process (mitochondrial ROS --> nuclear DNA damage --> mutation) contributes to aging and age-associated diseases.

Is defective electron transport at the hub of aging?

The bulwark of the mitochondrial theory of aging is that a defective respiratory chain initiates the death cascade. The increased production of superoxide is suggested to result in progressive oxidant damage to cellular components and particularly to mtDNA that encodes subunits assembled in respiratory complexes. Earlier studies of respiration in muscle mitochondria obtained from large cohorts of patients supported this notion by showing that either singly or in combinations, the respiratory complexes exhibited decreased activity in the elderly. The following critique of the most cited publications over the past decade points out the systematic errors that put earlier work at odds with recent findings. These later investigations indicate that aging has no overt effect on either the electron transport system or oxidative phosphorylation.

Coenzyme Q10 can prolong C. elegans lifespan by lowering oxidative stress

The mev-1 gene encodes cytochrome b, a large subunit of the Complex II enzyme succinate-CoQ oxidoreductase. The mev-1(kn1) mutants are hypersensitive to oxidative stress and age precociously, probably because of elevated superoxide anion production in mitochondria. Coenzyme Q (CoQ) is essential for the mitochondrial respiratory chain. Here, we show that CoQ(10) and Vitamin E extended the life span of wild-type Caenorhabditis elegans. Conversely, only CoQ(10) recovered the life shortening effects seen in mev-1. We also show that CoQ(10) but not Vitamin E reduced superoxide anion levels in wild type and mev-1. Another previously described phenotype of mev-1 animals is the presence of supernumerary apoptotic cells. We now demonstrate that CoQ(10) (but not Vitamin E) suppressed these supernumerary apoptoses. Collectively these data suggest that exogenously supplied CoQ(10) can play a significant anti-aging function. It may do so either by acting as an antioxidant to dismutate the free radical superoxide anion or by reducing the uncoupling of reactions during election transport that could otherwise result in superoxide anion production. The latter activity has not been ascribed to CoQ(10); however, it is known that conditions that uncouple electron transport reactions can lead to elevated superoxide anion production.

Oxidative stress in Caenorhabditis elegans: protective effects of superoxide dismutase/catalase mimetics

The lifespan of Caenorhabditis elegans can be extended by the administration of synthetic superoxide dismutase/ catalase mimetics (SCMs) without any effects on development or fertility. Here we demonstrate that the mimetics, Euk-134 and Euk-8, confer resistance to the oxidative stress-inducing agent, paraquat and to thermal stress. The protective effects of the compounds are apparent with treatments either during development or during adulthood and are independent of an insulin/IGF-I-like signalling pathway also known to affect thermal and oxidative stress resistance. Worms exposed to the compounds do not induce a cellular stress response and no detrimental effects are observed.

Genes of aging

According to developmental genetics theories, aging is a genetically programmed and controlled continuum of development and maturation. Being dynamic and malleable processes, development and aging are controlled not only by genes but also by environmental and epigenetic influences that predominate in the second half of life. Genetic mutations affect many phenotypes in flies, worms, rodents, and humans which share several diseases or their equivalents, including cancer, neurodegeneration, and infectious disorders as well as their susceptibility to them. Life span and stress resistance are closely linked. Oxidative stress actually constitutes a defined hypothesis of aging in that macromolecule oxidative damage accumulates with age and tends to be associated with life expectancy. DNA methylation, a force in the regulation of gene expression, is also one of the biomarkers of genetic damage. The mitotic clock of aging is marked, if not guided, by telomeres, essential genetic elements stabilizing natural chromosomic ends. The dream of humans to live longer, healthy lives is being tested by attempts to modify longevity in animal models, frequently by dietary manipulation. The quest continues to understand the mechanisms of healthy aging, one of the most compelling areas of research in the 21st century.