A Complex II Defect Affects Mitochondrial Structure, Leading to ced-3- and ced-4-dependent Apoptosis and Aging

Abstract

The mev-1(kn1) mutation of Caenorhabditis elegans is in Cyt-1, which encodes a subunit of succinate-coenzyme Q oxidoreductase in the mitochondrial electron transport chain. Mutants are hypersensitive to oxidative stress and age precociously in part because of increased superoxide anion production. Here, we show that mev-1 mutants are defective in succinate-coenzyme Q oxidoreductase, possess ultrastructural mitochondrial abnormalities (especially in muscle cells), show a loss of membrane potential, have altered CED-9 and Cyt-1 protein levels under hyperoxia, and contain ced-3-and ced-4-dependent supernumerary apoptotic cells. These defects likely explain the failure of mev-1 to complete embryonic development under hyperoxia as well as its reduced life span.The oxidative phosphorylation system or electron transport chain is located within the mitochondrial inner membrane and is intimately responsible for three important processes: (i) ATP production, (ii) generation of reactive oxygen species (ROS), 1 and (iii) regulation of programmed cell death or apoptosis. The proximal ROS is superoxide anion (O 2 . ), which derives from the Q cycle at rates estimated between 0.1 and 2% of the total electron flow (reviewed in Refs. 1 and 2). A variety of enzymatic and nonenzymatic defense mechanisms have evolved to either quench free radicals or ameliorate their resultant oxidative damage. Apoptosis (or programmed cell death) occurs by a mechanism that is conserved from nematodes to humans (reviewed in Refs. 3 and 4). In Caenorhabditis elegans, a largely invariant cell lineage (including 131 specific somatic cells destined to undergo apoptosis during hermaphroditic development) has provided fertile ground for the study of apoptosis. In other organisms, mitochondria are thought to provide a major initiation signal for apoptosis via the opening of the mitochondrial permeability transition pore, which leads to the release of a number of cell death-promoting factors, including cytochrome c. In C. elegans, normal, developmentally regulated apoptoses are not mediated by cytochrome c leakage from mitochondria. Mitochondrial oxidative phosphorylation system defects can cause a perplexing variety of diseases that exhibit a significant degree of clinical heterogeneity (reviewed in Refs. 5 and 6). For the most part, the pathophysiologies present themselves in tissues with high energy demands, such as heart and muscle. The interrelationship of the three activities described above (e.g. ATP generation, ROS production, and apoptosis initiation) may explain some of the perplexing pathophysiological features of mitochondrial diseases. We have previously described the mev-1(kn1) mutant of the nematode C. elegans as containing a point mutation in Cyt-1 (a large subunit of cytochrome b in complex II) (7). Isolated by screening for hypersensitivity to oxidative damage (8), mev-1 ages precociously at high oxygen concentrations and accumulates markers of aging (e.g. fluorescent materials and protein carbonyls) more rapidly compared with the wild ...

A Measurable Increase in Oxidative Damage Due to Reduction in Superoxide Detoxification Fails to Shorten the Life Span of Long-Lived Mitochondrial Mutants of Caenorhabditis elegans

“…Although the short life of mev-1 mutants has sometimes been hypothesized to result from increased oxidative stress, there is no immediate demonstration of this. In fact, recent data very strongly indicate that a large part of the effect of mev-1 on life span is due to its effect on increasing programmed cell death through a direct effect on the ex pression of ced-9 (Senoo- Matsuda et al 2001Matsuda et al , 2003. Our findings suggest that the degree by which sod RNAi treatment increases oxidative stress ( Figure 3E) in this mutant does not shorten its life span.…”

Autophagy and ageing: Insights from invertebrate model organisms

“…The mitochondrial theory of ageing, proposed first by Harman (1956Harman ( , 1972, posits that accumulating oxidative damage drives senescent decline and ageing. In C. elegans, long-lived daf-2 or isp-1 mutants display increased anti-oxidant enzyme expression (MnSOD and SOD-3 respectively; Feng et al, 2001;Honda and Honda, 1999), whilst short-lived gas-1 and mev-1 mutants show increased ROS levels as well as high mitochondrial and nuclear DNA mutation rates (Ishii et al, 1998;Kayser et al, 2004;Senoo-Matsuda et al, 2003). In Drosophila, the long-lived Methuselah (mth) mutants display increased resistance to oxidative stress (Lin et al, 1998), whilst overexpression of anti-oxidant enzymes extends lifespan (Sun et al, 2002;Sun and Tower, 1999).…”

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