To identify new components that regulate C. elegans lifespan, we performed a genetic screen for long-lived mutants. To eliminate the isolation of components of the well-characterized
daf-2 pathway, we screened for long life in a
daf-16 null mutant background. In a pilot screen of 2000 haploid genome, we isolated
mg312, which exhibited a dramatic increase in lifespan, up to two-fold greater than controls.
mg312 also displayed other pleiotropies, including reduced pumping and defecation rate, slow growth and sterility.
mg312 is a mutation in the mitochondrial leucyl-tRNA synthetase gene
lrs-2.
lrs-2(
mg312) is a probable null mutant because it is a nonsense mutation in the second exon of the gene. Using a rescuing
lrs-2::GFP fusion gene, we showed that LRS-2 is localized to the mitochondria of many different tissues, including neurons, intestine and body-wall muscle. To assess mitochondrial morphology, we used a GFP marker that is specifically expressed in the body-wall muscle mitochondria. In wild-type animals, this marker exhibits highly organized rod-like staining, but in
lrs-2(
mg312) animals, it was found in contorted and swollen patterns, suggesting that mitochondria were somehow compromised in these mutants. The mechanism contributing to the long-lived phenotype of
lrs-2(
mg312) is unclear. However, a prediction based on the oxidative theory of aging is that reduced reactive oxygen species (ROS) production will lead to reduced cellular damage and longer lifespan. To test this hypothesis, we subjected animals to the oxidative damage-inducing agent paraquat. We found that
lrs-2(
mg312) animals were more sensitive to paraquat treatment, suggesting that these mutants may in fact produce more endogenous ROS, perhaps due to a block in electron transport chain activity. We also utilized an RNA interference (RNAi) strategy to systematically analyze lifespan alterations following reduced expression of each of the predicted C. elegans gene. Inactivation of each gene of Chromosome I revealed that a large number (about 10%) of the RNAi candidates leading to a long-lived phenotype are annotated as mitochondrial genes. These results further indicate that mitochondria likely play an essential role in controlling C. elegans lifespan.