Increased production of reactive oxygen species (ROS) has long been considered a cause of aging. However, recent studies have implicated ROS as essential secondary messengers. Here we show that the site of ROS production significantly contributes to their apparent dual nature. We report that ROS increase with age as mitochondrial function deteriorates. However, we also demonstrate that increasing ROS production specifically through respiratory complex I reverse electron transport extends Drosophila lifespan. Reverse electron transport rescued pathogenesis induced by severe oxidative stress, highlighting the importance of the site of ROS production in signaling. Furthermore, preventing ubiquinone reduction, through knockdown of PINK1, shortens lifespan and accelerates aging; phenotypes that are rescued by increasing reverse electron transport. These results illustrate that the source of a ROS signal is vital in determining its effects on cellular physiology and establish that manipulation of ubiquinone redox state is a valid strategy to delay aging.

A point mutation in the Drosophila gene technical knockout (tko), encoding mitoribosomal protein S12, was previously shown to cause a phenotype of respiratory chain deficiency, developmental delay, and neurological abnormalities similar to those presented in many human mitochondrial disorders, as well as defective courtship behavior. Here, we describe a transcriptome-wide analysis of gene expression in tko^25t mutant flies that revealed systematic and compensatory changes in the expression of genes connected with metabolism, including up-regulation of lactate dehydrogenase and of many genes involved in the catabolism of fats and proteins, and various anaplerotic pathways. Gut-specific enzymes involved in the primary mobilization of dietary fats and proteins, as well as a number of transport functions, were also strongly up-regulated, consistent with the idea that oxidative phosphorylation OXPHOS dysfunction is perceived physiologically as a starvation for particular biomolecules. In addition, many stress-response genes were induced. Other changes may reflect a signature of developmental delay, notably a down-regulation of genes connected with reproduction, including gametogenesis, as well as courtship behavior in males; logically this represents a programmed response to a mitochondrially generated starvation signal. The underlying signalling pathway, if conserved, could influence many physiological processes in response to nutritional stress, although any such pathway involved remains unidentified. These studies indicate that general and organ-specific metabolism is transformed in response to mitochondrial dysfunction, including digestive and absorptive functions, and give important clues as to how novel therapeutic strategies for mitochondrial disorders might be developed.

Caenorhabditis elegans clk-1 mutants cannot produce coenzyme Q₉ and instead accumulate demethoxy-Q₉ (DMQ₉). DMQ₉ has been proposed to be responsible for the extended lifespan of clk-1 mutants, theoretically through its enhanced antioxidant properties and its decreased function in respiratory chain electron transport. In the present study, we assess the functional roles of DMQ₆ in the yeast Saccharomyces cerevisiae. Three mutations designed to mirror the clk-1 mutations of C. elegans were introduced into COQ7, the yeast homologue of clk-1: E233K, predicted to disrupt the di-iron carboxylate site considered essential for hydroxylase activity; L237Stop, a deletion of 36 amino acid residues from the carboxyl terminus; and P175Stop, a deletion of the carboxyl-terminal half of Coq7p. Growth on glycerol, quinone content, respiratory function, and response to oxidative stress were analyzed in each of the coq7 mutant strains. Yeast strains lacking Q₆ and producing solely DMQ were respiratory deficient and unable to support ⁶either NADH-cytochrome c reductase or succinate-cytochrome c reductase activities. DMQ₆ failed to protect cells against oxidative stress generated by H₂O₂ or linolenic acid. Thus, in the yeast model system, DMQ does not support respiratory activity and fails to act as an effective antioxidant. These results suggest that the life span extension observed in the C. elegans clk-1 mutants cannot be attributed to the presence of DMQ per se.