Historically, mitochondrial ROS (mtROS) production and oxidative damage have been associated with aging and age-related diseases such as Parkinson’s disease. In fact, the age-related increase in ROS has been viewed as a cause of the aging process while mitochondrial dysfunction is considered a hallmark of aging, as a consequence of ROS accumulation.
However, pioneering work in Caenorhabidits elegans has shown that mutations in genes encoding subunits of the electron transport chain (ETC) or genes required for biosynthesis of ubiquinone extend lifespan despite reducing mitochondrial function. The lifespan extension conferred by many of these alterations is ROS dependent, as reduction of ROS abolishes this effect. Moreover, chemical inhibition of glycolysis or exposure to metabolic poisons that block respiratory complex I (CI) (rotenone, paraquat, or piericidin A) or complex III (CIII) (e.g., antimycin A) also prolong lifespan in C. elegans in a ROS-dependent manner.
Various studies have shown that ROS act as secondary messengers in many cellular pathways, including those which protect against or repair damage. ROS-dependent activation of these protective pathways may explain their positive effect on lifespan. The confusion over the apparent dual nature of ROS may, in part, be due to a lack of resolution as without focused genetic or biochemical models it is impossible to determine the site from which ROS originate.
A promising path to resolving ROS production in vivo is the use of alternative respiratory enzymes, absent from mammals and flies, to modulate ROS generation at specific sites of the ETC. The alternative oxidase (AOX) of Ciona intestinalis is a cyanide-resistant terminal oxidase able to reduce oxygen to water with electrons from reduced ubiquinone (CoQ), thus bypassing CIII and complex IV (CIV).
NDI1 is a rotenone-insensitive alternative NADH dehydrogenase found in plants and fungi, which is present on the matrix-face of the mitochondrial inner membrane where it is able to oxidize NADH and reduce ubiquinone, effectively bypassing CI. Researchers have demonstrated that allotopic expression of NDI1 in Drosophila melanogaster can extend lifespan under a variety of conditions and rescue developmental lethality in flies with an RNAi-mediated decrease in CI levels.
To determine the role of increased ROS production in regulating longevity, authors utilized allotopic expression of NDI1 and AOX, along with Drosophila genetic tools to regulate ROS production from specific sites in the ETC.
Authors show that NDI1 over-reduces the CoQ pool and increases ROS via reverse electron transport (RET) through CI. Importantly, restoration of CoQ redox state via NDI1 expression rescued mitochondrial function and longevity.
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.
Mitochondrial ROS Produced via Reverse Electron Transport Extend Animal Lifespan
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