Free radicals run in lizard families: a mitochondrial uncoupling phenomenon

Olsson and co-authors have recently published interesting papers on oxidative stress in lizards. The aim is to understand the mechanisms defining ageing rates in free-ranging animals, particularly by looking at mitochondrial production of reactive oxygen species (ROS). Indeed, ROS, and more specifically superoxide, are chronically formed at different sites of the electron transport chain (ETC) during oxidative phosphorylation. The induced potential oxidative stress has been proposed by the ‘free radical theory of ageing’ to result in accumulation of macromolecule damage that causes ageing (Beckman & Ames 1998). Olsson’s studies report that differences in levels of ROS production exist among families and suggest that these differences can be driven by natural selection. In their recent paper (Olsson et al. 2008), the authors demonstrated that ROS production levels are heritable, a relevant result in understanding the physiological basis of life-history traits. This study is based on an innovative approach using cell biology methods to tackle a question relating to evolutionary biology. To evaluate ROS production within the mitochondria, superoxide production was measured by flow cytometry. Interestingly, mitochondrial ROS production was modified using carbonyl cyanide 3-chlorophenylhydrazone (CCCP), a proton-ionophore uncoupler that reduces the mitochondrial membrane potential by allowing the protons to cross back through the inner membrane by a pathway other than ATP synthase. CCCP is therefore accelerating the working rate of the ETC, resulting in an enhanced consumption of oxygen for an identical mitochondrial electrochemical gradient (uncoupling effect). The authors argue that ‘CCCP administration mildly uncouples mitochondria, which increases electron flow through the ETC [.] may lead to increased ROS production.’. Accordingly, they found an increased ROS production after CCCP treatment, which is heritable in lizard families. In this paper the authors conclude that using their ‘uncoupled superoxide measurement, the extremely high heritability of superoxide in combination with CCCP uncoupling, suggests strong family effects on how this molecule affects the proton gradient’ (Olsson et al. 2008).