Defining how mitochondria generate reactive oxygen species (ROS) and control its production

Mitochondria are dynamic semi-autonomous organelles of symbiotic origin that are involved in fulfilling a number of cell functions. These functions depend on mitochondrial bioenergetics and ATP and ROS production. ATP production begins with the oxidation of fuels by mitochondrial dehydrogenases and flavoproteins housed in the Krebs cycle and other metabolic pathways, which liberate electrons that are then passed to the respiratory chain. Energy released from the flow of electrons through the respiratory chain is then trapped for ATP production. Once formed, ATP is released into the cytoplasm to perform useful “work” throughout the cell. ROS production also depends on the same fuel oxidizing and electron transferring pathways required to make ATP. However, in contrast to ATP, ROS has a bi-functional relationship with mammalian cells; high levels of ROS can be deleterious but low amounts are required for cell signaling. This latter function is related to the capacity of ROS to oxidize protein cysteine thiols, also known as “cysteine switches”, which activates and deactivates proteins involved in cell division, growth, stress signaling, autophagy, or apoptosis. Like any other secondary messenger, ROS levels are controlled through the rate of its production and degradation. However, even though it is clear that mitochondria employ ROS as a “mitokine”, we still only have a rudimentary understanding of how mitochondria 1) produce ROS and which enzymes serve as the major generators in different tissues and 2) control its production. Here, I will provide an update on recently published and current research that demonstrates that non-conventional sites of production, like α-ketoglutarate dehydrogenase and pyruvate dehydrogenase, make substantial contributions to the overall amount of ROS released by mitochondria. Moreover, we have found that other sources outside of complexes I and III can make substantial contributions. My group has also observed that mouse sex and strain can influence how much ROS is released from individual sites of production and that ROS genesis from non-traditional sites can contribute to heart disease. I will also discuss findings demonstrating that the redox signal, protein S-glutathionylation, is required to regulate mitochondrial bioenergetics and ROS production and how manipulation of mitochondrial cysteine switches can protect mice from diet-induced obesity.