Student-Faculty Collaborative Research Grant Report
Analytical Chemistry | Biochemistry | Biology | Systems Biology
Mitochondria are essential organelles in most eukaryotic cells due to their role in metabolism and ATP production by the oxidative phosphorylation (OXPHOS) pathway, as well as other key cellular processes. Metal cofactors are incorporated into the OXPHOS protein complexes of yeast, and misincorporation or modulation of available metals, such as copper and iron, in yeast mitochondria leads to the production of reactive oxygen species (ROS). ROS are reactive molecules containing oxygen such as peroxides, superoxide, and hydroxyl radicals. Yeast are a good model for studying aging and the effect of ROS on lifespan because they are easy to grow, and many of the genes and proteins involved in determining yeast lifespan are conserved in humans and other mammals. Specifically, chronological lifespan assays in yeast rely on promoting a quiescent stationary phase by calorie restriction, a phase characterized by curtailed overall metabolic rate and a shift from fermentation to mitochondrial respiration. This shift in glucose utilization can be accomplished by reduced TOR (Target of Rapamycin) pathway signaling, a complex regulator of cell growth and cell cycle. Many downstream effects of TOR signaling can be silenced by cell treatment with Rapamycin, a drug that binds to Tor1p and inhibits kinase domain function, drastically increasing a cell culture’s chronological lifespan.
Mitochondrial OXPHOS complexes, whose activity and expression is modulated by TOR signaling, is the primary site of superoxide formation. A primary defense of free radical damage is the neutralization of superoxide by the enzyme superoxide dismutase 1 (Sod1), which requires copper and zinc as cofactors. Copper is additionally utilized in the OXPHOS complex cytochrome c oxidase as an electron transferring group. Given its roles in defending against and production of ROS, copper was modulated by extracellular supplementation to further elucidate its functionality in the context of Rapamycin treatment and SOD1 deletion strains. Copper supplementation on Rapamycin treated SOD1 mutants results in cell viability disadvantage over most of a chronological lifespan. SOD1 mutants’ sensitivity to ROS can be aggravated by increased intracellular copper, leading to metal toxicity. However, copper supplementation is advantageous to lifespan in SOD1 mutants when treated with Rapamycin. This indicates a functional consequence of Rapamycin treatment in suppressing overall metabolic activity. In cells lacking Rapamycin treatment there could be a greater metabolic demand for copper to function in cytochrome C oxidase, and supplementation ensures that more ETC machinery is able to operate, lessening electron leakage and subsequent ROS production.
Bestwick, Megan, "Student-Faculty Collaborative Research Grant Report" (2017). Post-Grant Reports. Report. Submission 111.