Project Summary/Abstract Mitochondria play essential roles in energy production and biosynthesis of a subset of macromolecules necessary for eukaryotic life. They also house potentially dangerous intracellular machinery capable of generating oxidative damage, triggering inflammatory signaling, and initiating programmed cell death. Mitochondrial homeostasis is critical for maintaining a healthy pool of metabolically active mitochondria and avoiding cellular damage caused by the accumulation of deteriorating mitochondria. One key component of mitochondrial homeostasis is the selective degradation of old, damaged, or superfluous mitochondria. Recent efforts have elucidated in great detail the molecular mechanisms through which experimentally damaged mitochondria are degraded in Parkin-expressing eukaryotic cells. However, it is clear that mammalian cells also undergo constant renewal of mitochondrial content through biogenesis of new mitochondria coupled to degradation of old mitochondria, even in the absence of exogenous damage. The mechanisms controlling this ongoing mitochondrial turnover are poorly understood. This proposal aims to illuminate the signal transduction pathway that drives mitochondrial degradation in unstressed mammalian cells, and to use this pathway as an entry point to understand the roles that basal mitochondrial degradation plays in cellular adaptation and stress resistance. This will be accomplished by answering three key questions: 1) What controls the rate of mitochondrial turnover in the absence of exogenous damage? 2) In the absence of exogenous damage, how are mitochondria selected for degradation? 3) What role does this pathway play in developmentally programmed mitochondrial clearance? These questions will be answered using molecular and genetic techniques on the biochemical, cellular, and organismal scales. Answering these questions will provide substantial insights into the mechanistic details and physiological roles of a fundamental cell biological process. This work will improve understanding of normal development and homeostasis as well as the etiologies of diverse human diseases against which mitochondrial homeostasis protects.