UC Berkeley

In order to promote sustained growth and health, the cell must carefully balance its metabolic state, responding to nutrient shortages, acute injury, and altered environment to maintain energetic homeostasis. The machinations of this task are extremely complex, and require extensive sensors and signals to diagnose cellular nutrient state, identify specific issues, and maintain functional balance. Two organelles, the lysosome and the mitochondrion, are central to cellular metabolic processes and host a wide variety of signaling pathways that are used to assess energetic health and capacity. Their coregulation is necessary to support cellular health in instances of energetic stress, and the breakdown of their communication often leads to debilitating degenerative diseases. While much work has been done to understand how the signals and functions of these two organelles overlap during stress, it is less clear what functional changes are initiated by lysosomal signaling itself, and how these may alter mitochondrial function to promote survivability.Lysosomes are the primary catabolic site of the cell, where the autophagic and endocytic pathways converge to deliver cargo for degradation. By this mechanism, the lysosomal lumen reflects the nutrient status of the cellular environment and can use this information to influence the metabolic state of the cell. To this purpose, the lysosomal surface hosts the active state of the kinase mechanistic Target of Rapamycin (mTOR) as part of mTOR Complex I (mTORC1). Here, mTORC1 responds to the presence of insulin, growth factors, and nutrients to promote anabolic processes for cell growth. If any of these inputs are depleted, mTORC1 is rendered inactive and removed from the lysosome, switching the cell to catabolic programming. With this process, mTORC1 is central to maintaining metabolic and energetic homeostasis in the cell. Meanwhile, the mitochondria is the cellular site of bulk metabolite and energetic production. Robust mitochondrial function is required for the survival of high-energy tissues, such as the heart and brain, and is necessary for cellular growth and differentiation. Mitochondrial stress results in depleted energetic supply, toxic redox species, and in some cases, the trigger of apoptotic cell death. Defects in mitochondrial function are thus detrimental to cellular and organismal health. Previous work has established that many signals that reflect mitochondrial stress interact with mTORC1, either directly or by pathway convergence, to coordinate energetic status with whole-cell programming. In many models of mitochondrial disease, coordinated mTORC1 function is required to allow survivability. While we generally understand how mTORC1 interacts with and supports the objectives of mitochondrial stress pathways, what is less clear is how mTORC1 signaling influences mitochondrial function at baseline. Here, I study how acute inhibition of mTORC1 by pharmacological intervention alters mitochondrial function in the absence of induced stress, thereby shedding light on mitochondrial pathways that may be influenced by mTORC1 status. Then, I study how these alterations map to mitochondria who have been injured by depletion of the iron-sulfur cluster biogenesis protein Frataxin (FXN) to identify which pathways may be valuable for mTORC1-supported mitochondrial survival under stress. With functional assays, we identify impaired function of Complex III of the mitochondrial electron transport chain as a result of FXN loss, with a converse restoration when mTORC1 is inhibited. Proteomic analysis of mitochondria in these conditions reveal an alteration of redox homeostasis upon mTORC1 inhibition, irrespective of FXN loss. Further exacerbation of the system by FXN loss then exposes a potential role of mTORC1 signaling for modulation of CoQ biosynthesis machinery, which may represent a newly discovered branch of mTORC1 influence.