Molecular Genetic Analysis of TORC1 and TORC2 Signaling in Neuronal Maintenance

Stanford University, Stanford CA

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Abstract

Project Summary Mitochondrial dysfunction and protein homeostasis (proteostasis) failure are common among diverse brain disorders, from neurodegenerative disease and stroke/traumatic brain injury to brain tumor and psychiatric diseases. It is not clear whether these seemingly disparate pathological features are mechanistically interconnected. Mitochondria play important roles in bioenergetics as well as other essential aspects of cellular physiology, such as calcium buffering, intermediary metabolism, and apoptosis. Maintaining mitochondrial quality and quantity is essential for tissue homeostasis, especially the highly energy-demanding neuromuscular tissues. Pten-induced kinase 1 (PINK1) and Parkin (encoding an E3 ubiquitin ligase), two genes associated with familial Parkinson's disease (PD), constitute a genetic pathway important for maintaining mitochondrial health and neuromuscular integrity. Identification of this pathway offers a much-needed entry point to decipher the relationship between mitochondrial dysfunction and other pathological hallmarks of disease. Our studies in previous funding cycles revealed that PINK1/Parkin directs an interconnected mitochondrial quality control process important for neuromuscular tissue integrity. This process encompasses biogenesis of respiratory chain, mitochondrial fission/fusion dynamics, transport, and mitophagy, and is regulated by mechanistic target of rapamycin complexes (mTORC1 and mTORC2). Importantly, in the last funding cycle, we found that ribosome-associated quality control (RQC), a recently recognized protein quality control mechanism that surveys the translating ribosomes for faulty translation products to safeguard proteostasis, is an important player in PINK1/Parkin-directed mitochondrial homeostasis. These new findings thus provide a mechanistic link between mitochondrial function and cellular proteostasis. We found that a highly conserved RQC factor responds to mitochondrial stress and PINK1 dysfunction and is required for stress adaptation. Activation of this RQC factor effectively rescued PINK1 mutant phenotypes. Moreover, our preliminary studies identified several upstream factors that regulate the post-translational modification (PTM) of this RQC factor. These results laid the foundation for understanding the normal physiological function of this key RQC factor and the upstream signaling mechanisms that regulate its activity in RQC and cellular homeostasis. We will use a powerful combination of molecular genetics, genomics, cell biology, and biochemistry approaches, and move between in vivo fly models, mouse models, and human iPSC-derived cell culture models to seek common mechanisms. We will test the hypothesis that this RQC factor directs the quality control of the translation of mitochondrial outer membrane-associated cytosolic mRNAs to promote mitochondria and neuromuscular tissue homeostasis under stress, and that it provides a signaling node for integrating diverse upstream regulatory inputs to ensure the efficiency and fidelity of this translation process.