Project Summary/Abstract Age-related neurodegenerative diseases are a class of incurable diseases that result in the progressive degeneration of neuronal cells. Mitochondria are essential cellular organelles that are key sources of ATP generated by oxidative phosphorylation. Mitochondria contain their own DNA and protein synthesis machinery, yet 99% of mitochondrial proteins are encoded by nuclear DNA and must be imported into the mitochondria. Mitochondria are dynamic in both their composition and structure, forming dynamic tubular networks through fission and fusion reactions. In neurodegenerative disorders such as Alzheimer’s, Parkinson’s and Huntington’s disease, there is a phenomenon known as mitochondrial fragmentation, in which the normal, elongated shape of mitochondria becomes disrupted and they instead take on a more fragmented, round appearance. How mitochondrial fragmentation leads to mitochondrial dysfunction is unknown, but it is important to understand as inhibition of mitochondrial fragmentation has been found to be protective in disease models of Alzheimer’s, Parkinson’s and Huntington’s disease. We propose a novel hypothesis that stable localization of nuclear-encoded mitochondrial mRNAs to the mitochondrial surface and co-translational targeting of proteins to individual mitochondrial fragments leads to heterogeneity in the expression of mitochondrial proteins across mitochondrial fragments driving mitochondrial dysfunction. This proposal seeks to establish basic principles of this hypothesis. To accomplish this, we will (Aim1) quantify the heterogeneity of protein expression across different mitochondrial morphologies driven by human disease inducing proteins Aβ42 and Htt103Q. Secondly, we will (Aim2) test the impact of mRNA localization and co- translation mitochondrial protein insertion on protein heterogeneity and mitochondrial dysfunction. Finally, we will (Aim3) establish tools for exploring this question in mammalian models by exploring mitochondrial mRNA localization and protein heterogeneity in mammalian cells. In the short term our goal is to establish a new model that explains how mitochondria become dysfunctional across different mitochondrial morphologies. In the long term we will build on these discoveries by conducting further investigations using mammalian models of neurodegeneration.