PROJECT SUMMARY/ABSTRACT Aging-associated brain disorders, including cognitive decline, are among the greatest public health challenges. DNA repair is emerging as a potential regulator of age-related cognitive decline and neurodegeneration, and may be a powerful potential target for effective therapeutic strategies in the future. The brain may be vulnerable to genomic alterations due to its network structure, the complexity of its transcriptome, and the low or absent turnover and long lifespan of neural cell types. This suggests genome maintenance pathways are crucial for brain health: persistent or incorrectly repaired DNA double-strand breaks (DSBs) could contribute to genomic alterations, thus promoting age-related cognitive impairment and neurodegenerative disorders. However, the role of post-developmental, neuronal DSB repair in brain physiology with age has not been addressed. The broad implication for this fundamental gap in knowledge is that crucial opportunities for development of therapeutics for treatment and prevention of brain disorders may be missed. This provides a strong rationale for elucidating the biology of neuronal DSB repair at multiple levels. Thus, our long-term goal is to determine the extent to which neuronal DNA double-strand break formation and repair impact brain function and brain disorders. We will elucidate the relationship between neural circuit function and the classical non-homologous end-joining (C-NHEJ) DNA repair machinery in neurons with age. Moreover, we will elucidate the extent to which post-developmental, neuronal DSB repair suppresses brain aging phenotypes related to chromatin structure, genome organization, and gene expression. The central hypothesis of the proposed project is that DNA double-strand break formation and repair in mature neurons impacts neural physiology. To test this hypothesis and to advance toward our long-term goal, we propose the following specific aims: (1) Define consequences of aging and C-NHEJ inactivation in neurons at the cellular and genomic level; (2) Elucidate impact of aging and C-NHEJ inactivation on the neuronal epigenomic landscape; and, (3) Determine impact of aging and C-NHEJ repair on circuit-level neuronal physiology. The proposed approach involves a comprehensive, multidisciplinary analysis of neuronal function at the genomic, epigenomic, organismal, and neural circuit level. The proposed project is significant because it uses innovative approaches to investigate emerging concepts with major implications for human brain health, age-related cognitive decline, and neurodegenerative diseases. Further, the project will lead to the development of new research tools and models. Insights gained from the proposed studies are also expected to inform research and knowledge in other fields related to genomic stability and aging.