PROJECT SUMMARY/ABSTRACT Over 6.5 million people in the United States currently live with progressive neurodegenerative Alzheimer’s disease (AD) and related dementias including dementia with Lewy bodies (DLB), yet no treatments to stop their progression are available. The presence of aggregated proteins in the brain (tau, amyloid β (Aβ), and α- synuclein) is thought to underlie disease, and clinical overlap in presentation of these diseases associates with the presence of more than one type of pathology. The factors influencing cellular and network vulnerability to each individual pathology or co-occurring pathologies are not well-understood. A proper understanding of vulnerability in the context of co-pathologies is necessary for the development and evaluation of novel disease- modifying treatments. This project will test the hypothesis that progression of intracellular α-synuclein and tau pathologies are bound by neuroanatomical connectivity but influenced by network and cell-level vulnerability. Network and cell-level vulnerability may also be influenced by additional factors such as the presence of co-pathologies, including extracellular amyloid β (Aβ) plaques. To test this hypothesis, the following aims will be pursued: (i) progression of α-synuclein and tau pathologies will be mapped in wildtype mice bearing single or multiple pathologies at high resolution using registration to a common neuroanatomical atlas, and network parameters of progression will be assessed with computational modeling (Specific Aim 1); quantitative pathology and network analysis will be utilized to assess the impact of Aβ plaques on the development, spread, and network vulnerability to α-synuclein and tau co-pathology in knock-in mice that express human tau and develop Aβ plaques and in a time-dependent manner (Specific Aim 2); spatial transcriptomics of human AD brain, DLB brain, and mouse co-pathology brain will be used to determine unique molecular signatures of neurons vulnerable to α-synuclein or tau pathologies. Vulnerability signatures will be validated with combined in situ hybridization-immunofluorescence and integrated with network vulnerability and regional gene expression to develop and evaluate a model of human co-pathology progression as a resource for understanding disease progression and evaluating therapeutic efficacy (Specific Aim 3). The proposed research is expected to improve our understanding of the spatiotemporal progression of co- pathologies and to develop improved tools to evaluate efficacy of novel therapeutic strategies that could slow the progression of neurodegenerative diseases.