PROJECT SUMMARY/ABSTRACT Alzheimer’s disease (AD) is a progressive neurodegenerative disease that spreads across the brain from its origin in the medial temporal lobe. The hippocampus is one of the earliest and most affected brain regions in AD and hippocampal atrophy has been linked to the severity of the behavioral symptoms. Although numerous theories have been put forth, the molecular underpinnings of hippocampal neurodegeneration remain unclear. This lack of understanding has stymied AD drug development from translating animal research into human treatment. I believe that a translational cellular atlas that bridges the gap between mouse and human AD research is needed to determine which specific hippocampal cell types are affected by AD. My previous research creating the mouse Hippocampus Gene Expression Atlas (HGEA) lays a strong foundation for this effort (Bienkowski et al., Nature Neuroscience, 2018). The HGEA defines 20 distinct genetic subdivisions of the hippocampus and subiculum based on mapped gene expression patterns, delineates each region’s input/output pathways, and demonstrates how each region contributes to brain-wide networks. Overall, I found that hippocampal gene expression patterns were highly related to specific anatomical connectivity patterns. Among many new insights, I discovered that subiculum gene expression patterns revealed hidden lamina of pyramidal neurons and demonstrated how this laminar architecture underlies a columnar organization similar to the cerebral cortex. Altogether, the HGEA demonstrates the multiscale organization of the hippocampus from individual genetic cell types to neuronal networks regulating animal behavior. I established a number of resources and tools on the Mouse Connectome Project website so that other hippocampal scientists around the world could use the HGEA to guide their own research experiments. The funding of this K01 Mentored Research Scientist Career Development Award application will provide me with training and expertise I need in order to develop a human version of the HGEA as a translational resource to understand AD neurodegeneration. Building on the mouse HGEA, the proposed research uses a cutting-edge spatial transcriptomics approach in thick optically-cleared tissue sections (STARmap) to reveal multiplexed gene expression patterns, build a human HGEA that can be registered to MRI data, and investigate changes to HGEA-defined neuronal cell types caused by hippocampal neurodegeneration within an AD mouse model (5XFAD mice) and humans with AD. To guide this project and my career development, I have assembled a world-class team of supportive mentors (Drs. Arthur Toga and Berislav Zlokovic) and collaborators (Dr. Carol Miller) to provide me with new training to investigate Alzheimer’s disease in both transgenic mouse models and human post-mortem tissue. Ultimately, the funding of this K01 proposal will complete my career development toward leading a translational neuroscience la...