The impact of Alzheimer's disease on novel genetic signatures of hippocampal memory traces

PROJECT SUMMARY / ABSTRACT Memory loss is a devastating symptom of age-related neurodegenerative disorders such as Alzheimer’s disease (AD). Memories are thought to be stored in the brain in neural ensembles called engrams, which are traces of individual memories that are active during both memory encoding and memory retrieval. In a mouse model of AD, prior research has shown that the APP/PS1 mice are impaired in retrieval of a contextual fear conditioning (CFC) memory, and this memory deficit is paralleled by impaired engram reactivation. While past studies have identified and manipulated these ensembles in the context of AD, researchers have not fully elucidated the molecular changes occurring in engram cells that contribute to this memory deficit. This proposal addresses this gap in knowledge by determining the gene expression landscape that distinguishes engram cells from non- engram cells in the HPC, and how this landscape changes with age and AD. These experiments will use cutting- edge labeling techniques and single-nuclear RNA sequencing (snRNA-seq) to test the hypothesis that engram cells show transcriptional signatures unique from those of cells active during memory encoding or retrieval alone, that these signatures are required for proper engram reactivation, and that they are lost in APP/PS1 mice. Aim 1A uses an intersectional transgenic approach with the activity-dependent labeling mouse line ArcCreERT2 x Sun1-GFP to indelibly tag nuclei of cells active during memory encoding and retrieval. HPC tissue will be dissociated, sorted via flow-activated nuclear sorting (FANS), and profiled via snRNA-seq to compare encoding- active, retrieval-active, and reactivated engram cells. In Aim 1B, select highly differentially enhanced genes (DEGs) in reactivated engram cells will be validated by fluorescence in situ hybridization (FISH), then artificially knocked down via virally induced RNA interference (RNAi) to test whether these key targets are necessary for engram reactivation and subsequent memory retrieval. Lastly, Aim 2 uses a similar tagging strategy and behavioral paradigm in 6-month-old APP/PS1 (+ or -) x ArcCreERT2 x Sun1-GFP mice to determine how engram transcriptomes diverge between control and disease states. This experiment tests the hypothesis that engrams of APP/PS1 mice lack the signatures that define engrams in age-matched controls, underlying their deficit in memory performance. Successful completion of these aims will illuminate with unprecedented resolution how memories are stored in the HPC and how they change with AD, unveiling a suite of key molecular players that are critical to memory storage and may serve as therapeutic targets. The training plan of this proposal includes several novel technical skills, including stereotaxic surgery, in situ hybridization, RNAi-mediated knockdown, and advanced bioinformatic analysis. Additionally, it describes myriad professional scientific skills such as grantsmanship, scientific writing, and oral presentation that will be fostered in the richly collaborative research environment of this application, setting the stage for a successful independent research career.