Dynamics of activity-induced transcription in single dentate granule cells

Project Summary In brain regions involved in learning and memory, a subset of neurons active in response to a novel experience is critical for encoding and later recalling a memory of that experience. These neurons, known as engram cells, have been identified as vital contributors to memory function in areas such as the hippocampus. Studies of engram cells have overwhelmingly used immediate-early gene (IEG) protein products to segregate active from inactive cells. Yet, the downstream processes that allow IEG-expressing cells to encode memories in a way that makes them selective to a particular event remain unknown. Single-nuclei RNA-Seq (snRNA-Seq) now offers an opportunity to reveal molecular changes in these engram cells at the single neuron level. Within the hippocampus, individual, active, IEG-expressing dentate granule cells (DGCs) show more dramatic transcriptional changes than other hippocampal cell types when mice explore a novel environment. Long after IEGs such as Fos and Arc decline, transcription of later waves of activity-regulated genes continues to distinguish active and inactive DGCs for at least 24hr after an initial stimulating event. While the hippocampus as a whole is critical for memory function, the unique contribution of the dentate gyrus is theorized to be discriminating between memories of similar events, a process termed pattern separation. This project will seek to link transcriptional changes in individual dentate gyrus engram cells to the dentate gyrus’ role in pattern separation. The underlying hypothesis is that activity-induced transcription sets the selectivity of DGC engram cells and in turn the effectiveness of the dentate gyrus in discriminating between similar memories. A series of experiments will compare activity-induced transcription and engram cell selectivity under standard cognitive conditions, enhanced cognitive conditions, and impaired cognitive conditions. First, a candidate gene upregulated in activated DGCs from previous snRNA-Seq results, proenkephalin (Penk), will be manipulated in healthy adult mice. Penk has received considerable attention in studies of reward and addiction, and this project will now test a novel role in influencing engram cell selectivity. Second, transcriptional changes in individual DGCs in control mice will be compared those in runners, who have enhanced dentate gyrus function and ability to discriminate similar memories. Finally, transcriptional changes in individual DGCs will be compared between wildtype mice and APP/PS1 mice, a model of Alzheimer’s Disease with impaired discrimination ability. The experiments described here will identify detailed transcriptional signatures of individual dentate gyrus engram cells during memory formation and connect these molecular changes to the dentate gyrus’ critical role in pattern separation. The results obtained from this work will reveal fundamental biological processes that can be targeted to enhance cognitive function in health and combat devastating memory decline in neurodegenerative disease.