Mapping the epigenomic and transcriptomic interplay during memory formation and recall in the hippocampal engram ensemble
Purpose: we utilized a mouse model that permanently labels neurons activated throughout a specific experience to decipher the interplay between chromatin accessibility, 3D-chromatin architecture and transcriptional changes across different memory phases. Non activated (basal) and activated neurons during memory encoding (early), consolidation (late) and recall (reactivated) were sorted and subjected to nuclear RNA sequencing (nRNA-seq) to determine gene expression, ATAC-seq (Assay for Transposase-Accessible Chromatin using sequencing) to assess chromatin accessibility, chromosome conformation capture (Hi-C) to identify global 3D -genome architecture and promoter capture Hi-C to identify the long range promoter-enhancer interactions.  Results: Our analysis revealed dynamic changes in compartments organization, where we observed re-localization of large chromatin segments from inactive to permissive environment (and vice versa) during the initial and late phase of memory formation. Interestingly, 52% of the regions in the early phase that switched from B to A maintained that state in the late phase (i.e. remained in state A. Moreover, nearly all these regions significantly overlapped with gained DARs identified in our ATAC-seq analysis, confirming the transition of sub-compartment from inactive to permissive environment. This data indicates that while some loci undergo sub-compartment switching across different memory phases, the majority are stable, correlate with chromatin accessibility and therefore might contribute to long-term changes in neuronal properties and function after initial activation. Overall design: We were interested in delineating the precise changes that occur in 3D nuclear organization to facilitate functionally-relevant promoter-enhancer interactions and transcriptional alterations. Hi-C libraries were prepared from 100,000 FACS sorted cells/per group (collected from two animals) in two biological replicates, using DovetailTM Hi-C kit manual (v.1.03, Dovetail Genomics, Chicago, USA).