Learning and Sleep Have Divergent Effects on Cytosolic and Membrane-Associated Ribosomal mRNA Profiles in Hippocampal Neurons

Abstract

The hippocampus plays an essential role in consolidating transient experiences into long-lasting memories. Memory consolidation can be facilitated by post-learning sleep, although the underlying cellular mechanisms are undefined. Here, we addressed this question using a mouse model of hippocampally-mediated, sleep-dependent memory consolidation (contextual fear memory; CFM), which is known to be disrupted by post-learning sleep loss. We used translating ribosome affinity purification (TRAP) to quantify ribosome-associated RNAs in different subcellular compartments (cytosol and membrane) and in different hippocampal cell populations (either whole hippocampus, Camk2a+ excitatory neurons, or highly active neurons expressing phosphorylated ribosomal subunit S6 [pS6+]). Using RNA-seq, we examined how these transcript profiles change as a function of sleep vs. sleep deprivation (SD) and as a function of prior learning (contextual fear conditioning; CFC). Surprisingly, we found that while many mRNAs on cytosolic ribosomes were altered by sleep loss, almost none were altered by learning. Of the few changes in cytosolic ribosomal transcript abundance following CFC, almost all were occluded by subsequent SD. This effect was particularly pronounced in pS6+ neurons with the highest level of neuronal activity following CFC, suggesting SD-induced disruption of post-learning transcript changes in putative "engram" neurons. In striking contrast, far fewer transcripts on membranebound (MB) ribosomes were altered by SD, and many more mRNAs (and lncRNAs) were altered on MB ribosomes as a function of prior learning. For hippocampal neurons, cellular pathways most significantly affected by CFC were involved in structural remodeling. Comparisons of post-CFC transcript profiles between freely-sleeping and SD mice implicated changes in cellular metabolism in Camk2a+ neurons, and increased protein synthesis capacity in pS6+ neurons, as biological processes disrupted by post-learning sleep loss.