Non-proteolytic roles of the ubiquitin-proteasome system in memory formation across the lifespan

Abstract

Memory formation and decline are driven by complex molecular mechanisms and region-specific changes in the brain. Epigenetic modifications, such as histone monoubiquitination and ubiquitin signaling, have emerged as key players in these processes. While monoubiquitination of histone H2B is a well-established regulator of increased gene transcription in memory formation, the role of histone H2A monoubiquitination (H2Aubi), a potent transcriptional repressor, remains largely unexplored. Similarly, although the canonical role of the ubiquitin-proteasome system (UPS) has been extensively studied during memory processing, less is known about its non-proteolytic functions, such as lysine-63 (K63) polyubiquitination. Here, we investigated the role of histone H2Aubi in memory formation and K63 polyubiquitination in age-related memory decline, focusing on the hippocampus and amygdala—two regions critical for memory processing. We observed global and gene-specific decreases in H2Aubi in the amygdala following fear conditioning. Notably, H2Aubi levels decreased at the Pten coding gene, a key inhibitor of PI3K-AKT-mTOR signaling, accompanied by increased PTEN protein expression. CRISPR-dCas9 mediated upregulation of the H2Aubi ligase, Ring1b, in the amygdala enhanced contextual memory. This suggests that reductions in H2Aubi constrain fear memory potentially through PTEN-mediated regulation of mTOR signaling. Additionally, we explored the role of K63 polyubiquitination in age-related memory decline. Using unbiased proteomic analysis, we observed significant increases in K63 polyubiquitination protein targets in the hippocampus across the lifespan. CRISPR-dCas13 mediated reduction of K63 polyubiquitination in the hippocampus of aged male rats reversed contextual fear memory impairments, whereas similar manipulations in middle-aged rats with normal memory had no effect, highlighting the age-specific role of K63 polyubiquitination in memory. Conversely, the amygdala exhibited consistent reductions in K63 polyubiquitination with age, and further decreasing K63 polyubiquitination improved memory retention in aged but not middle-aged. Together, our findings reveal novel region- and age-specific roles of histone H2Aubi and K63 polyubiquitination in modulating fear memory and age-related memory decline.