Increasing degradation-independent linear polyubiquitin in the hippocampus enhances memory in young adult but not aged rats

Age-related memory loss affects approximately 40% of the world's population after the age of 65 and is a significant risk factor for the development of dementia and Alzheimer's Disease (AD). Numerous studies have reported that late in life there decreases in the function of the ubiquitin-proteasome system, the main regulator of protein degradation in cells that is also critically involved in memory formation. However, ubiquitin can mark proteins for fates other than destruction by the proteasome. Importantly, it remains unknown how the aging process alters proteasome-independent forms of ubiquitination and how this could contribute to age-related memory loss. Here, using an unbiased proteomic approach, we found that linear polyubiquitination - the only non-lysine proteasome-independent form of polyubiquitination - is significantly increased in the aged hippocampus at rest relative to young adults. However, in response to learning there was a significant reduction in linear polyubiquitination in the aged hippocampus, which contrasted with increases seen in young adult animals following learning. CRISPR-dCas9 mediated upregulation of linear polyubiquitination in the hippocampus improved memory in young adult, but not aged, rats. Together, these data suggest that while linear polyubiquitination is a critical regulator of hippocampus-dependent memory, increasing it in the aged hippocampus is not sufficient to improve memory in advanced age. These findings advance our understanding of the molecular mechanisms regulating memory late in life and stimulate future research on the role of degradation-independent ubiquitination in this process.