ApoE isoforms differentially regulate neuronal membrane proteasomes to shift the threshold for pathological aggregation of endogenous Tau

Neuroproteasomes are a subset of 20S proteasomes that are localized to the neuronal plasma membrane and degrade newly synthesized proteins. To date, the molecular composition of neuroproteasomes is undefined, and moreover, whether neuroproteasomes can influence protein aggregation with relevance to neurodegenerative disorders remains unexplored. Using a Cre-dependent conditional knock-in mouse line to endogenously tag the proteasome, we find that neuroproteasomes co-purify with ApoE, the most significant risk factor for late-onset Alzheimers Disease (AD). We discover that neuroproteasome membrane localization is differentially modulated by ApoE isoforms (E4<E3<E2) in vitro, in vivo, and in human postmortem samples. We synthesized selective, neuroproteasome-specific inhibitors and discovered that neuroproteasome inhibition induces aggregation of endogenous mouse and human Tau, without the need for seeding or pathogenic mutations. Using hApoE-KI/hTau-KI crosses, we find that ApoE isoforms differentially shift the aggregation threshold for Tau. Neuroproteasome inhibition in vivo is sufficient to induce sarkosyl-insoluble and Thioflavin-S positive endogenous Tau aggregates in only three days, which are completely abrogated by co-application of cycloheximide. Newly synthesized Tau levels increase threefold after neuroproteasome inhibition, leading us to posit that newly synthesized Tau is uniquely susceptible to aggregation due to neuroproteasome dysfunction. Overall, our data define neuroproteasomes as a pivotal proteostasis mechanism underlying the formation of endogenous Tau aggregates, which is directly regulated by the largest genetic risk factor for late-onset Alzheimers Disease.