Investigating proteostasis in development and disease using IPSC‐neurons with MAPT mutations linked to FTD

Abstract Background Mutations in the microtubule‐associated protein tau (MAPT) are causative of Frontotemporal Dementia (FTD). Many of the features associated with the development of tau pathology, such as high levels of tau phosphorylation, are also present in early development. iPSC‐neurons have gene expression signatures similar to fetal neurons, and iPSC‐neurons with MAPT mutations do not develop tau aggregates. We hypothesise that iPSC‐neurons are resistant to developing tau aggregates due to high activity levels of the proteostasis network during early development. To test this hypothesis we investigated the RNA and protein levels of proteasome subunits (B5, RPT6, PSMD11) and associated proteins (TRIM11, BAG3) in developing MAPT mutated iPSC derived neurons. Methods Human cortical neurons were derived from isogenic iPSCs with the following MAPT genotypes: WT, 10+16 monoallelic, 10+16 biallelic and 10+16/P301S biallelic. RNA and protein analysis of proteasome subunits was performed by qPCR and Western blot at Days 0, 10, 30 and 100, spanning the different stages of neurogenesis. Results Gene and protein expression of the core proteasome subunit B5 remains consistent throughout development. The expression of regulatory proteasome subunits PSMD11 decreases at both the protein and RNA level. RPT6 decreases at the protein level but increases at the RNA level. BAG3 significantly decreases at the protein level. There is no significant difference in protein expression between disease and WT lines. Conclusion This data indicates that the core 20S proteasome unit remains at consistent levels throughout development, however the expression of regulatory subunits differ. A change in regulatory subunit expression could have a direct impact on the ability of the proteasome to degrade proteins. Ongoing work is using proteasome activity assays to determine how this changes through development and is impacted by MAPT mutations.