Mutations in the MAPT gene cause Frontotemporal dementia with parkinsonism-17 (FTDP-17) which is pathologically characterised by insoluble hyperphosphorylated tau aggregates. The mechanisms leading to tau aggregation are poorly understood, however, a decline in proteostasis pathways such as the ubiquitin-proteasome system (UPS) and autophagy, has been implicated in multiple neurodegenerative diseases. Induced pluripotent stem cell-derived neurons (iPSC-neurons) with MAPT mutations have been widely used to model tauopathy, however, these cells do not develop tau tangles. Based on transcriptomics and tau splicing signatures, iPSC- neurons largely resemble foetal neurons. The activities of the UPS and autophagy are high during early development, where the rapid turnover of morphogens is essential for neuronal patterning. Therefore, we hypothesised that disruption of proteaostasis degredation pathways may induce tau pathology.

We investigated the levels of proteasome subunits and proteasome activity during WT iPSC-neuron differentiation and examined the effects of proteasome and autophagy inhibition on tau and autophagy-associated proteins in iPSC-neurons with MAPT mutations linked to FTD.

We show that proteasome activity decreases during the differentiation of iPSCs to cortical neurons, accompanied by a significant reduction of the proteasome regulatory subunits Rpt6 and Rpn6. Proteasome inhibition of WT iPSC-neurons led to a change in tau solubility and an increase in the autophagy-associated proteins BAG3 and p62, together with an induction of tau cleaved by caspase-3 at Asp421. We observed a reduction in the cleavage of tau by caspase-3 in iPSC-neurons with 10+16 MAPT mutations after proteasome inhibition, likely due to reduced levels of caspase-3. In addition, iPSC-neurons with 10+16 MAPT mutations display lower levels of BAG3 and p62 after proteasome inhibition compared to WT iPSC-neurons. We were able to recapitulate a number of these observations in 10+16 patient-derived and age-matched control iPSC-neurons, confirming that the differences we observe are likely due to the occurrence of higher levels of 4R tau in 10+16 MAPT mutations.

In conclusion, mutations in the MAPT gene alter proteostasis pathways in iPSC- neurons, which could contribute to FTDP-17 progression. Flexible proteostasis in early development may protect against tau pathology, therefore, increasing the proteostasis capabilities of neurons could be an effective therapeutic strategy.