A human iPSC model of Tauopathies engineered for 4R Tau isoform expression endogenously develops late-stage neuronal Tau pathology
Tauopathies, such as Alzheimer's disease and Frontotemporal Dementia, are common neurodegenerative diseases characterized by misfolding, hyperphosphorylation, and aggregation of Tau. Molecular mechanisms underlying Tauopathies are still poorly understood, which is in part due to a lack of human models autonomously developing major disease hallmarks. Formation of late-stage disease phenotypes may require adult Tau isoform expression, which contributes to Tau pathogenesis but is challenging to replicate in human stem-cell-derived systems, thus impeding research on underlying mechanisms and drug development. Here we show that induction of adult human brain-like 4R Tau isoform expression enables cell-intrinsic formation of late-stage Tauopathy hallmarks in iPSC-derived neurons engineered to contain synergistic Tau mutations, without exogenous sources of Tau pathology. Neurons accumulated seeding-competent, hyper-phosphorylated, fibrillar Tau in tangle-like structures. Furthermore, exclusive expression of mutant 4R in the absence of the 3R Tau isoform disproportionately intensified pathology, resulting in highly abundant Tau misfolding and aggregation. Finally, we provide proof-of-principle that our model can be translationally applied both to test chemical disease modulators and evaluate human Tau PET tracers. Collectively, our model corroborates the central role of 4R Tau isoform expression for pathogenesis in human neurons and enables novel investigations to elucidate mechanisms underlying human Tauopathy formation. Moreover, it may also serve as a platform supporting urgently needed development of disease-modifying drugs. Overall design: Bulk RNA sequencing was performed on human induced pluriptent stem cell derived cells during dual-SMAD-inhibition directed cortical neuron differentiation to investigate changes occuring as a result of gene editing. Samples include wild type and 4 different lines with edits of the endogenous MAPT locus, as well as wild-type cells treated with Nocodazole as a control. Three biological replicates (i.e. independent differentiations) were analyzed for each sample at  day in vitro (DIV) 15 and 35.