A next-generation iPSC-derived forebrain organoid model of tauopathy with tau fibrils by AAV-mediated gene transfer

Summary: It is known that the human cellular models of Alzheimer’s disease (AD) and tauopathy can only recapitulate the very early stage of the disease. To overcome these limitations, we developed a technology to make forebrain organoids (FBOs) from feeder-free induced pluripotent stem cells (iPSC)s by regulating a FGF2 concentration and applied this method to generate FBOs from patients with familial AD (fAD FBOs). The obtained fAD FBOs recapitulated the amyloid-β pathology and increased tau phosphorylation but not tau aggregates. To fully induce the tau pathology, FBOs were injected with adeno-associated virus (AAV)-expressing P301L mutant tau. In these Tau-P301L FBOs, tau fibrils were observed in the neuronal cell body and neurites with immunoelectron microscopy, in addition to the sarkosyl-insoluble and thioflavin S-positive phospho-tau aggregates. Collectively, this model can be used as a platform for investigating pathogenetic mechanisms and evaluation of target molecules for drug discovery for tauopathy. Motivation: One of the reasons why drug development for Alzheimer’s disease (AD) and tauopathy has been difficult to date is the lack of appropriate models. Rodent models cannot fully recapitulate the disease pathology due to species differences between rodents and humans. Due to the low accessibility of patient brain for experimental use, human induced pluripotent stem cell (iPSC) technology is a promising approach for the generation of human cell models as an alternative research tool. However, even using human iPSCs, most cellular models of AD and tauopathy can only recapitulate the early stage of the disease, and it is challenging to display protein aggregation, such as amyloid-β plaques and tau fibrils, which are hallmarks of AD. To overcome these limitations, we made an improvement to increase the efficiency of neuroepithelia formation, leading to an increase in the efficiency of brain organoid generation. Furthermore, we established an efficient system of gene transfer by adeno-associated virus (AAV) injection into brain organoids and applied this to generate brain organoids with pronounced tau pathology, including tau fibrils.