Basic Science and Pathogenesis

Progressive supranuclear palsy (PSP) is the most common primary tauopathy, with a constellation of pathological features including 4R-tau positive neurofibrillary tangles and tufted astrocytes. Most PSP cases are sporadic and associated with common structural variation in the 17q21.31 MAPT locus as well as other loci, including EIF2AK3 which is critical for the integrated stress response (ISR). Despite these known genetic risk associations, mechanisms underlying disease pathogenesis are unclear. To investigate candidate mechanisms, there is a critical need for model systems that better recapitulate the cellular complexity of the human brain. Induced pluripotent stem cell (iPSC) patient-derived organoid models are a powerful tool to study molecular and cellular changes in a disease-relevant genomic context. Single-nucleus RNA sequencing (snRNA-seq) was performed in the subthalamic nucleus region from autopsy PSP and control brains. Transcriptional differences were validated by immunohistochemistry (IHC) using antibodies for ISR activation markers and phosphorylated tau (p-tau). Fibroblasts grown from sporadic PSP patient skin were reprogrammed into iPSCs, and midbrain organoids were generated in spinner flasks through pharmacological directed differentiation. Total tau, tau isoform, p-tau, and ISR activation markers were assessed in PSP and control organoids. Differential gene expression and pathway analysis in PSP brain snRNA-seq data identified dysregulated EIF2 signaling, a target of the ISR, in vulnerable cell types. Histological validation in autopsy brain tissue showed PSP-vulnerable brain regions had the highest frequency of ISR activation, while no activation was detected in protected brain regions. ISR activation positively correlated with tau burden and was localized to p-tau neurons and astrocytes. PSP organoids contained increased high molecular weight p-tau and 4R-tau, a higher ratio of p-tau:total tau, and different ISR activation levels compared to controls. SnRNA-seq and neurohistological data reveals ISR dysregulation in disease-affected cell types in PSP brain tissue. ISR activation positively associates with tau burden in vulnerable brain regions in PSP, providing a potential mechanistic link with EIF2AK3 genetic risk. PSP patient-derived organoids recapitulate key disease-relevant features, including elevation of toxic tau proteoforms and ISR dysregulation. This sporadic PSP organoid model will provide insight into cell-type specific dri