Investigating mechanisms of sporadic progressive supranuclear palsy in autopsy‐validated human iPSC‐derived midbrain organoids and human brain tissue

Abstract

AbstractBackgroundProgressive 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 risk loci, including EIF2AK3 which is critical for the unfolded protein response (UPR). 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. Human induced pluripotent stem cell (hiPSC) patient‐derived organoid models have emerged as powerful tools to study molecular and cellular changes in a disease‐relevant genomic context.MethodFibroblasts grown from sporadic PSP patient skin cells were cultured in vitro and reprogrammed into hiPSCs. HiPSC‐derived midbrain organoids were generated in suspension spinner flasks through pharmacological directed differentiation, processed and screened for patterning using qRT‐PCR, immunohistochemistry (IHC), and quantitative immunoblot. Total tau, tau isoform, hyperphosphorylated tau (p‐tau), and UPR activation markers were assessed in organoids and human brain tissue. Single‐nucleus RNA sequencing (snRNA‐seq) was performed on the subthalamic nucleus and adjacent structures in autopsy‐validated PSP brain tissue and clinically normal controls.ResultPSP and control organoids displayed a cytoarchitectural and gene expression pattern consistent with the developing neuroectoderm and midbrain. We observed disease‐relevant differences between PSP and control organoids, including increased high molecular weight p‐tau, a higher ratio of p‐tau:total tau, and increased levels of 4R‐tau in the PSP organoids. Human PSP brain snRNA‐seq data showed dysregulation of the UPR in astrocytes and neuronal subpopulations. We also observed different UPR activation levels in PSP organoids. These findings were validated by neurohistological characterization of the UPR in autopsy brain tissue.ConclusionSingle nucleus transcriptomic and neurohistological data reveals dysregulation of the UPR in key cell types affected in PSP. PSP patient‐derived organoids develop a characteristic midbrain cellular composition and recapitulate key disease‐relevant features, including elevation of toxic tau proteoforms and UPR dysregulation. This sporadic PSP organoid model will provide insight into cell‐type specific drivers of neurodegeneration that underlie sporadic tauopathy.