Determinants of Astrocytic Pathology in Stem Cell Models of Primary Tauopathies

Astrocytic tau aggregates are seen in several primary and secondary tauopathies, including progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and chronic traumatic encephalopathy (CTE). In all cases, astrocytic tau consists exclusively of the longer (4R) tau isoform, even when adjacent neuronal aggregates consist of a mixture of 3- and 4R tau, as in CTE. The reasons for this and the mechanisms by which astrocytic tau aggregates form remain unclear. We used a combination of RNA in situ hybridization and immunofluorescence in post-mortem human brain tissue, as well as tau uptake studies in human stem cell-derived astrocytes, to determine the origins of astrocytic tau in 4R tauopathies. We found that astrocytes across tauopathies do not upregulate tau mRNA expression between diseases or between tau-positive and -negative astrocytes within PSP. We then found that stem cell-derived astrocytes preferentially take up long isoform (4R) labeled recombinant tau and that this uptake is impaired by induction of reactivity with inflammatory stimuli or nutritional stress. Astrocytes exposed to either 3R or 4R tau also showed downregulation of genes related to astrocyte differentiation. Our findings suggest that astrocytes preferentially take up neuronal 4R tau from the extracellular space, which potentially explains why astrocytic tau aggregates contain only 4R tau, and that tau uptake is impaired by decreased nutrient availability or neuroinflammation, both of which are common in the aging brain.