Engineered Alzheimer Organoids Validate the Link Between Intracellular and Soluble p-Tau Biomarkers and Highlight the Contribution of Astrocytic Tau.

Soluble phosphorylated tau has become a key biomarker for Alzheimer's disease pathology, yet further mechanistic studies are needed beyond observational investigations to clarify the relationship between soluble p-tau species and intracellular tau pathology. Here, we utilized chimeric human cerebral organoids (chCOs) to generate a series of organoids in which the endogenous MAPT gene was CRISPR/Cas9 edited in different cell types to create phosphorylation-deficient mutants at the AD-associated sites. We found that the APPswe mutation increased tau phosphorylation in both neurons and astrocytes. Notably, astrocyte-specific phosphorylation-deficient mutations of tau in organoids reduced soluble p-tau181 and p-tau217 levels, as detected by single-molecule array. These findings indicate that astrocytic tau plays a substantial role in contributing to the pool of extracellular phosphorylated tau and suggest it may be an overlooked source of AD biomarkers. Moreover, our engineered chCOs offer a versatile platform for exploring how cell-type-specific pathologies correlate with changes in biomarker profiles.