Exosome-associated tau is secreted in tauopathy models and is selectively phosphorylated in cerebrospinal fluid in early Alzheimer disease.

Recent demonstrations that the secretion, uptake, and interneuronal transfer of tau can be modulated by disease-associated tau modifications suggest that secretion may be an important element in tau-induced neurodegeneration. Here, we show that much of the tau secreted by M1C cells occurs via exosomal release, a widely characterized mechanism that mediates unconventional secretion of other aggregation-prone proteins (α-synuclein, prion protein, and β-amyloid) in neurodegenerative disease. Exosome-associated tau is also present in human CSF samples and is phosphorylated at Thr-181 (AT270), an established phosphotau biomarker for Alzheimer disease (AD), in both M1C cells and in CSF samples from patients with mild (Braak stage 3) AD. A preliminary analysis of proteins co-purified with tau in secreted exosomes identified several that are known to be involved in disease-associated tau misprocessing. Our results suggest that exosome-mediated secretion of phosphorylated tau may play a significant role in the abnormal processing of tau and in the genesis of elevated CSF tau in early AD.