Tau aggregate replication occurs at the pre-synapse of cultured human neurons and increases with application of TNFa

Tau aggregation at synapses is a key process driving Alzheimer's disease but the mechanism(s) that cause this have not been established. We used a model system of forward-programming induced glutamatergic neurons (iNeurons) with three independent cell lines treated with TNFa. Using aggregate-specific SIMOA, STED microscopy, and SynPull to detect nanoscopic tau aggregates in bulk samples and at individual synapses, we found that TNFa-driven tau aggregation occurs preferentially at the pre-synapse, forming predominantly non-fibrillar aggregates that are larger than ones in the extra- and post-synaptic regions. Using mathematical models of aggregate formation, we fitted the frequency of AT8-positive tau aggregates in synaptosomes, which showed that aggregate replication is the dominant process and is much faster than de-novo aggregate formation, leading to rapid local amplification once one aggregate is formed. Our results provide direct evidence for tau aggregate replication at the pre-synapse, linking inflammation induced tau aggregation with synaptic pathology.