Seeding Alzheimer’s disease-associated tau pathology in MAPT knock-in primary neurons causes early axonopathy and synaptic dysfunction

The progressive accumulation of pathological tau is a hallmark of Alzheimer's disease (AD). The bulk of existing in vitro and in vivo evidence suggests that pathological tau forms can seed further aggregation of the protein. However, many of the subsequent functional consequences following the formation of pathogenic tau aggregates are not yet fully understood. Here, we utilized the tau seeding phenomenon to induce the formation of pathogenic tau and identify intracellular consequences in a neuron culture model of AD-associated tauopathy. Primary neurons from human tau knock-in (MAPT-KI) mice were seeded with human AD brain-derived insoluble tau (AD-tau). Microscopy and biochemical assays were used to characterize the pathological tau species formed, as well as the extent of neuronal, axonal and synaptic degeneration in seeded MAPT-KI neurons. In addition, high-density microelectrode arrays were used to assess synaptic functionality in seeded MAPT-KI neuron cultures. Human-derived AD-tau seeded intracellular endogenous tau inclusions that contained AD-associated modifications (i.e. phosphorylation at the PHF1, AT8, and pS422 antibody epitopes) and adopted multiple pathogenic conformations (i.e. oligomers and exposure of an N-terminal phosphatase activating domain; PAD). Tau inclusions, containing pS422 + and PAD-exposed tau, colocalized with active glycogen synthase kinase 3β (the kinase involved in PAD-mediated axonal transport impairment) and accumulations of axonal transport cargo proteins (i.e. synaptophysin and amyloid precursor protein) in dystrophic axons. While there was no overt axonal degeneration or cell loss, intact excitatory synapses were reduced in the AD-tau neurons. Neuron cultures treated with AD-tau exhibited an N-methyl-D-aspartate receptor-dependent increase in network burst frequency when activated with glutamate as measured through high-density microelectrode arrays. Together, the data demonstrate that the AD-tau seeded MAPT-KI neuron model exhibits features associated with neuronal dysfunction resembling those that occur early in human disease (i.e. axonal pathology and dystrophy, hyperexcitability and hypersynchrony), without causing overt neurodegeneration.