In several neurodegenerative diseases, including Alzheimer disease, the neuronal microtubule-associated protein tau becomes hyperphosphorylated, accumulates in the somatodendritic compartment, and aggregates into insoluble filaments. The consequences of the accumulation of hyperphosphorylated tau in the somatodendritic compartment remain poorly characterized at the early stage of disease before the formation of tau insoluble filaments.