Tau aggregates: toxic, inert, or protective species?

Alzheimer's disease brains are characterized by extracellular aggregates of the amyloid-beta peptide and intracellular neurofibrillary tangles, composed of aggregated hyperphosphorylated tau protein. The role of aggregated tau in neurodegeneration is still controversial, as evidence point to either a toxic or protective role in the disease. Here, we will first examine tau aggregation and its putative roles in Alzheimer's disease. We will then review the findings concerning different species of tau and their potential toxicity.