Multiple mechanisms of extracellular tau spreading in a non-transgenic tauopathy model.

While the interneuronal propagation of neurofibrillary lesions in Alzheimer's disease and other tauopathies now appears to involve the spreading of tau-associated toxicity, little is known about its mechanism. We characterized the movement of human tau through the brain of a non-transgenic lower vertebrate tauopathy model in which full-length wild type and mutant human tau isoforms were expressed in identified neurons, thus permitting the identification and localization of EC tau sources. We describe two distinct patterns of tau spreading that correspond to tau species that lack (MTBR-) and contain (MTBR+) the tau microtubule-binding region. These patterns illustrate the production, migration and uptake of EC tau and resemble some of the extracellular tau deposits typically seen in human brain after repeated traumatic injury in cases of chronic traumatic encephalopathy (CTE). We propose that misprocessed human tau can spread between CNS neurons via a variety of non-synaptic mechanisms as well as synaptically mediated mechanisms.