Remarkably, the behavioral impairments observed in mice overexpressing WT tau are significantly reduced when two proline residues are introduced near the second repeat in the microtubule-binding region. Compared to 2P tau, the enhanced toxic effects of WT tau correlate with higher levels of tau phosphorylation on several residues. Indeed, it has been already demonstrated that the accumulation of hyperphosphorylated forms of tau, most likely dissociated from microtubules, can have detrimental effects in neurons39,40. However, these toxic effects are unlikely to be caused by aggregated tau, which remains at low levels in mice overexpressing WT 4R0N tau. Similarly, a mouse model overexpressing 4R1N tau displays cognitive and motor deficits in the absence of any neurofibrillary degeneration, suggesting that phospho-tau and pre-tangle species can cause neuronal dysfunction22. In the Drosophila, overexpression of 3R0N tau can destabilize microtubules via the formation of soluble hyperphosphorylated species that sequester endogenous tau11. Here, we have not observed any significant loss of axonal microtubules in the cortex following AAV-WT injection (see Fig. 6). It is therefore plausible that other mechanisms are implicated, such as disrupted mitochondrial transport, calcium dyshomeostasis and synaptic dysfunction, that may contribute to the induced behavioral deficits41.