Pro-aggregant Tau impairs mossy fiber plasticity due to structural changes and Ca(++) dysregulation

We used an inducible mouse model expressing the Tau repeat domain with the pro-aggregant mutation ΔK280 to analyze presynaptic Tau pathology in the hippocampus. Expression of pro-aggregant Tau(RDΔ) leads to phosphorylation, aggregation and missorting of Tau in area CA3. To test presynaptic pathophysiology we used electrophysiology in the mossy fiber tract. Synaptic transmission was severely disturbed in pro-aggregant Tau(RDΔ) and Tau-knockout mice. Long-term depression of the mossy fiber tract failed in pro-aggregant Tau(RDΔ) mice. We observed an increase in bouton size, but a decline in numbers and presynaptic markers. Both pre-and postsynaptic structural deficits are preventable by inhibition of Tau(RDΔ) aggregation. Calcium imaging revealed progressive calcium dysregulation in boutons of pro-aggregant Tau(RDΔ) mice. In N2a cells we observed this even in cells without tangle load, whilst in primary hippocampal neurons transient Tau(RDΔ) expression alone caused similar Ca(++) dysregulation. Ultrastructural analysis revealed a severe depletion of synaptic vesicles pool in accordance with synaptic transmission impairments. We conclude that oligomer formation by Tau(RDΔ) causes pre- and postsynaptic structural deterioration and Ca(++) dysregulation which leads to synaptic plasticity deficits.