Distinguishing hallmarks of Alzheimer’s Disease include the accumulation of both amyloid beta (Aβ) plaques and neurofibrillary tangles (NFT’s) in specific regions of affected brains. The aggregation of Aβ peptides into insoluble fibrils, the principal components of plaques, involves the production of Aβ soluble oligomer intermediates. NFT’s are composed of insoluble aggregates of the microtubule associated protein tau. The amyloid cascade hypothesis suggests that Aβ soluble oligomers alter neuronal metabolism resulting in aberrant tau phosphorylation and truncation, ultimately leading to the production of NFT’s and eventual neuronal death.

In order to investigate specific effects of Aβ, cultured rat hippocampal neurons were treated with oligomerized Aβ and a time-course of biochemical samples were harvested prior to, and during, the period of cell death. Multiple phosphorylation epitopes on tau were analyzed for altered abundance via western blotting and 2D immunoblotting using site and phospho-specific antibodies. Surprisingly, none of the 8 phospho-epitopes analyzed revealed significantly altered levels compared within the time-course of Aβ treatment relative to controls even though altered signaling pathways suggest elevated Cdk5 and GSK3β activity within the neurons.

On the other hand, a prominent alteration to tau observed throughout the time-course of Aβ treatment was the loss of the full-length tau and accumulation of lower molecular weight fragments. These tau fragments remained stable in the later time points when significant levels of neuronal cell death was observed as assayed by both metabolic function and membrane integrity. Furthermore, pre-treatment of the neurons with calpain and caspase inhibitors block the production of low molecular weight tau fragments, and partially protect against Aβ mediated cell death. Surprisingly, tau appears to become dephosphorylated prior to degradation.

Overall, these data suggests a mechanism of Aβ mediated cell death where an undefined initial tau alteration is followed by significant tau fragmentation by calpain and caspase proteases; which collectively induce neuronal cell death. The significance of these findings is that tau hyperphosphorylation is likely to not be a driver of final neuronal death in AD, whereas proteolytic tau fragmentation combined with a yet to be identified initial tau dysfunction may converge to produce neurodegeneration.