Following Activation of the Amyloid Cascade, Apolipoprotein E4 Drives the in vivo Oligomerization of Amyloid-β Resulting in Neurodegeneration

According to the amyloid hypothesis, the accumulation of oligomerized amyloid-beta (A beta) is a primary event in the pathogenesis of Alzheimer's disease (AD). The trigger of the amyloid cascade and of A beta oligomerization in sporadic AD, the most prevalent form of the disease, remains elusive. Here, we examined the hypothesis that apolipoprotein E4 (ApoE4), the most prevalent genetic risk factor for AD, triggers the accumulation of intraneuronal oligomerized A beta following activation of the amyloid cascade. We investigated the intracellular organelles that are targeted by these processes and govern their pathological consequences. This revealed that activation of the amyloid cascade in vivo by inhibition of the A beta degrading enzyme neprilysin specifically results in accumulation of A beta and oligomerized A beta and of ApoE4 in the CA1 neurons of ApoE4 mice. This was accompanied by lysosomal and mitochondrial pathology and the co-localization of A beta, oligomerized A beta, and ApoE4 with enlarged lysosomes and of A beta and oligomerized A beta with mitochondria. The time course of the lysosomal effects paralleled that of the loss of CA1 neurons, whereas the mitochondrial effects reached an earlier plateau. These findings suggest that ApoE4 potentiates the pathological effects of A beta and the amyloid cascade by triggering the oligomerization of A beta, which in turn, impairs intraneuronal mitochondria and lysosomes and drives neurodegeneration.