Neuron loss in the 5XFAD mouse model of Alzheimer's disease correlates with intraneuronal Aβ₄₂ accumulation and Caspase-3 activation

Background: Although the mechanism of neuron loss in Alzheimer's disease (AD) is enigmatic, it is associated with cerebral accumulation of Aβ₄₂. The 5XFAD mouse model of amyloid deposition expresses five familial AD (FAD) mutations that are additive in driving Aβ₄₂ overproduction. 5XFAD mice exhibit intraneuronal Aβ₄₂ accumulation at 1.5 months, amyloid deposition at 2 months, and memory deficits by 4 months of age. Results: Here, we demonstrate by unbiased stereology that statistically significant neuron loss occurs by 9 months of age in 5XFAD mice. We validated two Aβ₄₂-selective antibodies by immunostaining 5XFAD; BACE1§ssup§-/-§esup§ bigenic brain sections and then used these antibodies to show that intraneuronal Aβ₄₂ and amyloid deposition develop in the same regions where neuron loss is observed in 5XFAD brain. In 5XFAD neuronal soma, intraneuronal Aβ₄₂ accumulates in puncta that co-label for Transferrin receptor and LAMP-1, indicating endosomal and lysosomal localization, respectively. In addition, in young 5XFAD brains, we observed activated Caspase-3 in the soma and proximal dendrites of intraneuronal Aβ₄₂-labeled neurons. In older 5XFAD brains, we found activated Caspase-3-positive punctate accumulations that co-localize with the neuronal marker class III β-tubulin, suggesting neuron loss by apoptosis. Conclusions: Together, our results indicate a temporal sequence of intraneuronal Aβ₄₂ accumulation, Caspase-3 activation, and neuron loss that implies a potential apoptotic mechanism of neuron death in the 5XFAD mouse.