Paradoxical effects of mutant ubiquitin on Aβ plaque formation in an Alzheimer mouse model.

Amyloid-β (Aβ) plaques are a prominent pathological hallmark of Alzheimer's disease (AD). They consist of aggregated Aβ peptides, which are generated through sequential proteolytic processing of the transmembrane protein amyloid precursor protein (APP) and several Aβ-associated factors. Efficient clearance of Aβ from the brain is thought to be important to prevent the development and progression of AD. The ubiquitin-proteasome system (UPS) is one of the major pathways for protein breakdown in cells and it has been suggested that impaired UPS-mediated removal of protein aggregates could play an important role in the pathogenesis of AD. To study the effects of an impaired UPS on Aβ pathology in vivo, transgenic APP/PS1ΔE9 mice (APPPS1) were crossed with transgenic mice expressing mutant ubiquitin (UBB), a protein-based inhibitor of the UPS. Surprisingly, the APPPS1/UBB crossbreed showed a remarkable decrease in Aβ plaque load during aging. Further analysis showed that UBB expression transiently restored PS1-NTF expression and γ-secretase activity in APPPS1 mice. Concurrently, UBB decreased levels of β-APP-CTF, which is a γ-secretase substrate. Although UBB reduced Aβ pathology in APPPS1 mice, it did not improve the behavioral deficits in these animals.