ABSTRACT: BACKGROUND:Self-assembly of the amyloid-? (A?) peptide into aggregates, from small oligomers to amyloid fibrils, is fundamentally linked with Alzheimer's disease (AD). However, it is clear that not all forms of A? are equally harmful and that linking a specific aggregate to toxicity also depends on the assays and model systems used (Haass et al., J Biol. Chem 269:17741-17748, 1994; Borchelt et al., Neuron 17:1005-1013, 1996). Though a central postulate of the amyloid cascade hypothesis, there remain many gaps in our understanding regarding the links between A? deposition and neurodegeneration. METHODS:In this study, we examined familial mutations of A? that increase aggregation and oligomerization, E22G and ?E22, and induce cerebral amyloid angiopathy, E22Q and D23N. We also investigated synthetic mutations that stabilize dimerization, S26C, and a phospho-mimetic, S8E, and non-phospho-mimetic, S8A. To that end, we utilized BRI2-A? fusion technology and rAAV2/1-based somatic brain transgenesis in mice to selectively express individual mutant A? species in vivo. In parallel, we generated PhiC31-based transgenic Drosophila melanogaster expressing wild-type (WT) and A?40 and A?42 mutants, fused to the Argos signal peptide to assess the extent of A?42-induced toxicity as well as to interrogate the combined effect of different A?40 and A?42 species. RESULTS:When expressed in the mouse brain for 6?months, A?42 E22G, A?42 E22Q/D23N, and A?42WT formed amyloid aggregates consisting of some diffuse material as well as cored plaques, whereas other mutants formed predominantly diffuse amyloid deposits. Moreover, while A?40WT showed no distinctive phenotype, A?40 E22G and E22Q/D23N formed unique aggregates that accumulated in mouse brains. This is the first evidence that mutant A?40 overexpression leads to deposition under certain conditions. Interestingly, we found that mutant A?42 E22G, E22Q, and S26C, but not A?40, were toxic to the eye of Drosophila. In contrast, flies expressing a copy of A?40 (WT or mutants), in addition to A?42WT, showed improved phenotypes, suggesting possible protective qualities for A?40. CONCLUSIONS:These studies suggest that while some A?40 mutants form unique amyloid aggregates in mouse brains, they do not exacerbate A?42 toxicity in Drosophila, which highlights the significance of using different systems for a better understanding of AD pathogenicity and more accurate screening for new potential therapies.
