Amyloid precursor protein-induced axonopathies are independent of amyloid-β peptides

Overexpression of amyloid precursor protein (APP), as well as mutations in the APP and presenilin genes, causes rare forms of Alzheimer's disease (AD). These genetic changes have been proposed to cause AD by elevating levels of amyloid-beta peptides (A beta), which are thought to be neurotoxic. Since overexpression of APP also causes defects in axonal transport, we tested whether defects in axonal transport were the result of A beta poisoning of the axonal transport machinery. Because directly varying APP levels also alters APP domains in addition to A beta, we perturbed A beta generation selectively by combining APP transgenes in Drosophila and mice with presenilin-1 (PS1) transgenes harboring mutations that cause familial AD (FAD). We found that combining FAD mutant PS1 with FAD mutant APP increased A beta 42/A beta 40 ratios and enhanced amyloid deposition as previously reported. Surprisingly, however, this combination suppressed rather than increased APP-induced axonal transport defects in both Drosophila and mice. In addition, neuronal apoptosis induced by expression of FAD mutant human APP in Drosophila was suppressed by co-expressing FAD mutant PS1. We also observed that directly elevating A beta with fusions to the Familial British and Danish Dementia-related BRI protein did not enhance axonal transport phenotypes in APP transgenic mice. Finally, we observed that perturbing A beta ratios in the mouse by combining FAD mutant PS1 with FAD mutant APP did not enhance APP-induced behavioral defects. A potential mechanism to explain these findings was suggested by direct analysis of axonal transport in the mouse, which revealed that axonal transport or entry of APP into axons is reduced by FAD mutant PS1. Thus, we suggest that APP-induced axonal defects are not caused by A beta.