BACE inhibitor reduces APP-β-C-terminal fragment accumulation in axonal swellings of okadaic acid-induced neurodegeneration

Emerging evidence suggests that not only β-amyloid but also other amyloid precursor protein (APP) fragments, such as the β-C-terminal fragment (βCTF), might be involved in Alzheimer's disease (AD). Treatment of neurons with okadaic acid (OA), a protein phosphatase-2A inhibitor, has been used to induce tau phosphorylation and neuronal death to create a research model of AD. In this study, we analyzed axonopathy and APP regulation in cultured rat neurons treated with OA. After OA treatment, the neurons presented with axonal swellings filled with vesicles, microtubule fragments, and transport molecules such as kinesin and synapsin-I. Western blotting showed that intracellular APP levels were increased and immunocytochemistry using antibodies against the APP C-terminus showed that APP accumulated in the axonal swellings. This APP C-terminus immunoreactivity disappeared when neurons were cotreated with a β-secretase inhibitor, but not with α- or γ-secretase inhibitors, indicating that the accumulation was primarily composed of APP-βCTF. These findings provide the first evidence that APP-βCTF can accumulate in the axons of OA-treated neurons, and may suggest that APP-βCTF is involved in the pathogenesis of AD.