β‐Amyloid‐Induced Neurotoxicity of a Hybrid Septal Cell Line Associated with Increased Tau Phosphorylation and Expression of β‐Amyloid Precursor Protein

Abstract

Recent evidence suggests that β‐amyloid peptide (β‐AP) may induce tau protein phosphorylation, resulting in loss of microtubule binding capacity and formation of paired helical filaments. The mechanism by which β‐AP increases tau phosphorylation, however, is unclear. Using a hybrid septal cell line, SN56, we demonstrate that aggregated β‐AP_1–40 treatment caused cell injury. Accompanying the cell injury, the levels of phosphorylated tau as well as total tau were enhanced as detected immunochemically by AT8, PHF‐1, Tau‐1, and Tau‐5 antibodies. Alkaline phosphatase treatment abolished AT8 and PHF‐1 immunoreactivity, confirming that the tau phosphorylation sites were at least at Ser^199/202 and Ser³⁹⁶. In association with the increase in tau phosphorylation, the immunoreactivity of cell‐associated and secreted β‐amyloid precursor protein (β‐APP) was markedly elevated. Application of antisense oligonucleotide to β‐APP reduced expression of β‐APP and immunoreactivity of phosphorylated tau. Control peptide β‐AP_1–28 did not produce significant effects on tau phosphorylation, although it slightly increased cell‐associated β‐APP. These results suggest that βAP_1–40‐induced tau phosphorylation may be associated with increased β‐APP expression in degenerated neurons.