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The carboxyl-terminal amino acids 272-299-truncated apoE4 (△272-299) is the main fragments of apoE4 hydrolysate in neurons. The effects of truncated-ApoE4 (△272-299) overexpression on tau phosphorylation in cultured N2a cells were investigated. The truncated-apoE4 (△272-299)cDNA was subcloned into pEGFP-c3 to form recombinant pEGFP-T-apoE4. pEGFP-c3, pEGFP-TapoE4 and pEGFP-apoE4 were transfected into N2a cells respectively by lipofectamine 2000 method. After 24-48 h, tau phosphorylation was detected by West blot assay and glycogen synthase kinase-3 (GSK-3) activity by using GSK-3 activity assay. The results showed that the overexpression of both full length-apoE4 and truncated apoE4 fragments in N2a cells induced a dramatic increase in phosphorylation of tau at Ser202 sites and the activation of GSK-3 as compared with untransfected cells, most significantly in the cells transfected with pEGFP-T-apoE4 (P＜0.05). It was concluded that in vitro overexpression of truncated-ApoE4 (△272-299) can result in tau hyperphosphorylation in N2a cells by activating GSK-3, suggesting truncated-ApoE4 (△272-299) might contribute the pathogenesis of Alzheimer disease.