P1-478: Isoform-specific effects of apolipoprotein E on the stability of Abeta protofibrils

Abstract

Alzheimer's disease (AD) is characterized by the deposition of amyloid beta peptides (Abeta) as senile plaques. Apolipoprotein E (apoE), one of the lipid transport proteins, co-deposits with senile plaque amyloids in AD brains. Three major alleles of human apoE gene, epsilon2, epsilon3 and epsilon4 have been recognized, of which epsilon4 has been known to be associated with an increased risk for developing AD. Mouse genetic studies by overexpression of human apoE3 and apoE4 in the absence of endogenous apoE showed that the fibrillar Abeta deposits in the hippocampus of apoE4-expressing mice were greater than those in apoE3-expressing mice, suggesting that human apoE4 facilitates amyloid fibril deposition thereby leading to AD. It has been well documented that apoE inhibits Abeta fibril formation in vitro especially in the nucleation phase, although the isoform-specific effects of apoE against Abeta fibrillization has not been well characterized. To evaluate the isoform-specific effects of apoE, we carried out an in vitro Abeta fibrillization assay and investigated the formation of low molecular weight Abeta, protofibrils, and fibrils using thioflavin T fluorescence and size exclusion chromatography. First, we showed that Abeta(1–42) fibrillized faster in the presence of recombinant apoE4 than apoE2 or apoE3, although the affinity of the three isoforms to native Abeta were at similar levels. We next found that apoE2 and apoE3 enhanced the persistence of Abeta protofibrils and delayed the start of Abeta fibril formation, whereas apoE4 did not. We also found that protofibrils comprised of Abeta and apoE3 were more stable than those of Abeta and apoE4. In addition, a SDS-stable 40-kDa band positive both for Abeta and apoE was detected in the Abeta protofibril fraction upon coincubation with apoE2 or apoE3, which was scarcely detected with apoE4. These data suggest that apoE2/3 is involved in the stability of Abeta protofibrils, whereas apoE4 lacks this ability. The isoform-specific effects of apoE on Abeta protofibrils may be one of the determinants for the aggregation and deposition of Abeta in AD brains.