β-Amyloid Peptides Increase the Binding and Internalization of Apolipoprotein E to Hippocampal Neurons

The frequency of the e4 allele of apolipoprotein E (apoE) is increased in late-onset and sporadic forms of Alzheimer's disease (AD). ApoE also binds to β-amyloid (Aβ) and both proteins are found in AD plaques. To further investigate the potential interaction of apoE and Aβ in the pathogenesis of AD, we have determined the binding, internalization, and degradation of human apoE isoforms in the presence and absence of Aβ peptides to rat primary hippocampal neurons. We demonstrate that the lipophilic Aβ peptides, in particular Aβ 1-42 , Aβ 1-40 , and Aβ 25-35 , increase significantly apoE-liposome binding to hippocampal neurons. For each Aβ peptide, the increase was significantly greater for the apoE4 isoform than for the apoE3 isoform. The most effective of the Aβ peptides to increase apoE binding, Aβ 25-35 , was further shown to increase significantly the internalization of both apoE3- and apoE4-liposomes, without affecting apoE degradation. Conversely, Aβ 1-40 uptake by hippocampal neurons was shown to be increased in the presence of apoE-liposomes, more so in the presence of the apoE4 than the apoE3 isoform. These results provide evidence that Aβ peptides interact directly with apoE lipoproteins, which may then be transported together into neuronal cells through apoE receptors.