Apolipoprotein E forms stable complexes with recombinant Alzheimer's disease β-amyloid precursor protein

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Abstract
Apolipoprotein E (apoE), a protein genetically linked to the incidence of Alzheimer's disease, forms SDS-stable complexes in vitro with β-amyloid peptide (Aβ), the primary component of senile plaques. In the present study, we investigated whether apoE was able to bind full-length Aβ precursor protein (APP). Using a maltose-binding-protein–APP fusion protein and human very-low-density lipoprotein (VLDL), we detected an interaction of apoE with APP that was inhibited by Aβ or anti-apoE antibody. Saturation-binding experiments indicated a single binding equilibrium with an apparent 1:1 stoichiometry and a dissociation constant of 15 nM. An interaction was also observed using apoE from cerebrospinal fluid or delipidated VLDL, as well as recombinant apoE. APP·apoE complexes were SDS-stable, and their formation was not inhibited by reducing conditions; however, they were dissociated by SDS under reducing conditions. ApoE·APP complexes formed high-molecular-mass aggregates, and competition experiments suggested that amino acids 14–23 of Aβ are responsible for complex-formation. Finally, no differences were found when studying the interaction of APP with apoE3 or apoE4. Taken together, our results demonstrate that apoE may form stable complexes with the Aβ moiety of APP with characteristics similar to those of complexes formed with isolated Aβ, and suggest the intriguing possibility that apoE–APP interactions may be pathologically relevant in vivo.

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