A differential association of Apolipoprotein E isoforms with the amyloid-β oligomer in solution.

The molecular pathogenesis of disorders arising from protein misfolding and aggregation is difficult to elucidate, involving a complex ensemble of intermediates, whose toxicity depends upon their state of progression along distinct processing pathways. To address the complex misfolding and aggregation that initiates the toxic cascade resulting in Alzheimer's disease (AD), we have developed a 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid spin-labeled amyloid-β (Aβ) peptide to observe its isoform-dependent interaction with the apoE protein. Although most individuals carry the E3 isoform of apoE, ∼15% of humans carry the E4 isoform, which is recognized as the most significant genetic determinant for Alzheimer's. ApoE is consistently associated with the amyloid plaque marker for AD. A vital question centers on the influence of the two predominant isoforms, E3 and E4, on Aβ peptide processing and hence Aβ toxicity. We used electron paramagnetic resonance (EPR) spectroscopy of incorporated spin labels to investigate the interaction of apoE with the toxic oligomeric species of Aβ in solution. EPR spectra of the spin-labeled side chain report on side chain and backbone dynamics as well as the spatial proximity of spins in an assembly. Our results indicate oligomer binding involves the C-terminal domain of apoE, with apoE3 reporting a much greater response through this conformational marker. Coupled with SPR binding measurements, apoE3 displays a higher affinity and capacity for the toxic Aβ oligomer. These findings support the hypothesis that apoE polymorphism and Alzheimer's risk can largely be attributed to the reduced ability of apoE4 to function as a clearance vehicle for the toxic form of Aβ.