High-affinity multivalent interactions between apolipoprotein E and the oligomers of amyloid-β

Although the interaction of apoE isoforms with amyloid-β (Aβ) peptides plays a critical role in the progression of Alzheimer's disease, how they interact with each other remains poorly understood. Here, we investigate the molecular mechanism of apoE-Aβ interactions by comparing the effects of the different domains of apoE on Aβ. The kinetics of aggregation of Aβ1-42 are delayed dramatically in the presence of substoichiometric, nanomolar concentrations of N-terminal fragment (NTF), C-terminal fragment (CTF) and full-length apoE both in lipid-free and in lipidated forms. However, interactions between apoE and Aβ as measured by intermolecular Förster resonance energy transfer (FRET) analysis were found to be minimal at t = 0 but to increase in a time-dependent manner. Thus, apoE must interact with one or more 'intermediates' rather than the monomers of Aβ. Kinetics of FRET between full-length apoE4 labelled with EDANS at position 62 or 139 or 210 or 247 or 276, and tetramethylrhodamine-labelled Aβ (TMR-Aβ), further support an involvement of all the three domains of apoE in the interactions. However, the above-mentioned residues do not appear to form a single pocket in the 3-dimensional structure of apoE. A competitive binding assay examining the effects of unlabelled fragments or full-length apoE on the FRET between EDANS-apoE and TMR-Aβ show that binding affinity of the full-length apoE to Aβ is much higher than that of the fragments. Furthermore, apoE4 is found to interact more strongly than apoE3. We hypothesize that high affinity of the apoE-Aβ interaction is attained due to multivalent binding mediated by multiple interactions between oligomeric Aβ and full-length apoE.