Binding of Apolipoprotein E Inhibits the Oligomer Growth of Amyloid-β Peptide in Solution as Determined by Fluorescence Cross-correlation Spectroscopy

One of the primary neuropathological hallmarks of Alzheimer disease is the presence of extracellular amyloid plaques resulting from the aggregation of amyloid-beta (A beta) peptides. The intrinsic disorder of the A beta peptide drives self-association and progressive reordering of the conformation in solution, and this dynamic distribution of A beta complicates biophysical studies. This property poses a challenge for understanding the interaction of A beta with apolipoprotein E (apoE). ApoE plays a pivotal role in the aggregation and clearance of A beta peptides in the brain, and the epsilon 4 allele of APOE is the most significant known genetic modulator of Alzheimer risk. Understanding the interaction between apoE and A beta will provide insight into the mechanism by which different apoE isoforms determine Alzheimer disease risk. Here we applied alternating laser excitation fluorescence cross-correlation spectroscopy to observe the single molecule interaction of A beta with apoE in the hydrated state. The diffusion time of freely diffusing A beta in the absence of apoE shows significant self-aggregation, whereas in the presence of apoE, binding of the protein results in a more stable complex. These results show that apoE slows down the oligomerization of A beta in solution and provide direct insight into the process by which apoE influences the deposition and clearance of A beta peptides in the brain. Furthermore, by developing an approach to remove signals arising from very large A beta aggregates, we show that real-time single particle observations provide access to information regarding the fraction of apoE bound and the stoichiometry of apoE and A beta in the complex.