Modulation of Amyloid Beta Peptide Aggregation by Apolipoprotein E Isoforms

Abstract

Apolipoprotein E4 (apoE) is a confirmed risk factor for late onset Alzheimer's disease (AD). Mouse model studies indicate enhanced depostion of amyloid beta (Aβ) peptides in the brain in presence of the e4 isoform of apoE. However, the mechanism of apoE-Aβ interaction and the biophysical nature of these complexes remain largely unclear. In addition, the relative specificity of this interaction with the other two common isoforms (apoE2 and apoE3) not associated with AD is poorly understood. Aβ aggregation followed by turbidity measurements show that the aggregation is accelerated by all three isoforms of apoE and apoE4 showed the greatest and apoE2 the least effects. Fluorescence correlation spectroscopy (FCS) measurements using fluorescently labeled Aβ show that diffusion time of Aβ increases upon incubation with wild type apoE proteins. FCS data confirm that apoE4 has the highest and apoE2 has the lowest affinity for Aβ. However, this interaction appears to be slow and does not reach completion even after several days of co-incubation. The diffusion time of the complexes are found to be large indicating involvement of multimeric forms of Aβ. In addition, a monomeric form of apoE prepared by 4 mutations in the apoE sequence does not bind to Aβ. Thus apoE multimers may be acting as nucleation sites for oligomerization of Aβ. We are currently investigating the molecular basis of the apoE-Aβ complex formations by employing Hydrogen-Deuterium Exchange by mass spectrometry (HDX-MS).