Molecular basis of Apolipoprotein E polymorphism in lipid metabolism and neurodegenerative diseases

A major risk determinant for late-onset Alzheimer's disease (AD) is polymorphism in apolipoprotein E (ApoE), a protein that plays a critical role in redistributing cholesterol and other lipids to neurons. Of the three isoforms, ApoE4 (R112, R158) is associated with an increased risk and ApoE2 (C112, C158) is associated with a decreased risk of AD, relative to the common ApoE3 (C112, R158) isoform. In addition, ApoE2 has been reported to benefit longevity in humans and mice regardless of AD. How single amino-acid changes can substantially alter AD pathology and longevity at a molecular level remains unclear, though several lines of evidence suggest that isoforms differ in lipid binding and subsequent binding to cell surface receptors as well as interaction with amyloid beta peptides. Direct evidence comparing structural ensembles between all three isoforms in their monomeric and lipid binding form is missing. Previous studies were performed at concentrations for which ApoE is oligomeric, or do not make direct structural comparisons between all three isoforms. We used single-molecule Förster Resonance Energy Transfer (smFRET), NMR spectroscopy and replica-exchange molecular dynamics (REMD) to characterize the conformational ensembles of full-length ApoE isoforms free in solution as monomers, tetramers, and in lipidated states. We identify compact and extended conformations of ApoE that differ depending on the isoform and oligomeric/lipidation state. These results, combined with our cell-based studies on ApoE isoform-associated metabolic and functional changes, provide insight into mechanisms by which ApoE polymorphisms influence disease risk.