An Investigation On Membrane Insertion And Oligomerization Of Amyloid-β Peptide

Alzheimer’s disease (AD) is a progressive, neurodegenerative disorder that is the leading cause of senile dementia, afflicting tens of millions of individuals worldwide. Amyloid-beta peptide (AP), the39to43amino acid peptide, is identified as the principal toxic entity in the progression of AD. Since Aβ aggregate, which was considered more toxic, is produced in a concentration-dependent manner that oligomerizes in response to increased Aβ levels in brain interstitial fluid (ISF), numerous attempts have been made to reduce endogenous Aβ production and deposition. But, so far, the molecular and cellular mechanisms underlying Aβ assembly into soluble oligomers in the brain is still unclear and remains one of the biggest challenges in the research of AD.Oligomerization and membrane insertion of AP are two key mechanisms underlying its pathologic role in AD. Although membrane insertion, oligomerization and neurotoxicity of Aβ appear to be tightly coupled, the correlation between soluble and membrane-embedded oligomers of Aβ remains elusive. In addition, it is currently unclear whether soluble Aβ oligomer can act by direct membrane incorporation. Hence, in order to clarify the interactive mechanism between Aβ and membrane, we systematically analyzed the interactions between different Aβ species with various phospholipids on model membrane systems.In this work, we mainly employ Langmuir monolayer and lipid bilayer vesicles (liposomes) technique, electrophoresis and western blotting technique, fluorescence spectral analysis and fluorescence resonance energy transfer (FRET) technique, negative stain electron microscopy technique to study the different behavior of Aβ monomer and oligomer species in their interactions with membrane, and further investigate the correlation of these two processes, i.e. membrane-insertion and oligomerization. We found that Aβ monomers interact strongly with lipids in vitro. Although Aβ monomer has strong aggregation ability in solution; in the presence of membrane, Aβ monomer prefers incorporating into membrane followed by rapid intramembrane assembly rather than oligomerizing in solution. By contrast, the oligomers assembled in buffer do not penetrate into membrane, and show lower affinity to membrane, suggesting that oligomers assembled from monomers within membrane are distinct from native Aβ oligomers formed in buffer.Based on results in this work, we present a dynamic model for Aβ monomer insertion and aggregation which involves peptide aggregation upon membrane contact. Furthermore, we indicate that the intra-and extra-membrane oligomerization of Aβ proceed via distinct competing pathways and may determine separate neurotoxic mechanisms, revealling a critical membrane regulation on the behavior of Aβ. Our finding will lead to a better understanding of AD pathogenesis and ultimately to more effective therapeutic strategies.