Single Molecule Study of the Oligomerization of Alzheimer's Aβ40 and Aβ42 on the Surface of Phospholipid Membranes

Abstract

The amyloid-β (Aβ) peptides 40 and 42 are believed to feature in the synaptic dysfunction and neuronal loss associated with Alzheimer’s disease. One proposed mechanism for the synaptic loss is via the interaction between Aβ and cell surface neurotransmitters and receptors, which subsequently affect the cell signaling. Another hypothesis is that formation of calcium-permeable channels by Aβ oligomers on the membrane directly disrupts calcium homeostasis and triggers cell death. Both mechanisms are likely to involve peptide-membrane interaction where the amphipathic cell membrane provides an extensive surface for amyloid interactions and is the primarily cellular structure that Aβ contacts with. Most of our knowledge comes from experiments done at high Aβ concentration (∼μM, as compared with the nM peptide concentrations in vivo) where peptide-peptide interactions in solution might bias the real peptide-membrane interaction. Using single molecule total internal reflection fluorescence microscopy, we observe single Aβ oligomers diffusing on the membrane at physiological concentration (∼4nM). Using single molecule photobleaching and fluorescence intensity to assess the oligomers’ sizes, we track individual peptide species as they diffuse in the membrane. Our studies reveal a mixture of freely diffusing and membrane-immobilized oligomers and show that the membrane accelerates Aβ dimer formation. A comparison of the membrane-bound oligomer species created by Aβ40 and Aβ42 yields information on how the two additional residues on Aβ42 affect the peptide-peptide and peptide-membrane interactions. These studies help further our understanding of the role of peptide-membrane interactions in the formation and growth of the amyloid-β oligomers that may contribute to Alzheimer’s pathology.