(230d) Quartz Crystal Microbalance Analysis of Amyloid-Beta Protein Assembly at a Biological Interface

Alzheimer's disease (AD) is a neurodegenerative disorder that affects approximately 4.5 million Americans over the age of 65: a statistic that is set to triple by 2050. The widely accepted amyloid hypothesis proposes that the disease is caused by the accumulation of aggregated forms of the amyloid-β protein within the brain. However, for the majority of AD cases, the trigger for amyloid-β aggregation is unknown.

Phospholipid bilayers that comprise cellular membranes are capable of supporting the accumulation of aggregated forms of Aβ, and the extent of this accumulation appears to be dependent upon bilayer composition. Still, the influence of phospholipid bilayer content upon the transformation of benign monomeric Aβ into toxic aggregates is not well understood. Predominant hurdles include both the isolation of surface-specific aggregate growth and the accurate quantification of aggregation in real time. The quartz crystal microbalance (QCM) is well suited as a biosensor to study Aβ aggregation at this biological interface as it provides a heterogeneous system that can measure mass changes in the nanogram range. QCM utilizes the piezoelectric effect in quartz crystals to detect changes in bound mass as a variation in the frequency of oscillation.

Initially, Aβ aggregates were coupled directly to the crystal surface using traditional avidin-biotin chemistry to examine the capability of QCM to quantify aggregate binding and study isolated aggregate growth in real time. The biosensor can detect binding of Aβ with sensitivity approaching the nanomolar range. Furthermore, measurements of aggregate growth fit a first-order reversible kinetic model, and the dissociation constant derived from this model reflects levels of Aβ observed at the transition between normal and AD brains.

Supported phospholipid bilayers were then constructed upon the crystal surface to study aggregate interactions with these surfaces as well as subsequent monomer addition to bound aggregates. Zwitterionic, but not anionic, phospholipid bilayers are capable of binding Aβ aggregates in a saturable fashion, and these bound aggregates can undergo growth via monomer addition in a dose-dependent manner. The fatty acid composition is also shown to effect aggregate binding as well as the propensity of these adhered aggregates to undergo growth via monomer addition.