Quartz crystal microbalance analysis of growth kinetics for aggregation intermediates of the amyloid-β protein

Evidence linking soluble aggregation intermediates of the amyloid-β protein (Aβ), as well as the ongoing growth of Aβ aggregates, to physiological responses characteristic of Alzheimer’s disease (AD) indicates that a kinetic description Aβ aggregation intermediate growth may be fundamental to understanding disease progression. Although the growth of mature Aβ fibrils has been investigated using several experimental platforms, the growth of Aβ aggregation intermediates has been less thoroughly explored. In the current study, a quartz crystal microbalance (QCM) was employed to analyze the real-time growth of Aβ1–40 aggregation intermediates selectively immobilized on the crystal surface. Immobilization permitted quantitative evaluation of Aβ1–40 aggregation intermediate growth under controlled solution conditions. Elongation of Aβ1–40 aggregation intermediates via monomer addition proceeded in a nonsaturable and reversible fashion. The rate of elongation was observed to vary linearly with both monomer concentration and immobilized aggregate density, to be elevated by increases in solution ionic strength, and to increase as solution pH became more acidic. Elongation was consistent with a first-order kinetic model for the single growth phase observed. These findings extend previous kinetic studies involving the growth of mature Aβ fibrils to describe the growth of Aβ1–40 aggregation intermediates via monomer addition.