(750c) Inhibition of Amyloid-β Protein Assembly Is Dependent Upon Environmental Conditions: A Quartz Crystal Microbalance Analysis
Alzheimer's disease (AD) is a neurodegenerative disorder that affects over 5 million Americans: 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-beta protein (Aβ) within the brain. However, for the majority of AD cases, the trigger for Aβ aggregation is unknown. Aβ aggregation is extremely sensitive to environmental conditions, with pH, ionic strength, temperature, and other molecular interactions all able to either promote, inhibit, or otherwise alter the process. However, in the context of evaluating specific aggregation mechanisms, thoroughly probing these effects is difficult, as multiple growth pathways are simultaneously active at many of these conditions and difficult to differentiate. This can compound the difficulty in evaluating potential aggregation inhibitor compounds, as their effect on individual growth mechanisms in a broad range of environmental conditions is difficult to quantify. The quartz crystal microbalance (QCM) offers the opportunity to isolate specific growth mechanisms over a broad range of conditions as well as to quantify aggregation in real time. QCM utilizes the piezoelectric effect in quartz crystals to detect changes in bound mass as a variation in the frequency of oscillation, providing sensitivity down to the nanogram range. In this way, Aβ aggregates immobilized onto the crystal surface can be monitored for addition of monomer, and solution conditions around the immobilized aggregates can be easily varied. Aβ aggregate growth via monomer aggregate interaction was measured over a range of solution pH, ionic strength, and temperature. The performance of a small aromatic inhibitor, which is effective at slowing monomer-aggregate binding in solution studies, was evaluated as a function of each of these variables. The effectiveness of inhibition was further tested on aggregates bound to zwitterionic supported phospholipid bilayer surfaces of varying hydrocarbon saturation. Temperature and pH were found to have a pronounced effect on inhibitor performance, to the extent of rendering the compound nonviable at some conditions. In contrast, ionic strength had little effect on the degree of inhibition in the window of conditions tested, indicating that purely electrostatic effects have little influence upon the action of this inhibitor. The inhibitor retained performance when acting on aggregates bound to phospholipid bilayers, and this performance was found to be dependent upon the degree of saturation of the lipids comprising the bilayer. These results demonstrate how this experimental platform can be used to identify which physiological conditions will be most influential when transitioning from in vitro to in vivo therapeutic screening.