Understanding Alzheimer's Disease: Mechanistic Studies of Amyloid-Beta Surface Assembly

Alzheimer's disease is the most common cause of dementia in the elderly, affecting 4.4 million people in the U.S. alone. Senile plaques of the protein amyloid-beta (A-beta) are a hallmark of the disease. Of A-beta's forms, namely insoluble fibrils, soluble protofibrils, and soluble monomers, evidence points to protofibrils as the toxic species. Two mechanisms of protofibril growth—elongation and association—have been studied in solution at micromolar concentrations. However, A-beta exists in vivo at nanomolar concentrations and interacts with cell membranes. Consequently, it is important to consider protofibril growth as a surface phenomenon. A quartz crystal microbalance (QCM) was employed to monitor both processes to determine how surface assembly of protofibril compares with solution mechanisms. The QCM relates a decrease in the resonance frequency of a quartz disk to an increase in bound mass. Protofibrils containing 10 mole% biotinylated monomer were immobilized on the crystal via high-affinity biotin-avidin binding, and elongation and association were studied. Experiments in solution verified that biotin does not alter protofibril assembly. These experiments validate the usefulness of QCM for studying protofibril growth at a surface and provide the groundwork for employing QCM to examine protofibril growth on a phospholipid bilayer model of the cell surface.