Assessing Lipid Membrane Interaction of Amyloid-Forming Proteins by Means of Colorimetric Biosensing Vesicles

Abstract

Protein misfolding diseases such as Alzheimer's disease (AD) and prion-related encephalopathies (PRE) are frequently identified by proteinaceous fibrillar aggregate deposits known as amyloids. Here we describe the utilization of a colorimetric, biomimetic, vesicle-binding assay to investigate the interactions of amyloid-forming proteins with lipid membranes. The assay is based on the assembly of vesicles in solution by incorporating a lipid within a polydiacetylene (PDA) matrix. When polymerized by UV irradiation, the lipid/PDA vesicle turns a blue color. The varied protein interaction with the lipid/PDA vesicles give rise to blue-to-red conversions of the PDA polymer when exposed to changes in the environment. Quantitatively, the value for this color transition is given by the colorimetric response. Time-resolved absorbance measurements of the “blue” (630 nm) and “red” (490 nm) wavelengths were obtained from several different types of lipid vesicles (total brain lipid extract (TBLE), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and 1,2-dimyristoyl-sn-glycero-3-phosphorylglycerol (DMPG) exposed to various amyloid-forming proteins such as prion peptide fragments and beta-amyloid. The measured colorimetric response of the lipid/PDA vesicles with amyloid-forming proteins correlates to the presence of protein-lipid interactions and varies with each protein and lipid. This may be due to protein or lipid structure, as well as the extent of protein aggregation and association with the lipid membrane. These studies provide valuable insight into the role of cellular surfaces in the mechanism of disease. We demonstrate the potential for this colorimetric assay as a biosensor and useful technique in studying biological systems.