A protein aggregation platform that distinguishes oligomers from amyloid fibrils.

Deposition of aggregated proteins is a pathological feature in many neurodegenerative disorders such as Alzheimer's and Parkinson's. In addition to insoluble amyloid fibrils, protein aggregation leads to the formation of soluble oligomers, which are more toxic and pathogenic than fibrils. However, it is challenging to screen for inhibitors targeting oligomers due to the overlapping processes of oligomerization and fibrillization. Here we report a protein aggregation platform that uses intact and split TEM-1 ß-lactamase proteins as reporters of protein aggregation. The intact ß-lactamase fused with an amyloid protein can report the overall protein aggregation, which leads to loss of lactamase activity. On the other hand, reconstitution of active ß-lactamase from the split lactamase construct requires the formation of amyloid oligomers, making the split lactamase system sensitive to oligomerization. Using Aß, a protein that forms amyloid plaques in Alzheimer's disease, we show that the growth curves of bacterial cells expressing either intact or split lactamase-Aß fusion proteins can report changes in the Aß aggregation. The cell lysate lactamase activity assays show that the oligomer fraction accounts for 20% of total activity for the split lactamase-Aß construct, but only 3% of total activity for the intact lactamase-Aß construct, confirming the sensitivity of the split lactamase to oligomerization. The combination of the intact and split lactamase constructs allows the distinction of aggregation modulators targeting oligomerization from those targeting overall aggregation. These low-cost bacterial cell-based and biochemical assays are suitable for high-throughput screening of aggregation inhibitors targeting oligomers of various amyloid proteins.

Preparation and Fractionation of Heterogeneous Aß42 Oligomers with Different Aggregation Properties.

Deposition of amyloid-ß (Aß) aggregates in the form of amyloid plaques is a central feature of Alzheimer's disease. The end products of Aß aggregation are amyloid fibrils. Soluble Aß aggregates called oligomers are also formed either on or off the pathway of fibril formation. The amyloid fibrils from different clinical subtypes of Alzheimer's disease have been found to adopt different structures, a phenomenon called fibril polymorphism. Meanwhile, different types of Aß oligomers have also been found. Recently, it has been shown that different types of Aß42 oligomers may form fibrils of different structures, linking oligomer heterogeneity to fibril polymorphism. In this chapter, we describe methods to prepare heterogeneous Aß42 oligomers and to quantify the concentration of these oligomers at a low micromolar range using a fluorescamine method. Fractionation of these oligomers by size using ultrafiltration filters allows for the formation of Aß42 fibrils with different structural properties.