Conformation-Specific Chemical Probes for Amyloid-

Abstract

The age-dependent accumulation of amyloid-beta (A-beta) is linked to the pathology of Alzheimer’s disease (AD). Both in vitro and in vivo, A-beta assembles into diverse quaternary conformations, including low-molecular weight structures (e.g. dimers, trimers, and tetramers), globular oligomers, protofibrils, and fibrils. Although these assemblies have been identified and partially characterized, the correlations between A-beta conformation and neurotoxicity remain unclear. Probes that differentiate between A-beta species are likely to be critical in answering this important question.

The purpose of this study is to develop small molecules that report on the presence of specific A-beta conformations. In this effort, we pursued three distinct avenues. In the first approach, we characterized the binding of well-known amyloid probes to different A-beta morphologies. In doing so, we established the profile of these widely used A-beta markers and found that most of them lack the ability to differentiate between A-beta conformers. Next, we used unbiased fluorescence screening methods to discover the first class of chemical probes, indoles, which selectively interact with pre-fibrillar amyloids. Further, we leveraged these compounds to develop a rapid, inexpensive spectroscopic assay for monitoring pre-fibril levels in vitro. Finally, we used computational design to build multivalent peptide-based ligands that target the earliest A-beta aggregates, including trimers and tetramers. We found that these probes do not recognize A-beta monomers or fibrils, and that they retain specificity in human cerebrospinal fluid.

Together these studies have established the first chemical probes that target specific A-beta conformations. This work demonstrated that small molecules are indeed capable of discerning between A-beta assemblies, which contributes to our basic knowledge about the properties of A-beta aggregates. Moreover, these scaffolds are expected to become useful tools for defining how A-beta structure correlates with neurotoxicity in AD.