Multiscale Infrared Spectroscopic Imaging of Amyloid Aggregates in Alzheimer’s Disease
Assistant Professor of Chemistry and Biochemistry
The University of Alabama
Aggregation of the amyloid beta (Aβ) protein into insoluble fibrillar deposits is one of main pathological features of Alzheimer’s disease (AD), but their exact role in AD and onset of dementia is unclear. It now believed that soluble Aβ oligomers and not mature fibrils are the primary neurotoxic species in AD, and that oligomers and prefibrillar aggregates are the likely key contributors to AD pathogenesis while mature fibrils possibly represent the end point of the disease. It is also well known that all amyloid fibrils are not equivalent: Aβ fibrils are polymorphic, with variations in molecular structure associated with different morphologies of fibrils which not only exhibit different levels of neurotoxicity, but also have been shown to be associated with different stages of AD.
Investigating structural ensembles of different amyloid aggregates is experimentally challenging, as current approaches for structure determination such as NMR and FTIR cannot spatially resolve spectra. Without spatial resolution, it is difficult to unequivocally attribute spectral features to specific aggregates or morphologies. This limitation can be circumvented by integrating microscopy with infrared (IR) spectroscopy, which provides the best of both worlds: spatial resolution and structural/chemical sensitivity. Two novel implementations of this approach will be presented, namely AFM-IR and IR confocal imaging. AFM-IR is a technique that integrates IR spectroscopy with an atomic force microscope and can examine nanoscale structural details by combining the spatial resolution of AFM and the chemical resolution of infrared. AFM-IR offers the unique capability of acquiring infrared spectra that are sensitive to protein structure with nanoscale resolution, so characteristic features of individual aggregates and fibrils can be measured in-vitro. IR confocal imaging, on the other hand, integrates an infrared laser with an optical microscope, and allows for investigating amyloid structure in plaques in AD tissues. I will present our work on applications of AFM-IR towards characterizing oligomeric and early-stage fibrillar aggregates of Aβ and tau and discuss the structural heterogeneities and polymorphism of each. However, the structure of amyloid aggregates in AD brain is poorly understood, and it is not always possible to determine if structures observed in-vitro can exist in plaques. To that end, we have used IR imaging to map secondary structures in plaques in AD tissues, and I will present IR imaging results which validate and complement in-vitro observations from isolated aggregates.