Label-free characterization of Amyloid-β-plaques and associated lipids in brain tissues using stimulated Raman scattering microscopy

The brains of patients with neurodegenerative diseases such as Alzheimers Disease (AD) often exhibit pathological alterations that involve abnormal aggregations of proteins and lipids. Here, we demonstrate that high-resolution, label-free, chemically-specific imaging using Stimulated Raman Scattering Microscopy (SRS) provides novel insights into the biophysical properties and biochemical composition of such pathological structures. In brain slices of a mouse model of AD, SRS reveals large numbers of Amyloid-{beta} plaques that commonly form a characteristic, three-dimensional core-shell structure, with a fibrillar proteinaceous core surrounded by a halo-like shell of lipid-rich deposits. SRS spectroscopic imaging allows for a clean, label-free visualization of the misfolded ({beta}-sheet) Amyloid-{beta} content in the plaque core. Surrounding lipid-rich deposits are found to contain comparatively high concentrations of membrane lipids (sphingomyelin, phosphatidylcholine), but lower levels of cholesterol than healthy white matter structures. Overall, the SRS spectra of plaque-associated lipids closely resemble those of nearby neurites, with the notable difference of a higher degree of lipid unsaturation compared to healthy brain structures. We hypothesize that plaque-associated lipid deposits may result from neuritic dystrophy associated with AD, and that the observed increased levels of unsaturation could help identify the kinds of pathological alterations taking place. Taken together, our results highlight the potential of Stimulated Raman Scattering microscopy to contribute to a deeper understanding of neurodegenerative diseases.