Fluorescence and Raman imaging of amyloid plaques reveals carotenoid accumulations

Alzheimer’s disease (AD) is characterized by the presence of extracellular deposits of amyloid-beta peptides (known as AD plaques). Its assessment is usually achieved post-mortem, requiring chemical pre-treatment via an antibody or indirect labelling. Label-free imaging techniques, like auto-fluorescence, spontaneous Raman (SpR) and stimulated Raman (SRS) imaging could be performed on tissue in its native state to study the biomolecular composition of AD plaques and contribute to a better understanding of the disease. Here we present imaging results of human brain amyloid core plaques. We show blue and green autofluorescence emission localized at the same plaque position while Raman spectroscopy revealed the presence of carotenoids at the same spot. For identifying the underlying carotenoids, first carotenoid reference spectra in hexane solution and then adsorbed on aggregated Aβ42 peptides were recorded. From the six carotenoids measured, lycopene matched closest with the Raman peak positions observed in the measured AD plaque. Furthermore, we used SRS to investigate the presence of a lipid halo around plaque locations as reported in literature for transgenic AD mice.