Investigation of amyloid plaques and enoxaparin brain penetration using Raman and infrared spectroscopy

Introduction: Alzheimer’s disease (AD) is a neurodegenerative disease associated with the deposition of β-amyloid (Aβ) dense core plaques in brain tissue. Peripheral administration of enoxaparin has been linked to decreased Aβ burden although the ability of enoxaparin fractions to enter the brain is unclear. Furthermore, Raman spectral mapping is a non-destructive tool which may have applications for dense-core plaque characterisation.

Aims: Our primary aim was to detect Fourier transform infrared (FTIR) and Raman spectral changes in the brain tissue of AD mouse models (Tg2576) associated with the administration of enoxaparin. Secondly, we aimed to investigate differences between plaque and non-plaque tissue using FTIR and Raman mapping.

Methods: Tg2576 mice were administered ip injections of enoxaparin (n=10) or water for injection (n=5), three times a week for five months. Cerebral cortex sections were prepared and capillaries were identified using differential interference contrast (DIC) microscopy. Raman spectra was collected at 0, 1, 2, 5, 10 and 20 µm from the capillary walls. FTIR spectra was acquired using 10 µm steps to a position 70 µm from the capillary walls. Differences in spectra were analysed using partial least squares (PLS) regression. FTIR and Raman spectral maps of dense core plaques were also assessed for spectral differences using clustering and principal component analysis.

Results: Spectral differences were detected between the enoxaparin-treated and control mice. Using PLS regression, a 17-factor model for the FTIR spectra and an 11-factor system for the Raman spectra explained these differences. FTIR and Raman mapping also detected spectral differences between non-plaque and plaque tissue at 2931, 2862 and 1685-1615cm-1 in the FTIR and 1667, 1433, 1124, 1082 and 1000cm-1 Raman spectral bands.

Discussion: This study detected differences in the brain chemistry of enoxaparin-treated and control mice suggesting that enoxaparin fractions can either cross the blood brain barrier or have peripheral effects which affect the brain. The FTIR and Raman mapping supports findings from previous studies regarding the distribution of phospholipids and changes in protein structure. Importantly, this study shows a shift in the phenylalanine band within the plaque core. The high levels of phospholipids with the phenylalanine shift in the dense core plaques indicates a potential interaction between components of cellular membranes and amyloid plaque formation.