In Vivo Hyperspectral CARS Imaging Reveals Photobiomodulation-Driven Remodeling of Fatty Acid Homeostasis in an AD Mouse Model

Alzheimer's disease (AD) is increasingly acknowledged to be associated with early lipid metabolic dysfunction. However, the exact timing and mechanism of the emergence, in vivo organization, and response to therapeutic intervention of fatty-acid saturation imbalance in the living brain remain unclear. In this study, a longitudinal in vivo hyperspectral coherent anti-Stokes Raman scattering (CARS) microscopy platform, integrated with multivariate curve resolution (MCR), was established to enable label-free phenotyping of fatty-acid composition in the cortex of APP/PS1 mice. Hyperspectral CARS imaging reveals abundant lipid-droplet deposition at early stages, followed by a gradual enhancement of saturated-fat-associated vibrational signatures and a distinct plaque-centric spatial redistribution around amyloid deposits. The unmixing results are verified by liquid chromatography-mass spectrometry, which identifies palmitic acid and stearic acid as the major saturated lipid species enriched within pathological droplets. Through the application of a photobiomodulation (PBM) regimen (transcranial 808 nm, 40 Hz), it is demonstrated that PBM significantly alleviates saturated-rich lipid accumulation adjacent to plaques and cerebral vessels, restores the balance between saturated and unsaturated fatty acids, and is accompanied by improved cognitive performance. Collectively, this research establishes an imaging-based framework to elucidate in vivo lipid-metabolic remodeling during AD progression and to quantitatively evaluate metabolic reprogramming induced by PBM-based intervention.