Monitoring photobiomodulation of amyloid-β aggregation in 3D cultured cells using label-free nonlinear optical imaging

The accumulation of beta-amyloid (Aβ) peptide aggregates, commonly known as plaques, is considered a key hallmark in the development of Alzheimer’s disease (AD). Recently, low-level light therapy (LLLT), also referred to as photobiomodulation (PBM), has emerged as a promising treatment approach for AD. Previous studies have shown that PBM reduces Aβ load primarily by enhancing the clearance capabilities of glia cells. However, it remains unclear whether PBM can directly reduce the formation of Aβ plaques in neuronal cells independent of the glia cell effect. In this study, we employed three-dimensional (3D) cultured HEK 293 APPsw cells as an AD model to investigate the impact of PBM on Aβ aggregation. We demonstrated that label-free two-photon excited fluorescence (TPEF) imaging and second harmonic generation (SHG) imaging are effective tools for monitoring Aβ aggregation in 3D cell models. The TPEF imaging results and subsequent quantification revealed that PBM, particularly with low-level near-infrared light from an 808 nm laser (compared to 1064, 1210, and 1470 nm lasers), significantly reduced Aβ aggregation, specifically plaques formation, in the 3D cultured cells, with the effect found to be dose-dependent. Moreover, a comprehensive analysis of protein expression in the 3D cultured cells revealed that PBM induces overexpression of the LRP1 receptor, which mediates Aβ degradation and thus leads to the reduction of Aβ aggregation. This study highlights the use of label-free nonlinear optical imaging to monitor Aβ aggregation in AD progression and provides novel insights into the effects of PBM on Aβ plaque formation in AD models.