Fibrillated Tau proteins are considered a hallmark of Alzheimer’s disease (AD) and may serve as a biosensor for its early diagnosis. Various studies have found that these proteins aggregate and accumulate in the retina in a manner similar to their deposition in the brains of AD patients. The alterations in retinal accumulation can be detected non-invasively by using AD-related scores. These scores were obtained from the reflectance measures of the individual’s fundus. However, the direct correlation between AD and the optical properties of fibrillated Tau proteins is still under investigation. A recent study published in Nature explained this association well using a novel optical technique.
Light Scattering Spectroscopy (LSS) was used in this study to detect the scattering characteristics of protein fibrils. This instrument consisted of an Optical System Light 2 (OSL2) Fiber Illuminator, which served as the primary light source, along with disposable hollow square borosilicate glass capillaries (0.80 × 0.80 mm) and two cameras (C1 and C2) for imaging. This study utilized the recombinant K18 domain of Tau protein isoform 0N4R. Prior to instrument analysis, dynamic light scattering (DLS), scanning transmission electron microscopy (STEM), and thioflavin T (ThT) fluorescence were applied to confirm the fibrillation of protein.
All spectroscopic characteristics were measured using a bench-side scattering spectroscope employing a 400–700 nm wavelength and a light source. The wavelength of protein fibrillation was determined using a customized side-scattering spectroscope. This should be visible in the reflection of the AD patient’s retina.
In this study, a bovine serum albumin (BSA) solution was chosen as a standard or model sample. These solutions were exposed to different aggregate durations, such as 0, 0.5, 1, 3, 5, and 6.5 h, at 63OC temperature.
In DLS analysis, at the start of aggregation (t = 0), the average hydrodynamic radius of BSA was found to be 4.43 ± 0.03 nm, after 0.5 h measurement increased to 6.7 ± 0.5 nm and after 5 h to 10.9 ± 0.7 nm. These measurements confirmed the formation and progressive growth of aggregates. Additionally, monodisperse of the sample was observed at t = 0, whereas aggregate size and polydispersity increased over time.
In STEM analysis, BSA exhibited the rapid formation of fibril under high temperatures. It was observed that within 30 minutes, the normalized ThT fluorescence intensity increased threefold, and the fourfold intensity increased after 3 h. At around 5 hours, it reached the plateau state. These results demonstrated the association of the protein cluster growth with the improvement of fluorescence intensity.
Tau protein solutions were prepared using polyphosphates (polyP) as an aggregating agent. The total incubation period was 35 days. Similar to BSA findings, fibrillation time was significantly increased with enhanced total side-scattering power (Ω), but the shape of the total side-scattering power curve differed from the BSA curve. The radius was found to be 140 ± 2 nm at the start of fibrillation and 1800 ± 200 nm at 7 days. However, the autocorrelation curve was not obtained after 35 days due to the fibrillation sedimentation of protein aggregates.
ThT fluorescence intensity in the fibrillated sample increased fourfold compared to non-fibrillated control after four days of fibrillation and eightfold after seven days. However, no intensity was increased in the sample compared to the control after 1 month. After 30 days, larger amorphous aggregates were identified exceeding 5 µm in size. The scattering slope of the curve increased with fibrillation time and showed stronger wavelength dependence as well as consistency with AD retina spectra.
In conclusion, this study highlights the spectral properties seen in AD patients’ retinas, which are reflected in the scattering of Tau protein fibrils. These discoveries are essential for expanding the knowledge, improving hyperspectral imaging tools, enhancing early AD detection, and expanding our understanding of retinal biomarkers.
Reference: Salajková Z, Barolo L, Baiocco P, et al. Optical signature of retinal Tau fibrillation. Sci Rep. 2025;15:7792. doi:10.1038/s41598-025-92565-w
