Amyloid models for quantitative x‐ray brain amyloid imaging

Abstract

AbstractBackgroundA range of human neurological disorders are related to amyloid fibrillar deposits including Alzheimer’s disease (AD). AD is linked to the presence of amyloid‐β (Aβ) fibrils and those are comprised of β‐sheet motifs. The precise mechanism through which Aβ leads to AD are not completely understood. The scarcity of human derived Aβ fibrils and high cost of recombinant Aβ fibrils further restrict the research in this area. However, intrinsically disordered proteins (IDPs) have been reported as amyloid model proteins. Even though, these models qualitatively mimic β‐sheet structure of Aβ fibrils but often fail to quantitatively match the β‐sheet content. Therefore, this study aimed to develop model proteins to closely mimic Aβ fibrils structure and produce them in a large quantity to evaluate emerging X‐ray methods to detect and quantify amyloid plaques.MethodsWe used a commercially available β‐lactoglobulin A and B genetic mixture (LGAB) protein as our amyloid model to mimic the structural properties of Aβ fibrils. Here, the alteration in the protein conformation was induced by altering the net charge on the polypeptide chain by changing the pH from 2 to 7. Additionally, we studied the fibrillization of these proteins in presence of more cooperative transitions at 90°C. The secondary structural compositions of LGAB were studied using attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy. The nanostructures of LGAB fibrils were characterized using small angle X‐ray scattering (SAXS). We further varied the mass fraction of LGAB fibrils (amyloid model) and quantitatively measured their amyloid content using a SAXS based method.ResultsATR‐FTIR (Fig.1) and SAXS (Fig.2) studies confirm that thermally aggregated LGAB fibrils exhibited parallel β‐sheet secondary structure similar to Aβ fibrils. Systematic variation in LGAB fibril concentration in the tissue mimicking environment showed a linear correlation between protein mass fraction and area under the curve of a peak (∼ 6 nm−1) corresponding to inter β‐sheet spacing (Fig.3).ConclusionOur study suggests that the amyloid model based on LGAB fibrils can likely be used to simulate Aβ fibrils in phantom studies for the evaluation of emerging X‐ray methods to detect and quantify amyloid plaques.