time when the intensity of ThT fluorescence reached 50% of the maximum; tgrow, time when the intensity of ThT fluorescence reached 90% of the maximum; ThT, thioflavin T; tlag, time when the intensity of ThT fluorescence reached 10% of the maximum.

Abrupt aggregation of amyloid β1‐42 (Aβ) peptide is a hallmark of Alzheimer's disease (AD), a severe pathology that affects more than 44 million people worldwide. A growing body of evidence suggests that lipids can uniquely alter rates of Aβ1‐42 aggregation. However, it remains unclear whether lipids only alter rates of protein aggregation or also uniquely modify the secondary structure and toxicity of Aβ1‐42 oligomers and fibrils. In this study, we investigated the effect of phosphatidylcholine (PC), cardiolipin (CL), and cholesterol (Chol) on Aβ1‐42 aggregation. We found that PC, CL and Chol strongly accelerated the rate of fibril formation compared to the rate of Aβ1‐42 aggregation in the lipid‐free environment. Furthermore, anionic CL enabled the strongest acceleration of Aβ1‐42 aggregation compared to zwitterionic PC and uncharged Chol. We also found that PC, CL and Chol uniquely altered the secondary structure of early‐, middle‐ and late‐stage Aβ1‐42 aggregates. Specifically, CL and Chol drastically increased the amount of parallel β‐sheet in Aβ1‐42 oligomers and fibrils grown in the presence of these lipids. This caused a significant increase in the toxicity of Aβ : CL and Aβ : Chol compared to the toxicity of Aβ : PC and Aβ1‐42 aggregates formed in the lipid‐free environment. These results demonstrate that toxicity of Aβ aggregates correlates with the amount of their β‐sheet content, which, in turn, is determined by the chemical structure of lipids present at the stage of Aβ1‐42 aggregation.

Biophysical properties of plasma membranes are determined by a chemical structure of phospholipids, including saturation of fatty acids and charge of polar heads of these molecules. Phospholipids not only determine fluidity and plasticity of membranes but also play an important role in abrupt aggregation of misfolded proteins. In this study, we investigate the role of the charge of the most abundant phospholipids in the plasma membrane on the aggregation properties of the lysozyme. We found that the charge of phospholipids determines the aggregation rate of lysozyme and the morphology of the protein aggregates. However, the secondary structure and toxicity of these protein specimens are determined by the chemical nature rather than the charge of phospholipids. These findings show that the charge of phospholipids can be a key factor that determines the stability and aggregation mechanism of amyloidogenic proteins.

Abrupt aggregation of amyloid beta (Aβ) peptide is strongly associated with Alzheimer's disease. In this study, we used atomic force microscopy−infrared (AFM-IR) spectroscopy to characterize the secondary structure of Aβ oligomers, protofibrils and fibrils formed at the early (4 h), middle (24 h), and late (72 h) stages of protein aggregation. This innovative spectroscopic approach allows for label-free nanoscale structural characterization of individual protein aggregates. Using AFM-IR, we found that at the early stage of protein aggregation, oligomers with parallel β-sheet dominated. However, these species exhibited slower rates of fibril formation compared to the oligomers with antiparallel β-sheet, which first appeared in the middle stage. These antiparallel β-sheet oligomers rapidly propagated into fibrils that were simultaneously observed together with parallel β-sheet fibrils at the late stage of protein aggregation. Our findings showed that aggregation of Aβ is a complex process that yields several distinctly different aggregates with dissimilar toxicities.

Abrupt aggregation of misfolded proteins is a hallmark of many medical pathologies including diabetes type 2, Alzheimer and Parkinson diseases. This results in the formation of amyloid fibrils, protein aggregates with distinct supramolecular chirality. A growing body of evidence suggests that lipids can alter rates of protein aggregation. In this study, we investigated whether lipids could alter the supramolecular chirality of amyloid fibrils. We found that if present at the stage of protein aggregation, phospho-and sphingolipids uniquely reversed supramolecular chirality of insulin and lysozyme fibrils. Furthermore, amyloid fibrils with opposite supramolecular chirality exerted distinctly different cell toxicity. Specifically, insulin and lysozyme fibrils with reversed supramolecular chirality were less toxic to cells than the aggregates with normal supramolecular chirality. These findings point on the important role of lipids and supramolecular chirality of amyloid fibrils in the onset and progression of amyloid diseases.