Aggregation behavior and peptide-lipid interaction in an amyloid model system

Abstract: Proteins constitute a major component of living cells and are essential to their function. Sometimes proteins misfold or aggregate in abnormal ways, which can give rise to diseases. Amyloids are a special type of highly ordered protein aggregates made of long intermolecular beta-sheets stacked into fibrils. Formation of amyloids is associated with many diseases, especially diseases related to neurodegeneration such as Alzheimer’s and Parkinson’s disease. That is one major reason for why there is a strong research interest in amyloids. The physiological environment, and especially neural tissue, is also very rich in lipids. Therefore, it is of special interest to understand more about how amyloids behave in the presence of lipids. For example, to what degree do amyloid forming proteins and peptides co-aggregate with lipids? How does the presence of lipids affect the kinetics of amyloid formation? Can amyloids have disruptive effects on lipid structures such as membranes? In this thesis a short amyloid forming peptide, known as NACore, has been used as a model peptide to study amyloid formation and amyloid-lipid interaction. NACore is an 11 amino acid residue peptide fragment from the protein alpha-synuclein, which forms amyloid fibrils in Parkinson’s disease. More specifically, NACore spans from residue 68 to 78 of alpha-synuclein and has the sequence GAVVTGVTAVA. It is part of the so-called non-amyloid-beta-component of alpha-synuclein, which is the central region of the protein. To investigate amyloid-lipid interaction, the peptide was studied in the presence of phospholipid vesicles and a fatty acid. We found that NACore readily forms amyloid fibrils in vitro and that the process can be controlled using pH. In terms of lipid interaction, we found that NACore and phospholipids co-aggregate on the aggregate level but remain mostly segregated on molecular length scales. The presence of lipids can substantially inhibit the fibrillation of the peptide. That was especially the case in the presence of the fatty acid linoleic acid, where even a very low lipid concentration resulted in substantial inhibition. Co-aggregation of amyloids and lipids could be important in mechanisms of toxicity to cells. For example, trapping of lipid membranes in amyloid fibril networks might disturb cellular processes such as vesicle trafficking. Clarifying the effects of lipids on the kinetics of amyloid formation might help us to better understand how amyloid formation is normally kept under control in the physiological environment. Our results obtained with this model peptide can also shed light on commonalities and differences between different amyloid forming proteins and peptides. Furthermore, they contribute to better understanding of biomolecular self-assembly in general.