Elucidation of the Effect of Phospholipid Charge on the Rate of Insulin Aggregation and Structure and Toxicity of Amyloid Fibrils

Abstract

The plasma membrane is a dynamic structure that separates the cell interior from the extracellular space. The fluidity and plasticity of the membrane determines a large number of physiologically important processes ranging from cell division to signal transduction. In turn, membrane fluidity is determined by phospholipids that possess different charges, lengths, and saturation states of fatty acids. A growing body of evidence suggests that phospholipids may play an important role in the aggregation of misfolded proteins, which causes pathological conditions that lead to severe neurodegenerative diseases. In this study, we investigate the role of the charge of the most abundant phospholipids in the plasma membrane: phosphatidylcholine and phosphatidylethanolamine, zwitterions: phosphatidylserine and phosphatidylglycerol, lipids that possess a negative charge, and cardiolipin that has double negative charge on its polar head. Our results show that both zwitterions strongly inhibit insulin aggregation, whereas negatively charged lipids accelerate fibril formation. We also found that in the equimolar presence of zwitterions insulin yields oligomers that exert significantly lower cell toxicity compared to fibrils that were grown in the lipid-free environment. Such aggregates were not formed in the presence of negatively charged lipids. Instead, long insulin fibrils that had strong cell toxicity were grown in the presence of such negatively charged lipids. However, our results showed no correlation between the charge of the lipid and secondary structure and toxicity of the aggregates formed in its presence. These findings show that the secondary structure and toxicity are determined by the chemical structure of the lipid rather than by the charge of the phospholipid polar head.

Phospholipids and Cholesterol Determine Molecular Mechanisms of Cytotoxicity of α-Synuclein Oligomers and Fibrils

“…Recently reported results by Zhaliazka and co-workers showed that such lipid-determined toxicity of oligomers and fibrils could be a general phenomenon typical for a large group of amyloidogenic proteins. , Specifically, the researchers showed that the secondary structure and toxicity of amyloid β 1–42 (Aβ 1–42 ) oligomers and fibrils could be uniquely altered by cholesterol (Cho), cardiolipin (CL), and PC . Similar results were also reported by Matveyenka and co-workers for insulin and lysozyme. − …”

Elucidating the Role of Lipids in the Aggregation of Amyloidogenic Proteins

“…Experimental results reported by our group and other research groups demonstrated that lipids uniquely altered the morphology and secondary structure of amyloid aggregates formed in their presence. ,,− ,, Specifically, both insulin and lysozyme formed spherical oligomers in the presence of PC and PE. , These aggregates dominated by unordered protein secondary structure possessing very few if any β-sheets. At the same time, insulin and lysozyme formed fibrils in the presence of negatively charged lipids, such as CL, PS, and PG. , These aggregates were dominated by a parallel β-sheet secondary structure.…”

“…There are also numerous pieces of experimental evidence indicating that lipids change the secondary structure and morphology of amyloid oligomers and fibrils formed in their presence. Such fibrils exert drastically different levels of cell toxicity compared to amyloid oligomers and fibrils formed in the lipid-free environments. ,,,, …”

“…The most recent findings summarized in this Account demonstrate that lipid bilayers strongly alter the rates of protein aggregation. ,,,, The extent to which lipids alter the rate of protein aggregation directly depends on the structure of the protein, the net charge of the lipid, and the length and saturation of FAs in the lipid. There are also numerous pieces of experimental evidence indicating that lipids change the secondary structure and morphology of amyloid oligomers and fibrils formed in their presence.…”

“…Our group also investigated the effect of different phospho- and sphingolipids on the rate of insulin aggregation. ,− It was found that negatively charged phospholipids, such as CL, PS, PA, and phosphatidylglycerol (PG), accelerated the rate of insulin aggregation whereas zwitterionic lipids, such as PC and phosphatidyl­ethanolamine (PE), on the opposite, strongly inhibited fibril formation . It should be noted that ceramide (CER) was found to enhance insulin aggregation, whereas sphingomyelin (SM), on the other hand, slowed down fibril formation .…”