Lipids reverse supramolecular chirality and reduce toxicity of amyloid fibrils

Macrophages and Natural Killers Degrade α-Synuclein Aggregates

Amyloid oligomers and fibrils are protein aggregates that exert a high cell toxicity. Efficient degradation of these protein aggregates can minimize the spread and progression of neurodegeneration. In this study, we investigate the properties of natural killer (NK) cells and macrophages in the degradation of α-synuclein (α-Syn) aggregates grown in a lipid-free environment and in the presence of phosphatidylserine and cholesterol (PS/Cho), which are lipids that are directly associated with the onset and progression of Parkinson's disease. We found that both types of α-Syn aggregates were endocytosed by neurons, which caused strong damage to cell endosomes. Our results also indicated that PS/Cho vesicles drastically increased the toxicity of α-Syn fibrils formed in their presence compared to the toxicity of α-Syn aggregates grown in a lipid-free environment. Both NK cells and macrophages were able to degrade α-Syn and α-Syn/Cho monomers, oligomers, and fibrils. Quantitative analysis of protein degradation showed that macrophages demonstrated substantially more efficient internalization and degradation of amyloid aggregates in comparison to NK cells. We also found that amyloid aggregates induced the proliferation of macrophages and NK cells and significantly changed the expression of their cytokines and chemokines.

The influence of zwitterionic and anionic phospholipids on protein aggregation

The progressive aggregation of misfolded proteins is the underlying molecular cause of numerous pathologies including Parkinson's disease and injection and transthyretin amyloidosis. A growing body of evidence indicates that protein deposits detected in organs and tissues of patients diagnosed with such pathologies contain fragments of lipid membranes. In vitro experiments also showed that lipid membranes could strongly change the aggregation rate of amyloidogenic proteins, as well as alter the secondary structure and toxicity of oligomers and fibrils formed in their presence. In this review, the effect of large unilamellar vesicles (LUVs) composed of zwitterionic and anionic phospholipids on the aggregation rate of insulin, lysozyme, transthyretin (TTR) and α- synuclein (α-syn) will be discussed. The manuscript will also critically review the most recent findings on the lipid-induced changes in the secondary structure of protein oligomers and fibrils, as well as reveal the extent to which lipids could alter the toxicity of protein aggregates formed in their presence.

Vibrational Optical Activity of Amyloid Fibrils

Amyloid fibrils are supramolecular systems showing distinct chirality at different levels of their complex multilayered architectures. Due to the regular long-range chiral organization, amyloid fibrils exhibit the most intense Vibrational Optical Activity (VOA) signal observed up to now, making VOA techniques: Vibrational Circular Dichroism (VCD) and Raman Optical Activity (ROA) very promising tools to explore their structures, handedness and intricate polymorphism. This concept article reviews up-to-date experimental studies on VOA applications to investigate amyloid fibrils highlighting its future potential in analyzing of these unique supramolecular systems, in particular in the context of biomedicine and nanotechnology.

Secondary structure and toxicity of transthyretin fibrils can be altered by unsaturated fatty acids

Transthyretin amyloidosis is a severe pathology characterized by the progressive accumulation of transthyretin (TTR) in various organs and tissues. This highly conserved through vertebrate evolution protein transports thyroid hormone thyroxine. In our bodies, TTR can interact with a large number of molecules, including ω-3 and ω-6 polyunsaturated fatty acids (PUFAs) that are broadly used as food supplies. In this study, we investigated the effect of ω-3 and ω-6 PUFAs, as well as their fully saturated analog, on TTR aggregation. Our results showed that both ω-3 and ω-6 PUFAs strongly decreased the rate of TTR aggregation. We also found that in the presence of PUFAs, TTR formed morphologically different fibrils compared to the lipid-free environment. Nano-Infrared imaging revealed that these fibrils had drastically different secondary structures compared to the secondary structure of TTR aggregates formed in the PUFAs-free environment. Furthermore, TTR fibrils formed in the presence of ω-3 and ω-6 PUFAs exerted significantly lower cell toxicity compared to the fibrils formed in the absence of fatty acids.

Cholesterol and Sphingomyelin Uniquely Alter the Rate of Transthyretin Aggregation and Decrease the Toxicity of Amyloid Fibrils

Transthyretin (TTR) is a small tetrameric protein that aggregates, forming highly toxic oligomers and fibrils. In the blood and cerebrospinal fluid, TTR can interact with various biomolecules, phospho- and sphingolipids, and cholesterol on the red blood cell plasma membrane. However, the role of these molecules in TTR aggregation remains unclear. In this study, we investigated the extent to which phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol (Cho), important components of plasma membranes, could alter the rate of TTR aggregation. We found that PC and SM inhibited TTR aggregation whereas Cho strongly accelerated it. The presence of these lipids during the stage of protein aggregation uniquely altered the morphology and secondary structure of the TTR fibrils, which changed the toxicity of these protein aggregates. These results suggest that interactions of TTR with red blood cells, whose membranes are rich with these lipids, can trigger irreversible aggregation of TTR and cause transthyretin amyloidosis.

Elucidation of the Role of Lipids in Late Endosomes on the Aggregation of Insulin

Abrupt aggregation of misfolded proteins is the underlying molecular cause of numerous pathologies including diabetes type 2 and injection amyloidosis. Although the exact cause of this process is unclear, a growing body of evidence suggests that protein aggregation is linked to a high protein concentration and the presence of lipid membranes. Endosomes are cell organelles that often possess high concentrations of proteins due to their uptake from the extracellular space. However, the role of endosomes in amyloid pathologies remains unclear. In this study, we used a set of biophysical methods to determine the role of bis(monoacylglycero)phosphate (BMP), the major lipid constituent of late endosomes on the aggregation properties of insulin. We found that both saturated and unsaturated BMP accelerated protein aggregation. However, very little if any changes in the secondary structure of insulin fibrils grown in the presence of BMP were observed. Therefore, no changes in the toxicity of these aggregates compared to the fibrils formed in the lipid-free environment were observed. We also found that the toxicity of insulin oligomers formed in the presence of a 77:23 mol/mol ratio of BMP/PC, which represents the lipid composition of late endosomes, was slightly higher than the toxicity of insulin oligomers formed in the lipid-free environment. However, the toxicity of mature insulin fibrils formed in the presence of BMP/PC mixture was found to be lower or similar to the toxicity of insulin fibrils formed in the lipid-free environment. These results suggest that late endosomes are unlikely to be the source of highly toxic protein aggregates if amyloid proteins aggregate in them.

Role of Saturation and Length of Fatty Acids of Phosphatidylserine in the Aggregation of Transthyretin

The progressive accumulation of transthyretin (TTR), a small protein that transports thyroxine, in various organs and tissues is observed upon transthyretin amyloidosis, a severe pathology that affects the central, peripheral, and autonomic nervous systems. Once expressed in the liver and choroid plexus, TTR is secreted into the bloodstream and cerebrospinal fluid. In addition to thyroxine, TTR interacts with a large number of molecules, including retinol-binding protein and lipids. In this study, we examined the extent to which phosphatidylserine (PS), a phospholipid that is responsible for the recognition of apoptotic cells by macrophages, could alter the stability of TTR. Using thioflavin T assay, we investigated the rates of TTR aggregation in the presence of PS with different lengths and saturation of fatty acids (FAs). We found that all analyzed lipids decelerated the rate of TTR aggregation. We also used a set of biophysical methods to investigate the extent to which the presence of PS altered the morphology and secondary structure of TTR aggregates. Our results showed that the length and saturation of fatty acids in PS uniquely altered the morphology and secondary structure of TTR fibrils. As a result, TTR fibrils that were formed in the presence of PS with different lengths and saturation of FAs exerted significantly lower cell toxicity compared with the TTR aggregates grown in the lipid-free environment. These findings help to reveal the role of PS in transthyretin amyloidosis and determine the role of the length and saturation of FAs in PS on the morphology and secondary structure of TTR fibrils.

Fatty Acids Reverse the Supramolecular Chirality of Insulin Fibrils

Long-chain unsaturated and polyunsaturated fatty acids (LCUFAs and LCPUFAs, respectively) are the essential components of phospholipids and sphingolipids, major building blocks of plasma and organelle membranes. These molecules are also involved in cell signaling and energy metabolism. Hence, both LCUFAs and LCPUFAs are broadly used as food supplements. However, the role of these fatty acids (FAs) in the self-assembly of misfolded proteins remains unclear. In this study, we investigated the effect of LCUFAs and LCPUFAs, as well as their saturated analogue, on insulin aggregation. Using vibrational circular dichroism, we found that all analyzed FAs reversed the supramolecular chirality of insulin fibrils. Molecular dynamics simulations showed that strong hydrophobic interactions were formed between the long aliphatic tails of FAs and hydrophobic amino acid residues of insulin. We infer that such insulin:FA complexes had different self-assembly mechanisms compared to that of insulin alone, which resulted in the observed reversal of the supramolecular chirality of the amyloid fibrils.

Unsaturated fatty acids uniquely alter aggregation rate of α-synuclein and insulin and change the secondary structure and toxicity of amyloid aggregates formed in their presence

Docosahexaenoic (DHA) and arachidonic acids (ARA) are omega-3 and omega-6 long-chain polyunsaturated fatty acids (LCPUFAs). These molecules constitute a substantial portion of phospholipids in plasma membranes. Therefore, both DHA and ARA are essential diet components. Once consumed, DHA and ARA can interact with a large variety of biomolecules, including proteins such as insulin and α-synuclein (α-Syn). Under pathological conditions known as injection amyloidosis and Parkinson's disease, these proteins aggregate forming amyloid oligomers and fibrils, toxic species that exert high cell toxicity. In this study, we investigate the role of DHA and ARA in the aggregation properties of α-Syn and insulin. We found that the presence of both DHA and ARA at the equimolar concentrations strongly accelerated aggregation rates of α-Syn and insulin. Furthermore, LCPUFAs substantially altered the secondary structure of protein aggregates, whereas no noticeable changes in the fibril morphology were observed. Nanoscale Infrared analysis of α-Syn and insulin fibrils grown in the presence of both DHA and ARA revealed the presence of LCPUFAs in these aggregates. We also found that such LCPUFAs-rich α-Syn and insulin fibrils exerted significantly greater toxicities compared to the aggregates grown in the LCPUFAs-free environment. These findings show that interactions between amyloid-associated proteins and LCPUFAs can be the underlying molecular cause of neurodegenerative diseases.

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

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.

Solid-to-Liposome Conformational Transition of Phosphatidylcholine and Phosphatidylserine Probed by Atomic Force Microscopy, Infrared Spectroscopy, and Density Functional Theory Calculations

Liposomes are emerging therapeutic formulations for site-specific delivery of chemotherapeutic drugs. The efficiency and selectivity of drug delivery by these carriers largely rely on their surface properties, shape, and size. There is a growing demand for analytical approaches that can be used for structural and morphological characterization of liposomes at the single-vesicle level. AFM-IR is a modern optical nanoscopic technique that combines the advantages of scanning probe microscopy and infrared spectroscopy. Our findings show that AFM-IR can be used to probe conformational changes in phospholipids that take place upon their assembly into liposomes. Such conclusions can be made based on the corresponding changes in intensities of the lipid vibrational bands as the molecules transition from a solid state into large unilamellar vesicles (LUVs). This spectroscopic analysis of LUV formation together with density functional theory calculations also reveals the extent to which the molecular conformation and local environment of the functional groups alter the AFM-IR spectra of phospholipids. Using melittin as a test protein, we also examined the extent to which LUVs can be used for protein internalization. We found that melittin enters LUVs nearly instantaneously, which protects it from possible structural modifications that are caused by a changing environment. This foundational work empowers AFM-IR analysis of liposomes and opens new avenues for determination of the molecular mechanisms of liposome-drug interactions.