The toxicities of A30P and A53T α-synuclein fibrils can be uniquely altered by the length and saturation of fatty acids in phosphatidylserine.

Progressive degeneration of dopaminergic neurons in the midbrain, hypothalamus, and thalamus is a hallmark of Parkinson's disease (PD). Neuronal death is linked to the abrupt aggregation of α-synuclein (α-syn), a small protein that regulates vesicle trafficking in synaptic clefts. Studies of families with a history of PD revealed several mutations in α-syn including A30P and A53T that are linked to the early onset of this pathology. Numerous pieces of evidence indicate that lipids can alter the rate of protein aggregation, as well as modify the secondary structure and toxicity of amyloid oligomers and fibrils. However, the role of lipids in the stability of α-syn mutants remains unclear. In this study, we investigate the effect of phosphatidylserine (PS), an anionic lipid that plays an important role in the recognition of apoptotic cells by macrophages, in the stability of WT, A30P, and A53T α-syn. We found PS with different lengths and saturation of fatty acids accelerated the rate of WT and A30P aggregation. At the same time, the opposite effect was observed for most PS on A53T. We also found that PS with different lengths and saturation of fatty acids change the secondary structure and toxicities of WT, A30P, and A53T fibrils. These results indicate that lipids can play an important role in the onset and spread of familial PD.

Secondary structure and toxicity of lysozyme fibrils are determined by the length and unsaturation of phosphatidic acid.

A progressive aggregation of misfolded proteins is a hallmark of numerous pathologies including diabetes Type 2, Alzheimer's disease, and Parkinson's disease. As a result, highly toxic protein aggregates, which are known as amyloid fibrils, are formed. A growing body of evidence suggests that phospholipids can uniquely alter the secondary structure and toxicity of amyloid aggregates. However, the role of phosphatidic acid (PA), a unique lipid that is responsible for cell signaling and activation of lipid-gated ion channels, in the aggregation of amyloidogenic proteins remains unclear. In this study, we investigate the role of the length and degree of unsaturation of fatty acids (FAs) in PA in the structure and toxicity of lysozyme fibrils formed in the presence of this lipid. We found that both the length and saturation of FAs in PA uniquely altered the secondary structure of lysozyme fibrils. However, these structural differences in PA caused very little if any changes in the morphology of lysozyme fibrils. We also utilized cell toxicity assays to determine the extent to which the length and degree of unsaturation of FAs in PA altered the toxicity of lysozyme fibrils. We found that amyloid fibrils formed in the presence of PA with C18:0 FAs exerted significantly higher cell toxicity compared to the aggregates formed in the presence of PA with C16:0 and C18:1 FAs. These results demonstrated that PA can be an important player in the onset and spread of amyloidogenic diseases.

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.

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.

Large Unilamellar Vesicles of Phosphatidic Acid Reduce the Toxicity of α-Synuclein Fibrils.

Parkinson's disease (PD) is a severe pathology that is caused by a progressive degeneration of dopaminergic neurons in substantia nigra pars compacta as well as other areas in the brain. These neurodegeneration processes are linked to the abrupt aggregation of α-synuclein (α-syn), a small protein that is abundant at presynaptic nerve termini, where it regulates cell vesicle trafficking. Due to the direct interactions of α-syn with cell membranes, a substantial amount of work was done over the past decade to understand the role of lipids in α-syn aggregation. However, the role of phosphatidic acid (PA), a negatively charged phospholipid with a small polar head, remains unclear. In this study, we examined the effect of PA large unilamellar vesicles (LUVs) on α-syn aggregation. We found that PA LUVs with 16:0, 18:0, and 18:1 FAs drastically reduced the toxicity of α-syn fibrils if were present in a 1:1 molar ratio with the protein. Our results also showed that the presence of these vehicles changed the rate of α-syn aggregation and altered the morphology and secondary structure of α-syn fibrils. These results indicate that PA LUVs can be used as a potential therapeutic strategy to reduce the toxicity of α-syn fibrils formed upon PD.

Lipids uniquely alter secondary structure and toxicity of lysozyme aggregates.

Abrupt aggregation of misfolded proteins is a hallmark of the large group of amyloid pathologies that include diabetes type 2, Alzheimer and Parkinson's diseases. Protein aggregation yields oligomers and fibrils, ß-sheet-rich structures that exert cell toxicity. Microscopic examination of amyloid deposits reveals the presence of lipids membranes, which suggests that lipids can be involved in the process of pathogenic protein assembly. In this study, we show that lipids can uniquely alter the aggregation rates of lysozyme, a protein that is associated with systemic amyloidosis. Specifically, cardiolipin (CL), ceramide (CER), and sphingomyelin (SM) accelerate, phosphatidylcholine (PC) strongly inhibits, whereas phosphatidylserine (PS) has no effect on the rate of protein aggregation. Furthermore, lipids uniquely alter the secondary structure of lysozyme aggregates. Furthermore, we found that lysozyme aggregates grown in the presence of CL, CER, SM, PS, and CL:PC mixtures exert significantly lower production of reactive oxygen species and mitochondrial dysfunction compared to lysozyme:PC aggregates and lysozyme fibrils grown in the lipid-free environment. These findings suggest that a change in the lipid composition of cell membranes, which is taken place upon neurodegeneration, may trigger the formation of toxic protein species that otherwise would not be formed.

Elucidation of molecular mechanisms by which amyloid ß1-42 fibrils exert cell toxicity.

Abrupt aggregation of amyloid ß1-42 (Aß1-42) peptide in the frontal lobe is the expected underlying cause of Alzheimer's disease (AD). ß-Sheet-rich oligomers and fibrils formed by Aß1-42 exert high cell toxicity. A growing body of evidence indicates that lipids can uniquely alter the secondary structure and toxicity of Aß1-42 aggregates. At the same time, underlying molecular mechanisms that determine this difference in toxicity of amyloid aggregates remain unclear. Using a set of molecular and biophysical assays to determine the molecular mechanism by which Aß1-42 aggregates formed in the presence of cholesterol, cardiolipin, and phosphatidylcholine exert cell toxicity. Our findings demonstrate that rat neuronal cells exposed to Aß1-42 fibrils formed in the presence of lipids with different chemical structure exert drastically different magnitude and dynamic of unfolded protein response (UPR) in the endoplasmic reticulum (ER) and mitochondria (MT). We found that the opposite dynamics of UPR in MT and ER in the cells exposed to Aß1-42: cardiolipin fibrils and Aß1-42 aggregates formed in a lipid-free environment. We also found that Aß1-42: phosphatidylcholine fibrils upregulated ER UPR simultaneously downregulating the UPR response of MT, whereas Aß1-42: cholesterol fibrils suppressed the UPR response of ER and upregulated UPR response of MT. We also observed progressively increasing ROS production that damages mitochondrial membranes and other cell organelles, ultimately leading to cell death.

Nanoscale Structural Characterization of Amyloid ß 1-42 Oligomers and Fibrils Grown in the Presence of Fatty Acids.

Mono- and polyunsaturated fatty acids (FAs) are broadly used as food supplements. However, their effect on the aggregation of amyloidogenic proteins remains unclear. In this study, we investigated the effect of a large number of mono- and polyunsaturated, as well as fully saturated FAs on the aggregation of amyloid ß1-42 (Aß1-42) peptide. A progressive aggregation of this peptide is the expected molecular cause of Alzheimer's disease (AD), one of the most common neurodegenerative pathologies in the world. We found that arachidonic and stearic acids delayed the aggregation of Aß1-42. Using Nano-Infrared spectroscopy, we found that FAs caused very little if any changes in the secondary structure of Aß1-42 oligomers and fibrils formed at different stages of protein aggregation. However, the analyzed mono- and polyunsaturated, as well as fully saturated FAs uniquely altered the toxicity of Aß1-42 fibrils. We found a direct relationship between the degree of FAs unsaturation and toxicity of Aß1-42 fibrils formed in their presence. Specifically, with an increase in the degree of unsaturation, the toxicity Aß1-42/FA fibrils increased. These results indicate that fully saturated or monounsaturated FAs could be used to decrease the toxicity of amyloid aggregates and, consequently, decelerate the development of AD.

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

Progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta, hypothalamus, and thalamus is a hallmark of Parkinson's disease. Neuronal death is linked to the abrupt aggregation of α-synuclein (α-Syn), a small membrane protein that regulates cell vesicle trafficking. α-Syn aggregation rate, as well as the secondary structure and toxicity of α-Syn fibrils, could be uniquely altered by lipids. However, molecular mechanisms that determine such a remarkable difference in the toxicity of α-Syn fibrils formed in the presence of lipids remain unclear. In this study, we used a set of molecular assays to determine the molecular mechanism by which α-Syn fibrils formed in the presence of phosphatidylcholine (PC), cardiolipin (CL), and cholesterol (Cho) exert cell toxicity. We found that rat dopaminergic cells exposed to α-Syn fibrils formed in the presence of different lipids exert drastically different magnitudes and dynamics of unfolded protein response (UPR) in the endoplasmic reticulum (ER) and mitochondria (MT). Specifically, α-Syn:CL were found to cause the strongest, whereas α-Syn fibrils formed in the absence of lipids had the lowest magnitude of the UPR cell response. We also found the opposite dynamics of the ER- and MT-UPR responses in rat dopaminergic cells exposed to protein aggregates. These results could suggest that facing severe ER stress, dopaminergic cells suppress MT-UPR response, enabling the maximal ATP production to restore their normal physiological function. These findings help to better understand complex mechanisms of cell toxicity of amyloid aggregates and ultimately find neuroprotective drug candidates that will be able to suppress the spread of Parkinson's disease.

Long-Chain Polyunsaturated Fatty Acids Accelerate the Rate of Insulin Aggregation and Enhance Toxicity of Insulin Aggregates.

Long-chain polyunsaturated fatty acids (LCPUFAs) are essential components of a human diet. These molecules are critically important for cognitive attention and memory, mood states, coronary circulation, and cirrhosis. However, recently reported findings demonstrated that docosahexaenoic (DHA) and arachidonic acids (ARA), ω-3 and ω-6 LCPUFAs, accelerated the aggregation rates of insulin and α-synuclein, proteins that are directly linked to diabetes type 2 and Parkinson's disease, respectively. Furthermore, both DHA and ARA uniquely altered the structure and toxicity of the corresponding protein aggregates. Our objective is to ascertain whether other LCPUFAs, alongside long-chain unsaturated fatty acid (LCUFA) proteins, exhibit similar effects on amyloidogenic proteins. To explore this matter, we investigated the effect of 10 different LCPUFAs and LCUFAs on the rate of insulin aggregation. We found that all of the analyzed fatty acids strongly accelerated insulin aggregation. Moreover, we found that protein aggregates that were formed in the presence of these fatty acids exerted significantly higher cell toxicity compared with insulin fibrils grown in the lipid-free environment. These findings show that interactions between amyloid-associated proteins and LCPUFAs can be the underlying molecular cause of neurodegenerative diseases.

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.

Nanoscale imaging of individual amyloid aggregates extracted from brains of Alzheimer and Parkinson patients reveals presence of lipids in α-synuclein but not in amyloid ß1-42 fibrils.

Abrupt aggregation of misfolded proteins is the underlying molecular cause of Alzheimer disease (AD) and Parkinson disease (PD). Both AD and PD are severe pathologies that affect millions of people around the world. A small 42 amino acid long peptide, known as amyloid ß (Aß), aggregates in the frontal cortex of AD patients forming oligomers and fibrils, highly toxic protein aggregates that cause progressive neuron death. Similar aggregates of α-synuclein (α-Syn), a small protein that facilitates neurotransmitter release, are observed in the midbrain, hypothalamus, and thalamus of people with PD. In this study, we utilized the innovative nano-Infrared imaging technique to investigate the structural organization of individual Aß and α-syn fibrils postmortem extracted from brains of AD and PD patients, respectively. We observed two morphologically different Aß and α-Syn fibril polymorphs in each patient's brain. One had twisted topology, whereas another exhibited flat tape-like morphology. We found that both polymorphs shared the same parallel ß-sheet-dominated secondary structure. These findings suggested that both fibril polymorphs were built from structurally similar if not identical filaments that coiled forming twisted fibrils or associated side-by-side in the case of straight Aß and α-Syn fibrils. Nano-Infrared analysis of individual protein aggregates also revealed the presence of lipids in the structure of both twisted and tape-like α-Syn fibrils that were not observed in any of the Aß fibril polymorphs. These findings demonstrate that lipid membranes can play a critically important role in the onset and progression of PD.

Nano-infrared analysis of amyloid ß1-42 fibrils formed in the presence of lipids with unsaturated fatty acids.

Alzheimer's disease (AD) is characterized by progressive memory loss and serious impairment of cognitive abilities. AD is the most common cause of dementia, affecting more than 44 million people around the world. The hallmark of AD is amyloid plaques, extracellular deposits primarily found in the frontal lobe, that are composed of amyloid ß (Aß) aggregates. In this study, we utilized nano-infrared spectroscopy, also known as Atomic Force Microscopy Infrared (AFM-IR) spectroscopy to investigate the effect of unsaturated phospholipids on the rate of Aß1-42 aggregation. We found that unsaturated phosphatidylcholine, phosphatidylserine, and cardiolipin strongly suppressed aggregation of Aß1-42. Furthermore, Aß1-42 fibrils formed in the presence of such lipids exerted significantly lower cell toxicity compared to the protein aggregates formed in the lipid-free environment. These findings suggest that dietary changes linked to the increased consumption of unsaturated phospholipids could be considered as a potential therapeutic approach that can decelerate the progression of AD. These results also suggest that large unilamellar vesicles with unsaturated lipids can be used as potential therapeutics to delay the onset and decelerate the progression of AD.

Lipids uniquely alter the secondary structure and toxicity of amyloid beta 1-42 aggregates.

Abrupt aggregation of amyloid ß1-42 (Aß) 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 Aß1-42 aggregation. However, it remains unclear whether lipids only alter rates of protein aggregation or also uniquely modify the secondary structure and toxicity of Aß1-42 oligomers and fibrils. In this study, we investigated the effect of phosphatidylcholine (PC), cardiolipin (CL), and cholesterol (Chol) on Aß1-42 aggregation. We found that PC, CL and Chol strongly accelerated the rate of fibril formation compared to the rate of Aß1-42 aggregation in the lipid-free environment. Furthermore, anionic CL enabled the strongest acceleration of Aß1-42 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 Aß1-42 aggregates. Specifically, CL and Chol drastically increased the amount of parallel ß-sheet in Aß1-42 oligomers and fibrils grown in the presence of these lipids. This caused a significant increase in the toxicity of Aß : CL and Aß : Chol compared to the toxicity of Aß : PC and Aß1-42 aggregates formed in the lipid-free environment. These results demonstrate that toxicity of Aß aggregates correlates with the amount of their ß-sheet content, which, in turn, is determined by the chemical structure of lipids present at the stage of Aß1-42 aggregation.

Concentration of Phosphatidylserine Influence Rates of Insulin Aggregation and Toxicity of Amyloid Aggregates In Vitro.

Phosphatidylserine (PS) is a negatively charged lipid that plays a critically important role in cell apoptosis. Under physiological conditions, PS is localized on the cytosolic side of plasma membranes via ATP-dependent flippase-mediated transport. A decrease in the ATP levels in the cell, which is taken place upon pathological processes, results in the increase in PS concentration on the exterior part of the cell membranes. PS on the outer membrane surfaces attracts and activates phagocytes, which trigger cell apoptosis. This programed irreversible cell death is observed upon the progressive neurodegeneration, a hallmark of numerous amyloid associated pathologies, such as diabetes type 2 and Alzheimer's disease. In this study, we investigate the extent to which the rates of protein aggregation, which occurs upon amyloid pathologies, can be altered by the concentration of PS in large unilamellar vesicles (LUVs). We found that with an increase in the concentration of PS from 20 to 40% relative to the concentration of phosphatidylcholine and phosphatidylethanolamine, the rate of insulin aggregation, protein linked to diabetes type 2, and injection amyloidosis drastically increased. Furthermore, the concentration of PS in LUVs determined the secondary structure of protein aggregates formed in their presence. We also found that these structurally different aggregates exerted distinctly different cell toxicities. These findings suggest that a substantial decrease in cell viability, which is likely to take place upon aging, results in the increase in the concentration of PS in the outer plasma membranes, where it triggers the irreversible self-assembly of amyloidogenic proteins, which, in turn, causes the progressive neurodegeneration.

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.

Saturation of fatty acids in phosphatidic acid uniquely alters transthyretin stability changing morphology and toxicity of amyloid fibrils.

Transthyretin (TTR) is a small, ß-sheet-rich tetrameric protein that transports thyroid hormone thyroxine and retinol. Phospholipids, including phosphatidic acid (PA), can uniquely alter the stability of amyloidogenic proteins. However, the role of PA in TTR aggregation remains unclear. In this study, we investigated the effect of saturation of fatty acids (FAs) in PA on the rate of TTR aggregation. We also reveal the extent to which PAs with different length and saturation of FAs altered the morphology and secondary structure of TTR aggregates. Our results showed that TTR aggregation in the equimolar presence of PAs with different length and saturation of FAs yielded structurally and morphologically different fibrils compared to those formed in the lipid-free environment. We also found that PAs drastically lowered the toxicity of TTR aggregates formed in the presence of this phospholipid. These results shed light on the role of PA in the stability of TTR and transthyretin amyloidosis.

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.

Protein-to-lipid ratio uniquely changes the rate of lysozyme aggregation but does not significantly alter toxicity of mature protein aggregates.

Irreversible aggregation of misfolded proteins is the underlying molecular cause of numerous pathologies, including diabetes type 2, Alzheimer's, and Parkinson's diseases. Such an abrupt protein aggregation results in the formation of small oligomers that can propagate into amyloid fibrils. A growing body of evidence suggests that protein aggregation can be uniquely altered by lipids. However, the role of the protein-to-lipid (P:L) ratio on the rate of protein aggregation, as well as the structure and toxicity of corresponding protein aggregates remains poorly understood. In this study, we investigate the role of the P:L ratio of five different phospho- and sphingolipids on the rate of lysozyme aggregation. We observed significantly different rates of lysozyme aggregation at 1:1, 1:5, and 1:10 P:L ratios of all analyzed lipids except phosphatidylcholine (PC). However, we found that at those P:L ratios, structurally and morphologically similar fibrils were formed. As a result, for all studies of lipids except PC, mature lysozyme aggregates exerted insignificantly different cell toxicity. These results demonstrate that the P:L ratio directly determines the rate of protein aggregation, however, has very little if any effect on the secondary structure of mature lysozyme aggregates. Furthermore, our results point to the lack of a direct relationship between the rate of protein aggregation, secondary structure, and toxicity of mature fibrils.

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.