Surface behavior of α-Synuclein and its interaction with phospholipids using the Langmuir monolayer technique: A comparison between monomeric and fibrillar α-Synuclein

Umbrella-like Helical Structure of α-Synuclein at the Air-Water Interface Observed with Experimental and Theoretical Sum Frequency Generation Spectroscopy

The misfolding of α-synuclein (αS) into amyloid aggregates is catalyzed by hydrophobic surfaces and associated with severe brain disorders, such as Parkinson's disease. Despite the important role of interfaces, the three-dimensional structure of αS at the interfaces is still not clear. We report interface-specific sum frequency generation (SFG) experiments of monomeric αS binding to the air-water interface, a model system for the important hydrophobic surfaces. We combine the SFG spectra with calculations of theoretical spectra based on molecular dynamics simulations to show that αS, which is an intrinsically disordered protein in solution, folds into a defined, mostly helical secondary structure at the air-water interface. The binding pose resembles an umbrella shape, where the C-terminus protrudes into the water phase, while the N-terminus and the NAC region span the canopy at the interface. In this binding pose, αS is prone to aggregate, which could explain the catalytic effect of hydrophobic interfaces and air bubbles on αS fibrillation.

Interfacial properties of α-synuclein's Parkinsonian variants

Human alpha-synuclein (αS) is associated with the occurrence of Parkinson's disease. In the past decade, six autosomally dominant mutations have been identified in αS (SNCA) gene that translate into A30P, E46K, H50Q, G51D, A53E, and A53T mutations in the protein. These mutations alter the electrostatics and hydrophobicity of a cardinal region of the protein. A comprehensive comparison of interfacial properties of these Parkinsonian αS variants is crucial to understand their membrane dynamics. Here, we investigated the interfacial activity of these αS variants at air-aqueous interface. All the αS variants were found to possess comparable surface activity of ∼20-22 mN/m. Compression/expansion isotherms reveal a very distinct behaviour of the A30P variant compared to others. The Blodgett-deposited films were analysed using CD and LD spectroscopy as well as the atomic force microscopy. All the variants adopted predominantly α-helical conformation in these films. Atomic force microscopy of the Langmuir-Blodgett films revealed self-assembly at the interface. The lipid-penetration activity was also investigated using zwitterionic and negatively charged lipid monolayers.

A study on the interaction of the amyloid fibrils of α-synuclein and hen egg white lysozyme with biological membranes

Alpha-synuclein (α-syn) aggregation and mitochondrial dysfunction are considered as two of the main factors associated with Parkinson's disease (PD). In the present investigation, the effectiveness of the amyloid fibrils obtained from α-syn with those of hen egg white lysozyme (HEWL), as disease-related and-unrelated proteins, to damage rat brain and rat liver mitochondria have been investigated. This was extended by looking at SH-SY5Y human neuroblastoma cells and erythrocytes, thereby investigating the significance of structural characteristics of amyloid fibrils related to their interactions with biomembranes obtained from various sources. Results presented clearly demonstrate substantial differences in the response of tested biomembranes to toxicity induced by α-syn/HEWL amyloid fibrils, highlighting a structure-function relationship. We found that fibrillar aggregates of α-syn, but not HEWL, caused a significant increase in mitochondrial ROS, loss of membrane potential, and mitochondrial swelling, in a dose-dependent manner. Toxicity was found to be more pronounced in brain mitochondria, as compared to liver mitochondria. For SH-SY5Y cells and erythrocytes, however, both α-syn and HEWL amyloid fibrils showed the capacity to induce toxicity. Taken together, these results may suggest selective toxicity of α-syn amyloid fibrils to mitochondria mediated likely by their direct interaction with the outer mitochondrial membrane, indicating a correlation between specific structural characteristics of α-syn fibrils and an organelle strongly implicated in PD pathology.

Insights into the molecular mechanism of amyloid filament formation: Segmental folding of α-synuclein on lipid membranes

Recent advances in the structural biology of disease-relevant α-synuclein fibrils have revealed a variety of structures, yet little is known about the process of fibril aggregate formation. Characterization of intermediate species that form during aggregation is crucial; however, this has proven very challenging because of their transient nature, heterogeneity, and low population. Here, we investigate the aggregation of α-synuclein bound to negatively charged phospholipid small unilamellar vesicles. Through a combination of kinetic and structural studies, we identify key time points in the aggregation process that enable targeted isolation of prefibrillar and early fibrillar intermediates. By using solid-state nuclear magnetic resonance, we show the gradual buildup of structural features in an α-synuclein fibril filament, revealing a segmental folding process. We identify distinct membrane-binding domains in α-synuclein aggregates, and the combined data are used to present a comprehensive mechanism of the folding of α-synuclein on lipid membranes.

N-terminal acetylation does not alter α-synuclein's interfacial properties

Alpha-synuclein (αS) is a membrane-binding protein found predominantly in neurons and erythrocytes. The protein remains unordered in aqueous solutions but folds into an α-helical structure when bound to membranes. Besides, it gets deposited as β-sheet rich aggregates in diseases known as synucleinopathies. The native αS has been reported to be acetylated at the N-terminus. Here, we compare the interfacial properties of the N-terminal acetylated αS (Ac-αS) with non-acetylated αS (NH₂-αS) at the air-water interface. Both the protein forms are highly surface-active, with surface pressure reaching up to ~30 mN/m upon compression. The pressure-area isotherms obtained from the repeated compression-expansion cycles display large hysteresis suggesting self-assembly at higher surface pressures. The expansion isotherm is characterized by a rapid decrease in surface pressure followed by a slower transition phase starting around 15-17 mN/m. These data suggest that the compressed monolayer breaks into small clusters upon expansion, followed by these clusters' loosening. The circular dichroism spectroscopic analysis of the Blodgett-deposited films suggests the protein to be in largely α-helical conformation. The linear dichroism investigations suggest the protein to be anisotropically deposited. Blodgett deposition of the Langmuir films, therefore, is a rather simple method for preparing oriented monolayers of surface-active macromolecules.

Single-molecule studies of amyloid proteins: from biophysical properties to diagnostic perspectives

In neurodegenerative diseases, a wide range of amyloid proteins or peptides such as amyloid-beta and α-synuclein fail to keep native functional conformations, followed by misfolding and self-assembling into a diverse array of aggregates. The aggregates further exert toxicity leading to the dysfunction, degeneration and loss of cells in the affected organs. Due to the disordered structure of the amyloid proteins, endogenous molecules, such as lipids, are prone to interact with amyloid proteins at a low concentration and influence amyloid cytotoxicity. The heterogeneity of amyloid proteinscomplicates the understanding of the amyloid cytotoxicity when relying only on conventional bulk and ensemble techniques. As complementary tools, single-molecule techniques (SMTs) provide novel insights into the different subpopulations of a heterogeneous amyloid mixture as well as the cytotoxicity, in particular as involved in lipid membranes. This review focuses on the recent advances of a series of SMTs, including single-molecule fluorescence imaging, single-molecule force spectroscopy and single-nanopore electrical recording, for the understanding of the amyloid molecular mechanism. The working principles, benefits and limitations of each technique are discussed and compared in amyloid protein related studies.. We also discuss why SMTs show great potential and are worthy of further investigation with feasibility studies as diagnostic tools of neurodegenerative diseases and which limitations are to be addressed.

Date palm (Phoenix dactylifera L.) fruit's polyphenols as potential inhibitors for human amylin fibril formation and toxicity in type 2 diabetes

BACKGROUND

β-Cell death is the key feature of type 2 diabetes mellitus (T2DM). The misfolding of human Islet Amyloid Polypeptide (hIAPP) is regarded as one of the causative factors of T2DM. Recent studies suggested that a diet based on date fruits presents various health benefits, as these fruits are naturally enriched in plant polyphenols.

METHOD

In this study, we used a broad biophysical approach, using cell biology techniques and bioinformatic tools, to demonstrate that various polyphenols from date palm (Phoenix dactylifera L.) fruit significantly inhibited hIAPP aggregation and cytotoxicity.

RESULT

Our results suggest that all of the polyphenols showed inhibitory effects, albeit varied, on the formation of toxic hIAPP amyloids. Correlation between cell viability assay, permeabilization of synthetic phospholipid vesicles tests, and ANS florescence measurements, revealed that both classes of polyphenols protected INS-1E cells from the toxicity of amylin aggregates. Docking results showed that the used polyphenols physically interacted with both hIAPP amyloidogenic region (residues Ser20-Ser29) and the non-amyloidogenic regions via hydrophobic and hydrogen interactions, thus reducing aggregation levels.

CONCLUSION

These findings highlight the benefits of consuming dates and the great potential of its polyphenols as a potential therapy for the prevention and treatment of T2DM as well as for many other amyloid-related diseases.

Inhibition of human islet amyloid polypeptide aggregation and cellular toxicity by oleuropein and derivatives from olive oil

Loss of β-cell function and β-cell death is the key feature of type 2 diabetes mellitus (T2DM). One hypothesis for the mechanism of this feature is amyloid formation by the human islet amyloid polypeptide (hIAPP). Despite the global prevalence of T2DM, there are no therapeutic strategies for the treatment of or prevention of amylin amyloidosis. Clinical trials and population studies indicate the healthy virtues of the Mediterranean diet, especially the extra virgin olive oil (EVOO) found in this diet. This oil is enriched in phenolic compounds shown to be effective against several aging and lifestyle diseases. Oleuropein (Ole), one of the most abundant polyphenols in EVOO, has been reported to be anti-diabetic. Some of Ole's main derivative have attracted our interest due to their multi-targetted effects, including interference with amyloid aggregation path. However, the structure-function relationship of Ole and its metabolites in T2DM are not yet clear. We report here a broad biophysical approach and cell biology techniques that enabled us to characterize the different molecular mechanisms by which tyrosol (TYR), hydroxytyrosol (HT), oleuropein (Ole) and oleuropein aglycone (OleA) modulate the hIAPP fibrillation in vitro and their effects on cell cytotoxicity. The OleA formed by enolic acid and hydroxytyrosol moiety was found to be more active than the Ole and HT at low micromolar concentrations. We further demonstrated that OleA inhibit the cytotoxicity induced by hIAPP aggregates by protecting more the cell membrane from permeabilization and then from death. These findings highlight the benefits of consuming EVOO and the great potential of its polyphenols, mainly OleA. Moreover, they support the possibility to validate and optimize the possible pharmacological use of EVOO polyphenols for T2DM prevention and therapy and also for many other amyloid related diseases.

Human islet amyloid polypeptide (hIAPP) aggregation in type 2 diabetes: Correlation between intrinsic physicochemical properties of hIAPP aggregates and their cytotoxicity

A large number of pathological diseases are known now to be associated with the misfolding and the aberrant oligomerization and deposition of peptides and proteins into various aggregates. One of these peptides is islet amyloid polypeptide (IAPP), which is responsible for amyloid formation in type 2 diabetes. The mechanism of IAPP amyloid formation in vivo and in vitro is not well understood and the factors behind the peptide aggregates toxicity are not fully defined. Therefore, the precise nature of toxic agents still remains to be elucidated. In this context, first we used a complementary biophysical approach to undertake a systematic study of the hIAPP aggregation process with focus on the lag phase, followed by the study of their degrees of toxicity when added to the extracellular medium of pancreatic cells. The structural properties of hIAPP aggregates are characterized by evaluating their size with DLS, their surface hydrophobicity with ANS, and the interactions between monomers through the intrinsic fluorescence of aromatic residues or by the quenching of these residues mainly the tyrosine in position 37. Our results indicate that despite the method used to study hIAPP aggregation, the obtained curve is easily well fitted in a sigmoidal curve but with some differences. In fact, the analysis of the kinetic parameters gives different information about the hIAPP aggregation process such as lag time and growth rate. Moreover, a high surface hydrophobicity and small size of the aggregates, mainly for the species formed during the lag time, shows strong correlation with the cytotoxicity. These findings provide new insights into the structural changes during hIAPP aggregation and are consistent with a model in which the exposure of hydrophobic surfaces and the small size of aggregates formed during the early stage of the process are crucial for their cytotoxicity.

Disruptive membrane interactions of alpha-synuclein aggregates

Alpha synuclein (αS) is a ~14 kDa intrinsically disordered protein. Decades of research have increased our knowledge on αS yet its physiological function remains largely elusive. The conversion of monomeric αS into oligomers and amyloid fibrils is believed to play a central role of the pathology of Parkinson's disease (PD). It is becoming increasingly clear that the interactions of αS with cellular membranes are important for both αS's functional and pathogenic actions. Therefore, understanding interactions of αS with membranes seems critical to uncover functional or pathological mechanisms. This review summarizes our current knowledge of how physicochemical properties of phospholipid membranes affect the binding and aggregation of αS species and gives an overview of how post-translational modifications and point mutations in αS affect phospholipid membrane binding and protein aggregation. We discuss the disruptive effects resulting from the interaction of αS aggregate species with membranes and highlight current approaches and hypotheses that seek to understand the pathogenic and/or protective role of αS in PD.

The C-terminal α-helices of mammalian Hsc70 play a critical role in the stabilization of α-synuclein binding and inhibition of aggregation

Protein misfolding, followed by aggregation and amyloid formation is an underlying pathological hallmark in a number of prevalent diseases, including Parkinson's (PD), Alzheimer's (AD) and Type 2 diabetes (T2D). In the case of PD, the aggregation of α-synuclein protein (α-syn) has been shown to be highly cytotoxic and to play a key role in the death of dopaminergic cells. Thus, inhibition of the aggregation process may be considered as an attractive avenue for therapeutic intervention. In this respect, molecular chaperones, known to promote proper folding of proteins, are able to inhibit protein aggregation thus preventing amyloid formation. In this work, the effect of the constitutively expressed chaperone Hsc70 and its various domains on α-syn aggregation have been investigated using different approaches. The results show that the C-terminal domain alone (residues 386-646) is as efficient in inhibiting α-syn aggregation as the entire Hsc70 protein, by increasing the lag phase for α-syn oligomeric nucleus formation, suggesting that the chaperone interacts with and stabilizes α-syn monomers and/or small aggregates. Deletion of the C-terminal helices (residues 510-646), which are known to play the role of a lid locking target peptide ligands in the peptide-binding site of the chaperone, strongly reduced the efficiency of inhibition of α-syn aggregation indicating that these helices play an essential in stabilizing the interaction between Hsc70 and α-syn. Furthermore, the effects of Hsc70 and its structural domains on aggregation appear to correlate with those on cytotoxicity, by reducing the fraction of α-syn toxic species to various degrees. Together these results suggest a mechanism in which inhibition of synuclein aggregation is the result of monomeric synuclein binding to the chaperone as any monomeric target unfolded protein or peptide binding to the chaperone.

The presence of an air-water interface affects formation and elongation of α-Synuclein fibrils

The aggregation of human α-Synuclein (α-Syn) into amyloid fibrils is related to the onset of multiple diseases termed synucleinopathies. Substantial evidence suggests that hydrophobic-hydrophilic interfaces promote the aggregation of amyloidogenic proteins and peptides in vitro. In this work the effect of the air-water interface (AWI) on α-Syn aggregation is investigated by means of thioflavin T binding measurements, dynamic light scattering, size-exclusion chromatography, electron microscopy, and atomic force microscopy. Measurements were performed with the monomeric protein alone or together with preformed seeds. In presence of the AWI, α-Syn aggregates readily into amyloid fibrils that remain adsorbed to the AWI. Instead, when the AWI is removed from the samples by replacing it with a solid-liquid interface, the interfacial aggregation of monomeric α-Syn is greatly reduced and no significant increase in ThT fluorescence is detected in the bulk, even at 900 μM concentration. Bulk aggregation is observed only when a sufficient amount of preformed seeds is added, and the initial slope of the kinetics scales with the amount of seeds as expected for first order kinetics. By contrast, in seeded experiments with the AWI, the initial slope is one order of magnitude lower and secondary nucleation pathways appear instead to be dominant. Thus, interfaces play multiple roles in the aggregation of α-Syn, influencing primary nucleation, aggregate elongation, and secondary nucleation processes. Interfacial effects must therefore be taken into account to achieve a complete understanding of protein aggregation events in vitro as well as in vivo.