Unraveling amyloid formation paths of Parkinson's disease protein α-synuclein triggered by anionic vesicles

Interplay of α-synuclein with Lipid Membranes: Cooperative Adsorption, Membrane Remodeling and Coaggregation

α-Synuclein is a small neuronal protein enriched at presynaptic termini. It is hypothesized to play a role in neurotransmitter release and synaptic vesicle cycling, while the formation of α-synuclein amyloid fibrils is associated with several neurodegenerative diseases, most notably Parkinson's Disease. The molecular mechanisms of both the physiological and pathological functions of α-synuclein remain to be fully understood, but in both cases, interactions with membranes play an important role. In this Perspective, we discuss several aspects of α-synuclein interactions with lipid membranes including cooperative adsorption, membrane remodeling and α-synuclein amyloid fibril formation in the presence of lipid membranes. We highlight the coupling between the different phenomena and their interplay in the context of physiological and pathological functions of α-synuclein.

The impact of hUC MSC-derived exosome-nanoliposome hybrids on α-synuclein fibrillation and neurotoxicity

Amyloid aggregation of α-synuclein (αSN) protein amplifies the pathogenesis of neurodegenerative diseases (NDs) such as Parkinson's disease (PD). Consequently, blocking aggregation or redirecting self-assembly to less toxic aggregates could be therapeutic. Here, we improve brain-specific nanocarriers using a hybrid of exosomes (Ex) from human umbilical cord mesenchymal stem cells (hUC MSCs) and nanoliposomes containing baicalein (Ex-NLP-Ba) and oleuropein (Ex-NLP-Ole). The hybrids contained both lipid membranes, Ex proteins, and baicalein or oleuropein. Fluorescence resonance energy transfer analysis confirmed their proper integration. The hybrids reduced the extent of αSN fibrillation and interfered with secondary nucleation and disaggregation. They not only reduced αSN pathogenicity but also enhanced drug internalization into cells, surpassing the efficacy of NLP alone, and also crossed the blood-brain barrier in a cellular model. We conclude that Ex can be successfully extracted and efficiently merged with NLPs while retaining its original properties, demonstrating great potential as a theranostic drug delivery vehicle against NDs like PD.

Roles of interfacial water states on advanced biomedical material design

When biomedical materials come into contact with body fluids, the first reaction that occurs on the material surface is hydration; proteins are then adsorbed and denatured on the hydrated material surface. The amount and degree of denaturation of adsorbed proteins affect subsequent cell behavior, including cell adhesion, migration, proliferation, and differentiation. Biomolecules are important for understanding the interactions and biological reactions of biomedical materials to elucidate the role of hydration in biomedical materials and their interaction partners. Analysis of the water states of hydrated materials is complicated and remains controversial; however, knowledge about interfacial water is useful for the design and development of advanced biomaterials. Herein, we summarize recent findings on the hydration of synthetic polymers, supramolecular materials, inorganic materials, proteins, and lipid membranes. Furthermore, we present recent advances in our understanding of the classification of interfacial water and advanced polymer biomaterials, based on the intermediate water concept.

Orientation of α-Synuclein at Negatively Charged Lipid Vesicles: Linear Dichroism Reveals Time-Dependent Changes in Helix Binding Mode

The neuronal protein α-synuclein, linked to Parkinson’s disease, binds to negatively charged vesicles adopting a partial α-helix structure, but helix arrangement at the vesicle surface is not fully understood. Using linear dichroism spectroscopy (LD), we study the interaction of monomeric α-synuclein with large unilamellar vesicles of 1,2-dioleoyl-sn-glycero-3-phospho-l-serine (DOPS), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS), and 1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (DOPG) under mild shear flow. The LD data of oriented lipid vesicles show that the long axis of the protein helix is oriented preferentially perpendicular to the membrane normal but deviates from a uniform in-plane distribution. Upon initial binding, a fraction of helices are oriented in the direction of least curvature for all ellipsoid-shaped vesicles at a lipid:protein molar ratio of 100. However, at a lower protein concentration the helices distribute uniformly on DOPS and POPS vesicles. In all cases, the α-synuclein helices rearrange with time (minute time scale) in the shear flow and begin to tilt into the vesicle membrane. Faster reorientation kinetics in the presence of flow suggests that modulation of membrane dynamics, by thermal or shear-dynamic activation, may overcome steric barriers by what may be called “flow catalysis”.

Single-vesicle imaging reveals lipid-selective and stepwise membrane disruption by monomeric α-synuclein

Neurodegenerative diseases are increasing among the world's population, but there are no cures. These disorders all involve proteins that assemble into amyloid fibers which results in brain cell death. Evidence suggests that association of these proteins with lipid membranes is crucial for both functional and pathological roles. In Parkinson's disease, the involved protein, α-synuclein, is thought to function in trafficking of lipid vesicles in the brain. In search of mechanistic origins, increasing focus is put on identifying neurotoxic reactions induced by membrane interactions. To contribute new clues to this question, we here employed a new surface-sensitive scattering microscopy technique. With this approach, we discovered that α-synuclein perturbs vesicles in a stepwise and lipid-dependent fashion already at very low protein coverage.

The interaction of the neuronal protein α-synuclein with lipid membranes appears crucial in the context of Parkinson’s disease, but the underlying mechanistic details, including the roles of different lipids in pathogenic protein aggregation and membrane disruption, remain elusive. Here, we used single-vesicle resolution fluorescence and label-free scattering microscopy to investigate the interaction kinetics of monomeric α-synuclein with surface-tethered vesicles composed of different negatively charged lipids. Supported by a theoretical model to account for structural changes in scattering properties of surface-tethered lipid vesicles, the data demonstrate stepwise vesicle disruption and asymmetric membrane deformation upon α-synuclein binding to phosphatidylglycerol vesicles at protein concentrations down to 10 nM (∼100 proteins per vesicle). In contrast, phosphatidylserine vesicles were only marginally affected. These insights into structural consequences of α-synuclein interaction with lipid vesicles highlight the contrasting roles of different anionic lipids, which may be of mechanistic relevance for both normal protein function (e.g., synaptic vesicle binding) and dysfunction (e.g., mitochondrial membrane interaction).

My journey in academia: things not on the CV

I am a professor at Chalmers University of Technology in Sweden. I trained in chemistry in Sweden but went to the USA for my postdoc. I remained there for 12 years, being faculty at two American universities, before I returned to Sweden for a professorship in the northern city of Umeå. More recently, I returned to my alma mater Chalmers University of Technology in Gothenburg, where I have taken on senior leadership roles. On paper, my career trajectory looks straightforward, but there are many detrimental aspects and lucky coincidences that are not listed on my CV. Life in academia is never easy, and one is never ‘done’. But working in academia is wonderful, as it provides so much freedom and creativity, including being very accommodating towards having kids. Here, I will describe my own personal journey, with the hope of inspiring young women to follow their own path in academia. Yes, there is still bias against women in academia, but change is happening, and the many benefits of being an academic beat such drawbacks.

Synaptic vesicle mimics affect the aggregation of wild-type and A53T α-synuclein variants differently albeit similar membrane affinity

α-Synuclein misfolding results in the accumulation of amyloid fibrils in Parkinson’s disease. Missense protein mutations (e.g. A53T) have been linked to early onset disease. Although α-synuclein interacts with synaptic vesicles in the brain, it is not clear what role they play in the protein aggregation process. Here, we compare the effect of small unilamellar vesicles (lipid composition similar to synaptic vesicles) on wild-type (WT) and A53T α-synuclein aggregation. Using biophysical techniques, we reveal that binding affinity to the vesicles is similar for the two proteins, and both interact with the helix long axis parallel to the membrane surface. Still, the vesicles affect the aggregation of the variants differently: effects on secondary processes such as fragmentation dominate for WT, whereas for A53T, fibril elongation is mostly affected. We speculate that vesicle interactions with aggregate intermediate species, in addition to monomer binding, vary between WT and A53T, resulting in different consequences for amyloid formation.

Transient domains of ordered water induced by divalent ions lead to lipid membrane curvature fluctuations

Cell membranes are composed of a hydrated lipid bilayer that is molecularly complex and diverse, and the link between molecular hydration structure and membrane macroscopic properties is not well understood, due to a lack of technology that can probe and relate molecular level hydration information to micro- and macroscopic properties. Here, we demonstrate a direct link between lipid hydration structure and macroscopic dynamic curvature fluctuations. Using high-throughput wide-field second harmonic (SH) microscopy, we observe the formation of transient domains of ordered water at the interface of freestanding lipid membranes. These domains are induced by the binding of divalent ions and their structure is ion specific. Using nonlinear optical theory, we convert the spatiotemporal SH intensity into maps of membrane potential, surface charge density, and binding free energy. Using an electromechanical theory of membrane bending, we show that transient electric field gradients across the membrane induce spatiotemporal membrane curvature fluctuations.

Correlation between Cellular Uptake and Cytotoxicity of Fragmented α-Synuclein Amyloid Fibrils Suggests Intracellular Basis for Toxicity

Aggregation and intracellular deposition of the protein α-synuclein is an underlying characteristic of Parkinson's disease. α-Synuclein assemblies also undergo cell-cell spreading, facilitating propagation of their cellular pathology. Understanding how cellular interactions and uptake of extracellular α-synuclein assemblies depend on their physical attributes is therefore important. We prepared fragmented fluorescently labeled α-synuclein amyloid fibrils of different average lengths (∼80 nm to >1 μm) and compared their interactions with SH-SY5Y cells. We report that fibrils of all lengths, but not monomers, bind avidly to the cell surface. Their uptake is inversely dependent on their average size, occurs via a heparan sulfate dependent endocytic route, and appears to have a size cutoff of ∼400 nm. The uptake of α-synuclein fibrils, but not monomers, correlates with their cytotoxicity as measured by reduction in metabolic activity, strongly suggesting an intracellular basis for α-synuclein fibril toxicity, likely involving endolysosomes.

Crosstalk Between Alpha-Synuclein and Other Human and Non-Human Amyloidogenic Proteins: Consequences for Amyloid Formation in Parkinson's Disease

It was recently shown (Sampson et al., Elife9, 2020) that an amyloidogenic protein, CsgA, present in E. coli biofilms in the gut can trigger Parkinson's disease in mice. This study emphasizes the possible role of the gut microbiome in modulation (and even initiation) of human neurodegenerative disorders, such as Parkinson's disease. As the CsgA protein was found to accelerate alpha-synuclein (the key amyloidogenic protein in Parkinson's disease) amyloid formation in vitro, this result suggests that also other amyloidogenic proteins from gut bacteria, and even from the diet (such as stable allergenic proteins), may be able to affect human protein conformations and thereby modulate amyloid-related diseases. In this review, we summarize what has been reported in terms of in vitro cross-reactivity studies between alpha-synuclein and other amyloidogenic human and non-human proteins. It becomes clear from the limited data that exist that there is a fine line between acceleration and inhibition, but that cross-reactivity is widespread, and it is more common for other proteins (among the studied cases) to accelerate alpha-synuclein amyloid formation than to block it. It is of high importance to expand investigations of cross-reactivity between amyloidogenic proteins to both reveal underlying mechanisms and links between human diseases, as well as to develop new treatments that may be based on an altered gut microbiome.

Interaction between Copper Chaperone Atox1 and Parkinson's Disease Protein α-Synuclein Includes Metal-Binding Sites and Occurs in Living Cells

Alterations in copper ion homeostasis appear coupled to neurodegenerative disorders, but mechanisms are unknown. The cytoplasmic copper chaperone Atox1 was recently found to inhibit amyloid formation in vitro of α-synuclein, the amyloidogenic protein in Parkinson's disease. As α-synuclein may have copper-dependent functions, and free copper ions promote α-synuclein amyloid formation, it is important to characterize the Atox1 interaction with α-synuclein on a molecular level. Here we applied solution-state nuclear magnetic resonance spectroscopy, with isotopically labeled α-synuclein and Atox1, to define interaction regions in both proteins. The α-synuclein interaction interface includes the whole N-terminal part up to Gln24; in Atox1, residues around the copper-binding cysteines (positions 11-16) are mostly perturbed, but additional effects are also found for residues elsewhere in both proteins. Because α-synuclein is N-terminally acetylated in vivo, we established that Atox1 also inhibits amyloid formation of this variant in vitro, and proximity ligation in human cell lines demonstrated α-synuclein-Atox1 interactions in situ. Thus, this interaction may provide the direct link between copper homeostasis and amyloid formation in vivo.

Folding of copper proteins: role of the metal?

Copper is a redox-active transition metal ion required for the function of many essential human proteins. For biosynthesis of proteins coordinating copper, the metal may bind before, during or after folding of the polypeptide. If the metal binds to unfolded or partially folded structures of the protein, such coordination may modulate the folding reaction. The molecular understanding of how copper is incorporated into proteins requires descriptions of chemical, thermodynamic, kinetic and structural parameters involved in the formation of protein-metal complexes. Because free copper ions are toxic, living systems have elaborate copper-transport systems that include particular proteins that facilitate efficient and specific delivery of copper ions to target proteins. Therefore, these pathways become an integral part of copper protein folding in vivo. This review summarizes biophysical-molecular in vitro work assessing the role of copper in folding and stability of copper-binding proteins as well as protein-protein copper exchange reactions between human copper transport proteins. We also describe some recent findings about the participation of copper ions and copper proteins in protein misfolding and aggregation reactions in vitro.

In Vitro Analysis of α-Synuclein Amyloid Formation and Cross-Reactivity

In vitro time-resolved characterization of protein aggregation into amyloid fibers and the effects of other proteins on the aggregation process are fundamentally important measurements to obtain a better understanding of the mechanisms contributing to neurodegeneration, as well as other diseases involving amyloid formation. Here, we describe how to perform in vitro aggregation experiments with α-synuclein, the amyloidogenic protein involved in Parkinson's disease, including how to assess the starting material, useful experimental/instrumental conditions, as well as how to set up cross-seeding and co-aggregation experiments. The high variability of data reported for in vitro α-synuclein amyloid formation may in part be explained by experimental differences.

Membrane-Protein-Hydration Interaction of α-Synuclein with Anionic Vesicles Probed via Angle-Resolved Second-Harmonic Scattering

Amyloid formation of the protein α-synuclein promotes neurodegeneration in Parkinson's disease. The normal function of α-synuclein includes synaptic vesicle transport and fusion, and the protein binds strongly to negatively charged vesicles in vitro. Here, we demonstrate that nonresonant angle-resolved second-harmonic scattering detects α-synuclein binding to liposomes through changes in water orientational correlations and can thus be used as a high-accuracy and high-throughput label-free probe of protein-liposome interactions. The obtained results support a binding model in which the N-terminus of α-synuclein adopts an α-helical conformation that lies flat on the vesicle surface while the negatively charged C-terminus remains in solution.

Alpha-Synuclein Modulates the Physical Properties of DNA

Fundamental research on Parkinson's disease (PD) most often focuses on the ability of α-synuclein (aS) to form oligomers and amyloids, and how such species promote brain cell death. However, there are indications that aS also plays a gene-regulatory role in the cell nucleus. Here, the interaction between monomeric aS and DNA in vitro has been investigated with single-molecule techniques. Using a nanofluidic channel system, it was discovered that aS binds to DNA and by studying the DNA-protein complexes at different confinements we determined that aS binding increases the persistence length of DNA from 70 to 90 nm at high coverage. By atomic force microscopy it was revealed that at low protein-to-DNA ratio, the aS binding occurs as small protein clusters scattered along the DNA; at high protein-to-DNA ratio, the DNA is fully covered by protein. As DNA-aS interactions may play roles in PD, it is of importance to characterize biophysical properties of such complexes in detail.

Ganglioside lipids accelerate α-synuclein amyloid formation

The deposition of α-synuclein fibrils is one hallmark of Parkinson's disease. Here, we investigate how ganglioside lipids, present in high amounts in neurons and exosomes, influence the aggregation kinetics of α-synuclein. Gangliosides, as well as, other anionic lipid species with small or large headgroups were found to induce conformational changes of α-synuclein monomers and catalyse their aggregation at mildly acidic conditions. Although the extent of this catalytic effect was slightly higher for gangliosides, the results imply that charge interactions are more important than headgroup chemistry in triggering aggregation. In support of this idea, uncharged lipids with large headgroups were not found to induce any conformational change and only weakly catalyse aggregation. Intriguingly, aggregation was also triggered by free ganglioside headgroups, while these caused no conformational change of α-synuclein monomers. Our data reveal that partially folded α-synuclein helical intermediates are not required species in triggering of α-synuclein aggregation.

Abundant fish protein inhibits α-synuclein amyloid formation

The most common allergen in fish, the highly-abundant protein β-parvalbumin, forms amyloid structures as a way to avoid gastrointestinal degradation and transit to the blood. In humans, the same amyloid structures are mostly associated with neurodegenerative disorders such as Alzheimer’s and Parkinson’s. We here assessed a putative connection between these amyloids using recombinant Atlantic cod β-parvalbumin and the key amyloidogenic protein in Parkinson’s disease, α-synuclein. Using a set of in vitro biophysical methods, we discovered that β-parvalbumin readily inhibits amyloid formation of α-synuclein. The underlying mechanism was found to involve α-synuclein binding to the surface of β-parvalbumin amyloid fibers. In addition to being a new amyloid inhibition mechanism, the data suggest that health benefits of fish may be explained in part by cross-reaction of β-parvalbumin with human amyloidogenic proteins.

Extracellular vesicles from human pancreatic islets suppress human islet amyloid polypeptide amyloid formation

Extracellular vesicles (EVs) are small vesicles released by cells to aid cell-cell communication and tissue homeostasis. Human islet amyloid polypeptide (IAPP) is the major component of amyloid deposits found in pancreatic islets of patients with type 2 diabetes (T2D). IAPP is secreted in conjunction with insulin from pancreatic β cells to regulate glucose metabolism. Here, using a combination of analytical and biophysical methods in vitro, we tested whether EVs isolated from pancreatic islets of healthy patients and patients with T2D modulate IAPP amyloid formation. We discovered that pancreatic EVs from healthy patients reduce IAPP amyloid formation by peptide scavenging, but T2D pancreatic and human serum EVs have no effect. In accordance with these differential effects, the insulin:C-peptide ratio and lipid composition differ between EVs from healthy pancreas and EVs from T2D pancreas and serum. It appears that healthy pancreatic EVs limit IAPP amyloid formation via direct binding as a tissue-specific control mechanism.