Specificity and kinetics of alpha-synuclein binding to model membranes determined with fluorescent excited state intramolecular proton transfer (ESIPT) probe

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 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.

Fluorescent Probes Based on Charge and Proton Transfer for Probing Biomolecular Environment

Fluorescent probes for sensing fundamental properties of biomolecular environment, such as polarity and hydration, help to study assembly of lipids into biomembranes, sensing interactions of biomolecules and imaging physiological state of the cells. Here, we summarize major efforts in the development of probes based on two photophysical mechanisms: (i) an excited-state intramolecular charge transfer (ICT), which is represented by fluorescent solvatochromic dyes that shift their emission band maximum as a function of environment polarity and hydration; (ii) excited-state intramolecular proton transfer (ESIPT), with particular focus on 5-membered cyclic systems, represented by 3-hydroxyflavones, because they exhibit dual emission sensitive to the environment. For both ICT and ESIPT dyes, the design of the probes and their biological applications are summarized. Thus, dyes bearing amphiphilic anchors target lipid membranes and report their lipid organization, while targeting ligands direct them to specific organelles for sensing their local environment. The labels, amino acid and nucleic acid analogues inserted into biomolecules enable monitoring their interactions with membranes, proteins and nucleic acids. While ICT probes are relatively simple and robust environment-sensitive probes, ESIPT probes feature high information content due their dual emission. They constitute a powerful toolbox for addressing multitude of biological questions.

Extracellular Vesicles as Biomarkers for Parkinson's Disease: How Far from Clinical Translation?

Parkinson's disease (PD) is a neurodegenerative disorder affecting about 10 million people worldwide with a prevalence of about 2% in the over-80 population. The disease brings in also a huge annual economic burden, recently estimated by the Michael J Fox Foundation for Parkinson's Research to be USD 52 billion in the United States alone. Currently, no effective cure exists, but available PD medical treatments are based on symptomatic prescriptions that include drugs, surgical approaches and rehabilitation treatment. Due to the complex biology of a PD brain, the design of clinical trials and the personalization of treatment strategies require the identification of accessible and measurable biomarkers to monitor the events induced by treatment and disease progression and to predict patients' responsiveness. In the present review, we strive to briefly summarize current knowledge about PD biomarkers, focusing on the role of extracellular vesicles as active or involuntary carriers of disease-associated proteins, with particular attention to those research works that envision possible clinical applications.

Multi-parametric sensing by multi-channel molecular fluorescent probes based on excited state intramolecular proton transfer and charge transfer processes

Considering the applications of fluorescent probes and the information they provide, their brightness of fluorescence and photostability are of paramount importance. However, in the case of steady-state fluorescence spectroscopy and fluorescence microscopy, the amount of information can be increased by the application of multi-channel probes, via a multi-band fluorophore introduced in the probe molecule. In most cases, the use of such a multi-band (or multi-channel) fluorophore can also be combined with the concomitant introduction of one or several analyte receptors. Most often, the design of ratiometric probes with multi-band fluorescence emission are based on phenomena such as photoinduced intramolecular charge transfer (ICT) or excited state intramolecular proton transfer (ESIPT). Although ICT probes were up to recently the most popular, ESIPT probes and among them 3-hydroxyflavone derivatives, were shown to be the most productive. Several general problems were resolved by this family of probes, as for example the measurement of local dielectric constant, local H-bond accepting ability, water local concentration and ATP concentration in small volumes. Incorporation of such multi-channel probes into lipid membranes allowed to measure the different membrane potentials and to detect cell apoptosis. Also, it enabled to recognize and characterize the rafts formation in different lipid bilayers and peculiar features of the charged membrane interface. Such probes are also able to provide a concentration-dependent fluorescence signals upon binding of H⁺, Mg²⁺and Ba²⁺ions, and thus to recognize these different cations. The multi-channel probes are effective tools in the study of interactions of macromolecules such as peptides, proteins and nucleic acids. The most useful feature is that they inform simultaneously about several physical parameters, in this way giving a better insight in the investigated system. Thus, by comparing the reviewed probes with other modern fluorescent approaches, it can be concluded they are more informative and accurate tools.

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.

Glycation of α-synuclein hampers its binding to synaptic-like vesicles and its driving effect on their fusion

Parkinson's disease (PD) is one of the most prevalent neurodegenerative disorders affecting the worldwide population. One of its hallmarks is the intraneuronal accumulation of insoluble Lewy bodies (LBs), which cause the death of dopaminergic neurons. α-Synuclein (αS) is the main component of these LBs and in them, it commonly contains non-enzymatic post-translational modifications, such as those resulting from its reaction with reactive carbonyl species arising as side products of the intraneuronal glycolysis (mainly methylglyoxal). Consequently, lysines of the αS found in LBs of diabetic individuals are usually carboxyethylated. A precise comprehension of the effect of Nε-(carboxyethyl)lysine (CEL) on the aggregation of αS and on its physiological function becomes crucial to fully understand the molecular mechanisms underlying the development of diabetes-induced PD. Consequently, we have here used a synthetic αS where all its Lys have been replaced by CEL moieties (αS-CEL), and we have studied how these modifications could impact on the neurotransmission mechanism. This study allows us to describe how the non-enzymatic glycosylation (glycation) affects the function of a protein like αS, involved in the pathogenesis of PD. CEL decreases the ability of αS to bind micelles, although the micelle-bound fraction of αS-CEL still displays an α-helical fold resembling that of the lipid-bound αS. However, CEL completely abolishes the affinity of αS towards synaptic-like vesicles and, consequently, it hampers its physiological function as a catalyst of the clustering and the fusion of the synaptic vesicles.

Monitoring Local pH of Membranous Aggregates via Ratiometric Color Changing Response

A series of oxidized di(indolyl)arylmethanes (DIAM) with polyaromatic signaling moieties have been designed for monitoring local pH at interfacial region of surfactant aggregates, such as micelles and vesicles etc. The oxidized DIAMs show changes in solution color from red to yellow when incorporated in cationic surfactants (at pH 7.4) and yellow to reddish pink when exposed to negatively-charged surfactants (at pH 5.0). The changes in surface charge can influence the interfacial pH (distinct from bulk pH of the medium) of the surfactant aggregates. The mechanistic studies indicate the red-shifted absorption maxima observed in the presence of anionic amphiphiles (acidic local pH) was originated from the protonated species. On the contrary, maxima in the blue region, triggered by positively charged amphiphiles (basic local pH), can be attributed to the zwitterionic species. Such prototropic equilibrium affects charge transfer states of the molecules along with their self-assembly properties. Thus, it is evident that probes can predict as well as quantify the local pH change using the pseudophase ion exchange formalism. Also, the probes can detect the presence of anionic amphiphiles even when bound to phospholipid membranes.

Fat and Protein Combat Triggers Immunological Weapons of Innate and Adaptive Immune Systems to Launch Neuroinflammation in Parkinson's Disease

Parkinson's disease (PD) is the second-most common neurodegenerative disease in the world, affecting up to 10 million people. This disease mainly happens due to the loss of dopaminergic neurons accountable for memory and motor function. Partial glucocerebrosidase enzyme deficiency and the resultant excess accumulation of glycosphingolipids and alpha-synuclein (α-syn) aggregation have been linked to predominant risk factors that lead to neurodegeneration and memory and motor defects in PD, with known and unknown causes. An increasing body of evidence uncovers the role of several other lipids and their association with α-syn aggregation, which activates the innate and adaptive immune system and sparks brain inflammation in PD. Here, we review the emerging role of a number of lipids, i.e., triglyceride (TG), diglycerides (DG), glycerophosphoethanolamines (GPE), polyunsaturated fatty acids (PUFA), sphingolipids, gangliosides, glycerophospholipids (GPL), and cholesterols, and their connection with α-syn aggregation as well as the induction of innate and adaptive immune reactions that trigger neuroinflammation in PD.

Interactions between SARS-CoV-2 N-Protein and α-Synuclein Accelerate Amyloid Formation

First cases that point at a correlation between SARS-CoV-2 infections and the development of Parkinson’s disease (PD) have been reported. Currently, it is unclear if there is also a direct causal link between these diseases. To obtain first insights into a possible molecular relation between viral infections and the aggregation of α-synuclein protein into amyloid fibrils characteristic for PD, we investigated the effect of the presence of SARS-CoV-2 proteins on α-synuclein aggregation. We show, in test tube experiments, that SARS-CoV-2 spike protein (S-protein) has no effect on α-synuclein aggregation, while SARS-CoV-2 nucleocapsid protein (N-protein) considerably speeds up the aggregation process. We observe the formation of multiprotein complexes and eventually amyloid fibrils. Microinjection of N-protein in SH-SY5Y cells disturbed the α-synuclein proteostasis and increased cell death. Our results point toward direct interactions between the N-protein of SARS-CoV-2 and α-synuclein as molecular basis for the observed correlation between SARS-CoV-2 infections and Parkinsonism.

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”.

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.

α-Synuclein: An All-Inclusive Trip Around its Structure, Influencing Factors and Applied Techniques

Alpha-synuclein (αSyn) is a highly expressed and conserved protein, typically found in the presynaptic terminals of neurons. The misfolding and aggregation of αSyn into amyloid fibrils is a pathogenic hallmark of several neurodegenerative diseases called synucleinopathies, such as Parkinson’s disease. Since αSyn is an Intrinsically Disordered Protein, the characterization of its structure remains very challenging. Moreover, the mechanisms by which the structural conversion of monomeric αSyn into oligomers and finally into fibrils takes place is still far to be completely understood. Over the years, various studies have provided insights into the possible pathways that αSyn could follow to misfold and acquire oligomeric and fibrillar forms. In addition, it has been observed that αSyn structure can be influenced by different parameters, such as mutations in its sequence, the biological environment (e.g., lipids, endogenous small molecules and proteins), the interaction with exogenous compounds (e.g., drugs, diet components, heavy metals). Herein, we review the structural features of αSyn (wild-type and disease-mutated) that have been elucidated up to present by both experimental and computational techniques in different environmental and biological conditions. We believe that this gathering of current knowledge will further facilitate studies on αSyn, helping the planning of future experiments on the interactions of this protein with targeting molecules especially taking into consideration the environmental conditions.

Influence of Lipid Membranes on α-Synuclein Aggregation

α-Synuclein is a neuronal protein involved in synaptic vesicle trafficking. During the course of Parkinson's disease, it aggregates, forming amyloid fibrils that accumulate in the midbrain. This pathological fibrillization process is strongly modulated by physiological interactions of α-synuclein with lipid membranes. However, the detailed mechanism of this effect remains unclear. In this work, we used environment-sensitive fluorescent dyes to study the influence of model lipid membranes on the kinetics of α-synuclein fibrillization. We observed that formation of the fibrils from α-synuclein monomers is strongly delayed even by small amounts of lipids. Furthermore, we found that membrane-bound α-synuclein monomers are not involved in fibril elongation. Hence, presence of lipids slows down fibril growth proportionally to the fraction of membrane-bound protein.

Cholesterol-containing lipid nanodiscs promote an α-synuclein binding mode that accelerates oligomerization

Dysregulation of the biosynthesis of cholesterol and other lipids has been implicated in many neurological diseases, including Parkinson's disease. Misfolding of α-synuclein (α-Syn), the main actor in Parkinson's disease, is associated with changes in a lipid environment. However, the exact molecular mechanisms underlying cholesterol effect on α-Syn binding to lipids as well as α-Syn oligomerization and fibrillation remain elusive, as does the relative importance of cholesterol compared to other factors. We probed the interactions and fibrillation behaviour of α-Syn using styrene-maleic acid nanodiscs, containing zwitterionic and anionic lipid model systems with and without cholesterol. Surface plasmon resonance and thioflavin T fluorescence assays were employed to monitor α-Syn binding, as well as fibrillation in the absence and presence of membrane models. 1 H-15 N-correlated NMR was used to monitor the fold of α-Syn in response to nanodisc binding, determining individual residue apparent affinities for the nanodisc-contained bilayers. The addition of cholesterol inhibited α-Syn interaction with lipid bilayers and, however, significantly promoted α-Syn fibrillation, with a more than a 20-fold reduction of lag times before fibrillation onset. When α-Syn bilayer interactions were analysed at an individual residue level by solution-state NMR, we observed two different effects of cholesterol. In nanodiscs made of DOPC, the addition of cholesterol modulated the NAC part of α-Syn, leading to stronger interaction of this region with the lipid bilayer. In contrast, in the nanodiscs comprising DOPC, DOPE and DOPG, the NAC part was mostly unaffected by the presence of cholesterol, while the binding of the N and the C termini was both inhibited.

Supramolecular and suprabiomolecular photochemistry: a perspective overview

In this perspective review article, we have attempted to bring out the important current trends of research in the areas of supramolecular and suprabiomolecular photochemistry. Since the spans of the subject areas are very vast, it is impossible to cover all the aspects within the limited space of this review article. Nevertheless, efforts have been made to assimilate the basic understanding of how supramolecular interactions can significantly change the photophysical and other related physiochemical properties of chromophoric dyes and drugs, which have enormous academic and practical implications. We have discussed with reference to relevant chemical systems where supramolecularly assisted modulations in the properties of chromophoric dyes and drugs can be used or have already been used in different areas like sensing, dye/drug stabilization, drug delivery, functional materials, and aqueous dye laser systems. In supramolecular assemblies, along with their conventional photophysical properties, the acid-base properties of prototropic dyes, as well as the excited state prototautomerization and related proton transfer behavior of proton donor/acceptor dye molecules, are also largely modulated due to supramolecular interactions, which are often reflected very explicitly through changes in their absorption and fluorescence characteristics, providing us many useful insights into these chemical systems and bringing out intriguing applications of such changes in different applied areas. Another interesting research area in supramolecular photochemistry is the excitation energy transfer from the donor to acceptor moieties in self-assembled systems which have immense importance in light harvesting applications, mimicking natural photosynthetic systems. In this review article, we have discussed varieties of these aspects, highlighting their academic and applied implications. We have tried to emphasize the progress made so far and thus to bring out future research perspectives in the subject areas concerned, which are anticipated to find many useful applications in areas like sensors, catalysis, electronic devices, pharmaceuticals, drug formulations, nanomedicine, light harvesting, and smart materials.

The Localization of Alpha-synuclein in the Endocytic Pathway

Alpha-synuclein (αS) is an intrinsically disordered protein (IDP) that is abundantly present in the brain and is associated with Parkinson's disease (PD). In spite of its abundance and its contribution to PD pathogenesis, the exact cellular function of αS remains largely unknown. The ability of αS to remodel phospholipid model membranes combined with biochemical and cellular studies suggests that αS is involved in endocytosis. To unravel with which route(s) and stage(s) of the endocytic pathway αS is associated, we quantified the colocalization between αS and endocytic marker proteins in differentiated SH-SY5Y neuronal cells, using an object based colocalization analysis. Comparison with randomized data allowed us to discriminate between structural and coincidental colocalizations. A large fraction of the αS positive vesicles colocalizes with caveolin positive vesicles, a smaller fraction colocalizes with EEA1 and Rab7. We find no structural colocalization between αS and clathrin and Rab11 positive vesicles. We conclude that in a physiological context, αS is structurally associated with caveolin dependent membrane vesiculation and is found further along the endocytic pathway, in decreasing amounts, on early and late endosomes. Our results not only shed new light on the function of αS, they also provide a possible link between αS function and vesicle trafficking malfunction in PD.

Fluorescent probes for bioimaging of potential biomarkers in Parkinson's disease

This review comprehensively summarizes various types of fluorescent probes for PD and their applications for detection of various PD biomarkers.

Interactions of α-synuclein oligomers with lipid membranes

Parkinson's disease is an increasingly prevalent and currently incurable neurodegenerative disorder. At the molecular level, this disease is characterized by the formation of aberrant intracellular protein deposits known as Lewy bodies. Oligomeric forms of the protein α-synuclein (αS), which are believed to be both intermediates and by-products of Lewy body formation, are considered to be the main pathogenic species. Interactions of such oligomers with lipid membranes are increasingly emerging as a major molecular pathway underpinning their toxicity. Here we review recent progress in our understanding of the interactions of αS oligomers with lipid membranes. We highlight key structural and biophysical features of αS oligomers, the effects of these features on αS oligomer membrane binding properties, and resultant implications for understanding the etiology of Parkinson's disease. We discuss mechanistic modes of αS oligomer-lipid membrane interactions and the effects of environmental factors to such modes. Finally, we provide an overview of the current understanding of the main molecular determinants of αS oligomer toxicity in vivo.

Gut-Brain axis in Parkinson's disease etiology: The role of lipopolysaccharide

Recent studies highlight the initiation of Parkinson's disease (PD) in the gastrointestinal tract, decades before the manifestations in the central nervous system (CNS). This gut-brain axis of neurodegenerative diseases defines the critical role played by the unique microbial composition of the "second brain" formed by the enteric nervous system (ENS). Compromise in the enteric wall can result in the translocation of gut-microbiota along with their metabolites into the system that can affect the homeostatic machinery. The released metabolites can associate with protein substrates affecting several biological pathways. Among these, the bacterial endotoxin from Gram-negative bacteria, i.e., Lipopolysaccharide (LPS), has been implicated to play a definite role in progressive neurodegeneration. The molecular interaction of the lipid metabolites can have a direct neuro-modulatory effect on homeostatic protein components that can be transported to the CNS via the vagus nerve. α-synuclein (α-syn) is one such partner protein, the molecular interactions with which modulate its overall fibrillation propensity in the system. LPS interaction has been shown to affect the protein's aggregation kinetics in an alternative inflammatory pathway of PD pathogenesis. Several other lipid contents from the bacterial membranes could also be responsible for the initiation of α-syn amyloidogenesis. The present review will focus on the intermolecular interactions of α-syn with bacterial lipid components, particularly LPS, with a definite clinical manifestation in PD pathogenesis. However, deconvolution of the sequence of interaction events from the ENS to its propagation in the CNS is not easy or obvious. Nevertheless, the characterization of these lipid-mediated structures is a step towards realizing the novel targets in the pre-emptive diagnoses of PD. This comprehensive description should prompt the correlation of potential risk of amyloidogenesis upon detection of specific paradigm shifts in the microbial composition of the gut.

Two C-terminal sequence variations determine differential neurotoxicity between human and mouse α-synuclein

BACKGROUND

α-Synuclein (aSyn) aggregation is thought to play a central role in neurodegenerative disorders termed synucleinopathies, including Parkinson's disease (PD). Mouse aSyn contains a threonine residue at position 53 that mimics the human familial PD substitution A53T, yet in contrast to A53T patients, mice show no evidence of aSyn neuropathology even after aging. Here, we studied the neurotoxicity of human A53T, mouse aSyn, and various human-mouse chimeras in cellular and in vivo models, as well as their biochemical properties relevant to aSyn pathobiology.

METHODS

Primary midbrain cultures transduced with aSyn-encoding adenoviruses were analyzed immunocytochemically to determine relative dopaminergic neuron viability. Brain sections prepared from rats injected intranigrally with aSyn-encoding adeno-associated viruses were analyzed immunohistochemically to determine nigral dopaminergic neuron viability and striatal dopaminergic terminal density. Recombinant aSyn variants were characterized in terms of fibrillization rates by measuring thioflavin T fluorescence, fibril morphologies via electron microscopy and atomic force microscopy, and protein-lipid interactions by monitoring membrane-induced aSyn aggregation and aSyn-mediated vesicle disruption. Statistical tests consisted of ANOVA followed by Tukey's multiple comparisons post hoc test and the Kruskal-Wallis test followed by a Dunn's multiple comparisons test or a two-tailed Mann-Whitney test.

RESULTS

Mouse aSyn was less neurotoxic than human aSyn A53T in cell culture and in rat midbrain, and data obtained for the chimeric variants indicated that the human-to-mouse substitutions D121G and N122S were at least partially responsible for this decrease in neurotoxicity. Human aSyn A53T and a chimeric variant with the human residues D and N at positions 121 and 122 (respectively) showed a greater propensity to undergo membrane-induced aggregation and to elicit vesicle disruption. Differences in neurotoxicity among the human, mouse, and chimeric aSyn variants correlated weakly with differences in fibrillization rate or fibril morphology.

CONCLUSIONS

Mouse aSyn is less neurotoxic than the human A53T variant as a result of inhibitory effects of two C-terminal amino acid substitutions on membrane-induced aSyn aggregation and aSyn-mediated vesicle permeabilization. Our findings highlight the importance of membrane-induced self-assembly in aSyn neurotoxicity and suggest that inhibiting this process by targeting the C-terminal domain could slow neurodegeneration in PD and other synucleinopathy disorders.

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).

Direct excitation of higher excited state and kinetics of photoreactions

Excited-state reactions (ESR) play an essential role in chemical, physical, and biological processes. The mathematical models are usually used to study ESR in kinetics and steady-state regimes. In these models, the excitation pulse populates the first excited state (the first singlet level) of the primary molecular form. Recently, researchers' paid growing attention to the reactions excited via the higher energy levels. We modeled these reactions using the system of linear differential equations. Exact analytical expressions of the kinetics of N* and P* populations were derived for the general case when excitation performed via the higher S_n singlet state by the delta pulse. The graphical forms of these expressions were N and P time-dependent pulses. We detected the changes of the pulses' shapes, their maxima locations, the time behavior of the populations, and the total yield of the P* population. The changes occur due to the populating of the product excited state in the kinetic and thermodynamic reaction regimes. Numerical analysis performed for different ESR parameters revealed peculiarities of the N* and P* populations. Kinetics properties of these population characterize systems with varying rates of reversible ESR and various contributions of anti-Kasha (AK) reaction (from the S_n state) to P* population. Modeling data presented in graphical form, allowed to understand better (a) the impact of the AK reaction on the kinetic properties of the excited states of the molecular systems operating in various mode of ESR (kinetic, reversible and intermediate); (b) the photochemical processes' mechanisms. Also, this modeling allowed establishing the criteria for revealing the effect of the AK reaction for improving the efficiency of anti-Kasha processes.

Effect of solvent environment on excited state intramolecular proton transfer in 2-(4-(dimethylamino)phenyl)-3-hydroxy-6,7-dimethoxy-4h-chromen-4-one

The new ratiometric fluorescent probe 2-(4-(dimethylamino)phenyl)-3-hydroxy-6,7-dimethoxy-4h-chromen-4-one (HOF) monitoring of methanol in biodiesel was discovered experimentally (T. Y. Qin et al., Sens. Actuators, B, 2018, 277, 484-491). But the experimental study did not report the reaction mechanism in detail. In this study, density functional theory (DFT) and time-density functional theory (TDDFT) methods were used to theoretically study the excited-state intramolecular proton transfer (ESIPT) process of the HOF molecule. The molecular structure in the ground state and the excited state was optimized, and the infrared vibrational spectra, the frontier molecular orbitals, the charge transfer, the potential energy curves and the transition-state structures were calculated. The calculated results prove that the solvent polarity has a great influence on the ESIPT reaction of the HOF molecule. As the solvent polarity increased, the intensity of the intramolecular hydrogen bond decreased, and ESIPT was more difficult to occur. This work has studied the mechanism of the ESIPT reaction in more detail, and paved the way for future research on HOF molecules.

Membrane Curvature Sensing by Amphipathic Helices: Insights from Implicit Membrane Modeling

Sensing and generation of lipid membrane curvature, mediated by the binding of specific proteins onto the membrane surface, play crucial roles in cell biology. A number of mechanisms have been proposed, but the molecular understanding of these processes is incomplete. All-atom molecular dynamics simulations have offered valuable insights but are extremely demanding computationally. Implicit membrane simulations could provide a viable alternative, but current models apply only to planar membranes. In this work, the implicit membrane model 1 is extended to spherical and tubular membranes. The geometric change from planar to curved shapes is straightforward but insufficient for capturing the full curvature effect, which includes changes in lipid packing. Here, these packing effects are taken into account via the lateral pressure profile. The extended implicit membrane model 1 is tested on the wild-types and mutants of the antimicrobial peptide magainin, the ALPS motif of arfgap1, α-synuclein, and an ENTH domain. In these systems, the model is in qualitative agreement with experiments. We confirm that favorable electrostatic interactions tend to weaken curvature sensitivity in the presence of strong hydrophobic interactions but may actually have a positive effect when those are weak. We also find that binding to vesicles is more favorable than binding to tubes of the same diameter and that the long helix of α-synuclein tends to orient along the axis of tubes, whereas shorter helices tend to orient perpendicular to it. Adoption of a specific orientation could provide a mechanism for coupling protein oligomerization to tubule formation.

Lipids at the Crossroad of α-Synuclein Function and Dysfunction: Biological and Pathological Implications

Since its discovery, the study of the biological role of α-synuclein and its pathological implications has been the subject of increasing interest. The propensity to adopt different conformational states governing its aggregation and fibrillation makes this small 14-kDa cytosolic protein one of the main etiologic factors associated with degenerative disorders known as synucleinopathies. The structure, function, and toxicity of α-synuclein and the possibility of different therapeutic approaches to target the protein have been extensively investigated and reviewed. One intriguing characteristic of α-synuclein is the different ways in which it interacts with lipids. Though in-depth studies have been carried out in this field, the information they have produced is puzzling and the precise role of lipids in α-synuclein biology and pathology and vice versa is still largely unknown. Here we provide an overview and discussion of the main findings relating to α-synuclein/lipid interaction and its involvement in the modulation of lipid metabolism and signaling.

Cooperation of Helix Insertion and Lateral Pressure to Remodel Membranes

Nature has developed different protein mediated mechanisms to remodel cellular membranes. One of the proteins that is implicated in these processes is α-synuclein (αS). Here we investigate if besides αS’s membrane bound amphipathic helix the disordered, solvent exposed tail of the protein contributes to membrane reshaping. We produced αS variants with elongated or truncated disordered solvent exposed domains. We observe a transformation of opaque multi lamellar vesicle solutions into nonscattering solutions containing smaller structures upon addition of all αS variants. Experimental data combined with model calculations show that the cooperation of helix insertion and lateral pressure exerted by the disordered domain makes the full length protein decidedly more efficient in membrane remodeling than the truncated version. Using disordered domains may not only be cost-efficient, it may also add a new level of control over vesicle fusion/fission by expansion or compaction of the domain.

A designed protein binding-pocket to control excited-state intramolecular proton transfer fluorescence

Excited-state intramolecular proton transfer involves a photochemical isomerization and creates the opportunity for the emission of two distinct wavelengths of light from a single fluorophore. The selectivity between these two wavelengths of emission is dependent on the environment around the fluorophore and suggests the possibility for ratiometric monitoring of protein microenvironments. Unfortunately, nonspecific binding of ESIPT fluorophores does not often lead to dramatic changes in the ratio between the two wavelengths of emission. A protein binding pocket was designed to selectively discriminate between the two channels of emission available to an ESIPT fluorophore. This work is significant because it demonstrates that specific interactions between the protein and the fluorophore are essential to realize strong ratiometric differences between the two possible wavelengths of emission. The design strategies proposed here lead to an ESIPT fluorophore that can discern subtle differences in the interface between two proteins.

Role of Parkinson's Disease-Linked Mutations and N-Terminal Acetylation on the Oligomerization of α-Synuclein Induced by 3,4-Dihydroxyphenylacetaldehyde

Identifying the mechanisms by which the presynaptic protein α-synuclein (aSyn) is associated with neurodegeneration of dopamine neurons is a major priority in the Parkinson's disease (PD) field. Studies indicate that DOPAL (3,4-dihydroxyphenylacetaldehyde), an aldehyde generated from the enzymatic oxidation of dopamine, may convert aSyn monomer into a neurotoxin via formation of covalently stabilized toxic oligomers. Herein we investigated the role of N-terminal acetylation and familial aSyn mutations (A30P, A53T, E46K, G51D, and H50Q) on DOPAL-induced oligomerization of the protein. Our results indicate that the wild-type (WT) N-terminally acetylated aSyn (Ac-aSyn) is less prone to form oligomers upon incubation with DOPAL than the non-N-terminally acetylated protein. On the other hand, familial mutants from Ac-aSyn, particularly A53T, E46K, and H50Q increased the formation of DOPAL-derived aSyn oligomers, especially large oligomers. Binding of aSyn to synaptic-like small unilamellar vesicles (SUVs) protected distinctive aSyn variants against the effects of DOPAL. While N-terminal acetylation increased the protective action of SUVs against DOPAL-induced aSyn oligomerization, A53T, A30P, and H50Q mutations in Ac-aSyn had an opposite effect. This means that PD-linked mutations may not only perturb the affinity of aSyn for membranes but also influence the formation of DOPAL-mediated oligomers. Overall, our findings provide important evidence for the existence of a connection between familial mutations of aSyn, their distinct affinity to lipid membranes, and the formation of potentially toxic oligomers of the protein mediated by DOPAL.

The Role of Lipids in Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative disease characterized by a progressive loss of dopaminergic neurons from the nigrostriatal pathway, formation of Lewy bodies, and microgliosis. During the past decades multiple cellular pathways have been associated with PD pathology (i.e., oxidative stress, endosomal-lysosomal dysfunction, endoplasmic reticulum stress, and immune response), yet disease-modifying treatments are not available. We have recently used genetic data from familial and sporadic cases in an unbiased approach to build a molecular landscape for PD, revealing lipids as central players in this disease. Here we extensively review the current knowledge concerning the involvement of various subclasses of fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, sterols, and lipoproteins in PD pathogenesis. Our review corroborates a central role for most lipid classes, but the available information is fragmented, not always reproducible, and sometimes differs by sex, age or PD etiology of the patients. This hinders drawing firm conclusions about causal or associative effects of dietary lipids or defects in specific steps of lipid metabolism in PD. Future technological advances in lipidomics and additional systematic studies on lipid species from PD patient material may improve this situation and lead to a better appreciation of the significance of lipids for this devastating disease.

The effect of different environments on excited-state intramolecular proton transfer in 4′-methoxy-3-hydroxyflavone

Excited state intramolecular proton transfer reaction occurs with increasing difficulty in the solvents tested in the order toluene → ACN → DMF.

Excited state proton transfer based fluorescent molecular probes and their application in studying lipid bilayer membranes

The distribution and prototropic equilibria of 1-naphthol (NpOH) in lipid bilayer membrane.

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 processes of α-synuclein amyloid protein complexes involved in the pathogenesis of Parkinson's disease]

AIM

To analyze interactions between α-synuclein (αS) protein and lipids using biophysical methods.

MATERIAL AND METHODS

Recombinant α-synuclein synthesized in prokaryotic cells was used. To characterize the interaction of αS with negatively charged vesicles of DOPS (1,2-dioleoyl-sn-glycero-3-phospho-L-serine, sodium salt) and DOPG (1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol), sodium salt) and the consequences of such interactions on αS amyloid formation, combined circular dichroism, fluorescence and imaging methods in vitro were applied.

RESULTS AND CONCLUSION

Lipid head-group chemistry modulates αS interactions and also affects amyloid fiber formation. Pre-formed αS oligomers, typically present in a small amount in the αS starting material, acted as templates for linear growth of anomalous amyloid fibers in the presence of vesicles. At the same time, the remaining αS monomers were restricted from vesicle-mediated nucleation of amyloid fibers. Although not a dominant process in bulk experiments, this hidden αS aggregation pathway may be of importance in vivo.

Ultrafast photophysics of the environment-sensitive 4'-methoxy-3-hydroxyflavone fluorescent dye

The excited state intramolecular proton transfer (ESIPT) of 3-hydroxyflavone derivatives results in a fluorescence spectrum composed of two emission bands, the relative intensity of which is strongly influenced by the interaction with the local environment. We use time-resolved fluorescence and ultrafast transient absorption spectroscopies to investigate the photophysics of 4'-methoxy-3-hydroxyflavone in different solvents characterized by various polarities and hydrogen (H) bonding capabilities. We evidence that in this compound, the ESIPT reaction rate varies by more than 3 orders of magnitude, depending on the H-bonding capability of its local environment. This remarkable property is attributed to the moderate electron-donating strength of the 4'-methoxy substituent, and turns this fluorescent dye into a very promising fluorescent probe of biomolecular structures and interactions, where local structural heterogeneity may possibly be revealed by resolving a distribution of ESIPT reaction rates.

Non-uniform self-assembly: On the anisotropic architecture of α-synuclein supra-fibrillar aggregates

Although the function of biopolymer hydrogels in nature depends on structural anisotropy at mesoscopic length scales, the self-assembly of such anisotropic structures in vitro is challenging. Here we show that fibrils of the protein α-synuclein spontaneously self-assemble into structurally anisotropic hydrogel particles. While the fibrils in the interior of these supra-fibrillar aggregates (SFAs) are randomly oriented, the fibrils in the periphery prefer to cross neighboring fibrils at high angles. This difference in organization coincides with a significant difference in polarity of the environment in the central and peripheral parts of the SFA. We rationalize the structural anisotropy of SFAs in the light of the observation that αS fibrils bind a substantial amount of counterions. We propose that, with the progress of protein polymerization into fibrils, this binding of counterions changes the ionic environment which triggers a change in fibril organization resulting in anisotropy in the architecture of hydrogel particles.

Membrane-Bound Alpha Synuclein Clusters Induce Impaired Lipid Diffusion and Increased Lipid Packing

The aggregation of membrane-bound α-synuclein (αS) into oligomers and/or amyloid fibrils has been suggested to cause membrane damage in in vitro model phospholipid membrane systems and in vivo. In this study, we investigate how αS interactions that precede the formation of well-defined aggregates influence physical membrane properties. Using three truncated variants of αS with different aggregation propensities and comparable phospholipid membrane binding affinities we show, using fluorescence recovery after photobleaching (FRAP) and fluorescence anisotropy measurements, that formation of αS clusters on supported lipid bilayers (SLBs) impairs lateral lipid diffusion and increases lipid packing beneath the αS clusters. Formation of protein clusters starts immediately after monomer addition. The magnitudes of the changes in effective lipid diffusion and lipid order increase with the protein cluster size. Our results show that the combination of inter-αS and αS-membrane interactions can drive the formation of more ordered lipid domains. Considering the functional involvement of membrane micro-domains in biological membranes, αS-induced domain formation may be relevant for alternative disease mechanisms.

ESIPT and FRET probes for monitoring nanoparticle polymer coating stability

Coating strategies of inorganic nanoparticles (NPs) can provide properties unavailable to the NP core alone, such as targeting, specific sensing, and increased biocompatibility. Non-covalent amphiphilic NP capping polymers function via hydrophobic interactions with surface ligands and are extensively used to transfer NPs to aqueous media. For applications of coated NPs as actuators (sensors, markers, or for drug delivery) in a complex environment, such as biological systems, it is important to achieve a deep understanding of the factors affecting coating stability and behavior. We have designed a system that tests the coating stability of amphiphilic polymers through a simple fluorescent readout using either polarity sensing ESIPT (excited state intramolecular proton transfer) dyes or NP FRET (Förster resonance energy transfer). The stability of the coating was determined in response to changes in polarity, pH and ionic strength in the medium. Using the ESIPT system we observed linear changes in signal up to ∼20-25% v/v of co-solvent addition, constituting a break point. Based on such data, we propose a model for coating instability and the important adjustable parameters, such as the electrical charge distribution. FRET data provided confirmatory evidence for the model. The ESIPT dyes and FRET based methods represent new, simple tools for testing NP coating stability in complex environments.

An extract of the marine alga Alaria esculenta modulates α-synuclein folding and amyloid formation

The conversion of α-synuclein from its natively unfolded and α-helical tetrameric forms to an amyloid conformation is central to the emergence of Parkinson's disease. Therefore, prevention of this conversion may offer an effective way of avoiding the onset of this disease or delaying its progress. At different concentrations, an aqueous extract from the edible winged kelp (Alaria esculenta), was shown to lower and to raise the melting point of α-synuclein. Size fractionation of the extract resulted in the separation of these distinct activities. The fraction below 5kDa decreased the melting point of α-synuclein, whereas the fraction above 10kDa raised the melting point. Both of these fractions were found to inhibit the formation of amyloid aggregates by α-synuclein, measured by thioflavin T dye-binding assays; this effect was further confirmed by transmission electron microscopy showing the inhibition of fibril formation. Circular dichroism analysis suggested that the incubation of α-synuclein under fibrillation conditions resulted in the loss of substantial native helical structure in the presence and absence of the fractions. It is therefore likely that the fractions inhibit fibrillation by interacting with the unfolded form of α-synuclein.

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

Amyloid formation of the synaptic brain proteinα-synuclein (αS) is related to degeneration of dopaminergic neurons in Parkinson's disease patients.αS is thought to function in vesicle transport and fusion and it binds strongly to negatively charged vesiclesin vitro. Here we combined circular dichroism, fluorescence and imaging methodsin vitroto characterize the interaction ofαS with negatively charged vesicles of DOPS (1,2-dioleoyl-sn-glycero-3-phospho-L-serine, sodium salt) and DOPG (1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol), sodium salt) and the consequences of such interactions onαS amyloid formation. We found that lipid head-group chemistry modulatesαS interactions and also affects amyloid fiber formation. During the course of the experiments, we made the unexpected discovery that pre-formedαS oligomers, typically present in a small amount in theαS starting material, acted as templates for linear growth of anomalous amyloid fibers in the presence of vesicles. At the same time, the remainingαS monomers were restricted from vesicle-mediated nucleation of amyloid fibers. Although not a dominant process in bulk experiments, this hiddenαS aggregation pathway may be of importancein vivo.

The Impact of N-terminal Acetylation of α-Synuclein on Phospholipid Membrane Binding and Fibril Structure

Human α-synuclein (αS) has been shown to be N terminally acetylated in its physiological state. This modification is proposed to modulate the function and aggregation of αS into amyloid fibrils. Using bacterially expressed acetylated-αS (NTAc-αS) and endogenous αS (Endo-αS) from human erythrocytes, we show that N-terminal acetylation has little impact on αS binding to anionic membranes and thus likely not relevant for regulating membrane affinity. N-terminal acetylation does have an effect on αS aggregation, resulting in a narrower distribution of the aggregation lag times and rates. 2D-IR spectra show that acetylation changes the secondary structure of αS in fibrils. This difference may arise from the slightly higher helical propensity of acetylated-αS in solution leading to a more homogenous fibril population with different fibril structure than non-acetylated αS. We speculate that N-terminal acetylation imposes conformational restraints on N-terminal residues in αS, thus predisposing αS toward specific interactions with other binding partners or alternatively decrease nonspecific interactions.

V63 and N65 of overexpressed α-synuclein are involved in mitochondrial dysfunction

Parkinson's Disease (PD) is one of the most common neurodegenerative diseases. α-Synuclein (α-Syn)-encoded by SNCA, the first-identified PD-related gene-is the main component of Lewy bodies, which are a pathological hallmark of PD. We previously reported that α-Syn accumulates in mitochondria in PD, causing mitochondrial abnormalities and disrupting mitochondrial membrane potential (Δψm) and mitochondrial potential transition pore (mPTP) opening by interacting with the voltage-dependent anion channel (VDAC) and adenine nucleotide translocator. However, the mechanistic basis of mitochondrial impairment caused by α-Syn has yet to be elucidated. It has been suggested that the amino acid residues Q62, V63, and N65 of α-Syn are important for the interaction of the protein with membranes. To investigate whether this underlies the mitochondrial dysfunction induced by α-Syn overexpression, we mutated these residues to alanine and transfected HEK293T and MN9D cells with the mutated forms of α-Syn protein. The V63A and N65A mutations prevented mitochondrial Ca(2+) overload and Δψm dysregulation as well as complex I inactivation and reactive oxygen species production while blocking mPTP opening and caspase 9 activation, possibly by reducing α-Syn accumulation in mitochondria. These results indicate that V63 and N65 are critical residues mediating mitochondrial inactivation. These findings provide novel insight into the molecular events contributing to PD pathogenesis.

Nanoscale dynamics of phospholipids reveals an optimal assembly mechanism of pore-forming proteins in bilayer membranes

Using STED-FCS, we show that the fluidity of the membrane controls the induced dynamical heterogeneity in model membranes upon interacting with pore-forming toxins.