The mode of α-synuclein binding to membranes depends on lipid composition and lipid to protein ratio

Elevated concentrations cause upright alpha-synuclein conformation at lipid interfaces

The amyloid aggregation of α-synuclein (αS), related to Parkinson's disease, can be catalyzed by lipid membranes. Despite the importance of lipid surfaces, the 3D-structure and orientation of lipid-bound αS is still not known in detail. Here, we report interface-specific vibrational sum-frequency generation (VSFG) experiments that reveal how monomeric αS binds to an anionic lipid interface over a large range of αS-lipid ratios. To interpret the experimental data, we present a frame-selection method ("ViscaSelect") in which out-of-equilibrium molecular dynamics simulations are used to generate structural hypotheses that are compared to experimental amide-I spectra via excitonic spectral calculations. At low and physiological αS concentrations, we derive flat-lying helical structures as previously reported. However, at elevated and potentially disease-related concentrations, a transition to interface-protruding αS structures occurs. Such an upright conformation promotes lateral interactions between αS monomers and may explain how lipid membranes catalyze the formation of αS amyloids at elevated protein concentrations.

Cryo-EM reveals the membrane-binding phenomenon of EspB, a virulence factor of the Mycobacterial Type VII secretion system

Mycobacterium tuberculosis (Mtb) utilizes sophisticated machinery called the type VII secretion system to translocate virulence factors across its complex lipid membrane. EspB, a ∼36 kDa secreted substrate of the ESX-1 apparatus, was shown to cause ESAT-6-independent host cell death. Despite the current wealth of high-resolution structural information of the ordered N-terminal domain, the mechanism of EspB-mediated virulence remains poorly characterized. Here we document EspB interaction with phosphatidic acid (PA) and phosphatidylserine (PS) in the context of membranes, through a biophysical approach including TEM and cryo-EM. We were also able to show PA, PS-dependent conversion of monomers to oligomers at physiological pH. Our data suggest that EspB adheres to biological membranes with limited PA and PS. Electron microscopy of yeast mitochondria with EspB indicates a mitochondrial-membrane binding property of this ESX-1 substrate. Further, we determined the 3D structures of EspB with and without PA and observed plausible stabilization of the low complexity C-terminal domain in the presence of PA. Collectively, our cryo-EM-based structural and functional studies of EspB provide further insight into the host-Mtb interaction.

Probing the interactions between amyloidogenic proteins and bio-membranes

Protein misfolding diseases (PMDs) in humans are characterized by the deposition of protein aggregates in tissues, including Alzheimer's disease, Parkinson's disease, type 2 diabetes, and amyotrophic lateral sclerosis. Misfolding and aggregation of amyloidogenic proteins play a central role in the onset and progression of PMDs, and these processes are regulated by multiple factors, especially the interaction between proteins and bio-membranes. Bio-membranes induce conformational changes in amyloidogenic proteins and affect their aggregation; on the other hand, the aggregates of amyloidogenic proteins may cause membrane damage or dysfunction leading to cytotoxicity. In this review, we summarize the factors that affect the binding of amyloidogenic proteins and membranes, the effects of bio-membranes on the aggregation of amyloidogenic proteins, mechanisms of membrane disruption by amyloidogenic aggregates, technical approaches for detecting these interactions, and finally therapeutic strategies targeting membrane damage caused by amyloidogenic proteins.

Dual Effects of Presynaptic Membrane Mimetics on α-Synuclein Amyloid Aggregation

Aggregation of intrinsically disordered α-synuclein (αSN) under various conditions is closely related to synucleinopathies. Although various biological membranes have shown to alter the structure and aggregation propensity of αSN, a thorough understanding of the molecular and mechanical mechanism of amyloidogenesis in membranes remains unanswered. Herein, we examined the structural changes, binding properties, and amyloidogenicity of three variations of αSN mutants under two types of liposomes, 1,2-Dioleoyl-sn-glycero-3-Phosphocholine (DOPC) and presynaptic vesicle mimetic (Mimic) membranes. While neutrally charged DOPC membranes elicited marginal changes in the structure and amyloid fibrillation of αSNs, negatively charged Mimic membranes induced dramatic helical folding and biphasic amyloid generation. At low concentration of Mimic membranes, the amyloid fibrillation of αSNs was promoted in a dose-dependent manner. However, further increases in the concentration constrained the fibrillation process. These results suggest the dual effect of Mimic membranes on regulating the amyloidogenesis of αSN, which is rationalized by the amyloidogenic structure of αSN and condensation-dilution of local αSN concentration. Finally, we propose physicochemical properties of αSN and membrane surfaces, and their propensity to drive electrostatic interactions as decisive factors of amyloidogenesis.

Glycoconjugate journal special issue on: the glycobiology of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder that affects over 10 million aging people worldwide. This condition is characterized by the degeneration of dopaminergic neurons in the pars compacta region of the substantia nigra (SNpc) and by aggregation of proteins, commonly α-synuclein (SNCA). The formation of Lewy bodies that encapsulate aggregated proteins in lipid vesicles is a hallmark of PD. Glycosylation of proteins and neuroinflammation are involved in the pathogenesis. SNCA has many posttranslational modifications and interacts with components of membranes that affect aggregation. The large membrane lipid dolichol accumulates in the brain upon age and has a significant effect on membrane structure. The replacement of dopamine and dopaminergic neurons are at the forefront of therapeutic development. This review examines the role of membrane lipids, glycolipids, glycoproteins and dopamine in the aggregation of SNCA and development of PD. We discuss the SNCA-dopamine-neuromelanin-dolichol axis and the role of membranes in neuronal stem cells that could be a regenerative therapy for PD patients.

Impact of Fatty Acid-Binding Proteins in α-Synuclein-Induced Mitochondrial Injury in Synucleinopathy

Synucleinopathies are diverse diseases with motor and cognitive dysfunction due to progressive neuronal loss or demyelination, due to oligodendrocyte loss in the brain. While the etiology of neurodegenerative disorders (NDDs) is likely multifactorial, mitochondrial injury is one of the most vital factors in neuronal loss and oligodendrocyte dysfunction, especially in Parkinson’s disease, dementia with Lewy body, multiple system atrophy, and Krabbe disease. In recent years, the abnormal accumulation of highly neurotoxic α-synuclein in the mitochondrial membrane, which leads to mitochondrial dysfunction, was well studied. Furthermore, fatty acid-binding proteins (FABPs), which are members of a superfamily and are essential in fatty acid trafficking, were reported to trigger α-synuclein oligomerization in neurons and glial cells and to target the mitochondrial outer membrane, thereby causing mitochondrial loss. Here, we provide an updated overview of recent findings on FABP and α-synuclein interactions and mitochondrial injury in NDDs.

Intracellular Protein-Lipid Interactions Studied by Rapid-Scan Electron Paramagnetic Resonance Spectroscopy

Protein-membrane interactions play key roles in essential cellular processes; studying these interactions in the cell is a challenging task of modern biophysical chemistry. A prominent example is the interaction of human α-synuclein (αS) with negatively charged membranes. It has been well-studied in vitro, but in spite of the huge amount of lipid membranes in the crowded environment of biological cells, to date, no interactions have been detected in cells. Here, we use rapid-scan (RS) electron paramagnetic resonance (EPR) spectroscopy to study αS interactions with negatively charged vesicles in vitro and upon transfection of the protein and lipid vesicles into model cells, i.e., oocytes of Xenopus laevis. We show that protein-vesicle interactions are reflected in RS spectra in vitro and in cells, which enables time-resolved monitoring of protein-membrane interaction upon transfection into cells. Our data suggest binding of a small fraction of αS to endogenous membranes.

Neuroprotective effect of damaurone D in a C. elegans model of Parkinson's disease

In this study, we evaluated the protective effects of damaurone D (DaD), a dihydropyranoaurone compound, on dopaminergic (DA) neurodegeneration in Caenorhabditis elegans. The results showed that DaD treatment could successfully increase the survival rate of the worms under MPP⁺ exposure. Additionally, DaD protected against the MPP⁺-induced neurodegeneration in all eight DA neurons of the worms. Similarly, diminished DA neuronal damage was observed in the DaD-fed transgenic mutant overexpressing tyrosine hydroxylase. In addition, the corresponding behavioral impairment induced by MPP⁺ was strongly improved in the DaD treated worms, implying DaD has protective properties for DA neuronal function. Then, we further investigated the effect of DaD on α-synuclein aggregation, a key pathogenesis of Parkinson's disease (PD). In this study, DaD reduced the fluorescence signals of transgenic mutants that carried YFP-fused α-synuclein. A similar reduction in expressions of α-synuclein was observed by Western blot. Interestingly, our result from the dot-blot assay demonstrated that the formation of oligomers was significantly attenuated by the DaD treatment. Furthermore, DaD improved the abnormal fat storage and shortened lifespan of the animals with the same genetic background which supports the beneficial action of DaD on the α-synuclein-induced DA neurodegeneration. These results demonstrate that DaD could protect against both chemical- and genetic-induced DA neurodegeneration possibly through the modulation of oxidative stress, DA metabolism, and α-synuclein toxicity. Based on our present findings, we suggest that DaD might have a potential therapeutic role in Parkinson's disease.

Metal ions shape α-synuclein

α-Synuclein is an intrinsically disordered protein that can self-aggregate and plays a major role in Parkinson's disease (PD). Elevated levels of certain metal ions are found in protein aggregates in neurons of people suffering from PD, and environmental exposure has also been linked with neurodegeneration. Importantly, cellular interactions with metal ions, particularly Ca²⁺, have recently been reported as key for α-synuclein's physiological function at the pre-synapse. Here we study effects of metal ion interaction with α-synuclein at the molecular level, observing changes in the conformational behaviour of monomers, with a possible link to aggregation pathways and toxicity. Using native nano-electrospray ionisation ion mobility-mass spectrometry (nESI-IM-MS), we characterize the heterogeneous interactions of alkali, alkaline earth, transition and other metal ions and their global structural effects on α-synuclein. Different binding stoichiometries found upon titration with metal ions correlate with their specific binding affinity and capacity. Subtle conformational effects seen for singly charged metals differ profoundly from binding of multiply charged ions, often leading to overall compaction of the protein depending on the preferred binding sites. This study illustrates specific effects of metal coordination, and the associated electrostatic charge patterns, on the complex structural space of the intrinsically disordered protein α-synuclein.

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.

Avidity within the N-terminal anchor drives α-synuclein membrane interaction and insertion

In the brain, α-synuclein (aSN) partitions between free unbound cytosolic and membrane bound forms modulating both its physiological and pathological role and complicating its study due to structural heterogeneity. Here, we use an interdisciplinary, synergistic approach to characterize the properties of aSN:lipid mixtures, isolated aSN:lipid co-structures, and aSN in mammalian cells. Enabled by the isolation of the membrane-bound state, we show that within the previously described N-terminal membrane anchor, membrane interaction relies both on an N-terminal tail (NTT) head group layer insertion of 14 residues and a folded-upon-binding helix at the membrane surface. Both binding events must be present; if, for example, the NTT insertion is lost, the membrane affinity of aSN is severely compromised and formation of aSN:lipid co-structures hampered. In mammalian cells, compromised cooperativity results in lowered membrane association. Thus, avidity within the N-terminal anchor couples N-terminal insertion and helical surface binding, which is crucial for aSN membrane interaction and cellular localization, and may affect membrane fusion.

Shaping membranes with disordered proteins

Membrane proteins control and shape membrane trafficking processes. The role of protein structure in shaping cellular membranes is well established. However, a significant fraction of membrane proteins is disordered or contains long disordered regions. It becomes more and more clear that these disordered regions contribute to the function of membrane proteins. While the fold of a structured protein is essential for its function, being disordered seems to be a crucial feature of membrane bound intrinsically disordered proteins and protein regions. Here we outline the motifs that encode function in disordered proteins and discuss how these functional motifs enable disordered proteins to modulate membrane properties. These changes in membrane properties facilitate and regulate membrane trafficking processes which are highly abundant in eukaryotes.

Assembly of α-synuclein aggregates on phospholipid bilayers

The spontaneous self-assembly of α-synuclein (α-syn) into aggregates of different morphologies is associated with the development of Parkinson's disease. However, the mechanism behind the spontaneous assembly remains elusive. The current study shows a novel effect of phospholipid bilayers on the assembly of the α-syn aggregates. Using time-lapse atomic force microscopy, it was discovered that α-syn assembles into aggregates on bilayer surfaces, even at the nanomolar concentration range. The efficiency of the aggregation process depends on the membrane composition, with the greatest efficiency observed for of 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS). Importantly, assembled aggregates can dissociate from the surface, suggesting that on-surface aggregation is a mechanism by which pathological aggregates may be produced. Computational modeling revealed that dimers of α-syn assembled rapidly, through the membrane-bound monomer on POPS bilayer, due to an aggregation-prone orientation of α-syn. Interaction of α-syn with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) leads to a binding mode that does not induce a fast assembly of the dimer. Based on these findings, we propose a model in which the interaction of α-syn with membranes plays a critical role initiating the formation of α-syn aggregates and the overall aggregation process.

East Indian sandalwood (Santalum album L.) oil confers neuroprotection and geroprotection in Caenorhabditis elegans via activating SKN-1/Nrf2 signaling pathway

East Indian Sandalwood Oil (EISO) has diverse beneficial effects and has been used for thousands of years in traditional folk-medicine for treatment of different human ailments. However, there has been no in-depth scientific investigation to decipher the neuroprotective and geroprotective mechanism of EISO and its principle components, α- and β-santalol. Hence the current study was undertaken to assess the protective effects of EISO, and α- and β-santalol against neurotoxic (6-OHDA/6-hydroxydopamine) and proteotoxic (α-synuclein) stresses in a Caenorhabditis elegans model. Initially, we found that EISO and its principle components exerted an excellent antioxidant and antiapoptotic activity as it was able to extend the lifespan, and inhibit the ROS generation, and germline cell apoptosis in 6-OHDA-intoxicated C. elegans. Further, we showed that supplementation of EISO, and α- and β-santalol reduced the 6-OHDA and α-synuclein-induced Parkinson's disease associated pathologies and improved the physiological functions. The genetic and reporter gene expression analysis revealed that an EISO, or α- and β-santalol-mediated protective effect does not appear to rely on DAF-2/DAF-16, but selectively regulates SKN-1 and its downstream targets involved in antioxidant defense and geroprotective processes. Together, our findings indicated that EISO and its principle components are worth exploring further as a candidate redox-based neuroprotectant for the prevention and management of age-related neurological disorders.

[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.

The Close Encounter Between Alpha-Synuclein and Mitochondria

The presynaptic protein alpha-synuclein (α-syn) is unequivocally linked to the development of Parkinson’s disease (PD). Not only it is the major component of amyloid fibrils found in Lewy bodies but mutations and duplication/triplication in its gene are responsible for the onset of familial autosomal dominant forms of PD. Nevertheless, the precise mechanisms leading to neuronal degeneration are not fully understood. Several lines of evidence suggest that impaired autophagy clearance and mitochondrial dysfunctions such as bioenergetics and calcium handling defects and alteration in mitochondrial morphology might play a pivotal role in the etiology and progression of PD, and indicate the intriguing possibility that α-syn could be involved in the control of mitochondrial function both in physiological and pathological conditions. In favor of this, it has been shown that a fraction of cellular α-syn can selectively localize to mitochondrial sub-compartments upon specific stimuli, highlighting possible novel routes for α-syn action. A plethora of mitochondrial processes, including cytochrome c release, calcium homeostasis, control of mitochondrial membrane potential and ATP production, is directly influenced by α-syn. Eventually, α-syn localization within mitochondria may also account for its aggregation state, making the α-syn/mitochondria intimate relationship a potential key for the understanding of PD pathogenesis. Here, we will deeply survey the recent literature in the field by focusing our attention on the processes directly controlled by α-syn within mitochondrial sub-compartments and its potential partners providing possible hints for future therapeutic targets.

Different Conformational Subensembles of the Intrinsically Disordered Protein α-Synuclein in Cells

The intrinsically disordered protein α-synuclein (αS) is thought to play an important role in cellular membrane processes. Although in vitro experiments indicate that this initially disordered protein obtains structure upon membrane binding, NMR and EPR studies in cells could not single out any conformational subensemble. Here we microinjected small amounts of αS, labeled with a Förster resonance energy transfer (FRET) pair, into SH-SY5Y cells to investigate conformational changes upon membrane binding. Our FRET studies show a clear conformational difference between αS in the cytosol and when bound to small vesicles. The identification of these different conformational subensembles inside cells resolves the apparent contradiction between in vitro and in vivo experiments and shows that at least two different conformational subensembles of αS exist in cells. The existence of conformational subensembles supports the idea that αS can obtain different functions which can possibly be dynamically addressed with changing intracellular physicochemical conditions.

Insights into the Molecular Mechanisms of Alzheimer's and Parkinson's Diseases with Molecular Simulations: Understanding the Roles of Artificial and Pathological Missense Mutations in Intrinsically Disordered Proteins Related to Pathology

Amyloid-β and α-synuclein are intrinsically disordered proteins (IDPs), which are at the center of Alzheimer's and Parkinson's disease pathologies, respectively. These IDPs are extremely flexible and do not adopt stable structures. Furthermore, both amyloid-β and α-synuclein can form toxic oligomers, amyloid fibrils and other type of aggregates in Alzheimer's and Parkinson's diseases. Experimentalists face challenges in investigating the structures and thermodynamic properties of these IDPs in their monomeric and oligomeric forms due to the rapid conformational changes, fast aggregation processes and strong solvent effects. Classical molecular dynamics simulations complement experiments and provide structural information at the atomic level with dynamics without facing the same experimental limitations. Artificial missense mutations are employed experimentally and computationally for providing insights into the structure-function relationships of amyloid-β and α-synuclein in relation to the pathologies of Alzheimer's and Parkinson's diseases. Furthermore, there are several natural genetic variations that play a role in the pathogenesis of familial cases of Alzheimer's and Parkinson's diseases, which are related to specific genetic defects inherited in dominant or recessive patterns. The present review summarizes the current understanding of monomeric and oligomeric forms of amyloid-β and α-synuclein, as well as the impacts of artificial and pathological missense mutations on the structural ensembles of these IDPs using molecular dynamics simulations. We also emphasize the recent investigations on residual secondary structure formation in dynamic conformational ensembles of amyloid-β and α-synuclein, such as β-structure linked to the oligomerization and fibrillation mechanisms related to the pathologies of Alzheimer's and Parkinson's diseases. This information represents an important foundation for the successful and efficient drug design studies.

Membrane-induced initial structure of α-synuclein control its amyloidogenesis on model membranes

Amyloid fibrillation causes serious neurodegenerative diseases and amyloidosis; however, the detailed mechanisms by which the structural states of precursor proteins in a lipid membrane-associated environment contribute to amyloidogenesis still remains to be elucidated. We examined the relationship between structural states of intrinsically-disordered wild-type and mutant α-synuclein (αSN) and amyloidogenesis on two-types of model membranes. Highly-unstructured wild-type αSN (αSN_WT) and a C-terminally-truncated mutant lacking negative charges (αSN₁₀₃) formed amyloid fibrils on both types of membranes, the model membrane mimicking presynaptic vesicles (Mimic membrane) and the model membrane of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC membrane). Unstructured αSN_WT and αSN₁₀₃ both bound to Mimic membranes in a helical conformation with similar binding affinity. Promotion and then inhibition of amyloidogenesis of αSN_WT were observed as the concentration of Mimic lipids increased. We explain this by the two-state binding model: at lower lipid concentrations, binding of αSN_WT to membranes enhances amyloidogenicity by increasing the local concentration of membrane-bound αSN and so promoting amyloid nucleation; at higher lipid concentrations, membrane-bound αSN_WT is actually in a sense diluted by increasing the number of model membranes, which blocks amyloid fibrillation due to an insufficient bound population for productive nucleation. Meanwhile, αSN₁₀₃ formed amyloid fibrils over the whole concentration of Mimic lipids used here without inhibition, revealing the importance of helical structures for binding affinity and negatively charged unstructured C-terminal region for modulating amyloidogenesis. We propose that membrane binding-induced initial conformations of αSN, its overall charge states, and the population of membrane-bound αSN are key determinants of amyloidogenesis on membranes.

Interaction of misfolded proteins and mitochondria in neurodegenerative disorders

The number of the people affected by neurodegenerative disorders is growing dramatically due to the ageing of population. The major neurodegenerative diseases share some common pathological features including the involvement of mitochondria in the mechanism of pathology and misfolding and the accumulation of abnormally aggregated proteins. Neurotoxicity of aggregated β-amyloid, tau, α-synuclein and huntingtin is linked to the effects of these proteins on mitochondria. All these misfolded aggregates affect mitochondrial energy metabolism by inhibiting diverse mitochondrial complexes and limit ATP availability in neurones. β-Amyloid, tau, α-synuclein and huntingtin are shown to be involved in increased production of reactive oxygen species, which can be generated in mitochondria or can target this organelle. Most of these aggregated proteins are capable of deregulating mitochondrial calcium handling that, in combination with oxidative stress, lead to opening of the mitochondrial permeability transition pore. Despite some of the common features, aggregated β-amyloid, tau, α-synuclein and huntingtin have diverse targets in mitochondria that can partially explain neurotoxic effect of these proteins in different brain regions.

Holothuria scabra extracts exhibit anti-Parkinson potential in C. elegans: A model for anti-Parkinson testing

OBJECTIVES

Parkinson's disease (PD) is associated with aggregation of α-synuclein and selective death of dopaminergic (DA) neurons in the substantia nigra, thereby leading to cognitive and motor impairments. Nowadays, the drugs commonly used for PD treatment, such as levodopa, provide only symptomatic relief. Therefore, seeking new drugs against PD, especially from plants and marine organisms, is one of the major research areas to be explored. This study aimed to investigate the anti-Parkinson activity of the extracts from the sea cucumber, Holothuria scabra, by using Caenorhabditis elegans as a model.

METHODS

H. scabra was solvent-extracted and subdivided into six fractions including whole body-ethyl acetate (WBEA), body wall-ethyl acetate (BWEA), viscera-ethyl acetate (VIEA), whole body-butanol (WBBU), body wall-butanol (BWBU), and viscera-butanol (VIBU). The extracts were tested in C. elegans BZ555 strain expressing the green fluorescent protein (GFP) specifically in the DA neurons and NL5901 strain expressing human α-synuclein in the muscle cells.

RESULTS

WBEA, BWEA, and WBBU fractions of H. scabra extracts at 500 µg/ml significantly attenuated DA neuron-degeneration induced by selective cathecholamine neurotoxin 6-hydroxydopamine (6-OHDA) in the BZ555 strain. Moreover, the extracts also reduced α-synuclein aggregation and restored lipid content in NL5901, as well as improved food-sensing behavior and prolonged lifespan in the 6-OHDA-treated wild-type strain.

DISCUSSION

The study indicated that the H. scabra extracts have anti-Parkinson potential in the C. elegans model. These findings encourage further investigations on using the H. scabra extract, as well as its active constituent compounds, as a possible preventive and/or therapeutic intervention against PD.

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 Contribution of α-Synuclein Spreading to Parkinson's Disease Synaptopathy

Synaptopathies are diseases with synapse defects as shared pathogenic features, encompassing neurodegenerative disorders such as Parkinson's disease (PD). In sporadic PD, the most common age-related neurodegenerative movement disorder, nigrostriatal dopaminergic deficits are responsible for the onset of motor symptoms that have been related to α-synuclein deposition at synaptic sites. Indeed, α-synuclein accumulation can impair synaptic dopamine release and induces the death of nigrostriatal neurons. While in physiological conditions the protein can interact with and modulate synaptic vesicle proteins and membranes, numerous experimental evidences have confirmed that its pathological aggregation can compromise correct neuronal functioning. In addition, recent findings indicate that α-synuclein pathology spreads into the brain and can affect the peripheral autonomic and somatic nervous system. Indeed, monomeric, oligomeric, and fibrillary α-synuclein can move from cell to cell and can trigger the aggregation of the endogenous protein in recipient neurons. This novel “prion-like” behavior could further contribute to synaptic failure in PD and other synucleinopathies. This review describes the major findings supporting the occurrence of α-synuclein pathology propagation in PD and discusses how this phenomenon could induce or contribute to synaptic injury and degeneration.

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.

The number of α-synuclein proteins per vesicle gives insights into its physiological function

Although it is well established that the protein α-synuclein (αS) plays an important role in Parkinson’s disease, its physiological function remains largely unknown. It has been reported to bind membranes and to play a role in membrane remodeling processes. The mechanism by which αS remodels membranes is still debated; it may either affect its physical properties or act as a chaperone for other membrane associated proteins. To obtain insight into the role of αS in membrane remodeling we investigated the number of αS proteins associated with single small vesicles in a neuronal cell model. Using single-molecule microscopy and photo-bleaching approaches, we most frequently found 70 αS-GFPs per vesicle. Although this number is high enough to modulate physical membrane properties, it is also strikingly similar to the number of synaptobrevins, a putative interaction partner of αS, per vesicle. We therefore hypothesize a dual, synergistic role for αS in membrane remodeling.

High expression of α-synuclein in damaged mitochondria with PLA2G6 dysfunction

To clarify the role of α-synuclein (αSyn) in neuronal membrane remodeling, we analyzed the expression of αSyn in neurons with a dysfunction of PLA2G6, which is indispensable for membrane remodeling. αSyn/phosphorylated-αSyn (PαSyn) distribution and neurodegeneration were quantitatively estimated in PLA2G6-knockout (KO) mice, which demonstrate marked mitochondrial membrane degeneration. We also assessed the relationship between αSyn deposits and mitochondria in brain tissue from patients with PLA2G6-associated neurodegeneration (PLAN) and Parkinson’s disease (PD), and quantitatively examined Lewy bodies (LBs) and neurons. The expression level of αSyn was elevated in PLA2G6-knockdown cells and KO mouse neurons. Strong PαSyn expression was observed in neuronal granules in KO mice before onset of motor symptoms. The granules were mitochondrial outer membrane protein (TOM20)-positive. Ultramicroscopy revealed that PαSyn-positive granules were localized to mitochondria with degenerated inner membranes. After symptom onset, TOM20-positive granules were frequently found in ubiquitinated spheroids, where PαSyn expression was low. Axons were atrophic, but the neuronal loss was not evident in KO mice. In PLAN neurons, small PαSyn-positive inclusions with a TOM20-positive edge were frequently observed and clustered into LBs. The surfaces of most LBs were TOM20-positive in PLAN and TOM20-negative in PD brains. The high proportion of LB-bearing neurons and the preserved neuronal number in PLAN suggested long-term survival of LB-bearing neurons. Elevated expression of αSyn/PαSyn in mitochondria appears to be the early response to PLA2G6-deficiency in neurons. The strong affinity of αSyn for damaged mitochondrial membranes may promote membrane stabilization of mitochondria and neuronal survival in neurons.

Oligomerization and Membrane-binding Properties of Covalent Adducts Formed by the Interaction of α-Synuclein with the Toxic Dopamine Metabolite 3,4-Dihydroxyphenylacetaldehyde (DOPAL)

Oxidative deamination of dopamine produces the highly toxic aldehyde 3,4-dihydroxyphenylacetaldehyde (DOPAL), enhanced production of which is found in post-mortem brains of Parkinson disease patients. When injected into the substantia nigra of rat brains, DOPAL causes the loss of dopaminergic neurons accompanied by the accumulation of potentially toxic oligomers of the presynaptic protein α-synuclein (aS), potentially explaining the synergistic toxicity described for dopamine metabolism and aS aggregation. In this work, we demonstrate that DOPAL interacts with aS via formation of Schiff-base and Michael-addition adducts with Lys residues, in addition to causing oxidation of Met residues to Met-sulfoxide. DOPAL modification leads to the formation of small aS oligomers that may be cross-linked by DOPAL. Both monomeric and oligomeric DOPAL adducts potently inhibit the formation of mature amyloid fibrils by unmodified aS. The binding of aS to either lipid vesicles or detergent micelles, which results in a gain of α-helix structure in its N-terminal lipid-binding domain, protects the protein against DOPAL adduct formation and, consequently, inhibits DOPAL-induced aS oligomerization. Functionally, aS-DOPAL monomer exhibits a reduced affinity for small unilamellar vesicles with lipid composition similar to synaptic vesicles, in addition to diminished membrane-induced α-helical content in comparison with the unmodified protein. These results suggest that DOPAL could compromise the functionality of aS, even in the absence of protein oligomerization, by affecting the interaction of aS with lipid membranes and hence its role in the regulation of synaptic vesicle traffic in neurons.

Effects of impaired membrane interactions on α-synuclein aggregation and neurotoxicity

The post-mortem brains of individuals with Parkinson's disease (PD) and other synucleinopathy disorders are characterized by the presence of aggregated forms of the presynaptic protein α-synuclein (aSyn). Understanding the molecular mechanism of aSyn aggregation is essential for the development of neuroprotective strategies to treat these diseases. In this study, we examined how interactions between aSyn and phospholipid vesicles influence the protein's aggregation and toxicity to dopaminergic neurons. Two-dimensional NMR data revealed that two familial aSyn mutants, A30P and G51D, populated an exposed, membrane-bound conformer in which the central hydrophobic region was dissociated from the bilayer to a greater extent than in the case of wild-type aSyn. A30P and G51D had a greater propensity to undergo membrane-induced aggregation and elicited greater toxicity to primary dopaminergic neurons compared to the wild-type protein. In contrast, the non-familial aSyn mutant A29E exhibited a weak propensity to aggregate in the presence of phospholipid vesicles or to elicit neurotoxicity, despite adopting a relatively exposed membrane-bound conformation. Our findings suggest that the aggregation of exposed, membrane-bound aSyn conformers plays a key role in the protein's neurotoxicity in PD and other synucleinopathy disorders.

Structural characterization of toxic oligomers that are kinetically trapped during α-synuclein fibril formation

We describe the isolation and detailed structural characterization of stable toxic oligomers of α-synuclein that have accumulated during the process of amyloid formation. Our approach has allowed us to identify distinct subgroups of oligomers and to probe their molecular architectures by using cryo-electron microscopy (cryoEM) image reconstruction techniques. Although the oligomers exist in a range of sizes, with different extents and nature of β-sheet content and exposed hydrophobicity, they all possess a hollow cylindrical architecture with similarities to certain types of amyloid fibril, suggesting that the accumulation of at least some forms of amyloid oligomers is likely to be a consequence of very slow rates of rearrangement of their β-sheet structures. Our findings reveal the inherent multiplicity of the process of protein misfolding and the key role the β-sheet geometry acquired in the early stages of the self-assembly process plays in dictating the kinetic stability and the pathological nature of individual oligomeric species.

Molecular determinants of α-synuclein mutants' oligomerization and membrane interactions

Parkinson's disease (PD) is associated with the formation of toxic α-synuclein oligomers that can penetrate the cell membrane. Familial forms of PD are caused by the point mutations A53T, A30P, E46K, and H50Q. Artificial point mutations E35K and E57K also increase oligomerization and pore formation. We generated structural conformations of α-synuclein and the above-mentioned mutants using molecular dynamics. We elucidated four main regions in these conformers contacting the membrane and found that the region including residues 39-45 (Zone2) may have maximum membrane penetration. E57K mutant had the highest rate of interaction with the membrane, followed by A53T, E46K, and E35K mutants and wild type (wt) α-synuclein. The mutant A30P had the smallest percentage of conformers that contact the membrane by Zone 2 than all other mutants and wt α-synuclein. These results were confirmed experimentally in vitro. We identified the key amino acids that can interact with the membrane (Y38, E62, and N65 (first hydrophilic layer); E104, E105, and D115 (second hydrophilic layer), and V15 and V26 (central hydrophobic layer)) and the residues that are involved in the interprotein contacts (L38, V48, V49, Q62, and T64). Understanding the molecular interactions of α-synuclein mutants is important for the design of compounds blocking the formation of toxic oligomers.

Early sodium dodecyl sulfate induced collapse of α-synuclein correlates with its amyloid formation

The aggregation of α-synuclein (A-syn) has been implicated strongly in Parkinson's disease (PD). In vitro studies established A-syn to be a member of the intrinsically disordered protein (IDP) family. This protein undergoes structural interconversion between an extended and a compact state, and this equilibrium influences the mechanism of its aggregation. A combination of fluorescence resonance energy transfer (FRET) and fluorescence correlation spectroscopy (FCS) has been used to study the membrane induced conformational reorganization and aggregation of A-syn. Different structural and conformational events, including the early collapse, the formation of the secondary structure, and aggregation have been identified and characterized using FCS and other biophysical methods. In addition, the concentrations of glycerol and urea have been varied to study the effect of solution conditions on the above conformational events. Further, we have extended this study on a number of A-syn mutants, namely, A30P, A53T, and E46K. These mutants are chosen because of their known implications in the disease pathology. The variation of solution conditions and mutational analyses suggest a strong correlation between the extent of early collapse and the onset of aggregation in PD.

Lipid vesicles trigger α-synuclein aggregation by stimulating primary nucleation

α-Synuclein (α-syn) is a 140-residue intrinsically disordered protein that is involved in neuronal and synaptic vesicle plasticity, but its aggregation to form amyloid fibrils is the hallmark of Parkinson's disease (PD). The interaction between α-syn and lipid surfaces is believed to be a key feature for mediation of its normal function, but under other circumstances it is able to modulate amyloid fibril formation. Using a combination of experimental and theoretical approaches, we identify the mechanism through which facile aggregation of α-syn is induced under conditions where it binds a lipid bilayer, and we show that the rate of primary nucleation can be enhanced by three orders of magnitude or more under such conditions. These results reveal the key role that membrane interactions can have in triggering conversion of α-syn from its soluble state to the aggregated state that is associated with neurodegeneration and to its associated disease states.

Membrane interactions and fibrillization of α-synuclein play an essential role in membrane disruption

We studied α-synuclein (αS) aggregation in giant vesicles, and observed dramatic membrane disintegration, as well as lipid incorporation into micrometer-sized suprafibrillar aggregates. In the presence of dye-filled vesicles, dye leakage and fibrillization happen concurrently. However, growing fibrils do not impair the integrity of phospholipid vesicles that have a low affinity for αS. Seeding αS aggregation accelerates dye leakage, indicating that oligomeric species are not required to explain the observed effect. The evolving picture suggests that fibrils that appear in solution bind membranes and recruit membrane-bound monomers, resulting in lipid extraction, membrane destabilization and the formation of lipid-containing suprafibrillar aggregates.

α-Synuclein oligomers distinctively permeabilize complex model membranes

α-Synuclein oligomers are increasingly considered to be responsible for the death of dopaminergic neurons in Parkinson's disease. The toxicity mechanism of α-synuclein oligomers likely involves membrane permeabilization. Even though it is well established that α-synuclein oligomers bind and permeabilize vesicles composed of negatively-charged lipids, little attention has been given to the interaction of oligomers with bilayers of physiologically relevant lipid compositions. We investigated the interaction of α-synuclein with bilayers composed of lipid mixtures that mimic the composition of plasma and mitochondrial membranes. In the present study, we show that monomeric and oligomeric α-synuclein bind to these membranes. The resulting membrane leakage differs from that observed for simple artificial model bilayers. Although the addition of oligomers to negatively-charged lipid vesicles displays fast content release in a bulk permeabilization assay, adding oligomers to vesicles with compositions mimicking mitochondrial membranes shows a much slower loss of content. Oligomers are unable to induce leakage in the artificial plasma membranes, even after long-term incubation. CD experiments indicate that binding to lipid bilayers initially induces conformational changes in both oligomeric and monomeric α-synuclein, which show little change upon long-term incubation of oligomers with membranes. The results of the present study demonstrate that the mitochondrial model membranes are more vulnerable to permeabilization by oligomers than model plasma membranes reconstituted from brain-derived lipids; this preference may imply that increasingly complex membrane components, such as those in the plasma membrane mimic used in the present study, are less vulnerable to damage by oligomers.

Number and Brightness analysis of alpha-synuclein oligomerization and the associated mitochondrial morphology alterations in live cells

BACKGROUND

Alpha-synuclein oligomerization is associated to Parkinson's disease etiopathogenesis. The study of alpha-synuclein oligomerization properties in live cell and the definition of their effects on cellular viability are among fields expected to provide the knowledge required to unravel the mechanism(s) of toxicity that lead to the disease.

METHODS

We used Number and Brightness method, which is a method based on fluorescence fluctuation analysis, to monitor alpha-synuclein tagged with EGFP aggregation in living SH-SY5Y cells. The presence of alpha-synuclein oligomers detected with this method was associated with intracellular structure conditions, evaluated by fluorescence confocal imaging.

RESULTS

Cells overexpressing alpha-synuclein-EGFP present a heterogeneous ensemble of oligomers constituted by less than 10 monomers, when the protein approaches a threshold concentration value of about 90nM in the cell cytoplasm. We show that the oligomeric species are partially sequestered by lysosomes and that the mitochondria morphology is altered in cells presenting oligomers, suggesting that these mitochondria may be dysfunctional.

CONCLUSIONS

We showed that alpha-synuclein overexpression in SH-SY5Y causes the formation of alpha-synuclein oligomeric species, whose presence is associated with mitochondrial fragmentation and autophagic-lysosomal pathway activation in live cells.

GENERAL SIGNIFICANCE

The unique capability provided by the Number and Brightness analysis to study alpha-synuclein oligomer distribution and properties, and the study of their association to intracellular components in single live cells is important to forward our understanding of the molecular mechanisms of Parkinson's disease and it may be of general significance when applied to the study of other aggregating proteins in cellular models.

n-butylidenephthalide protects against dopaminergic neuron degeneration and α-synuclein accumulation in Caenorhabditis elegans models of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is the second most common degenerative disorder of the central nervous system that impairs motor skills and cognitive function. To date, the disease has no effective therapies. The identification of new drugs that provide benefit in arresting the decline seen in PD patients is the focus of much recent study. However, the lengthy time frame for the progression of neurodegeneration in PD increases both the time and cost of examining potential therapeutic compounds in mammalian models. An alternative is to first evaluate the efficacy of compounds in Caenorhabditis elegans models, which reduces examination time from months to days. n-Butylidenephthalide is the naturally-occurring component derived from the chloroform extract of Angelica sinensis. It has been shown to have anti-tumor and anti-inflammatory properties, but no reports have yet described the effects of n-butylidenephthalide on PD. The aim of this study was to assess the potential for n-butylidenephthalide to improve PD in C. elegans models.

METHODOLOGY/PRINCIPAL FINDINGS

In the current study, we employed a pharmacological strain that expresses green fluorescent protein specifically in dopaminergic neurons (BZ555) and a transgenic strain that expresses human α-synuclein in muscle cells (OW13) to investigate the antiparkinsonian activities of n-butylidenephthalide. Our results demonstrate that in PD animal models, n-butylidenephthalide significantly attenuates dopaminergic neuron degeneration induced by 6-hydroxydopamine; reduces α-synuclein accumulation; recovers lipid content, food-sensing behavior, and dopamine levels; and prolongs life-span of 6-hydroxydopamine treatment, thus revealing its potential as a possible antiparkinsonian drug. n-Butylidenephthalide may exert its effects by blocking egl-1 expression to inhibit apoptosis pathways and by raising rpn-6 expression to enhance the activity of proteasomes.

CONCLUSIONS/SIGNIFICANCE

n-Butylidenephthalide may be one of the effective neuroprotective agents for PD.

A four-amino acid linker between repeats in the α-synuclein sequence is important for fibril formation

α-Synuclein is a 140-amino acid protein that can switch conformation among intrinsically disordered in solution, helical on a membrane, and β-sheet in amyloid fibrils. Using the fluorescence of single-tryptophan mutants, we determined the immersion of different regions of the protein into lipid membranes. Our results suggest the presence of a flexible break close to residues 52-55 between two helical domains. The four-amino acid linker is not necessary for membrane binding but is important for fibril formation. A deletion mutant lacking this linker aggregates extremely slowly and slightly inhibits wild-type aggregation, possibly by blocking the growing ends of fibrils.