α-Synuclein oligomers distinctively permeabilize complex model membranes

Peripheral membrane protein endophilin B1 probes, perturbs and permeabilizes lipid bilayers

Bin/Amphiphysin/Rvs167 (BAR) domain containing proteins are peripheral membrane proteins that regulate intracellular membrane curvature. BAR protein endophilin B1 plays a key role in multiple cellular processes critical for oncogenesis, including autophagy and apoptosis. Amphipathic regions in endophilin B1 drive membrane association and tubulation through membrane scaffolding. Our understanding of exactly how BAR proteins like endophilin B1 promote highly diverse intracellular membrane remodeling events in the cell is severely limited due to lack of high-resolution structural information. Here we present the highest resolution cryo-EM structure of a BAR protein to date and the first structures of a BAR protein bound to a lipid bicelle. Using neural networks, we can effectively sort particle species of different stoichiometries, revealing the tremendous flexibility of post-membrane binding, pre-polymer BAR dimer organization and membrane deformation. We also show that endophilin B1 efficiently permeabilizes negatively charged liposomes that contain mitochondria-specific lipid cardiolipin and propose a new model for Bax-mediated cell death.

27-Hydroxycholesterol Drives the Spread of α-Synuclein Pathology in Parkinson's Disease

BACKGROUND

The accumulation and aggregation of α-synuclein (α-Syn) are characteristic of Parkinson's disease (PD). Epidemiological evidence indicates that hyperlipidemia is associated with an increased risk of PD. The levels of 27-hydroxycholesterol (27-OHC), a cholesterol oxidation derivative, are increased in the brain and cerebrospinal fluid of patients with PD. However, whether 27-OHC plays a role in α-Syn aggregation and propagation remains elusive.

OBJECTIVE

The aim of this study was to determine whether 27-OHC regulates α-Syn aggregation and propagation.

METHODS

Purified recombinant α-Syn, neuronal cultures, and α-Syn fibril-injected mouse model of PD were treated with 27-OHC. In addition, CYP27A1 knockout mice were used to investigate the effect of lowering 27-OHC on α-Syn pathology in vivo.

RESULTS

27-OHC accelerates the aggregation of α-Syn and enhances the seeding activity of α-Syn fibrils. Furthermore, the 27-OHC-modified α-Syn fibrils localize to the mitochondria and induce mitochondrial dysfunction and neurotoxicity. Injection of 27-OHC-modified α-Syn fibrils induces enhanced spread of α-Syn pathology and dopaminergic neurodegeneration compared with pure α-Syn fibrils. Similarly, subcutaneous administration of 27-OHC facilitates the seeding of α-Syn pathology. Genetic deletion of cytochrome P450 27A1 (CYP27A1), the enzyme that converts cholesterol to 27-OHC, ameliorates the spread of pathologic α-Syn, degeneration of the nigrostriatal dopaminergic pathway, and motor impairments. These results indicate that the cholesterol metabolite 27-OHC plays an important role in the pathogenesis of PD.

CONCLUSIONS

27-OHC promotes the aggregation and spread of α-Syn. Strategies aimed at inhibiting the CYP27A1-27-OHC axis may hold promise as a disease-modifying therapy to halt the progression of α-Syn pathology in PD. © 2023 International Parkinson and Movement Disorder Society.

The Effects of Lipids on α-Synuclein Aggregation In Vitro

The small neuronal protein α-synuclein (αS) is found in pre-synaptic terminals and plays a role in vesicle recycling and neurotransmission. Fibrillar aggregates of αS are the hallmark of Parkinson's disease and related neurodegenerative disorders. In both health and disease, interactions with lipids influence αS's structure and function, prompting much study of the effects of lipids on αS aggregation. A comprehensive collection (126 examples) of aggregation rate data for various αS/lipid combinations was presented, including combinations of lipid variations and mutations or post-translational modifications of αS. These data were interpreted in terms of lipid structure to identify general trends. These tabulated data serve as a resource for the community to help in the interpretation of aggregation experiments with lipids and to be potentially used as inputs for computational models of lipid effects on aggregation.

α-Synuclein fibril and synaptic vesicle interactions lead to vesicle destruction and increased lipid-associated fibril uptake into iPSC-derived neurons

Monomeric alpha-synuclein (aSyn) is a well characterised protein that importantly binds to lipids. aSyn monomers assemble into amyloid fibrils which are localised to lipids and organelles in insoluble structures found in Parkinson's disease patient's brains. Previous work to address pathological aSyn-lipid interactions has focused on using synthetic lipid membranes, which lack the complexity of physiological lipid membranes. Here, we use physiological membranes in the form of synaptic vesicles (SV) isolated from rodent brain to demonstrate that lipid-associated aSyn fibrils are more easily taken up into iPSC-derived cortical i3Neurons. Lipid-associated aSyn fibril characterisation reveals that SV lipids are an integrated part of the fibrils and while their fibril morphology differs from aSyn fibrils alone, the core fibril structure remains the same, suggesting the lipids lead to the increase in fibril uptake. Furthermore, SV enhance the aggregation rate of aSyn, yet increasing the SV:aSyn ratio causes a reduction in aggregation propensity. We finally show that aSyn fibrils disintegrate SV, whereas aSyn monomers cause clustering of SV using small angle neutron scattering and high-resolution imaging. Disease burden on neurons may be impacted by an increased uptake of lipid-associated aSyn which could enhance stress and pathology, which in turn may have fatal consequences for neurons.

Alpha Synuclein: Neurodegeneration and Inflammation

Alpha-Synuclein (α-Syn) is one of the most important molecules involved in the pathogenesis of Parkinson's disease and related disorders, synucleinopathies, but also in several other neurodegenerative disorders with a more elusive role. This review analyzes the activities of α-Syn, in different conformational states, monomeric, oligomeric and fibrils, in relation to neuronal dysfunction. The neuronal damage induced by α-Syn in various conformers will be analyzed in relation to its capacity to spread the intracellular aggregation seeds with a prion-like mechanism. In view of the prominent role of inflammation in virtually all neurodegenerative disorders, the activity of α-Syn will also be illustrated considering its influence on glial reactivity. We and others have described the interaction between general inflammation and cerebral dysfunctional activity of α-Syn. Differences in microglia and astrocyte activation have also been observed when in vivo the presence of α-Syn oligomers has been combined with a lasting peripheral inflammatory effect. The reactivity of microglia was amplified, while astrocytes were damaged by the double stimulus, opening new perspectives for the control of inflammation in synucleinopathies. Starting from our studies in experimental models, we extended the perspective to find useful pointers to orient future research and potential therapeutic strategies in neurodegenerative disorders.

Quantitative Seed Amplification Assay: A Proof-of-Principle Study

Amyloid fibrils of the protein α-synuclein (αS) have recently been identified as a biomarker for Parkinson's disease (PD). To detect the presence of these amyloid fibrils, seed amplification assays (SAAs) have been developed. SAAs allow for the detection of αS amyloid fibrils in biomatrices such as cerebral spinal fluid and are promising for PD diagnosis by providing a dichotomous (yes/no) response. The additional quantification of the number of αS amyloid fibrils may enable clinicians to evaluate and follow the disease progression and severity. Developing quantitative SAAs has been shown to be challenging. Here, we report on a proof-of-principle study on the quantification of αS fibrils in fibril-spiked model solutions of increasing compositional complexity including blood serum. We show that parameters derived from standard SAAs can be used for fibril quantification in these solutions. However, interactions between the monomeric αS reactant that is used for amplification and biomatrix components such as human serum albumin have to be taken into account. We demonstrate that quantification of fibrils is possible even down to the single fibril level in a model sample consisting of fibril-spiked diluted blood serum.

pH Effects Can Dominate Chemical Shift Perturbations in 1H,15N-HSQC NMR Spectroscopy for Studies of Small Molecule/α-Synuclein Interactions

¹H,¹⁵N-Heteronuclear Single Quantum Coherence (HSQC) NMR is a powerful technique that has been employed to characterize small-molecule interactions with intrinsically disordered monomeric α-Synuclein (aSyn). We report how solution pH can impact the interpretation of aSyn HSQC NMR spectra and demonstrate that small-molecule formulations (e.g., complexation with acidic salts) can lower sample pH and confound interpretation of drug binding and concomitant protein structural changes. Through stringent pH control, we confirm that several previously identified compounds (EGCG, Baicalin, and Dopamine (DOPA)) as well as a series of potent small-molecule inhibitors of aSyn pathology (Demeclocycline, Ro90-7501, and (±)-Bay K 8644) are capable of direct target engagement of aSyn. Previously, DOPA-aSyn interactions have been shown to elicit a dramatic chemical shift perturbation (CSP) localized to aSyn's H50 at low DOPA concentrations then expanding to aSyn's acidic C-terminal residues at increasing DOPA levels. Interestingly, this CSP profile mirrors our pH titration, where a small reduction in pH affects H50 CSP, and large pH changes induce robust C-terminal CSP. In contrast, under tightly controlled pH 5.0, DOPA induces significant CSPs observed at both ionizable and nonionizable residues. These results suggest that previous interpretations of DOPA-aSyn interactions were conflated with pH-induced CSP, highlighting the need for stringent pH control to minimize potential false-positive interpretations of ligand interactions in HSQC NMR experiments. Furthermore, DOPA's preferential interaction with aSyn under acidic pH represents a novel understanding of DOPA-aSyn interactions that may provide insight into the potential gain of toxic function of aSyn misfolding in α-synucleinopathies.

DA5-CH and Semaglutide Protect against Neurodegeneration and Reduce α-Synuclein Levels in the 6-OHDA Parkinson's Disease Rat Model

Insulin desensitization has been observed in the brains of patients with Parkinson's disease (PD), which is a progressive neurodegenerative disorder for which there is no cure. Semaglutide is a novel long-actingglucagon-likepeptide-1 (GLP-1) receptor agonist that is on the market as a treatment for type 2 diabetes. It is in a phase II clinical trial in patients with PD. Two previous phase II trials in PD patients showed good effects with the older GLP-1 receptor agonists, exendin-4 and liraglutide. We have developed a dual GLP-1/GIP receptor agonist (DA5-CH) that can cross the blood-brain barrier (BBB) at a higher rate than semaglutide. We tested semaglutide and DA5-CH in the 6-OHDA-lesion rat model of PD. Treatment was semaglutide or DA5-CH (25 nmol/kg, i.p.) daily for 30 days postlesion. Both drugs reduced the apomorphine-induced rotational behavior and alleviated dopamine depletion and the inflammation response in the lesioned striatum as shown in reduced IL-1β and TNF-α levels, with DA5-CH being more effective. In addition, both drugs protected dopaminergic neurons and increased TH expression in the substantia nigra. Furthermore, the level of monomer and aggregated α-synuclein was reduced by the drugs, and insulin resistance as shown in reduced pIRS-1ser312 phosphorylation was also attenuated after drug treatment, with DA5-CH being more effective. Therefore, while semaglutide showed good effects in this PD model, DA5-CH was superior and may be a better therapeutic drug for neurodegenerative disorders such as PD than GLP-1 receptor agonists that do not easily cross the BBB.

Oligomeropathies, inflammation and prion protein binding

The central role of oligomers, small soluble aggregates of misfolded proteins, in the pathogenesis of neurodegenerative disorders is recognized in numerous experimental conditions and is compatible with clinical evidence. To underline this concept, some years ago we coined the term oligomeropathies to define the common mechanism of action of protein misfolding diseases like Alzheimer, Parkinson or prion diseases. Using simple experimental conditions, with direct application of synthetic β amyloid or α-synuclein oligomers intraventricularly at micromolar concentrations, we could detect differences and similarities in the biological consequences. The two oligomer species affected cognitive behavior, neuronal dysfunction and cerebral inflammatory reactions with distinct mechanisms. In these experimental conditions the proposed mediatory role of cellular prion protein in oligomer activities was not confirmed. Together with oligomers, inflammation at different levels can be important early in neurodegenerative disorders; both β amyloid and α-synuclein oligomers induce inflammation and its control strongly affects neuronal dysfunction. This review summarizes our studies with β-amyloid or α-synuclein oligomers, also considering the potential curative role of doxycycline, a well-known antibiotic with anti-amyloidogenic and anti-inflammatory activities. These actions are analyzed in terms of the therapeutic prospects.

Distribution of α-Synuclein Aggregation in the Peripheral Tissues

Parkinson's disease (PD) is a chronic neurodegenerative disease mainly characterized by movement disorders and other non-motor symptoms, including the loss of dopaminergic neurons in the substantia nigra parts. Abnormal α-synuclein aggregation in the brain is closely associated with the loss of dopaminergic neurons. α-synuclein can propagate in the central nervous system (CNS) and periphery under pathological conditions. Many researches have focused on its aggregation and distribution in the CNS and explored its relationship with PD. But in recent years, the distribution of α-synuclein in peripheral tissues have been paid much attention. This review summarized the distribution of α-synuclein in the choroid plexus, blood, saliva, gastrointestine and other tissues, and discussed the potential mechanism of α-synuclein aggregation, providing a basis for the early diagnosis and intervention of PD.

The Pathological G51D Mutation in Alpha-Synuclein Oligomers Confers Distinct Structural Attributes and Cellular Toxicity

A wide variety of oligomeric structures are formed during the aggregation of proteins associated with neurodegenerative diseases. Such soluble oligomers are believed to be key toxic species in the related disorders; therefore, identification of the structural determinants of toxicity is of upmost importance. Here, we analysed toxic oligomers of α-synuclein and its pathological variants in order to identify structural features that could be related to toxicity and found a novel structural polymorphism within G51D oligomers. These G51D oligomers can adopt a variety of β-sheet-rich structures with differing degrees of α-helical content, and the helical structural content of these oligomers correlates with the level of induced cellular dysfunction in SH-SY5Y cells. This structure–function relationship observed in α-synuclein oligomers thus presents the α-helical structure as another potential structural determinant that may be linked with cellular toxicity in amyloid-related proteins.

Parkinson Disease: Translating Insights from Molecular Mechanisms to Neuroprotection

Parkinson disease (PD) used to be considered a nongenetic condition. However, the identification of several autosomal dominant and recessive mutations linked to monogenic PD has changed this view. Clinically manifest PD is then thought to occur through a complex interplay between genetic mutations, many of which have incomplete penetrance, and environmental factors, both neuroprotective and increasing susceptibility, which variably interact to reach a threshold over which PD becomes clinically manifested. Functional studies of PD gene products have identified many cellular and molecular pathways, providing crucial insights into the nature and causes of PD. PD originates from multiple causes and a range of pathogenic processes at play, ultimately culminating in nigral dopaminergic loss and motor dysfunction. An in-depth understanding of these complex and possibly convergent pathways will pave the way for therapeutic approaches to alleviate the disease symptoms and neuroprotective strategies to prevent disease manifestations. This review is aimed at providing a comprehensive understanding of advances made in PD research based on leveraging genetic insights into the pathogenesis of PD. It further discusses novel perspectives to facilitate identification of critical molecular pathways that are central to neurodegeneration that hold the potential to develop neuroprotective and/or neurorestorative therapeutic strategies for PD. SIGNIFICANCE STATEMENT: A comprehensive review of PD pathophysiology is provided on the complex interplay of genetic and environmental factors and biologic processes that contribute to PD pathogenesis. This knowledge identifies new targets that could be leveraged into disease-modifying therapies to prevent or slow neurodegeneration in PD.

PGC-1s in the Spotlight with Parkinson's Disease

Parkinson's disease is one of the most common neurodegenerative disorders worldwide, characterized by a progressive loss of dopaminergic neurons mainly localized in the substantia nigra pars compacta. In recent years, the detailed analyses of both genetic and idiopathic forms of the disease have led to a better understanding of the molecular and cellular pathways involved in PD, pointing to the centrality of mitochondrial dysfunctions in the pathogenic process. Failure of mitochondrial quality control is now considered a hallmark of the disease. The peroxisome proliferator-activated receptor gamma coactivator 1 (PGC-1) family acts as a master regulator of mitochondrial biogenesis. Therefore, keeping PGC-1 level in a proper range is fundamental to guarantee functional neurons. Here we review the major findings that tightly bond PD and PGC-1s, raising important points that might lead to future investigations.

Extract from the Marine Seaweed Padina pavonica Protects Mitochondrial Biomembranes from Damage by Amyloidogenic Peptides

The identification of compounds which protect the double-membrane of mitochondrial organelles from disruption by toxic confomers of amyloid proteins may offer a therapeutic strategy to combat human neurodegenerative diseases. Here, we exploited an extract from the marine brown seaweed Padina pavonica (PPE) as a vital source of natural bioactive compounds to protect mitochondrial membranes against insult by oligomeric aggregates of the amyloidogenic proteins amyloid-β (Aβ), α-synuclein (α-syn) and tau, which are currently considered to be major targets for drug discovery in Alzheimer's disease (AD) and Parkinson's disease (PD). We show that PPE manifested a significant inhibitory effect against swelling of isolated mitochondria exposed to the amyloid oligomers, and attenuated the release of cytochrome c from the mitochondria. Using cardiolipin-enriched synthetic lipid membranes, we also show that dye leakage from fluorophore-loaded vesicles and formation of channel-like pores in planar bilayer membranes are largely prevented by incubating the oligomeric aggregates with PPE. Lastly, we demonstrate that PPE curtails the ability of Aβ42 and α-syn monomers to self-assemble into larger β-aggregate structures, as well as potently disrupts their respective amyloid fibrils. In conclusion, the mito-protective and anti-aggregator biological activities of Padina pavonica extract may be of therapeutic value in neurodegenerative proteinopathies, such as AD and PD.

Sigma-2 receptor antagonists rescue neuronal dysfunction induced by Parkinson's patient brain-derived α-synuclein

α‐Synuclein oligomers are thought to have a pivotal role in sporadic and familial Parkinson's disease (PD) and related α‐synucleinopathies, causing dysregulation of protein trafficking, autophagy/lysosomal function, and protein clearance, as well as synaptic function impairment underlying motor and cognitive symptoms of PD. Moreover, trans‐synaptic spread of α‐synuclein oligomers is hypothesized to mediate disease progression. Therapeutic approaches that effectively block α‐synuclein oligomer‐induced pathogenesis are urgently needed. Here, we show for the first time that α‐synuclein species isolated from human PD patient brain and recombinant α‐synuclein oligomers caused similar deficits in lipid vesicle trafficking rates in cultured rat neurons and glia, while α‐synuclein species isolated from non‐PD human control brain samples did not. Recombinant α‐synuclein oligomers also increased neuronal expression of lysosomal‐associated membrane protein‐2A (LAMP‐2A), the lysosomal receptor that has a critical role in chaperone‐mediated autophagy. Unbiased screening of several small molecule libraries (including the NIH Clinical Collection) identified sigma‐2 receptor antagonists as the most effective at blocking α‐synuclein oligomer‐induced trafficking deficits and LAMP‐2A upregulation in a dose‐dependent manner. These results indicate that antagonists of the sigma‐2 receptor complex may alleviate α‐synuclein oligomer‐induced neurotoxicity and are a novel therapeutic approach for disease modification in PD and related α‐synucleinopathies.

α-Synuclein in traumatic and vascular diseases of the central nervous system

α-Synuclein (α-Syn) is a small, soluble, disordered protein that is widely expressed in the nervous system. Although its physiological functions are not yet fully understood, it is mainly involved in synaptic vesicle transport, neurotransmitter synthesis and release, cell membrane homeostasis, lipid synthesis, mitochondrial and lysosomal activities, and heavy metal removal. The complex and inconsistent pathological manifestations of α-Syn are attributed to its structural instability, mutational complexity, misfolding, and diverse posttranslational modifications. These effects trigger mitochondrial dysfunction, oxidative stress, and neuroinflammatory responses, resulting in neuronal death and neurodegeneration. Several recent studies have discovered the pathogenic roles of α-Syn in traumatic and vascular central nervous system diseases, such as traumatic spinal cord injury, brain injury, and stroke, and in aggravating the processes of neurodegeneration. This review aims to highlight the structural and pathophysiological changes in α-Syn and its mechanism of action in traumatic and vascular diseases of the central nervous system.

Multiplicity of α-Synuclein Aggregated Species and Their Possible Roles in Disease

α-Synuclein amyloid aggregation is a defining molecular feature of Parkinson's disease, Lewy body dementia, and multiple system atrophy, but can also be found in other neurodegenerative disorders such as Alzheimer's disease. The process of α-synuclein aggregation can be initiated through alternative nucleation mechanisms and dominated by different secondary processes giving rise to multiple amyloid polymorphs and intermediate species. Some aggregated species have more inherent abilities to induce cellular stress and toxicity, while others seem to be more potent in propagating neurodegeneration. The preference for particular types of polymorphs depends on the solution conditions and the cellular microenvironment that the protein encounters, which is likely related to the distinct cellular locations of α-synuclein inclusions in different synucleinopathies, and the existence of disease-specific amyloid polymorphs. In this review, we discuss our current understanding on the nature and structure of the various types of α-synuclein aggregated species and their possible roles in pathology. Precisely defining these distinct α-synuclein species will contribute to understanding the molecular origins of these disorders, developing accurate diagnoses, and designing effective therapeutic interventions for these highly debilitating neurodegenerative diseases.

Interaction of Alpha-Synuclein With Lipids: Mitochondrial Cardiolipin as a Critical Player in the Pathogenesis of Parkinson's Disease

Alpha-Synuclein (α-Syn) is a central protein in the pathogenesis of synucleinopathies, a group of neurodegenerative disorders including Parkinson’s disease (PD). Although its role in neurotransmission is well established, the precise role of this protein in disease pathogenesis is still not fully understood. It is, however, widely regarded to be associated with the misfolding and accumulation of toxic intracellular aggregates. In fact, α-Syn is the most abundant protein component of Lewy bodies and Lewy neurites, which are also characterized by a high lipid content. Lipids, the main constituents of cellular membranes, have been implicated in many aspects of PD-related processes. α-Syn interacts with membrane phospholipids and free fatty acids via its N-terminal domain, and altered lipid-protein complexes might enhance both its binding to synaptic and mitochondrial membranes and its oligomerization. Several studies have highlighted a specific interaction of α-Syn with the phospholipid cardiolipin (CL), a major constituent of mitochondrial membranes. By interacting with CL, α-Syn is able to disrupt mitochondrial membrane integrity, leading to mitochondrial dysfunction. Additionally, externalized CL is able to facilitate the refolding of toxic α-Syn species at the outer mitochondrial membrane. In this review, we discuss how α-Syn/lipid interactions, in particular the α-Syn/CL interaction at the mitochondrial membrane, may affect α-Syn aggregation and mitochondrial dysfunction and may thus represent an important mechanism in the pathogenesis of PD.

Functional Reciprocity of Amyloids and Antimicrobial Peptides: Rethinking the Role of Supramolecular Assembly in Host Defense, Immune Activation, and Inflammation

Pathological self-assembly is a concept that is classically associated with amyloids, such as amyloid-β (Aβ) in Alzheimer's disease and α-synuclein in Parkinson's disease. In prokaryotic organisms, amyloids are assembled extracellularly in a similar fashion to human amyloids. Pathogenicity of amyloids is attributed to their ability to transform into several distinct structural states that reflect their downstream biological consequences. While the oligomeric forms of amyloids are thought to be responsible for their cytotoxicity via membrane permeation, their fibrillar conformations are known to interact with the innate immune system to induce inflammation. Furthermore, both eukaryotic and prokaryotic amyloids can self-assemble into molecular chaperones to bind nucleic acids, enabling amplification of Toll-like receptor (TLR) signaling. Recent work has shown that antimicrobial peptides (AMPs) follow a strikingly similar paradigm. Previously, AMPs were thought of as peptides with the primary function of permeating microbial membranes. Consistent with this, many AMPs are facially amphiphilic and can facilitate membrane remodeling processes such as pore formation and fusion. We show that various AMPs and chemokines can also chaperone and organize immune ligands into amyloid-like ordered supramolecular structures that are geometrically optimized for binding to TLRs, thereby amplifying immune signaling. The ability of amphiphilic AMPs to self-assemble cooperatively into superhelical protofibrils that form structural scaffolds for the ordered presentation of immune ligands like DNA and dsRNA is central to inflammation. It is interesting to explore the notion that the assembly of AMP protofibrils may be analogous to that of amyloid aggregates. Coming full circle, recent work has suggested that Aβ and other amyloids also have AMP-like antimicrobial functions. The emerging perspective is one in which assembly affords a more finely calibrated system of recognition and response: the detection of single immune ligands, immune ligands bound to AMPs, and immune ligands spatially organized to varying degrees by AMPs, result in different immunologic outcomes. In this framework, not all ordered structures generated during multi-stepped AMP (or amyloid) assembly are pathological in origin. Supramolecular structures formed during this process serve as signatures to the innate immune system to orchestrate immune amplification in a proportional, situation-dependent manner.

Targeting α-synuclein for PD Therapeutics: A Pursuit on All Fronts

Parkinson's Disease (PD) is characterized both by the loss of dopaminergic neurons in the substantia nigra and the presence of cytoplasmic inclusions called Lewy Bodies. These Lewy Bodies contain the aggregated α-synuclein (α-syn) protein, which has been shown to be able to propagate from cell to cell and throughout different regions in the brain. Due to its central role in the pathology and the lack of a curative treatment for PD, an increasing number of studies have aimed at targeting this protein for therapeutics. Here, we reviewed and discussed the many different approaches that have been studied to inhibit α-syn accumulation via direct and indirect targeting. These analyses have led to the generation of multiple clinical trials that are either completed or currently active. These clinical trials and the current preclinical studies must still face obstacles ahead, but give hope of finding a therapy for PD with time.

Protective effects of trehalose against Mn-induced α-synuclein oligomerization in mice: Involvement of oxidative stress and autophagy

Overexposure to manganese (Mn) is widely known to induce alpha-synuclein (α-Syn) oligomerization, which has been attributed to the oxidative damage of α-Syn protein. Trehalose has been shown to induce autophagy and serve as a chemical chaperone, but little information has been reported about its effect on Mn-induced α-Syn oligomerization. In this study, we investigate whether trehalose can effectively interfere with Mn-induced α-Syn oligomerization, using different concentrations of trehalose (2% and 4% (g/vol [mL])) in a mouse model of manganism. After 6 weeks of exposure to Mn, both oxidative stress and autophagy were activated and resulted in α-Syn oligomerization and neuronal cell damage in the mouse brain tissue. Our results also revealed that pretreatment with trehalose significantly reduced the oxidative damage to α-Syn protein and increased autophagy activation. These findings clearly demonstrated that trehalose can relieve Mn-induced α-Syn oligomerization and neuronal cell damage through its anti-oxidative and autophagy-inducing effects.

Cardiolipin Promotes Pore-Forming Activity of Alpha-Synuclein Oligomers in Mitochondrial Membranes

Aggregation of the amyloid-forming α-synuclein (αS) protein is closely associated with the etiology of Parkinson's disease (PD), the most common motor neurodegenerative disorder. Many studies have shown that soluble aggregation intermediates of αS, termed oligomers, permeabilize a variety of phospholipid membranes; thus, membrane disruption may represent a key pathogenic mechanism of αS toxicity. Given the centrality of mitochondrial dysfunction in PD, we therefore probed the formation of ion-permeable pores by αS oligomers in planar lipid bilayers reflecting the complex phospholipid composition of mitochondrial membranes. Using single-channel electrophysiology, we recorded distinct multilevel conductances (100-400 pS) with stepwise current transitions, typical of protein-bound nanopores, in mitochondrial-like membranes. Crucially, we observed that the presence of cardiolipin (CL), the signature phospholipid of mitochondrial membranes, enhanced αS-lipid interaction and the membrane pore-forming activity of αS oligomers. Further, preincubation of isolated mitochondria with a CL-specific dye protected against αS oligomer-induced mitochondrial swelling and release of cytochrome c. Hence, we favor a scenario in which αS oligomers directly porate a local lipid environment rich in CL, for instance outer mitochondrial contact sites or the inner mitochondrial membrane, to induce mitochondrial dysfunction. Pharmacological modulation of αS pore complex formation might thus preserve mitochondrial membrane integrity and alleviate mitochondrial dysfunction in PD.

Autophagy in Synucleinopathy: The Overwhelmed and Defective Machinery

Alpha-synuclein positive-intracytoplasmic inclusions are the common denominators of the synucleinopathies present as Lewy bodies in Parkinson’s disease, dementia with Lewy bodies, or glial cytoplasmic inclusions in multiple system atrophy. These neurodegenerative diseases also exhibit cellular dyshomeostasis, such as autophagy impairment. Several decades of research have questioned the potential link between the autophagy machinery and alpha-synuclein protein toxicity in synucleinopathy and neurodegenerative processes. Here, we aimed to discuss the active participation of autophagy impairment in alpha-synuclein accumulation and propagation, as well as alpha-synuclein-independent neurodegenerative processes in the field of synucleinopathy. Therapeutic approaches targeting the restoration of autophagy have started to emerge as relevant strategies to reverse pathological features in synucleinopathies.

Cerebral inoculation of human A53T α-synuclein reduces spatial memory decline and amyloid-β aggregation in APP/PS1 transgenic mice of Alzheimer's disease

Amyloid-β (Aβ) peptide and α-synuclein (α-syn) are major components of senile plaques in Alzheimer's disease (AD) and Lewy bodies in Parkinson's disease (PD), respectively. Co-occurrence of Aβ and α-syn in the senile brains of AD and LB diseases suggests interactions between the two proteins. However, the significance of the overlapping deposition, especially the effects of α-syn on the Aβ aggregation, still remains to be clarified. In the present study, we investigated the effects of α-syn pre-formed fibrils (PFFs) injection on the cognitive behaviors and Aβ deposition in the brain of APP/PS1 transgenic AD mice by using Morris water maze (MWM) test, immunohistochemistry and western blot techniques. We found that APP/PS1 transgenic mice exhibited an obvious elevation in the α-syn load, as well as Aβ deposition in the brain compared with wild type of C57 BL littermates. 5 months after cerebral injection of exogenous α-syn, MWM tests showed an alleviation in cognitive impairments in APP/PS1 mice; western blot and immunohistochemistry experiments also exhibited a significant reduction in Aβ level in the brain of APP/PS1 mice injected with α-syn. These results suggest that α-syn aggregated in the brain of AD may act as a protective factor and defend the brain tissue from early Aβ deposition and cognitive deficits.

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.

Intrinsic Conformational Preferences and Interactions in α-Synuclein Fibrils: Insights from Molecular Dynamics Simulations

Amyloid formation by the intrinsically disordered α-synuclein protein is the hallmark of Parkinson's disease. We present atomistic Molecular Dynamics simulations of the core of α-synuclein using enhanced sampling techniques to describe the conformational and binding free energy landscapes of fragments implicated in fibril stabilization. The theoretical framework is derived to combine the free energy profiles of the fragments into the reaction free energy of a protein binding to a fibril. Our study shows that individual fragments in solution have a propensity toward attaining non-β conformations, indicating that in a fibril β-strands are stabilized by interactions with other strands. We show that most dimers of hydrogen-bonded fragments are unstable in solution, while hydrogen bonding stabilizes the collective binding of five fragments to the end of a fibril. Hydrophobic effects make further contributions to the stability of fibrils. This study is the first of its kind where structural and binding preferences of the five major fragments of the hydrophobic core of α-synuclein have been investigated. This approach improves sampling of intrinsically disordered proteins, provides information on the binding mechanism between the core sequences of α-synuclein, and enables the parametrization of coarse grained models.

Cardiolipin in Central Nervous System Physiology and Pathology

Cardiolipin, an anionic phospholipid found primarily in the inner mitochondrial membrane, has many well-defined roles within the peripheral tissues, including the maintenance of mitochondrial membrane fluidity and the regulation of mitochondrial functions. Within the central nervous system (CNS), cardiolipin is found within both neuronal and non-neuronal glial cells, where it regulates metabolic processes, supports mitochondrial functions, and promotes brain cell viability. Furthermore, cardiolipin has been shown to act as an elimination signal and participate in programmed cell death by apoptosis of both neurons and glia. Since cardiolipin is associated with regulating brain homeostasis, the modification of its structure, or even a decrease in the overall levels of cardiolipin, can result in mitochondrial dysfunction, which is a characteristic feature of many diseases. In this review, we outline the various functions of cardiolipin within the cells of the CNS, including neurons, astrocytes, microglia, and oligodendrocytes. In addition, we discuss the role cardiolipin may play in the pathogenesis of the neurodegenerative disorders Alzheimer's disease and Parkinson's disease, as well as traumatic brain injury.

Mechanisms of Parkinson's disease-related proteins in mediating secondary brain damage after cerebral ischemia

Both Parkinson's disease (PD) and stroke are debilitating conditions that result in neuronal death and loss of neurological functions. These two conditions predominantly affect aging populations with the deterioration of the quality of life for the patients themselves and a tremendous burden to families. While the neurodegeneration and symptomology of PD develop chronically over the years, post-stroke neuronal death and dysfunction develop rapidly in days. Despite the discrepancy in the pathophysiological time frame and severity, both conditions share common molecular mechanisms that include oxidative stress, mitochondrial dysfunction, inflammation, endoplasmic reticulum stress, and activation of various cell death pathways (apoptosis/necrosis/autophagy) that synergistically modulate the neuronal death. Emerging evidence indicates that several proteins associated with early-onset familial PD play critical roles in mediating the neuronal death. Importantly, mutations in the genes encoding Parkin, PTEN-induced putative kinase 1 and DJ-1 mediate autosomal recessive forms of PD, whereas mutations in the genes encoding leucine-rich repeat kinase 2 and α-synuclein are responsible for autosomal dominant PD. This review discusses the significance of these proteins with the emphasis on the role of α-synuclein in mediating post-ischemic brain damage.

Familial Mutations May Switch Conformational Preferences in α-Synuclein Fibrils

The pathogenesis of Parkinson's disease is closely associated with the aggregation of the α-synuclein protein. Several familial mutants have been identified and shown to affect the aggregation kinetics of α-synuclein through distinct molecular mechanisms. Quantitative evaluation of the relative stabilities of the wild type and mutant fibrils is crucial for understanding the aggregation process and identifying the key component steps. In this work, we examined two topologically different α-synuclein fibril structures that are either determined by solid-state NMR method or modeled based on solid-state NMR data, and characterized their conformational properties and thermodynamic stabilities using molecular dynamics simulations. We show that the two fibril morphologies have comparable size, solvent exposure, secondary structures, and similar molecule/peptide binding modes; but different stabilities. Familial mutations do not significantly alter the overall fibril structures but shift their relative stabilities. Distinct mutations display altered fibril conformational behavior, suggesting different propagation preferences, reminiscent of cross-seeding among prion strains and tau deletion mutants. The simulations quantify the hydrophobic and electrostatic interactions, as well as N-terminal dynamics, that may contribute to the divergent aggregation kinetics that has been observed experimentally. Our results indicate that small molecule and peptide inhibitors may share the same binding region, providing molecular recognition that is independent of fibril conformation.

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.

α-Synuclein Oligomers Stabilize Pre-Existing Defects in Supported Bilayers and Propagate Membrane Damage in a Fractal-Like Pattern

Phospholipid vesicles are commonly used to get insights into the mechanism by which oligomers of amyloidogenic proteins damage membranes. Oligomers of the protein α-synuclein (αS) are thought to create pores in phospholipid vesicles containing a high amount of anionic phospholipids but fail to damage vesicle membranes at low surface charge densities. The current understanding of how αS oligomers damage the membranes is thus incomplete. This incomplete understanding may, in part, result from the choice of model membrane systems. The use of free-standing membranes such as vesicles may interfere with the unraveling of some damage mechanisms because the line tension at the edge of a membrane defect or pore ensures defect closure. Here, we have used supported lipid bilayers (SLBs) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPC/POPS) to study the membrane damage caused by αS oligomers. Although αS oligomers were not able to initiate the disruption of POPC/POPS vesicles or intact SLBs, oligomers did stabilize and enlarge pre-existing SLB defects. The increased exposure of lipid acyl chains at the edges of defects very likely facilitates membrane-oligomer interactions, resulting in the growth of fractal domains devoid of lipids. Concomitant with the appearance of the fractal membrane damage patterns, lipids appear in solution, directly implicating αS oligomers in the observed lipid extraction. The growth of the membrane damage patterns is not limited by the binding of lipids to the oligomer. The analysis of the shape and growth of the lipid-free domains suggests the involvement of an oligomer-dependent diffusion-limited extraction mechanism. The observed αS oligomer-induced propagation of membrane defects offers new insights into the mechanisms by which αS oligomers can contribute to the loss in membrane integrity.

Immunomodulators as Therapeutic Agents in Mitigating the Progression of Parkinson's Disease

Parkinson’s disease (PD) is a common neurodegenerative disorder that primarily afflicts the elderly. It is characterized by motor dysfunction due to extensive neuron loss in the substantia nigra pars compacta. There are multiple biological processes that are negatively impacted during the pathogenesis of PD, and are implicated in the cell death in this region. Neuroinflammation is evidently involved in PD pathology and mitigating the inflammatory cascade has been a therapeutic strategy. Age is the number one risk factor for PD and thus needs to be considered in the context of disease pathology. Here, we discuss the role of neuroinflammation within the context of aging as it applies to the development of PD, and the potential for two representative compounds, fractalkine and astaxanthin, to attenuate the pathophysiology that modulates neurodegeneration that occurs in Parkinson’s disease.

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.

Fluorescence Methods for Unraveling Oligomeric Amyloid Intermediates

Amyloid oligomers are considered to be the relevant toxic species in many amyloid diseases and much research effort has been devoted to fully characterize these oligomers. Despite their importance, oligomers have proven to be difficult to characterize structurally. Information on their aggregation number is scarce, largely because standard techniques struggle to provide reliable results. In this chapter, we present two different methods that reproducibly yield fluorescently labeled α-Synuclein oligomers. We then discuss a new approach, combining single-molecule photobleaching and sub-stoichiometric fluorescent labeling, that we have developed to determine the aggregation number of supramolecular protein assemblies.

Interaction of α-synuclein with biomembranes in Parkinson's disease--role of cardiolipin

One of the key molecular events underlying the pathogenesis of Parkinson's disease (PD) is the aberrant misfolding and aggregation of the α-synuclein (αS) protein into higher-order oligomers that play a key role in neuronal dysfunction and degeneration. A wealth of experimental data supports the hypothesis that the neurotoxicity of αS oligomers is intrinsically linked with their ability to interact with, and disrupt, biological membranes; especially those membranes having negatively-charged surfaces and/or lipid packing defects. Consequences of αS-lipid interaction include increased membrane tension, permeation by pore formation, membrane lysis and/or leakage due to the extraction of lipids from the bilayer. Moreover, we assert that the interaction of αS with a liquid-disordering phospholipid uniquely enriched in mitochondrial membranes, namely cardiolipin (1,3-diphosphatidyl-sn-glycerol, CL), helps target the αS oligomeric complexes intracellularly to mitochondria. Binding mediated by CL may thus represent an important pathomechanism by which cytosolic αS could physically associate with mitochondrial membranes and disrupt their integrity. Impaired mitochondrial function culminates in a cellular bioenergetic crisis and apoptotic death. To conclude, we advocate the accelerated discovery of new drugs targeting this pathway in order to restore mitochondrial function in PD.

Parkinson's Protein α-Synuclein Binds Efficiently and with a Novel Conformation to Two Natural Membrane Mimics

Binding of human α-Synuclein, a protein associated with Parkinson’s disease, to natural membranes is thought to be crucial in relation to its pathological and physiological function. Here the binding of αS to small unilamellar vesicles mimicking the inner mitochondrial and the neuronal plasma membrane is studied in situ by continuous wave and pulsed electron paramagnetic resonance. Local binding information of αS spin labeled by MTSL at positions 56 and 69 respectively shows that also helix 2 (residues 50–100) binds firmly to both membranes. By double electron-electron resonance (DEER) on the mutant spin labeled at positions 27 and 56 (αS 27/56) a new conformation on the membrane is found with a distance of 3.6 nm/ 3.7 nm between residues 27 and 56. In view of the low negative charge density of these membranes, the strong interaction is surprising, emphasizing that function and pathology of αS could involve synaptic vesicles and mitochondria.

Membrane Curvature-sensing and Curvature-inducing Activity of Islet Amyloid Polypeptide and Its Implications for Membrane Disruption

Islet amyloid polypeptide (IAPP) is a 37-amino acid amyloid protein intimately associated with pancreatic islet β-cell dysfunction and death in type II diabetes. In this study, we combine spectroscopic methods and microscopy to investigate α-helical IAPP-membrane interactions. Using light scattering and fluorescence microscopy, we observe that larger vesicles become smaller upon treatment with human or rat IAPP. Electron microscopy shows the formation of various highly curved structures such as tubules or smaller vesicles in a membrane-remodeling process, and spectrofluorometric detection of vesicle leakage shows disruption of membrane integrity. This effect is stronger for human IAPP than for the less toxic rat IAPP. From CD spectra in the presence of different-sized vesicles, we also uncover the membrane curvature-sensing ability of IAPP and find that it transitions from inducing to sensing membrane curvature when lipid negative charge is decreased. Our in vivo EM images of immunogold-labeled rat IAPP and human IAPP show both forms to localize to mitochondrial cristae, which contain not only locally curved membranes but also phosphatidylethanolamine and cardiolipin, lipids with high spontaneous negative curvature. Disruption of membrane integrity by induction of membrane curvature could apply more broadly to other amyloid proteins and be responsible for membrane damage observed in other amyloid diseases as well.

Intracellular Dynamics of Synucleins: "Here, There and Everywhere"

Synucleins are small, soluble proteins expressed primarily in neural tissue and in certain tumors. The synuclein family consists of three members: α-, β-, and γ-synucleins present only in vertebrates. Members of the synuclein family have high sequence identity, especially in the N-terminal regions. The synuclein gene family came into the spotlight, when one of its members, α-synuclein, was found to be associated with Parkinson's disease and other neurodegenerative disorders, whereas γ-synuclein was linked to several forms of cancer. There are a lot of controversy and exciting debates concerning members of the synuclein family, including their normal functions, toxicity, role in pathology, transmission between cells and intracellular localization. Important findings which remain undisputable for many years are synuclein localization in synapses and their role in the regulation of synaptic vesicle trafficking, whereas their presence and function in mitochondria and nucleus is a debated topic. In this review, we present the data on the localization of synucleins in two intracellular organelles: the nucleus and mitochondria.

Protein aggregation and neurodegeneration in prototypical neurodegenerative diseases: Examples of amyloidopathies, tauopathies and synucleinopathies

Alzheimer's and Parkinson's diseases are the most prevalent neurodegenerative diseases that generate important health-related direct and indirect socio-economic costs. They are characterized by severe neuronal losses in several disease-specific brain regions associated with deposits of aggregated proteins. In Alzheimer's disease, β-amyloid peptide-containing plaques and intraneuronal neurofibrillary tangles composed of hyperphosphorylated microtubule-associated protein tau are the two main neuropathological lesions, while Parkinson's disease is defined by the presence of Lewy Bodies that are intraneuronal proteinaceous cytoplasmic inclusions. α-Synuclein has been identified as a major protein component of Lewy Bodies and heavily implicated in the pathogenesis of Parkinson's disease. In the past few years, evidence has emerged to explain how these aggregate-prone proteins can undergo spontaneous self-aggregation, propagate from cell to cell, and mediate neurotoxicity. Current research now indicates that oligomeric forms are probably the toxic species. This article discusses recent progress in the understanding of the pathogenesis of these diseases, with a focus on the underlying mechanisms of protein aggregation, and emphasizes the pathophysiological molecular mechanisms leading to cellular toxicity. Finally, we present the putative direct link between β-amyloid peptide and tau in causing toxicity in Alzheimer's disease as well as α-synuclein in Parkinson's disease, along with some of the most promising therapeutic strategies currently in development for those incurable neurodegenerative disorders.

Alpha-synuclein amyloid oligomers act as multivalent nanoparticles to cause hemifusion in negatively charged vesicles

Multivalent membrane binding sites on the α-synuclein oligomer result in clustering of vesicles and hemifusion of negatively charged model membranes. These multivalent, biological nanoparticles are reminiscent of inorganic nanoparticles in their interactions with membranes. Alpha-synuclein oligomers induce lipid exchange efficiently, with fewer than 10 oligomers/vesicle required to complete hemifusion. No full fusion or vesicle content mixing is observed.

Mitochondrial dysfunction and mitophagy in Parkinson's: from familial to sporadic disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterised by the preferential loss of dopaminergic neurons in the substantia nigra. Mitochondrial dysfunction is increasingly appreciated as a key determinant of dopaminergic neuronal susceptibility in PD and is a feature of both familial and sporadic disease, as well as in toxin-induced Parkinsonism. Recently, the mechanisms by which PD-associated mitochondrial proteins phosphatase and tensin homolog deleted on chromosome 10 (PTEN)-induced putative kinase 1 (PINK1) and parkin function and induce neurodegeneration have been identified. In addition, increasing evidence implicates other PD-associated proteins such as α-synuclein (α-syn) and leucine-rich repeat kinase 2 (LRRK2) in mitochondrial dysfunction in genetic cases of PD with the potential for a large functional overlap with sporadic disease. This review highlights how recent advances in understanding familial PD-associated proteins have identified novel mechanisms and therapeutic strategies for addressing mitochondrial dysfunction in PD.