N-alpha-acetylation of α-synuclein increases its helical folding propensity, GM1 binding specificity and resistance to aggregation

Reverse engineering Lewy bodies: how far have we come and how far can we go?

Lewy bodies (LBs) are α-synuclein (α-syn)-rich intracellular inclusions that are an important pathological hallmark of Parkinson disease and several other neurodegenerative diseases. Increasing evidence suggests that the aggregation of α-syn has a central role in LB formation and is one of the key processes that drive neurodegeneration and pathology progression in Parkinson disease. However, little is known about the mechanisms underlying the formation of LBs, their biochemical composition and ultrastructural properties, how they evolve and spread with disease progression, and their role in neurodegeneration. In this Review, we discuss current knowledge of α-syn pathology, including the biochemical, structural and morphological features of LBs observed in different brain regions. We also review the most used cellular and animal models of α-syn aggregation and pathology spreading in relation to the extent to which they reproduce key features of authentic LBs. Finally, we provide important insights into molecular and cellular determinants of LB formation and spreading, and highlight the critical need for more detailed and systematic characterization of α-syn pathology, at both the biochemical and structural levels. This would advance our understanding of Parkinson disease and other neurodegenerative diseases and allow the development of more-reliable disease models and novel effective therapeutic strategies.

α-Synuclein and neuronal membranes: Conformational flexibilities in health and disease

Parkinson's disease (PD) is the second most common neurodegenerative disease. Currently, PD has no treatment. The neuronal protein α-synuclein (αS) plays an important role in PD. However, the molecular mechanisms governing its physiological and pathological roles are not fully understood. It is becoming widely acknowledged that the biological roles of αS involve interactions with biological membranes. In these biological processes there is a fine-tuned interplay between lipids affecting the properties of αS and αS affecting lipid metabolism, αS binding to membranes, and membrane damage. In this review, the intricate interactions between αS and membranes will be reviewed and a discussion of the relationship between αS and neuronal membrane structural plasticity in health and disease will be made. It is proposed that in healthy neurons the conformational flexibilities of αS and the neuronal membranes are coupled to assist the physiological roles of αS. However, in circumstances where their conformational flexibilities are decreased or uncoupled, there is a shift toward cell toxicity. Strategies to modulate toxic αS-membrane interactions are potential approaches for the development of new therapies for PD. Future work using specific αS molecular species as well as membranes with specific physicochemical properties should widen our understanding of the intricate biological roles of αS which, in turn, would propel the development of new strategies for the treatment of PD.

Glycosphingolipids

Glycosphingolipids are amphiphilic plasma membrane components formed by a glycan linked to a specific lipid moiety. In this chapter we report on these compounds, on their role played in our cells to maintain the correct cell biology.In detail, we report on their structure, on their metabolic processes, on their interaction with proteins and from this, their property to modulate positively in health and negatively in disease, the cell signaling and cell biology.

Gangliosides in the Brain: Physiology, Pathophysiology and Therapeutic Applications

Gangliosides are glycosphingolipids highly abundant in the nervous system, and carry most of the sialic acid residues in the brain. Gangliosides are enriched in cell membrane microdomains ("lipid rafts") and play important roles in the modulation of membrane proteins and ion channels, in cell signaling and in the communication among cells. The importance of gangliosides in the brain is highlighted by the fact that loss of function mutations in ganglioside biosynthetic enzymes result in severe neurodegenerative disorders, often characterized by very early or childhood onset. In addition, changes in the ganglioside profile (i.e., in the relative abundance of specific gangliosides) were reported in healthy aging and in common neurological conditions, including Huntington's disease (HD), Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), stroke, multiple sclerosis and epilepsy. At least in HD, PD and in some forms of epilepsy, experimental evidence strongly suggests a potential role of gangliosides in disease pathogenesis and potential treatment. In this review, we will summarize ganglioside functions that are crucial to maintain brain health, we will review changes in ganglioside levels that occur in major neurological conditions and we will discuss their contribution to cellular dysfunctions and disease pathogenesis. Finally, we will review evidence of the beneficial roles exerted by gangliosides, GM1 in particular, in disease models and in clinical trials.

Gangliosides and Neuroblastomas

The focus of this review is the ganglio-series of glycosphingolipids found in neuroblastoma (NB) and the myriad of unanswered questions associated with their possible role(s) in this cancer. NB is one of the more common solid malignancies of children. Five-year survival for those diagnosed with low risk NB is 90-95%, while that for children with high-risk NB is around 40-50%. Much of the survival rate reflects age of diagnosis with children under a year having a much better prognosis than those over two. Identification of expression of GD2 on the surface of most NB cells led to studies of the effectiveness and subsequent approval of anti-GD2 antibodies as a treatment modality. Despite much success, a subset of patients, possibly those whose tumors fail to express concentrations of gangliosides such as GD1b and GT1b found in tumors from patients with a good prognosis, have tumors refractory to treatment. These observations support discussion of what is known about control of ganglioside synthesis, and their actual functions in NB, as well as their possible relationship to treatment response.

Genetic Variation in Complex Traits in Transgenic α-Synuclein Strains of Caenorhabditis elegans

Different genetic backgrounds can modify the effect of mutated genes. Human α-synuclein (SNCA) gene encodes α-synuclein, and its oligomeric complexes accumulate with age and mediate the disruption of cellular homeostasis, resulting in the neuronal death that is characteristic of Parkinson’s Disease. Polymorphic variants modulate this complex pathologic mechanism. Previously, we constructed five transgenic introgression lines of a Caenorhabditis elegans model of α-synuclein using genetic backgrounds that are genetically diverse from the canonical wild-type Bristol N2. A gene expression analysis revealed that the α-synuclein transgene differentially affects genome-wide transcription due to background modifiers. To further investigate how complex traits are affected in these transgenic lines, we measured the α-synuclein transgene expression, the overall accumulation of the fusion protein of α-synuclein and yellow fluorescent protein (YFP), the lysosome-related organelles, and the body size. By using quantitative PCR (qPCR), we demonstrated stable and similar expression levels of the α-synuclein transgene in different genetic backgrounds. Strikingly, we observed that the levels of the a-synuclein:YFP fusion protein vary in different genetic backgrounds by using the COPAS™ biosorter. The quantification of the Nile Red staining assay demonstrates that α-synuclein also affects lysosome-related organelles and body size. Our results show that the same α-synuclein introgression in different C. elegans backgrounds can produces differing effects on complex traits due to background modifiers.

Extracellular vesicle-associated lipids in central nervous system disorders

Increasing evidence indicates that lipid metabolism is disturbed in central nervous system (CNS) disorders, such as multiple sclerosis, Alzheimer's, and Parkinson's disease. Extracellular vesicles (EVs), including exosomes and microvesicles, are nanosized particles that play an essential role in intercellular communication and tissue homeostasis by transporting diverse biologically active molecules, including a large variety of lipid species. In the last decade, studies defined that changes in the EV lipidome closely correlate with disease-progression and -remission in CNS disorders. In this review, we summarize and discuss these changes in the EV lipidome and elaborate on the impact of different EV-associated lipids on pathological processes in CNS disorders. We conclude that EV-associated lipids are closely associated with neuroinflammation, CNS repair, and pathological protein aggregation in CNS disorders, and that modulation of the EV lipidome represents a promising therapeutic strategy to halt disease progression in multiple sclerosis, Alzheimer's, and Parkinson's disease. Moreover, we predict that disease-stage specific EV-associated lipid signatures can be invaluable markers for the diagnosis and early detection of CNS disorders in the future.

Lipid rafts and neurodegeneration: structural and functional roles in physiologic aging and neurodegenerative diseases

Lipid rafts are small, dynamic membrane areas characterized by the clustering of selected membrane lipids as the result of the spontaneous separation of glycolipids, sphingolipids, and cholesterol in a liquid-ordered phase. The exact dynamics underlying phase separation of membrane lipids in the complex biological membranes are still not fully understood. Nevertheless, alterations in the membrane lipid composition affect the lateral organization of molecules belonging to lipid rafts. Neural lipid rafts are found in brain cells, including neurons, astrocytes, and microglia, and are characterized by a high enrichment of specific lipids depending on the cell type. These lipid rafts seem to organize and determine the function of multiprotein complexes involved in several aspects of signal transduction, thus regulating the homeostasis of the brain. The progressive decline of brain performance along with physiological aging is at least in part associated with alterations in the composition and structure of neural lipid rafts. In addition, neurodegenerative conditions, such as lysosomal storage disorders, multiple sclerosis, and Parkinson's, Huntington's, and Alzheimer's diseases, are frequently characterized by dysregulated lipid metabolism, which in turn affects the structure of lipid rafts. Several events underlying the pathogenesis of these diseases appear to depend on the altered composition of lipid rafts. Thus, the structure and function of lipid rafts play a central role in the pathogenesis of many common neurodegenerative diseases.jlr;61/5/636/F1F1f1.

Do Post-Translational Modifications Influence Protein Aggregation in Neurodegenerative Diseases: A Systematic Review

The accumulation of abnormal protein aggregates represents a universal hallmark of neurodegenerative diseases (NDDs). Post-translational modifications (PTMs) regulate protein structure and function. Dysregulated PTMs may influence the propensity for protein aggregation in NDD-proteinopathies. To investigate this, we systematically reviewed the literature to evaluate effects of PTMs on aggregation propensity for major proteins linked to the pathogenesis and/or progression of NDDs. A search of PubMed, MEDLINE, EMBASE, and Web of Science Core Collection was conducted to retrieve studies that investigated an association between PTMs and protein aggregation in seven NDDs: Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), amyotrophic lateral sclerosis (ALS), spinocerebellar ataxias, transmissible spongiform encephalopathy, and multiple sclerosis. Together, 1222 studies were identified, of which 69 met eligibility criteria. We identified that the following PTMs, in isolation or combination, potentially act as modulators of proteinopathy in NDDs: isoaspartate formation in Aβ, phosphorylation of Aβ or tau in AD; acetylation, 4-hydroxy-2-neonal modification, O-GlcNAcylation or phosphorylation of α-synuclein in PD; acetylation or phosphorylation of TAR DNA-binding protein-43 in ALS, and SUMOylation of superoxide dismutase-1 in ALS; and phosphorylation of huntingtin in HD. The potential pharmacological manipulation of these aggregation-modulating PTMs represents an as-yet untapped source of therapy to treat NDDs.

A multi-faceted genotoxic network of alpha-synuclein in the nucleus and mitochondria of dopaminergic neurons in Parkinson's disease: Emerging concepts and challenges

α-Synuclein is a hallmark amyloidogenic protein component of the Lewy bodies (LBs) present in dopaminergic neurons affected by Parkinson's disease (PD). Despite an enormous increase in emerging knowledge, the mechanism(s) of α-synuclein neurobiology and crosstalk among pathological events that are critical for PD progression remains enigmatic, creating a roadblock for effective intervention strategies. One confounding question is about the potential link between α-synuclein toxicity and genome instability in PD. We previously reported that pro-oxidant metal ions, together with reactive oxygen species (ROS), act as a "double whammy" in dopaminergic neurons by not only inducing genome damage but also inhibiting their repair. Our recent studies identified a direct role for chromatin-bound, oxidized α-synuclein in the induction of DNA strand breaks, which raised the question of a paradoxical role for α-synuclein's DNA binding in neuroprotection versus neurotoxicity. Furthermore, recent advances in our understanding of α-synuclein mediated mitochondrial dysfunction warrants revisiting the topics of α-synuclein pathophysiology in order to devise and assess the efficacy of α-synuclein-targeted interventions. In this review article, we discuss the multi-faceted neurotoxic role of α-synuclein in the nucleus and mitochondria with a particular emphasis on the role of α-synuclein in DNA damage/repair defects. We utilized a protein-DNA binding simulation to identify potential residues in α-synuclein that could mediate its binding to DNA and may be critical for its genotoxic functions. These emerging insights and paradigms may guide new drug targets and therapeutic modalities.

GM1 Ganglioside Is A Key Factor in Maintaining the Mammalian Neuronal Functions Avoiding Neurodegeneration

Many species of ganglioside GM1, differing for the sialic acid and ceramide content, have been characterized and their physico-chemical properties have been studied in detail since 1963. Scientists were immediately attracted to the GM1 molecule and have carried on an ever-increasing number of studies to understand its binding properties and its neurotrophic and neuroprotective role. GM1 displays a well balanced amphiphilic behavior that allows to establish strong both hydrophobic and hydrophilic interactions. The peculiar structure of GM1 reduces the fluidity of the plasma membrane which implies a retention and enrichment of the ganglioside in specific membrane domains called lipid rafts. The dynamism of the GM1 oligosaccharide head allows it to assume different conformations and, in this way, to interact through hydrogen or ionic bonds with a wide range of membrane receptors as well as with extracellular ligands. After more than 60 years of studies, it is a milestone that GM1 is one of the main actors in determining the neuronal functions that allows humans to have an intellectual life. The progressive reduction of its biosynthesis along the lifespan is being considered as one of the causes underlying neuronal loss in aged people and severe neuronal decline in neurodegenerative diseases. In this review, we report on the main knowledge on ganglioside GM1, with an emphasis on the recent discoveries about its bioactive component.

N-terminal acetylation mutants affect alpha-synuclein stability, protein levels and neuronal toxicity

Alpha-synuclein (aSyn) protein levels are sufficient to drive Parkinson's disease (PD) and other synucleinopathies. Despite the biomedical/therapeutic potential of aSyn protein regulation, little is known about mechanisms that limit/control aSyn levels. Here, we investigate the role of a post-translational modification, N-terminal acetylation, in aSyn neurotoxicity. N-terminal acetylation occurs in all aSyn molecules and has been proposed to determine its lipid binding and aggregation capacities; however, its effect in aSyn stability/neurotoxicity has not been evaluated. We generated N-terminal mutants that alter or block physiological aSyn N-terminal acetylation in wild-type or pathological mutant E46K aSyn versions and confirmed N-terminal acetylation status by mass spectrometry. By optical pulse-labeling in living primary neurons we documented a reduced half-life and accumulation of aSyn N-terminal mutants. To analyze the effect of N-terminal acetylation mutants in neuronal toxicity we took advantage of a neuronal model where aSyn toxicity was scored by longitudinal survival analysis. Salient features of aSyn neurotoxicity were previously investigated with this approach. aSyn-dependent neuronal death was recapitulated either by higher aSyn protein levels in the case of WT aSyn, or by the combined effect of protein levels and enhanced neurotoxicity conveyed by the E46K mutation. aSyn N-terminal mutations decreased E46K aSyn-dependent neuronal death both by reducing protein levels and, importantly, by reducing the intrinsic E46K aSyn toxicity, being the D2P mutant the least toxic. Together, our results illustrate that the N-terminus determines, most likely through its acetylation, aSyn protein levels and toxicity, identifying this modification as a potential therapeutic target.

Acetylation of Aβ40 Alters Aggregation in the Presence and Absence of Lipid Membranes

A hallmark of Alzheimer's disease (AD) is the formation of senile plaques comprised of the β-amyloid (Aβ) peptide. Aβ fibrillization is a complex nucleation-dependent process involving a variety of metastable intermediate aggregates and features the formation of inter- and intramolecular salt bridges involving lysine residues, K16 and K28. Cationic lysine residues also mediate protein-lipid interactions via association with anionic lipid headgroups. As several toxic mechanisms attributed to Aβ involve membrane interactions, the impact of acetylation on Aβ₄₀ aggregation in the presence and absence of membranes was determined. Using chemical acetylation, varying mixtures of acetylated and nonacetylated Aβ₄₀ were produced. With increasing acetylation, fibril and oligomer formation decreased, eventually completely arresting fibrillization. In the presence of total brain lipid extract (TBLE) vesicles, acetylation reduced the interaction of Aβ₄₀ with membranes; however, fibrils still formed at near complete levels of acetylation. Additionally, the combination of TBLE and acetylated Aβ promoted annular aggregates. Finally, toxicity associated with Aβ₄₀ was reduced with increasing acetylation in a cell culture assay. These results suggest that in the absence of membranes that the cationic character of lysine plays a major role in fibril formation. However, acetylation promotes unique aggregation pathways in the presence of lipid membranes.

Emerging Role of Sirtuin 2 in Parkinson's Disease

Parkinson’s disease (PD), the main risk factor of which is age, is one of the most common neurodegenerative diseases, thus presenting a substantial burden on the health of affected individuals as well as an economic burden. Sirtuin 2 (SIRT2), a subtype in the family of sirtuins, belongs to class III histone deacetylases (HDACs). It is known that SIRT2 levels increase with aging, and a growing body of evidence has been accumulating, showing that the activity of SIRT2 mediates various processes involved in PD pathogenesis, including aggregation of α-synuclein (α-syn), microtubule function, oxidative stress, inflammation, and autophagy. There have been conflicting reports about the role of SIRT2 in PD, in that some studies indicate its potential to induce the death of dopaminergic (DA) neurons, and that inhibition of SIRT2 may, therefore, have protective effects in PD. Other studies suggest a protective role of SIRT2 in the context of neuronal damage. As current treatments for PD are directed at alleviating symptoms and are very limited, a comprehensive understanding of the enzymology of SIRT2 in PD may be essential for developing novel therapeutic agents for the treatment of this disease. This review article will provide an update on our knowledge of the structure, distribution, and biological characteristics of SIRT2, and highlight its role in the pathogenesis of PD.

In-Cell NMR of Intrinsically Disordered Proteins in Mammalian Cells

In-cell NMR enables structural insights at atomic resolution of proteins in their natural environment. To date, very few methods have been developed to study proteins by in-cell NMR in mammalian systems. Here we describe a detailed protocol to conduct in-cell NMR on the intrinsically disordered protein of alpha-Synuclein (αSyn) in mammalian cells. This chapter includes a simplified expression and purification protocol of recombinant αSyn and its delivery into mammalian cells. The chapter also describes how to assess the cell leakage that might occur to the cells, the setup of the instrument, and how to perform basic analyses with the obtained NMR data.

Parkinson's disease recovery by GM1 oligosaccharide treatment in the B4galnt1+/- mouse model

Given the recent in vitro discovery that the free soluble oligosaccharide of GM1 is the bioactive portion of GM1 for neurotrophic functions, we investigated its therapeutic potential in the B4galnt1^+/− mice, a model of sporadic Parkinson’s disease. We found that the GM1 oligosaccharide, systemically administered, reaches the brain and completely rescues the physical symptoms, reduces the abnormal nigral α-synuclein content, restores nigral tyrosine hydroxylase expression and striatal neurotransmitter levels, overlapping the wild-type condition. Thus, this study supports the idea that the Parkinson’s phenotype expressed by the B4galnt1^+/− mice is due to a reduced level of neuronal ganglioside content and lack of interactions between the oligosaccharide portion of GM1 with specific membrane proteins. It also points to the therapeutic potential of the GM1 oligosaccharide for treatment of sporadic Parkinson’s disease.

Mechanisms of alpha-synuclein toxicity: An update and outlook

Alpha-synuclein (aSyn) was identified as the main component of inclusions that define synucleinopathies more than 20 years ago. Since then, aSyn has been extensively studied in an attempt to unravel its roles in both physiology and pathology. Today, studying the mechanisms of aSyn toxicity remains in the limelight, leading to the identification of novel pathways involved in pathogenesis. In this chapter, we address the molecular mechanisms involved in synucleinopathies, from aSyn misfolding and aggregation to the various cellular effects and pathologies associated. In particular, we review our current understanding of the mechanisms involved in the spreading of aSyn between different cells, from the periphery to the brain, and back. Finally, we also review recent studies on the contribution of inflammation and the gut microbiota to pathology in synucleinopathies. Despite significant advances in our understanding of the molecular mechanisms involved, we still lack an integrated understanding of the pathways leading to neurodegeneration in PD and other synucleinopathies, compromising our ability to develop novel therapeutic strategies.

Membrane interactions of intrinsically disordered proteins: The example of alpha-synuclein

Peripheral membrane proteins associate reversibly with biological membranes that, compared to protein binding partners, are structurally labile and devoid of specific binding pockets. Membranes in different subcellular compartments vary primarily in their chemical composition and physical properties, and recognition of these features is therefore critical for allowing such proteins to engage their proper membrane targets. Intrinsically disordered proteins (IDPs) are well-suited to accomplish this task using highly specific and low- to moderate-affinity interactions governed by recognition principles that are both similar to and different from those that mediate the membrane interactions of rigid proteins. IDPs have also evolved multiple mechanisms to regulate membrane (and other) interactions and achieve their impressive functional diversity. Moreover, IDP-membrane interactions may have a kinetic advantage in fast processes requiring rapid control of such interactions, such as synaptic transmission or signaling. Herein we review the biophysics, regulation and functional implications of IDP-membrane interactions and include a brief overview of some of the methods that can be used to study such interactions. At each step, we use the example of alpha-synuclein, a protein involved in the pathogenesis of Parkinson's disease and one of the best characterized membrane-binding IDP, to illustrate some of the principles discussed.

N-Terminal Acetylation Affects α-Synuclein Fibril Polymorphism

Parkinson's disease etiology involves amyloid formation by α-synuclein (αSyn). In vivo, αSyn is constitutively acetylated at the α-amino N-terminus. Here, we find N-terminally acetylated αSyn (Ac-αSyn) aggregates more slowly than non-acetylated αSyn (NH₃-αSyn) with significantly reduced sensitivity to thioflavin T (ThT). Fibril differences were characterized by transmission electron microscopy, circular dichroism spectroscopy, and limited proteolysis. Interestingly, the low-ThT Ac-αSyn fibrils seed both acetylated and non-acetylated αSyn and faithfully propagate the low-ThT character through several generations, indicating a stable fibril polymorph. In contrast, the high-ThT NH₃-αSyn seeds lose fidelity over subsequent generations. Despite it being outside of the amyloid core, the chemical nature of the N-terminus modulates αSyn aggregation and fibril polymorphism.

Effects of alpha-synuclein post-translational modifications on metal binding

Parkinson's disease is the second most common neurodegenerative disorder worldwide. Neurodegeneration in this pathology is characterized by the loss of dopaminergic neurons in the substantia nigra, coupled with cytoplasmic inclusions known as Lewy bodies containing α-synuclein. The brain is an organ that concentrates metal ions, and there is emerging evidence that a break-down in metal homeostasis may be a critical factor in a variety of neurodegenerative diseases. α-synuclein has emerged as an important metal-binding protein in the brain, whereas these interactions play an important role in its aggregation and might represent a link between protein aggregation, oxidative damage, and neuronal cell loss. Additionally, α-synuclein undergoes several post-translational modifications that regulate its structure and physiological function, and may be linked to the aggregation and/or oligomer formation. This review is focused on the interaction of this protein with physiologically relevant metal ions, highlighting the cases where metal-AS interactions profile as key modulators for its structural, aggregation, and membrane-binding properties. The impact of α-synuclein phosphorylation and N-terminal acetylation in the metal-binding properties of the protein are also discussed, underscoring a potential interplay between PTMs and metal ion binding in regulating α-synuclein physiological functions and its role in pathology. This article is part of the Special Issue "Synuclein".

Optically sensing phospholipid induced coil-helix transitions in the phosphoinositide-binding motif of gelsolin

We present a systematic experimental and computational study of phospholipid induced peptide coil-helix transitions which are relevant in the context of proteins mediating cytoskeletal rearrangement via membrane binding. We developed a sensitive Förster resonance energy transfer (FRET) based assay to address whether coil-helix transitions in phospholipid binding motifs of actin-binding proteins can be induced by physiologically-relevant concentrations (1-20 μM) of phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) phospholipids. Based on inter-residue distance constraints obtained from Molecular Dynamics (MD) simulations of a 20 residue peptide (Gel 150-169) from the actin-severing protein gelsolin, we synthetized and labeled the peptide with a tryptophan donor and IAEDANS acceptor pair. Upon addition of PI(4,5)P2 micelles and mixed vesicles containing PI(4,5)P2 and phosphatidylcholine to the peptide, we observed a decrease in the tryptophan emission intensity with increasing concentrations of PI(4,5)P2. The IAEDANS emission spectra showed a more complex profile exhibiting a blue shift of the emission peak and non-monotonic changes in the intensity profile with increasing concentrations of PI(4,5)P2. We showed that the IAEDANS acceptor emission response is a result of both intrinsic polarity sensitivity of the acceptor in the vicinity of the membrane surface and fluorescence energy transfer from the donor. Importantly, the fluorescence lifetime of the donor (tryptophan) showed a monotonous decrease with increasing mol% of PI(4,5)P2 from 1.13 ± 0.10 ns in the absence of phospholipids to 0.25 ± 0.03 ns in the presence of 100% PI(4,5)P2 micelles. We also showed a concomitant increase in FRET efficiency with increasing PI(4,5)P2 levels indicating a PI(4,5)P2 concentration dependent coil-helix transition. Our studies demonstrate that membrane PI(4,5)P2 concentrations as low as 2.5-5 μM can trigger helix-coil conformational changes in gelsolin relevant for triggering regulatory processes in the cell.

GM1 Ganglioside Modifies α-Synuclein Toxicity and is Neuroprotective in a Rat α-Synuclein Model of Parkinson's Disease

While GM1 may interact with α-synuclein in vitro to inhibit aggregation, the ability of GM1 to protect against α-synuclein toxicity in vivo has not been investigated. We used targeted adeno-associated viral vector (AAV) overexpression of human mutant α-synuclein (A53T) in the rat substantia nigra (SN) to produce degeneration of SN dopamine neurons, loss of striatal dopamine levels, and behavioral impairment. Some animals received daily GM1 ganglioside administration for 6 weeks, beginning 24 hours after AAV-A53T administration or delayed start GM1 administration for 5 weeks beginning 3 weeks after AAV-A53T administration. Both types of GM1 administration protected against loss of SN dopamine neurons and striatal dopamine levels, reduced α-synuclein aggregation, and delayed start administration of GM1 reversed early appearing behavioral deficits. These results extend prior positive results in MPTP models, are consistent with the results of a small clinical study of GM1 in PD patients that showed slowing of symptom progression with chronic use, and argue for the continued refinement and development of GM1 as a potential disease modifying therapy for PD.

A novel NAA10 p.(R83H) variant with impaired acetyltransferase activity identified in two boys with ID and microcephaly

Background

N-terminal acetylation is a common protein modification in human cells and is catalysed by N-terminal acetyltransferases (NATs), mostly cotranslationally. The NAA10-NAA15 (NatA) protein complex is the major NAT, responsible for acetylating ~ 40% of human proteins. Recently, NAA10 germline variants were found in patients with the X-linked lethal Ogden syndrome, and in other familial or de novo cases with variable degrees of developmental delay, intellectual disability (ID) and cardiac anomalies.

Methods

Here we report a novel NAA10 (NM_003491.3) c.248G > A, p.(R83H) missense variant in NAA10 which was detected by whole exome sequencing in two unrelated boys with intellectual disability, developmental delay, ADHD like behaviour, very limited speech and cardiac abnormalities. We employ in vitro acetylation assays to functionally test the impact of this variant on NAA10 enzyme activity.

Results

Functional characterization of NAA10-R83H by in vitro acetylation assays revealed a reduced enzymatic activity of monomeric NAA10-R83H. This variant is modelled to have an altered charge density in the acetyl-coenzyme A (Ac-CoA) binding region of NAA10.

Conclusions

We show that NAA10-R83H has a reduced monomeric catalytic activity, likely due to impaired enzyme-Ac-CoA binding. Our data support a model where reduced NAA10 and/or NatA activity cause the phenotypes observed in the two patients.

Electronic supplementary material

The online version of this article (10.1186/s12881-019-0803-1) contains supplementary material, which is available to authorized users.

The role of lipids in α-synuclein misfolding and neurotoxicity

The misfolding and aggregation of α-synuclein (αsyn) in the central nervous system is associated with a group of neurodegenerative disorders referred to as the synucleinopathies. In addition to being a pathological hallmark of disease, it is now well-established that upon misfolding, αsyn acquires pathogenic properties, such as neurotoxicity, that can contribute to disease development. The mechanisms that produce αsyn misfolding and the molecular events underlying the neuronal damage caused by these misfolded species are not well-defined. A consistent observation that may be relevant to αsyn's pathogenicity is its ability to associate with lipids. This appears important not only to how αsyn aggregates, but also to the mechanism by which the misfolded protein causes intracellular damage. This review discusses the current literature reporting a role of lipids in αsyn misfolding and neurotoxicity in various synucleinopathy disorders and provides an overview of current methods to assess protein misfolding and pathogenicity both in vitro and in vivo.

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

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

Interplay between α-synuclein amyloid formation and membrane structure

Amyloid formation is a pathological hallmark of many neurodegenerative diseases, including Alzheimer's, Parkinson's, and Huntington's. While it is unknown how these disorders are initiated, in vitro and cellular experiments confirm the importance of membranes. Ubiquitous in vivo, membranes induce conformational changes in amyloidogenic proteins and in some cases, facilitate aggregation. Reciprocally, perturbations in the bilayer structure can be induced by amyloid formation. Here, we review studies in the last 10 years describing α-synuclein (α-syn) and its interactions with membranes, detailing the roles of anionic and zwitterionic lipids in aggregation, and their contribution to Parkinson's disease. We summarize the impact of α-syn - comparing monomeric, oligomeric, and fibrillar forms - on membrane structure, and the effect of membrane remodeling on amyloid formation. Finally, perspective on future studies investigating the interplay between α-syn aggregation and membranes is discussed. This article is part of a Special Issue entitled: Amyloids.

Conformational ensemble of native α-synuclein in solution as determined by short-distance crosslinking constraint-guided discrete molecular dynamics simulations

Combining structural proteomics experimental data with computational methods is a powerful tool for protein structure prediction. Here, we apply a recently-developed approach for de novo protein structure determination based on the incorporation of short-distance crosslinking data as constraints in discrete molecular dynamics simulations (CL-DMD) for the determination of conformational ensemble of the intrinsically disordered protein α-synuclein in the solution. The predicted structures were in agreement with hydrogen-deuterium exchange, circular dichroism, surface modification, and long-distance crosslinking data. We found that α-synuclein is present in solution as an ensemble of rather compact globular conformations with distinct topology and inter-residue contacts, which is well-represented by movements of the large loops and formation of few transient secondary structure elements. Non-amyloid component and C-terminal regions were consistently found to contain β-structure elements and hairpins.

As the population ages, neurodegenerative diseases such as Parkinson’s disease will become an increasing problem in many countries. Aggregation of the protein α-synuclein is the primary cause of Parkinson’s disease, but there is still a dearth of structural information pertaining to the native, non-aggregating form of this protein. A better understanding the structural state of the native protein may prove useful for the design of new therapeutics to combat this disease. In order to obtain more structural information on this protein, we have recently modelled the native α-synuclein protein. These models were generated using a novel approach which combines protein crosslinking and discrete molecular dynamics simulations. We have found that the α-synuclein protein can adopt several shapes, all with a similar topology, resembling a three fingered closed claw. A region of the protein important for aggregation was found to be protected from the surrounding biological environment in these conformations, and the stabilization of these structures may be a fruitful avenue for future drug research into mitigating the cause and effect of Parkinson’s disease.

C-Terminal CuII Coordination to α-Synuclein Enhances Aggregation

The structurally dynamic amyloidogenic protein α-synuclein (αS) is universally recognized as a key player in Parkinson's disease (PD). Copper, which acts as a neuronal signaling agent, is also an effector of αS structure, aggregation, and localization in vivo. In humans, αS is known to carry an acetyl group on the starting methionine residue, capping the N-terminal free amine which was a known high-affinity Cu^II binding site. We now report the first detailed characterization data using electron paramagnetic resonance (EPR) spectroscopy to describe the Cu^II coordination modes of N-terminally acetylated αS (^NAcαS). Through use of EPR hyperfine structure analyses and the Peisach-Blumberg correlation, an N3O1 binding mode was established that involves the single histidine residue at position 50 and a lower population of a second Cu^II-binding mode that may involve a C-terminal contribution. We additionally generated an N-terminally acetylated disease-relevant variant, ^NAcH50Q, that promotes a shift in the Cu^II binding site to the C-terminus of the protein. Moreover, fibrillar ^NAcH50Q-Cu^II exhibits enhanced parallel β-sheet character and increased hydrophobic surface area compared to ^NAcαS-Cu^II and to both protein variants that lack a coordinated cupric ion. The results presented herein demonstrate the differential impact of distinct Cu^II binding sites within ^NAcαS, revealing that C-terminal Cu^II binding exacerbates the structural consequences of the H50Q missense mutation. Likewise, the global structural modifications that result from N-terminal capping augment the properties of Cu^II coordination. Hence, consideration of the effect of Cu^II on ^NAcαS and ^NAcH50Q misfolding may shed light on the extrinsic or environmental factors that influence PD pathology.

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

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

The Role of Lipids in Parkinson's Disease

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

Circular Dichroism and Isothermal Titration Calorimetry to Study the Interaction of α-Synuclein with Membranes

α-Synuclein's physiology and pathology have been linked by numerous reports to its ability to bind and remodel membranes, especially at synaptic terminals. It is therefore critical for researchers investigating the determinants of these interactions to rely on methods capable of providing an accurate and complete physicochemical snapshot of the binding events. Circular dichroism (CD) and isothermal titration calorimetry (ITC) are established techniques for the study of binding equilibria in biological systems and, especially when used in combination, allow a thorough characterization of the protein-lipid interplay.Here we provide general guidelines and describe some common pitfalls of these experiments. This protocol describes the preparation of small unilamellar vesicles (SUVs), mimicking the curved bilayers α-synuclein normally interacts with, the CD-monitored titration of α-synuclein with SUVs, the ITC (lipid-into-protein) experiment, and the subsequent data analysis using an n independent binding site model.

Could α-Synuclein Modulation of Insulin and Dopamine Identify a Novel Link Between Parkinson's Disease and Diabetes as Well as Potential Therapies?

Characterizing the normal function(s) of the protein α-Synuclein (aSyn) has the potential to illuminate links between Parkinson’s disease (PD) and diabetes and also point the way toward new therapies for these disorders. Here we provide a perspective for consideration based on our discovery that aSyn normally acts to inhibit insulin secretion from pancreatic β-cells by interacting with the Kir6.2 subunit of the ATP-sensitive potassium channel (K-ATP). It is also known that K-ATP channels act to inhibit brain dopamine secretion, and we have also shown that aSyn is a normal inhibitor of dopamine synthesis. The finding, that aSyn modulates Kir6.2 and other proteins involved in dopamine and insulin secretion, suggests that aSyn interacting proteins may be negatively impacted when aSyn aggregates inside cells, whether in brain or pancreas. Furthermore, identifying therapies for PD that can counteract dysfunction found in diabetes, would be highly beneficial. One such compound may be the multiple sclerosis drug, FTY720, which like aSyn can stimulate the activity of the catalytic subunit of protein phosphatase 2A (PP2Ac) as well as insulin secretion. In aging aSyn transgenic mice given long term oral FTY720, the mice had reduced aSyn pathology and increased levels of the protective molecule, brain derived neurotrophic factor (BDNF) (Vidal-Martinez et al., 2016). In collaboration with medicinal chemists, we made two non-immunosuppressive FTY720s that also enhance PP2Ac activity, and BDNF expression (Vargas-Medrano et al., 2014; Enoru et al., 2016; Segura-Ulate et al., 2017a). FTY720 and our novel FTY720-based-derivatives, may thus have therapeutic potential for both diabetes and PD.

The role of posttranslational modifications of α-synuclein and LRRK2 in Parkinson's disease: Potential contributions of environmental factors

Parkinson's disease (PD) is the second most common neurodegenerative disease after Alzheimer's disease (AD), and the most prevalent movement disorder. PD is characterized by dopaminergic neurodegeneration in the substantia nigra, but its etiology has yet to be established. Among several genetic variants contributing to PD pathogenesis, α-synuclein and leucine-rich repeat kinase (LRRK2) are widely associated with neuropathological phenotypes in familial and sporadic PD. α-Synuclein and LRRK2 found in Lewy bodies, a pathogenic hallmark of PD, are often posttranslationally modified. As posttranslational modifications (PTMs) are key processes in regulating the stability, localization, and function of proteins, PTMs have emerged as important modulators of α-synuclein and LRRK2 pathology. Aberrant PTMs altering phosphorylation, ubiquitination, nitration and truncation of these proteins promote PD pathogenesis, while other PTMs such as sumoylation may be protective. Although the causes of many aberrant PTMs are unknown, environmental risk factors may contribute to their aberrancy. Environmental toxicants such as rotenone and paraquat have been shown to interact with these proteins and promote their abnormal PTMs. Notably, manganese (Mn) exposure leads to a PD-like neurological disorder referred to as manganism-and induces pathogenic PTMs of α-synuclein and LRRK2. In this review, we highlight the role of PTMs of α-synuclein and LRRK2 in PD pathogenesis and discuss the impact of environmental risk factors on their aberrancy.

Native Top-Down Mass Spectrometry and Ion Mobility MS for Characterizing the Cobalt and Manganese Metal Binding of α-Synuclein Protein

Structural characterization of intrinsically disordered proteins (IDPs) has been a major challenge in the field of protein science due to limited capabilities to obtain full-length high-resolution structures. Native ESI-MS with top-down MS was utilized to obtain structural features of protein-ligand binding for the Parkinson's disease-related protein, α-synuclein (αSyn), which is natively unstructured. Binding of heavy metals has been implicated in the accelerated formation of αSyn aggregation. Using high-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry, native top-down MS with various fragmentation methods, including electron capture dissociation (ECD), collisional activated dissociation (CAD), and multistage tandem MS (MS³), deduced the binding sites of cobalt and manganese to the C-terminal region of the protein. Ion mobility MS (IM-MS) revealed a collapse toward compacted states of αSyn upon metal binding. The combination of native top-down MS and IM-MS provides structural information of protein-ligand interactions for intrinsically disordered proteins. Graphical Abstract ᅟ.

N-terminal Acetylation Levels Are Maintained During Acetyl-CoA Deficiency in Saccharomyces cerevisiae

Nt-acetylation is a prevalent protein modification catalyzed by N-terminal acetyltransferases using acetyl-CoA as acetyl donor. Here, we performed a global analysis of Nt-acetylation in yeast following nutrient starvation. Contrary to histone acetylation, which is sensitive to acetyl-CoA levels, we demonstrate that Nt-acetylation remains largely unaffected to changes in cellular metabolism. We did, however, identify two protein groups that were differentially Nt-acetylated, one showing the same sensitivity to acetyl-CoA as histones. We propose that specific, rather than global, Nt-acetylation events are subject to metabolic regulation.

N-terminal acetylation (Nt-acetylation) is a highly abundant protein modification in eukaryotes and impacts a wide range of cellular processes, including protein quality control and stress tolerance. Despite its prevalence, the mechanisms regulating Nt-acetylation are still nebulous. Here, we present the first global study of Nt-acetylation in yeast cells as they progress to stationary phase in response to nutrient starvation. Surprisingly, we found that yeast cells maintain their global Nt-acetylation levels upon nutrient depletion, despite a marked decrease in acetyl-CoA levels. We further observed two distinct sets of protein N termini that display differential and opposing Nt-acetylation behavior upon nutrient starvation, indicating a dynamic process. The first protein cluster was enriched for annotated N termini showing increased Nt-acetylation in stationary phase compared with exponential growth phase. The second protein cluster was conversely enriched for alternative nonannotated N termini (i.e. N termini indicative of shorter N-terminal proteoforms) and, like histones, showed reduced acetylation levels in stationary phase when acetyl-CoA levels were low. Notably, the degree of Nt-acetylation of Pcl8, a negative regulator of glycogen biosynthesis and two components of the pre-ribosome complex (Rsa3 and Rpl7a) increased during starvation. Moreover, the steady-state levels of these proteins were regulated both by starvation and NatA activity. In summary, this study represents the first comprehensive analysis of metabolic regulation of Nt-acetylation and reveals that specific, rather than global, Nt-acetylation events are subject to metabolic regulation.

Control of protein degradation by N-terminal acetylation and the N-end rule pathway

Nα-terminal acetylation (Nt-acetylation) occurs very frequently and is found in most proteins in eukaryotes. Despite the pervasiveness and universality of Nt-acetylation, its general functions in terms of physiological outcomes remain largely elusive. However, several recent studies have revealed that Nt-acetylation has a significant impact on protein stability, activity, folding patterns, cellular localization, etc. In addition, Nt-acetylation marks specific proteins for degradation by a branch of the N-end rule pathway, a subset of the ubiquitin-mediated proteolytic system. The N-end rule associates a protein's in vivo half-life with its N-terminal residue or modifications on its N-terminus. This review provides a current understanding of intracellular proteolysis control by Nt-acetylation and the N-end rule pathway.

Spotlight on protein N-terminal acetylation

N-terminal acetylation (Nt-acetylation) is a widespread protein modification among eukaryotes and prokaryotes alike. By appending an acetyl group to the N-terminal amino group, the charge, hydrophobicity, and size of the N-terminus is altered in an irreversible manner. This alteration has implications for the lifespan, folding characteristics and binding properties of the acetylated protein. The enzymatic machinery responsible for Nt-acetylation has been largely described, but significant knowledge gaps remain. In this review, we provide an overview of eukaryotic N-terminal acetyltransferases (NATs) and the impact of Nt-acetylation. We also discuss other functions of known NATs and outline methods for studying Nt-acetylation.

Minimotifs dysfunction is pervasive in neurodegenerative disorders

Minimotifs are modular contiguous peptide sequences in proteins that are important for posttranslational modifications, binding to other molecules, and trafficking to specific subcellular compartments. Some molecular functions of proteins in cellular pathways can be predicted from minimotif consensus sequences identified through experimentation. While a role for minimotifs in regulating signal transduction and gene regulation during disease pathogenesis (such as infectious diseases and cancer) is established, the therapeutic use of minimotif mimetic drugs is limited. In this review, we discuss a general theme identifying a pervasive role of minimotifs in the pathomechanism of neurodegenerative diseases. Beyond their longstanding history in the genetics of familial neurodegeneration, minimotifs are also major players in neurotoxic protein aggregation, aberrant protein trafficking, and epigenetic regulation. Generalizing the importance of minimotifs in neurodegenerative diseases offers a new perspective for the future study of neurodegenerative mechanisms and the investigation of new therapeutics.

pTSara-NatB, an improved N-terminal acetylation system for recombinant protein expression in E. coli

N-terminal acetylation is one of the most common co- and post-translational modifications of the eukaryotic proteome and regulates numerous aspects of cellular physiology, such as protein folding, localization and turnover. In particular α-synuclein, whose dyshomeostasis has been tied to the pathogenesis of several neurodegenerative disorders, is completely Nα-acetylated in nervous tissue. In this work, building on previous reports, we develop and characterize a bacterial N-terminal acetylation system based on the expression of the yeast N-terminal acetyltransferase B (NatB) complex under the control of the PBAD (L-arabinose-inducible) promoter. We show its functionality and the ability to completely Nα-acetylate our model substrate α-synuclein both upon induction of the construct with L-arabinose and also by only relying on the constitutive expression of the NatB genes.

Investigating the functionality of a ribosome-binding mutant of NAA15 using Saccharomyces cerevisiae

Objective

N-terminal acetylation is a common protein modification that occurs preferentially co-translationally as the substrate N-terminus is emerging from the ribosome. The major N-terminal acetyltransferase complex A (NatA) is estimated to N-terminally acetylate more than 40% of the human proteome. To form a functional NatA complex the catalytic subunit NAA10 must bind the auxiliary subunit NAA15, which properly folds NAA10 for correct substrate acetylation as well as anchors the entire complex to the ribosome. Mutations in these two genes are associated with various neurodevelopmental disorders in humans. The aim of this study was to investigate the in vivo functionality of a Schizosaccharomyces pombe NAA15 mutant that is known to prevent NatA from associating with ribosomes, but retains NatA-specific activity in vitro.

Results

Here, we show that Schizosaccharomyces pombe NatA can functionally replace Saccharomyces cerevisiae NatA. We further demonstrate that the NatA ribosome-binding mutant Naa15 ΔN K6E is unable to rescue the temperature-sensitive growth phenotype of budding yeast lacking NatA. This finding indicates the in vivo importance of the co-translational nature of NatA-mediated N-terminal acetylation.

Altered expression of genes involved in ganglioside biosynthesis in substantia nigra neurons in Parkinson's disease

Reduced expression of GM1 and other major brain gangliosides GD1a, GD1b and GT1b have been reported in Parkinson's disease (PD) brain. Mechanisms underlying these changes are unclear but may be due to a deficit in the ganglioside biosynthetic process. The present study examined the extent to which deficits in gene expression of key biosynthetic enzymes involved in synthesis of GM1 and GD1b (B3galt4) and GD1a and GT1b (St3gal2) exist in neuromelanin-containing neurons in the PD substantia nigra (SN). In situ hybridization histochemistry was used to examine gene expression of B3GALT4 and ST3GAL2 in neuromelanin-containing neurons in the SN in 8 normal controls (61-92 yrs.) and 7 PD subjects (77-95 yrs). There was a significant decrease in both B3GALT4 and ST3GAL2 gene expression in residual neuromelanin-containing cells in the SN of PD patients compared to age-matched neurologically normal controls. These changes appeared to be cell-type specific as abundant B3GALT4 and ST3GAL2 gene expression was observed in non-neuromelanin containing neurons located outside of the SN in the PD brain. These data show that residual neuromelanin-containing neurons in the PD SN have decreased expression of the ganglioside biosynthetic genes B3GALT4 and ST3GAL2, consistent with previous reports of decreased levels of gangliosides GM1, GD1a, GD1b and GT1b in the PD SN. These changes may increase the vulnerability of these neurons to degeneration in response to a variety of potential stressors.

The Role of Lipids Interacting with α-Synuclein in the Pathogenesis of Parkinson's Disease

α-synuclein is a small protein abundantly expressed in the brain and mainly located in synaptic terminals. The conversion of α-synuclein into oligomers and fibrils is the hallmark of a range of neurodegenerative disorders including Parkinson's disease and dementia with Lewy bodies. α-synuclein is disordered in solution but can adopt an α-helical conformation upon binding to lipid membranes. This lipid-protein interaction plays an important role in its proposed biological function, i.e., synaptic plasticity, but can also entail the aggregation of the protein. Both the chemical properties of the lipids and the lipid-to-protein-ratio have been reported to modulate the aggregation propensity of α-synuclein. In this review, the influence of changes in the nature and levels of lipids on the aggregation propensity of α-synuclein in vivo and in vitro will be discussed within a common general framework. In particular, while biophysical measurements and kinetic analyses of the time courses of α-synuclein aggregation in the presence of different types of lipid vesicles allow a mechanistic dissection of the influence of the lipids on α-synuclein aggregation, biological studies of cellular and animal models of Parkinson's disease allow the determination of changes in lipid levels and properties associated with the disease.

Acetylome in Human Fibroblasts From Parkinson's Disease Patients

Parkinson's disease (PD) is a multifactorial neurodegenerative disorder. The pathogenesis of this disease is associated with gene and environmental factors. Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most frequent genetic cause of familial and sporadic PD. Moreover, posttranslational modifications, including protein acetylation, are involved in the molecular mechanism of PD. Acetylation of lysine proteins is a dynamic process that is modulated in PD. In this descriptive study, we characterized the acetylated proteins and peptides in primary fibroblasts from idiopathic PD (IPD) and genetic PD harboring G2019S or R1441G LRRK2 mutations. Identified acetylated peptides are modulated between individuals' groups. Although acetylated nuclear proteins are the most represented in cells, they are hypoacetylated in IPD. Results display that the level of hyperacetylated and hypoacetylated peptides are, respectively, enhanced in genetic PD and in IPD cells.

Iron Redox Chemistry Promotes Antiparallel Oligomerization of α-Synuclein

Brain metal dyshomeostasis and altered structural dynamics of the presynaptic protein α-synuclein (αS) are both implicated in the pathology of Parkinson's disease (PD), yet a mechanistic understanding of disease progression in the context of αS structure and metal interactions remains elusive. In this Communication, we detail the influence of iron, a prevalent redox-active brain biometal, on the aggregation propensity and secondary structure of N-terminally acetylated αS (^NAcαS), the physiologically relevant form in humans. We demonstrate that under aerobic conditions, Fe(II) commits ^NAcαS to a PD-relevant oligomeric assembly, verified by the oligomer-selective A11 antibody, that does not have any parallel β-sheet character but contains a substantial right-twisted antiparallel β-sheet component based on CD analyses and descriptive deconvolution of the secondary structure. This ^NAcαS-Fe^II oligomer does not develop into the β-sheet fibrils that have become hallmarks of PD, even after extended incubation, as verified by TEM imaging and the fibril-specific OC antibody. Thioflavin T (ThT), a fluorescent probe for β-sheet fibril formation, also lacks coordination to this antiparallel conformer. We further show that this oligomeric state is not observed when O₂ is excluded, indicating a role for iron(II)-mediated O₂ chemistry in locking this dynamic protein into a conformation that may have physiological or pathological implications.

Cell Biology and Pathophysiology of α-Synuclein

α-Synuclein is an abundant neuronal protein that is highly enriched in presynaptic nerve terminals. Genetics and neuropathology studies link α-synuclein to Parkinson's disease (PD) and other neurodegenerative disorders. Accumulation of misfolded oligomers and larger aggregates of α-synuclein defines multiple neurodegenerative diseases called synucleinopathies, but the mechanisms by which α-synuclein acts in neurodegeneration are unknown. Moreover, the normal cellular function of α-synuclein remains debated. In this perspective, we review the structural characteristics of α-synuclein, its developmental expression pattern, its cellular and subcellular localization, and its function in neurons. We also discuss recent progress on secretion of α-synuclein, which may contribute to its interneuronal spread in a prion-like fashion, and describe the neurotoxic effects of α-synuclein that are thought to be responsible for its role in neurodegeneration.

Gangliosides, α-Synuclein, and Parkinson's Disease

This review addresses the role of α-synuclein (αSyn) in the etiopathology of Parkinson's disease (PD), with emphasis on its interaction with GM1 ganglioside. We begin with a brief review of some of the milestone discoveries that helped to elucidate PD neuropathology, including the fibrous inclusions of Lewy that characterize the degenerating dopaminergic neurons of the substantia nigra and the presence of αSyn as a major constituent of these Lewy bodies and neurites. This enabled Braak et al. to define the progressive nature of PD in developing their staging hypothesis which described the topographically predictable sequence of neuropathological changes giving rise to prodromal nonmotor symptoms that precede the classical motor dysfunctions. We recount recent studies demonstrating strong, specific binding of αSyn to GM1 that serves to inhibit fibril formation and the key role of N-acetylation of αSyn in enhancing GM1 binding and specificity. The consequences of insufficient GM1 are illustrated in a newly presented mouse model of PD based on partial deletion of this ganglioside due to heterologous disruption of B4galnt1 (GM2/GD2 synthase), such mice presenting accurate recapitulation of the PD phenotype. A key feature of these mice was marked elevation of αSyn aggregates which accompanied motor impairment, both aggregates and motor dysfunction being corrected by GM1 replacement therapy. Such therapy was achieved with high dosage of GM1 and more effectively with lower doses of LIGA20, a membrane permeable analog of GM1. The accuracy of this mouse model was emphasized by the finding that various central nervous system and noncentral nervous system tissues from PD patients manifested similar GM1 deficiency as the B4galnt1^+/- mouse. A mechanism is proposed whereby the GM1 deficiency detected in PD patients gives rise to αSyn aggregation and facilitation by the latter in blocking glial cell-derived neurotrophic factor neuroprotection.