Role of post-translational modifications in modulating the structure, function and toxicity of alpha-synuclein: implications for Parkinson's disease pathogenesis and therapies

Discovery, validation and optimization of cerebrospinal fluid biomarkers for use in Parkinson's disease

INTRODUCTION

Parkinson's disease (PD) is a complex and phenotypically heterogeneous neurodegenerative disease, for which the diagnosis is mainly based on clinical parameters (even if neuroimaging plays a role in diagnostic assessment); as a consequence, misdiagnosis is common, especially in early stages. Thus, there is an urgent need of having available biomarkers in order to achieve an early and accurate diagnosis. Since molecular changes in the brain are reliably and timely reflected in cerebrospinal fluid (CSF), CSF represents an ideal source for biomarkers of different pathophysiological processes characterizing the disease since its early phases. Areas covered: The aim of this review is to provide an update on the role, development and validation of most studied CSF biomarkers showing a role in the diagnosis and/or prognosis of PD. Oligomeric alpha-synuclein, DJ-1, lysosomal enzymes (namely, glucocerebrosidase) show consistent evidence as potential diagnostic biomarkers of PD. Neurofilament light chain may also have a significant role in differentiating PD from other parkinsonisms. Amyloid beta peptide 1-42 has consistently shown a prognostic value in terms of development of cognitive impairment and dementia in PD patients. Expert commentary: CSF biomarkers represent a very promising approach to early and differential diagnosis of PD. The biomarkers available so far need preanalytical and analytical validation in order to have these CSF biomarkers ready for clinical use.

O-GlcNAc modification inhibits the calpain-mediated cleavage of α-synuclein

The major protein associated with Parkinson's disease (PD) is α-synuclein, as it can form toxic amyloid-aggregates that are a hallmark of many neurodegenerative diseases. α-Synuclein is a substrate for several different posttranslational modifications (PTMs) that have the potential to affect its biological functions and/or aggregation. However, the biophysical effects of many of these modifications remain to be established. One such modification is the addition of the monosaccharide N-acetyl-glucosamine, O-GlcNAc, which has been found on several α-synuclein serine and threonine residues in vivo. We have previously used synthetic protein chemistry to generate α-synuclein bearing two of these physiologically relevant O-GlcNAcylation events at threonine 72 and serine 87 and demonstrated that both of these modifications inhibit α-synuclein aggregation. Here, we use the same synthetic protein methodology to demonstrate that these same O-GlcNAc modifications also inhibit the cleavage of α-synuclein by the protease calpain. This further supports a role for O-GlcNAcylation in the modulation of α-synuclein biology, as proteolysis has been shown to potentially affect both protein aggregation and degradation.

Looking at the recent advances in understanding α-synuclein and its aggregation through the proteoform prism

Despite attracting the close attention of multiple researchers for the past 25 years, α-synuclein continues to be an enigma, hiding sacred truth related to its structure, function, and dysfunction, concealing mechanisms of its pathological spread within the affected brain during disease progression, and, above all, covering up the molecular mechanisms of its multipathogenicity, i.e. the ability to be associated with the pathogenesis of various diseases. The goal of this article is to present the most recent advances in understanding of this protein and its aggregation and to show that the remarkable structural, functional, and dysfunctional multifaceted nature of α-synuclein can be understood using the proteoform concept.

α-Synuclein binds to cytoplasmic vesicles in U251 glioblastoma cells

α-Synuclein is the major component of Lewy bodies, Lewy neurites, and glial cytoplasmic inclusions. It plays an important role in neurodegenerative diseases such as Parkinson's disease, multiple system atrophy, and other synucleinopathies. However, the pathogenesis and neurodegenerative effects of α-synuclein remain unknown. In this study, we established an α-synuclein and an α-synuclein-EGFP overexpressing U251 cell line. α-Synuclein overexpression increases oxidative stress and alters the cell surface and mitochondrial morphologies. We provide fluorescent-protein tagging, immunofluorescence and ultrastructural evidence showing that α-synuclein accumulations are associated with clusters of cytoplasmic vesicles and the diameter of these vesicles increases by H₂O₂ in a time- and dose-dependent manner.

Implication of Alpha-Synuclein Phosphorylation at S129 in Synucleinopathies: What Have We Learned in the Last Decade?

Abnormal accumulation of proteinaceous intraneuronal inclusions called Lewy bodies (LBs) is the neurpathological hallmark of Parkinson’s disease (PD) and related synucleinopathies. These inclusions are mainly constituted of a presynaptic protein, α-synuclein (α-syn). Over the past decade, growing amounts of studies reported an aberrant accumulation of phosphorylated α-syn at the residue S129 (pS129) in the brain of patients suffering from PD, as well as in transgenic animal models of synucleinopathies. Whereas only a small fraction of α-syn (<4%) is phosphorylated in healthy brains, a dramatic accumulation of pS129 (>90%) has been observed within LBs, suggesting that this post-translational modification may play an important role in the regulation of α-syn aggregation, LBs formation and neuronal degeneration. However, whether phosphorylation at S129 suppresses or enhances α-syn aggregation and toxicity in vivo remains a subject of active debate. The answer to this question has important implications for understanding the role of phosphorylation in the pathogenesis of synucleinopathies and determining if targeting kinases or phosphatases could be a viable therapeutic strategy for the treatment of these devastating neurological disorders. In the present review, we explore recent findings from in vitro, cell-based assays and in vivo studies describing the potential implications of pS129 in the regulation of α-syn physiological functions, as well as its implication in synucleinopathies pathogenesis and diagnosis.

Anti-α-synuclein immunotherapy reduces α-synuclein propagation in the axon and degeneration in a combined viral vector and transgenic model of synucleinopathy

Neurodegenerative disorders such as Parkinson's Disease (PD), PD dementia (PDD) and Dementia with Lewy bodies (DLB) are characterized by progressive accumulation of α-synuclein (α-syn) in neurons. Recent studies have proposed that neuron-to-neuron propagation of α-syn plays a role in the pathogenesis of these disorders. We have previously shown that antibodies against the C-terminus of α-syn reduce the intra-neuronal accumulation of α-syn and related deficits in transgenic models of synucleinopathy, probably by abrogating the axonal transport and accumulation of α-syn in in vivo models. Here, we assessed the effect of passive immunization against α-syn in a new mouse model of axonal transport and accumulation of α-syn. For these purpose, non-transgenic, α-syn knock-out and mThy1-α-syn tg (line 61) mice received unilateral intra-cerebral injections with a lentiviral (LV)-α-syn vector construct followed by systemic administration of the monoclonal antibody 1H7 (recognizes amino acids 91-99) or control IgG for 3 months. Cerebral α-syn accumulation and axonopathy was assessed by immunohistochemistry and effects on behavior were assessed by Morris water maze. Unilateral LV-α-syn injection resulted in axonal propagation of α-syn in the contra-lateral site with subsequent behavioral deficits and axonal degeneration. Passive immunization with 1H7 antibody reduced the axonal accumulation of α-syn in the contra-lateral side and ameliorated the behavioral deficits. Together this study supports the notion that immunotherapy might improve the deficits in models of synucleinopathy by reducing the axonal propagation and accumulation of α-syn. This represents a potential new mode of action through which α-syn immunization might work.

Interaction of LRRK2 and α-Synuclein in Parkinson's Disease

Parkinson's disease (PD) is a progressively debilitating neurodegenerative syndrome. It is best described as a movement disorder characterized by motor dysfunctions, progressive degeneration of dopaminergic neurons of the substantia nigra pars compacta, and abnormal intraneuronal protein aggregates, named Lewy bodies and Lewy neurites. Nevertheless, knowledge of the molecular events leading to this pathophysiology is incomplete. To date, only mutations in the α-synuclein and LRRK2-encoding genes have been associated with typical findings of clinical and pathologic PD. LRRK2 appears to have a central role in the pathogenesis of PD as it is associated with α-synuclein pathology and other proteins implicated in neurodegeneration. Thus, LRRK2 dysfunction may influence the accumulation of α-synuclein and its pathology through diverse pathomechanisms altering cellular functions and signaling pathways, including immune system, autophagy, vesicle trafficking, and retromer complex modulation. Consequently, development of novel LRRK2 inhibitors can be justified to treat the neurodegeneration associated with abnormal α-synuclein accumulation.

Alpha-synuclein at the intracellular and the extracellular side: functional and dysfunctional implications

Alpha-synuclein (α-syn) is an abundant neuronal protein whose physiological function, even if still not completely understood, has been consistently related to synaptic function and vesicle trafficking. A group of disorders known as synucleinopathies, among which Parkinson’s disease (PD), is deeply associated with the misfolding and aggregation of α-syn, which can give rise to proteinaceous inclusion known as Lewy bodies (LB). Proteostasis stress is a relevant aspect in these diseases and, currently, the presence of oligomeric α-syn species rather than insoluble aggregated forms, appeared to be associated with cytotoxicity. Many observations suggest that α-syn is responsible for neurodegeneration by interfering with multiple signaling pathways. α-syn protein can directly form plasma membrane channels or modify with their activity, thus altering membrane permeability to ions, abnormally associate with mitochondria and cause mitochondrial dysfunction (i.e. mitochondrial depolarization, Ca2+ dys-homeostasis, cytochrome c release) and interfere with autophagy regulation. The picture is further complicated by the fact that single point mutations, duplications and triplication in α-syn gene are linked to autosomal dominant forms of PD. In this review we discuss the multi-faced aspect of α-syn biology and address the main hypothesis at the basis of its involvement in neuronal degeneration.

Traumatic Brain Injury Leads to Development of Parkinson's Disease Related Pathology in Mice

Traumatic brain injury (TBI) is a major health and socio-economic problem that affects all societies. This condition results from the application of external physical strength to the brain that leads to transitory or permanent structural and functional impairments. Moreover, TBI is a risk factor for neurodegeneration and can e.g., increase the risk for Parkinson's disease (PD), a late-onset neurodegenerative disorder with loss of dopaminergic neurons in substantia nigra. In this study, we wanted to explore the possible development of PD-related pathology within the context of an experimental model of TBI. Traumatic brain injury was induced in mice by controlled cortical impact. At different time points behavioral tests (open field, elevated plus maze tests, and Barnes maze) were performed: The animals were sacrificed 30 days after the impact and the brains were processed for Western blot and immunohistochemical analyses. Following TBI there was a significant decrease in expression of tyrosine hydroxylase and dopamine transporter in the substantia nigra as well as significant behavioral alterations. In addition, a strong increase in neuroinflammation was evident, as shown by increased levels of cyclooxygenase-2 and inducible nitric oxide synthase as well as IκB-α degradation and nuclear-κB translocation. Moreover, neurotrophic factors such as brain-derived neurotrophic factor, neurotrophin-3, nerve growth factor, and glial cell line-derived neurotrophic factor were decreased 30 days post-TBI. Interestingly, we observed a significant accumulation of α-synuclein in microglia compared to astrocytes. This study suggests that PD-related molecular events can be triggered upon TBI. The biological mechanisms linking brain trauma and neurodegenerative diseases need to be further investigated.

α-synuclein aggregation and its modulation

Parkinson's disease (PD) is a neurological disorder marked by the presence of cytoplasmic inclusions, Lewy bodies (LBs) and Lewy neurites (LNs) as well as the degeneration of dopamine producing neurons in the substantia nigra region of the brain. The LBs and LNs in PD are mainly composed of aggregated form of a presynaptic protein, α-synuclein (α-Syn). However, the mechanisms of α-Syn aggregation and actual aggregated species responsible for the degeneration of dopaminergic neurons have not yet been resolved. Despite the fact that α-Syn aggregation in LBs and LNs is crucial and mutations of α-Syn are associated with early onset PD, it is really a challenging task to establish a correlation between α-Syn aggregation rate and PD pathogenesis. Regardless of strong genetic contribution, PD is mostly sporadic and familial forms of the disease represent only a minor part (<10%) of all cases. The complexity in PD further increases due to the involvement of several cellular factors in the pathogenesis of the disease as well as the environmental factors associated with the risk of developing PD. Therefore, effect of these factors on α-Syn aggregation pathway and how these factors modulate the properties of wild type (WT) as well as mutated α-Syn should be collectively taken into account. The present review specifically provides an overview of recent research on α-Syn aggregation pathways and its modulation by several cellular factors potentially relevant to PD pathogenesis. We also briefly discuss about effect of environmental risk factors on α-Syn aggregation.

Alpha-synuclein activates BV2 microglia dependent on its aggregation state

Synucleinopathies such as Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA) are defined by the presence of intracellular alpha-synuclein aggregates in neurons and/or oligodendrocytes. In addition, post mortem tissue analysis revealed profound changes in microglial morphology, indicating microglial activation and neuroinflammation. Thus, alpha-synuclein may directly activate microglia, leading to increased production of key pro-inflammatory cytokines like tumor necrosis factor-alpha (TNF-α) and interleukin-1beta (IL-1β), which in turn modulates the disease progression. The distinct alpha-synuclein species, which mediates the activation of microglia, is not well defined. We hypothesized that microglial activation depends on a specific aggregation state of alpha-synuclein. Here, we show that primarily human fibrillar alpha-synuclein increased the production and secretion of pro-inflammatory cytokines by microglial BV2 cells compared to monomeric and oligomeric alpha-synuclein. BV2 cells also preferentially phagocytosed fibrillar alpha-synuclein compared to alpha-synuclein monomers and oligomers. Microglial uptake of alpha-synuclein fibrils and the consequent activation were time- and concentration-dependent. Moreover, the degree of fibrillization determined the efficiency of microglial internalization. Taken together, our study highlights the specific crosstalk of distinct alpha-synuclein species with microglial cells.

Two-Dimensional Crowding Uncovers a Hidden Conformation of α-Synuclein

The intrinsically disordered protein (IDP), α-synuclein (αS), is well-known for phospholipid membrane binding-coupled folding into tunable helical conformers. Here, using single-molecule experiments in conjunction with ensemble assays and a theoretical model, we present a unique case demonstrating that the interaction-folding landscape of αS can be tuned by two-dimensional (2D) crowding through simultaneous binding of a second protein on the bilayer surface. Unexpectedly, the experimental data show a clear deviation from a simple competitive inhibition model, but are consistent with a bimodal inhibition mechanism wherein membrane binding of a second protein (a membrane interacting chaperone, Hsp27, in this case) differentially inhibits two distinct modules of αS-membrane interaction. As a consequence, αS molecules are forced to access a hidden conformational state on the phospholipid bilayer in which only the higher-affinity module remains membrane-bound. Our results demonstrate that macromolecular crowding in two dimensions can play a significant role in shaping the conformational landscape of membrane-binding IDPs with multiple binding modes.

Caspase-1 causes truncation and aggregation of the Parkinson's disease-associated protein α-synuclein

The aggregation of α-synuclein (aSyn) leading to the formation of Lewy bodies is the defining pathological hallmark of Parkinson's disease (PD). Rare familial PD-associated mutations in aSyn render it aggregation-prone; however, PD patients carrying wild type (WT) aSyn also have aggregated aSyn in Lewy bodies. The mechanisms by which WT aSyn aggregates are unclear. Here, we report that inflammation can play a role in causing the aggregation of WT aSyn. We show that activation of the inflammasome with known stimuli results in the aggregation of aSyn in a neuronal cell model of PD. The insoluble aggregates are enriched with truncated aSyn as found in Lewy bodies of the PD brain. Inhibition of the inflammasome enzyme caspase-1 by chemical inhibition or genetic knockdown with shRNA abated aSyn truncation. In vitro characterization confirmed that caspase-1 directly cleaves aSyn, generating a highly aggregation-prone species. The truncation-induced aggregation of aSyn is toxic to neuronal culture, and inhibition of caspase-1 by shRNA or a specific chemical inhibitor improved the survival of a neuronal PD cell model. This study provides a molecular link for the role of inflammation in aSyn aggregation, and perhaps in the pathogenesis of sporadic PD as well.

C-Terminal Tyrosine Residue Modifications Modulate the Protective Phosphorylation of Serine 129 of α-Synuclein in a Yeast Model of Parkinson's Disease

Parkinson´s disease (PD) is characterized by the presence of proteinaceous inclusions called Lewy bodies that are mainly composed of α-synuclein (αSyn). Elevated levels of oxidative or nitrative stresses have been implicated in αSyn related toxicity. Phosphorylation of αSyn on serine 129 (S129) modulates autophagic clearance of inclusions and is prominently found in Lewy bodies. The neighboring tyrosine residues Y125, Y133 and Y136 are phosphorylation and nitration sites. Using a yeast model of PD, we found that Y133 is required for protective S129 phosphorylation and for S129-independent proteasome clearance. αSyn can be nitrated and form stable covalent dimers originating from covalent crosslinking of two tyrosine residues. Nitrated tyrosine residues, but not di-tyrosine-crosslinked dimers, contributed to αSyn cytotoxicity and aggregation. Analysis of tyrosine residues involved in nitration and crosslinking revealed that the C-terminus, rather than the N-terminus of αSyn, is modified by nitration and di-tyrosine formation. The nitration level of wild-type αSyn was higher compared to that of A30P mutant that is non-toxic in yeast. A30P formed more dimers than wild-type αSyn, suggesting that dimer formation represents a cellular detoxification pathway in yeast. Deletion of the yeast flavohemoglobin gene YHB1 resulted in an increase of cellular nitrative stress and cytotoxicity leading to enhanced aggregation of A30P αSyn. Yhb1 protected yeast from A30P-induced mitochondrial fragmentation and peroxynitrite-induced nitrative stress. Strikingly, overexpression of neuroglobin, the human homolog of YHB1, protected against αSyn inclusion formation in mammalian cells. In total, our data suggest that C-terminal Y133 plays a major role in αSyn aggregate clearance by supporting the protective S129 phosphorylation for autophagy and by promoting proteasome clearance. C-terminal tyrosine nitration increases pathogenicity and can only be partially detoxified by αSyn di-tyrosine dimers. Our findings uncover a complex interplay between S129 phosphorylation and C-terminal tyrosine modifications of αSyn that likely participates in PD pathology.

Parkinson’s disease is characterized by loss of dopaminergic neurons in midbrain and the presence of αSyn protein inclusions. Human αSyn mimics the disease pathology in yeast resulting in cytotoxicity and aggregate formation. αSyn is abundantly phosphorylated at serine S129 and possesses four tyrosines (Y39, Y125, Y133, and Y136) that can be posttranslationally modified by nitration or phosphorylation. The consequence of each of these possible modifications is still unclear. Nitration as consequence of oxidative stress is a hallmark for neurodegenerative diseases. Here, we addressed the molecular mechanism, how tyrosine posttranslational modifications affect αSyn cytotoxicity. Tyrosine nitration can contribute to αSyn toxicity or can be part of a cellular salvage pathway when di-tyrosine-crosslinked dimers are formed. The Y133 residue, which can be either phosphorylated or nitrated, determines whether S129 is protectively phosphorylated and αSyn inclusions are cleared. This interplay with S129 phosphorylation demonstrates a dual role for C-terminal tyrosine residues. Yeast flavohemoglobin Yhb1 and its human counterpart neuroglobin NGB protect cells against cytotoxicity and aggregate formation. These novel insights into the molecular pathways responsible for αSyn cytotoxicity indicate NGB as a potential target for therapeutic intervention in PD.

The contribution of alpha synuclein to neuronal survival and function - Implications for Parkinson's disease

The aggregation of alpha synuclein (α-syn) is a neuropathological feature that defines a spectrum of disorders collectively termed synucleinopathies, and of these, Parkinson's disease (PD) is arguably the best characterized. Aggregated α-syn is the primary component of Lewy bodies, the defining pathological feature of PD, while mutations or multiplications in the α-syn gene result in familial PD. The high correlation between α-syn burden and PD has led to the hypothesis that α-syn aggregation produces toxicity through a gain-of-function mechanism. However, α-syn has been implicated to function in a diverse range of essential cellular processes such as the regulation of neurotransmission and response to cellular stress. As such, an alternative hypothesis with equal explanatory power is that the aggregation of α-syn results in toxicity because of a toxic loss of necessary α-syn function, following sequestration of functional forms α-syn into insoluble protein aggregates. Within this review, we will provide an overview of the literature linking α-syn to PD and the knowledge gained from current α-syn-based animal models of PD. We will then interpret these data from the viewpoint of the α-syn loss-of-function hypothesis and provide a potential mechanistic model by which loss of α-syn function could result in at least some of the neurodegeneration observed in PD. By providing an alternative perspective on the etiopathogenesis of PD and synucleinopathies, this may reveal alternative avenues of research in order to identify potential novel therapeutic targets for disease modifying strategies. The correlation between α-synuclein burden and Parkinson's disease pathology has led to the hypothesis that α-synuclein aggregation produces toxicity through a gain-of-function mechanism. However, in this review, we discuss data supporting the alternative hypothesis that the aggregation of α-synuclein results in toxicity because of loss of necessary α-synuclein function at the presynaptic terminal, following sequestration of functional forms of α-synuclein into aggregates.

Structural studies on the mechanism of protein aggregation in age related neurodegenerative diseases

The progression of many neurodegenerative diseases is assumed to be caused by misfolding of specific characteristic diseases related proteins, resulting in aggregation and fibril formation of these proteins. Protein misfolding associated age related diseases, although different in disease manifestations, share striking similarities. In all cases, one disease protein aggregates and loses its function or additionally shows a toxic gain of function. However, the clear link between these individual amyloid-like protein aggregates and cellular toxicity is often still uncertain. The similar features of protein misfolding and aggregation in this group of proteins, all involved in age related neurodegenerative diseases, results in high interest in characterization of their structural properties. We review here recent findings on structural properties of some age related disease proteins, in the context of their biological importance in disease.

Induction of de novo α-synuclein fibrillization in a neuronal model for Parkinson's disease

Lewy bodies (LBs) are intraneuronal inclusions consisting primarily of fibrillized human α-synuclein (hα-Syn) protein, which represent the major pathological hallmark of Parkinson's disease (PD). Although doubling hα-Syn expression provokes LB pathology in humans, hα-Syn overexpression does not trigger the formation of fibrillar LB-like inclusions in mice. We hypothesized that interactions between exogenous hα-Syn and endogenous mouse synuclein homologs could be attenuating hα-Syn fibrillization in mice, and therefore, we systematically assessed hα-Syn aggregation propensity in neurons derived from α-Syn-KO, β-Syn-KO, γ-Syn-KO, and triple-KO mice lacking expression of all three synuclein homologs. Herein, we show that hα-Syn forms hyperphosphorylated (at S129) and ubiquitin-positive LB-like inclusions in primary neurons of α-Syn-KO, β-Syn-KO, and triple-KO mice, as well as in transgenic α-Syn-KO mouse brains in vivo. Importantly, correlative light and electron microscopy, immunogold labeling, and thioflavin-S binding established their fibrillar ultrastructure, and fluorescence recovery after photobleaching/photoconversion experiments showed that these inclusions grow in size and incorporate soluble proteins. We further investigated whether the presence of homologous α-Syn species would interfere with the seeding and spreading of α-Syn pathology. Our results are in line with increasing evidence demonstrating that the spreading of α-Syn pathology is most prominent when the injected preformed fibrils and host-expressed α-Syn monomers are from the same species. These findings provide insights that will help advance the development of neuronal and in vivo models for understanding mechanisms underlying hα-Syn intraneuronal fibrillization and its contribution to PD pathogenesis, and for screening pharmacologic and genetic modulators of α-Syn fibrillization in neurons.

Intracellular repair of oxidation-damaged α-synuclein fails to target C-terminal modification sites

Cellular oxidative stress serves as a common denominator in many neurodegenerative disorders, including Parkinson's disease. Here we use in-cell NMR spectroscopy to study the fate of the oxidation-damaged Parkinson's disease protein alpha-synuclein (α-Syn) in non-neuronal and neuronal mammalian cells. Specifically, we deliver methionine-oxidized, isotope-enriched α-Syn into cultured cells and follow intracellular protein repair by endogenous enzymes at atomic resolution. We show that N-terminal α-Syn methionines Met1 and Met5 are processed in a stepwise manner, with Met5 being exclusively repaired before Met1. By contrast, C-terminal methionines Met116 and Met127 remain oxidized and are not targeted by cellular enzymes. In turn, persisting oxidative damage in the C-terminus of α-Syn diminishes phosphorylation of Tyr125 by Fyn kinase, which ablates the necessary priming event for Ser129 modification by CK1. These results establish that oxidative stress can lead to the accumulation of chemically and functionally altered α-Syn in cells.

α-synuclein is a protein linked to the occurrence of Parkinson's disease. Here, the authors use time-resolved in-cell NMR spectroscopy to study the repair of methionine-oxidized α-synuclein by endogenous cellular enzymes.

Semisynthesis and Enzymatic Preparation of Post-translationally Modified α-Synuclein

Posttranslational modifications (PTMs) serve as molecular switches for regulating protein folding, function, and interactome and have been implicated in the misfolding and amyloid formation by several proteins linked to neurodegenerative diseases, including Alzheimer's and Parkinson's disease. Understanding the role of individual PTMs in protein misfolding and aggregation requires the preparation of site-specifically modified proteins, as well as the identification of the enzymes involved in regulating these PTMs. Recently, our group has pioneered the development of enzymatic, synthetic, and semisynthetic strategies that allow site-specific introduction of PTMs at single or multiple sites and generation of modified proteins in milligram quantities. In this chapter, we provide detailed description of enzymatic and semisynthetic strategies for the generation of the phosphorylated α-Synuclein (α-Syn) at S129, (pS129), which has been identified as a pathological hallmark of Parkinson's disease. The semisynthetic method described for generation of α-Syn-pS129 requires expertise with protein chemical ligation, but can be used to incorporate other PTMs (single or multiple) within the α-Syn C-terminus if desired. On the other hand, the in vitro kinase-mediated phosphorylation strategy does not require any special setup and is rather easy to apply, but its application is restricted to the generation of α-Syn_pS129. These methods have the potential to increase the availability of pure and homogenous modified α-Syn reagents, which may be used as standards in numerous applications, including the search for potential biomarkers of synucleinopathies.

Dynamic structural flexibility of α-synuclein

α-Synuclein is a conserved, abundantly expressed protein that is partially localized in pre-synaptic terminals in the central nervous system. The precise biological function(s) and structure of α-synuclein are under investigation. Recently, the native conformation and the presence of naturally occurring multimeric assemblies have come under debate. These are important deliberations because α-synuclein assembles into highly organized amyloid-like fibrils and non-amyloid amorphous aggregates that constitute the neuronal inclusions in Parkinson's disease and related disorders. Therefore understanding the nature of the native and pathological conformations is pivotal from the standpoint of therapeutic interventions that could maintain α-synuclein in its physiological state. In this review, we will discuss the existing evidence that define the physiological states of α-synuclein and highlight how the inherent structural flexibility of this protein may be important in health and disease.

Development of Passive Immunotherapies for Synucleinopathies

Immunotherapy using antibodies targeting alpha-synuclein has proven to be an effective strategy for ameliorating pathological and behavioral deficits induced by excess pathogenic alpha-synuclein in various animal and/or cellular models. However, the process of selecting the anti-alpha-synuclein antibody with the best potential to treat synucleinopathies in humans is not trivial. Critical to this process is a better understanding of the pathological processes involved in the synucleinopathies and how antibodies are able to influence these. We will give an overview of the first proof-of-concept studies in rodent disease models and discuss challenges associated with developing antibodies against alpha-synuclein resulting from the distribution and structural characteristics of the protein. We will also provide a status on the passive immunization approaches targeting alpha-synuclein that have entered, or are expected to enter, clinical evaluation.

α-Synuclein and nonhuman primate models of Parkinson's disease

Accumulation of α-synuclein (α-syn) leading to the formation of insoluble intracellular aggregates named Lewy bodies is proposed to have a significant role in Parkinson's disease (PD) pathology. Nonhuman primate (NHP) models of PD have proven essential for understanding the neurobiological basis of the disease and for the preclinical evaluation of first-in-class and invasive therapies. In addition to neurotoxin, aging and intracerebral gene transfer models, a new generation of models using inoculations of α-syn formulations, as well as transgenic methods is emerging. Understanding of their advantages and limitations will be essential when choosing a platform to evaluate α-syn-related pathology and interpreting the test results of new treatments targeting α-syn aggregation. In this review we aim to provide insight on this issue by critically analyzing the differences in endogenous α-syn, as well as α-syn pathology in PD and PD NHP models.

Alpha-synuclein Toxicity in the Early Secretory Pathway: How It Drives Neurodegeneration in Parkinsons Disease

Alpha-synuclein is a predominant player in the pathogenesis of Parkinson's Disease. However, despite extensive study for two decades, its physiological and pathological mechanisms remain poorly understood. Alpha-synuclein forms a perplexing web of interactions with lipids, trafficking machinery, and other regulatory factors. One emerging consensus is that synaptic vesicles are likely the functional site for alpha-synuclein, where it appears to facilitate vesicle docking and fusion. On the other hand, the dysfunctions of alpha-synuclein are more dispersed and numerous; when mutated or over-expressed, alpha-synuclein affects several membrane trafficking and stress pathways, including exocytosis, ER-to-Golgi transport, ER stress, Golgi homeostasis, endocytosis, autophagy, oxidative stress, and others. Here we examine recent developments in alpha-synuclein's toxicity in the early secretory pathway placed in the context of emerging themes from other affected pathways to help illuminate its underlying pathogenic mechanisms in neurodegeneration.

Bent out of shape: α-Synuclein misfolding and the convergence of pathogenic pathways in Parkinson's disease

Protein inclusions made up primarily of misfolded α-synuclein (α-Syn) are the hallmark of a set of disorders known as synucleinopathies, most notably Parkinson's disease (PD). It is becoming increasingly appreciated that α-Syn misfolding can spread to anatomically connected regions in a prion-like manner. The protein aggregates that ensue are correlated with neurodegeneration in the various yet select neuronal populations that are affected. Recent advances have begun to shed light on the spreading and toxicity mechanisms that may be occurring in PD. Several key emerging themes are arising from this work suggesting that α-Syn mediated neurodegeneration is due to a combination of relative α-Syn expression level, connectivity to affected brain regions, and intrinsic vulnerability to pathology.

Cellular response of human neuroblastoma cells to α-synuclein fibrils, the main constituent of Lewy bodies

BACKGROUND

α-Synuclein (α-Syn) fibrils are the main constituent of Lewy bodies and a neuropathological hallmark of Parkinson's disease (PD). The propagation of α-Syn assemblies from cell to cell suggests that they are involved in PD progression. We previously showed that α-Syn fibrils are toxic because of their ability to bind and permeabilize cell membranes. Here, we document the cellular response in terms of proteome changes of SH-SY5Y cells exposed to exogenous α-Syn fibrils.

METHODS

We compare the proteomes of cells of neuronal origin exposed or not either to oligomeric or fibrillar α-Syn using two dimensional differential in-gel electrophoresis (2D-DIGE) and mass spectrometry.

RESULTS

Only α-Syn fibrils induce significant changes in the proteome of SH-SY5Y cells. In addition to proteins associated to apoptosis and toxicity, or proteins previously linked to neurodegenerative diseases, we report an overexpression of proteins involved in intracellular vesicle trafficking. We also report a remarkable increase in fibrillar α-Syn heterogeneity, mainly due to C-terminal truncations.

CONCLUSIONS

Our results show that cells of neuronal origin adapt their proteome to exogenous α-Syn fibrils and actively modify those assemblies.

GENERAL SIGNIFICANCE

Cells of neuronal origin adapt their proteome to exogenous toxic α-Syn fibrils and actively modify those assemblies. Our results bring insights into the cellular response and clearance events the cells implement to face the propagation of α-Syn assemblies associated to pathology.

O-GlcNAc modification blocks the aggregation and toxicity of the protein α-synuclein associated with Parkinson's disease

Several aggregation-prone proteins associated with neurodegenerative diseases can be modified by O-linked N-acetyl-glucosamine (O-GlcNAc) in vivo. One of these proteins, α-synuclein, is a toxic aggregating protein associated with synucleinopathies, including Parkinson's disease. However, the effect of O-GlcNAcylation on α-synuclein is not clear. Here, we use synthetic protein chemistry to generate both unmodified α-synuclein and α-synuclein bearing a site-specific O-GlcNAc modification at the physiologically relevant threonine residue 72. We show that this single modification has a notable and substoichiometric inhibitory effect on α-synuclein aggregation, while not affecting the membrane binding or bending properties of α-synuclein. O-GlcNAcylation is also shown to affect the phosphorylation of α-synuclein in vitro and block the toxicity of α-synuclein that was exogenously added to cells in culture. These results suggest that increasing O-GlcNAcylation may slow the progression of synucleinopathies and further support a general function for O-GlcNAc in preventing protein aggregation.

Dopaminergic lesioning impairs adult hippocampal neurogenesis by distinct modification of α-synuclein

Nonmotor symptoms of cognitive and affective nature are present in premotor and motor stages of Parkinson's disease (PD). Neurogenesis, the generation of new neurons, persists throughout the mammalian life span in the hippocampal dentate gyrus. Adult hippocampal neurogenesis may be severely affected in the course of PD, accounting for some of the neuropsychiatric symptoms such as depression and cognitive impairment. Two important PD-related pathogenic factors have separately been attributed to contribute to both PD and adult hippocampal neurogenesis: dopamine depletion and accumulation of α-synuclein (α-syn). In the acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model, altered neurogenesis has been linked merely to a reduced dopamine level. Here, we seek to determine whether a distinct endogenous α-syn expression pattern is associated, possibly contributing to the hippocampal neurogenic deficit. We observed a persistent reduction of striatal dopamine and a loss of tyrosine hydroxylase-expressing neurons in the substantia nigra pars compacta in contrast to a complete recovery of tyrosine hydroxylase-immunoreactive dopaminergic fibers within the striatum. However, dopamine levels in the hippocampus were significantly decreased. Survival of newly generated neurons was significantly reduced and paralleled by an accumulation of truncated, membrane-associated, insoluble α-syn within the hippocampus. Specifically, the presence of truncated α-syn species was accompanied by increased activity of calpain-1, a calcium-dependent protease. Our results further substantiate the broad effects of dopamine loss in PD-susceptible brain nuclei, gradually involved in the PD course. Our findings also indicate a detrimental synergistic interplay between dopamine depletion and posttranslational modification of α-syn, contributing to impaired hippocampal plasticity in PD.

A novel link between the conformations, exposure of specific epitopes, and subcellular localization of α-synuclein

BACKGROUND

Genetic studies and the abundance of alpha-synuclein (α-Syn) in presynaptic terminals suggest that α-Syn plays a critical role in maintaining synaptic vesicle pools. However, there are still few experimental tools for elucidating its physiological roles.

METHODS

Unexpectedly, we detected various cellular distribution patterns of endogenous α-Syn by immunofluorescence assays (IFAs). To provide new molecular insights into α-Syn research, we identified associations between epitopes, conformations, and subcellular localization of α-Syn and categorized them.

RESULTS

The α-Syn exposing Y125 was found to coexist with F-actin at the edge of the cells, including the plasma membrane. α-Syn conformations exposing P128 or both F94 and K97 were partly localized to the mitochondria. These results indicate that various conformations of α-Syn are associated with specific subcellular localizations. Intriguingly, we demonstrate for the first time that the phosphorylated α-Syn at Ser129, also known as a Parkinson's disease (PD)-causing form, is targeted to the mitochondria.

CONCLUSIONS

Our study showed that different subcellular distribution patterns of α-Syn reflect the existence of various α-Syn conformations under normal conditions.

GENERAL SIGNIFICANCE

This study provides novel clues for deciphering the physiological function of α-Syn in connection with subcellular localization. Dissecting the specific α-Syn conformations may lead to useful strategies in PD therapy and diagnosis.

Combination therapies: The next logical Step for the treatment of synucleinopathies?

Currently there are no disease-modifying alternatives for the treatment of most neurodegenerative disorders. The available therapies for diseases such as Parkinson's disease (PD), PD dementia (PDD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA), in which the protein alpha-synuclein (α-Syn) accumulates within neurons and glial cells with toxic consequences, are focused on managing the disease symptoms. However, using strategic drug combinations and/or multi-target drugs might increase the treatment efficiency when compared with monotherapies. Synucleinopathies are complex disorders that progress through several stages, and toxic α-Syn aggregates exhibit prion-like behavior spreading from cell to cell. Therefore, it follows that these neurodegenerative disorders might require equally complex therapeutic approaches to obtain significant and long-lasting results. Hypothetically, therapies aimed at reducing α-Syn accumulation and cell-to-cell transfer, such as immunotherapy against α-Syn, could be combined with agents that reduce neuroinflammation with potential synergistic outcomes. Here we review the current evidence supporting this type of approach, suggesting that such rational therapy combinations, together with the use of multi-target drugs, may hold promise as the next logical step for the treatment of synucleinopathies.

The phosphorylation of α-synuclein: development and implication for the mechanism and therapy of the Parkinson's disease

Parkinson's disease (PD) is cited to be the second most common neuronal degenerative disorders; however, the exact mechanism of PD is still unclear. α-synuclein is one of the key proteins in PD pathogenesis as it's the main component of the PD hallmark Lewy bodies (LBs). Nowadays, the study of α-synuclein phosphorylation mechanism related to the PD pathology has become a research hotspot, given that 90% of α-synuclein deposition in LBs is phosphorylated at Ser129, whereas in normal brains, only 4% or less of α-synuclein is phosphorylated at the residue. Here, we review the related study of PD pathological mechanism involving the phosphorylation of α-synuclein mainly at Ser129, Ser87, and Tyr125 residues in recent years, as well as some explorations relating to potential clinical application, in an attempt to describe the development and implication for the mechanism and therapy of PD. Given that some of the studies have yielded paradoxical results, there is need for more comprehensive research in the field. The phosphorylation of α-synuclein might provide a breakthrough for PD mechanism study and even supply a new therapeutic strategy. The milestone study on the phosphorylation of α-synuclein mainly at Ser129, Ser87, and Tyr125 relating to PD in recent years as well as some clinical application exploration are overviewed. The potential pathways of the phosphorylated α-synuclein related to PD are also summarized. The review may supply more ideas and thinking on this issue for the scientists in related research field.

Pathogenic mechanisms of neurodegeneration in Parkinson disease

The last 2 decades represent a period of unparalleled advancement in the understanding of the pathogenesis of Parkinson disease (PD). The discovery of several forms of familial parkinsonism with mendelian inheritance has elucidated insights into the mechanisms underlying the degeneration of dopaminergic neurons of the substantia nigra that histologically characterize PD. α-Synuclein, the principal component of Lewy bodies, remains the presumed pathogen at the heart of the current model; however, concurrently, a diverse range of other mechanisms have been implicated. The creation of a coherent disease model will be crucial to the development of effective disease modifying therapies for sporadic PD.

Role of synucleins in traumatic brain injury — an experimental in vitro and in vivo study in mice

Synucleins are small prone to aggregate proteins associated with several neurodegenerative diseases (NDDs), however their role in traumatic brain injury (TBI) is an emerging area of investigation. Using in vitro scratch injury model and in vivo mouse weight-drop model we have found that the injury causes alterations in the expression and localization of synucleins near the damaged area. Before injury, α-synuclein is diffused in the cytoplasm of neurons and γ-synuclein is both in the cytoplasm and nucleus of oligodendrocytes. After the scratch injury of the mixed neuronal and glial culture, α-synuclein forms punctate structures in the cytoplasm of neurons and γ-synuclein is almost completely localized to the nucleus of the oligodendrocytes. Furthermore, the amount of post-translationally modified Met38-oxidized γ-synuclein is increased 3.8 fold 24 h after the scratch. α- and γ-synuclein containing cells increased in the initially cell free scratch zone up to 24 h after the scratch.Intracellular expression and localization of synucleins are also changed in a mouse model of focal closed head injury, using a standardized weight drop device. γ-Synuclein goes from diffuse to punctate staining in a piriform cortex near the amygdala, which may reflect the first steps in the formation of deposits/inclusions. Surprisingly, oxidized γ-synuclein co-localizes with cofilin-actin rods in the thalamus, which are absent in all other regions of the brain. These structures reach their peak amounts 7 days after injury. The changes in γ-synuclein localization are accompanied by injury-induced alterations in the morphology of both astrocytes and neurons.

Synthetic Proteins and Peptides for the Direct Interrogation of α-Synuclein Posttranslational Modifications

α-Synuclein is the aggregation-prone protein associated with Parkinson’s disease (PD) and related neurodegenerative diseases. Complicating both its biological functions and toxic aggregation are a variety of posttranslational modifications. These modifications have the potential to either positively or negatively affect α-synuclein aggregation, raising the possibility that the enzymes that add or remove these modifications could be therapeutic targets in PD. Synthetic protein chemistry is uniquely positioned to generate site-specifically and homogeneously modified proteins for biochemical study. Here, we review the application of synthetic peptides and proteins towards understanding the effects of α-synuclein posttranslational modifications.

Post-translational modification of α-synuclein in Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease, and the most prevalent degenerative movement disorder. It is estimated that the prevalence of such age-related neurodegenerative diseases will double in the next 25 years. While the etiology of Parkinson's disease is not entirely clear, a common link between both inherited and sporadic forms of disease is the protein α-synuclein. In PD brains, α-synuclein is typically found in large, insoluble protein aggregates referred to as Lewy bodies and Lewy neurites. The exact role of α-synuclein is still unknown, but it has been shown to undergo a variety of post-translational modifications, which impact α-synuclein aggregation and oligomer formation in different ways. This review highlights key post-translational modifications and the impact they have on α-synuclein aggregation and toxicity, elucidating potential mechanisms for PD pathogenesis and targets for future therapeutics. This article is part of a Special Issue entitled SI: Neuroprotection.

Activation of MyD88-dependent TLR1/2 signaling by misfolded α-synuclein, a protein linked to neurodegenerative disorders

Synucleinopathies, such as Parkinson's disease and diffuse Lewy body disease, are progressive neurodegenerative disorders characterized by selective neuronal death, abnormal accumulation of misfolded α-synuclein, and sustained microglial activation. In addition to inducing neuronal toxicity, higher-ordered oligomeric α-synuclein causes proinflammatory responses in the brain parenchyma by triggering microglial activation, which may exacerbate pathogenic processes by establishing a chronic neuroinflammatory milieu. We found that higher-ordered oligomeric α-synuclein induced a proinflammatory microglial phenotype by directly engaging the heterodimer TLR1/2 (Toll-like receptor 1 and 2) at the cell membrane, leading to the nuclear translocation of NF-κB (nuclear factor κB) and the increased production of the proinflammatory cytokines TNF-α (tumor necrosis factor-α) and IL-1β (interleukin-1β) in a MyD88-dependent manner. Blocking signaling through the TLR1/2 heterodimer with the small-molecule inhibitor CU-CPT22 reduced the nuclear translocation of NF-κB and secretion of TNF-α from cultured primary mouse microglia. Candesartan cilexetil, a drug approved for treating hypertension and that inhibits the expression of TLR2, reversed the activated proinflammatory phenotype of primary microglia exposed to oligomeric α-synuclein, supporting the possibility of repurposing this drug for synucleinopathies.

Proteinopathies, a core concept for understanding and ultimately treating degenerative disorders?

The current review covers proteinopathies an umbrella term for neurodegenerative disorders that are characterized by the accumulation of specific proteins within neurons or in the brain parenchyma. Most prevalent examples for typical proteinopathies are Alzheimer's disease and Parkinson's disease. In healthy brain, these proteins are unstructured as a monomer, serving most likely as the physiological form. In a disease condition, the unstructured proteins experience a conformational change leading to small oligomers that eventually will aggregate into higher order structures. Prion disease is an exception within the family of proteinopathies as the aggregated prion protein is highly infectious and can self-aggregate and propagate. Recent reports might implicate a prion-like spread of misfolded proteins in Alzheimer's and Parkinson's disease; however there are evident differences in comparison to prion diseases. As proteinopathies are caused by the aggregation of disease-typical proteins with an ordered structure, active and passive immunization protocols have been used to expose model systems to therapeutic antibodies that bind to the aggregates thereby inhibiting the prolongation into higher ordered fibrils or dissolving the existing fibrillar structure. While most of the immunization treatments have been only carried out in preclinical model systems overexpressing the disease-relevant aggregating protein, other approaches are already in clinical testing. Taking the core concept of proteinopathies with conformationally altered protein aggregates into account, immunization appears to be a very promising therapeutic option for neurodegenerative disorders.

Posttranslational Modifications and Clearing of α-Synuclein Aggregates in Yeast

The budding yeast Saccharomyces cerevisiae represents an established model system to study the molecular mechanisms associated to neurodegenerative disorders. A key-feature of Parkinson’s disease is the formation of Lewy bodies, which are cytoplasmic protein inclusions. Misfolded α-synuclein is one of their main constituents. Expression of α-synuclein protein in yeast leads to protein aggregation and cellular toxicity, which is reminiscent to Lewy body containing human cells. The molecular mechanism involved in clearance of α-synuclein aggregates is a central question for elucidating the α-synuclein-related toxicity. Cellular clearance mechanisms include ubiquitin mediated 26S proteasome function as well as lysosome/vacuole associated degradative pathways as autophagy. Various modifications change α-synuclein posttranslationally and alter its inclusion formation, cytotoxicity and the distribution to different clearance pathways. Several of these modification sites are conserved from yeast to human. In this review, we summarize recent findings on the effect of phosphorylation and sumoylation of α-synuclein to the enhanced channeling to either the autophagy or the proteasome degradation pathway in yeast model of Parkinson’s disease.

The Interplay between Alpha-Synuclein Clearance and Spreading

Parkinson's Disease (PD) is a complex neurodegenerative disorder classically characterized by movement impairment. Pathologically, the most striking features of PD are the loss of dopaminergic neurons and the presence of intraneuronal protein inclusions primarily composed of alpha-synuclein (α-syn) that are known as Lewy bodies and Lewy neurites in surviving neurons. Though the mechanisms underlying the progression of PD pathology are unclear, accumulating evidence suggests a prion-like spreading of α-syn pathology. The intracellular homeostasis of α-syn requires the proper degradation of the protein by three mechanisms: chaperone-mediated autophagy, macroautophagy and ubiquitin-proteasome. Impairment of these pathways might drive the system towards an alternative clearance mechanism that could involve its release from the cell. This increased release to the extracellular space could be the basis for α-syn propagation to different brain areas and, ultimately, for the spreading of pathology and disease progression. Here, we review the interplay between α-syn degradation pathways and its intercellular spreading. The understanding of this interplay is indispensable for obtaining a better knowledge of the molecular basis of PD and, consequently, for the design of novel avenues for therapeutic intervention.

Ser129 phosphorylation of endogenous α-synuclein induced by overexpression of polo-like kinases 2 and 3 in nigral dopamine neurons is not detrimental to their survival and function

Phosphorylation of the α-synuclein (α-syn) protein at Ser129 [P(S129)-α-syn] was found to be the most abundant form in intracellular inclusions in brains from Parkinson's disease (PD) patients. This finding suggests that P(S129)-α-syn plays a central role in the pathogenesis of PD. However, it is at present unclear whether P(S129)-α-syn is pathogenic driving the neurodegenerative process. Rodent studies using neither the phosphomimics of human α-syn nor co-expression of human wild-type α-syn and kinases phosphorylating α-syn at Ser129 gave consistent results. One major concern in interpreting these findings is that human α-syn was expressed above physiological levels inducing neurodegeneration in rat nigral neurons. In order to exclude this confounding factor, we took a different approach and increased the phosphorylation level of endogenous α-syn. For this purpose, we took advantage of recombinant adeno-associated viral (rAAV) vectors to deliver polo-like kinase (PLK) 2 or PLK3 in the substantia nigra and investigated whether increased levels of P(S129)-α-syn compromised the function and survival of nigral dopaminergic neurons. Interestingly, we observed that hyperphosphorylated α-syn did not induce nigral dopaminergic cell death, as assessed at 1 and 4months. Furthermore, histological analysis did not show any accumulation of α-syn protein or formation of inclusions. Using in vivo microdialysis, we found that the only measurable functional alteration was the depolarisation-induced release of dopamine, while the in vivo synthesis rate of DOPA and dopamine baseline release remained unaltered. Taken together, our results suggest that phosphorylation of α-syn at Ser129 does not confer a toxic gain of function per se.

Posttranslational modification and mutation of histidine 50 trigger alpha synuclein aggregation and toxicity

Background

Aggregation and aggregation-mediated formation of toxic alpha synuclein (aSyn) species have been linked to the pathogenesis of sporadic and monogenic Parkinson’s disease (PD). A novel H50Q mutation of aSyn, resulting in the substitution of histidine by glutamine, has recently been identified in PD patients. We have previously shown that the lipid peroxidation product 4-hydroxy-2-nonenal (HNE) induces the formation of HNE-aSyn adducts, thereby promoting aSyn oligomerization and increasing its extracellular toxicity to human dopaminergic neurons. Intriguingly, we identified histidine 50 (H50) of aSyn as one of the HNE modification target residues. These converging lines of evidence support the hypothesis that changes in H50 via posttranslational modification (PTM) and mutation trigger the formation of aggregated, toxic aSyn species, which interfere with cellular homeostasis. In the present study, we aim to elucidate 1) the role of H50 in HNE-mediated aSyn aggregation and toxicity, and 2) the impact of H50 mutation on aSyn pathology. Besides the PD-related H50Q, we analyze a PD-unrelated control mutation, in which H50 is replaced by an arginine residue (H50R).

Results

Analysis of HNE-treated aSyn revealed that H50 is the most susceptible residue of aSyn to HNE modification and is crucial for HNE-mediated aSyn oligomerization. Overexpression of aSyn with substituted H50 in H4 neuroglioma cells reduced HNE-induced cell damage, indicating a pivotal role of H50 in HNE modification-induced aSyn toxicity.

Furthermore, we showed in vitro that H50Q/R mutations substantially increase the formation of high density and fibrillar aSyn species, and potentiate the oligomerization propensity of aSyn in the presence of a nitrating agent. Cell-based experiments also revealed that overexpression of H50Q aSyn in H4 cells promotes aSyn oligomerization. Importantly, overexpression of both H50Q/R aSyn mutants in H4 cells significantly increased cell death when compared to wild type aSyn. This increase in cell death was further exacerbated by the application of H₂O₂.

Conclusion

A dual approach addressing alterations of H50 showed that either H50 PTM or mutation trigger aSyn aggregation and toxicity, suggesting an important role of aSyn H50 in the pathogenesis of both sporadic and monogenic PD.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-015-0004-0) contains supplementary material, which is available to authorized users.

Parkinson disease mutant E46K enhances α-synuclein phosphorylation in mammalian cell lines, in yeast, and in vivo

Although α-synuclein (α-syn) phosphorylation has been considered as a hallmark of sporadic and familial Parkinson disease (PD), little is known about the effect of PD-linked mutations on α-syn phosphorylation. In this study, we investigated the effects of the A30P, E46K, and A53T PD-linked mutations on α-syn phosphorylation at residues Ser-87 and Ser-129. Although the A30P and A53T mutants slightly affected Ser(P)-129 levels compared with WT α-syn, the E46K mutation significantly enhanced Ser-129 phosphorylation in yeast and mammalian cell lines. This effect was not due to the E46K mutant being a better kinase substrate nor due to alterations in endogenous kinase levels, but was mostly linked with enhanced nuclear and endoplasmic reticulum accumulation. Importantly, lentivirus-mediated overexpression in mice also showed enhanced Ser-129 phosphorylation of the E46K mutant compared to WT α-syn, thus providing in vivo validation of our findings. Altogether, our findings suggest that the different PD-linked mutations may contribute to PD pathogenesis via different mechanisms.

The Synaptic Function of α-Synuclein

α-Synuclein is an abundant neuronal protein which localizes predominantly to presynaptic terminals, and is strongly linked genetically and pathologically to Parkinson's disease and other neurodegenerative diseases. While the accumulation of α-synuclein in the form of misfolded oligomers and large aggregates defines multiple neurodegenerative diseases called "synucleinopathies", its cellular function has remained largely unclear, and is the subject of intense investigation. In this review, I focus on the structural characteristics of α-synuclein, its cellular and subcellular localization, and discuss how this relates to its function in neurons, in particular at the neuronal synapse.

Hydrodynamic size-based separation and characterization of protein aggregates from total cell lysates

Herein we describe a protocol that uses hollow-fiber flow field-flow fractionation (FFF) coupled with multiangle light scattering (MALS) for hydrodynamic size-based separation and characterization of complex protein aggregates. The fractionation method, which requires 1.5 h to run, was successfully modified from the analysis of protein aggregates, as found in simple protein mixtures, to complex aggregates, as found in total cell lysates. In contrast to other related methods (filter assay, analytical ultracentrifugation, gel electrophoresis and size-exclusion chromatography), hollow-fiber flow FFF coupled with MALS allows a flow-based fractionation of highly purified protein aggregates and simultaneous measurement of their molecular weight, r.m.s. radius and molecular conformation (e.g., round, rod-shaped, compact or relaxed). The polyethersulfone hollow fibers used, which have a 0.8-mm inner diameter, allow separation of as little as 20 μg of total cell lysates. In addition, the ability to run the samples in different denaturing and nondenaturing buffer allows defining true aggregates from artifacts, which can form during sample preparation. The protocol was set up using Paraquat-induced carbonylation, a model that induces protein aggregation in cultured cells. This technique will advance the biochemical, proteomic and biophysical characterization of molecular-weight aggregates associated with protein mutations, as found in many CNS degenerative diseases, or chronic oxidative stress, as found in aging, and chronic metabolic and inflammatory conditions.