Physiological C-terminal truncation of α-synuclein potentiates the prion-like formation of pathological inclusions

Alzheimer's disease neuropathology and its estimation with fluid and imaging biomarkers

Alzheimer's disease (AD) is neuropathologically characterized by the extracellular deposition of the amyloid-β peptide (Aβ) and the intraneuronal accumulation of abnormal phosphorylated tau (τ)-protein (p-τ). Most frequently, these hallmark lesions are accompanied by other co-pathologies in the brain that may contribute to cognitive impairment, such as vascular lesions, intraneuronal accumulation of phosphorylated transactive-response DNA-binding protein 43 (TDP-43), and/or α-synuclein (αSyn) aggregates. To estimate the extent of these AD and co-pathologies in patients, several biomarkers have been developed. Specific tracers target and visualize Aβ plaques, p-τ and αSyn pathology or inflammation by positron emission tomography. In addition to these imaging biomarkers, cerebrospinal fluid, and blood-based biomarker assays reflecting AD-specific or non-specific processes are either already in clinical use or in development. In this review, we will introduce the pathological lesions of the AD brain, the related biomarkers, and discuss to what extent the respective biomarkers estimate the pathology determined at post-mortem histopathological analysis. It became evident that initial stages of Aβ plaque and p-τ pathology are not detected with the currently available biomarkers. Interestingly, p-τ pathology precedes Aβ deposition, especially in the beginning of the disease when biomarkers are unable to detect it. Later, Aβ takes the lead and accelerates p-τ pathology, fitting well with the known evolution of biomarker measures over time. Some co-pathologies still lack clinically established biomarkers today, such as TDP-43 pathology or cortical microinfarcts. In summary, specific biomarkers for AD-related pathologies allow accurate clinical diagnosis of AD based on pathobiological parameters. Although current biomarkers are excellent measures for the respective pathologies, they fail to detect initial stages of the disease for which post-mortem analysis of the brain is still required. Accordingly, neuropathological studies remain essential to understand disease development especially in early stages. Moreover, there is an urgent need for biomarkers reflecting co-pathologies, such as limbic predominant, age-related TDP-43 encephalopathy-related pathology, which is known to modify the disease by interacting with p-τ. Novel biomarker approaches such as extracellular vesicle-based assays and cryptic RNA/peptides may help to better detect these co-pathologies in the future.

Refining α-synuclein seed amplification assays to distinguish Parkinson's disease from multiple system atrophy

BACKGROUND

Parkinson's disease (PD) and multiple system atrophy (MSA) are two distinct α-synucleinopathies traditionally differentiated through clinical symptoms. Early diagnosis of MSA is problematic, and seed amplification assays (SAAs), such as real-time quaking-induced conversion (RT-QuIC), offer the potential to distinguish these diseases through their underlying α-synuclein (α-Syn) pathology and proteoforms. Currently, SAAs provide a binary result, signifying either the presence or absence of α-Syn seeds. To enhance the diagnostic potential and biological relevance of these assays, there is a pressing need to incorporate quantification and stratification of α-Syn proteoform-specific aggregation kinetics into current SAA pipelines.

METHODS

Optimal RT-QuIC assay conditions for α-Syn seeds extracted from PD and MSA patient brains were determined, and assay kinetics were assessed for α-Syn seeds from different pathologically relevant brain regions (medulla, substantia nigra, hippocampus, middle temporal gyrus, and cerebellum). The conformational profiles of disease- and region-specific α-Syn proteoforms were determined by subjecting the amplified reaction products to concentration-dependent proteolytic digestion with proteinase K.

RESULTS

Using our protocol, PD and MSA could be accurately delineated using proteoform-specific aggregation kinetics, including α-Syn aggregation rate, maximum relative fluorescence, the gradient of amplification, and core protofilament size. MSA cases yielded significantly higher values than PD cases across all four kinetic parameters in brain tissues, with the MSA-cerebellar phenotype having higher maximum relative fluorescence than the MSA-Parkinsonian phenotype. Statistical significance was maintained when the data were analysed regionally and when all regions were grouped.

CONCLUSIONS

Our RT-QuIC protocol and analysis pipeline can distinguish between PD and MSA, and between MSA phenotypes. MSA α-Syn seeds induce faster propagation and exhibit higher aggregation kinetics than PD α-Syn, mirroring the biological differences observed in brain tissue. With further validation of these quantitative parameters, we propose that SAAs could advance from a yes/no diagnostic to a theranostic biomarker that could be utilised in developing therapeutics.

From onset to advancement: the temporal spectrum of α-synuclein in synucleinopathies

This review provides an in-depth analysis of the complex role of alpha-synuclein (α-Syn) in the development of α-synucleinopathies, with a particular focus on its structural diversity and the resulting clinical variability. The ability of α-Syn to form different strains or polymorphs and undergo various post-translational modifications significantly contributes to the wide range of symptoms observed in disorders such as Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA), as well as in lesser-known non-classical α-synucleinopathies. The interaction between genetic predispositions and environmental factors further complicates α-synucleinopathic disease pathogenesis, influencing the disease-specific onset and progression. Despite their common pathological hallmark of α-Syn accumulation, the clinical presentation and progression of α-synucleinopathies differ significantly, posing challenges for diagnosis and treatment. The intricacies of α-Syn pathology highlight the critical need for a deeper understanding of its biological functions and interactions within the neuronal environment to develop targeted therapeutic strategies. The precise point at which α-Syn aggregation transitions from being a byproduct of initial disease triggers to an active and independent driver of disease progression - through the propagation and acceleration of pathogenic processes - remains unclear. By examining the role of α-Syn across various contexts, we illuminate its dual role as both a marker and a mediator of disease, offering insights that could lead to innovative approaches for managing α-synucleinopathies.

N-terminus α-synuclein detection reveals new and more diverse aggregate morphologies in multiple system atrophy and Parkinson's disease

BACKGROUND

Parkinson's disease (PD) and multiple system atrophy (MSA) are classified as α-synucleinopathies and are primarily differentiated by their clinical phenotypes. Delineating these diseases based on their specific α-synuclein (α-Syn) proteoform pathologies is crucial for accurate antemortem biomarker diagnosis. Newly identified α-Syn pathologies in PD raise questions about whether MSA exhibits a similar diversity. This prompted the need for a comparative study focusing on α-Syn epitope-specific immunoreactivities in both diseases, which could clarify the extent of pathological overlap and diversity, and guide more accurate biomarker development.

METHODS

We utilised a multiplex immunohistochemical approach to detect multiple structural domains of α-Syn proteoforms across multiple regions prone to pathological accumulation in MSA (n = 10) and PD (n = 10). Comparison of epitope-specific α-Syn proteoforms was performed in the MSA medulla, inferior olivary nucleus, substantia nigra, hippocampus, and cerebellum, and in the PD olfactory bulb, medulla, substantia nigra, hippocampus, and entorhinal cortex.

RESULTS

N-terminus and C-terminus antibodies detected significantly more α-Syn pathology in MSA than antibodies for phosphorylated (pS129) α-Syn, which are classically used to detect α-Syn. Importantly, C-terminus immunolabelling is more pronounced in MSA compared to PD. Meanwhile, N-terminus immunolabelling consistently detected the highest percentage of α-Syn across pathologically burdened regions of both diseases, which could be of biological significance. As expected, oligodendroglial involvement distinguished MSA from PD, but in contrast to PD, no substantial astrocytic or microglial α-Syn accumulation in MSA occurred. These data confirm glial-specific changes between these diseases when immunolabelling the N-terminus epitope. In comparison, N-terminus neuronal α-Syn was present in PD and MSA, with most MSA neurons lacking pS129 α-Syn proteoforms. This explains why characterisation of neuronal MSA pathologies is lacking and challenges the reliance on pS129 antibodies for the accurate quantification of α-Syn pathological load across α-synucleinopathies.

CONCLUSIONS

These findings underscore the necessity of utilising a multiplex approach to detect α-Syn, most importantly including the N-terminus, to capture the entire spectrum of α-Syn proteoforms in α-synucleinopathies. The data provide novel insights toward the biological differentiation of these α-synucleinopathies and pave the way for more refined antemortem diagnostic methods to facilitate early identification and intervention of these neurodegenerative diseases.

Exploring the Impact of Physiological C-Terminal Truncation on α-Synuclein Conformations to Unveil Mechanisms Regulating Pathological Aggregation

Emerging evidence suggests that physiological C-terminal truncation of α-synuclein (αS) plays a critical role in regulating liquid-liquid phase separation and promoting amyloid aggregation, processes implicated in neurodegenerative diseases such as Parkinson's disease (PD). However, the molecular mechanisms through which C-terminal truncation influences αS conformation and modulates its aggregation remain poorly understood. In this study, we investigated the impact of C-terminal truncation on αS conformational dynamics by comparing full-length αS1-140 with truncated αS1-103 monomers using atomistic discrete molecular dynamics simulations. Our findings revealed that both αS1-140 and αS1-103 primarily adopted helical conformations around residues 7-32, while residues 36-95, located in the second half of the N-terminal and NAC domains, predominantly formed a dynamic β-sheet core. The C-terminus of αS1-140 was largely unstructured and dynamically wrapped around the β-sheet core. While residues 1-95 exhibited similar secondary structure propensities in both αS1-140 and αS1-103, the dynamic capping by the C-terminus in αS1-140 slightly enhanced β-sheet formation around residues 36-95. In contrast, key aggregation-driving regions (residues 2-9, 36-42, 45-57, and 68-78) were dynamically shielded by the C-terminus in αS1-140, reducing their exposure and potentially preventing interpeptide interactions that drive aggregation. C-terminal truncation, on the other hand, increased the exposed surface area of these aggregation-prone regions, thereby enhancing interpeptide interactions, phase separation, and amyloid aggregation. Overall, our simulations provide valuable insights into the conformational effects of C-terminal truncation on αS and its role in promoting pathological aggregation.

Liquid-liquid phase separation and conformational strains of α-Synuclein: implications for Parkinson's disease pathogenesis

Parkinson's disease (PD) and other synucleinopathies are characterized by the aggregation and deposition of alpha-synuclein (α-syn) in brain cells, forming insoluble inclusions such as Lewy bodies (LBs) and Lewy neurites (LNs). The aggregation of α-syn is a complex process involving the structural conversion from its native random coil to well-defined secondary structures rich in β-sheets, forming amyloid-like fibrils. Evidence suggests that intermediate species of α-syn aggregates formed during this conversion are responsible for cell death. However, the molecular events involved in α-syn aggregation and its relationship with disease onset and progression remain not fully elucidated. Additionally, the clinical and pathological heterogeneity observed in various synucleinopathies has been highlighted. Liquid-liquid phase separation (LLPS) and condensate formation have been proposed as alternative mechanisms that could underpin α-syn pathology and contribute to the heterogeneity seen in synucleinopathies. This review focuses on the role of the cellular environment in α-syn conformational rearrangement, which may lead to pathology and the existence of different α-syn conformational strains with varying toxicity patterns. The discussion will include cellular stress, abnormal LLPS formation, and the potential role of LLPS in α-syn pathology.

Nanorod-associated plasmonic circular dichroism monitors the handedness and composition of α-synuclein fibrils from Parkinson's disease models and post-mortem brain

Human full-length (fl) αSyn fibrils, key neuropathological hallmarks of Parkinson's disease (PD), generate intense optical activity corresponding to the surface plasmon resonance of interacting gold nanorods. Herein, we analysed fibril-enriched protein extracts from mouse and human brain samples as well as from SK-N-SH cell lines with or without human fl and C-terminally truncated (Ctt) αSyn overexpression and exposed them to αSyn monomers, recombinant fl αSyn fibrils or Ctt αSyn fibrils. In vitro-generated human recombinant fl and Ctt αSyn fibrils and fibrils purified from SK-N-SH cells with fl or Ctt αSyn overexpression were also analysed using transmission electron microscopy (TEM) to gain insights into the nanorod-fibril complexes. We found that under the same experimental conditions, bisignate circular dichroism (CD) spectra of Ctt αSyn fibrils exhibited a blue-wavelength shift compared to that of fl αSyn fibrils. TEM results supported that this could be attributed to the different properties of nanorods. In our experimental conditions, fibril-enriched PD brain extract broadened the longitudinal surface plasmonic band with a bisignate CD couplet centred corresponding to the absorption band maximum. Plasmonic CD (PCD) couplets of in vivo- and in vitro-generated fibrils displayed sign reversal, suggesting their opposite handedness. Moreover, the incubation of in vitro-generated human recombinant fl αSyn fibrils in mouse brain extracts from αSyn null mice resulted in PCD couplet inversion, indicating that the biological environment may shape the handedness of αSyn fibrils. These findings support that nanorod-based PCD can provide useful information on the composition and features of αSyn fibrils from biological materials.

Neuronal tissue collection from intra-cranial instruments used in deep brain stimulation surgery for Parkinson's disease with implications for study of alpha-synuclein

Alpha-synuclein (αSyn) forms pathologic aggregates in Parkinson's disease (PD) and is implicated in mechanisms underlying neurodegeneration. While pathologic αSyn has been extensively studied, there is currently no method to evaluate αSyn within the brains of living patients. Patients with PD are often treated with deep brain stimulation (DBS) surgery in which surgical instruments are in direct contact with neuronal tissue; herein, we describe a method by which tissue is collected from DBS surgical instruments in PD and essential tremor (ET) patients and demonstrate that αSyn is detected. 24 patients undergoing DBS surgery for PD (17 patients) or ET (7 patients) were enrolled; from patient samples, 81.2 ± 44.8 µg of protein (n = 15), on average, was collected from surgical instruments. Light microscopy revealed axons, capillaries, and blood cells as the primary components of purified tissue (n = 3). ELISA assay further confirmed the presence of neuronal and glial tissue in DBS samples (n = 4). Further analysis was conducted using western blot, demonstrating that multiple αSyn antibodies are reactive in PD (n = 5) and ET (n = 3) samples; truncated αSyn (1-125 αSyn) was significantly increased in PD (n = 5) compared to ET (n = 3), in which αSyn misfolding is not expected (0.64 ± 0.25 vs. 0.25 ± 0.12, P = 0.046), thus showing that multiple forms of αSyn can be detected from living PD patients with this method.

VAMP2 chaperones α-synuclein in synaptic vesicle co-condensates

α-Synuclein (α-Syn) aggregation is closely associated with Parkinson's disease neuropathology. Physiologically, α-Syn promotes synaptic vesicle (SV) clustering and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex assembly. However, the underlying structural and molecular mechanisms are uncertain and it is not known whether this function affects the pathological aggregation of α-Syn. Here we show that the juxtamembrane region of vesicle-associated membrane protein 2 (VAMP2)-a component of the SNARE complex that resides on SVs-directly interacts with the carboxy-terminal region of α-Syn through charged residues to regulate α-Syn's function in clustering SVs and promoting SNARE complex assembly by inducing a multi-component condensed phase of SVs, α-Syn and other components. Moreover, VAMP2 binding protects α-Syn against forming aggregation-prone oligomers and fibrils in these condensates. Our results suggest a molecular mechanism that maintains α-Syn's function and prevents its pathological amyloid aggregation, the failure of which may lead to Parkinson's disease.

A multiverse of α-synuclein: investigation of prion strain properties with carboxyl-terminal truncation specific antibodies in animal models

Synucleinopathies are a group of neurodegenerative disorders characterized by the presence of misfolded α-Synuclein (αSyn) in the brain. These conditions manifest with diverse clinical and pathophysiological characteristics. This disease diversity is hypothesized to be driven by αSyn strains with differing biophysical properties, potentially influencing prion-type propagation and consequentially the progression of illness. Previously, we investigated this hypothesis by injecting brain lysate (seeds) from deceased individuals with various synucleinopathies or human recombinant αSyn preformed fibrils (PFFs) into transgenic mice overexpressing either wild type or A53T human αSyn. In the studies herein, we expanded on these experiments, utilizing a panel of antibodies specific for the major carboxyl-terminally truncated forms of αSyn (αSynΔC). These modified forms of αSyn are found enriched in human disease brains to inform on potential strain-specific proteolytic patterns. With monoclonal antibodies specific for human αSyn cleaved at residues 103, 114, 122, 125, and 129, we demonstrate that multiple system atrophy (MSA) seeds and PFFs induce differing neuroanatomical spread of αSyn pathology associated with host specific profiles. Overall, αSyn cleaved at residue 103 was most widely present in the induced pathological inclusions. Furthermore, αSynΔC-positive inclusions were present in astrocytes, but more frequently in activated microglia, with patterns dependent on host and inoculum. These findings support the hypothesis that synucleinopathy heterogeneity might stem from αSyn strains with unique biochemical properties that include proteolytic processing, which could result in dominant strain properties.

Internalized α-synuclein fibrils become truncated and resist degradation in neurons while glial cells rapidly degrade α-synuclein fibrils

Parkinson's disease (PD) and other α-synucleinopathies are characterized by the accumulation of α-synuclein (αS) pathology that can spread via the cell-to-cell transmission of αS aggregates. To better understand how various brain cells contribute to the spreading of αS pathology, we examined the metabolism of αS aggreges or pre-formed fibrils (PFFs) in neuronal and glial cells (microglia, astrocytes, and oligodendrocytes). In neurons, while the full-length αS rapidly disappeared following αS PFF uptake, truncated αS accumulated with a half-life of days rather than hours. Epitope mapping and fractionation studies indicate that αS PFF was truncated at the C-terminal region following uptake and remained insoluble/aggregated. In contrast, microglia and astrocytes rapidly metabolized αS PFF as the half-lives of αS PFF in these glial cells were <6 hours. Differential processing of αS by neurons was recapitulated in cell lines as differentiated CLU neuronal cell lines stably accumulate truncated αS while undifferentiated cells rapidly metabolize αS. Immunolocalization and subcellular fractionation studies show that internalized αS PFF is initially localized to endosomes followed by lysosomes. The lysosome is largely responsible for the degradation of internalized αS PFF as the inhibition of lysosomal function leads to the stabilization of αS in all cell types. Significantly, αS PFF causes lysosomal dysfunction in neurons. In summary, we show that neurons are inefficient in metabolizing internalized αS aggregates, partially because αS aggregates cause lysosomal dysfunction, potentially generating aggregation-prone truncated αS. In contrast, glial cells may protect neurons from αS aggregates by rapidly clearing αS aggregates.

Analysis of alpha-synuclein harvested from intracranial instruments used in deep brain stimulation surgery for Parkinson's disease

Alpha-synuclein (αSyn) forms pathologic aggregates in Parkinson's disease (PD) and is implicated in mechanisms underlying neurodegeneration. While pathologic αSyn has been extensively studied, there is currently no method to evaluate αSyn within the brains of living patients. Patients with PD are often treated with deep brain stimulation (DBS) surgery in which surgical instruments are in direct contact with neuronal tissue; herein, we describe a method by which tissue is purified from DBS surgical instruments in PD and essential tremor (ET) patients and demonstrate that αSyn is robustly detected. 24 patients undergoing DBS surgery for PD (17 patients) or ET (7 patients) were enrolled; from patient samples, 81.2 ± 44.8 μg protein (n=15) is able to be purified, with immunoblot assays specific for αSyn reactive in all tested samples. Light microscopy revealed axons and capillaries as the primary components of purified tissue (n=3). Further analysis was conducted using western blot, demonstrating that truncated αSyn (1-125 αSyn) was significantly increased in PD (n=5) compared to ET (n=3), in which αSyn misfolding is not expected (0.64 ± 0.25 vs. 0.25 ± 0.12, P = 0.046), thus showing that pathologic αSyn can be reliably purified from living PD patients with this method.

A glimpse into the structural properties of α-synuclein oligomers

α-Synuclein (αS) aggregation is the main neurological hallmark of a group of debilitating neurodegenerative disorders, collectively referred to as synucleinopathies, of which Parkinson's disease is the most prevalent. αS oligomers formed during the initial stages of aggregation are considered key pathogenic drivers of disease onset and progression, standing as privileged targets for therapeutic intervention and diagnosis. However, the structure of αS oligomers and the mechanistic basis of oligomer to fibril conversion are yet poorly understood, thereby precluding the rational formulation of strategies aimed at targeting oligomeric species. In this review, we delve into the recent advances in the structural and mechanistic characterization of αS oligomers. We also discuss how these advances are transforming our understanding of these elusive species and paving the way for oligomer-targeting therapeutics and diagnosis.

Genetic and pharmacological reduction of CDK14 mitigates synucleinopathy

Parkinson's disease (PD) is a debilitating neurodegenerative disease characterized by the loss of midbrain dopaminergic neurons (DaNs) and the abnormal accumulation of α-Synuclein (α-Syn) protein. Currently, no treatment can slow nor halt the progression of PD. Multiplications and mutations of the α-Syn gene (SNCA) cause PD-associated syndromes and animal models that overexpress α-Syn replicate several features of PD. Decreasing total α-Syn levels, therefore, is an attractive approach to slow down neurodegeneration in patients with synucleinopathy. We previously performed a genetic screen for modifiers of α-Syn levels and identified CDK14, a kinase of largely unknown function as a regulator of α-Syn. To test the potential therapeutic effects of CDK14 reduction in PD, we ablated Cdk14 in the α-Syn preformed fibrils (PFF)-induced PD mouse model. We found that loss of Cdk14 mitigates the grip strength deficit of PFF-treated mice and ameliorates PFF-induced cortical α-Syn pathology, indicated by reduced numbers of pS129 α-Syn-containing cells. In primary neurons, we found that Cdk14 depletion protects against the propagation of toxic α-Syn species. We further validated these findings on pS129 α-Syn levels in PD patient neurons. Finally, we leveraged the recent discovery of a covalent inhibitor of CDK14 to determine whether this target is pharmacologically tractable in vitro and in vivo. We found that CDK14 inhibition decreases total and pathologically aggregated α-Syn in human neurons, in PFF-challenged rat neurons and in the brains of α-Syn-humanized mice. In summary, we suggest that CDK14 represents a novel therapeutic target for PD-associated synucleinopathy.

Modulation of α-synuclein in vitro aggregation kinetics by its alternative splice isoforms

The misfolding and aggregation of α-synuclein is linked to a family of neurodegenerative disorders known as synucleinopathies, the most prominent of which is Parkinson's disease (PD). Understanding the aggregation process of α-synuclein from a mechanistic point of view is thus of key importance. SNCA, the gene encoding α-synuclein, comprises six exons and produces various isoforms through alternative splicing. The most abundant isoform is expressed as a 140-amino acid protein (αSyn-140), while three other isoforms, αSyn-126, αSyn-112, and αSyn-98, are generated by skipping exon 3, exon 5, or both exons, respectively. In this study, we performed a detailed biophysical characterization of the aggregation of these four isoforms. We found that αSyn-112 and αSyn-98 exhibit accelerated aggregation kinetics compared to αSyn-140 and form distinct aggregate morphologies, as observed by transmission electron microscopy. Moreover, we observed that the presence of relatively small amounts of αSyn-112 accelerates the aggregation of αSyn-140, significantly reducing the aggregation half-time. These results indicate a potential role of alternative splicing in the pathological aggregation of α-synuclein and provide insights into how this process could be associated with the development of synucleinopathies.

Computational investigation on the conformational dynamics of C-terminal truncated α-synuclein bound to membrane

Accelerated progression rates in Parkinson's disease (PD) have been linked to C-terminal domain (CTD) truncations of monomeric α-Synuclein (α-Syn), which have been suggested to increase amyloid aggregation in vivo and in vitro. In the brain of PD patients, CTD truncated α-Syn was found to have lower cell viability and tends to increase in the formation of fibrils. The CTD of α-Syn acts as a guard for regulating the normal functioning of α-Syn. The absence of the CTD may allow the N-terminal of α-Syn to interact with the membrane thereby affecting the normal functioning of α-Syn, and all of which will affect the etiology of PD. In this study, the conformational dynamics of CTD truncated α-Syn (1-99 and 1-108) monomers and their effect on the protein-membrane interactions were demonstrated using the all-atom molecular dynamics (MD) simulation method. From the MD analyses, it was noticed that among the two truncated monomers, α-Syn (1-108) was found to be more stable, shows rigidness at the N-terminal region and contains a significant number of intermolecular hydrogen bonds between the non-amyloid β-component (NAC) region and membrane, and lesser number of extended strands. Further, the bending angle in the N-terminal domain was found to be lesser in the α-Syn (1-108) in comparison with the α-Syn (1-99). Our findings suggest that the truncation on the CTD of α-Syn affects its interaction with the membrane and subsequently has an impact on the aggregation.

Aggregate-prone brain regions in Parkinson's disease are rich in unique N-terminus α-synuclein conformers with high proteolysis susceptibility

In Parkinson's disease (PD), and other α-synucleinopathies, α-synuclein (α-Syn) aggregates form a myriad of conformational and truncational variants. Most antibodies used to detect and quantify α-Syn in the human brain target epitopes within the C-terminus (residues 96-140) of the 140 amino acid protein and may fail to capture the diversity of α-Syn variants present in PD. We sought to investigate the heterogeneity of α-Syn conformations and aggregation states in the PD human brain by labelling with multiple antibodies that detect epitopes along the entire length of α-Syn. We used multiplex immunohistochemistry to simultaneously immunolabel tissue sections with antibodies mapping the three structural domains of α-Syn. Discrete epitope-specific immunoreactivities were visualised and quantified in the olfactory bulb, medulla, substantia nigra, hippocampus, entorhinal cortex, middle temporal gyrus, and middle frontal gyrus of ten PD cases, and the middle temporal gyrus of 23 PD, and 24 neurologically normal cases. Distinct Lewy neurite and Lewy body aggregate morphologies were detected across all interrogated regions/cases. Lewy neurites were the most prominent in the olfactory bulb and hippocampus, while the substantia nigra, medulla and cortical regions showed a mixture of Lewy neurites and Lewy bodies. Importantly, unique N-terminus immunoreactivity revealed previously uncharacterised populations of (1) perinuclear, (2) glial (microglial and astrocytic), and (3) neuronal lysosomal α-Syn aggregates. These epitope-specific N-terminus immunoreactive aggregate populations were susceptible to proteolysis via time-dependent proteinase K digestion, suggesting a less stable oligomeric aggregation state. Our identification of unique N-terminus immunoreactive α-Syn aggregates adds to the emerging paradigm that α-Syn pathology is more abundant and complex in human brains with PD than previously realised. Our findings highlight that labelling multiple regions of the α-Syn protein is necessary to investigate the full spectrum of α-Syn pathology and prompt further investigation into the functional role of these N-terminus polymorphs.

Physicochemical mechanisms of aggregation and fibril formation of α-synuclein and apolipoprotein A-I

Deposition and accumulation of amyloid fibrils is a hallmark of a group of diseases called amyloidosis and neurodegenerative disorders. Although polypeptides potentially have a fibril-forming propensity, native proteins have evolved into proper functional conformations to avoid aggregation and fibril formation. Understanding the mechanism for regulation of fibril formation of native proteins provides clues for the rational design of molecules for inhibiting fibril formation. Although fibril formation is a complex multistep reaction, experimentally obtained fibril formation curves can be fitted with the Finke-Watzky (F-W) two-step model for homogeneous nucleation followed by autocatalytic fibril growth. The resultant F-W rate constants for nucleation and fibril formation provide information on the chemical kinetics of fibril formation. Using the F-W two-step model analysis, we investigated the physicochemical mechanisms of fibril formation of a Parkinson's disease protein α-synuclein (αS) and a systemic amyloidosis protein apolipoprotein A-I (apoA-I). The results indicate that the C-terminal region of αS enthalpically and entropically suppresses nucleation through the intramolecular interaction with the N-terminal region and the intermolecular interaction with existing fibrils. In contrast, the nucleation of the N-terminal fragment of apoA-I is entropically driven likely due to dehydration of large hydrophobic segments in the molecule. Based on our recent findings, we discuss the similarity and difference of the fibril formation mechanisms of αS and the N-terminal fragment of apoA-I from the physicochemical viewpoints.

Saturation mutagenesis of α-synuclein reveals monomer fold that modulates aggregation

α-Synuclein (aSyn) aggregation underlies neurodegenerative synucleinopathies. aSyn seeds are proposed to replicate and propagate neuronal pathology like prions. Seeding of aSyn can be recapitulated in cellular systems of aSyn aggregation; however, the mechanism of aSyn seeding and its regulation are not well understood. We developed an mEos-based aSyn seeding assay and performed saturation mutagenesis to identify with single-residue resolution positive and negative regulators of aSyn aggregation. We not only found the core regions that govern aSyn aggregation but also identified mutants outside of the core that enhance aggregation. We identified local structure within the N terminus of aSyn that hinders the fibrillization propensity of its aggregation-prone core. Based on the screen, we designed a minimal aSyn fragment that shows a ~4-fold enhancement in seeding activity and enabled discrimination of synucleinopathies. Our study expands the basic knowledge of aSyn aggregation and advances the design of cellular systems of aSyn aggregation to diagnose synucleinopathies based on protein conformation.

Tuning Hsp104 specificity to selectively detoxify α-synuclein

Hsp104 is an AAA+ protein disaggregase that solubilizes and reactivates proteins trapped in aggregated states. We have engineered potentiated Hsp104 variants to mitigate toxic misfolding of α-synuclein, TDP-43, and FUS implicated in fatal neurodegenerative disorders. Though potent disaggregases, these enhanced Hsp104 variants lack substrate specificity and can have unfavorable off-target effects. Here, to lessen off-target effects, we engineer substrate-specific Hsp104 variants. By altering Hsp104 pore loops that engage substrate, we disambiguate Hsp104 variants that selectively suppress α-synuclein toxicity but not TDP-43 or FUS toxicity. Remarkably, α-synuclein-specific Hsp104 variants emerge that mitigate α-synuclein toxicity via distinct ATPase-dependent mechanisms involving α-synuclein disaggregation or detoxification of soluble α-synuclein conformers. Importantly, both types of α-synuclein-specific Hsp104 variant reduce dopaminergic neurodegeneration in a C. elegans model of Parkinson's disease more effectively than non-specific variants. We suggest that increasing the substrate specificity of enhanced disaggregases could be applied broadly to tailor therapeutics for neurodegenerative disease.

Truncation or proteolysis of α-synuclein in Parkinsonism

Posttranslational modifications of α-synuclein, such as truncation or abnormal proteolysis, are implicated in Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). A key focus of this article includes the proteases responsible for inducing truncation, the specific sites susceptible to truncation, and the resultant influence of these truncated species on the seeding and aggregation of endogenous α-synuclein. We also shed light on the unique structural attributes of these truncated species, and how these modifications can lead to distinctive forms of synucleinopathies. In addition, we explore the comparative toxic potentials of various α-synuclein species. An extensive analysis of available evidence of truncated α-synuclein species in human-synucleinopathy brains is also provided. Lastly, we delve into the detrimental impact of truncated species on key cellular structures such as the mitochondria and endoplasmic reticulum. Our article discusses enzymes involved in α-synuclein truncation, including 20 S proteasome, cathepsins, asparagine endopeptidase, caspase-1, calpain-1, neurosin/kallikrein-6, matrix metalloproteinase-1/-3, and plasmin. Truncation patterns impact α-synuclein aggregation - C-terminal truncation accelerates aggregation with larger truncations correlated with shortened aggregation lag times. N-terminal truncation affects aggregation differently based on the truncation location. C-terminally truncated α-synuclein forms compact, shorter fibrils compared to the full-length (FL) protein. N-terminally truncated monomers form fibrils similar in length to FL α-synuclein. Truncated forms show distinct fibril morphologies, increased β-sheet structures, and greater protease resistance. Misfolded α-synuclein can adopt various conformations, leading to unique aggregates and distinct synucleinopathies. Fibrils, with prion-like transmission, are potentially more toxic than oligomers, though this is still debated. Different α-synuclein variants with N- and C-terminal truncations, namely 5-140, 39-140, 65-140, 66-140, 68-140, 71-140, 1-139, 1-135, 1-133, 1-122, 1-119, 1-115, 1-110, and 1-103 have been found in PD, DLB, and MSA patients' brains. In Parkinsonism, excess misfolded α-synuclein overwhelms the proteasome degradation system, resulting in truncated protein production and accumulation in the mitochondria and endoplasmic reticulum.

The Bidirectional Interplay of α-Synuclein with Lipids in the Central Nervous System and Its Implications for the Pathogenesis of Parkinson's Disease

The alteration and aggregation of alpha-synuclein (α-syn) play a crucial role in neurodegenerative diseases collectively termed as synucleinopathies, including Parkinson's disease (PD). The bidirectional interaction of α-syn with lipids and biomembranes impacts not only α-syn aggregation but also lipid homeostasis. Indeed, lipid composition and metabolism are severely perturbed in PD. One explanation for lipid-associated alterations may involve structural changes in α-syn, caused, for example, by missense mutations in the lipid-binding region of α-syn as well as post-translational modifications such as phosphorylation, acetylation, nitration, ubiquitination, truncation, glycosylation, and glycation. Notably, different strategies targeting the α-syn-lipid interaction have been identified and are able to reduce α-syn pathology. These approaches include the modulation of post-translational modifications aiming to reduce the aggregation of α-syn and modify its binding properties to lipid membranes. Furthermore, targeting enzymes involved in various steps of lipid metabolism and exploring the neuroprotective potential of lipids themselves have emerged as novel therapeutic approaches. Taken together, this review focuses on the bidirectional crosstalk of α-syn and lipids and how alterations of this interaction affect PD and thereby open a window for therapeutic interventions.

Cellular processing of α-synuclein fibrils results in distinct physiological C-terminal truncations with a major cleavage site at residue Glu 114

α-synuclein (αS) is an abundant, neuronal protein that assembles into fibrillar pathological inclusions in a spectrum of neurodegenerative diseases that include Lewy body diseases (LBD) and Multiple System Atrophy (MSA). The cellular and regional distributions of pathological inclusions vary widely between different synucleinopathies contributing to the spectrum of clinical presentations. Extensive cleavage within the carboxy (C)-terminal region of αS is associated with inclusion formation, although the events leading to these modifications and the implications for pathobiology are of ongoing study. αS preformed fibrils can induce prion-like spread of αS pathology in both in vitro and animal models of disease. Using C truncation-specific antibodies, we demonstrated here that prion-like cellular uptake and processing of αS preformed fibrils resulted in two major cleavages at residues 103 and 114. A third cleavage product (122 αS) accumulated upon application of lysosomal protease inhibitors. In vitro, both 1-103 and 1-114 αS polymerized rapidly and extensively in isolation and in the presence of full-length αS. 1-103 αS also demonstrated more extensive aggregation when expressed in cultured cells. Furthermore, we used novel antibodies to αS cleaved at residue Glu114, to assess x-114 αS pathology in postmortem brain tissue from patients with LBD and MSA, as well as three different transgenic αS mouse models of prion-like induction. The distribution of x-114 αS pathology was distinct from that of overall αS pathology. These studies reveal the cellular formation and behavior of αS C-truncated at residues 114 and 103 as well as the disease dependent distribution of x-114 αS pathology.

α-Synuclein decoy peptide protects mice against α-synuclein-induced memory loss

AIMS

We previously found that a decoy peptide derived from the C-terminal sequence of α-Synuclein (αSyn) prevents cytotoxic αSyn aggregation caused by fatty acid-binding protein 3 (FABP3) in vitro. In this study, we continued to utilize αSyn-derived peptides to further validate their effects on αSyn neurotoxicity and behavioral impairments in αSyn preformed fibrils (PFFs)-injected mouse model of Parkinson's disease (PD).

METHODS

Mice were injected with αSyn PFFs in the bilateral olfactory bulb (OB) and then were subjected to behavioral analysis at 2-week intervals post-injection. Peptides nasal administration was initiated one week after injection. Changes in phosphorylation of αSyn and neuronal damage in the OB were measured using immunostaining at week 4. The effect of peptides on the interaction between αSyn and FABP3 was examined using co-immunoprecipitation.

RESULTS

αSyn PFF-injected mice showed significant memory loss but no motor function impairment. Long-term nasal treatment with peptides effectively prevented memory impairment. In peptide-treated αSyn PFF-injected mice, the peptides entered the OB smoothly through the nasal cavity and were mainly concentrated in neurons in the mitral cell layer, significantly suppressing the excessive phosphorylation of αSyn and reducing the formation of αSyn-FABP3 oligomers, thereby preventing neuronal death. The addition of peptides also blocked the interaction of αSyn and FABP3 at the recombinant protein level, and its effect was strongest at molar concentrations comparable to those of αSyn and FABP3.

CONCLUSIONS

Our findings suggest that the αSyn decoy peptide represents a novel therapeutic approach for reducing the accumulation of toxic αSyn-FABP3 oligomers in the brain, thereby preventing the progression of synucleinopathies.

Pyrogallol, Corilagin and Chebulagic acid target the "fuzzy coat" of alpha-synuclein to inhibit the fibrillization of the protein

The accumulation of the intrinsically disordered protein alpha-synuclein (αSyn) in the form of insoluble fibrillar aggregates in the central nervous system is linked to a variety of neurodegenerative disorders such as Parkinson's disease, Lewy body dementia, and multiple system atrophy. Here we show that Pyrogallol, Corilagin and Chebulagic acid, compounds containing a different number of catechol rings, are independently capable of delaying and reducing the extent of αSyn fibrillization. The efficiency of inhibition was found to correlate with the number of catechol rings. Further, our NMR studies reveal that these compounds interact with the N-terminal region of αSyn which is unstructured even in the fibrillar form of the protein and is known as the "fuzzy coat" of fibrils. Thus, Corilagin and Chebulagic acid target the fuzzy coat of αSyn and not the amyloid core which is a common target for the inhibition of protein fibrillization. Our results indicate that the N-terminus also plays a key role in the fibrillization of αSyn.

Inhibition of α-Synuclein Seeding-Dependent Aggregation by ssDNA Aptamers Specific to C-Terminally Truncated α-Synuclein Fibrils

Neuropathologically, Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are characterized by the accumulation of insoluble aggregates of α-synuclein (α-syn) in the Lewy bodies (LBs). In addition to full-length α-syn fibrils, C-terminally truncated α-syn is also abundant in the LBs that acts as seeds and facilitates the aggregation of the full-length α-syn in vitro and in vivo and induces toxicity. Hence, identifying molecules that can inhibit the seeding activity of these truncated forms is of great importance. Here, we report the first in vitro selection of aptamers targeting the fibrillar forms of different C-terminally truncated α-syn using systematic evolution by an exponential enrichment method followed by quantitative high-throughput DNA sequencing. We identify a panel of aptamers that bound with high specificity to different truncated forms of α-syn fibrils with no cross-reactivity toward other amyloid fibrils. Interestingly, two of the aptamers (named Apt11 and Apt15) show higher affinity to most C-terminally truncated forms of α-syn fibrils with an evident inhibition of α-syn-seeded aggregation in vitro by Apt11. This inhibition is further confirmed by circular dichroism, Congo red binding assay, and electronic microscopy. Moreover, Apt11 is also found to reduce the insoluble phosphorylated form of α-syn at Ser-129 (pS129-α-syn) in the cell model and also can inhibit α-syn aggregation using RT-QuIC reactions seeded with brain homogenates extracted from patients affected by PD. The aptamers discovered in this study represent potential useful tools for research and diagnostics or therapy toward PD and DLB.

miRNA and antisense oligonucleotide-based α-synuclein targeting as disease-modifying therapeutics in Parkinson's disease

α-synuclein is the synaptic protein majorly involved in neuronal dysfunction and death and it is well known for the last two decades as a hallmark of Parkinson's disease. Alpha-synuclein is involved in neurodegeneration mediated through various neurotoxic pathways, majorly including autophagy or lysosomal dysregulation, mitochondrial disruption, synaptic dysfunction, and oxidative stress. Moreover, the alpha-synuclein aggregation has been associated with the development of several neurodegenerative conditions such as various forms of Parkinson's disease. The recent discovery in oligonucleotide chemistry has developed potential alpha-synuclein targeting molecules for the treatment of neurodegenerative diseases. The present review article focuses on recent advances in the applications of oligonucleotides acting via alpha-synuclein targeting mechanisms and their implication in combating Parkinson's disease. Moreover, the article emphasizes the potential of miRNAs, and antisense oligonucleotides and the challenges associated with their use in the therapeutical management of Parkinson's disease.

DOPAnization of tyrosine in α-synuclein by tyrosine hydroxylase leads to the formation of oligomers

Parkinson's disease is a progressive neurodegenerative disorder characterized by the preferential loss of tyrosine hydroxylase (TH)-expressing dopaminergic neurons in the substantia nigra. Although the abnormal accumulation and aggregation of α-synuclein have been implicated in the pathogenesis of Parkinson's disease, the underlying mechanisms remain largely elusive. Here, we found that TH converts Tyr136 in α-synuclein into dihydroxyphenylalanine (DOPA; Y136DOPA) through mass spectrometric analysis. Y136DOPA modification was clearly detected by a specific antibody in the dopaminergic neurons of α-synuclein-overexpressing mice as well as human α-synucleinopathies. Furthermore, dopanized α-synuclein tended to form oligomers rather than large fibril aggregates and significantly enhanced neurotoxicity. Our findings suggest that the dopanization of α-synuclein by TH may contribute to oligomer and/or seed formation causing neurodegeneration with the potential to shed light on the pathogenesis of Parkinson's disease.

DOPAnization of tyrosine in α-synuclein by tyrosine hydroxylase leads to the formation of oligomers

Parkinson's disease is a progressive neurodegenerative disorder characterized by the preferential loss of tyrosine hydroxylase (TH)-expressing dopaminergic neurons in the substantia nigra. Although the abnormal accumulation and aggregation of α-synuclein have been implicated in the pathogenesis of Parkinson's disease, the underlying mechanisms remain largely elusive. Here, we found that TH converts Tyr136 in α-synuclein into dihydroxyphenylalanine (DOPA; Y136DOPA) through mass spectrometric analysis. Y136DOPA modification was clearly detected by a specific antibody in the dopaminergic neurons of α-synuclein-overexpressing mice as well as human α-synucleinopathies. Furthermore, dopanized α-synuclein tended to form oligomers rather than large fibril aggregates and significantly enhanced neurotoxicity. Our findings suggest that the dopanization of α-synuclein by TH may contribute to oligomer and/or seed formation causing neurodegeneration with the potential to shed light on the pathogenesis of Parkinson's disease.

Revisiting the specificity and ability of phospho-S129 antibodies to capture alpha-synuclein biochemical and pathological diversity

Antibodies against phosphorylated alpha-synuclein (aSyn) at S129 have emerged as the primary tools to investigate, monitor, and quantify aSyn pathology in the brain and peripheral tissues of patients with Parkinson's disease and other neurodegenerative diseases. Herein, we demonstrate that the co-occurrence of multiple pathology-associated C-terminal post-translational modifications (PTMs) (e.g., phosphorylation at Tyrosine 125 or truncation at residue 133 or 135) differentially influences the detection of pS129-aSyn species by pS129-aSyn antibodies. These observations prompted us to systematically reassess the specificity of the most commonly used pS129 antibodies against monomeric and aggregated forms of pS129-aSyn in mouse brain slices, primary neurons, mammalian cells and seeding models of aSyn pathology formation. We identified two antibodies that are insensitive to pS129 neighboring PTMs. Although most pS129 antibodies showed good performance in detecting aSyn aggregates in cells, neurons and mouse brain tissue containing abundant aSyn pathology, they also showed cross-reactivity towards other proteins and often detected non-specific low and high molecular weight bands in aSyn knock-out samples that could be easily mistaken for monomeric or high molecular weight aSyn species. Our observations suggest that not all pS129 antibodies capture the biochemical and morphological diversity of aSyn pathology, and all should be used with the appropriate protein standards and controls when investigating aSyn under physiological conditions. Finally, our work underscores the need for more pS129 antibodies that are not sensitive to neighboring PTMs and more thorough characterization and validation of existing and new antibodies.

C-terminal truncation modulates α-Synuclein's cytotoxicity and aggregation by promoting the interactions with membrane and chaperone

α-Synuclein (α-syn) is the main protein component of Lewy bodies, the major pathological hallmarks of Parkinson's disease (PD). C-terminally truncated α-syn is found in the brain of PD patients, reduces cell viability and tends to form fibrils. Nevertheless, little is known about the mechanisms underlying the role of C-terminal truncation on the cytotoxicity and aggregation of α-syn. Here, we use nuclear magnetic resonance spectroscopy to show that the truncation alters α-syn conformation, resulting in an attractive interaction of the N-terminus with membranes and molecular chaperone, protein disulfide isomerase (PDI). The truncated protein is more toxic to mitochondria than full-length protein and diminishes the effect of PDI on α-syn fibrillation. Our findings reveal a modulatory role for the C-terminus in the cytotoxicity and aggregation of α-syn by interfering with the N-terminus binding to membranes and chaperone, and provide a molecular basis for the pathological role of C-terminal truncation in PD pathogenesis.

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

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

PTEN-Induced Putative Kinase 1 Dysfunction Accelerates Synucleinopathy

Background:

Mutations in PTEN-induced putative kinase 1 (PINK1) cause autosomal recessive Parkinson’s disease (PD) and contribute to the risk of sporadic PD. However, the relationship between PD-related PINK1 mutations and alpha-synuclein (α-syn) aggregation—a main pathological component of PD—remains unexplored.

Objective:

To investigate whether α-syn pathology is exacerbated in the absence of PINK1 after α-syn preformed fibril (PFF) injection in a PD mouse model and its effects on neurodegeneration.

Methods:

In this study, 10-week-old Pink1 knockout (KO) and wildtype (WT) mice received stereotaxic unilateral striatal injection of recombinant mouse α-syn PFF. Then, α-syn pathology progression, inflammatory responses, and neurodegeneration were analyzed via immunohistochemistry, western blot analysis, and behavioral testing.

Results:

After PFF injection, the total α-syn levels significantly increased, and pathological α-syn was markedly aggregated in Pink1 KO mice compared with Pink1 WT mice. Then, earlier and more severe neuronal loss and motor deficits occurred. Moreover, compared with WT mice, Pink1 KO mice had evident microglial/astrocytic immunoreactivity and prolonged astrocytic activation, and a higher rate of protein phosphatase 2A phosphorylation, which might explain the greater α-syn aggravation and neuronal death.

Conclusion:

The loss of Pink1 function accelerated α-syn aggregation, accumulation and glial activation, thereby leading to early and significant neurodegeneration and behavioral impairment in the PD mouse model. Therefore, our findings support the notion that PINK1 dysfunction increases the risk of synucleinopathy.

The Role of Extracellular Matrix Components in the Spreading of Pathological Protein Aggregates

Several neurodegenerative diseases are characterized by the accumulation of aggregated misfolded proteins. These pathological agents have been suggested to propagate in the brain via mechanisms similar to that observed for the prion protein, where a misfolded variant is transferred from an affected brain region to a healthy one, thereby inducing the misfolding and/or aggregation of correctly folded copies. This process has been characterized for several proteins, such as α-synuclein, tau, amyloid beta (Aβ) and less extensively for huntingtin and TDP-43. α-synuclein, tau, TDP-43 and huntingtin are intracellular proteins, and their aggregates are located in the cytosol or nucleus of neurons. They have been shown to spread between cells and this event occurs, at least partially, via secretion of these protein aggregates in the extracellular space followed by re-uptake. Conversely, Aβ aggregates are found mainly extracellularly, and their spreading occurs in the extracellular space between brain regions. Due to the inherent nature of their spreading modalities, these proteins are exposed to components of the extracellular matrix (ECM), including glycans, proteases and core matrix proteins. These ECM components can interact with or process pathological misfolded proteins, potentially changing their properties and thus regulating their spreading capabilities. Here, we present an overview of the documented roles of ECM components in the spreading of pathological protein aggregates in neurodegenerative diseases with the objective of identifying the current gaps in knowledge and stimulating further research in the field. This could potentially lead to the identification of druggable targets to slow down the spreading and/or progression of these pathologies.

α-Synuclein phase separation and amyloid aggregation are modulated by C-terminal truncations

The aggregation of α-synuclein (α-Syn) is a key pathological hallmark of Parkinson's disease (PD). α-Syn undergoes liquid-liquid phase separation (LLPS) to drive amyloid aggregation. How the LLPS of α-Syn is regulated remains largely unknown. Here, we discovered that the C-terminal region modulates α-Syn phase separation through electrostatic interactions. The wild-type (WT) and PD disease-related truncated α-Syn can co-exist in the condensates. The truncated α-Syn could dramatically promote WT α-Syn phase separation. Further studies demonstrated that the truncated α-Syn accelerated WT α-Syn turning to amyloid aggregates by modulation of phase separation. Together, our findings disclose the role of the C-terminal domain in the LLPS of α-Syn and pave the path for understanding the mechanism of truncated α-Syn in PD pathology.

Mechanisms of enhanced aggregation and fibril formation of Parkinson's disease-related variants of α-synuclein

Aggregation of α-synuclein (α-syn) into amyloid fibrils is closely associated with Parkinson’s disease (PD). Familial mutations or posttranslational truncations in α-syn are known as risk factor for PD. Here, we examined the effects of the PD-related A30P or A53T point mutation and C-terminal 123–140 or 104–140 truncation on the aggregating property of α-syn based on the kinetic and thermodynamic analyses. Thioflavin T fluorescence measurements indicated that A53T, Δ123‒140, and Δ104–140 variants aggregated faster than WT α-syn, in which the A53T mutation markedly increases nucleation rate whereas the Δ123‒140 or Δ104‒140 truncation significantly increases both nucleation and fibril elongation rates. Ultracentrifugation and western blotting analyses demonstrated that these mutations or truncations promote the conversion of monomer to aggregated forms of α-syn. Analysis of the dependence of aggregation reaction of α-syn variants on the monomer concentration suggested that the A53T mutation enhances conversion of monomers to amyloid nuclei whereas the C-terminal truncations, especially the Δ104–140, enhance autocatalytic aggregation on existing fibrils. In addition, thermodynamic analysis of the kinetics of nucleation and fibril elongation of α-syn variants indicated that both nucleation and fibril elongation of WT α-syn are enthalpically and entropically unfavorable. Interestingly, the unfavorable activation enthalpy of nucleation greatly decreases for the A53T and becomes reversed in sign for the C-terminally truncated variants. Taken together, our results indicate that the A53T mutation and the C-terminal truncation enhance α-syn aggregation by reducing unfavorable activation enthalpy of nucleation, and the C-terminal truncation further triggers the autocatalytic fibril elongation on the fibril surfaces.

Change in the Oligomeric State of α-Synuclein Variants in Living Cells

The accumulation of β-sheet-rich α-synuclein (α-Syn) protein in human brain cells is a pathological hallmark of Parkinson's disease (PD). Moreover, it has been reported that familial PD mutations (A30P, E46K, H50Q, G51D, and A53T) accumulate at an accelerated rate both in vivo and in vitro. In addition, accumulations of various C-terminal α-Syn truncations, such as C-terminal-truncated N103 α-synuclein (N103), were found in an aggregated form in the brain tissue of PD patients. Fluorescent protein-tagged wild-type α-Syn, A30P, E46K, H50Q, G51D, A53T, and N103 were transfected into HEK293T and SHSY5Y cells, and their diffusion behaviors were investigated with a custom-built fluorescence microscope system. Based on our experimental results, the oligomerization of α-Syn is a time-dependent process in both HEK293T and SHSY5Y cells, and the oligomer state approaches a plateau after 48 h of transfection. The change in the oligomeric state of E46K, H50Q, and G51D exhibited a similar trend to the wild type at a lower concentration but became intense at a higher concentration. A53T and N103 possess smaller diffusion coefficients than wild-type α-synuclein and other family PD mutations, indicating that these two mutants could form higher oligomeric states or stronger interactions in HEK293T and SHSY5Y cells. In contrast, the smallest oligomer and the lowest intracellular interaction among all investigated α-Syn variants were found for A30P. These phenomena indicated the presence of different pathogeneses among familial PD mutants and C-terminal α-Syn truncations.

The C-terminal tail of α-synuclein protects against aggregate replication but is critical for oligomerization

Aggregation of the 140-residue protein α-synuclein (αSN) is a key factor in the etiology of Parkinson’s disease. Although the intensely anionic C-terminal domain (CTD) of αSN does not form part of the amyloid core region or affect membrane binding ability, truncation or reduction of charges in the CTD promotes fibrillation through as yet unknown mechanisms. Here, we study stepwise truncated CTDs and identify a threshold region around residue 121; constructs shorter than this dramatically increase their fibrillation tendency. Remarkably, these effects persist even when as little as 10% of the truncated variant is mixed with the full-length protein. Increased fibrillation can be explained by a substantial increase in self-replication, most likely via fragmentation. Paradoxically, truncation also suppresses toxic oligomer formation, and oligomers that can be formed by chemical modification show reduced membrane affinity and cytotoxicity. These remarkable changes correlate to the loss of negative electrostatic potential in the CTD and highlight a double-edged electrostatic safety guard.

Farzadfard et al. present a comprehensive analysis of a range of C-terminal truncations of aSN, linking the importance of high C-terminus charge for decreased fibrillation rates. The ability to formation oligomers, to disrupt synthetic vesicles and cell toxicity was reduced with truncated aSN, aiding in understanding of the intramolecular interactions of aSN which promote/inhibit aggregation.

Digesting recent findings: gut alpha-synuclein, microbiome changes in Parkinson's disease

Two hallmarks of Parkinson's disease (PD) are the widespread deposition of misfolded alpha-synuclein (αSyn) protein in the nervous system and loss of substantia nigra dopamine neurons. Recent research has suggested that αSyn aggregates in the enteric nervous system (ENS) lead to prodromal gastrointestinal (GI) symptoms such as constipation in PD, then propagating to the brain stem and eventually triggering neurodegeneration and motor symptoms. Additionally, whether the microbiome changes in PD contribute to the primary pathogenesis or, alternatively, are consequential to either the disease process or medication is still unclear. In this review, we discuss the possible roles of αSyn and microbiome changes in the GI system in PD and consider if and how the changes interact and contribute to the disease process and symptoms.

Targeting α-Synuclein in Parkinson's Disease by Induced Pluripotent Stem Cell Models

Parkinson's disease (PD) is a progressive, neurodegenerative disorder characterized by motor and non-motor symptoms. To date, no specific treatment to halt disease progression is available, only medication to alleviate symptoms can be prescribed. The main pathological hallmark of PD is the development of neuronal inclusions, positive for α-synuclein (α-syn), which are termed Lewy bodies (LBs) or Lewy neurites. However, the cause of the inclusion formation and the loss of neurons remain largely elusive. Various genetic determinants were reported to be involved in PD etiology, including SNCA, DJ-1, PRKN, PINK1, LRRK2, and GBA. Comprehensive insights into pathophysiology of PD critically depend on appropriate models. However, conventional model organisms fall short to faithfully recapitulate some features of this complex disease and as a matter-of-fact access to physiological tissue is limiting. The development of disease models replicating PD that are close to human physiology and dynamic enough to analyze the underlying molecular mechanisms of disease initiation and progression, as well as the generation of new treatment options, is an important and overdue step. Recently, the establishment of induced pluripotent stem cell (iPSC)-derived neural models, particularly from genetic PD-variants, developed into a promising strategy to investigate the molecular mechanisms regarding formation of inclusions and neurodegeneration. As these iPSC-derived neurons can be generated from accessible biopsied samples of PD patients, they carry pathological alterations and enable the possibility to analyze the differences compared to healthy neurons. This review focuses on iPSC models carrying genetic PD-variants of α-syn that will be especially helpful in elucidating the pathophysiological mechanisms of PD. Furthermore, we discuss how iPSC models can be instrumental in identifying cellular targets, potentially leading to the development of new therapeutic treatments. We will outline the enormous potential, but also discuss the limitations of iPSC-based α-syn models.

Truncation-Driven Lateral Association of α-Synuclein Hinders Amyloid Clearance by the Hsp70-Based Disaggregase

The aggregation of α-synuclein is the hallmark of a collective of neurodegenerative disorders known as synucleinopathies. The tendency to aggregate of this protein, the toxicity of its aggregation intermediates and the ability of the cellular protein quality control system to clear these intermediates seems to be regulated, among other factors, by post-translational modifications (PTMs). Among these modifications, we consider herein proteolysis at both the N- and C-terminal regions of α-synuclein as a factor that could modulate disassembly of toxic amyloids by the human disaggregase, a combination of the chaperones Hsc70, DnaJB1 and Apg2. We find that, in contrast to aggregates of the protein lacking the N-terminus, which can be solubilized as efficiently as those of the WT protein, the deletion of the C-terminal domain, either in a recombinant context or as a consequence of calpain treatment, impaired Hsc70-mediated amyloid disassembly. Progressive removal of the negative charges at the C-terminal region induces lateral association of fibrils and type B* oligomers, precluding chaperone action. We propose that truncation-driven aggregate clumping impairs the mechanical action of chaperones, which includes fast protofilament unzipping coupled to depolymerization. Inhibition of the chaperone-mediated clearance of C-truncated species could explain their exacerbated toxicity and higher propensity to deposit found in vivo.

The cellular modifier MOAG-4/SERF drives amyloid formation through charge complementation

While aggregation-prone proteins are known to accelerate aging and cause age-related diseases, the cellular mechanisms that drive their cytotoxicity remain unresolved. The orthologous proteins MOAG-4, SERF1A, and SERF2 have recently been identified as cellular modifiers of such proteotoxicity. Using a peptide array screening approach on human amyloidogenic proteins, we found that SERF2 interacted with protein segments enriched in negatively charged and hydrophobic, aromatic amino acids. The absence of such segments, or the neutralization of the positive charge in SERF2, prevented these interactions and abolished the amyloid-promoting activity of SERF2. In protein aggregation models in the nematode worm Caenorhabditis elegans, protein aggregation and toxicity were suppressed by mutating the endogenous locus of MOAG-4 to neutralize charge. Our data indicate that MOAG-4 and SERF2 drive protein aggregation and toxicity by interactions with negatively charged segments in aggregation-prone proteins. Such charge interactions might accelerate primary nucleation of amyloid by initiating structural changes and by decreasing colloidal stability. Our study points at charge interactions between cellular modifiers and amyloidogenic proteins as potential targets for interventions to reduce age-related protein toxicity.

Mechanistic Insight from Preclinical Models of Parkinson's Disease Could Help Redirect Clinical Trial Efforts in GDNF Therapy

Parkinson’s disease (PD) is characterized by four pathognomonic hallmarks: (1) motor and non-motor deficits; (2) neuroinflammation and oxidative stress; (3) pathological aggregates of the α-synuclein (α-syn) protein; (4) neurodegeneration of the nigrostriatal system. Recent evidence sustains that the aggregation of pathological α-syn occurs in the early stages of the disease, becoming the first trigger of neuroinflammation and subsequent neurodegeneration. Thus, a therapeutic line aims at striking back α-synucleinopathy and neuroinflammation to impede neurodegeneration. Another therapeutic line is restoring the compromised dopaminergic system using neurotrophic factors, particularly the glial cell-derived neurotrophic factor (GDNF). Preclinical studies with GDNF have provided encouraging results but often lack evaluation of anti-α-syn and anti-inflammatory effects. In contrast, clinical trials have yielded imprecise results and have reported the emergence of severe side effects. Here, we analyze the discrepancy between preclinical and clinical outcomes, review the mechanisms of the aggregation of pathological α-syn, including neuroinflammation, and evaluate the neurorestorative properties of GDNF, emphasizing its anti-α-syn and anti-inflammatory effects in preclinical and clinical trials.

α-Synuclein inhibits Snx3-retromer retrograde trafficking of the conserved membrane-bound proprotein convertase Kex2 in the secretory pathway of Saccharomyces cerevisiae

We tested the ability of alpha-synuclein (α-syn) to inhibit Snx3-retromer mediated retrograde trafficking of Kex2 and Ste13 between late endosomes and the trans-Golgi (TGN) using a Saccharomyces cerevisiae model of Parkinson's disease (PD). Kex2 and Ste13 are a conserved, membrane-bound proprotein convertase and dipeptidyl aminopeptidase, respectively, that process pro-α-factor and pro-killer toxin. Each of these proteins contains a cytosolic tail that binds to sorting nexin Snx3. Using a combination of techniques, including fluorescence microscopy, western blotting and a yeast mating assay, we found that α-syn disrupts Snx3-retromer trafficking of Kex2-GFP and GFP-Ste13 from the late endosome to the TGN, resulting in these two proteins transiting to the vacuole by default. Using three α-syn variants (A53T, A30P, and α-synΔC, which lacks residues 101-140), we further found that A53T and α-synΔC, but not A30P, reduce Snx3-retromer trafficking of Kex2-GFP, which is likely to be due to weaker binding of A30P to membranes. Degradation of Kex2 and Ste13 in the vacuole should result in the secretion of unprocessed, inactive forms of α-factor, which will reduce mating efficiency between MATa and MATα cells. We found that wild-type α-syn but not A30P significantly inhibited the secretion of α-factor. Collectively, our results support a model in which the membrane-binding ability of α-syn is necessary to disrupt Snx3-retromer retrograde recycling of these two conserved endopeptidases.

Similarity of the non-amyloid-β component and C-terminal tail of monomeric and tetrameric alpha-synuclein with 14-3-3 sigma

Alpha-synuclein (αSyn) is often described as a predominantly disordered protein that has a propensity to self-assemble into toxic oligomers that are found in patients with Parkinson's and Alzheimer's diseases. αSyn's chaperone behavior and tetrameric structure are proposed to be protective against toxic oligomerization. In this paper, we extended the previously proposed similarity between αSyn and 14-3-3 proteins to the α-helical tetrameric species of αSyn in detail. 14-3-3 proteins are a family of well-folded proteins with seven human isoforms, and function in signal transduction and as molecular chaperones. We investigated protein homology, using sequence alignment, amyloid, and disorder prediction, as well as three-dimensional visualization and protein-interaction networks. Our results show sequence homology and structural similarity between the aggregation-prone non-amyloid-β component (NAC) residues Val-52 to Gly-111 in αSyn and 14-3-3 sigma residues Leu-12 to Gly-78. We identified an additional region of sequence homology in the C-terminal region of αSyn (residues Ser-129 to Asp-135) and a C-terminal loop of 14-3-3 between helix αH and αI (residues Ser-209 to Asp-215). This data indicates αSyn shares conserved domain architecture with small heat shock proteins. We show predicted regions of high amyloidogenic propensity and intrinsic structural disorder in αSyn coincide with amyloidogenic and disordered predictions for 14-3-3 proteins. The homology in the NAC region aligns with residues involved in dimer- and tetramerization of the non-amyloidogenic 14-3-3 proteins. Because 14-3-3 proteins are generally not prone to misfolding, our results lend further support to the hypothesis that the NAC region is critical to the assembly of αSyn into the non-toxic tetrameric state.

The N terminus of α-synuclein dictates fibril formation

The generation of α-synuclein (α-syn) truncations from incomplete proteolysis plays a significant role in the pathogenesis of Parkinson's disease. It is well established that C-terminal truncations exhibit accelerated aggregation and serve as potent seeds in fibril propagation. In contrast, mechanistic understanding of N-terminal truncations remains ill defined. Previously, we found that disease-related C-terminal truncations resulted in increased fibrillar twist, accompanied by modest conformational changes in a more compact core, suggesting that the N-terminal region could be dictating fibril structure. Here, we examined three N-terminal truncations, in which deletions of 13-, 35-, and 40-residues in the N terminus modulated both aggregation kinetics and fibril morphologies. Cross-seeding experiments showed that out of the three variants, only ΔN13-α-syn (14‒140) fibrils were capable of accelerating full-length fibril formation, albeit slower than self-seeding. Interestingly, the reversed cross-seeding reactions with full-length seeds efficiently promoted all but ΔN40-α-syn (41-140). This behavior can be explained by the unique fibril structure that is adopted by 41-140 with two asymmetric protofilaments, which was determined by cryogenic electron microscopy. One protofilament resembles the previously characterized bent β-arch kernel, comprised of residues E46‒K96, whereas in the other protofilament, fewer residues (E61‒D98) are found, adopting an extended β-hairpin conformation that does not resemble other reported structures. An interfilament interface exists between residues K60‒F94 and Q62‒I88 with an intermolecular salt bridge between K80 and E83. Together, these results demonstrate a vital role for the N-terminal residues in α-syn fibril formation and structure, offering insights into the interplay of α-syn and its truncations.

Disease-, region- and cell type specific diversity of α-synuclein carboxy terminal truncations in synucleinopathies

Synucleinopathies, including Parkinson's disease (PD), Lewy body dementia (LBD), Alzheimer's disease with amygdala restricted Lewy bodies (AD/ALB), and multiple system atrophy (MSA) comprise a spectrum of neurodegenerative disorders characterized by the presence of distinct pathological α-synuclein (αSyn) inclusions. Experimental and pathological studies support the notion that αSyn aggregates contribute to cellular demise and dysfunction with disease progression associated with a prion-like spread of αSyn aggregates via conformational templating. The initiating event(s) and factors that contribute to diverse forms of synucleinopathies remain poorly understood. A major post-translational modification of αSyn associated with pathological inclusions is a diverse array of specific truncations within the carboxy terminal region. While these modifications have been shown experimentally to induce and promote αSyn aggregation, little is known about their disease-, region- and cell type specific distribution. To this end, we generated a series of monoclonal antibodies specific to neo-epitopes in αSyn truncated after residues 103, 115, 119, 122, 125, and 129. Immunocytochemical investigations using these new tools revealed striking differences in the αSyn truncation pattern between different synucleinopathies, brain regions and specific cellular populations. In LBD, neuronal inclusions in the substantia nigra and amygdala were positive for αSyn cleaved after residues 103, 119, 122, and 125, but not 115. In contrast, in the same patients' brain αSyn cleaved at residue 115, as well as 103, 119 and 122 were abundant in the dorsal motor nucleus of the vagus. In patients with AD/ALB, these modifications were only weakly or not detected in amygdala αSyn inclusions. αSyn truncated at residues 103, 115, 119, and 125 was readily present in MSA glial cytoplasmic inclusions, but 122 cleaved αSyn was only weakly or not present. Conversely, MSA neuronal pathology in the pontine nuclei was strongly reactive to the αSyn x-122 neo-epitope but did not display any reactivity for αSyn 103 cleavage. These studies demonstrate significant disease-, region- and cell type specific differences in carboxy terminal αSyn processing associated with pathological inclusions that likely contributes to their distinct strain-like prion properties and promotes the diversity displayed in the degrees of these insidious diseases.

α-Synuclein-induced dysregulation of neuronal activity contributes to murine dopamine neuron vulnerability

Pathophysiological damages and loss of function of dopamine neurons precede their demise and contribute to the early phases of Parkinson's disease. The presence of aberrant intracellular pathological inclusions of the protein α-synuclein within ventral midbrain dopaminergic neurons is one of the cardinal features of Parkinson's disease. We employed molecular biology, electrophysiology, and live-cell imaging to investigate how excessive α-synuclein expression alters multiple characteristics of dopaminergic neuronal dynamics and dopamine transmission in cultured dopamine neurons conditionally expressing GCaMP6f. We found that overexpression of α-synuclein in mouse (male and female) dopaminergic neurons altered neuronal firing properties, calcium dynamics, dopamine release, protein expression, and morphology. Moreover, prolonged exposure to the D2 receptor agonist, quinpirole, rescues many of the alterations induced by α-synuclein overexpression. These studies demonstrate that α-synuclein dysregulation of neuronal activity contributes to the vulnerability of dopaminergic neurons and that modulation of D2 receptor activity can ameliorate the pathophysiology. These findings provide mechanistic insights into the insidious changes in dopaminergic neuronal activity and neuronal loss that characterize Parkinson's disease progression with significant therapeutic implications.