The structural differences between patient-derived α-synuclein strains dictate characteristics of Parkinson's disease, multiple system atrophy and dementia with Lewy bodies

Rational selection of the monoclonal α-synuclein antibody amlenetug (Lu AF82422) for the treatment of α-synucleinopathies

Amlenetug (Lu AF82422) is a human monoclonal antibody targeting α-synuclein in clinical development for multiple system atrophy. We describe a series of studies that characterize its functional properties and supported its selection as a viable clinical candidate. Amlenetug inhibits seeding induced in mouse primary neurons by various α-synuclein fibrillar assemblies and by aggregates isolated from MSA brain homogenate. In vivo, both co-injection of amlenetug with α-synuclein assemblies in mouse brain and peripheral administration inhibit α-synuclein seeding. Amlenetug inhibits uptake of α-synuclein seeds as well as accumulation of C-terminal truncated α-synuclein seeds and demonstrates binding to monomeric, aggregated, and truncated forms of human α-synuclein. The epitope of amlenetug was mapped to amino acids 112-117 and further characterized by crystallographic structure analysis. Based on our data, we hypothesize that targeting α-synuclein will potentially slow further disease progression by inhibiting further pathology development but be without impact on established pathology and symptoms.

Intracellular Inclusions Induced by Patient-Derived and Amplified α-Synuclein Aggregates Are Morphologically Indistinguishable

Lewy Body Disease (LBD) and Multiple System Atrophy (MSA) are synucleinopathies with distinct prognoses and neuropathologies, however, with overlapping clinical symptoms. Different disease characteristics are proposed to be determined by distinct conformations of alpha-synuclein (α-Syn) aggregates, which can self-propagate and spread between cells via a prion-like mechanism. The goal of this study is to investigate whether α-syn aggregates amplified from brain and CSF samples of LBD and MSA patients using the Seed Amplification Assay (SAA) maintain α-Syn seeding properties similar to those of α-syn aggregates derived from patients' brains. To address this, SAA-amplified and un-amplified α-Syn aggregates from LBD and MSA patients' brains, as well as SAA-amplified α-Syn aggregates from LBD and MSA patients' CSF samples, were used to treat synuclein biosensor cells, and induced intracellular α-Syn inclusions were analyzed by confocal microscopy. Our data indicate that induced α-Syn aggregates from LBD and MSA patients' brains have similar seeding properties and morphological characteristics in the α-Syn biosensor cells as those amplified from LBD and MSA patients' brains, as well as those amplified from LBD and MSA patients' CSF samples. In this study, we demonstrated that, regardless of the source of aggregates, the seeds from LBD and MSA produce cellular accumulation of α-Syn with distinct morphologies, confirming the presence of different conformational strains of α-Syn in LBD and MSA and allowing us to differentiate synucleinopathies based on the morphology of aggregates and seeding properties.

Fibril-seeded animal models of synucleinopathies: Pathological mechanisms, disease modeling, and therapeutic implications

Accumulating evidence suggests that prion-like spread of misfolded α-Synuclein (αSyn) underlies the pathological progression of Lewy body diseases (LBD). Animal models injected with αSyn preformed fibrils (PFFs) have provided strong evidence for the prion hypothesis in LBD. Moreover, αSyn PFFs can be administered to various hosts and regions, contributing to the elucidation of pathological mechanisms and disease modeling. These models have also been used to identify biomarkers and develop new disease-modifying therapies for LBD. In contrast, it remains unknown how the prion-like properties of αSyn contribute to the pathogenesis of multiple system atrophy (MSA). Recent studies indicate that conformationally distinct αSyn fibrils induce different pathological features in animals, supporting the strain hypothesis, which suggests that conformational variations in αSyn fibrils contribute to the clinicopathological heterogeneity in synucleinopathies. However, the study of disease-specific αSyn fibrils in pathological mechanisms and disease modeling is still in its early stages. This review aims to highlight recent advances in αSyn fibril-seeded animal models with an emphasis on their unique features and utility in exploring pathological mechanisms and identifying novel disease-modifying therapies. In addition, I discuss future directions for refining these models in light of the emerging strain hypothesis in synucleinopathies.

Local Ionic Conditions Modulate the Aggregation Propensity and Influence the Structural Polymorphism of α-Synuclein

Parkinson's disease (PD) is linked to the aggregation of the intrinsically disordered protein α-synuclein (aSyn), but the precise triggers and mechanisms driving this process remain unclear. Local environmental factors, such as ion concentrations, can influence aSyn's conformational ensemble and its tendency to aggregate. In this study, we explore how physiologically relevant ions, mainly Ca2+ and Na+, affect aSyn aggregation, monomer structural dynamics, and fibril polymorphism. ThT fluorescence assays show that all ions speed up aggregation, with Ca2+ having the strongest effect. Using heteronuclear single quantum correlation nuclear magnetic resonance (1H-15N HSQC NMR) spectroscopy, we validate that Ca2+ binds at the C-terminus while Na+ interacts nonspecifically across the sequence. Small-angle neutron scattering (SANS) and hydrogen-deuterium exchange mass spectrometry (HDX-MS) show that Na+ leads to more extended aSyn structures, while Ca2+ results in moderate extension. Molecular dynamics (MD) simulations support this, showing Na+ increases extension between the NAC region and C-terminus, whereas Ca2+ biases the ensemble toward a moderately elongated structure. MD also shows that Ca2+ increases water persistence times in the hydration shell, indicating that aSyn aggregation propensity is due to a combination of conformational bias of the monomer and solvent mobility. Atomic force microscopy (AFM) points toward the formation of distinct fibril polymorphs under different ionic conditions, suggesting ion-induced monomer changes contribute to the diversity of fibril structures. These findings underscore the pivotal influence of the local ionic milieu in shaping the structure and aggregation propensity of aSyn, offering insights into the molecular underpinnings of PD and potential therapeutic strategies targeting aSyn dynamics.

Distinct alpha-synuclein strains derived from Parkinson's disease patient tissues trigger differential inclusion pathology in a novel biosensor cell model

Background

α-Synuclein (αSyn) can misfold and aggregate to form fibrillar ß-sheet-rich aggregates ("strains") that are phosphorylated (p-αSyn) and deposited into intracellular inclusions in the brain, the pathological hallmark of synucleinopathies including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). Previously, we reported that seed amplification assays such as real-time quaking-induced conversion (RT-QuIC) amplifies and detects αSyn strains from the patient skin. However, whether skin-derived αSyn strains induce disease-specific pathological features in a biological system is unknown.

Methods

We generated a U251 human glioblastoma cell line expressing fluorescently tagged αSyn carrying the PD-linked A53T mutation and Förster resonance energy transfer (FRET)-based U251 biosensor cells. Using fluorescence microscopy coupled with in situ detergent extraction, FRET-Flow cytometry and high-content confocal imaging, we examined the pathological burden and morphology of p-αSyn inclusions seeded by RT-QuIC-amplified patient skin and brain αSyn strains in αSyn-expressing U251 cells, FRET-based αSyn biosensor cells and αSyn biosensor cell-derived neurons.

Results

U251 cells allow robust and rapid in situ detection of detergent-insoluble intracellular αSyn inclusions triggered by exogenous αSyn seeds. In U251 FRET-based biosensor cells, PD skin-amplified strains induce a greater pathological burden and distinct p-αSyn inclusion morphology from PD brain-amplified and DLB skin-amplified strains. Inclusion morphology of DLB and MSA skin- and brain-amplified strains are comparable. Furthermore, skin-amplified αSyn strains induce neuronal inclusions and cause degeneration of induced neurons reprogrammed from the U251 biosensor cells. Finally, biosensor cell-propagated PD skin αSyn strains induce higher seeding activity measured by RT-QuIC than PD brain and DLB skin αSyn strains, linking intracellular pathological burden to in vitro seeding activity.

Conclusions

We report the detection of distinct PD strains derived from patient skin and brain tissues that trigger unique pathological phenotypes in U251 αSyn biosensor cells and cause degeneration of reprogrammed neurons from the same cell lineage. Moreover, DLB and MSA skin αSyn strains mimic pathological features of their brain αSyn strains in these biosensor cells. Therefore, the U251 αSyn biosensor cell model is a robust tool to potentially discriminate PD and DLB synucleinopathies and to study αSyn tissue- and strain-specific etiology and pathogenesis.

Graphical abstract

Pathomechanisms of neuropsychiatric disturbances in atypical parkinsonian disorders: a current view

Multiple system atrophy (MSA), corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP) are the most common atypical parkinsonisms. These adult-onset and lethal neurodegenerative disorders of unknown etiology are clinically characterized by varying combinations of autonomic, levodopa-poorly responsive parkinsonsm, motor, non-motor, cerebellar syndromes, behavioral, cognitive and other neuropsychiatric disorders. Although their pathological hallmarks are different-MSA α-synucleinopathy, CBD and PSP 4-repeat (4R) tauopathies-their neuropsychiatric disturbances include anxiety, depression, agitations, attention-executive dysfunctions, less often compulsive and REM sleep behavior disorders (RBD), which may contribute to disease progression and reduced quality of life (QoL) of patients and caregivers. The present paper reviews the prevalence and type of neuropsychiatric profile in these atypical parkinsonian syndromes, their neuroimaging, and pathogenic backgrounds based on extensive literature research. MSA patients show anxiety, apathy (depression), initial RBD, attentional and executive dysfunction; PSP patients present with apathy, depression, disinhibition, and to a lesser extent, anxiety and agitation; CBD patients are featured by executive and visuospatial dysfunctions, irritability, alien limb phenomena, sleep and language disorders. Neuropsychiatric disorders in these syndromes are often similar, due to disruption of prefronto-subcortical (limbic) and striato-thalamo-cortical circuitries or default mode and attention network disorder. This supports the concept that they are brain network disorders due to complex pathogenic mechanisms related to the basic proteinopathies that are still poorly understood. Psychotic symptoms, hallucinations and delusions are rare. Neuropsychiatric changes in these disorders are often premature and anticipate motor dysfunctions; their assessment and further elucidation of their pathogenesis are warranted as a basis for early diagnosis and adequate treatment of these debilitating comorbidities.

A reciprocal relationship between markers of genomic DNA damage and alpha-synuclein pathology in dementia with Lewy bodies

BACKGROUND

DNA damage and DNA damage repair (DDR) dysfunction are insults with broad implications for cellular physiology and have been implicated in various neurodegenerative diseases. Alpha-synuclein (aSyn), a pre-synaptic and nuclear protein associated with neurodegenerative disorders known as synucleinopathies, has been associated with DNA double strand break (DSB) repair. However, although nuclear aSyn pathology has been observed in cortical tissue of dementia with Lewy body (DLB) cases, whether such nuclear pathology coincides with the occurrence of DNA damage has not previously been investigated. Moreover, the specific types of DNA damage elevated in DLB cases and the contribution of DNA damage towards Lewy body (LB) formation is unknown.

METHODS

DNA damage and aSyn pathology were assessed in fixed lateral temporal cortex from clinically and neuropathologically confirmed DLB cases and controls, as well as in cortical tissue from young 3-month-old presymptomatic A30P-aSyn mice. Frozen lateral temporal cortex from DLB and control cases was subject to nuclear isolation, western blotting, aSyn seed amplification and proteomic characterisation via mass spectrometry.

RESULTS

We detected seed-competent nuclear aSyn, and elevated nuclear serine-129 phosphorylation in DLB temporal cortex, alongside the accumulation of DSBs in neuronal and non-neuronal cellular populations. DNA damage was also present in cortical tissue from presymptomatic A30P mice, demonstrating it is an early insult closely associated with pathogenic aSyn. Strikingly, in postmortem DLB tissue, markers of genomic DNA damage-derived cytoplasmic DNA (CytoDNA) were evident within the majority of LBs examined. The observed cellular pathology was consistent with nuclear upregulation of associated DDR proteins, particularly those involved in base excision repair and DSB repair pathways.

CONCLUSIONS

Collectively our study demonstrates the accumulation of seed-competent pathological nuclear associated aSyn, alongside nuclear DNA damage and the potential involvement of DNA damage derived cytoDNA species in cytoplasmic aSyn pathology. Ultimately, our study supports the hypothesis of a reciprocal relationship between aSyn pathology and nuclear DNA damage and highlights a potential underlying role for DNA damage in pathological mechanisms relevant to DLB, as well as other synucleinopathies, opening novel possibilities for diagnosis and treatment.

Neuropathology meets chemical and genetic pathology head-on: a personal perspective

Medical science has made revolutionary discoveries around the nosology and aetiology of the neurodegenerative diseases. Dementia is the second leading cause of death in Australia. My colleagues and I are now looking at therapeutics which potentially can delay or prevent the onset of Alzheimer's disease (AD). Advances in diagnosis allow detection of preclinical AD. Neuropathologists working closely with chemical and genetic pathologists have a major role to play in pushing diagnostics and therapeutics to the forefront of clinical management of these diseases.

Fibrinogen degradation products exacerbate alpha-synuclein aggregation by inhibiting autophagy via downregulation of Beclin1 in multiple system atrophy

Multiple system atrophy (MSA) is a rapidly progressive neurodegenerative disease arising from accumulation of the α-synuclein and aberrant protein clearance in oligodendrocytes. The mechanisms of autophagy involved in the progression of MSA remain poorly understood. It is reported that MSA patients have blood-brain barrier impairments, which may increase the entry of fibrinogen into the brain. However, the roles of fibrinogen and its degradation products (FDPs) on autophagy and α-synuclein accumulation in MSA remain unknown. Here, we established the MSA animal model by intraperitoneal injection of 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP) and 3-nitropropionic acid (3-NP), and cellular models by adding fibrillar α-syn into oligodendrocytes to investigate the mechanisms of FDPs on autophagy and accumulation of α-synuclein in oligodendrocytes. We found that FDPs inhibit the entry of α-synuclein into lysosomes for degradation, increasing aggregation of α-synuclein in oligodendrocytes (OLN-93). Our findings indicated that in OLN-93, FDPs inhibited the expressions of Beclin1 and Bif-1, which could promote the fusion of autophagosomes with lysosomes. Furthermore, the expression of α-synuclein was elevated in FDPs-injected mice, accompanied by an increase in the protein level of p62. We detected elevated expression of FDPs in the striatum of MSA mice. Finally, FDPs inhibited the expression of Beclin1 and Bif-1, which led to aberrant autophagic degradation and increased aggregation of α-synuclein and phospho-α-synuclein in MSA mice. Our study illustrates that FDPs can cause aggregation of α-synuclein in MSA by inhibiting Beclin1-mediated autophagy, which may exacerbate disease progression. These results provide a new therapeutic approach for MSA, that targets the inhibitory effect of FDPs on oligodendrocyte autophagy.

Transcriptional dysregulation in the cerebellum triggered by oligodendroglial α-synucleinopathy: insights from a transgenic mouse into the early disease mechanisms of MSA

Multiple system atrophy (MSA) is a fatal neurodegenerative disorder characterized by abnormal accumulation of α-synuclein, progressive neuronal loss, motor impairment and widespread pathological changes, which include significant involvement of the cerebellum. To understand the early molecular mechanisms that might underlie α-synuclein-triggered MSA cerebellar pathology, we performed RNA sequencing (RNA-Seq) of cerebellar samples from a well-established model of MSA. RNA-Seq and differential gene expression analysis was conducted in the PLP-αSyn model of MSA. Cerebellum from two and 12-month-old MSA and wildtype mice were used. Gene ontology (GO) and KEGG enrichment analyses of the differentially expressed genes (DEGs) were performed to explore processes involved in MSA-like disease progression. The overlap between transcriptional changes in MSA and those associated with aging was also evaluated. RNA-Seq analysis demonstrated significant transcriptional dysregulation in cerebellum from MSA mice, even at early stages. GO and KEGG analyses of DEGs point to a potential role of synaptic dysfunction, cellular signaling dysregulation and inflammation in the cerebellar pathology of MSA mice. In addition, those changes exacerbate with disease progression. Additionally, our analysis of aging in both control and PLP-αSyn mice showed that age-related transcriptional changes in mid-aged controls seem to be present in young MSA mice. Thus, MSA-like pathology might lead to an acceleration of aging-related mechanisms. Our findings demonstrate significant cerebellar transcriptional dysregulation triggered by oligodendroglial α-synucleinopathy in PLP-αSyn mice, revealing pathways that might be critical for the early cerebellar pathology of MSA, and that may serve as potential molecular targets for therapeutic interventions in this devastating disorder.

Deciphering motor dysfunction and microglial activation in mThy1-α-synuclein mice: a comprehensive study of behavioral, gene expression, and methylation changes

Introduction

Growing recognition of microglia's role in neurodegenerative disorders has accentuated the need to characterize microglia profiles and their influence on pathogenesis. To understand changes observed in the microglial profile during the progression of synucleinopathies, microglial gene expression and DNA methylation were examined in the mThy1-α-synuclein mouse model.

Methods

Disease progression was determined using behavioral tests evaluating locomotor deficits before DNA and RNA extraction at 7 and 10 months from isolated microglia for enzymatic methyl-sequencing and RNA-sequencing.

Results

Pathway analysis of these changes at 7 months indicates a pro-inflammatory profile and changes in terms related to synaptic maintenance. Expression and methylation at both 7 and 10 months included terms regarding mitochondrial and metabolic stress. While behavior symptoms progressed at 10 months, we see many previously activated pathways being inhibited in microglia at a later stage, with only 8 of 53 shared pathways predicted to be directionally concordant. Despite the difference in pathway directionality, 21 of the 22 genes that were differentially expressed and annotated to differentially methylated regions at both 7 and 10 months had conserved directionality changes.

Discussion

These results highlight a critical period in disease progression, during which the microglia respond to α-synuclein, suggesting a transition in the role of microglia from the early to late stages of the disease.

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.

Biological definitions of synucleinopathies should be anchored in clinical trajectories and encompass the complex biology of the disease

Recently, two proposals for defining Parkinson's disease and its related pathogenic processes have been published. In this viewpoint, we discuss the primary drivers behind these efforts, the future directions, and the challenges that must be addressed. While finding biomarkers is a mandatory step for better precision medicine and optimal patient stratification in therapeutic trials, we argue that a biological definition of Parkinson's disease based on a single biomarker will struggle to account for the complexity of the mechanisms involved in developing the disease. Additionally, a biological definition of asymptomatic patients should rely on a thorough understanding of patients' clinical trajectories, which is currently not the case in synucleinopathies.

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.

The Spectrum of Cognitive Impairment in Atypical Parkinsonism Syndromes: A Comprehensive Review of Current Understanding and Research

Multiple system atrophy (MSA), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD) are the most common atypical parkinsonism (AP) syndromes. They are clinically characterized by varying combinations of levodopa-poorly responsive parkinsonism, motor, cerebellar, and other signs. They are associated with a wide spectrum of non-motor symptoms, including prominent cognitive impairment such as global cognitive deficits, memory, executive, attentional, visuospatial, language, and non-verbal reasoning dysfunctions. Within the APs, their cognitive functioning is distributed along a continuum from MSA with the least impaired cognitive profile (similar to Parkinson's disease) to PSP and CBD with the greatest decline in global cognitive and executive domains. Although their pathological hallmarks are different-MSA α-synucleinopathy, CBD, and PSP 4-repeat tauopathies-cognitive dysfunctions in APs show both overlaps and dissimilarities. They are often preceding and anticipate motor dysfunctions, finally contributing to reduced quality of life of patients and caregivers. The present paper will review the current evidence of the prevalence and type of cognitive impairment in these AP syndromes, their neuroimaging, pathogenic backgrounds, and current management options based on extensive literature research. Cognitive dysfunctions in APs are due to disruption of prefronto-subcortical and striato-thalamo-cortical circuitries and multiple essential brain networks. This supports the concept that they are brain network disorders due to complex pathogenic mechanisms related to the basic proteinopathies that are still poorly understood. Therefore, the pathophysiology and pathogenesis of cognitive impairment in APs deserve further elucidation as a basis for early diagnosis and adequate treatment of these debilitating comorbidities.

Recursive seed amplification detects distinct α-synuclein strains in cerebrospinal fluid of patients with Parkinson's disease

Parkinson's disease (PD) is a heterogeneous neurodegenerative disorder with a wide range of clinical phenotypes. Pathologically, it is characterized by neuronal inclusions containing misfolded, fibrillar alpha-synuclein (aSyn). Prion-like properties of aSyn contribute to the spread of aSyn pathology throughout the nervous system as the disease progresses. Utilizing these properties, seed amplification assays (SAA) enable the detection of aSyn pathology in living patients. We hypothesized that structurally distinct aSyn aggregates, or strains, may underlie the clinical heterogeneity of PD. To test this hypothesis, we recursively amplified aSyn fibrils from the cerebrospinal fluid (CSF) of 54 patients (34 people with PD and 20 controls). These fibrils were then characterized regarding SAA kinetic properties and detergent resistance. In addition, cultured cells were transfected with SAA products, and the extent of seeded aSyn pathology was quantified by staining for phosphorylated aSyn followed by automated high-throughput microscopy and image analysis. We found that fibrils, amplified from CSF by recursive SAA, exhibit two types of distinct biophysical properties and have different seeding capacities in cells. These properties are associated with clinical parameters and may therefore help explain the clinical heterogeneity in PD. Measuring aSyn strains may be relevant for prognosis and for therapies targeting aSyn pathology.

Multiple System Atrophy: Pathology, Pathogenesis, and Path Forward

Multiple system atrophy (MSA) is a fatal neurodegenerative disease characterized by autonomic failure and motor impairment. The hallmark pathologic finding in MSA is widespread oligodendroglial cytoplasmic inclusions rich in aggregated α-synuclein (αSyn). MSA is widely held to be an oligodendroglial synucleinopathy, and we outline lines of evidence to support this assertion, including the presence of early myelin loss. However, we also consider emerging data that support the possibility of neuronal or immune dysfunction as a primary driver of MSA. These hypotheses are placed in the context of a major recent discovery that αSyn is conformationally distinct in MSA versus other synucleinopathies such as Parkinson's disease. We outline emerging techniques in epidemiology, genetics, and molecular pathology that will shed more light on this mysterious disease. We anticipate a future in which cutting-edge developments in personalized disease modeling, including with pluripotent stem cells, bridge mechanistic developments at the bench and real benefits at the bedside.

α-Synuclein pathology as a target in neurodegenerative diseases

α-Synuclein misfolds into pathological forms that lead to various neurodegenerative diseases known collectively as α-synucleinopathies. In this Review, we provide a comprehensive overview of pivotal advances in α-synuclein research. We examine structural features and physiological functions of α-synuclein and summarize current insights into key post-translational modifications, such as nitration, phosphorylation, ubiquitination, sumoylation and truncation, considering their contributions to neurodegeneration. We also highlight the existence of disease-specific α-synuclein strains and their mechanisms of pathological spread, and discuss seed amplification assays and PET tracers as emerging diagnostic tools for detecting pathological α-synuclein in clinical settings. We also discuss α-synuclein aggregation and clearance mechanisms, and review cell-autonomous and non-cell-autonomous processes that contribute to neuronal death, including the roles of adaptive and innate immunity in α-synuclein-driven neurodegeneration. Finally, we highlight promising therapeutic approaches that target pathological α-synuclein and provide insights into emerging areas of research.

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.

Multiple system atrophy: advances in pathophysiology, diagnosis, and treatment

Multiple system atrophy is an adult-onset, sporadic, and progressive neurodegenerative disease. People with this disorder report a wide range of motor and non-motor symptoms. Overlap in the clinical presentation of multiple system atrophy with other movement disorders (eg, Parkinson's disease and progressive supranuclear palsy) is a concern for accurate and timely diagnosis. Over the past 5 years, progress has been made in understanding key pathophysiological events in multiple system atrophy, including the seeding of α-synuclein inclusions and the detection of disease-specific α-synuclein strains. Diagnostic criteria were revised in 2022 with the intention to improve the accuracy of a diagnosis of multiple system atrophy, particularly for early disease stages. Early signals of efficacy in clinical trials have indicated the potential for disease-modifying therapies for multiple system atrophy, although no trial has yet provided unequivocal evidence of neuroprotection in this rare disease. The advances in pathophysiology could play a part in biomarker discovery for early diagnosis as well as in the development of disease-modifying therapies.

Spatial-temporal dynamic evolution of lewy body dementia by metabolic PET imaging

PURPOSE

Lewy body dementia (LBD) is a neurodegenerative disease with high heterogeneity and complex pathogenesis. Our study aimed to use disease progression modeling to uncover spatial-temporal dynamic evolution of LBD in vivo, and to explore differential profiles of clinical features, glucose metabolism, and dopaminergic function among different evolution-related subtypes.

METHODS

A total of 123 participants (31 healthy controls and 92 LBD patients) who underwent 18F-FDG PET scans were retrospectively enrolled. 18F-FDG PET-based Subtype and Stage Inference (SuStaIn) model was established to illustrate spatial-temporal evolutionary patterns and categorize relevant subtypes. Then subtypes and stages were further related to clinical features, glucose metabolism, and dopaminergic function of LBD patients.

RESULTS

This 18F-FDG PET imaging-based approach illustrated two distinct patterns of neurodegenerative evolution originating from the neocortex and basal ganglia in LBD and defined them as subtype 1 and subtype 2, respectively. There were obvious differences between subtypes. Compared with subtype 1, subtype 2 exhibited a greater proportion of male patients (P = 0.045) and positive symptoms such as visual hallucinations (P = 0.033) and fluctuating cognitions (P = 0.033). Cognitive impairment, metabolic abnormalities, dopaminergic dysfunction and progression were all more severe in subtype 2 (all P < 0.05). In addition, a strong association was observed between SuStaIn subtypes and two clinical phenotypes (Parkinson's disease dementia and dementia with Lewy bodies) (P = 0.005).

CONCLUSIONS

Our findings based on 18F-FDG PET and data-driven model illustrated spatial-temporal dynamic evolution of LBD and categorized novel subtypes with different evolutionary patterns, clinical and imaging features in vivo. The evolution-related subtypes are associated with LBD clinical phenotypes, which supports the perspective of existence of distinct entities in LBD spectrum.

Fluid Biomarkers in Dementia Diagnosis

OBJECTIVE

This article familiarizes neurologists with the currently available CSF and plasma biomarkers for the diagnosis of dementia and diagnosis-dependent treatment decisions.

LATEST DEVELOPMENTS

For Alzheimer disease, the recent US Food and Drug Administration (FDA) approval of monoclonal antibody therapy has increased the urgency of confirming the pathologic diagnosis with biomarkers before initiating therapy. The new availability of disease-modifying therapies also highlights the need for biomarkers to monitor efficacy over time. Both of these needs have been partially addressed by the emergence of improved blood-based biomarkers for Alzheimer disease. Regarding other forms of dementia, the latest development is a CSF assay for aggregated α-synuclein, which permits the biomarker confirmation of synuclein pathology in Lewy body dementia.

ESSENTIAL POINTS

CSF biomarkers for the diagnosis of Alzheimer disease, Lewy body dementia, and Creutzfeldt-Jakob disease are well established. Blood-based biomarkers for dementia diagnosis are emerging and rapidly evolving. Sensitivity and specificity for diagnosis continue to improve, and they are being incorporated into diagnostic decisions. Fluid biomarkers for monitoring the efficacy of therapy are not yet established. Because serial CSF examinations are impractical, the validation of blood-based biomarkers of disease activity will be critical for addressing this unmet need.

Brain-derived and in vitro-seeded alpha-synuclein fibrils exhibit distinct biophysical profiles

The alpha-synuclein (αSyn) seeding amplification assay (SAA) that allows the generation of disease-specific in vitro seeded fibrils (SAA fibrils) is used as a research tool to study the connection between the structure of αSyn fibrils, cellular seeding/spreading, and the clinicopathological manifestations of different synucleinopathies. However, structural differences between human brain-derived and SAA αSyn fibrils have been recently highlighted. Here, we characterize the biophysical properties of the human brain-derived αSyn fibrils from the brains of patients with Parkinson's disease with and without dementia (PD, PDD), dementia with Lewy bodies (DLB), multiple system atrophy (MSA), and compare them to the 'model' SAA fibrils. We report that the brain-derived αSyn fibrils show distinct biochemical profiles, which were not replicated in the corresponding SAA fibrils. Furthermore, the brain-derived αSyn fibrils from all synucleinopathies displayed a mixture of 'straight' and 'twisted' microscopic structures. However, the PD, PDD, and DLB SAA fibrils had a 'straight' structure, whereas MSA SAA fibrils showed a 'twisted' structure. Finally, the brain-derived αSyn fibrils from all four synucleinopathies were phosphorylated (S129). Interestingly, phosphorylated αSyn were carried over to the PDD and DLB SAA fibrils. Our findings demonstrate the limitation of the SAA fibrils modeling the brain-derived αSyn fibrils and pay attention to the necessity of deepening the understanding of the SAA fibrillation methodology.

Distinct ultrastructural phenotypes of glial and neuronal alpha-synuclein inclusions in multiple system atrophy

Multiple system atrophy is characterized pathologically by the accumulation of alpha-synuclein (aSyn) into glial cytoplasmic inclusions (GCIs). The mechanism underlying the formation of GCIs is not well understood. In this study, correlative light and electron microscopy was employed to investigate aSyn pathology in the substantia nigra and putamen of post-mortem multiple system atrophy brain donors. Three distinct types of aSyn immuno-positive inclusions were identified in oligodendrocytes, neurons and dark cells presumed to be dark microglia. Oligodendrocytes contained fibrillar GCIs that were consistently enriched with lysosomes and peroxisomes, supporting the involvement of the autophagy pathway in aSyn aggregation in multiple system atrophy. Neuronal cytoplasmic inclusions exhibited ultrastructural heterogeneity resembling both fibrillar and membranous inclusions, linking multiple systems atrophy and Parkinson's disease. The novel aSyn pathology identified in the dark cells, displayed GCI-like fibrils or non-GCI-like ultrastructures suggesting various stages of aSyn accumulation in these cells. The observation of GCI-like fibrils within dark cells suggests these cells may be an important contributor to the origin or spread of pathological aSyn in multiple system atrophy. Our results suggest a complex interplay between multiple cell types that may underlie the formation of aSyn pathology in multiple system atrophy brain and highlight the need for further investigation into cell-specific disease pathologies in multiple system atrophy.

Mouse α-synuclein fibrils are structurally and functionally distinct from human fibrils associated with Lewy body diseases

The intricate process of α-synuclein aggregation and fibrillization holds pivotal roles in Parkinson's disease (PD) and multiple system atrophy (MSA). While mouse α-synuclein can fibrillize in vitro, whether these fibrils commonly used in research to induce this process or form can reproduce structures in the human brain remains unknown. Here, we report the first atomic structure of mouse α-synuclein fibrils, which was solved in parallel by two independent teams. The structure shows striking similarity to MSA-amplified and PD-associated E46K fibrils. However, mouse α-synuclein fibrils display altered packing arrangements, reduced hydrophobicity, and heightened fragmentation sensitivity and evoke only weak immunological responses. Furthermore, mouse α-synuclein fibrils exhibit exacerbated pathological spread in neurons and humanized α-synuclein mice. These findings provide critical insights into the structural underpinnings of α-synuclein pathogenicity and emphasize a need to reassess the role of mouse α-synuclein fibrils in the development of related diagnostic probes and therapeutic interventions.

α-Synuclein Strain Dynamics Correlate with Cognitive Shifts in Parkinson's Disease

α-Synuclein (α-syn) strains can serve as discriminators between Parkinson's disease (PD) from other α-synucleinopathies. The relationship between α-syn strain dynamics and clinical performance as patients transition from normal cognition (NC) to cognitive impairment (CI) is not known. Here, we show that the biophysical properties and neurotoxicity of α-syn strains change as PD cognitive status transitions from NC to mild cognitive impairment (PD-MCI) and dementia (PD-D). Both cross-sectional and longitudinal analyses reveal distinct α-syn strains in PD patients correlating to their level of cognitive impairment. This study presents evidence that individuals with PD have different α-syn strains that change in accordance with their cognitive status and highlights the potential of α-syn strain dynamics to guide future diagnosis, management, and stratification of PD patients.

Different neurotoxicity and seeding activity between α-synuclein oligomers formed in plasma of patients with Parkinson's disease and multiple system atrophy

Previous studies have shown that α-synuclein (α-Syn) aggregates derived from the brains of patients with Parkinson's disease (PD) and multiple system atrophy (MSA) exhibit different phosphorylation, cytotoxicity, and seeding activity. However, the mechanism underlying the differences remains poorly understood. Here, recombinant human α-Syn was incubated in the plasma of patients with PD and MSA, and the oligomers formed in the plasma (PD-O-α-Syn and MSA-O-α-Syn) were purified and analyzed for their phosphorylation, cytotoxicity and seeding activity. In vitro assays revealed that both PD-O-α-Syn and MSA-O-α-Syn were phosphorylated at serine 129. However, the phosphorylation degree of MSA-O-α-Syn was significantly higher than that of PD-O-α-Syn. In addition, MSA-O-α-Syn exhibited stronger cytotoxicity and seeding activity compared with PD-O-α-Syn. In vivo experiments showed that mice receiving intrastriatal inoculation of MSA-O-α-Syn developed more severe motor dysfunction and dopaminergic degeneration than mice receiving intrastriatal inoculation of PD-O-α-Syn. Compared with the mice inoculated with PD-O-α-Syn, the mice inoculated with MSA-O-α-Syn accumulated more phosphorylated and oligomerized α-Syn in the striatum and brain regions (substantia nigra, hippocampus and prefrontal cortex) away from the inoculated site. The results obtained suggest that α-Syn oligomers formed in PD and MSA plasma are different in phosphorylation, cytotoxicity, and seeding activity.

Single-Molecule Fingerprinting Reveals Different Growth Mechanisms in Seed Amplification Assays for Different Polymorphs of α-Synuclein Fibrils

α-Synuclein (αSyn) aggregates, detected in the biofluids of patients with Parkinson's disease (PD), have the ability to catalyze their own aggregation, leading to an increase in the number and size of aggregates. This self-templated amplification is used by newly developed assays to diagnose Parkinson's disease and turns the presence of αSyn aggregates into a biomarker of the disease. It has become evident that αSyn can form fibrils with slightly different structures, called "strains" or polymorphs, but little is known about their differential reactivity in diagnostic assays. Here, we compared the properties of two well-described αSyn polymorphs. Using single-molecule techniques, we observed that one of the polymorphs had an increased tendency to undergo secondary nucleation and we showed that this could explain the differences in reactivity observed in in vitro seed amplification assay and cellular assays. Simulations and high-resolution microscopy suggest that a 100-fold difference in the apparent rate of growth can be generated by a surprisingly low number of secondary nucleation "points" (1 every 2000 monomers added by elongation). When both strains are present in the same seeded reaction, secondary nucleation displaces proportions dramatically and causes a single strain to dominate the reaction as the major end product.

α-Synuclein Conformations in Plasma Distinguish Parkinson's Disease from Dementia with Lewy Bodies

Aggregation of misfolded α-synuclein (aSyn) within the brain is the pathologic hallmark of Lewy body diseases (LBD), including Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Evidence exists for aSyn "strains" - conformations with distinct biological properties. However, biomarkers for PD vs. DLB, including potential aSyn strain differences, are lacking. Here, we used two monoclonal antibodies selective for different in vitro aSyn species - termed Strain A and B - to evaluate human brain tissue, cerebrospinal fluid (CSF), and plasma. Surprisingly, levels of Strain A and B aSyn species differed in plasma from individuals with PD vs. DLB in two independent cohorts. Lower plasma aSyn Strain A species also predicted subsequent PD cognitive decline. Strain A and Strain B aSyn species were undetectable in CSF, but plasma aSyn species could template aSyn fibrillization, particularly in PD. Our findings suggest that aSyn strains may impact LBD clinical presentation and originate outside the brain.

Rapid iPSC inclusionopathy models shed light on formation, consequence, and molecular subtype of α-synuclein inclusions

The heterogeneity of protein-rich inclusions and its significance in neurodegeneration is poorly understood. Standard patient-derived iPSC models develop inclusions neither reproducibly nor in a reasonable time frame. Here, we developed screenable iPSC "inclusionopathy" models utilizing piggyBac or targeted transgenes to rapidly induce CNS cells that express aggregation-prone proteins at brain-like levels. Inclusions and their effects on cell survival were trackable at single-inclusion resolution. Exemplar cortical neuron α-synuclein inclusionopathy models were engineered through transgenic expression of α-synuclein mutant forms or exogenous seeding with fibrils. We identified multiple inclusion classes, including neuroprotective p62-positive inclusions versus dynamic and neurotoxic lipid-rich inclusions, both identified in patient brains. Fusion events between these inclusion subtypes altered neuronal survival. Proteome-scale α-synuclein genetic- and physical-interaction screens pinpointed candidate RNA-processing and actin-cytoskeleton-modulator proteins like RhoA whose sequestration into inclusions could enhance toxicity. These tractable CNS models should prove useful in functional genomic analysis and drug development for proteinopathies.

On the pH-dependence of α-synuclein amyloid polymorphism and the role of secondary nucleation in seed-based amyloid propagation

The aggregation of the protein α-synuclein is closely associated with several neurodegenerative disorders and as such the structures of the amyloid fibril aggregates have high scientific and medical significance. However, there are dozens of unique atomic-resolution structures of these aggregates, and such a highly polymorphic nature of the α-synuclein fibrils hampers efforts in disease-relevant in vitro studies on α-synuclein amyloid aggregation. In order to better understand the factors that affect polymorph selection, we studied the structures of α-synuclein fibrils in vitro as a function of pH and buffer using cryo-EM helical reconstruction. We find that in the physiological range of pH 5.8-7.4, a pH-dependent selection between Type 1, 2, and 3 polymorphs occurs. Our results indicate that even in the presence of seeds, the polymorph selection during aggregation is highly dependent on the buffer conditions, attributed to the non-polymorph-specific nature of secondary nucleation. We also uncovered two new polymorphs that occur at pH 7.0 in phosphate-buffered saline. The first is a monofilament Type 1 fibril that highly resembles the structure of the juvenile-onset synucleinopathy polymorph found in patient-derived material. The second is a new Type 5 polymorph that resembles a polymorph that has been recently reported in a study that used diseased tissues to seed aggregation. Taken together, our results highlight the shallow amyloid energy hypersurface that can be altered by subtle changes in the environment, including the pH which is shown to play a major role in polymorph selection and in many cases appears to be the determining factor in seeded aggregation. The results also suggest the possibility of producing disease-relevant structure in vitro.

Aptamer binding footprints discriminate α-synuclein fibrillar polymorphs from different synucleinopathies

Synucleinopathies, including dementia with Lewy bodies (DLB), Parkinson's disease (PD), and multiple system atrophy (MSA), are characterized by the presence of α-synuclein (α-syn) aggregates in the central nervous system. Recent evidence suggests that the heterogeneity of synucleinopathies may be partly explained by the fact that patients may have different α-syn fibrillar polymorphs with structural differences. In this study, we identify nuclease resistant 2'fluoro-pyrimidine RNA aptamers that can differentially bind to structurally distinct α-syn fibrillar polymorphs. Moreover, we introduce a method, AptaFOOT-Seq, designed to rapidly assess the affinity of a mixture of these aptamers for different α-SYN fibrillar polymorphs using next-generation sequencing. Our findings reveal that the binding behavior of aptamers can be very different when they are tested separately or in the presence of other aptamers. In this case, competition and cooperation can occur, providing a higher level of information, which can be exploited to obtain specific 'footprints' for different α-Syn fibrillar polymorphs. Notably, these footprints can distinguish polymorphs obtained from patients with PD, DLB or MSA. This result suggests that aptaFOOT-Seq could be used for the detection of misfolded or abnormal protein conformations to improve the diagnosis of synucleinopathies.

Protein modification in neurodegenerative diseases

Posttranslational modifications play a crucial role in governing cellular functions and protein behavior. Researchers have implicated dysregulated posttranslational modifications in protein misfolding, which results in cytotoxicity, particularly in neurodegenerative diseases such as Alzheimer disease, Parkinson disease, and Huntington disease. These aberrant posttranslational modifications cause proteins to gather in certain parts of the brain that are linked to the development of the diseases. This leads to neuronal dysfunction and the start of neurodegenerative disease symptoms. Cognitive decline and neurological impairments commonly manifest in neurodegenerative disease patients, underscoring the urgency of comprehending the posttranslational modifications' impact on protein function for targeted therapeutic interventions. This review elucidates the critical link between neurodegenerative diseases and specific posttranslational modifications, focusing on Tau, APP, α-synuclein, Huntingtin protein, Parkin, DJ-1, and Drp1. By delineating the prominent aberrant posttranslational modifications within Alzheimer disease, Parkinson disease, and Huntington disease, the review underscores the significance of understanding the interplay among these modifications. Emphasizing 10 key abnormal posttranslational modifications, this study aims to provide a comprehensive framework for investigating neurodegenerative diseases holistically. The insights presented herein shed light on potential therapeutic avenues aimed at modulating posttranslational modifications to mitigate protein aggregation and retard neurodegenerative disease progression.

20S proteasome enhancers prevent cytotoxic tubulin polymerization-promoting protein induced α-synuclein aggregation

Synucleinopathies are a class of neurodegenerative diseases defined by the presence of α-synuclein inclusions. The location and composition of these α-synuclein inclusions directly correlate to the disease pattern. The inclusions in Multiple System Atrophy are located predominantly in oligodendrocytes and are rich in a second protein, p25α. P25α plays a key role in neuronal myelination by oligodendrocytes. In healthy oligodendrocytes, there is little to no α-synuclein present. If aberrant α-synuclein is present, p25α leaves the myelin sheaths and quickly co-aggregates with α-synuclein, resulting in the disruption of the cellular process and ultimately cell death. Herein, we report that p25α is susceptible for 20S proteasome-mediated degradation and that p25α induces α-synuclein aggregation, resulting in proteasome impairment and cell death. In addition, we identified small molecules 20S proteasome enhancers that prevent p25α induced α-synuclein fibrilization, restore proteasome impairment, and enhance cell viability.

Cryo-EM structures of pathogenic fibrils and their impact on neurodegenerative disease research

Neurodegenerative diseases are commonly associated with the formation of aberrant protein aggregates within the brain, and ultrastructural analyses have revealed that the proteins within these inclusions often assemble into amyloid filaments. Cryoelectron microscopy (cryo-EM) has emerged as an effective method for determining the near-atomic structure of these disease-associated filamentous proteins, and the resulting structures have revolutionized the way we think about aberrant protein aggregation and propagation during disease progression. These structures have also revealed that individual fibril conformations may dictate different disease conditions, and this newfound knowledge has improved disease modeling in the lab and advanced the ongoing pursuit of clinical tools capable of distinguishing and targeting different pathogenic entities within living patients. In this review, we summarize some of the recently developed cryo-EM structures of ex vivo α-synuclein, tau, β-amyloid (Aβ), TAR DNA-binding protein 43 (TDP-43), and transmembrane protein 106B (TMEM106B) fibrils and discuss how these structures are being leveraged toward mechanistic research and therapeutic development.

Effect of host and strain factors on α-synuclein prion pathogenesis

Prion diseases are a group of neurodegenerative disorders caused by misfolding of proteins into pathogenic conformations that self-template to spread disease. Although this mechanism is largely associated with the prion protein (PrP) in classical prion diseases, a growing literature indicates that other proteins, including α-synuclein, rely on a similar disease mechanism. Notably, α-synuclein misfolds into distinct conformations, or strains, that cause discrete clinical disorders including multiple system atrophy (MSA) and Parkinson's disease (PD). Because the recognized similarities between PrP and α-synuclein are increasing, this review article draws from research on PrP to identify the host and strain factors that impact disease pathogenesis, predominantly in rodent models, and focuses on key considerations for future research on α-synuclein prions.

α-Synuclein seed amplification technology for Parkinson's disease and related synucleinopathies

Synucleinopathies are a group of neurodegenerative diseases (NDs) associated with cerebral accumulation of α-synuclein (αSyn) misfolded aggregates. At this time, there is no effective treatment to stop or slow down disease progression, which in part is due to the lack of an early and objective biochemical diagnosis. In the past 5 years, the seed amplification technology has emerged for highly sensitive identification of these diseases, even at the preclinical stage of the illness. Much research has been done in multiple laboratories to validate the efficacy and reproducibility of this assay. This article provides a comprehensive review of this technology, including its conceptual basis and its multiple applications for disease diagnosis, as well for understanding of the disease biology and therapeutic development.

α-Synuclein pathology from the body to the brain: so many seeds so close to the central soil

ABSTRACT

α-Synuclein is a protein that mainly exists in the presynaptic terminals. Abnormal folding and accumulation of α-synuclein are found in several neurodegenerative diseases, including Parkinson's disease. Aggregated and highly phosphorylated α-synuclein constitutes the main component of Lewy bodies in the brain, the pathological hallmark of Parkinson's disease. For decades, much attention has been focused on the accumulation of α-synuclein in the brain parenchyma rather than considering Parkinson's disease as a systemic disease. Recent evidence demonstrates that, at least in some patients, the initial α-synuclein pathology originates in the peripheral organs and spreads to the brain. Injection of α-synuclein preformed fibrils into the gastrointestinal tract triggers the gut-to-brain propagation of α-synuclein pathology. However, whether α-synuclein pathology can occur spontaneously in peripheral organs independent of exogenous α-synuclein preformed fibrils or pathological α-synuclein leakage from the central nervous system remains under investigation. In this review, we aimed to summarize the role of peripheral α-synuclein pathology in the pathogenesis of Parkinson's disease. We also discuss the pathways by which α-synuclein pathology spreads from the body to the brain.

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.

A novel mouse model for investigating α-synuclein aggregates in oligodendrocytes: implications for the glial cytoplasmic inclusions in multiple system atrophy

The aggregated alpha-synuclein (αsyn) in oligodendrocytes (OLGs) is one of the pathological hallmarks in multiple system atrophy (MSA). We have previously reported that αsyn accumulates not only in neurons but also in OLGs long after the administration of αsyn preformed fibrils (PFFs) in mice. However, detailed spatial and temporal analysis of oligodendroglial αsyn aggregates was technically difficult due to the background neuronal αsyn aggregates. The aim of this study is to create a novel mouse that easily enables sensitive and specific detection of αsyn aggregates in OLGs and the comparable analysis of the cellular tropism of αsyn aggregates in MSA brains. To this end, we generated transgenic (Tg) mice expressing human αsyn-green fluorescent protein (GFP) fusion proteins in OLGs under the control of the 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNP) promoter (CNP-SNCAGFP Tg mice). Injection of αsyn PFFs in these mice induced distinct GFP-positive aggregates in the processes of OLGs as early as one month post-inoculation (mpi), and their number and size increased in a centripetal manner. Moreover, MSA-brain homogenates (BH) induced significantly more oligodendroglial αsyn aggregates than neuronal αsyn aggregates compared to DLB-BH in CNP-SNCAGFP Tg mice, suggestive of their potential tropism of αsyn seeds for OLGs. In conclusion, CNP-SNCAGFP Tg mice are useful for studying the development and tropism of αsyn aggregates in OLGs and could contribute to the development of therapeutics targeting αsyn aggregates in OLGs.

Neither alpha-synuclein fibril strain nor host murine genotype influences seeding efficacy

Parkinson's disease (PD) is a neurodegenerative disease characterized by progressive motor symptoms and alpha-synuclein (αsyn) aggregation in the nervous system. For unclear reasons, PD patients with certain GBA1 mutations (GBA-PD) have a more aggressive clinical progression. Two testable hypotheses that can potentially account for this phenomenon are that GBA1 mutations promote αsyn spread or drive the generation of highly pathogenic αsyn polymorphs (i.e., strains). We tested these hypotheses by treating homozygous GBA1 D409V knockin (KI) mice with human α-syn-preformed fibrils (PFFs) and treating wild-type mice (WT) with several αsyn-PFF polymorphs amplified from brain autopsy samples collected from patients with idiopathic PD and GBA-PD patients with either homozygous or heterozygous GBA1 mutations. Robust phosphorylated-αsyn (PSER129) positive pathology was observed at the injection site (i.e., the olfactory bulb granule cell layer) and throughout the brain six months following PFF injection. The PFF seeding efficiency and degree of spread were similar regardless of the mouse genotype or PFF polymorphs. We found that PFFs amplified from the human brain, regardless of patient genotype, were generally more effective seeders than wholly synthetic PFFs (i.e., non-amplified); however, PFF concentration differed between these two studies, which might also account for the observed differences. To investigate whether the molecular composition of pathology differed between different seeding conditions, we performed Biotinylation by Antibody Recognition on PSER129 (BAR-PSER129). We found that for BAR-PSER129, the endogenous PSER129 pool dominated identified interactions, and thus, very few potential interactions were explicitly identified for seeded pathology. However, we found Dynactin Subunit 2 (Dctn2) interaction was shared across all PFF conditions, and NCK Associated Protein 1 (Nckap1) and Adaptor Related Protein Complex 3 Subunit Beta 2 (Ap3b2) were unique to PFFs amplified from GBA-PD brains of heterozygous mutation carriers. In conclusion, both the genotype and αsyn strain had little effect on overall seeding efficacy and global PSER129-interactions.

Nicotine-mediated therapy for Parkinson's disease in transgenic Caenorhabditis elegans model

Parkinson's disease resultant in the degeneration of Dopaminergic neurons and accumulation of α-synuclein in the substantia nigra pars compacta. The synthetic therapeutics for Parkinson's disease have moderate symptomatic benefits but cannot prevent or delay disease progression. In this study, nicotine was employed by using transgenic Caenorhabditis elegans Parkinson's disease models to minimize the Parkinson's disease symptoms. The results showed that the nicotine at 100, 150, and 200 μM doses reduced degeneration of Dopaminergic neurons caused by 6-hydroxydopamine (14, 33, and 40%), lowered the aggregative toxicity of α-synuclein by 53, 56, and 78%, respectively. The reduction in food-sensing behavioral disabilities of BZ555 was observed to be 18, 49, and 86%, respectively, with nicotine concentrations of 100 μM, 150 μM, and 200 μM. Additionally, nicotine was found to enhance Daf-16 nuclear translocation by 14, 31, and 49%, and dose-dependently increased SOD-3 expression by 10, 19, and 23%. In summary, the nicotine might a promising therapy option for Parkinson's disease.

Ligand Profiling as a Diagnostic Tool to Differentiate Patient-Derived α-Synuclein Polymorphs

Amyloid fibrils are characteristic of many neurodegenerative diseases, including Alzheimer's and Parkinson's diseases. While different diseases may have fibrils formed of the same protein, the supramolecular morphology of these fibrils is disease-specific. Here, a method is reported to distinguish eight morphologically distinct amyloid fibrils based on differences in ligand binding properties. Eight fibrillar polymorphs of α-synuclein (αSyn) were investigated: five generated de novo using recombinant αSyn and three generated using protein misfolding cyclic amplification (PMCA) of recombinant αSyn seeded with brain homogenates from deceased patients diagnosed with Parkinson's disease (PD), multiple system atrophy (MSA), and dementia with Lewy bodies (DLB). Fluorescence binding assays were carried out for each fibril using a toolkit of six different ligands. The fibril samples were separated into five categories based on a binary classification of whether they bound specific ligands or not. Quantitative binding measurements then allowed every fibrillar polymorph to be uniquely identified, and the PMCA fibrils derived from PD, MSA, and DLB patients could be unambiguously distinguished. This approach constitutes a novel and operationally simple method to differentiate amyloid fibril morphologies and to identify disease states using PMCA fibrils obtained by seeding with patient samples.

α-Synuclein Conformations in Plasma Distinguish Parkinson's Disease from Dementia with Lewy Bodies

Spread and aggregation of misfolded α-synuclein (aSyn) within the brain is the pathologic hallmark of Lewy body diseases (LBD), including Parkinson's disease (PD) and dementia with Lewy bodies (DLB). While evidence exists for multiple aSyn protein conformations, often termed "strains" for their distinct biological properties, it is unclear whether PD and DLB result from aSyn strain differences, and biomarkers that differentiate PD and DLB are lacking. Moreover, while pathological forms of aSyn have been detected outside the brain ( e.g., in skin, gut, blood), the functional significance of these peripheral aSyn species is unclear. Here, we developed assays using monoclonal antibodies selective for two different aSyn species generated in vitro - termed Strain A and Strain B - and used them to evaluate human brain tissue, cerebrospinal fluid (CSF), and plasma, through immunohistochemistry, enzyme-linked immunoassay, and immunoblotting. Surprisingly, we found that plasma aSyn species detected by these antibodies differentiated individuals with PD vs. DLB in a discovery cohort (UPenn, n=235, AUC 0.83) and a multi-site replication cohort (Parkinson's Disease Biomarker Program, or PDBP, n=200, AUC 0.72). aSyn plasma species detected by the Strain A antibody also predicted rate of cognitive decline in PD. We found no evidence for aSyn strains in CSF, and ability to template aSyn fibrillization differed for species isolated from plasma vs. brain, and in PD vs. DLB. Taken together, our findings suggest that aSyn conformational differences may impact clinical presentation and cortical spread of pathological aSyn. Moreover, the enrichment of these aSyn strains in plasma implicates a non-central nervous system source.

LETC inhibits α-Syn aggregation and ameliorates motor deficiencies in the L62 mouse model of synucleinopathy

Alpha-Synuclein (α-Syn) aggregation is a pathological feature of synucleinopathies, neurodegenerative disorders that include Parkinson's disease (PD). Here, we explored the efficacy of N,N,N',N'-tetraethyl-10H-phenothiazine-3,7-diamine dihydrochloride (LETC), a protein aggregation inhibitor, on α-Syn aggregation. In both cellular models and transgenic mice, α-Syn aggregation was achieved by the overexpression of full-length human α-Syn fused with a signal sequence peptide. α-Syn accumulated in transfected DH60.21 neuroblastoma cells and α-Syn aggregation was inhibited by LETC with an EC50 of 0.066 ± 0.047 μM. Full-length human α-Syn overexpressing Line 62 (L62) mice accumulated neuronal α-Syn that was associated with a decreased motor performance in the open field and automated home cage. LETC, administered orally for 6 weeks at 10 mg/kg significantly decreased α-Syn-positive neurons in multiple brain regions and this resulted in a rescue of movement deficits in the open field in these mice. LETC however, did not improve activity deficits of L62 mice in the home cage environment. The results suggest that LETC may provide a potential disease modification therapy in synucleinopathies through the inhibition of α-Syn aggregation.

O-GlcNAc forces an α-synuclein amyloid strain with notably diminished seeding and pathology

Amyloid-forming proteins such α-synuclein and tau, which are implicated in Alzheimer's and Parkinson's disease, can form different fibril structures or strains with distinct toxic properties, seeding activities and pathology. Understanding the determinants contributing to the formation of different amyloid features could open new avenues for developing disease-specific diagnostics and therapies. Here we report that O-GlcNAc modification of α-synuclein monomers results in the formation of amyloid fibril with distinct core structure, as revealed by cryogenic electron microscopy, and diminished seeding activity in seeding-based neuronal and rodent models of Parkinson's disease. Although the mechanisms underpinning the seeding neutralization activity of the O-GlcNAc-modified fibrils remain unclear, our in vitro mechanistic studies indicate that heat shock proteins interactions with O-GlcNAc fibril inhibit their seeding activity, suggesting that the O-GlcNAc modification may alter the interactome of the α-synuclein fibrils in ways that lead to reduce seeding activity in vivo. Our results show that posttranslational modifications, such as O-GlcNAc modification, of α-synuclein are key determinants of α-synuclein amyloid strains and pathogenicity.

Cyclic Ion Mobility-Mass Spectrometry and Tandem Collision Induced Unfolding for Quantification of Elusive Protein Biomarkers

Sensitive analytical techniques that are capable of detecting and quantifying disease-associated biomolecules are indispensable in our efforts to understand disease mechanisms and guide therapeutic intervention through early detection, accurate diagnosis, and effective monitoring of disease. Parkinson's Disease (PD), for example, is one of the most prominent neurodegenerative disorders in the world, but the diagnosis of PD has primarily been based on the observation of clinical symptoms. The protein α-synuclein (α-syn) has emerged as a promising biomarker candidate for PD, but a lack of analytical methods to measure complex disease-associated variants of α-syn has prevented its widespread use as a biomarker. Antibody-based methods such as immunoassays and mass spectrometry-based approaches have been used to measure a limited number of α-syn forms; however, these methods fail to differentiate variants of α-syn that display subtle differences in only the sequence and structure. In this work, we developed a cyclic ion mobility-mass spectrometry method that combines multiple stages of activation and timed ion selection to quantify α-syn variants using both mass- and structure-based measurements. This method can allow for the quantification of several α-syn variants present at physiological levels in biological fluid. Taken together, this approach can be used to galvanize future efforts aimed at understanding the underlying mechanisms of PD and serves as a starting point for the development of future protein-structure-based diagnostics and therapeutic interventions.

Stochastic Optical Reconstruction Microscopy Imaging of Multiple System Atrophy Inclusions Suggests Stepwise α-Synuclein Aggregation

BACKGROUND

The architecture and composition of glial (GCI) and neuronal (NCI) α-synuclein inclusions observed in multiple system atrophy (MSA) remain to be precisely defined to better understand the disease.

METHODS

Here, we used stochastic optical reconstruction microscopy (STORM) to characterize the nanoscale organization of glial (GCI) and neuronal (NCI) α-synuclein inclusions in cryopreserved brain sections from MSA patients.

RESULTS

STORM revealed a dense cross-linked internal structure of α-synuclein in all GCI and NCI. The internal architecture of hyperphosphorylated α-synuclein (p-αSyn) inclusions was similar in glial and neuronal cells, suggesting a common aggregation mechanism. A similar sequence of p-αSyn stepwise intracellular aggregation was defined in oligodendrocytes and neurons, starting from the perinuclear area and growing inside the cells. Consistent with this hypothesis, we found a higher mitochondrial density in GCI and NCI compared to oligodendrocytes and neurons from unaffected donors (P < 0.01), suggesting an active recruitment of the organelles during the aggregation process.

CONCLUSIONS

These first STORM images of GCI and NCI suggest stepwise α-synuclein aggregation in MSA. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

The potential of phosphorylated α-synuclein as a biomarker for the diagnosis and monitoring of multiple system atrophy

INTRODUCTION

Multiple system atrophy (MSA) is a rapidly progressive neurodegenerative disorder characterized by the presence of glial cytoplasmic inclusions (GCIs) containing aggregated α-synuclein (α-Syn). Accurate diagnosis and monitoring of MSA present significant challenges, which can lead to potential misdiagnosis and inappropriate treatment. Biomarkers play a crucial role in improving the accuracy of MSA diagnosis, and phosphorylated α-synuclein (p-syn) has emerged as a promising biomarker for aiding in diagnosis and disease monitoring.

METHODS

A literature search was conducted on PubMed, Scopus, and Google Scholar using specific keywords and MeSH terms without imposing a time limit. Inclusion criteria comprised various study designs including experimental studies, case-control studies, and cohort studies published only in English, while conference abstracts and unpublished sources were excluded.

RESULTS

Increased levels of p-syn have been observed in various samples from MSA patients, such as red blood cells, cerebrospinal fluid, oral mucosal cells, skin, and colon biopsies, highlighting their diagnostic potential. The α-Syn RT-QuIC assay has shown sensitivity in diagnosing MSA and tracking its progression. Meta-analyses and multicenter investigations have confirmed the diagnostic value of p-syn in cerebrospinal fluid, demonstrating high specificity and sensitivity in distinguishing MSA from other neurodegenerative diseases. Moreover, combining p-syn with other biomarkers has further improved the diagnostic accuracy of MSA.

CONCLUSION

The p-syn stands out as a promising biomarker for MSA. It is found in oligodendrocytes and shows a correlation with disease severity and progression. However, further research and validation studies are necessary to establish p-syn as a reliable biomarker for MSA. If proven, p-syn could significantly contribute to early diagnosis, disease monitoring, and assessing treatment response.

Solubility of α-synuclein species in the L62 mouse model of synucleinopathy

The accumulation of α-synuclein (α-Syn) into Lewy bodies is a hallmark of synucleinopathies, a group of neurological disorders that include Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Small oligomers as well as larger fibrils of α-Syn have been suggested to induce cell toxicity leading to a degenerative loss of neurones. A richer understanding of α-Syn aggregation in disease, however, requires the identification of the different α-Syn species and the characterisation of their biochemical properties. We here aimed at a more in-depth characterisation of the α-Syn transgenic mice, Line 62 (L62), and examined the deposition pattern and solubility of human and murine α-Syn in these mice using immunohistochemical and biochemical methods. Application of multiple antibodies confirmed mAb syn204 as the most discriminatory antibody for human α-Syn in L62. Syn204 revealed an intense and widespread immunohistochemical α-Syn labelling in parietal cortex and hippocampus, and to a lower level in basal forebrain and hindbrain regions. The labelled α-Syn represented somatic inclusions as well as processes and synaptic endings. Biochemical analysis revealed a Triton-resistant human α-Syn pool of large oligomers, a second pool of small oligomers that was not resistant to solubilization with urea/Triton. A third SDS-soluble pool of intermediate sized aggregates containing a mixture of both, human and mouse α-Syn was also present. These data suggest that several pools of α-Syn can exist in neurones, most likely in different cellular compartments. Information about these different pools is important for the development of novel disease modifying therapies aimed at α-Syn.