Widespread microglial activation in multiple system atrophy

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.

Reanalysis of DIA Data Demonstrates the Capabilities of MS/MS-Free Proteomics to Reveal New Biological Insights in Disease-Related Samples

Data-independent acquisition (DIA) at the shortened data acquisition time is becoming a method of choice for quantitative proteomic applications requiring high throughput analysis of large cohorts of samples. With the advent of the combination of high resolution mass spectrometry with an asymmetric track lossless analyzer, these DIA capabilities were further extended with the recent demonstration of quantitative analyses at the speed of up to hundreds of samples per day. In particular, the proteomic data for the brain samples related to multiple system atrophy disease were acquired using 7 and 28 min chromatography gradients (Guzman et al., Nat. Biotech. 2024). In this work, we applied the recently introduced DirectMS1 method to reanalysis of these data using only MS1 spectra. Both DirectMS1 and DIA results were matched against long gradient DDA analysis from the earlier study of the same sample cohort. While the quantitation efficiency of DirectMS1 was comparable with DIA on the same data sets, we found an additional five proteins of biological significance relevant to the analyzed tissue samples. Among the findings, DirectMS1 was able to detect decreased caspase activity for Vimentin protein in the multiple system atrophy samples missed by the MS/MS-based quantitation methods. Our study suggests that DirectMS1 can be an efficient MS1-only addition to the analysis of DIA data in high-throughput quantitative proteomic studies.

Tissue resident memory CD8+ T cells are present but not critical for demyelination and neurodegeneration in a mouse model of multiple system atrophy

Multiple system atrophy (MSA) is rare, fast progressing, and fatal synucleinopathy with alpha-synuclein (α-syn) inclusions located within oligodendroglia called glial cytoplasmic inclusions (GCI). Along with GCI pathology there is severe demyelination, neurodegeneration, and neuroinflammation. In post-mortem tissue, there is significant infiltration of CD8+ T cells into the brain parenchyma, however their role in disease progression is unknown. To determine the role of CD8+ T cells, a modified AAV, Olig001-SYN, was used to selectively overexpress α-syn in oligodendrocytes modeling MSA in mice. Four weeks post transduction, we observed significant CD8+ T cell infiltration into the striatum of Olig001-SYN transduced mice recapitulating the CD8+ T cell infiltration observed in post-mortem tissue. To understand the role of CD8+ T cells, a CD8 knockout mice were transduced with Olig001-SYN. Six months post transduction into a mouse lacking CD8+ T cells, demyelination and neurodegeneration were unchanged. Four weeks post transduction, neuroinflammation and demyelination were enhanced in CD8 knockout mice compared to wild type controls. Applying unbiased spectral flow cytometry, CD103+, CD69+, CD44+, CXCR6+, CD8+ T cells were identified when α-syn was present in oligodendrocytes, suggesting the presence of tissue resident memory CD8+ T (Trm) cells during MSA disease progression. This study indicates that CD8+ T cells are not critical in driving MSA pathology but are needed to modulate the neuroinflammation and demyelination response.

Systemic inflammation activates coagulation and immune cell infiltration pathways in brains with propagating α-synuclein fibril aggregates

Synucleinopathies are a group of diseases characterized by brain aggregates of α-synuclein (α-syn). The gradual accumulation of α-syn and the role of inflammation in early-stage pathogenesis remain poorly understood. We explored this interaction by inducing chronic inflammation in a common pre-clinical synucleinopathy mouse model. Three weeks post unilateral intra-striatal injections of human α-syn pre-formed fibrils (PFF), mice underwent repeated intraperitoneal injections of 1 mg/ml lipopolysaccharide (LPS) for 3 weeks. Histological examinations of the ipsilateral site showed phospho-α-syn regional spread and LPS-induced neutrophil recruitment to the brain vasculature. Biochemical assessment of the contralateral site confirmed spreading of α-syn aggregation to frontal cortex and a rise in intracerebral TNF-α, IL-1β, IL-10 and KC/GRO cytokines levels due to LPS. No LPS-induced exacerbation of α-syn pathology load was observed at this stage. Proteomic analysis was performed contralateral to the PFF injection site using LC-MS/MS. Subsequent downstream Reactome Gene-Set Analysis indicated that α-syn pathology alters mitochondrial metabolism and synaptic signaling. Chronic LPS-induced inflammation further lead to an overrepresentation of pathways related to fibrin clotting as well as integrin and B cell receptor signaling. Western blotting confirmed a PFF-induced increase in fibrinogen brain levels and a PFF + LPS increase in Iba1 levels, indicating activated microglia. Splenocyte profiling revealed changes in T and B cells, monocytes, and neutrophils populations due to LPS treatment in PFF injected animals. In summary, early α-syn pathology impacts energy homeostasis pathways, synaptic signaling and brain fibrinogen levels. Concurrent mild systemic inflammation may prime brain immune pathways in interaction with peripheral immunity.

Neuroinflammatory gene expression profiles of reactive glia in the substantia nigra suggest a multidimensional immune response to alpha synuclein inclusions

Parkinson's disease (PD) pathology is characterized by alpha-synuclein (α-syn) aggregates, degeneration of dopamine neurons in the substantia nigra pars compacta (SNpc), and neuroinflammation. The presence of reactive glia correlates with deposition of pathological α-syn in early-stage PD. Thus, understanding the neuroinflammatory response of microglia and astrocytes to synucleinopathy may identify therapeutic targets. Here we characterized the neuroinflammatory gene expression profile of reactive microglia and astrocytes in the SNpc during early synucleinopathy in the rat α-syn pre-formed fibril (PFF) model. Rats received intrastriatal injection of α-syn PFFs and expression of immune genes was quantified with droplet digital PCR (ddPCR), after which fluorescent in situ hybridization (FISH) was used to localize gene expression to microglia or astrocytes in the SNpc. Genes previously associated with reactive microglia (Cd74, C1qa, Stat1, Axl, Casp1, Il18, Lyz2) and reactive astrocytes (C3, Gbp2, Serping1) were significantly upregulated in the SN of PFF injected rats. Localization of gene expression to SNpc microglia near α-syn aggregates identified a unique α-syn aggregate microglial gene expression profile characterized by upregulation of Cd74, Cxcl10, Rt-1a2, Grn, Csf1r, Tyrobp, C3, C1qa, Serping1 and Fcer1g. Importantly, significant microglial upregulation of Cd74 and C3 were only observed following injection of α-syn PFFs, not α-syn monomer, confirming specificity to α-syn aggregation. Serping1 expression also localized to astrocytes in the SNpc. Interestingly, C3 expression in the SNpc localized to microglia at 2- and 4-months post-PFF, but to astrocytes at 6-months post-PFF. We also observed expression of Rt1-a2 and Cxcl10 in SNpc dopamine neurons. Cumulatively our results identify a dynamic, yet reproducible gene expression profile of reactive microglia and astrocytes associated with early synucleinopathy in the rat SNpc.

Infections in the Etiology of Parkinson's Disease and Synucleinopathies: A Renewed Perspective, Mechanistic Insights, and Therapeutic Implications

Increasing evidence suggests a potential role for infectious pathogens in the etiology of synucleinopathies, a group of age-related neurodegenerative disorders including Parkinson's disease (PD), multiple system atrophy and dementia with Lewy bodies. In this review, we discuss the link between infections and synucleinopathies from a historical perspective, present emerging evidence that supports this link, and address current research challenges with a focus on neuroinflammation. Infectious pathogens can elicit a neuroinflammatory response and modulate genetic risk in PD and related synucleinopathies. The mechanisms of how infections might be linked with synucleinopathies as well as the overlap between the immune cellular pathways affected by virulent pathogens and disease-related genetic risk factors are discussed. Here, an important role for α-synuclein in the immune response against infections is emerging. Critical methodological and knowledge gaps are addressed, and we provide new future perspectives on how to address these gaps. Understanding how infections and neuroinflammation influence synucleinopathies will be essential for the development of early diagnostic tools and novel therapies.

Quantitative cellular changes in multiple system atrophy brains

Multiple system atrophy (MSA) is a neurodegenerative disorder characterised by a combined symptomatology of parkinsonism, cerebellar ataxia, autonomic failure and corticospinal dysfunction. In brains of MSA patients, the hallmark lesion is the aggregation of misfolded alpha-synuclein in oligodendrocytes. Even though the underlying pathological mechanisms remain poorly understood, the evidence suggests that alpha-synuclein aggregation in oligodendrocytes may contribute to the neurodegeneration seen in MSA. The primary aim of this review is to summarise the published stereological data on the total number of neurons and glial cell subtypes (oligodendrocytes, astrocytes and microglia) and volumes in brains from MSA patients. Thus, we include in this review exclusively the reports of unbiased quantitative data from brain regions including the neocortex, nuclei of the cerebrum, the brainstem and the cerebellum. Furthermore, we compare and discuss the stereological results in the context of imaging findings and MSA symptomatology. In general, the stereological results agree with the common neuropathological findings of neurodegeneration and gliosis in brains from MSA patients and support a major loss of nigrostriatal neurons in MSA patients with predominant parkinsonism (MSA-P), as well as olivopontocerebellar atrophy in MSA patients with predominant cerebellar ataxia (MSA-C). Surprisingly, the reports indicate only a minor loss of oligodendrocytes in sub-cortical regions of the cerebrum (glial cells not studied in the cerebellum) and negligible changes in brain volumes. In the past decades, the use of stereological methods has provided a vast amount of accurate information on cell numbers and volumes in the brains of MSA patients. Combining different techniques such as stereology and diagnostic imaging (e.g. MRI, PET and SPECT) with clinical data allows for a more detailed interdisciplinary understanding of the disease and illuminates the relationship between neuropathological changes and MSA symptomatology.

Increased microglia activation in late non-central nervous system cancer survivors links to chronic systemic symptomatology

Prolonged inflammatory expression within the central nervous system (CNS) is recognized by the brain as a molecular signal of "sickness", that has knock-on effects to the blood-brain barrier, brain-spinal barrier, blood-cerebrospinal fluid barrier, neuro-axonal structures, neurotransmitter activity, synaptic plasticity, neuroendocrine function, and resultant systemic symptomatology. It is concurred that the inflammatory process associated with cancer and cancer treatments underline systemic symptoms present in a large portion of survivors, although this concept is largely theoretical from disparate and indirect evidence and/or clinical anecdotal reports. We conducted a proof-of-concept study to link for the first time late non-CNS cancer survivors presenting chronic systemic symptoms and the presence of centralized inflammation, or neuroinflammation, using TSPO-binding PET tracer [11 C]-PBR28 to visualize microglial activation. We compared PBR28 SUVR in 10 non-CNS cancer survivors and 10 matched healthy controls. Our data revealed (1) microglial activation was significantly higher in caudate, temporal, and occipital regions in late non-central nervous system/CNS cancer survivors compared to healthy controls; (2) increased neuroinflammation in cancer survivors was not accompanied by significant differences in plasma cytokine markers of peripheral inflammation; (3) increased neuroinflammation was not accompanied by reduced fractional anisotropy, suggesting intact white matter microstructural integrity, a marker of neurovascular fiber tract organization; and (4) the presentation of chronic systemic symptoms in cancer survivors was significantly connected with microglial activation. We present the first data empirically supporting the concept of a peripheral-to-centralized inflammatory response in non-CNS cancer survivors, specifically those previously afflicted with head and neck cancer. Following resolution of the initial peripheral inflammation from the cancer/its treatments, in some cases damage/toxification to the central nervous system occurs, ensuing chronic systemic symptoms.

Microglia in neurodegenerative diseases: mechanism and potential therapeutic targets

Microglia activation is observed in various neurodegenerative diseases. Recent advances in single-cell technologies have revealed that these reactive microglia were with high spatial and temporal heterogeneity. Some identified microglia in specific states correlate with pathological hallmarks and are associated with specific functions. Microglia both exert protective function by phagocytosing and clearing pathological protein aggregates and play detrimental roles due to excessive uptake of protein aggregates, which would lead to microglial phagocytic ability impairment, neuroinflammation, and eventually neurodegeneration. In addition, peripheral immune cells infiltration shapes microglia into a pro-inflammatory phenotype and accelerates disease progression. Microglia also act as a mobile vehicle to propagate protein aggregates. Extracellular vesicles released from microglia and autophagy impairment in microglia all contribute to pathological progression and neurodegeneration. Thus, enhancing microglial phagocytosis, reducing microglial-mediated neuroinflammation, inhibiting microglial exosome synthesis and secretion, and promoting microglial conversion into a protective phenotype are considered to be promising strategies for the therapy of neurodegenerative diseases. Here we comprehensively review the biology of microglia and the roles of microglia in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, multiple system atrophy, amyotrophic lateral sclerosis, frontotemporal dementia, progressive supranuclear palsy, corticobasal degeneration, dementia with Lewy bodies and Huntington's disease. We also summarize the possible microglia-targeted interventions and treatments against neurodegenerative diseases with preclinical and clinical evidence in cell experiments, animal studies, and clinical trials.

Alpha-synuclein fibrils amplified from multiple system atrophy and Parkinson's disease patient brain spread after intracerebral injection into mouse brain

Parkinson's disease (PD), multiple system atrophy (MSA), and dementia with Lewy bodies (DLB) are neurodegenerative disorders with alpha-synuclein (α-syn) aggregation pathology. Different strains of α-syn with unique properties are suggested to cause distinct clinical and pathological manifestations resulting in PD, MSA, or DLB. To study individual α-syn spreading patterns, we injected α-syn fibrils amplified from brain homogenates of two MSA patients and two PD patients into the brains of C57BI6/J mice. Antibody staining against pS129-α-syn showed that α-syn fibrils amplified from the brain homogenates of the four different patients caused different levels of α-syn spreading. The strongest α-syn pathology was triggered by α-syn fibrils of one of the two MSA patients, followed by comparable pS129-α-syn induction by the second MSA and one PD patient material. Histological analysis using an antibody against Iba1 further showed that the formation of pS129-α-syn is associated with increased microglia activation. In contrast, no differences in dopaminergic neuron numbers or co-localization of α-syn in oligodendrocytes were observed between the different groups. Our data support the spreading of α-syn pathology in MSA, while at the same time pointing to spreading heterogeneity between different patients potentially driven by individual patient immanent factors.

Inflammation in multiple system atrophy

Misfolding protein aggregation inside or outside cells is the major pathological hallmark of several neurodegenerative diseases. Among proteinopathies are neurodegenerative diseases with atypical Parkinsonism and an accumulation of insoluble fibrillary alpha-synuclein (synucleinopathies) or hyperphosphorylated tau protein fragments (tauopathies). As there are no therapies available to slow or halt the progression of these disea ses, targeting the inflammatory process is a promising approach. The inflammatory biomarkers could also help in the differential diagnosis of Parkinsonian syndromes. Here, we review inflammation's role in multiple systems atrophy pathogenesis, diagnosis, and treatment.

Multiple system atrophy: at the crossroads of cellular, molecular and genetic mechanisms

Multiple system atrophy (MSA) is a rare oligodendroglial α-synucleinopathy characterized by neurodegeneration in striatonigral and olivopontocerebellar regions and autonomic brain centres. It causes complex cumulative motor and non-motor disability with fast progression and effective therapy is currently lacking. The difficulties in the diagnosis and treatment of MSA are largely related to the incomplete understanding of the pathogenesis of the disease. The MSA pathogenic landscape is complex, and converging findings from genetic and neuropathological studies as well as studies in experimental models of MSA have indicated the involvement of genetic and epigenetic changes; α-synuclein misfolding, aggregation and spreading; and α-synuclein strain specificity. These studies also indicate the involvement of myelin and iron dyshomeostasis, neuroinflammation, mitochondrial dysfunction and other cell-specific aspects that are relevant to the fast progression of MSA. In this Review, we discuss these findings and emphasize the implications of the complexity of the multifactorial pathogenic cascade for future translational research and its impact on biomarker discovery and treatment target definitions.

CSF1R-Mediated Myeloid Cell Depletion Prolongs Lifespan But Aggravates Distinct Motor Symptoms in a Model of Multiple System Atrophy

As the CNS-resident macrophages and member of the myeloid lineage, microglia fulfill manifold functions important for brain development and homeostasis. In the context of neurodegenerative diseases, they have been implicated in degenerative and regenerative processes. The discovery of distinct activation patterns, including increased phagocytosis, indicated a damaging role of myeloid cells in multiple system atrophy (MSA), a devastating, rapidly progressing atypical parkinsonian disorder. Here, we analyzed the gene expression profile of microglia in a mouse model of MSA (MBP29-hα-syn) and identified a disease-associated expression profile and upregulation of the colony-stimulating factor 1 (Csf1). Thus, we hypothesized that CSF1 receptor-mediated depletion of myeloid cells using PLX5622 modifies the disease progression and neuropathological phenotype in this mouse model. Intriguingly, sex-balanced analysis of myeloid cell depletion in MBP29-hα-syn mice revealed a two-faced outcome comprising an improved survival rate accompanied by a delayed onset of neurological symptoms in contrast to severely impaired motor functions. Furthermore, PLX5622 reversed gene expression profiles related to myeloid cell activation but reduced gene expression associated with transsynaptic signaling and signal release. While transcriptional changes were accompanied by a reduction of dopaminergic neurons in the SNpc, striatal neuritic density was increased upon myeloid cell depletion in MBP29-hα-syn mice. Together, our findings provide insight into the complex, two-faced role of myeloid cells in the context of MSA emphasizing the importance to carefully balance the beneficial and adverse effects of CSF1R inhibition in different models of neurodegenerative disorders before its clinical translation.SIGNIFICANCE STATEMENT Myeloid cells have been implicated as detrimental in the disease pathogenesis of multiple system atrophy. However, long-term CSF1R-dependent depletion of these cells in a mouse model of multiple system atrophy demonstrates a two-faced effect involving an improved survival associated with a delayed onset of disease and reduced inflammation which was contrasted by severely impaired motor functions, synaptic signaling, and neuronal circuitries. Thus, this study unraveled a complex role of myeloid cells in multiple system atrophy, which indicates important functions beyond the previously described disease-associated, destructive phenotype and emphasized the need of further investigation to carefully and individually fine-tune immunologic processes in different neurodegenerative diseases.

Quantitative susceptibility mapping and blood neurofilament light chain differentiate between parkinsonian disorders

Objectives

We employed quantitative susceptibility mapping (QSM) to assess iron deposition in parkinsonian disorders and explored whether combining QSM values and neurofilament light (NfL) chain levels can improve the accuracy of distinguishing Parkinson’s disease (PD) from multiple system atrophy (MSA) and progressive supranuclear palsy (PSP).

Materials and methods

Forty-seven patients with PD, 28 patients with MSA, 18 patients with PSP, and 28 healthy controls (HC) were enrolled, and QSM data were reconstructed. Susceptibility values in the bilateral globus pallidus (GP), putamen (PUT), caudate nucleus (CN), red nucleus (RN), substantia nigra (SN), and dentate nucleus (DN) were obtained. Plasma NfL levels of 47 PD, 18 MSA, and 14 PSP patients and 22 HC were measured by ultrasensitive Simoa technology.

Results

The highest diagnostic accuracy distinguishing MSA from PD patients was observed with increased susceptibility values in CN (AUC: 0.740). The susceptibility values in RN yielded the highest diagnostic performance for distinguishing PSP from PD patients (AUC: 0.829). Plasma NfL levels were significantly higher in the MSA and PSP groups than in PD and HC groups. Combining the susceptibility values in the RN and plasma NfL levels improved the diagnostic performance for PSP vs. PD (AUC: 0.904), whereas plasma NfL levels had higher diagnostic accuracy for MSA vs. PD (AUC: 0.877).

Conclusion

The exploratory study indicates different patterns of iron accumulation in deep gray matter nuclei in Parkinsonian disorders. Combining QSM values with NfL levels may be a promising biomarker for distinguishing PSP from PD, whereas plasma NfL may be a reliable biomarker for differentiating MSA from PD. QSM and NfL measures appeared to have low accuracy for separating PD from controls.

18 F-Florzolotau Tau Positron Emission Tomography Imaging in Patients with Multiple System Atrophy-Parkinsonian Subtype

BACKGROUND

Anecdotal evidence suggests that patients diagnosed with the parkinsonian subtype of multiple system atrophy (MSA-P) may show uptake of the second-generation tau positron emission tomography (PET) tracer ¹⁸ F-Florzolotau (previously known as ¹⁸ F-APN-1607) in the putamen.

OBJECTIVES

This study systematically investigated the localization and magnitude of ¹⁸ F-Florzolotau uptake in a relatively large cohort of patients with MSA-P.

METHODS

¹⁸ F-Florzolotau PET imaging was performed in 31 patients with MSA-P, 24 patients with Parkinson's disease (PD), and 20 age-matched healthy controls. ¹⁸ F-Florzolotau signal in the striatum was analyzed by visual inspection and classified as either positive or negative. Regional ¹⁸ F-Florzolotau binding was also expressed as standardized uptake value ratio (SUVR) to assess whether it was associated with core symptoms of MSA-P after adjustment for potential confounders.

RESULTS

By visual inspection and semiquantitative SUVR comparisons, patients with MSA-P showed elevated ¹⁸ F-Florzolotau uptake in the putamen, globus pallidus, and dentate-a finding that was not observed in PD. This increased signal was significantly associated with the core symptoms of MSA-P. In addition, patients with MSA-P with cerebellar ataxia showed an elevated ¹⁸ F-Florzolotau uptake in the cerebellar dentate.

CONCLUSIONS

¹⁸ F-Florzolotau tau PET imaging findings may reflect the clinical severity of MSA-P and can potentially discriminate between this condition and PD. © 2022 International Parkinson and Movement Disorder Society.

Oligoclonal Bands in Multiple System Atrophy: Case Report and Proposed Mechanisms of Immunogenicity

Multiple System Atrophy (MSA) is a neurodegenerative disease with heterogeneous manifestations and is therefore difficult to diagnose definitively. Because of this, oftentimes an extensive workup for mimickers is undertaken. We herein report a case where the history and cerebrospinal fluid (CSF) findings of oligoclonal bands suggested an inflammatory disorder. Immunomodulatory therapy failed to ameliorate symptoms or alter the trajectory of continued physical decline, prompting re-visitation of the diagnosis. Oligoclonal bands, while generally viewed as specific to multiple sclerosis or other inflammatory conditions, may be seen in other disease processes. Therefore, this finding should not exclude consideration of neurodegenerative disease.

Heterogeneity of Multiple System Atrophy: An Update

Multiple system atrophy (MSA) is a fatal, rapidly progressing neurodegenerative disease of uncertain etiology, clinically characterized by various combinations of Levodopa unresponsive parkinsonism, cerebellar, autonomic and motor dysfunctions. The morphological hallmark of this α-synucleinopathy is the deposition of aberrant α-synuclein in both glia, mainly oligodendroglia (glial cytoplasmic inclusions /GCIs/) and neurons, associated with glioneuronal degeneration of the striatonigral, olivopontocerebellar and many other neuronal systems. Typical phenotypes are MSA with predominant parkinsonism (MSA-P) and a cerebellar variant (MSA-C) with olivocerebellar atrophy. However, MSA can present with a wider range of clinical and pathological features than previously thought. In addition to rare combined or "mixed" MSA, there is a broad spectrum of atypical MSA variants, such as those with a different age at onset and disease duration, "minimal change" or prodromal forms, MSA variants with Lewy body disease or severe hippocampal pathology, rare forms with an unusual tau pathology or spinal myoclonus, an increasing number of MSA cases with cognitive impairment/dementia, rare familial forms, and questionable conjugal MSA. These variants that do not fit into the current classification of MSA are a major challenge for the diagnosis of this unique proteinopathy. Although the clinical diagnostic accuracy and differential diagnosis of MSA have improved by using combined biomarkers, its distinction from clinically similar extrapyramidal disorders with other pathologies and etiologies may be difficult. These aspects should be taken into consideration when revising the current diagnostic criteria. This appears important given that disease-modifying treatment strategies for this hitherto incurable disorder are under investigation.

Can SARS-CoV-2 Infection Lead to Neurodegeneration and Parkinson's Disease?

The SARS-CoV-2 pandemic has affected the daily life of the worldwide population since 2020. Links between the newly discovered viral infection and the pathogenesis of neurodegenerative diseases have been investigated in different studies. This review aims to summarize the literature concerning COVID-19 and Parkinson's disease (PD) to give an overview on the interface between viral infection and neurodegeneration with regard to this current topic. We will highlight SARS-CoV-2 neurotropism, neuropathology and the suspected pathophysiological links between the infection and neurodegeneration as well as the psychosocial impact of the pandemic on patients with PD. Some evidence discussed in this review suggests that the SARS-CoV-2 pandemic might be followed by a higher incidence of neurodegenerative diseases in the future. However, the data generated so far are not sufficient to confirm that COVID-19 can trigger or accelerate neurodegenerative diseases.

The Concept of α-Synuclein Strains and How Different Conformations May Explain Distinct Neurodegenerative Disorders

In the past few years, an increasing amount of studies primarily based on experimental models have investigated the existence of distinct α-synuclein strains and their different pathological effects. This novel concept could shed light on the heterogeneous nature of α-synucleinopathies, a group of disorders that includes Parkinson's disease, dementia with Lewy bodies and multiple system atrophy, which share as their key-molecular hallmark the abnormal aggregation of α-synuclein, a process that seems pivotal in disease pathogenesis according to experimental observations. However, the etiology of α-synucleinopathies and the initial events leading to the formation of α-synuclein aggregates remains elusive. Hence, the hypothesis that structurally distinct fibrillary assemblies of α-synuclein could have a causative role in the different disease phenotypes and explain, at least to some extent, their specific neurodegenerative, disease progression, and clinical presentation patterns is very appealing. Moreover, the presence of different α-synuclein strains might represent a potential biomarker for the diagnosis of these neurodegenerative disorders. In this regard, the recent use of super resolution techniques and protein aggregation assays has offered the possibility, on the one hand, to elucidate the conformation of α-synuclein pathogenic strains and, on the other hand, to cyclically amplify to detectable levels low amounts of α-synuclein strains in blood, cerebrospinal fluid and peripheral tissue from patients. Thus, the inclusion of these techniques could facilitate the differentiation between α-synucleinopathies, even at early stages, which is crucial for successful therapeutic intervention. This mini-review summarizes the current knowledge on α-synuclein strains and discusses its possible applications and potential benefits.

Glia Imaging Differentiates Multiple System Atrophy from Parkinson's Disease: A Positron Emission Tomography Study with [11 C]PBR28 and Machine Learning Analysis

BACKGROUND

The clinical diagnosis of multiple system atrophy (MSA) is challenged by overlapping features with Parkinson's disease (PD) and late-onset ataxias. Additional biomarkers are needed to confirm MSA and to advance the understanding of pathophysiology. Positron emission tomography (PET) imaging of the translocator protein (TSPO), expressed by glia cells, has shown elevations in MSA.

OBJECTIVE

In this multicenter PET study, we assess the performance of TSPO imaging as a diagnostic marker for MSA.

METHODS

We analyzed [¹¹ C]PBR28 binding to TSPO using imaging data of 66 patients with MSA and 24 patients with PD. Group comparisons were based on regional analysis of parametric images. The diagnostic readout included visual reading of PET images against clinical diagnosis and machine learning analyses. Sensitivity, specificity, and receiver operating curves were used to discriminate MSA from PD and cerebellar from parkinsonian variant MSA.

RESULTS

We observed a conspicuous pattern of elevated regional [¹¹ C]PBR28 binding to TSPO in MSA as compared with PD, with "hotspots" in the lentiform nucleus and cerebellar white matter. Visual reading discriminated MSA from PD with 100% specificity and 83% sensitivity. The machine learning approach improved sensitivity to 96%. We identified MSA subtype-specific TSPO binding patterns.

CONCLUSIONS

We found a pattern of significantly increased regional glial TSPO binding in patients with MSA. Intriguingly, our data are in line with severe neuroinflammation in MSA. Glia imaging may have potential to support clinical MSA diagnosis and patient stratification in clinical trials on novel drug therapies for an α-synucleinopathy that remains strikingly incurable. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Current experimental disease-modifying therapeutics for multiple system atrophy

Multiple system atrophy (MSA) is a challenging neurodegenerative disorder with a difficult and often inaccurate early diagnosis, still lacking effective treatment. It is characterized by a highly variable clinical presentation with parkinsonism, cerebellar ataxia, autonomic dysfunction, and pyramidal signs, with a rapid progression and an aggressive clinical course. The definite MSA diagnosis is only possible post-mortem, when the presence of distinctive oligodendroglial cytoplasmic inclusions (GCIs), mainly composed of misfolded and aggregated α-Synuclein (α-Syn) is demonstrated. The process of α-Syn accumulation and aggregation within oligodendrocytes is accepted one of the main pathological events underlying MSA. However, MSA is considered a multifactorial disorder with multiple pathogenic events acting together including neuroinflammation, oxidative stress, and disrupted neurotrophic support, among others. The discussed here treatment approaches are based on our current understanding of the pathogenesis of MSA and the results of preclinical and clinical therapeutic studies conducted over the last 2 decades. We summarize leading disease-modifying approaches for MSA including targeting α-Syn pathology, modulation of neuroinflammation, and enhancement of neuroprotection. In conclusion, we outline some challenges related to the need to overcome the gap in translation between preclinical and clinical studies towards a successful disease modification in MSA.

Neurons and Glia Interplay in α-Synucleinopathies

Accumulation of the neuronal presynaptic protein alpha-synuclein within proteinaceous inclusions represents the key histophathological hallmark of a spectrum of neurodegenerative disorders, referred to by the umbrella term a-synucleinopathies. Even though alpha-synuclein is expressed predominantly in neurons, pathological aggregates of the protein are also found in the glial cells of the brain. In Parkinson's disease and dementia with Lewy bodies, alpha-synuclein accumulates mainly in neurons forming the Lewy bodies and Lewy neurites, whereas in multiple system atrophy, the protein aggregates mostly in the glial cytoplasmic inclusions within oligodendrocytes. In addition, astrogliosis and microgliosis are found in the synucleinopathy brains, whereas both astrocytes and microglia internalize alpha-synuclein and contribute to the spread of pathology. The mechanisms underlying the pathological accumulation of alpha-synuclein in glial cells that under physiological conditions express low to non-detectable levels of the protein are an area of intense research. Undoubtedly, the presence of aggregated alpha-synuclein can disrupt glial function in general and can contribute to neurodegeneration through numerous pathways. Herein, we summarize the current knowledge on the role of alpha-synuclein in both neurons and glia, highlighting the contribution of the neuron-glia connectome in the disease initiation and progression, which may represent potential therapeutic target for a-synucleinopathies.

Recent developments on PET radiotracers for TSPO and their applications in neuroimaging

The 18 kDa translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor, is predominately localized to the outer mitochondrial membrane in steroidogenic cells. Brain TSPO expression is relatively low under physiological conditions, but is upregulated in response to glial cell activation. As the primary index of neuroinflammation, TSPO is implicated in the pathogenesis and progression of numerous neuropsychiatric disorders and neurodegenerative diseases, including Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), multiple sclerosis (MS), major depressive disorder (MDD) and obsessive compulsive disorder (OCD). In this context, numerous TSPO-targeted positron emission tomography (PET) tracers have been developed. Among them, several radioligands have advanced to clinical research studies. In this review, we will overview the recent development of TSPO PET tracers, focusing on the radioligand design, radioisotope labeling, pharmacokinetics, and PET imaging evaluation. Additionally, we will consider current limitations, as well as translational potential for future application of TSPO radiopharmaceuticals. This review aims to not only present the challenges in current TSPO PET imaging, but to also provide a new perspective on TSPO targeted PET tracer discovery efforts. Addressing these challenges will facilitate the translation of TSPO in clinical studies of neuroinflammation associated with central nervous system diseases.

From Synaptic Protein to Prion: The Long and Controversial Journey of α-Synuclein

Since its discovery 30 years ago, α-synuclein (α-syn) has been one of the most studied proteins in the field of neuroscience. Dozens of groups worldwide have tried to reveal not only its role in the CNS but also in other organs. α-syn has been linked to several processes essential in brain homeostasis such as neurotransmitter release, synaptic function, and plasticity. However, despite the efforts made in this direction, the main function of α-syn is still unknown. Moreover, α-syn became a protein of interest for neurologists and neuroscientists when mutations in its gene were found associated with Parkinson's disease (PD) and even more when α-syn protein deposits were observed in the brain of PD, dementia with Lewy bodies (DLB), and multiple system atrophy (MSA) patients. At present, the abnormal accumulation of α-syn constitutes one of the pathological hallmarks of these disorders, also referred to as α-synucleinopathies, and it is used for post-mortem diagnostic criteria. Whether α-syn aggregation is cause or consequence of the pathogenic events underlying α-synucleinopathies remains unclear and under discussion. Recently, different in vitro and in vivo studies have shown the ability of pathogenic α-syn to spread between cells, not only within the CNS but also from peripheral locations such as the gut, salivary glands, and through the olfactory network into the CNS, inducing abnormal misfolding of endogenous α-syn and leading to neurodegeneration and motor and cognitive impairment in animal models. Thus, it has been suggested that α-syn should be considered a prion protein. Here we present an update of what we know about α-syn function, aggregation and spreading, and its role in neurodegeneration. We also discuss the rationale and findings supporting the hypothetical prion nature of α-syn, its weaknesses, and future perspectives for research and the development of disease-modifying therapies.

T cell infiltration in both human multiple system atrophy and a novel mouse model of the disease

Multiple system atrophy (MSA) is a progressive neurodegenerative disorder characterized by abnormal accumulation of alpha-synuclein (α-syn) in oligodendrocytes accompanied by inflammation, demyelination, and subsequent synapse and neuronal loss. Little is known about the mechanisms of neurodegeneration in MSA. However, recent work has highlighted the important role of the immune system to the pathophysiology of other synuclein-related diseases such as Parkinson’s disease. In this study, we investigated postmortem brain tissue from MSA patients and control subjects for evidence of immune activation in the brain. We found a significant increase of HLA-DR⁺ microglia in the putamen and substantia nigra of MSA patient tissue compared to controls, as well as significant increases in CD3⁺, CD4⁺, and CD8⁺ T cells in these same brain regions. To model MSA in vivo, we utilized a viral vector that selectively overexpresses α-syn in oligodendrocytes (Olig001-SYN) with > 95% tropism in the dorsal striatum of mice, resulting in demyelination and neuroinflammation similar to that observed in human MSA. Oligodendrocyte transduction with this vector resulted in a robust inflammatory response, which included increased MHCII expression on central nervous system (CNS) resident microglia, and infiltration of pro-inflammatory monocytes into the CNS. We also observed robust infiltration of CD4 T cells into the CNS and antigen-experienced CD4 T cells in the draining cervical lymph nodes. Importantly, genetic deletion of TCR-β or CD4 T cells attenuated α-syn-induced inflammation and demyelination in vivo. These results suggest that T cell priming and infiltration into the CNS are key mechanisms of disease pathogenesis in MSA, and therapeutics targeting T cells may be disease modifying.

Insights into the pathogenesis of multiple system atrophy: focus on glial cytoplasmic inclusions

Multiple system atrophy (MSA) is a debilitating and fatal neurodegenerative disorder. The disease severity warrants urgent development of disease-modifying therapy, but the disease pathogenesis is still enigmatic. Neurodegeneration in MSA brains is preceded by the emergence of glial cytoplasmic inclusions (GCIs), which are insoluble α-synuclein accumulations within oligodendrocytes (OLGs). Thus, preventive strategies against GCI formation may suppress disease progression. However, although numerous studies have tried to elucidate the molecular pathogenesis of GCI formation, difficulty remains in understanding the pathological interaction between the two pivotal aspects of GCIs; α-synuclein and OLGs. The difficulty originates from several enigmas: 1) what triggers the initial generation and possible propagation of pathogenic α-synuclein species? 2) what contributes to OLG-specific accumulation of α-synuclein, which is abundantly expressed in neurons but not in OLGs? and 3) how are OLGs and other glial cells affected and contribute to neurodegeneration? The primary pathogenesis of GCIs may involve myelin dysfunction and dyshomeostasis of the oligodendroglial cellular environment such as autophagy and iron metabolism. We have previously reported that oligodendrocyte precursor cells are more prone to develop intracellular inclusions in the presence of extracellular fibrillary α-synuclein. This finding implies a possibility that the propagation of GCI pathology in MSA brains is mediated through the internalization of pathological α-synuclein into oligodendrocyte precursor cells. In this review, in order to discuss the pathogenesis of GCIs, we will focus on the composition of neuronal and oligodendroglial inclusions in synucleinopathies. Furthermore, we will introduce some hypotheses on how α-synuclein pathology spreads among OLGs in MSA brains, in the light of our data from the experiments with primary oligodendrocyte lineage cell culture. While various reports have focused on the mysterious source of α-synuclein in GCIs, insights into the mechanism which regulates the uptake of pathological α-synuclein into oligodendroglial cells may yield the development of the disease-modifying therapy for MSA. The interaction between glial cells and α-synuclein is also highlighted with previous studies of post-mortem human brains, cultured cells, and animal models, which provide comprehensive insight into GCIs and the MSA pathomechanisms.

CSF-1 controls cerebellar microglia and is required for motor function and social interaction

Microglia, the brain resident macrophages, critically shape forebrain neuronal circuits. However, their precise function in the cerebellum is unknown. Here we show that human and mouse cerebellar microglia express a unique molecular program distinct from forebrain microglia. Cerebellar microglial identity was driven by the CSF-1R ligand CSF-1, independently of the alternate CSF-1R ligand, IL-34. Accordingly, CSF-1 depletion from Nestin⁺ cells led to severe depletion and transcriptional alterations of cerebellar microglia, while microglia in the forebrain remained intact. Strikingly, CSF-1 deficiency and alteration of cerebellar microglia were associated with reduced Purkinje cells, altered neuronal function, and defects in motor learning and social novelty interactions. These findings reveal a novel CSF-1-CSF-1R signaling-mediated mechanism that contributes to motor function and social behavior.