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

Vanillin Mitigates the MPTP-Induced α-Synucleinopathy in a Mouse Model of Parkinson's Disease: Insights into the Involvement of Wnt/β-Catenin Signaling

BACKGROUND

The abnormal aggregation of α-synuclein (α-syn) in the substantia nigra pars compacta (SNpc) region of the brain is characteristic of Parkinson's disease (PD), leading to the selective demise of neurons. Modifications in the post-translational processing of α-syn, phosphorylation at Ser129 in particular, are implicated in α-syn aggregation and are considered key hallmarks of PD. Furthermore, dysregulated Wnt/β-catenin signaling, influenced by glycogen synthase kinase-3 beta (GSK-3β), is implicated in PD pathogenesis. Inhibition of GSK-3β holds promise in promoting neuroprotection by enhancing the Wnt/β-catenin pathway.

METHODS

In our previous study utilizing 1-methyl-4-phenylpyridinium (MPP+)-administered differentiated SH-SY5Y cells and a PD mouse model, we explored Vanillin's neuroprotective properties and related mechanisms against neuronal loss induced by MPP+/1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration. In the current study, we elucidated the mitigating effects of Vanillin on motor impairments, P-Ser129-α-syn expression, Wnt/β-catenin signaling, and autophagic neuron death induced by MPTP in a mouse model of PD by performing motor function tests, western blot analysis and immunostaining.

RESULTS

Our results show that Vanillin effectively modulated the motor dysfunctions, GSK-3β expression, and activity, activated the Wnt/β-catenin signaling, and reduced autophagic neuronal demise in the MPTP-lesioned mice, highlighting its neuroprotective effects.

CONCLUSIONS

These findings underscore the complex interplay between α-syn pathology, GSK-3β, Wnt/β-catenin signaling, and autophagic-cell death in PD pathogenesis. Targeting these pathways, particularly with Vanillin, can be a promising therapeutic strategy for restoring dopaminergic (DA-ergic) neuronal homeostasis and slowing the progression of PD. Further research is crucial to resolving existing disputes and translating these discoveries into effective therapeutic interventions for PD patients.

α-Synuclein species in plasma neuron-derived extracellular vesicles as biomarkers for iRBD

OBJECTIVE

Isolated REM sleep behavior disorder (iRBD) is considered as the strongest predictor of Parkinson's disease (PD). Reliable and accurate biomarkers for iRBD detection and the prediction of phenoconversion are in urgent need. This study aimed to investigate whether α-Synuclein (α-Syn) species in plasma neuron-derived extracellular vesicles (NDEVs) could differentiate between iRBD patients and healthy controls (HCs).

METHODS

Nanoscale flow cytometry was used to detect α-Syn-containing NDEVs in plasma.

RESULTS

A total of 54 iRBD patients and 53 HCs were recruited. The concentrations of total α-Syn, α-Syn aggregates, and phosphorylated α-Syn at Ser129 (pS129)-containing NDEVs in plasma of iRBD individuals were significantly higher than those in HCs (p < 0.0001 for all). In distinguishing between iRBD and HCs, the area under the receiver operating characteristic (ROC) curve (AUC) for an integrative model incorporating the levels of α-Syn, pS129, and α-Syn aggregate-containing NDEVs in plasma was 0.965. This model achieved a sensitivity of 94.3% and a specificity of 88.9%. In iRBD group, the concentrations of α-Syn aggregate-containing NDEVs exhibited a negative correlation with Sniffin' Sticks olfactory scores (r = -0.351, p = 0.039). Smokers with iRBD exhibited lower levels of α-Syn aggregates and pS129-containing NDEVs in plasma compared to nonsmokers (pα-Syn aggregates = 0.014; ppS129 = 0.003).

INTERPRETATION

The current study demonstrated that the levels of total α-Syn, α-Syn aggregates, and pS129-containing NDEVs in the plasma of individuals with iRBD were significantly higher compared to HCs. The levels of α-Syn species-containing NDEVs in plasma may serve as biomarkers for iRBD.

Navigating the Neurobiology of Parkinson's: The Impact and Potential of α-Synuclein

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease worldwide; therefore, since its initial description, significant progress has been made, yet a mystery remains regarding its pathogenesis and elusive root cause. The widespread distribution of pathological α-synuclein (αSyn) aggregates throughout the body raises inquiries regarding the etiology, which has prompted several hypotheses, with the most prominent one being αSyn-associated proteinopathy. The identification of αSyn protein within Lewy bodies, coupled with genetic evidence linking αSyn locus duplication, triplication, as well as point mutations to familial Parkinson's disease, has underscored the significance of αSyn in initiating and propagating Lewy body pathology throughout the brain. In monogenic and sporadic PD, the presence of early inflammation and synaptic dysfunction leads to αSyn aggregation and neuronal death through mitochondrial, lysosomal, and endosomal functional impairment. However, much remains to be understood about αSyn pathogenesis, which is heavily grounded in biomarkers and treatment strategies. In this review, we provide emerging new evidence on the current knowledge about αSyn's pathophysiological impact on PD, and its presumable role as a specific disease biomarker or main target of disease-modifying therapies, highlighting that this understanding today offers the best potential of disease-modifying therapy in the near future.

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

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

Tilorone mitigates the propagation of α-synucleinopathy in a midbrain-like organoid model

BACKGROUND

Parkinson's disease (PD) is a neurodegenerative condition characterized by the loss of dopaminergic neurons and the accumulation of Lewy-body protein aggregates containing misfolded α-synuclein (α-syn) in a phosphorylated form. The lack of effective models for drug screens has hindered drug development studies for PD. However, the recent development of in vitro brain-like organoids provides a new opportunity for evaluating therapeutic agents to slow the progression of this chronic disease.

METHODS

In this study, we used a 3D brain-like organoid model to investigate the potential of repurposing Tilorone, an anti-viral drug, for impeding the propagation of α-synucleinopathy. We assessed the effect of Tilorone on the uptake of fluorescently labeled α-syn preformed fibrils (sPFF) and sPFF-induced apoptosis using confocal microscopy. We also examined Tilorone's impact on the phosphorylation of endogenous α-syn induced by pathogenic sPFF by immunoblotting midbrain-like organoid extracts. Additionally, quantitative RT-PCR and proteomic profiling of sPFF-treated organoids were conducted to evaluate the global impact of Tilorone treatment on tissue homeostasis in the 3D organoid model.

RESULTS

Tilorone inhibits the uptake of sPFF in both mouse primary neurons and human midbrain-like organoids. Tilorone also reduces the phosphorylation of endogenous α-syn induced by pathogenic α-syn fibrils and mitigates α-syn fibril-induced apoptosis in midbrain-like organoids. Proteomic profiling of fibril-treated organoids reveals substantial alterations in lipid homeostasis by α-syn fibrils, which are reversed by Tilorone treatment. Given its safety profile in clinics, Tilorone may be further developed as a therapeutic intervention to alleviate the propagation of synucleinopathy in PD patients.

Comparing GBA1-Parkinson's disease and idiopathic Parkinson's disease: α-Synuclein oligomers and synaptic density as biomarkers in the skin biopsy

The main genetic risk factors for Parkinson's disease (PD) are presently represented by variants in GBA1 gene encoding for the β-glucocerebrosidase (GCase). Searching for a peripheral biomarker that can be used for selecting and monitoring patients in clinical trials targeting GBA1-associated PD (GBA1-PD) is a current challenge. We previously demonstrated that α-synuclein oligomers expressed as proximity ligation assay (PLA) score in synaptic terminals of skin biopsy are a reliable biomarker for distinguishing idiopathic PD (iPD) from healthy controls (HC). This cross-sectional study investigates an unexplored cohort of GBA1-PD (n = 27) compared to 28 HC, and 36 iPD cases to (i) analyze α-synuclein oligomers and quantify them throughout PLA score, (ii) investigate GCase expression in brain and synaptic terminals targeting the sweat gland, (iii) unravel indicators that could differentiate patients with specific GBA1 mutations. PLA score discriminates GBA1-PD from HC with sensitivity = 88.9% (95% CI 70.84-97.65), specificity = 88.5% (95% CI 69.85-97.55), and PPV = 88.9% (95% CI 73.24-95.90), AUC value = 0.927 (95% CI 0.859-0.996). No difference was found between GBA1-PD patients and iPD, suggesting a common pathological pathway based on α-synuclein oligomers. GCase score did not differ in GBA1-PD, iPD, and HC in the synaptic terminals, whereas a positive correlation was found between PLA score and GCase score. Moreover, a significant increase in synaptic density was observed in GBA1-PD compared to iPD and HC (P < 0.0001). Employing ROC curve to discriminate GBA1-PD from iPD, we found an AUC value for synaptic density = 0.855 (95% CI 0.749-0.961) with sensitivity = 85.2% (95% CI 66.27%-95.81%), specificity = 77.1% (95% CI 59.86%-89.58%), and PPV = 74.19% (60.53%-84.35%). The highest synaptic density values were observed in p.N409S patients. This work points out to the value of both PLA score and synaptic density in distinguishing GBA1-PD from iPD and to their potential to stratify and monitor patients in the context of new pathway-specific therapeutic options.

Intra-striatal infusion of the small molecule alpha-synuclein aggregator, FN075, does not enhance parkinsonism in a subclinical AAV-alpha-synuclein rat model

Numerous challenges hinder the development of neuroprotective treatments for Parkinson's disease, with a regularly identified issue being the lack of clinically relevant animal models. Viral vector overexpression of α-synuclein is widely considered the most relevant model; however, this has been limited by high variability and inconsistency. One potential method of optimisation is pairing it with a secondary insult such as FN075, a synthetic molecule demonstrated to accelerate α-synucleinopathy. Thus, the aim of this study was to investigate if sequential infusion of adeno-associated virus (AAV)-α-synuclein and FN075 into the rat brain can replicate α-synucleinopathy, nigrostriatal pathology and motor dysfunction associated with Parkinson's disease. Rats received a unilateral injection of AAV-α-synuclein (or AAV-green fluorescent protein) into two sites in the substantia nigra, followed 4 weeks later by unilateral injection of FN075 (or vehicle) into the striatum. Animals underwent behavioural testing every 4 weeks until sacrifice at 20 weeks, followed by immunohistochemistry assessment post-mortem. As anticipated, AAV-α-synuclein led to extensive overexpression of human α-synuclein throughout the nigrostriatal pathway, as well as elevated levels of phosphorylated and aggregated forms of the protein. However, the sequential administration of FN075 into the striatum did not exacerbate any of the α-synuclein pathology. Furthermore, despite the extensive α-synuclein pathology, neither administration of AAV-α-synuclein nor FN075, alone or in combination, was sufficient to induce dopaminergic degeneration or motor deficits. In conclusion, this approach did not replicate the key characteristics of Parkinson's disease, and further studies are required to create more representational models for testing of novel compounds and treatments for Parkinson's disease.

N-acetylation of α-synuclein enhances synaptic vesicle clustering mediated by α-synuclein and lysophosphatidylcholine

Post-translational modifications (PTMs) of α-synuclein (α-syn) such as acetylation and phosphorylation play important yet distinct roles in regulating α-syn conformation, membrane binding, and amyloid aggregation. However, how PTMs regulate α-syn function in presynaptic terminals remains unclear. Previously, we reported that α-syn clusters synaptic vesicles (SV)1, and neutral phospholipid lysophosphatidylcholine (LPC) can mediate this clustering2. Here, based on our previous findings, we further demonstrate that N-terminal acetylation, which occurs under physiological conditions and is irreversible in mammalian cells, significantly enhances the functional activity of α-syn in clustering SVs. Mechanistic studies reveal that this enhancement is caused by the N-acetylation-promoted insertion of α-syn's N-terminus and increased intermolecular interactions on the LPC-containing membrane. Our work demonstrates that N-acetylation fine-tunes α-syn-LPC interaction for mediating α-syn's function in SV clustering.

Current insights and assumptions on α-synuclein in Lewy body disease

Lewy body disorders are heterogeneous neurological conditions defined by intracellular inclusions composed of misshapen α-synuclein protein aggregates. Although α-synuclein aggregates are only one component of inclusions and not strictly coupled to neurodegeneration, evidence suggests they seed the propagation of Lewy pathology within and across cells. Genetic mutations, genomic multiplications, and sequence polymorphisms of the gene encoding α-synuclein are also causally linked to Lewy body disease. In nonfamilial cases of Lewy body disease, the disease trigger remains unidentified but may range from industrial/agricultural toxicants and natural sources of poisons to microbial pathogens. Perhaps due to these peripheral exposures, Lewy inclusions appear at early disease stages in brain regions connected with cranial nerves I and X, which interface with inhaled and ingested environmental elements in the nasal or gastrointestinal cavities. Irrespective of its identity, a stealthy disease trigger most likely shifts soluble α-synuclein (directly or indirectly) into insoluble, cross-β-sheet aggregates. Indeed, β-sheet-rich self-replicating α-synuclein multimers reside in patient plasma, cerebrospinal fluid, and other tissues, and can be subjected to α-synuclein seed amplification assays. Thus, clinicians should be able to capitalize on α-synuclein seed amplification assays to stratify patients into potential responders versus non-responders in future clinical trials of α-synuclein targeted therapies. Here, we briefly review the current understanding of α-synuclein in Lewy body disease and speculate on pathophysiological processes underlying the potential transmission of α-synucleinopathy across the neuraxis.

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.

Immunomodulator-Derived Nanoparticles Induce Neuroprotection and Regulatory T Cell Action to Alleviate Parkinsonism

Post-translational modification, mitochondrial abruptions, neuroinflammation, and α-synuclein (α-Syn) aggregation are considered as major causes of Parkinson's disease (PD) pathogenesis. The recent literature highlights neuroimmune cross talk and the negative role of immune effector T (Teff) and positive regulation by regulatory T (Treg) cells in PD treatment. Herein, a strategy to endow Treg action paves the path for development of PD treatment. Thus, we explored the neuroprotective efficiency of the immunomodulator and PP2A (protein phosphatase 2) activator, FTY720 nanoparticles in in vivo experimental PD models. Repurposing of FTY720 for PD is known due to its protective effect by reducing PD and its camouflaged role in endowing EZH2-mediated epigenetic regulation of PD. EZH2-FOXP3 interaction is necessary for the neuroprotective Treg cell activity. Therefore, we synthesized FTY720 nanoparticles to improve FTY720 protective efficacy in an in vivo PD model to explore the PP2A mediated signaling. We confirmed the formation of FTY720NPs, and the results of the behavioral and protein expression study showed the significant neuroprotective efficiency of our nanoformulations. In the exploration of neuroprotective mechanism, several lines of evidence confirmed FTY720NPs mediated induction of PP2A/EZH2/FOXP3 signaling in the induction of Treg cells effect in in vivo PD treatment. In summary, our nanoformulations have novel potential to alleviate PD by inducing PP2A-induced epigenetic regulation-mediated neuroimmunomodulation at the clinical setup.

Human tripartite cortical network model for temporal assessment of alpha-synuclein aggregation and propagation in Parkinson's Disease

Previous studies have shown that aggregated alpha-synuclein (α-s) protein, a key pathological marker of Parkinson's disease (PD), can propagate between cells, thus participating in disease progression. This prion-like propagation has been widely studied using in vivo and in vitro models, including rodent and human cell cultures. In this study, our focus was on temporal assessment of functional changes during α-s aggregation and propagation in human induced pluripotent stem cell (hiPSC)-derived neuronal cultures and in engineered networks. Here, we report an engineered circular tripartite human neuronal network model in a microfluidic chip integrated with microelectrode arrays (MEAs) as a platform to study functional markers during α-s aggregation and propagation. We observed progressive aggregation of α-s in conventional neuronal cultures and in the exposed (proximal) compartments of circular tripartite networks following exposure to preformed α-s fibrils (PFF). Furthermore, aggregated forms propagated to distal compartments of the circular tripartite networks through axonal transport. We observed impacts of α-s aggregation on both the structure and function of neuronal cells, such as in presynaptic proteins, mitochondrial motility, calcium oscillations and neuronal activity. The model enabled an assessment of the early, middle, and late phases of α-s aggregation and its propagation during a 13-day follow-up period. While our temporal analysis suggested a complex interplay of structural and functional changes during the in vitro propagation of α-s aggregates, further investigation is required to elucidate the underlying mechanisms. Taken together, this study demonstrates the technical potential of our introduced model for conducting in-depth analyses for revealing such mechanisms.

An antioxidant and anti-ER stress combination therapy elevates phosphorylation of α-Syn at serine 129 and alleviates post-TBI PD-like pathology in a sex-specific manner in mice

Clinical studies have shown that traumatic brain injury (TBI) increases the onset of Parkinson's disease (PD) in later life by >50%. Oxidative stress, endoplasmic reticulum (ER) stress, and inflammation are the major drivers of both TBI and PD pathologies. We presently evaluated if curtailing oxidative stress and ER stress concomitantly using a combination of apocynin and tert-butylhydroquinone and salubrinal during the acute stage after TBI in mice reduces the severity of late-onset PD-like pathology. The effect of multiple low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on post-TBI neurodegeneration was also evaluated. The combo therapy elevated the level of phosphorylation at serine 129 (pS129) of α-Syn in the pericontusional cortex of male mice at 72 h post-TBI. Motor and cognitive deficits induced by TBI lasted at least 3 months and the combo therapy curtailed these deficits in both sexes. At 3 months post-TBI, male mice given combo therapy exhibited significantly lesser α-Syn aggregates in the SN and higher TH+ cells in the SNpc, compared to vehicle control. However, the aggregate number was not significantly different between groups of female mice. Moreover, TBI-induced loss of TH+ cells was negligible in female mice irrespective of treatment. The MPTP treatment aggravated PD-like pathology in male mice but had a negligible effect on the loss of TH+ cells in female mice. Thus, the present study indicates that mitigation of TBI-induced oxidative stress and ER stress at the acute stage could potentially reduce the risk of post-TBI PD-like pathology at least in male mice, plausibly by elevating pS129-α-Syn level.

MicroRNA-124-3p Modulates Alpha-Synuclein Expression Levels in a Paraquat-Induced in vivo Model for Parkinson's Disease

Parkinson's disease (PD) is the second most prevalent neurodegenerative disease and the most common movement disorder. Although PD etiology is not fully understood, alpha (α)-synuclein is a key protein involved in PD pathology. MicroRNAs (miRNA), small gene regulatory RNAs that control gene expression, have been identified as biomarkers and potential therapeutic targets for brain diseases, including PD. In particular, miR-124 is downregulated in the plasma and brain samples of PD patients. Recently we showed that the brain delivery of miR-124 counteracts 6-hydroxydopamine-induced motor deficits. However, its role in α-synuclein pathology has never been addressed. Here we used paraquat (PQ)-induced rat PD model to evaluate the role of miR-124-3p in α-synuclein accumulation and dopaminergic neuroprotection. Our results showed that an intranigral administration of miR-124-3p reduced the expression and aggregation of α-synuclein in the substantia nigra (SN) of rats exposed to PQ. NADPH oxidases (NOX), responsible for reactive oxygen species generation, have been considered major players in the development of α-synuclein pathology. Accordingly, miR-124-3p decreased protein expression levels of NOX1 and its activator, small GTPase Rac1, in the SN of PQ-lesioned rats. Moreover, miR-124-3p was able to counteract the reduced levels of pituitary homeobox 3 (PITX3), a protein required for the dopaminergic phenotype, induced by PQ in the SN. This is the first study showing that miR-124-3p decreases PQ-induced α-synuclein levels and the associated NOX1/Rac1 signaling pathway, and impacts PITX3 protein levels, supporting the potential of miR-124-3p as a disease-modifying agent for PD and related α-synucleinopathies.

Physiological roles of α-synuclein serine-129 phosphorylation - not an oxymoron

α-Synuclein (αS) is an abundant presynaptic protein that regulates neurotransmission. It is also a key protein implicated in a broad class of neurodegenerative disorders termed synucleinopathies, including Parkinson's disease (PD) and Lewy body dementia (LBD). Pathological αS deposits in these diseases, Lewy bodies (LBs)/neurites (LNs), contain about 90% of αS in its phospho-serine129 (pS129) form. Therefore, pS129 is widely used as a surrogate marker of pathology. However, recent findings demonstrate that pS129 is also physiologically triggered by neuronal activity to positively regulate synaptic transmission. In this opinion article, we contrast the literature on pathological and physiological pS129, with a special focus on the latter. We emphasize that pS129 is ambiguous and knowledge about the context is necessary to correctly interpret changes in pS129.

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

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

Physiological and pathological effects of phase separation in the central nervous system

Phase separation, also known as biomolecule condensate, participates in physiological processes such as transcriptional regulation, signal transduction, gene expression, and DNA damage repair by creating a membrane-free compartment. Phase separation is primarily caused by the interaction of multivalent non-covalent bonds between proteins and/or nucleic acids. The strength of molecular multivalent interaction can be modified by component concentration, the potential of hydrogen, posttranslational modification, and other factors. Notably, phase separation occurs frequently in the cytoplasm of mitochondria, the nucleus, and synapses. Phase separation in vivo is dynamic or stable in the normal physiological state, while abnormal phase separation will lead to the formation of biomolecule condensates, speeding up the disease progression. To provide candidate suggestions for the clinical treatment of nervous system diseases, this review, based on existing studies, carefully and systematically represents the physiological roles of phase separation in the central nervous system and its pathological mechanism in neurodegenerative diseases.

Sensor-integrated brain-on-a-chip platforms: Improving the predictive validity in neurodegenerative research

Affecting millions of individuals worldwide, neurodegenerative diseases (NDDs) pose a significant and growing health concern in people over the age of 60 years. Contributing to this trend are the steady increase in the aging population coupled with a persistent lack of disease-altering treatment strategies targeting NDDs. The absence of efficient therapeutics can be attributed to high failure rates in clinical trials and the ineptness of animal models in preceding preclinical studies. To that end, in recent years, significant research effort has been dedicated to the development of human cell-based preclinical disease models characterized by a higher degree of predictive validity. However, a key requirement of any in vitro model constitutes the precise knowledge and replication of the target tissues' (patho-)physiological microenvironment. Herein, microphysiological systems have demonstrated superiority over conventional static 2D/3D in vitro cell culture systems, as they allow for the emulation and continuous monitoring of the onset, progression, and remission of disease-associated phenotypes. This review provides an overview of recent advances in the field of NDD research using organ-on-a-chip platforms. Specific focus is directed toward non-invasive sensing strategies encompassing electrical, electrochemical, and optical sensors. Additionally, promising on- and integrable off-chip sensing strategies targeting key analytes in NDDs will be presented and discussed in detail.

4-Oxo-2-Nonenal- and Agitation-Induced Aggregates of α-Synuclein and Phosphorylated α-Synuclein with Distinct Biophysical Properties and Biomedical Applications

α-Synuclein (α-syn) can form oligomers, protofibrils, and fibrils, which are associated with the pathogenesis of Parkinson's disease and other synucleinopathies. Both the lipid peroxidation product 4-oxo-2-nonenal (ONE) and agitation can induce aggregation of α-syn and phosphorylated α-syn. Thus, clarification of the characteristics of different α-syn species could help to select suitable aggregates for diagnosis and elucidate the pathogenesis of diseases. Here, we characterized ONE-induced wild-type (WT) α-syn aggregates (OW), ONE-induced phosphorylated α-syn (p-α-syn) aggregates (OP), agitation-induced α-syn preformed fibrils (PFF), and agitation-induced p-α-syn preformed fibrils (pPFF). Thioflavin T (ThT) dying demonstrated that OW and OP had fewer fibrils than the PFF and pPFF. Transmission electron microscopy revealed that the lengths of PFF and pPFF were similar, but the diameters differed. OW and OP had more compact structures than PFF and pPFF. Aggregation of p-α-syn was significantly faster than WT α-syn. Furthermore, OW and OP were more sodium dodecyl sulfate-stable and proteinase K-resistant, suggesting greater stability and compactness, while aggregates of PFF and pPFF were more sensitive to proteinase K treatment. Both ONE- and agitation-induced aggregates were cytotoxic when added exogenously to SH-SY5Y cells with increasing incubation times, but the agitation-induced aggregates caused cell toxicity in a shorter time and more p-α-syn inclusions. Similarly, p-proteins were more cytotoxic than non-p-proteins. Finally, all four aggregates were used as standard antigens to establish sandwich enzyme-linked immunosorbent assay (ELISA). The results showed that the recognition efficiency of OW and OP was more sensitive than that of PFF and pPFF. The OW- and OP-specific ELISA for detection of p-α-syn and α-syn in plasma samples of Thy1-α-syn transgenic mice showed that the content of aggregates could reflect the extent of disease. ONE and agitation induced the formation of α-syn aggregates with distinct biophysical properties and biomedical applications.

Rapid FRET Assay for the Early Detection of Alpha-Synuclein Aggregation in Parkinson's Disease

Alpha-synuclein (α-Syn) is a key protein of Parkinson's disease (PD). Oligomers formed by misfolding and aggregation of α-Syn can cause many pathological phenomena and aggravate the development of PD. Therefore, sensitive and accurate detection of oligomers is essential to understanding the pathology of PD and beneficial to screening and developing new drugs against PD. Here, we demonstrated a simple and sensitive method to detect the early aggregation of α-Syn via Förster resonance energy transfer (FRET) technology. We performed systematic investigations of the FRET sensitizations, efficiencies, and donor-to-acceptor distances during α-Syn aggregation, which was proved to be more sensitive to reflect small distance changes in the early stage of α-Syn aggregation, especially for α-Syn oligomers. The FRET assays were also applied to study the influence of Ser129 phosphorylation (pS129) on the aggregation rate of α-Syn. Our results showed that pS129 modification promotes α-Syn aggregation and enhances the ability of preformed fibrils to induce monomer aggregation. pS129 also increased the cytotoxicity of α-Syn. These results are of great significance for a better understanding of the pathological mechanisms of PD and future PD drug development.

Widespread nuclear lamina injuries defeat proteostatic purposes of α-synuclein amyloid inclusions

Biogenesis of inclusion bodies (IBs) facilitates protein quality control (PQC). Canonical aggresomes execute degradation of misfolded proteins while non-degradable amyloids sequester into insoluble protein deposits. Lewy bodies (LBs) are filamentous amyloid inclusions of α-synuclein, but PQC benefits and drawbacks associated with LB-like IBs remain underexplored. Here, we report that crosstalk between filamentous LB-like IBs and aggresome-like IBs of α-synuclein (Syn-aggresomes) buffer the load, aggregation state, and turnover of the amyloidogenic protein in mouse primary neurons and HEK293T cells. Filamentous LB-like IBs possess unorthodox PQC capacities of self-quarantining α-synuclein amyloids and being degradable upon receding fresh amyloidogenesis. Syn-aggresomes equilibrate biogenesis of filamentous LB-like IBs by facilitating spontaneous degradation of α-synuclein and conditional turnover of disintegrated α-synuclein amyloids. Thus, both types of IB primarily contribute to PQC. Incidentally, the overgrown perinuclear LB-like IBs become degenerative once these are misidentified by BICD2, a cargo-adapter for the cytosolic motor-protein dynein. Microscopy indicates that microtubules surrounding the perinuclear filamentous inclusions are also distorted, misbalancing the cytoskeleton-nucleoskeleton tension leading to widespread lamina injuries. Together, nucleocytoplasmic mixing, DNA damage, and deregulated transcription of stress chaperones defeat the proteostatic purposes of the filamentous amyloids of α-synuclein.

An artificial protein modulator reprogramming neuronal protein functions

Reversible protein phosphorylation, regulated by protein phosphatases, fine-tunes target protein function and plays a vital role in biological processes. Dysregulation of this process leads to aberrant post-translational modifications (PTMs) and contributes to disease development. Despite the widespread use of artificial catalysts as enzyme mimetics, their direct modulation of proteins remains largely unexplored. To address this gap and enable the reversal of aberrant PTMs for disease therapy, we present the development of artificial protein modulators (APROMs). Through atomic-level engineering of heterogeneous catalysts with asymmetric catalytic centers, these modulators bear structural similarities to protein phosphatases and exhibit remarkable ability to destabilize the bridging μ3-hydroxide. This activation of catalytic centers enables spontaneous hydrolysis of phospho-substrates, providing precise control over PTMs. Notably, APROMs, with protein phosphatase-like characteristics, catalytically reprogram the biological function of α-synuclein by directly hydrolyzing hyperphosphorylated α-synuclein. Consequently, synaptic function is reinforced in Parkinson's disease. Our findings offer a promising avenue for reprogramming protein function through de novo PTMs strategy.

Young human alpha synuclein transgenic (BAC-SNCA) mice display sex- and gene-dose-dependent phenotypic disturbances

Parkinson's disease (PD) is a common neurodegenerative movement disorder, characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and the accumulation of aggregated alpha synuclein (aSyn). The disease often presents with early prodromal non-motor symptoms and later motor symptoms. Diagnosing PD based purely on motor symptoms is often too late for successful intervention, as a significant neuronal loss has already occurred. Furthermore, the lower prevalence of PD in females is not well understood, highlighting the need for a better understanding of the interaction between sex and aSyn, the crucial protein for PD pathogenesis. Here, we conducted a comprehensive phenotyping study in 1- to 5-month-old mice overexpressing human aSyn gene (SNCA) in a bacterial artificial chromosome (BAC-SNCA). We demonstrate a SNCA gene-dose-dependent increase of human aSyn and phosphorylated aSyn, as well as a decrease in tyrosine hydroxylase expression in BAC-SNCA mice, with more pronounced effects in male mice. Phosphorylated aSyn was already found in the dorsal motor nucleus of the vagus nerve of 2-month-old mice. This was time-wise associated with significant gait altrations in BAC-SNCA mice as early as 1 and 3 months of age using CatWalk gait analysis. Furthermore, anxiety-related behavioral tests revealed an increase in anxiety levels in male BAC-SNCA mice. Finally, 5-month-old male BAC-SNCA mice exhibited a SNCA gene-dose-dependent elevation in energy expenditure in automated home-cage monitoring. For the first time, these findings describe early-onset, sex- and gene-dose-dependent, aSyn-mediated disturbances in BAC-SNCA mice, providing a model for sex-differences, early-onset neuropathology, and prodromal symptoms of PD.

Role of the Gut Microbiome and Bacterial Amyloids in the Development of Synucleinopathies

Less than ten years ago, evidence began to accumulate about association between the changes in the composition of gut microbiota and development of human synucleinopathies, in particular sporadic form of Parkinson's disease. We collected data from more than one hundred and thirty experimental studies that reported similar results and summarized the frequencies of detection of different groups of bacteria in these studies. It is important to note that it is extremely rare that a unidirectional change in the population of one or another group of microorganisms (only an elevation or only a reduction) was detected in the patients with Parkinson's disease. However, we were able to identify several groups of bacteria that were overrepresented in the patients with Parkinson's disease in the analyzed studies. There are various hypotheses about the molecular mechanisms that explain such relationships. Usually, α-synuclein aggregation is associated with the development of inflammatory processes that occur in response to the changes in the microbiome. However, experimental evidence is accumulating on the influence of bacterial proteins, including amyloids (curli), as well as various metabolites, on the α-synuclein aggregation. In the review, we provided up-to-date information about such examples.

In silico analysis of alpha-synuclein protein variants and posttranslational modifications related to Parkinson's disease

Parkinson's disease (PD) is among the most prevalent neurodegenerative disorders, affecting over 10 million people worldwide. The protein encoded by the SNCA gene, alpha-synuclein (ASYN), is the major component of Lewy body (LB) aggregates, a histopathological hallmark of PD. Mutations and posttranslational modifications (PTMs) in ASYN are known to influence protein aggregation and LB formation, possibly playing a crucial role in PD pathogenesis. In this work, we applied computational methods to characterize the effects of missense mutations and PTMs on the structure and function of ASYN. Missense mutations in ASYN were compiled from the literature/databases and underwent a comprehensive predictive analysis. Phosphorylation and SUMOylation sites of ASYN were retrieved from databases and predicted by algorithms. ConSurf was used to estimate the evolutionary conservation of ASYN amino acids. Molecular dynamics (MD) simulations of ASYN wild-type and variants A30G, A30P, A53T, and G51D were performed using the GROMACS package. Seventy-seven missense mutations in ASYN were compiled. Although most mutations were not predicted to affect ASYN stability, aggregation propensity, amyloid formation, and chaperone binding, the analyzed mutations received relatively high rates of deleterious predictions and predominantly occurred at evolutionarily conserved sites within the protein. Moreover, our predictive analyses suggested that the following mutations may be possibly harmful to ASYN and, consequently, potential targets for future investigation: K6N, T22I, K34E, G36R, G36S, V37F, L38P, G41D, and K102E. The MD analyses pointed to remarkable flexibility and essential dynamics alterations at nearly all domains of the studied variants, which could lead to impaired contact between NAC and the C-terminal domain triggering protein aggregation. These alterations may have functional implications for ASYN and provide important insight into the molecular mechanism of PD, supporting the design of future biomedical research and improvements in existing therapies for the disease.

ZCCHC17 Modulates Neuronal RNA Splicing and Supports Cognitive Resilience in Alzheimer's Disease

ZCCHC17 is a putative master regulator of synaptic gene dysfunction in Alzheimer's disease (AD), and ZCCHC17 protein declines early in AD brain tissue, before significant gliosis or neuronal loss. Here, we investigate the function of ZCCHC17 and its role in AD pathogenesis using data from human autopsy tissue (consisting of males and females) and female human cell lines. Co-immunoprecipitation (co-IP) of ZCCHC17 followed by mass spectrometry analysis in human iPSC-derived neurons reveals that ZCCHC17's binding partners are enriched for RNA-splicing proteins. ZCCHC17 knockdown results in widespread RNA-splicing changes that significantly overlap with splicing changes found in AD brain tissue, with synaptic genes commonly affected. ZCCHC17 expression correlates with cognitive resilience in AD patients, and we uncover an APOE4-dependent negative correlation of ZCCHC17 expression with tangle burden. Furthermore, a majority of ZCCHC17 interactors also co-IP with known tau interactors, and we find a significant overlap between alternatively spliced genes in ZCCHC17 knockdown and tau overexpression neurons. These results demonstrate ZCCHC17's role in neuronal RNA processing and its interaction with pathology and cognitive resilience in AD, and suggest that the maintenance of ZCCHC17 function may be a therapeutic strategy for preserving cognitive function in the setting of AD pathology.

Distinct tau and alpha-synuclein molecular signatures in Alzheimer's disease with and without Lewy bodies and Parkinson's disease with dementia

Alpha-synuclein (aSyn) pathology is present in approximately 50% of Alzheimer's disease (AD) cases at autopsy and might impact the age-of-onset and disease progression in AD. Here, we aimed to determine whether tau and aSyn profiles differ between AD cases with Lewy bodies (AD-LB), pure AD and Parkinson's disease with dementia (PDD) cases using epitope-, post-translational modification- (PTM) and isoform-specific tau and aSyn antibody panels spanning from the N- to C-terminus. We included the middle temporal gyrus (MTG) and amygdala (AMY) of clinically diagnosed and pathologically confirmed cases and performed dot blotting, western blotting and immunohistochemistry combined with quantitative and morphological analyses. All investigated phospho-tau (pTau) species, except pT181, were upregulated in AD-LB and AD cases compared to PDD and control cases, but no significant differences were observed between AD-LB and AD subjects. In addition, tau antibodies targeting the proline-rich regions and C-terminus showed preferential binding to AD-LB and AD brain homogenates. Antibodies targeting C-terminal aSyn epitopes and pS129 aSyn showed stronger binding to AD-LB and PDD cases compared to AD and control cases. Two pTau species (pS198 and pS396) were specifically detected in the soluble protein fractions of AD-LB and AD subjects, indicative of early involvement of these PTMs in the multimerization process of tau. Other phospho-variants for both tau (pT212/S214, pT231 and pS422) and aSyn (pS129) were only detected in the insoluble protein fraction of AD-LB/AD and AD-LB/PDD cases, respectively. aSyn load was higher in the AMY of AD-LB cases compared to PDD cases, suggesting aggravated aSyn pathology under the presence of AD pathology, while tau load was similar between AD-LB and AD cases. Co-localization of pTau and aSyn could be observed within astrocytes of AD-LB cases within the MTG. These findings highlight a unique pathological signature for AD-LB cases compared to pure AD and PDD cases.

Disruption of lysosomal proteolysis in astrocytes facilitates midbrain organoid proteostasis failure in an early-onset Parkinson's disease model

Accumulation of advanced glycation end products (AGEs) on biopolymers accompanies cellular aging and drives poorly understood disease processes. Here, we studied how AGEs contribute to development of early onset Parkinson's Disease (PD) caused by loss-of-function of DJ1, a protein deglycase. In induced pluripotent stem cell (iPSC)-derived midbrain organoid models deficient for DJ1 activity, we find that lysosomal proteolysis is impaired, causing AGEs to accumulate, α-synuclein (α-syn) phosphorylation to increase, and proteins to aggregate. We demonstrated these processes are at least partly driven by astrocytes, as DJ1 loss reduces their capacity to provide metabolic support and triggers acquisition of a pro-inflammatory phenotype. Consistently, in co-cultures, we find that DJ1-expressing astrocytes are able to reverse the proteolysis deficits of DJ1 knockout midbrain neurons. In conclusion, astrocytes' capacity to clear toxic damaged proteins is critical to preserve neuronal function and their dysfunction contributes to the neurodegeneration observed in a DJ1 loss-of-function PD model.

Excess phosphoserine-129 α-synuclein induces synaptic vesicle trafficking and declustering defects at a vertebrate synapse

α-Synuclein is a presynaptic protein that regulates synaptic vesicle (SV) trafficking. In Parkinson's disease (PD) and dementia with Lewy bodies (DLB), α-synuclein aberrantly accumulates throughout neurons, including at synapses. During neuronal activity, α-synuclein is reversibly phosphorylated at serine 129 (pS129). While pS129 comprises ∼4% of total α-synuclein under physiological conditions, it dramatically increases in PD and DLB brains. The impacts of excess pS129 on synaptic function are currently unknown. We show here that compared with wild-type (WT) α-synuclein, pS129 exhibits increased binding and oligomerization on synaptic membranes and enhanced vesicle "microclustering" in vitro. Moreover, when acutely injected into lamprey reticulospinal axons, excess pS129 α-synuclein robustly localized to synapses and disrupted SV trafficking in an activity-dependent manner, as assessed by ultrastructural analysis. Specifically, pS129 caused a declustering and dispersion of SVs away from the synaptic vicinity, leading to a significant loss of total synaptic membrane. Live imaging further revealed altered SV cycling, as well as microclusters of recently endocytosed SVs moving away from synapses. Thus, excess pS129 caused an activity-dependent inhibition of SV trafficking via altered vesicle clustering/reclustering. This work suggests that accumulation of pS129 at synapses in diseases like PD and DLB could have profound effects on SV dynamics.

Exploring the impact of mutation and post-translational modification on α-Synuclein: Insights from molecular dynamics simulations with and without copper

We report molecular dynamics simulations of two modifications to α-Synuclein, namely A53T mutation and phosphorylation at Ser129, which have been observed in Parkinson's disease patients. Both modifications are close to known metal binding sites, so as well as each modified peptide we also study Cu(II) bound to N-terminal and C-terminal residues. We show that A53T is predicted to cause increased β-sheet content of the peptide, with a persistent β-hairpin between residues 35-55 particularly notable. Phosphorylation has less effect on secondary structure but is predicted to significantly increase the size of the peptide, especially when bound to Cu(II), which is ascribed to reduced interaction of C-terminal sequence with central non-amyloid component. In addition, estimate of binding free energy to Cu(II) indicates A53T has little effect on metal-ion affinity, whereas phosphorylation markedly enhances the strength of binding. We suggest that the predicted changes in spatial extent and secondary structure of α-Synuclein may have implications for aggregation into Lewy bodies.

Non-invasive systemic viral delivery of human alpha-synuclein mimics selective and progressive neuropathology of Parkinson's disease in rodent brains

BACKGROUND

Alpha-synuclein (α-syn) aggregation into proteinaceous intraneuronal inclusions, called Lewy bodies (LBs), is the neuropathological hallmark of Parkinson's disease (PD) and related synucleinopathies. However, the exact role of α-syn inclusions in PD pathogenesis remains elusive. This lack of knowledge is mainly due to the absence of optimal α-syn-based animal models that recapitulate the different stages of neurodegeneration.

METHODS

Here we describe a novel approach for a systemic delivery of viral particles carrying human α-syn allowing for a large-scale overexpression of this protein in the mouse brain. This approach is based on the use of a new generation of adeno-associated virus (AAV), AAV-PHP.eB, with an increased capacity to cross the blood-brain barrier, thus offering a viable tool for a non-invasive and large-scale gene delivery in the central nervous system.

RESULTS

Using this model, we report that widespread overexpression of human α-syn induced selective degeneration of dopaminergic (DA) neurons, an exacerbated neuroinflammatory response in the substantia nigra and a progressive manifestation of PD-like motor impairments. Interestingly, biochemical analysis revealed the presence of insoluble α-syn oligomers in the midbrain. Together, our data demonstrate that a single non-invasive systemic delivery of viral particles overexpressing α-syn prompted selective and progressive neuropathology resembling the early stages of PD.

CONCLUSIONS

Our new in vivo model represents a valuable tool to study the role of α-syn in PD pathogenesis and in the selective vulnerability of nigral DA neurons; and offers the opportunity to test new strategies targeting α-syn toxicity for the development of disease-modifying therapies for PD and related disorders.

Skin Biopsy as a Diagnostic Tool for Synucleinopathies

Studies published in the last decade identified skin biopsies as a promising source of material for detecting alpha-synuclein (αSN). Alpha-synuclein gets deposited in the skin of patients with synucleinopathies, and therefore, a skin biopsy can be used to diagnose and confirm these diseases histopathologically. A skin biopsy can also be helpful for studies focusing on the nature of αSN deposits. The most important aspects of a biomarker are sensitivity, specificity, and technical feasibility. The potential for a skin biopsy to become the clinical tool of choice as a reliable biomarker for diagnosing synucleinopathies appears to be high, with consistently high sensitivity (>80%) and specificity approaching 100%. The review aims to provide an overview of the factors impacting skin biopsy's sensitivity, specificity, and feasibility in detecting dermal αSN deposits.

Alpha synuclein modulates mitochondrial Ca2+ uptake from ER during cell stimulation and under stress conditions

Alpha synuclein (a-syn) is an intrinsically disordered protein prevalent in neurons, and aggregated forms are associated with synucleinopathies including Parkinson's disease (PD). Despite the biomedical importance and extensive studies, the physiological role of a-syn and its participation in etiology of PD remain uncertain. We showed previously in model RBL cells that a-syn colocalizes with mitochondrial membranes, depending on formation of N-terminal helices and increasing with mitochondrial stress1. We have now characterized this colocalization and functional correlates in RBL, HEK293, and N2a cells. We find that expression of a-syn enhances stimulated mitochondrial uptake of Ca2+ from the ER, depending on formation of its N-terminal helices but not on its disordered C-terminal tail. Our results are consistent with a-syn acting as a tether between mitochondria and ER, and we show increased contacts between these two organelles using structured illumination microscopy. We tested mitochondrial stress caused by toxins related to PD, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP/MPP+) and carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and found that a-syn prevents recovery of stimulated mitochondrial Ca2+ uptake. The C-terminal tail, and not N-terminal helices, is involved in this inhibitory activity, which is abrogated when phosphorylation site serine-129 is mutated (S129A). Correspondingly, we find that MPTP/MPP+ and CCCP stress is accompanied by both phosphorylation (pS129) and aggregation of a-syn. Overall, our results indicate that a-syn can participate as a tethering protein to modulate Ca2+ flux between ER and mitochondria, with potential physiological significance. A-syn can also prevent cellular recovery from toxin-induced mitochondrial dysfunction, which may represent a pathological role of a-syn in the etiology of PD.

Peptide immunization against the C-terminal of alpha-synuclein reduces locomotor activity in mice overexpressing alpha-synuclein

Abnormal accumulation of alpha-synuclein (αSyn) in the remaining nigra dopaminergic neurons is a common neuropathological feature found in patients with Parkinson's disease (PD). Antibody-based immunotherapy has been considered a potential approach for PD treatment. This study aims to investigate the effectiveness of active immunization against αSyn in a mouse model of PD. Adult mice were immunized with or without a synthetic peptide containing the C-terminal residues of human αSyn and activation epitopes, followed by an intranigral injection of adeno-associated virus vectors for overexpressing human αSyn. Upon the peptide injection, αSyn-specific antibodies were raised, accompanied by degeneration of dopaminergic neurons and motor deficits. Furthermore, the induction of neuroinflammation was postulated by the elevation of astroglial and microglial markers in the immunized mice. Instead of lessening αSyn toxicity, this peptide vaccine caused an increase in the pathogenic species of αSyn. Our data demonstrated the potential adverse effects of active immunization to raise antibodies against the C-terminal fragment of αSyn. This drawback highlights the need for further investigation to weigh the pros and cons of immunotherapy in PD. Applying the αSyn C-terminal peptide vaccine for PD treatment should be cautiously exercised. This study provides valuable insights into the intricate interplay among immune intervention, αSyn accumulation, and neurodegeneration.

Flanking regions, amyloid cores, and polymorphism: the potential interplay underlying structural diversity

The β-sheet-rich amyloid core is the defining feature of protein aggregates associated with neurodegenerative disorders. Recent investigations have revealed that there exist multiple examples of the same protein, with the same sequence, forming a variety of amyloid cores with distinct structural characteristics. These structural variants, termed as polymorphs, are hypothesized to influence the pathological profile and the progression of different neurodegenerative diseases, giving rise to unique phenotypic differences. Thus, identifying the origin and properties of these structural variants remain a focus of studies, as a preliminary step in the development of therapeutic strategies. Here, we review the potential role of the flanking regions of amyloid cores in inducing polymorphism. These regions, adjacent to the amyloid cores, show a preponderance for being structurally disordered, imbuing them with functional promiscuity. The dynamic nature of the flanking regions can then manifest in the form of conformational polymorphism of the aggregates. We take a closer look at the sequences flanking the amyloid cores, followed by a review of the polymorphic aggregates of the well-characterized proteins amyloid-β, α-synuclein, Tau, and TDP-43. We also consider different factors that can potentially influence aggregate structure and how these regions can be viewed as novel targets for therapeutic strategies by utilizing their unique structural properties.

Mutant α-synuclein propagates via the lymphatic system of the brain in the monomeric state

Prion-like protein propagation is considered a common pathogenic mechanism in neurodegenerative diseases. Here we investigate the in vivo propagation pattern and aggregation state of mutant α-synuclein by injecting adeno-associated viral (AAV)-α-synuclein-A53T-EGFP into the mouse olfactory cortex. Comparison of aggregation states in various brain regions at multiple time points after injection using western blot analyses shows that the monomeric state of the mutant/misfolded protein propagates to remote brain regions by 2 weeks and that the propagated proteins aggregate in situ after being incorporated into neurons. Moreover, injection of Alexa 488-labeled α-synuclein-A53T confirms the monomeric propagation at 2 weeks. Super-resolution microscopy shows that both α-synuclein-A53T proteins propagate via the lymphatic system, penetrate perineuronal nets, and reach the surface of neurons. Electron microscopy shows that the propagated mutant/misfolded monomer forms fibrils characteristic of Parkinson's disease after its incorporation into neurons. These findings suggest a mode of propagation different from that of aggregate-dependent propagation.

Parkinson's disease neurons exhibit alterations in mitochondrial quality control proteins

Mitochondrial dysfunction has been suggested to contribute to Parkinson's disease pathogenesis, though an understanding of the extent or exact mechanism of this contribution remains elusive. This has been complicated by challenging nature of pathway-based analysis and an inability simultaneously study multiple related proteins within human brain tissue. We used imaging mass cytometry (IMC) to overcome these challenges, measuring multiple protein targets, whilst retaining the spatial relationship between targets in post-mortem midbrain sections. We used IMC to simultaneously interrogate subunits of the mitochondrial oxidative phosphorylation complexes, and several key signalling pathways important for mitochondrial homoeostasis, in a large cohort of PD patient and control cases. We revealed a generalised and synergistic reduction in mitochondrial quality control proteins in dopaminergic neurons from Parkinson's patients. Further, protein-protein abundance relationships appeared significantly different between PD and disease control tissue. Our data showed a significant reduction in the abundance of PINK1, Parkin and phosphorylated ubiquitinSer65, integral to the mitophagy machinery; two mitochondrial chaperones, HSP60 and PHB1; and regulators of mitochondrial protein synthesis and the unfolded protein response, SIRT3 and TFAM. Further, SIRT3 and PINK1 did not show an adaptive response to an ATP synthase defect in the Parkinson's neurons. We also observed intraneuronal aggregates of phosphorylated ubiquitinSer65, alongside increased abundance of mitochondrial proteases, LONP1 and HTRA2, within the Parkinson's neurons with Lewy body pathology, compared to those without. Taken together, these findings suggest an inability to turnover mitochondria and maintain mitochondrial proteostasis in Parkinson's neurons. This may exacerbate the impact of oxidative phosphorylation defects and ageing related oxidative stress, leading to neuronal degeneration. Our data also suggest that that Lewy pathology may affect mitochondrial quality control regulation through the disturbance of mitophagy and intramitochondrial proteostasis.

Alternative splicing in neurodegenerative disease and the promise of RNA therapies

Alternative splicing generates a myriad of RNA products and protein isoforms of different functions from a single gene. Dysregulated alternative splicing has emerged as a new mechanism broadly implicated in the pathogenesis of neurodegenerative diseases such as Alzheimer disease, amyotrophic lateral sclerosis, frontotemporal dementia, Parkinson disease and repeat expansion diseases. Understanding the mechanisms and functional outcomes of abnormal splicing in neurological disorders is vital in developing effective therapies to treat mis-splicing pathology. In this Review, we discuss emerging research and evidence of the roles of alternative splicing defects in major neurodegenerative diseases and summarize the latest advances in RNA-based therapeutic strategies to target these disorders.

Assessment of Aggregated and Exosome-Associated α-Synuclein in Brain Tissue and Cerebrospinal Fluid Using Specific Immunoassays

Even though it is currently well-established that α-synuclein aggregation is closely associated with the pathological events in Parkinson's disease (PD) and several other neurodegenerative disorders, collectively called synucleinopathies, the mechanistic link between α-synuclein aggregates and the onset and progression of neurodegeneration in these diseases remain unclear. The process of aggregation initiates from a structurally distorted monomer that gradually oligomerizes to generate a repertoire of fibrillar and oligomeric multimers that deposit within diseased cells in the brain. Total α-synuclein has been proposed as a potential biomarker in PD, but most of the studies do not discriminate between distinct α-synuclein conformers. To correlate protein measurements to disease pathology, we have developed a conformation-specific ELISA method that selectively detects fibrillar and oligomeric forms of α-synuclein without cross-reacting with monomers. We have used this assay to determine the levels of aggregated α-synuclein in human and mouse brain tissue as well as in CSF and CSF-derived exosomes from patients with synucleinopathy and control subjects. Our results verify the ability of the new assay to detect aggregated α-synuclein in complex matrices and support the idea that the levels of these conformers are related to the age of onset in PD patients, while CSF analysis showed that these species exist in low abundance in CSF and CSF-derived exosomes. Future studies will be required to fully assess the diagnostic usefulness of this ELISA in synucleinopathies.

Phosphorylation of α-synuclein at T64 results in distinct oligomers and exerts toxicity in models of Parkinson's disease

α-Synuclein accumulates in Lewy bodies, and this accumulation is a pathological hallmark of Parkinson's disease (PD). Previous studies have indicated a causal role of α-synuclein in the pathogenesis of PD. However, the molecular and cellular mechanisms of α-synuclein toxicity remain elusive. Here, we describe a novel phosphorylation site of α-synuclein at T64 and the detailed characteristics of this post-translational modification. T64 phosphorylation was enhanced in both PD models and human PD brains. T64D phosphomimetic mutation led to distinct oligomer formation, and the structure of the oligomer was similar to that of α-synuclein oligomer with A53T mutation. Such phosphomimetic mutation induced mitochondrial dysfunction, lysosomal disorder, and cell death in cells and neurodegeneration in vivo, indicating a pathogenic role of α-synuclein phosphorylation at T64 in PD.

Single point mutations at the S129 residue of α-synuclein and their effect on structure, aggregation, and neurotoxicity

Parkinson's disease is an age-related neurological disorder, and the pathology of the disease is linked to different types of aggregates of α-synuclein or alpha-synuclein (aS), which is an intrinsically disordered protein. The C-terminal domain (residues 96-140) of the protein is highly fluctuating and possesses random/disordered coil conformation. Thus, the region plays a significant role in the protein's solubility and stability by an interaction with other parts of the protein. In the current investigation, we examined the structure and aggregation behavior of two artificial single point mutations at a C-terminal residue at position 129 that represent a serine residue in the wild-type human aS (wt aS). Circular Dichroism (CD) and Raman spectroscopy were performed to analyse the secondary structure of the mutated proteins and compare it to the wt aS. Thioflavin T assay and atomic force microscopy imaging helped in understanding the aggregation kinetics and type of aggregates formed. Finally, the cytotoxicity assay gave an idea about the toxicity of the aggregates formed at different stages of incubation due to mutations. Compared to wt aS, the mutants S129A and S129W imparted structural stability and showed enhanced propensity toward the α-helical secondary structure. CD analysis showed proclivity of the mutant proteins toward α-helical conformation. The enhancement of α-helical propensity lengthened the lag phase of fibril formation. The growth rate of β-sheet-rich fibrillation was also reduced. Cytotoxicity tests on SH-SY5Y neuronal cell lines established that the S129A and S129W mutants and their aggregates were potentially less toxic than wt aS. The average survivability rate was ∼40% for cells treated with oligomers (presumably formed after 24 h of incubation of the freshly prepared monomeric protein solution) produced from wt aS and ∼80% for cells treated with oligomers obtained from mutant proteins. The relative structural stability with α-helical propensity of the mutants could be a plausible reason for their slow rate of oligomerization and fibrillation, and this was also the possible reason for reduced toxicity to neuronal cells.

Alpha Synuclein Modulates Mitochondrial Ca 2+ Uptake from ER During Cell Stimulation and Under Stress Conditions

Alpha synuclein (a-syn) is an intrinsically disordered protein prevalent in neurons, and aggregated forms are associated with synucleinopathies including Parkinson' disease (PD). Despite the biomedical importance and extensive studies, the physiological role of a-syn and its participation in etiology of PD remain uncertain. We showed previously in model RBL cells that a-syn colocalizes with mitochondrial membranes, depending on formation of N-terminal helices and increasing with mitochondrial stress. 1 We have now characterized this colocalization and functional correlates in RBL, HEK293, and N2a cells. We find that expression of a-syn enhances stimulated mitochondrial uptake of Ca 2+ from the ER, depending on formation of its N-terminal helices but not on its disordered C-terminal tail. Our results are consistent with a-syn acting as a tether between mitochondria and ER, and we show increased contacts between these two organelles using structured illumination microscopy. We tested mitochondrial stress caused by toxins related to PD, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP/MPP+) and carbonyl cyanide m-chlorophenyl hydrazone (CCCP), and found that a-syn prevents recovery of stimulated mitochondrial Ca 2+ uptake. The C-terminal tail, and not N-terminal helices, is involved in this inhibitory activity, which is abrogated when phosphorylation site serine-129 is mutated (S129A). Correspondingly, we find that MPTP/MPP+ and CCCP stress is accompanied by both phosphorylation (pS129) and aggregation of a-syn. Overall, our results indicate that a-syn can participate as a tethering protein to modulate Ca 2+ flux between ER and mitochondria, with potential physiological significance. A-syn can also prevent cellular recovery from toxin-induced mitochondrial dysfunction, which may represent a pathological role of a-syn in the etiology of PD.

Contribution of A-to-I RNA editing, M6A RNA Methylation, and Alternative Splicing to physiological brain aging and neurodegenerative diseases

Aging is a physiological and progressive phenomenon in all organisms' life cycle, characterized by the accumulation of degenerative processes triggered by several alterations within molecular pathways. These changes compromise cell fate, resulting in the loss of functions in tissues throughout the body, including the brain. Physiological brain aging has been linked to structural and functional alterations, as well as to an increased risk of neurodegenerative diseases. Post-transcriptional RNA modifications modulate mRNA coding properties, stability, translatability, expanding the coding capacity of the genome, and are involved in all cellular processes. Among mRNA post-transcriptional modifications, the A-to-I RNA editing, m6A RNA Methylation and Alternative Splicing play a critical role in all the phases of a neuronal cell life cycle and alterations in their mechanisms of action significantly contribute to aging and neurodegeneration. Here we review our current understanding of the contribution of A-to-I RNA editing, m6A RNA Methylation, and Alternative Splicing to physiological brain aging process and neurodegenerative diseases.

ZCCHC17 modulates neuronal RNA splicing and supports cognitive resilience in Alzheimer's disease

ZCCHC17 is a putative master regulator of synaptic gene dysfunction in Alzheimer's Disease (AD), and ZCCHC17 protein declines early in AD brain tissue, before significant gliosis or neuronal loss. Here, we investigate the function of ZCCHC17 and its role in AD pathogenesis. Co-immunoprecipitation of ZCCHC17 followed by mass spectrometry analysis in human iPSC-derived neurons reveals that ZCCHC17's binding partners are enriched for RNA splicing proteins. ZCCHC17 knockdown results in widespread RNA splicing changes that significantly overlap with splicing changes found in AD brain tissue, with synaptic genes commonly affected. ZCCHC17 expression correlates with cognitive resilience in AD patients, and we uncover an APOE4 dependent negative correlation of ZCCHC17 expression with tangle burden. Furthermore, a majority of ZCCHC17 interactors also co-IP with known tau interactors, and we find significant overlap between alternatively spliced genes in ZCCHC17 knockdown and tau overexpression neurons. These results demonstrate ZCCHC17's role in neuronal RNA processing and its interaction with pathology and cognitive resilience in AD, and suggest that maintenance of ZCCHC17 function may be a therapeutic strategy for preserving cognitive function in the setting of AD pathology.

Functional and neuropathological changes induced by injection of distinct alpha-synuclein strains: A pilot study in non-human primates

The role of alpha-synuclein in Parkinson's disease has been heavily investigated since its discovery as a component of Lewy bodies. Recent rodent data demonstrate that alpha-synuclein strain structure is critical for differential propagation and toxicity. Based on these findings, we have compared, for the first time, in this pilot study, the capacity of two alpha-synuclein strains and patient-derived Lewy body extracts to model synucleinopathies after intra-putaminal injection in the non-human primate brain. Functional alterations triggered by these injections were evaluated in vivo using glucose positron emission tomography imaging. Post-mortem immunohistochemical and biochemical analyses were used to detect neuropathological alterations in the dopaminergic system and alpha-synuclein pathology propagation. In vivo results revealed a decrease in glucose metabolism more pronounced in alpha-synuclein strain-injected animals. Histology showed a decreased number of dopaminergic tyrosine hydroxylase-positive cells in the substantia nigra to different extents according to the inoculum used. Biochemistry revealed that alpha-synuclein-induced aggregation, phosphorylation, and propagation in different brain regions are strain-specific. Our findings show that distinct alpha-synuclein strains can induce specific patterns of synucleinopathy in the non-human primate, changes in the nigrostriatal pathway, and functional alterations that resemble early-stage Parkinson's disease.

Lewy Body-like Pathology and Loss of Dopaminergic Neurons in Midbrain Organoids Derived from Familial Parkinson's Disease Patient

Progressive accumulation of α-Synuclein (αSyn) in Lewy bodies (LBs) and loss of dopaminergic (DA) neurons are the hallmark pathological features of Parkinson's disease (PD). Although currently available in vitro and in vivo models have provided crucial information about PD pathogenesis, the mechanistic link between the progressive accumulation of αSyn into LBs and the loss of DA neurons is still unclear. To address this, it is critical to model LB formation and DA neuron loss, the two key neuropathological aspects of PD, in a relevant in vitro system. In this study, we developed a human midbrain-like organoid (hMBO) model of PD. We demonstrated that hMBOs generated from induced pluripotent stem cells (hiPSCs), derived from a familial PD (fPD) patient carrying αSyn gene (SNCA) triplication accumulate pathological αSyn over time. These cytoplasmic inclusions spatially and morphologically resembled diverse stages of LB formation and were composed of key markers of LBs. Importantly, the progressive accumulation of pathological αSyn was paralleled by the loss of DA neurons and elevated apoptosis. The model developed in this study will complement the existing in vitro models of PD and will provide a unique platform to study the spatiotemporal events governing LB formation and their relation with neurodegeneration. Furthermore, this model will also be beneficial for in vitro screening and the development of therapeutic compounds.

Liquid-liquid Phase Separation of α-Synuclein: A New Mechanistic Insight for α-Synuclein Aggregation Associated with Parkinson's Disease Pathogenesis

Aberrant aggregation of the misfolded presynaptic protein, α-Synuclein (α-Syn) into Lewy body (LB) and Lewy neuritis (LN) is a major pathological hallmark of Parkinson's disease (PD) and other synucleinopathies. Numerous studies have suggested that prefibrillar and fibrillar species of the misfolded α-Syn aggregates are responsible for cell death in PD pathogenesis. However, the precise molecular events during α-Syn aggregation, especially in the early stages, remain elusive. Emerging evidence has demonstrated that liquid-liquid phase separation (LLPS) of α-Syn occurs in the nucleation step of α-Syn aggregation, which offers an alternate non-canonical aggregation pathway in the crowded microenvironment. The liquid-like α-Syn droplets gradually undergo an irreversible liquid-to-solid phase transition into amyloid-like hydrogel entrapping oligomers and fibrils. This new mechanism of α-Syn LLPS and gel formation might represent the molecular basis of cellular toxicity associated with PD. This review aims to demonstrate the recent development of α-Syn LLPS, the underlying mechanism along with the microscopic events of aberrant phase transition. This review further discusses how several intrinsic and extrinsic factors regulate the thermodynamics and kinetics of α-Syn LLPS and co-LLPS with other proteins, which might explain the pathophysiology of α-Syn in various neurodegenerative diseases.

CSF-derived extracellular vesicles from patients with Parkinson's disease induce symptoms and pathology

Parkinson's disease is characterized by the gradual appearance of intraneuronal inclusions that are primarily composed of misfolded α-synuclein protein, leading to cytotoxicity and neural death. Recent in vitro and in vivo studies suggest that misfolded α-synuclein may spread transcellularly in a prion-like manner, inducing pathological aggregates in healthy neurons, and is disseminated via secretion of extracellular vesicles. Accordingly, extracellular vesicles derived from brain lysates and CSF of patients with Parkinson's disease were shown to facilitate α-synuclein aggregation in healthy cells. Prompted by the hypothesis of Braak and colleagues that the olfactory bulb is one of the primary propagation sites for the initiation of Parkinson's disease, we sought to investigate the role of extracellular vesicles in the spread of α-synuclein and progression of Parkinson's disease through the olfactory bulb. Extracellular vesicles derived from the CSF of patients diagnosed with Parkinson's disease or with a non-synucleinopathy neurodegenerative disorder were administered intranasally to healthy mice, once daily over 4 days. Three months later, mice were subjected to motor and non-motor tests. Functional impairments were elucidated by histochemical analysis of midbrain structures relevant to Parkinson's disease pathology, 8 months after EVs treatment. Mice treated with extracellular vesicles from the patients with Parkinson's disease displayed multiple symptoms consistent with prodromal and clinical-phase Parkinson's disease such as hyposmia, motor behaviour impairments and high anxiety levels. Furthermore, their midbrains showed widespread α-synuclein aggregations, dopaminergic neurodegeneration, neuroinflammation and altered autophagy activity. Several unconventional pathologies were also observed, such as α-synuclein aggregations in the red nucleus, growth of premature grey hair and astrogliosis. Collectively, these data indicate that intranasally administered extracellular vesicles derived from the CSF of patients with Parkinson's disease can propagate α-synuclein aggregation in vivo and trigger Parkinson's disease-like symptoms and pathology in healthy mice.

Specific Detection of Physiological S129 Phosphorylated α-Synuclein in Tissue Using Proximity Ligation Assay

BACKGROUND

Synucleinopathies are a group of neurodegenerative disorders that are pathologically characterized by intracellular aggregates called Lewy bodies. Lewy bodies are primarily composed of α-synuclein (asyn) protein, which is mostly phosphorylated at serine 129 (pS129) when aggregated and therefore used as a marker for pathology. Currently commercial antibodies against pS129 asyn stain aggregates well but in healthy brains cross react with other proteins, thus making it difficult to specifically detect physiological pS129 asyn.

OBJECTIVE

To develop a staining procedure that detects endogenous and physiological relevant pS129 asyn with high specificity and low background.

METHODS

We used the fluorescent and brightfield in situ proximity ligation assay (PLA) to specifically detect pS129 asyn in cell culture, mouse, and human brain sections.

RESULTS

The pS129 asyn PLA specifically stained physiological and soluble pS129 asyn in cell culture, mouse brain sections, and human brain tissue without significant cross-reactivity or background signal. However, this technique was not successful in detecting Lewy bodies in human brain tissue.

CONCLUSION

We successfully developed a novel PLA method that can, in the future, be used on in vitro and in vivo samples as a tool to explore and better understand the cellular localization and function of pS129 asyn in health and disease.

Hippocampal Reduction of α-Synuclein via RNA Interference Improves Neuropathology in Alzheimer's Disease Mice

BACKGROUND

Alzheimer's disease (AD) cases are often characterized by the pathological accumulation of α-synuclein (α-syn) in addition to amyloid-β (Aβ) and tau hallmarks. The role of α-syn has been extensively studied in synucleinopathy disorders, but less so in AD. Recent studies have shown that α-syn may also play a role in AD and its downregulation may be protective against the toxic effects of Aβ accumulation.

OBJECTIVE

We hypothesized that selectively knocking down α-syn via RNA interference improves the neuropathological and biochemical findings in AD mice.

METHODS

Here we used amyloid precursor protein transgenic (APP-Tg) mice to model AD and explore pathologic and behavioral phenotypes with knockdown of α-syn using RNA interference. We selectively reduced α-syn levels by stereotaxic bilateral injection of either LV-shRNA α-syn or LV-shRNA-luc (control) into the hippocampus of AD mice.

RESULTS

We found that downregulation of α-syn results in significant reduction in the number of Aβ plaques. In addition, mice treated with LV-shRNA α-syn had amelioration of abnormal microglial activation (Iba1) and astrocytosis (GFAP) phenotypes in AD mice.

CONCLUSION

Our data suggests a novel link between Aβ and α-syn pathology as well as a new therapeutic angle for targeting AD.