Development of Parkinson's Disease (PD) is linked with a history of traumatic brain injury (TBI), although the mechanisms driving this remain unclear. Of note, many key parallels have been identified between the pathologies of PD and TBI; in particular, PD is characterised by loss of dopaminergic neurons from the substantia nigra (SN), accompanied by broader changes to dopaminergic signalling, disruption of the Locus Coeruleus (LC) and noradrenergic system, and accumulation of aggregated α-synuclein in Lewy Bodies, which spreads in a stereotypical pattern throughout the brain. Widespread disruptions to the dopaminergic and noradrenergic systems, including progressive neuronal loss from the SN and LC, have been observed acutely following injury, some of which have also been identified chronically in TBI patients and preclinical models. Furthermore, changes to α-synuclein expression are also seen both acutely and chronically following injury throughout the brain, although detailed characterisation of these changes and spread of pathology is limited. In this review, we detail the current literature regarding dopaminergic and noradrenergic disruption and α-synuclein pathology following injury, with particular focus on how these changes may predispose individuals to prolonged pathology and progressive neurodegeneration, particularly the development of PD. While it is increasingly clear that TBI is a key risk factor for the development of PD, significant gaps remain in current understanding of neurodegenerative pathology following TBI, particularly chronic manifestations of injury.

Two major neuropathological features of Parkinson's disease (PD) are α-synuclein Lewy pathology and mitochondrial dysfunction. Although both α-synuclein pathology and mitochondrial dysfunction may independently contribute to PD pathogenesis, the interaction between these two factors is not yet fully understood. In this review, we discuss the physiological functions of α-synuclein and mitochondrial homeostasis in neurons as well as the pathological defects that ensue when these functions are disturbed in PD. Recent studies have highlighted that dysfunctional mitochondria can become sequestered within Lewy bodies, and cell biology studies have suggested that α-synuclein can directly impair mitochondrial function. There are also PD cases caused by genetic or environmental perturbation of mitochondrial homeostasis. Together, these studies suggest that mitochondrial dysfunction may be a common pathway to neurodegeneration in PD, triggered by multiple insults. We review the literature surrounding the interaction between α-synuclein and mitochondria and highlight open questions in the field that may be explored to advance our understanding of PD and develop novel, disease-modifying therapies.

The emergence of promising biomarkers of α-synuclein Lewy pathology has led to new biological definitions and staging systems for Parkinson's disease and dementia with Lewy bodies. These research frameworks aim to enhance patient selection for studies of biomarkers and disease-modifying therapies. Building on approaches developed for Alzheimer's disease, these new frameworks focus on hallmark neuropathological findings in Lewy body diseases, including abnormal α-synuclein aggregates and neurodegeneration, particularly nigrostriatal dopaminergic loss. Understanding the temporal inter-relationships between Lewy pathology, Alzheimer's disease, and other co-pathologies and symptom manifestation is central to any biological staging system. Neuropathological and in vivo evidence demonstrates substantial temporal and biological heterogeneity in the progression of clinical and pathological events across Lewy body disorders, highlighting knowledge gaps. Staging systems must incorporate this evidence into a nuanced conceptual framework of biological progression. Such revision will be crucial for the appropriate selection of participants and correct timing of targeted interventions in clinical research.

α-Synuclein (α-syn), a pathological hallmark of PD, is emerging as a bridging element at the crossroads between neuro/immune-inflammatory responses and neurodegeneration in PD. Several evidence show that pathological α-syn accumulates in neuronal and non-neuronal cells (i.e., neurons, microglia, macrophages, skin cells, and intestinal cells) in central and peripheral tissues since the prodromal phase of the disease, contributing to brain pathology. Indeed, pathological α-syn deposition can promote neurogenic/immune-inflammatory responses that contribute to systemic and central neuroinflammation associated with PD. After providing an overview of the structure and functions of physiological α-syn as well as its pathological forms, we review current studies about the role of neuronal and non-neuronal α-syn at the crossroads between neuroinflammation and neurodegeneration in PD. In addition, we provide an overview of the correlation between the accumulation of α-syn in central and peripheral tissues and PD, related symptoms, and neuroinflammation. Special attention was paid to discussing whether targeting α-syn can represent a suitable therapeutical approach for PD.

Both genetic and environmental factors modulate the risk of Parkinson's disease. In this article, all these pathophysiologic processes that contribute to damages at the tissue, cellular, organelle, and molecular levels, and their effects are talked about.

Intracerebral grafting of dopamine-producing cells is proposed as a strategy to replace the typical neurons lost to Parkinson's disease (PD) and improve PD motor symptoms. Non-human primate studies have provided clues on the relationship between the host's immune response and grafting success. Herein, we discuss how the host's immune system differentially affects the graft depending on the origin of the cells and reflect on the advantages and limitations of the immune paradigms utilized to assess graft-related outcomes. We also consider new strategies to minimize or circumvent the host's immunological response and related preclinical research needed to identify the most promising new approaches to be translated into the clinic.

Research into the crosstalk between α-synuclein (α-syn) and synaptic vesicles (SVs) has gained considerable attention. Notably, the recently discovered liquid-liquid phase separation of α-syn involving SVs is crucial for performing their physiological functions and mediating the transition to pathological aggregates. This review first examines the functional interactions between α-syn and SVs in the context of α-syn's condensation state. It then explores how these interactions become disrupted under pathological conditions, leading to α-syn aggregation and subsequent synaptic dysfunction. Finally, the review discusses the therapeutic potential of targeting α-syn-SV interactions to restore synaptic function in diseased states. By connecting α-syn's physiological roles with its pathological effects, the article aims to shed light on its dual role as both a regulator of SVs and a driver of neurodegeneration.

The cytopathological hallmark of Parkinson's disease (PD) is a neuronal cytoplasmic inclusion called Lewy body (LB). Lewy bodies are composed of alpha-synuclein (aSyn), a 140 aa protein that is predominantly expressed in the presynaptic terminal and which is implicated in neurotransmitter release. Recently, aSyn was found to propagate from neuron to neuron in a trans-synaptic manner. Although the precise molecular mechanisms are unclear, the propagation of aSyn is believed to play a major role in the progression of Lewy pathology in PD. Neuropathologically, the initial Lewy pathology has been shown to be formed in the dorsal motor nucleus of the vagus (DMV) or olfactory bulb by neuropathological studies. Since the DMV innervates the enteric nervous system (ENS) and LBs are formed in the gut nerve plexuses, it is conceivable that LBs propagate from the gut to the DMV and then to other regions of the brain. In this article, clinical, neuropathological, and experimental evidence supporting or negating the idea that aSyn propagation from the ENS to the brain leads to PD is reviewed. Moreover, the propagation of aSyn seeds through systemic circulation or multifocal generation of aSyn seeds is discussed as a potential alternative scenario for aSyn spreading.

Parkinson's disease is a progressive neurodegenerative condition with a considerable health and economic burden1. It is characterized by the loss of midbrain dopaminergic neurons and a diminished response to symptomatic medical or surgical therapy as the disease progresses2. Cell therapy aims to replenish lost dopaminergic neurons and their striatal projections by intrastriatal grafting. Here, we report the results of an open-label phase I clinical trial (NCT04802733) of an investigational cryopreserved, off-the-shelf dopaminergic neuron progenitor cell product (bemdaneprocel) derived from human embryonic stem (hES) cells and grafted bilaterally into the putamen of patients with Parkinson's disease. Twelve patients were enrolled sequentially in two cohorts-a low-dose (0.9 million cells, n = 5) and a high-dose (2.7 million cells, n = 7) cohort-and all of the participants received one year of immunosuppression. The trial achieved its primary objectives of safety and tolerability one year after transplantation, with no adverse events related to the cell product. At 18 months after grafting, putaminal 18Fluoro-DOPA positron emission tomography uptake increased, indicating graft survival. Secondary and exploratory clinical outcomes showed improvement or stability, including improvement in the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS) Part III OFF scores by an average of 23 points in the high-dose cohort. There were no graft-induced dyskinesias. These data demonstrate safety and support future definitive clinical studies.

Parkinson's disease is caused by the loss of dopamine neurons, causing motor symptoms. Initial cell therapies using fetal tissues showed promise but had complications and ethical concerns1-5. Pluripotent stem (PS) cells emerged as a promising alternative for developing safe and effective treatments6. In this phase I/II trial at Kyoto University Hospital, seven patients (ages 50-69) received bilateral transplantation of dopaminergic progenitors derived from induced PS (iPS) cells. Primary outcomes focused on safety and adverse events, while secondary outcomes assessed motor symptom changes and dopamine production for 24 months. There were no serious adverse events, with 73 mild to moderate events. Patients' anti-parkinsonian medication doses were maintained unless therapeutic adjustments were required, resulting in increased dyskinesia. Magnetic resonance imaging showed no graft overgrowth. Among six patients subjected to efficacy evaluation, four showed improvements in the Movement Disorder Society Unified Parkinson's Disease Rating Scale part III OFF score, and five showed improvements in the ON scores. The average changes of all six patients were 9.5 (20.4%) and 4.3 points (35.7%) for the OFF and ON scores, respectively. Hoehn-Yahr stages improved in four patients. Fluorine-18-L-dihydroxyphenylalanine (18F-DOPA) influx rate constant (Ki) values in the putamen increased by 44.7%, with higher increases in the high-dose group. Other measures showed minimal changes. This trial (jRCT2090220384) demonstrated that allogeneic iPS-cell-derived dopaminergic progenitors survived, produced dopamine and did not form tumours, therefore suggesting safety and potential clinical benefits for Parkinson's disease.

The water exchange between brain parenchyma and cerebrospinal fluid (CSF) is considered to be responsible for glymphatic clearance of solutes and metabolic wastes from the brain, including amyloid-β, a biomarker in neurodegeneration. Despite the potential significance, no noninvasive technique for in vivo measurement of parenchyma-CSF water exchange has been demonstrated in humans, capable of investigating age-related changes in glymphatic clearance.

To demonstrate a noninvasive, translatable MRI technique capable of measuring glymphatic water exchange in humans and to apply this technique to examine age-related changes in the glymphatic exchange measures in healthy subjects.

Repeating on-resonance magnetization transfer (MT) RF pulses were applied to saturate macromolecules within the brain parenchyma and label its interstitial water, followed by measuring partial CSF saturation resulting from the parenchyma-CSF water exchange. Bloch simulations and phantom experiments determined the extent of direct CSF saturation by the MT pulses. An additional labeling nulling experiment was performed by preemptively saturating parenchyma spins to disable the following MT-based spin labeling, to examine non-exchange contributions to the observed CSF saturation. These techniques were applied to young (n = 6; ages 25-41) and elder (n = 6; ages 53-66) healthy participants to examine age-related changes in their saturation-based exchange measurements.

Both Bloch simulations and phantom experiments indicated small (0.4-0.7 %) direct CSF saturation when B0 inhomogeneities and CSF T2 variations were considered. A statistically significant (P = 0.037) difference was observed in the average CSF saturation ratio within the subarachnoid space (SAS) between the young (4.7 %±0.5 %) and the elder (3.5 %±1.2 %) subjects, with their ages negatively correlating with this exchange metric (R2=0.34, P = 0.046). The substantial saturation reductions in the labeling nulling experiment (40-50 % in young; 10-30 % in elder) suggested parenchyma-CSF exchange as a substantial source of the observed saturation signal. These findings survived when the exchange metrics were compensated for potential atrophy-related dilution effect caused by variations in intra-voxel CSF volume.

Optimized MT-based parenchyma spin labeling followed by CSF partial saturation measurement demonstrated feasibility of a noninvasive MRI approach to detect glymphatic water exchange between human brain parenchyma and CSF in vivo, with statistically significant findings of age-related differences in the exchange measures.

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

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

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

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

Chronic neuroinflammation and misfolded α-synuclein (αSyn) have been identified as key pathological correlates driving Parkinson's disease (PD) pathogenesis; however, the contribution of ion channels to microglia activation in the context of α-synucleinopathy remains elusive. Herein, we show that KCa3.1, a calcium-activated potassium channel, is robustly upregulated within microglia in multiple preclinical models of PD and, most importantly, in human PD and dementia with Lewy bodies (DLB) brains. Pharmacological inhibition of KCa3.1 via senicapoc or TRAM-34 inhibits KCa3.1 channel activity and the associated reactive microglial phenotype in response to aggregated αSyn, as well as ameliorates of PD like pathology in diverse PD mouse models. Additionally, proteomic and transcriptomic profiling of microglia revealed that senicapoc ameliorates aggregated αSyn-induced, inflammation-associated pathways and dysregulated metabolism in primary microglial cells. Mechanistically, FYN kinase in a STAT1 dependent manner regulates KCa3.1 mediated the microglial reactive activation phenotype after α-synucleinopathy. Moreover, reduced neuroinflammation and subsequent PD-like neuropathology were observed in SYN AAV inoculated KCa3.1 knockout mice. Together, these findings suggest that KCa3.1 inhibition represents a novel therapeutic strategy for treating patients with PD and related α-synucleinopathies.

Cytoplasmic alpha-synuclein (αSyn) aggregates are a typical feature of Parkinson's disease (PD). Extracellular insoluble αSyn can induce pathology in healthy neurons suggesting that PD neurodegeneration may spread through cell-to-cell transfer of αSyn proteopathic seeds. Early pro-homeostatic reaction of microglia to toxic forms of αSyn remains elusive, which is especially relevant considering the recently uncovered microglial molecular diversity. Here, we show that periventricular microglia of the subependymal neurogenic niche monitor the cerebrospinal fluid and can rapidly phagocytize and degrade different aggregated forms of αSyn delivered into the lateral ventricle. However, this clearing ability worsens with age, leading to an increase in microglia with aggregates in aged treated mice, an accumulation also observed in human PD samples. We also show that exposure of aged microglia to aggregated αSyn isolated from human PD samples results in the phosphorylation of the endogenous protein and the generation of αSyn seeds that can transmit the pathology to healthy neurons. Our data indicate that while microglial phagocytosis rapidly clears toxic αSyn, aged microglia can contribute to synucleinopathy spreading.

Lewy body diseases, including Parkinson's disease (PD), are characterized by the spread of alpha-synuclein (αSyn) between neurons across synapses, a process crucial for understanding their pathophysiology and developing effective treatments. In this study, we aimed to investigate the role of neuronal activity in releasing αSyn from human induced pluripotent stem cell-derived dopaminergic neurons. We examined human induced pluripotent stem cell-derived dopaminergic neurons, both healthy and those with the αSyn gene mutation associated with PD. We employed pharmacological agents and optogenetic techniques and demonstrated that increased neuronal activity, induced by bicuculline or optogenetic stimulation, significantly enhances αSyn release. However, suppression of neuronal activity with cyanquixaline reduces αSyn secretion. These findings underscore the pivotal role of neuronal activity in αSyn transmission between neurons, showing its potential impact on the spread of Lewy pathology in patients with neurodegenerative diseases like PD. Therefore, this study advances our understanding of PD and opens new avenues for therapeutic strategies to mitigate Lewy body disease progression.

Parkinson's disease (PD) is a neurodegenerative condition characterized by the loss of midbrain dopaminergic neurons, leading to motor symptoms. While current treatments provide limited relief, they don't alter disease progression. Stem cell technology, involving patient-specific stem cell-derived neurons, offers a promising avenue for research and personalized regenerative therapies. This article reviews the potential of stem cell-based research in PD, summarizing ongoing efforts, their limitations, and introducing innovative research models. The integration of stem cell technology and advanced models promises to enhance our understanding and treatment strategies for PD.

Despite known sex differences in human synucleinopathies such as Parkinson's disease, the impact of sex on alpha-synuclein pathology in mouse models has been largely overlooked. To address this need, we examine sex differences in whole brain signatures of neurodegeneration due to aSyn toxicity in the M83 mouse model using longitudinal magnetic resonance imaging (MRI; T1-weighted; 100 μm3 isotropic voxel; -7, 30, 90 and 120 days post-injection [dpi]; n ≥ 8 mice/group/sex/time point). To initiate aSyn spreading, M83 mice are inoculated with recombinant human aSyn preformed fibrils (Hu-PFF) or phosphate buffered saline in the right striatum. We observe more aggressive neurodegenerative profiles over time for male Hu-PFF-injected mice when examining voxel-wise trajectories. However, at 90 dpi, we observe widespread patterns of neurodegeneration in the female Hu-PFF-injected mice. These differences are not accompanied by any differences in motor symptom onset between the sexes. However, male Hu-PFF-injected mice reached their humane endpoint sooner. These findings suggest that post-motor symptom onset, despite accelerated disease trajectories for male Hu-PFF-injected mice, neurodegeneration may appear sooner in the female Hu-PFF-injected mice (prior to motor symptomatology). These findings suggest that sex-specific synucleinopathy phenotypes urgently need to be considered to improve our understanding of neuroprotective and neurodegenerative mechanisms.

α-Synuclein (αSyn) is a major component of Lewy bodies (LBs) and Lewy neurites (LNs), which are pathological features of Parkinson's disease (PD) and dementia with Lewy bodies. In the PD brain, with disease progression, LB/LN formation is propagated from the lower brainstem to the cerebral cortex. Prion-like cell-to-cell seed transmission has been implicated as an underlying mechanism for Lewy-pathology propagation. However, the biochemical properties and production mechanism of those pathogenic seeds are unelucidated. In this study, we ascertained that the seeds released from pathological neurons that harbor LB/LN-like aggregates have the N-terminally truncated form of αSyn. This N-terminal truncation is directly catalyzed by SENP2, which is a well-known deSUMOylation enzyme. After SENP2 processing of recombinant αSyn, the SDS-resistant high-molecular oligomer formation was promoted in vitro. Inhibition of SENP2 activity suppressed aggregate formation and propagation in cultured neurons and mouse brains. Thus, SENP2 might be a therapeutic target in LB diseases.

The transplantation of human neurons into the central nervous system (CNS) offers transformative opportunities for modeling neurodegenerative diseases in vivo. This study evaluated the survival, integration, and functional properties of cryopreserved forebrain GABAergic neurons (iGABAs) derived from human induced pluripotent stem cells (iPSCs) across three species used in translational research. iGABAs were stereotactically injected into the striatum of Sprague-Dawley rats, immunodeficient RNU rats, R6/2 Huntington's disease (HD) mice, wild-type controls, and Cynomolgus monkeys. Post-transplantation, long-term assessments revealed robust neuronal survival, extensive neurite outgrowth, and integration with host CNS environments. In immunodeficient rats, iGABAs innervated the neuraxis, extending from the prefrontal cortex to the midbrain, while maintaining mature neuronal phenotypes without uncontrolled proliferation. Similarly, grafts in nonhuman primates showed localized survival and stable phenotype at one month. In the neurodegenerative milieu of HD mice, iGABAs survived up to six months, projecting into the host striatum and white matter, with evidence of mutant huntingtin aggregates localized within the graft, indicating pathological protein transfer. These findings underscore the utility of cryopreserved iGABAs as a reproducible, scalable model for disease-specific CNS investigations and mechanistic studies of proteinopathic propagation. This work establishes a critical platform for studying neurodegenerative diseases and developing therapeutic interventions.

Neurodegenerative diseases (ND) are characterized by the accumulation of aggregated proteins. The glymphatic system, through its rapid exchange mechanisms between cerebrospinal fluid (CSF) and interstitial fluid (ISF), facilitates the movement of metabolic substances within the brain, serving functions akin to those of the peripheral lymphatic system. This emerging waste clearance mechanism offers a novel perspective on the removal of pathological substances in ND. This article elucidates recent discoveries regarding the glymphatic system and updates relevant concepts within its model. It discusses the potential roles of the glymphatic system in ND, including Alzheimer's disease (AD), Parkinson's disease (PD), and multiple system atrophy (MSA), and proposes the glymphatic system as a novel therapeutic target for these conditions.

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

α-Synucleinopathies constitute a spectrum of neurodegenerative disorders, including Parkinson's disease (PD), Lewy body dementia (LBD), Multiple System Atrophy (MSA), and Alzheimer's disease concurrent with LBD (AD-LBD). These disorders are unified by a pathological hallmark: aberrant misfolding and accumulation of α-synuclein (α-syn). This review delves into the pivotal role of α-syn, the key agent in α-synucleinopathy pathophysiology, and provides a survey of potential therapeutics that target cell-to-cell spread of pathologic α-syn. Recognizing the intricate complexity and multifactorial etiology of α-synucleinopathy, the review illuminates the potential of various membrane receptors, proteins, intercellular spreading pathways, and pathological agents for therapeutic interventions. While significant progress has been made in understanding α-synucleinopathy, the pursuit of efficacious treatments remains challenging. Several strategies involving decreasing α-syn production and aggregation, increasing α-syn degradation, lowering extracellular α-syn, and inhibiting cellular uptake of α-syn are presented. The paper underscores the necessity of meticulous and comprehensive investigations to advance our knowledge of α-synucleinopathy pathology and ultimately develop innovative therapeutic strategies for α-synucleinopathies.

Aggregation of α-synuclein (α-Syn) and Lewy body (LB) formation are the key pathological events implicated in Parkinson's disease (PD) that spread in a prion-like manner. However, biophysical and structural characteristics of toxic α-Syn species and molecular events that drive early events in the propagation of α-Syn amyloids in a prion-like manner remain elusive. We used a neuronal cell model to demonstrate the size-dependent native biological activities of α-Syn fibril seeds. Biophysical characterization of the fibril seeds generated by controlled fragmentation indicated that increased fragmentation leads to a reduction in fibril size, correlating directly with the extent of fragmentation events. Although the size-based complexity of amyloid fibrils modulates their biological activities and fibril amplification pathways, it remains unclear how the variability of fibril seed size dictates its specific uptake mechanism into the cells. The present study elucidates the mechanism of α-Syn fibril internalization and how it is regulated by the size of fibril seeds. Further, we demonstrate that size-dependent endocytic pathways (dynamin-dependent clathrin/caveolin-mediated) are more prominent for the differential uptake of short fibril seeds compared to their longer counterparts. This size-dependent preference might contribute to the enhanced uptake and transcellular propagation of short α-Syn fibril seeds in a prion-like manner. Overall, the present study suggests that the physical dimension of α-Syn amyloid fibril seeds significantly influences their cellular uptake and pathological responses in the initiation and progression of PD.

ΑBSTRACT: In Parkinson's disease (PD), Lewy pathology deposits in the cerebral cortex, but how the pathology disrupts cortical circuit integrity and function remains poorly understood. To begin to address this question, we injected α-synuclein (αSyn) preformed fibrils (PFFs) into the dorsolateral striatum of mice to seed αSyn pathology in the cortical cortex and induce degeneration of midbrain dopaminergic neurons. We reported that αSyn aggregates accumulate in the motor cortex in a layer- and cell-subtype-specific pattern. Specifically, αSyn aggregates-bearing intratelencephalic neurons (ITNs) showed hyperexcitability, increased input resistance, and decreased cell capacitance, which were associated with impaired HCN channel function. Morphologically, the αSyn aggregates-bearing ITNs showed shrinkage of cell bodies and loss of dendritic spines. Last, we showed that partial dopamine depletion is not sufficient to alter thalamocortical transmission to cortical pyramidal neurons. Our results provide a novel mechanistic understanding of cortical circuit dysfunction in PD.

Parkinson's disease (PD) involves both the central nervous system (CNS) and enteric nervous system (ENS), and their interaction is important for understanding both the clinical manifestations of the disease and the underlying disease pathophysiology. Although the neuroanatomical distribution of pathology strongly suggests that the ENS is involved in disease pathophysiology, there are significant gaps in knowledge about the underlying mechanisms. In this article, we review the clinical presentation and management of gastrointestinal dysfunction in PD. In addition, we discuss the current understanding of disease pathophysiology in the gut, including controversies about early involvement of the gut in disease pathogenesis. We also review current knowledge about gut α-synuclein and the microbiome, discuss experimental models of PD-linked gastrointestinal pathophysiology, and highlight areas for further research. Finally, we discuss opportunities to use the gut-brain axis for the development of biomarkers and disease-modifying treatments.

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

Parkinson's disease (PD) is a prevalent, chronic neurodegenerative disorder characterised by the progressive loss of dopaminergic neurons in the substantia nigra and other brain regions. The aggregation of alpha-synuclein (α-syn) into Lewy bodies and neurites is a key pathological feature associated with PD and its progression. Many therapeutic studies aim to target these aggregated forms of α-syn to potentially slow down or stop disease progression in PD. This review provides a comprehensive analysis of recent clinical trials involving vaccines and monoclonal antibodies targeting α-syn. Specifically, UB-312, AFFITOPE PD01A, PD03A and ACI-7104.056 are designed to provoke an immune response against α-syn (active immunisation), while Prasinezumab and Cinpanemab, MEDI1341 and Lu AF82422 focus on directly targeting α-syn aggregates (passive immunisation). Despite some promising results, challenges such as variable efficacy and trial discontinuations persist. Future research must address these challenges to advance disease-modifying therapies for PD around this therapeutic target.

Parkinson's Disease (PD) is a prevalent and escalating neurodegenerative disorder with significant societal implications. Despite being considered a proteinopathy, in which the aggregation of α-synuclein is the main pathological change, the intricacies of PD initiation remain elusive. Recent evidence suggests a potential link between gut microbiota and PD initiation, emphasizing the need to explore the effects of microbiota-derived molecules on neuronal cells. In this study, we exposed dopaminergic-differentiated SH-SY5Y cells to microbial molecules such as lipopolysaccharide (LPS), rhamnolipid, curli CsgA and phenol soluble modulin α-1 (PSMα1). We assessed cellular viability, cytotoxicity, growth curves and α-synuclein levels by performing MTS, LDH, real-time impedance readings, qRT-PCR and Western Blot assays respectively. Statistical analysis revealed that rhamnolipid exhibited concentration-dependent effects, reducing viability and inducing cytotoxicity at higher concentrations, increasing α-synuclein mRNA and protein levels with negative effects on cell morphology and adhesion. Furthermore, LPS exposure also increased α-synuclein levels. Curli CsgA and PSMα-1 showed minimal or no changes. Our findings suggest that microbiota-derived molecules, particularly rhamnolipid and LPS, impact dopaminergic neurons by increasing α-synuclein levels. This study highlights the potential involvement of gut microbiota in initiating the upregulation of α-synuclein that may further initiate PD, indicating the complex interplay between microbiota and neuronal cells.

Parkinson's disease (PD) is characterized by the loss of the dopaminergic nigrostriatal pathway which has led to the successful development of drug therapies that replace or stimulate this network pharmacologically. Although these drugs work well in the early stages of the disease, over time they produce side effects along with less consistent clinical benefits to the person with Parkinson's (PwP). As such there has been much interest in repairing this pathway using transplants of dopamine neurons. This work which began 50 years ago this September is still ongoing and has now moved to first in human trials using human pluripotent stem cell-derived dopaminergic neurons. The results of these trials are eagerly awaited although proof of principle data has already come from trials using human fetal midbrain dopamine cell transplants. This data has shown that developing dopamine cells when transplanted in the brain of a PwP can survive long term with clinical benefits lasting decades and with restoration of normal dopaminergic innervation in the grafted striatum. In this article, we discuss the history of this field and how this has now led us to the recent stem cell trials for PwP.

Excessive exposure to manganese (Mn) increases the risk of chronic neurological diseases, including Parkinson's disease (PD) and other related Parkinsonisms. Aggregated α-synuclein (αSyn), a hallmark of PD, can spread to neighboring cells by exosomal release from neurons. We previously discovered that Mn enhances its spread, triggering neuroinflammatory and neurodegenerative processes. To better understand the Mn-induced release of exosomal αSyn, we examined the effect of Mn on endosomal trafficking and misfolded protein degradation. Exposing MN9D dopaminergic neuronal cells stably expressing human wild-type (WT) αSyn to 300 μM Mn for 24 h significantly suppressed protein and mRNA expression of Rab11a, thereby downregulating endosomal recycling, forcing late endosomes to mature into multivesicular bodies (MVBs). Ectopic expression of WT Rab11a significantly mitigated exosome release, whereas ectopic mutant Rab11a (S25N) increased it. Our in vitro and in vivo studies reveal that Mn exposure upregulated (1) mRNA and protein levels of endosomal Rab27a, which mediates the fusion of MVBs with the plasma membrane; and (2) expression of the autophagosomal markers Beclin-1 and p62, but downregulated the lysosomal marker LAMP2, thereby impairing autophagolysosome formation as confirmed by LysoTracker, cathepsin, and acridine orange assays. Our novel findings demonstrate that Mn promotes the exosomal release of misfolded αSyn by impairing endosomal trafficking and protein degradation.

Synucleinopathies are neurodegenerative disorders characterized by the accumulation of α-synuclein containing Lewy bodies. Ubiquitination, a key post-translational modification, has been recognized as a pivotal regulator of α-synuclein's cellular dynamics, influencing its degradation, aggregation, and associated neurotoxicity. This review examines comprehensively the current understanding of α-synuclein ubiquitination and its role in the pathogenesis of synucleinopathies, particularly in the context of Parkinson's disease. We explore the molecular mechanisms responsible for α-synuclein ubiquitination, with a focus on the roles of E3 ligases and deubiquitinases implicated in the degradation process which occurs primarily through the endosomal lysosomal pathway. The review further discusses how the dysregulation of these mechanisms contributes to α-synuclein aggregation and LB formation and offers suggestions for future investigations into the role of α-synuclein ubiquitination. Understanding these processes may shed light on potential therapeutic avenues that can modulate α-synuclein ubiquitination to alleviate its pathological impact in synucleinopathies.

Protein molecules organize into an intricate alphabet of twenty amino acids and five architecture levels. The jargon "one structure, one functionality" has been challenged, considering the amount of intrinsically disordered proteins in the human genome and the requirements of hierarchical hetero- and homo-protein complexes in cell signaling. The assembly of large protein structures in health and disease is now viewed through the lens of phase separation and transition phenomena. What drives protein misfolding and aggregation? Or, more fundamentally, what hinders proteins from maintaining their native conformations, pushing them toward aggregation? Here, we explore the principles of protein folding, phase separation, and aggregation, which hinge on crucial events such as the reorganization of solvents, the chemical properties of amino acids, and their interactions with the environment. We focus on the dynamic shifts between functional and dysfunctional states of proteins and the conditions that promote protein misfolding, often leading to disease. By exploring these processes, we highlight potential therapeutic avenues to manage protein aggregation and reduce its harmful impacts on health.

Parkinson's Disease (PD) is a chronic and progressive neurodebilitating disorder that affects both motor and non-motor functions. PD is the second most commonly occurring brain disorder after Alzheimer's disease. The incidence rate of PD was found to be 17 per 100000 per year. The prevalence of the disease is at its peak at age 70 and older. One of the major reasons for the failure to devise a complete therapeutic cure for PD is an inability to identify the exact pathological cause. Recent research has also stated that PD originates in the gut way before the symptoms begin to manifest in an affected person. This might be due to the transmission of pathological alpha-synuclein (α-syn) from the gut to the brain via the vagus nerve. The involvement of toxic environmental exposure in the generation of major disorders like cancer, brain disorders etc, is not an entirely new notion. Our genes are affected directly by the environment. Simultaneously, a number of environmental pollutants may contribute significantly to the trigger of alpha-synuclein misfolding in the brain during PD. In the present review, we will mainly focus on understanding the pathological cascade of PD and how it is triggered by environmental pollutants.

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor deficits due to the depletion of nigrostriatal dopamine. Stem cell differentiation therapy emerges as a promising treatment option for sustained symptom relief. In this study, we successfully developed a one-step differentiation system using the YFBP cocktail (Y27632, Forskolin, SB431542, and SP600125) to effectively convert human umbilical cord mesenchymal stem cells (hUCMSCs) into dopaminergic neurons without genetic modification. This approach addresses the challenge of rapidly and safely generating functional neurons on a large scale. After a 7-day induction period, over 80 % of the cells were double-positive for TUBB3 and NEUN. Transcriptome analysis revealed the dual roles of the cocktail in inducing fate erasure in mesenchymal stem cells and activating the neuronal program. Notably, these chemically induced cells (CiNs) did not express HLA class II genes, preserving their immune-privileged status. Further study indicated that YFBP significantly downregulated p53 signaling and accelerated the differentiation process when Pifithrin-α, a p53 signaling inhibitor, was applied. Additionally, Wnt/β-catenin signaling was transiently activated within one day, but the prolonged activation hindered the neuronal differentiation of hUCMSCs. Upon transplantation into the striatum of mice, CiNs survived well and tested positive for dopaminergic neuron markers. They exhibited typical action potentials and sodium and potassium ion channel activity, demonstrating neuronal electrophysiological activity. Furthermore, CiNs treatment significantly increased the number of tyrosine hydroxylase-positive cells and the concentration of dopamine in the striatum, effectively ameliorating movement disorders in PD mice. Overall, our study provides a secure and reliable framework for cell replacement therapy for Parkinson's disease.

This comprehensive review elucidates the critical role of antioxidants to mitigate oxidative stress, a common denominator in an array of neurodegenerative disorders. Oxidative stress-induced damage has been linked to the development of diseases such as Alzheimer's, Parkinson's, Huntington's disease and amyotrophic lateral sclerosis. This article examines a wide range of scientific literature and methodically delineates the several methods by which antioxidants exercise their neuroprotective benefits. It also explores into the complex relationship between oxidative stress and neuroinflammation, focusing on how antioxidants can alter signaling pathways and transcription factors to slow neurodegenerative processes. Key antioxidants, such as vitamins C and E, glutathione, and polyphenolic compounds, are tested for their ability to combat reactive oxygen and nitrogen species. The dual character of antioxidants, which operate as both direct free radical scavengers and regulators of cellular redox homeostasis, is investigated in terms of therapeutic potential. Furthermore, the study focuses on new antioxidant-based therapy techniques and their mechanisms including Nrf-2, PCG1α, Thioredoxin etc., which range from dietary interventions to targeted antioxidant molecules. Insights into ongoing clinical studies evaluating antioxidant therapies in neurodegenerative illnesses offer an insight into the translational potential of antioxidant research. Finally, this review summarizes our present understanding of antioxidant processes in neurodegenerative illnesses, providing important possibilities for future study and treatment development.

The aggregation of alpha-synuclein (aSyn) represents a neuropathological hallmark observed in a group of neurodegenerative disorders collectively known as synucleinopathies. Despite their shared characteristics, these disorders manifest diverse clinical and pathological phenotypes. The mechanism underlying this heterogeneity is thought to be due to the diversity in the aSyn strains present across the diseases. In this perspective, we will explore recent findings on aSyn strains and discuss recent discoveries about Lewy bodies' composition. We further discuss the current hypothesis for aSyn spreading and emphasize the emerging biomarker field demonstrating promising results. A comprehension of these mechanisms holds substantial promise for future clinical applications. This understanding can pave the way for the development of personalized medicine strategies, specifically targeting the unique underlying causes of each synucleinopathy. Such advancements can revolutionize therapeutic approaches and significantly contribute to more effective interventions in the intricate landscape of neurodegenerative disorders.

An emerging theme in Parkinson's disease (PD) is the propagation of α-synuclein pathology as the disease progresses. Research involving the injection of preformed α-synuclein fibrils (PFFs) in animal models has recapitulated the pathological spread observed in PD patients. At the cellular and molecular levels, this intercellular spread requires the translocation of α-synuclein across various membrane barriers. Recent studies have identified subcellular organelles and protein machineries that facilitate these processes. In this review, we discuss the proposed pathways for α-synuclein intercellular transmission, including unconventional secretion, receptor-mediated uptake, endosome escape and nanotube-mediated transfer. In addition, we advocate for a rigorous examination of the evidence for the localization of α-synuclein in extracellular vesicles.

Brain protein aggregates are a hallmark of neurodegenerative disease. Previous work indicates that specific protein components of these aggregates are toxic, including tau (encoded by MAPT) in Alzheimer's disease and related tauopathies. Increasing evidence also indicates that these toxic proteins traffic between cells in a prion-like fashion, thereby spreading pathology from one brain region to another. However, the mechanisms involved in trafficking are poorly understood. We therefore developed a transgenic Drosophila model to facilitate rapid evaluation of candidate tau trafficking modifiers. Our model uses the bipartite Q system to drive co-expression of tau and GFP in the fly eye. We found age-dependent spread of tau into the brain, represented by detection of tau, but not of GFP. We also found that tau trafficking was attenuated upon inhibition of the endocytic factor dynamin (encoded by shi) or knockdown of glycogen synthase kinase-3β (GSK-3β, encoded by sgg). Further work revealed that dynamin promoted tau uptake in recipient tissues, whereas GSK-3β appeared to promote tau spread via direct phosphorylation of tau. Our robust and flexible system will promote the identification of tau-trafficking components involved in the pathogenesis of neurodegenerative diseases.

The cellular prion protein, PrPC, has been postulated to function as a receptor for α-synuclein, potentially facilitating cell-to-cell spreading and/or toxicity of α-synuclein aggregates in neurodegenerative disorders such as Parkinson's disease. Previously, we generated the "Salt (S)" and "No Salt (NS)" strains of α-synuclein aggregates that cause distinct pathological phenotypes in M83 transgenic mice overexpressing A53T-mutant human α-synuclein. To test the hypothesis that PrPC facilitates the propagation of α-synuclein aggregates, we produced M83 mice that either express or do not express PrPC. Following intracerebral inoculation with the S or NS strain, the absence of PrPC in M83 mice did not prevent disease development and had minimal influence on α-synuclein strain-specified attributes such as the extent of cerebral α-synuclein deposition, selective targeting of specific brain regions and cell types, the morphology of induced α-synuclein deposits, and the structural fingerprints of protease-resistant α-synuclein aggregates. Likewise, there were no appreciable differences in disease manifestation between PrPC-expressing and PrPC-lacking M83 mice following intraperitoneal inoculation of the S strain. Interestingly, intraperitoneal inoculation with the NS strain resulted in two distinct disease phenotypes, indicative of α-synuclein strain evolution, but this was also independent of PrPC expression. Overall, these results suggest that PrPC plays at most a minor role in the propagation, neuroinvasion, and evolution of α-synuclein strains in mice that express A53T-mutant human α-synuclein. Thus, other putative receptors or cell-to-cell propagation mechanisms may have a larger effect on the spread of α-synuclein aggregates during disease.

Aggregation of α-synuclein (αSyn) and its accumulation as Lewy bodies play a central role in the pathogenesis of Parkinson's disease (PD). However, the mechanism by which αSyn aggregates in the brain remains unclear. Biochemical studies have demonstrated that αSyn interacts with lipids, and these interactions affect the aggregation process of αSyn. Furthermore, genetic studies have identified mutations in lipid metabolism-associated genes such as glucocerebrosidase 1 (GBA1) and synaptojanin 1 (SYNJ1) in sporadic and familial forms of PD, respectively. In this review, we focus on the role of lipids in triggering αSyn aggregation in the pathogenesis of PD and propose the possibility of modulating the interaction of lipids with αSyn as a potential therapy for PD.

Prion diseases are caused by prions, which are proteinaceous infectious particles that have been identified as causative factors of transmissible spongiform encephalopathies such as Creutzfeldt-Jakob disease (CJD). Prion diseases are devastating neurodegenerative disorders in humans and many animals, including sheep, cows, deer, cats, and camels. Prion diseases are classified into sporadic and genetic forms. Additionally, a third, environmentally acquired category exists. This type includes kuru, iatrogenic CJD caused by human dura mater grafts or human pituitary-derived hormones, and variant CJD transmitted through food contaminated with bovine spongiform encephalopathy prions. Bovine spongiform encephalopathy and variant CJD have nearly been controlled, but chronic wasting disease, a prion disease affecting deer, is spreading widely in North America and South Korea and recently in Northern Europe. Recently, amyloid-beta, alpha-synuclein, and other proteins related to Alzheimer's disease, Parkinson's disease, and other neurodegenerative diseases were reported to have prion features such as transmission to animals. Amyloid-beta transmission to humans has been suggested in iatrogenic CJD cases and in cerebral amyloid angiopathy cases with cerebral bleeding occurring long after childhood neurosurgery with or without cadaveric dura mater transplantation. These findings indicate that diseases caused by various prions, namely various transmissible proteins, appear to be a threat, particularly in the current longevity society. Prion disease represented by CJD has obvious transmissibility and is considered to be an "archetype of various neurodegenerative diseases". Overcoming prion diseases is a top priority currently in our society, and this strategy will certainly contribute to elucidating pathomechanism of other neurodegenerative diseases and developing new therapies for them.