Oral and intravenous transmission of α-synuclein fibrils to mice

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

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

Vaccination with structurally adapted fungal protein fibrils induces immunity to Parkinson's disease

The pathological misfolding and aggregation of soluble α-synuclein into toxic oligomers and insoluble amyloid fibrils causes Parkinson's disease, a progressive age-related neurodegenerative disease for which there is no cure. HET-s is a soluble fungal protein that can form assembled amyloid fibrils in its prion state. We engineered HET-s(218-298) to form four different fibrillar vaccine candidates, each displaying a specific conformational epitope present on the surface of α-synuclein fibrils. Vaccination with these four vaccine candidates prolonged the survival of immunized TgM83+/- mice challenged with α-synuclein fibrils by 8% when injected into the brain to model brain-first Parkinson's disease or by 21% and 22% when injected into the peritoneum or gut wall, respectively, to model body-first Parkinson's disease. Antibodies from fully immunized mice recognized α-synuclein fibrils and brain homogenates from patients with Parkinson's disease, dementia with Lewy bodies and multiple system atrophy. Conformation-specific vaccines that mimic epitopes present only on the surface of pathological fibrils but not on soluble monomers, hold great promise for protection against Parkinson's disease, related synucleinopathies and other amyloidogenic protein misfolding disorders.

Is There a Place for Lewy Bodies before and beyond Alpha-Synuclein Accumulation? Provocative Issues in Need of Solid Explanations

In the last two decades, alpha-synuclein (alpha-syn) assumed a prominent role as a major component and seeding structure of Lewy bodies (LBs). This concept is driving ongoing research on the pathophysiology of Parkinson's disease (PD). In line with this, alpha-syn is considered to be the guilty protein in the disease process, and it may be targeted through precision medicine to modify disease progression. Therefore, designing specific tools to block the aggregation and spreading of alpha-syn represents a major effort in the development of disease-modifying therapies in PD. The present article analyzes concrete evidence about the significance of alpha-syn within LBs. In this effort, some dogmas are challenged. This concerns the question of whether alpha-syn is more abundant compared with other proteins within LBs. Again, the occurrence of alpha-syn compared with non-protein constituents is scrutinized. Finally, the prominent role of alpha-syn in seeding LBs as the guilty structure causing PD is questioned. These revisited concepts may be helpful in the process of validating which proteins, organelles, and pathways are likely to be involved in the damage to meso-striatal dopamine neurons and other brain regions involved in PD.

Addition of α-synuclein aggregates to the intestinal environment recapitulates Parkinsonian symptoms in model systems

The gut-brain axis plays a vital role in Parkinson's disease (PD). The mechanisms of gut-brain transmission mainly focus on α-synuclein deposition, intestinal inflammation and microbiota function. A few studies have shown the trigger of PD pathology in the gut. α-Synuclein is highly conserved in food products, which was able to form β-folded aggregates and to infect the intestinal mucosa. In this study we investigated whether α-synuclein-preformed fibril (PFF) exposure could modulate the intestinal environment and induce rodent models replicating PD pathology. We first showed that PFF could be internalized into co-cultured Caco-2/HT29/Raji b cells in vitro. Furthermore, we demonstrated that PFF perfusion caused the intestinal inflammation and activation of enteric glial cells in an ex vivo intestinal organ culture and in an in vivo intestinal mouse coloclysis model. Moreover, we found that PFF exposure through regular coloclysis induced PD pathology in wild-type (WT) and A53T α-synuclein transgenic mice with various phenotypes. Particularly in A53T mice, PFF induced significant behavioral disorders, intestinal inflammation, α-synuclein deposition, microbiota dysbiosis, glial activation as well as degeneration of dopaminergic neurons in the substantia nigra. In WT mice, however, the PFF induced only mild behavioral abnormalities, intestinal inflammation, α-synuclein deposition, and glial activation, without significant changes in microbiota and dopaminergic neurons. Our results reveal the possibility of α-synuclein aggregates binding to the intestinal mucosa and modeling PD in mice. This study may shed light on the investigation and early intervention of the gut-origin hypothesis in neurodegenerative diseases.

Colonoscopy and Subsequent Risk of Parkinson's Disease

Background

Parkinson's disease (PD) is caused by the misfolding and aggregation of α-synuclein in neurons into toxic oligomers and fibrils that have prion-like properties allowing them to infect healthy neurons and to be transmitted to animal models of PD by injection or oral exposure. Given α-synuclein fibrils' potential transmission on the gut-brain axis, α-synuclein may be transmitted through colonoscopy procedures.

Objective

This study examines a possible association between colonoscopy and PD.

Methods

Longitudinal health insurance data of 250,000 individuals aged 50+ from 2004-2019 was analyzed. Cox proportional hazard and competing risk models with death as a competing event were estimated to calculate the risk of PD. Colonoscopy was categorized as never receiving colonoscopy, colorectal cancer (CRC) screening without or with biopsy, destruction or excision (BDE), and diagnostic colonoscopy without or with BDE.

Results

We identified 6,422 new cases of PD among 221,582 individuals. The Cox model revealed a significantly increased risk of PD for patients who ever had a diagnostic colonoscopy without or with BDE (HR = 1.31; 95% CI: [1.23-1.40]; HR = 1.32 [1.22-1.42]) after adjustment for age and sex. After controlling for covariates and death, persons who ever underwent CRC screening had a 40% reduced risk of PD (CRHR = 0.60 [0.54-0.67]), while persons who underwent diagnostic colonoscopy had a 20% reduced risk of PD (CRHR = 0.81 [0.75-0.88]).

Conclusions

Colonoscopy does not increase the risk of PD, after adjusting for death and covariates. Individuals who underwent only CRC screening had the lowest risk of PD, which may be a result of a more health-conscious lifestyle.

Structure specific neuro-toxicity of α-synuclein oligomer

Parkinson's disease (PD) is linked to α-synuclein (aS) aggregation and deposition of amyloid in the substantia nigra region of the brain tissues. In the current investigation we produced two distinct classes of aS oligomer of differed protein conformation, stability and compared their toxic nature to cultured neuronal cells. Lyophilized oligomer (LO) was produced in storage of aS at-20 °C for 7 days and it was enriched with loosely hold molten globule like structure with residues having preferences for α-helical conformational space. The size of the oligomer was 4-5.5 nm under AFM. This kind of oligomer exhibited potential toxicity towards neuronal cell lines and did not transform into compact β-sheet rich amyloid fiber even after incubation at 37 °C for several days. Formation of another type of oligomer was often observed in the lag phase of aS fibrillation that often occurred at an elevated temperature (37 °C). This kind of heat induced oligomer (IO) was more hydrophobic and relatively less toxic to neuronal cells compared to lyophilized oligomer (LO). Importantly, initiation of hydrophobic zipping of aS caused the transformation of IO into thermodynamically stable β-sheet rich amyloid fibril. On the other hand, the presence of molten globule like conformation in LO, rendered greater toxicity to cultured neuronal cells.

Protease-Sensitive and -Resistant Forms of Human and Murine Alpha-Synucleins in Distinct Brain Regions of Transgenic Mice (M83) Expressing the Human Mutated A53T Protein

Human neurodegenerative diseases associated with the misfolding of the alpha-synuclein (aS) protein (synucleinopathies) are similar to prion diseases to the extent that lesions are spread by similar molecular mechanisms. In a transgenic mouse model (M83) overexpressing a mutated (A53T) form of human aS, we had previously found that Protein Misfolding Cyclic Amplification (PMCA) triggered the aggregation of aS, which is associated with a high resistance to the proteinase K (PK) digestion of both human and murine aS, a major hallmark of the disease-associated prion protein. In addition, PMCA was also able to trigger the aggregation of murine aS in C57Bl/6 mouse brains after seeding with sick M83 mouse brains. Here, we show that intracerebral inoculations of M83 mice with C57Bl/6-PMCA samples strikingly shortens the incubation period before the typical paralysis that develops in this transgenic model, demonstrating the pathogenicity of PMCA-aggregated murine aS. In the hind brain regions of these sick M83 mice containing lesions with an accumulation of aS phosphorylated at serine 129, aS also showed a high PK resistance in the N-terminal part of the protein. In contrast to M83 mice, old APPxM83 mice co-expressing human mutated amyloid precursor and presenilin 1 proteins were seen to have an aggregation of aS, especially in the cerebral cortex, hippocampus and striatum, which also contained the highest load of aS phosphorylated at serine 129. This was proven by three techniques: a Western blot analysis of PK-resistant aS; an ELISA detection of aS aggregates; or the identification of aggregates of aS using immunohistochemical analyses of cytoplasmic/neuritic aS deposits. The results obtained with the D37A6 antibody suggest a higher involvement of murine aS in APPxM83 mice than in M83 mice. Our study used novel tools for the molecular study of synucleinopathies, which highlight similarities with the molecular mechanisms involved in prion diseases.

Cerebral Microvascular Injury Induced by Lag3-Dependent α-Synuclein Fibril Endocytosis Exacerbates Cognitive Impairment in a Mouse Model of α-Synucleinopathies

The pathological accumulation of α-synuclein (α-Syn) and the transmission of misfolded α-Syn underlie α-synucleinopathies. Increased plasma α-Syn levels are associated with cognitive impairment in Parkinson's disease, multiple system atrophy, and dementia with Lewy bodies, but it is still unknown whether the cognitive deficits in α-synucleinopathies have a common vascular pathological origin. Here, it is reported that combined injection of α-Syn preformed fibrils (PFFs) in the unilateral substantia nigra pars compacta, hippocampus, and cerebral cortex results in impaired spatial learning and memory abilities at 6 months post-injection and that this cognitive decline is related to cerebral microvascular injury. Moreover, insoluble α-Syn inclusions are found to form in primary mouse brain microvascular endothelial cells (BMVECs) through lymphocyte-activation gene 3 (Lag3)-dependent α-Syn PFFs endocytosis, causing poly(ADP-ribose)-driven cell death and reducing the expression of tight junction proteins in BMVECs. Knockout of Lag3 in vitro prevents α-Syn PFFs from entering BMVECs, thereby reducing the abovementioned response induced by α-Syn PFFs. Deletion of endothelial cell-specific Lag3 in vivo reverses the negative effects of α-Syn PFFs on cerebral microvessels and cognitive function. In short, this study reveals the effectiveness of targeting Lag3 to block the spread of α-Syn fibrils to endothelial cells in order to improve cognition.

Myo1d promotes alpha-synuclein transfer from brain microvascular endothelial cells to pericytes through tunneling nanotubes

α-Synuclein preformed fibrils (α-syn PFF) in the blood can cross the blood-brain barrier and invade the central nervous system. Our previous study proved that α-syn PFF can be taken up by brain microvascular endothelial cells (BMVECs). Here, we found that α-syn PFF spread from BMVECs to pericytes with the highest transmission efficiency. We observed abundant tunneling nanotubes (TNTs) connecting BMVECs and pericytes, and α-syn PFF transmitted through these TNTs. Furthermore, α-syn PFF accumulation in BMVECs did not promote TNT formation, but activated the molecular motor Myo1d. Inhibition of Myo1d prevented α-syn PFF transfer from BMVECs to pericytes and decreased the colocalization of Myo1d and F-actin in BMVECs. In summary, we are the first to demonstrate that α-syn PFF spread from BMVECs to pericytes through a mechanism involving TNTs and myosin. Targeting Myo1d may be a promising approach to prevent α-syn spreading from the blood to the brain.

Experimental models of gut-first Parkinson's disease: A systematic review

BACKGROUND

There is strong support from studies in humans and in animal models that Parkinson's disease (PD) may begin in the gut. This brings about a unique opportunity for researchers in the field of neurogastroenterology to contribute to advancing the field and making contributions that could lead to the ability to diagnose and treat PD in the premotor stages. Lack of familiarity with some of the aspects of the experimental approaches used in these studies may present a barrier for neurogastroenterology researchers to enter the field. Much remains to be understood about intestinal-specific components of gut-first PD pathogenesis and the field would benefit from contributions of enteric and central nervous system neuroscientists.

PURPOSE

To address these issues, we have conducted a systematic review of the two most frequently used experimental models of gut-first PD: transneuronal propagation of α-synuclein preformed fibrils and oral exposure to environmental toxins. We have reviewed the details of these studies and present methodological considerations for the use of these models. Our aim is that this review will serve as a framework and useful reference for neuroscientists, gastroenterologists, and neurologists interested in applying their expertise to advancing our understanding of gut-first PD.

Propagative α-synuclein seeds as serum biomarkers for synucleinopathies

Abnormal α-synuclein aggregation is a key pathological feature of a group of neurodegenerative diseases known as synucleinopathies, which include Parkinson's disease (PD), dementia with Lewy bodies and multiple system atrophy (MSA). The pathogenic β-sheet seed conformation of α-synuclein is found in various tissues, suggesting potential as a biomarker, but few studies have been able to reliably detect these seeds in serum samples. In this study, we developed a modified assay system, called immunoprecipitation-based real-time quaking-induced conversion (IP/RT-QuIC), which enables the detection of pathogenic α-synuclein seeds in the serum of individuals with synucleinopathies. In our internal first and second cohorts, IP/RT-QuIC showed high diagnostic performance for differentiating PD versus controls (area under the curve (AUC): 0.96 (95% confidence interval (CI) 0.95-0.99)/AUC: 0.93 (95% CI 0.84-1.00)) and MSA versus controls (AUC: 0.64 (95% CI 0.49-0.79)/AUC: 0.73 (95% CI 0.49-0.98)). IP/RT-QuIC also showed high diagnostic performance in differentiating individuals with PD (AUC: 0.86 (95% CI 0.74-0.99)) and MSA (AUC: 0.80 (95% CI 0.65-0.97)) from controls in a blinded external cohort. Notably, amplified seeds maintained disease-specific properties, allowing the differentiation of samples from individuals with PD versus MSA. In summary, here we present a novel platform that may allow the detection of individuals with synucleinopathies using serum samples.

Gut-to-Brain α-Synuclein Transmission in Parkinson's Disease: Evidence for Prion-like Mechanisms

Parkinson's disease (PD) is a multifactorial disorder involving both motor and non-motor symptoms caused by the progressive death of distinct neuronal populations, including dopaminergic neurons in the substantia nigra. The deposition of aggregated α-synuclein protein into Lewy body inclusions is a hallmark of the disorder, and α-synuclein pathology has been found in the enteric nervous system (ENS) of PD patients up to two decades prior to diagnosis. In combination with the high occurrence of gastrointestinal dysfunction in early stages of PD, current evidence strongly suggests that some forms of PD may originate in the gut. In this review, we discuss human studies that support ENS Lewy pathology as a characteristic feature of PD, and present evidence from humans and animal model systems that α-synuclein aggregation may follow a prion-like spreading cascade from enteric neurons, through the vagal nerve, and into the brain. Given the accessibility of the human gut to pharmacologic and dietary interventions, therapeutic strategies aimed at reducing pathological α-synuclein in the gastrointestinal tract hold significant promise for PD treatment.

PET imaging in animal models of Parkinson's disease

Alpha-synucleinopathies, such as Parkinson's disease, dementia with Lewy bodies and multiple system atrophy, are characterized by aberrant accumulation of alpha-synuclein and synaptic dysfunction leading to motor and cognitive deficits. Animal models of alpha-synucleinopathy have greatly facilitated the mechanistic understanding of the disease and the development of therapeutics. Various transgenic, alpha-synuclein fibril-injected, and toxin-injected animal models of Parkinson's disease and multiple system atrophy that recapitulate the disease pathology have been developed and widely used. Recent advances in positron emission tomography have allowed the noninvasive visualization of molecular alterations, underpinning behavioral dysfunctions in the brains of animal models and the longitudinal monitoring of treatment effects. Imaging studies in these disease animal models have employed multi-tracer PET designs to reveal dopaminergic deficits together with other molecular alterations. This review focuses on the development of new positron emission tomography tracers and studies of alpha-synuclein, synaptic vesicle glycoprotein 2A neurotransmitter receptor deficits such as dopaminergic receptor, dopaminergic transporter, serotonergic receptor, vesicular monoamine transporter 2, hypometabolism, neuroinflammation, mitochondrial dysfunction and leucine rich repeat kinase 2 in animal models of Parkinson's disease. The outstanding challenges and emerging applications are outlined, such as investigating the gut-brain-axis by using positron emission tomography in animal models, and provide a future outlook.

Patients with isolated REM-sleep behavior disorder have elevated levels of alpha-synuclein aggregates in stool

Misfolded and aggregated α-synuclein is a neuropathological hallmark of Parkinson's disease (PD). Thus, α-synuclein aggregates are regarded as a biomarker for the development of diagnostic assays. Quantification of α-synuclein aggregates in body fluids is challenging, and requires highly sensitive and specific assays. Recent studies suggest that α-synuclein aggregates may be shed into stool. We used surface-based fluorescence intensity distribution analysis (sFIDA) to detect and quantify single particles of α-synuclein aggregates in stool of 94 PD patients, 72 isolated rapid eye movement sleep behavior disorder (iRBD) patients, and 51 healthy controls. We measured significantly elevated concentrations of α-synuclein aggregates in stool of iRBD patients versus those of controls (p = 0.024) or PD patients (p < 0.001). Our results show that α-synuclein aggregates are excreted in stool and can be measured using the sFIDA assay, which could support the diagnosis of prodromal synucleinopathies.

Effects of a probiotic suspension Symprove™ on a rat early-stage Parkinson's disease model

An increasing number of studies in recent years have focused on the role that the gut may play in Parkinson's Disease (PD) pathogenesis, suggesting that the maintenance of a healthy gut may lead to potential treatments of the disease. The health of microbiota has been shown to be directly associated with parameters that play a potential role in PD including gut barrier integrity, immunity, function, metabolism and the correct functioning of the gut-brain axis. The gut microbiota (GM) may therefore be employed as valuable indicators for early diagnosis of PD and potential targets for preventing or treating PD symptoms. Preserving the gut homeostasis using probiotics may therefore lead to a promising treatment strategy due to their known benefits in improving constipation, motor impairments, inflammation, and neurodegeneration. However, the mechanisms underlying the effects of probiotics in PD are yet to be clarified. In this project, we have tested the efficacy of an oral probiotic suspension, Symprove™, on an established animal model of PD. Symprove™, unlike many commercially available probiotics, has been shown to be resistant to gastric acidity, improve symptoms in gastrointestinal diseases and improve gut integrity in an in vitro PD model. In this study, we used an early-stage PD rat model to determine the effect of Symprove™ on neurodegeneration and neuroinflammation in the brain and on plasma cytokine levels, GM composition and short chain fatty acid (SCFA) release. Symprove™ was shown to significantly influence both the gut and brain of the PD model. It preserved the gut integrity in the PD model, reduced plasma inflammatory markers and changed microbiota composition. The treatment also prevented the reduction in SCFAs and striatal inflammation and prevented tyrosine hydroxylase (TH)-positive cell loss by 17% compared to that observed in animals treated with placebo. We conclude that Symprove™ treatment may have a positive influence on the symptomology of early-stage PD with obvious implications for the improvement of gut integrity and possibly delaying/preventing the onset of neuroinflammation and neurodegeneration in human PD patients.

Neuroinflammation, immune response and α-synuclein pathology: how animal models are helping us to connect dots

INTRODUCTION

A key pathological event occurring in Parkinson's disease (PD) is the transneuronal spreading of alpha-synuclein (α-syn). Other hallmarks of PD include neurodegeneration, glial activation, and immune cell infiltration in susceptible brain regions. Although preclinical models can mimic most of the key characteristics of PD, it is crucial to know the biological bases of individual differences between them when choosing one over another, to ensure proper interpretation of the results and to positively influence the outcome of the experiments.

AREAS COVERED

This review provides an overview of current preclinical models actively used to study the interplay between α-syn pathology, neuroinflammation and immune response in PD but also to explore new potential preclinical models or emerging therapeutic strategies intended to fulfill the unmet medical needs in this disease. Lastly, this review also considers the current state of the ongoing clinical trials of new drugs designed to target these processes and delay the initiation or progression of the disease.

EXPERT OPINION

Anti-inflammatory and immunomodulatory agents have been demonstrated to be very promising candidates for reducing disease progression; however, more efforts are needed to reduce the enormous gap between these and dopaminergic drugs, which have dominated the therapeutic market for the last sixty years.

Footprints of a microbial toxin from the gut microbiome to mesencephalic mitochondria

OBJECTIVE

Idiopathic Parkinson's disease (PD) is characterised by alpha-synuclein (aSyn) aggregation and death of dopaminergic neurons in the midbrain. Recent evidence posits that PD may initiate in the gut by microbes or their toxins that promote chronic gut inflammation that will ultimately impact the brain. In this work, we sought to demonstrate that the effects of the microbial toxin β-N-methylamino-L-alanine (BMAA) in the gut may trigger some PD cases, which is especially worrying as this toxin is present in certain foods but not routinely monitored by public health authorities.

DESIGN

To test the hypothesis, we treated wild-type mice, primary neuronal cultures, cell lines and isolated mitochondria with BMAA, and analysed its impact on gut microbiota composition, barrier permeability, inflammation and aSyn aggregation as well as in brain inflammation, dopaminergic neuronal loss and motor behaviour. To further examine the key role of mitochondria, we also determined the specific effects of BMAA on mitochondrial function and on inflammasome activation.

RESULTS

BMAA induced extensive depletion of segmented filamentous bacteria (SFB) that regulate gut immunity, thus triggering gut dysbiosis, immune cell migration, increased intestinal inflammation, loss of barrier integrity and caudo-rostral progression of aSyn. Additionally, BMAA induced in vitro and in vivo mitochondrial dysfunction with cardiolipin exposure and consequent activation of neuronal innate immunity. These events primed neuroinflammation, dopaminergic neuronal loss and motor deficits.

CONCLUSION

Taken together, our results demonstrate that chronic exposure to dietary BMAA can trigger a chain of events that recapitulate the evolution of the PD pathology from the gut to the brain, which is consistent with 'gut-first' PD.

The Hidden Cell-to-Cell Trail of α-Synuclein Aggregates

The progressive accumulation of insoluble aggregates of the presynaptic protein alpha-synuclein (α-Syn) is a hallmark of neurodegenerative disorders including Parkinson's disease (PD), Multiple System Atrophy, and Dementia with Lewy Bodies, commonly referred to as synucleinopathies. Despite considerable progress on the structural biology of these aggregates, the molecular mechanisms mediating their cell-to-cell transmission, propagation, and neurotoxicity remain only partially understood. Numerous studies have highlighted the stereotypical spatiotemporal spreading of pathological α-Syn aggregates across different tissues and anatomically connected brain regions over time. Experimental evidence from various cellular and animal models indicate that α-Syn transfer occurs in two defined steps: the release of pathogenic α-Syn species from infected cells, and their uptake via passive or active endocytic pathways. Once α-Syn aggregates have been internalized, little is known about what drives their toxicity or how they interact with the endogenous protein to promote its misfolding and subsequent aggregation. Similarly, unknown genetic factors modulate different cellular responses to the aggregation and accumulation of pathogenic α-Syn species. Here we discuss the current understanding of the molecular phenomena associated with the intercellular spreading of pathogenic α-Syn seeds and summarize the evidence supporting the transmission hypothesis. Understanding the molecular mechanisms involved in α-Syn aggregates transmission is essential to develop novel targeted therapeutics against PD and related synucleinopathies.

Heparan sulfate proteoglycans mediate prion-like α-synuclein toxicity in Parkinson's in vivo models

This study offers new Caenorhabditis elegans models in which feeding with α-synuclein pre-formed fibrils results in prion-like aggregation of host α-synuclein and dopamine neuron degeneration, and these disease phenotypes are regulated by the heparan sulfate proteoglycan pathway.

Parkinson’s disease (PD) is a debilitating neurodegenerative disorder characterized by progressive motor decline and the aggregation of α-synuclein protein. Growing evidence suggests that α-synuclein aggregates may spread from neurons of the digestive tract to the central nervous system in a prion-like manner, yet the mechanisms of α-synuclein transmission and neurotoxicity remain poorly understood. Animal models that are amenable to high-throughput investigations are needed to facilitate the discovery of disease mechanisms. Here we describe the first Caenorhabditis elegans models in which feeding with α-synuclein preformed fibrils (PFFs) induces dopaminergic neurodegeneration, prion-like seeding of aggregation of human α-synuclein expressed in the host, and an associated motor decline. RNAi-mediated knockdown of the C. elegans syndecan sdn-1, or other enzymes involved in heparan sulfate proteoglycan synthesis, protected against PFF-induced α-synuclein aggregation, motor dysfunction, and dopamine neuron degeneration. This work offers new models by which to investigate gut-derived α-synuclein spreading and propagation of disease.

Molecular and Cellular Interactions in Pathogenesis of Sporadic Parkinson Disease

An increasing number of the population all around the world suffer from age-associated neurodegenerative diseases including Parkinson’s disease (PD). This disorder presents different signs of genetic, epigenetic and environmental origin, and molecular, cellular and intracellular dysfunction. At the molecular level, α-synuclein (αSyn) was identified as the principal molecule constituting the Lewy bodies (LB). The gut microbiota participates in the pathogenesis of PD and may contribute to the loss of dopaminergic neurons through mitochondrial dysfunction. The most important pathogenetic link is an imbalance of Ca²⁺ ions, which is associated with redox imbalance in the cells and increased generation of reactive oxygen species (ROS). In this review, genetic, epigenetic and environmental factors that cause these disorders and their cause-and-effect relationships are considered. As a constituent of environmental factors, the example of organophosphates (OPs) is also reviewed. The role of endothelial damage in the pathogenesis of PD is discussed, and a ‘triple hit hypothesis’ is proposed as a modification of Braak’s dual hit one. In the absence of effective therapies for neurodegenerative diseases, more and more evidence is emerging about the positive impact of nutritional structure and healthy lifestyle on the state of blood vessels and the risk of developing these diseases.

Ischemic stroke causes Parkinson's disease-like pathology and symptoms in transgenic mice overexpressing alpha-synuclein

The etiology of Parkinson's disease is poorly understood and is most commonly associated with advancing age, genetic predisposition, or environmental toxins. Epidemiological findings suggest that patients have a higher risk of developing Parkinson's disease after ischemic stroke, but this potential causality lacks mechanistic evidence. We investigated the long-term effects of ischemic stroke on pathogenesis in hemizygous TgM83 mice, which express human α-synuclein with the familial A53T mutation without developing any neuropathology or signs of neurologic disease for more than 600 days. We induced transient focal ischemia by middle cerebral artery occlusion in 2-month-old TgM83^+/- mice and monitored their behavior and health status for up to 360 days post surgery. Groups of mice were sacrificed at 14, 30, 90, 180, and 360 days after surgery for neuropathological analysis of their brains. Motor deficits first appeared 6 months after focal ischemia and worsened until 12 months afterward. Immunohistochemical analysis revealed ischemia-induced neuronal loss in the infarct region and astrogliosis and microgliosis indicative of an inflammatory response, which was most pronounced at 14 days post surgery. Infarct volume and inflammation gradually decreased in size and severity until 180 days post surgery. Surprisingly, neuronal loss and inflammation were increased again by 360 days post surgery. These changes were accompanied by a continuous increase in α-synuclein aggregation, its neuronal deposition, and a late loss of dopaminergic neurons in the substantia nigra, which we detected at 360 days post surgery. Control animals that underwent sham surgery without middle cerebral artery occlusion showed no signs of disease or neuropathology. Our results establish a mechanistic link between ischemic stroke and Parkinson's disease and provide an animal model for studying possible interventions.

α-Synuclein in Parkinson's disease and advances in detection

Parkinson's disease (PD) is a threatening neurodegenerative disorder that seriously affects patients' life quality. Substantial evidence links the overexpression and abnormal aggregation of alpha-synuclein (α-Syn) to PD. α-Syn has been identified as a characteristic biomarker of PD, which indicates its great value of diagnosis and designing effective therapeutic strategy. This article systematically summarizes the pathogenic process of α-Syn based on recent researches, outlines and compares commonly used analysis and detection technologies of α-Syn. Specifically, the detection of α-Syn by new electrochemical, photochemical, and crystal biosensors is mainly examined. Furthermore, the speculation of future study orientation is discussed, which provides reference for the further research and application of α-Syn as biomarker.

Mitochondria-Microbiota Interaction in Neurodegeneration

Alzheimer’s and Parkinson’s are the two best-known neurodegenerative diseases. Each is associated with the excessive aggregation in the brain and elsewhere of its own characteristic amyloid proteins. Yet the two afflictions have much in common and often the same amyloids play a role in both. These amyloids need not be toxic and can help regulate bile secretion, synaptic plasticity, and immune defense. Moreover, when they do form toxic aggregates, amyloids typically harm not just patients but their pathogens too. A major port of entry for pathogens is the gut. Keeping the gut’s microbe community (microbiota) healthy and under control requires that our cells’ main energy producers (mitochondria) support the gut-blood barrier and immune system. As we age, these mitochondria eventually succumb to the corrosive byproducts they themselves release, our defenses break down, pathogens or their toxins break through, and the side effects of inflammation and amyloid aggregation become problematic. Although it gets most of the attention, local amyloid aggregation in the brain merely points to a bigger problem: the systemic breakdown of the entire human superorganism, exemplified by an interaction turning bad between mitochondria and microbiota.

Body-first Parkinson's disease and variant Creutzfeldt-Jakob disease - similar or different?

In several neurodegenerative disorders, proteins that typically exhibit an α-helical structure misfold into an amyloid conformation rich in β-sheet content. Through a self-templating mechanism, these amyloids are able to induce additional protein misfolding, facilitating their propagation throughout the central nervous system. This disease mechanism was originally identified for the prion protein (PrP), which misfolds into PrP^Sc in a number of disorders, including variant Creutzfeldt-Jakob disease (vCJD) and bovine spongiform encephalopathy (BSE). More recently, the prion mechanism of disease was expanded to include other proteins that rely on this self-templating mechanism to cause progressive degeneration, including α-synuclein misfolding in Parkinson's disease (PD). Several studies now suggest that PD patients can be subcategorized based on where in the body misfolded α-synuclein originates, either the brain or the gut, similar to patients developing sporadic CJD or vCJD. In this review, we discuss the human and animal model data indicating that α-synuclein and PrP^Sc misfolding originates in the gut in body-first PD and vCJD, and summarize the data identifying the role of the autonomic nervous system in the gut-brain axis of both diseases.

Brain regions susceptible to alpha-synuclein spreading

The spreading of misfolded alpha-synuclein (α-syn) protein has been observed in animal models of Parkinson's disease (PD) and other α-synucleinopathies that mimic human PD pathologies. In animal models, the spreading of α-syn has been associated with motor dysfunction and neuronal death. However, variability in both susceptible brain regions and cellular populations limits our understanding of the consequences of α-syn spreading and the development of associated therapies. Here, we have reviewed the physiological and pathological functions of α-syn and summarized the susceptible brain regions and cell types identified from human postmortem studies and exogenous α-syn injection-based animal models. We have reviewed the methods for inducing α-syn aggregation, the specific hosts, the inoculation sites, the routes of propagation, and other experimental settings that may affect the spreading pattern of α-syn, as reported in current studies. Understanding the spread of α-syn to produce a consistent PD animal model is vital for future drug discovery.

Acute Exposure to the Food-Borne Pathogen Listeria monocytogenes Does Not Induce α-Synuclein Pathology in the Colonic ENS of Nonhuman Primates

INTRODUCTION

Gastrointestinal (GI) inflammation elicited by environmental factors is proposed to trigger Parkinson's disease (PD) by stimulating accumulation of pathological α-synuclein (α-syn) in the enteric nervous system (ENS), which then propagates to the central nervous system via the vagus nerve. The goal of this study was to model, in nonhuman primates, an acute exposure to a common food-borne pathogen in order to assess whether the related acute GI inflammation could initiate persistent α-syn pathology in the ENS, ultimately leading to PD.

METHODS

Adult female cynomolgus macaques were inoculated by oral gavage with 1×108 colony-forming units (CFUs) Listeria monocytogenes (LM, n=10) or vehicle (mock, n=3) and euthanized 2 weeks later. Evaluations included clinical monitoring, blood and fecal shedding of LM, and postmortem pathological analysis of colonic and cecal tissues.

RESULTS

LM inoculation of healthy adult cynomolgus macaques induced minimal to mild clinical signs of infection; LM shedding in feces was not seen in any of the animals nor was bacteremia detected. Colitis varied from none to moderate in LM-treated subjects and none to minimal in mock-treated subjects. Expression of inflammatory markers (HLA-DR, CD3, CD20), oxidative stress (8-OHDG), α-syn, and phosphorylated-α-syn in the enteric ganglia was not significantly different between treatment groups.

DISCUSSION

Our results demonstrate that cynomolgus macaques orally inoculated with LM present with a clinical response that resembles human LM exposure. They also suggest that acute exposure to food-borne pathogens is not sufficient to induce significant and persistent α-syn changes in healthy adult female subjects. Based on the results of this limited experimental setting, we propose that, if LM has a role in PD pathology, other underlying factors or conditions, such as male sex, inflammatory bowel disease, exposure to toxins, dysbiosis, and/or aging, are needed to be present.

Assembly of α-synuclein and neurodegeneration in the central nervous system of heterozygous M83 mice following the peripheral administration of α-synuclein seeds

Peripheral administration (oral, intranasal, intraperitoneal, intravenous) of assembled A53T α-synuclein induced synucleinopathy in heterozygous mice transgenic for human mutant A53T α-synuclein (line M83). The same was the case when cerebellar extracts from a case of multiple system atrophy with type II α-synuclein filaments were administered intraperitoneally, intravenously or intramuscularly. We observed abundant immunoreactivity for pS129 α-synuclein in nerve cells and severe motor impairment, resulting in hindlimb paralysis and shortened lifespan. Filaments immunoreactive for pS129 α-synuclein were in evidence. A 70% loss of motor neurons was present five months after an intraperitoneal injection of assembled A53T α-synuclein or cerebellar extract with type II α-synuclein filaments from an individual with a neuropathologically confirmed diagnosis of multiple system atrophy. Microglial cells changed from a predominantly ramified to a dystrophic appearance. Taken together, these findings establish a close relationship between the formation of α-synuclein inclusions in nerve cells and neurodegeneration, accompanied by a shift in microglial cell morphology. Propagation of α-synuclein inclusions depended on the characteristics of both seeds and transgenically expressed protein.

Extracellular Vesicles Physiological Role and the Particular Case of Disease-Spreading Mechanisms in Polyglutamine Diseases

Recent research demonstrated pathological spreading of the disease-causing proteins from one focal point across other brain regions for some neurodegenerative diseases, such as Parkinson's and Alzheimer's disease. Spreading mediated by extracellular vesicles is one of the proposed disease-spreading mechanisms. Extracellular vesicles are cell membrane-derived vesicles, used by cells for cell-to-cell communication and excretion of toxic components. Importantly, extracellular vesicles carrying pathological molecules, when internalized by "healthy" cells, may trigger pathological pathways and, consequently, promote disease spreading to neighboring cells. Polyglutamine diseases are a group of genetic neurodegenerative disorders characterized by the accumulation of mutant misfolded proteins carrying an expanded tract of glutamines, including Huntington's and Machado-Joseph disease. The pathological spread of the misfolded proteins or the corresponding mutant mRNA has been explored. The understanding of the disease-spreading mechanism that plays a key role in the pathology progression of these diseases can result in the development of effective therapeutic approaches to stop disease progression, arresting the spread of the toxic components and disease aggravation. Therefore, the present review's main focus is the disease-spreading mechanisms with emphasis on polyglutamine diseases and the putative role played by extracellular vesicles in this process.

The Bacterial Amyloids Phenol Soluble Modulins from Staphylococcus aureus Catalyze Alpha-Synuclein Aggregation

Aggregated α-synuclein (α-syn) is the main constituent of Lewy bodies, which are a pathological hallmark of Parkinson's disease (PD). Environmental factors are thought to be potential triggers capable of initiating the aggregation of the otherwise monomeric α-syn. Braak's seminal work redirected attention to the intestine and recent reports of dysbiosis have highlighted the potential causative role of the microbiome in the initiation of pathology of PD. Staphylococcus aureus is a bacterium carried by 30-70% of the general population. It has been shown to produce functional amyloids, called phenol soluble modulins (PSMαs). Here, we studied the kinetics of α-syn aggregation under quiescent conditions in the presence or absence of four different PSMα peptides and observed a remarkable shortening of the lag phase in their presence. Whereas pure α-syn monomer did not aggregate up to 450 h after initiation of the experiment in neither neutral nor mildly acidic buffer, the addition of different PSMα peptides resulted in an almost immediate increase in the Thioflavin T (ThT) fluorescence. Despite similar peptide sequences, the different PSMα peptides displayed distinct effects on the kinetics of α-syn aggregation. Kinetic analyses of the data suggest that all four peptides catalyze α-syn aggregation through heterogeneous primary nucleation. The immunogold electron microscopic analyses showed that the aggregates were fibrillar and composed of α-syn. In addition of the co-aggregated materials to a cell model expressing the A53T α-syn variant fused to GFP was found to catalyze α-syn aggregation and phosphorylation in the cells. Our results provide evidence of a potential trigger of synucleinopathies and could have implications for the prevention of the diseases.

Is Multiple System Atrophy a Prion-like Disorder?

Multiple system atrophy (MSA) is a rapidly progressive, fatal neurodegenerative disease of uncertain aetiology that belongs to the family of α-synucleinopathies. It clinically presents with parkinsonism, cerebellar, autonomic, and motor impairment in variable combinations. Pathological hallmarks are fibrillary α-synuclein (αSyn)-rich glial cytoplasmic inclusions (GCIs) mainly involving oligodendroglia and to a lesser extent neurons, inducing a multisystem neurodegeneration, glial activation, and widespread demyelinization. The neuronal αSyn pathology of MSA has molecular properties different from Lewy bodies in Parkinson's disease (PD), both of which could serve as a pool of αSyn (prion) seeds that could initiate and drive the pathogenesis of synucleinopathies. The molecular cascade leading to the "prion-like" transfer of "strains" of aggregated αSyn contributing to the progression of the disease is poorly understood, while some presented evidence that MSA is a prion disease. However, this hypothesis is difficult to reconcile with postmortem analysis of human brains and the fact that MSA-like pathology was induced by intracerebral inoculation of human MSA brain homogenates only in homozygous mutant 53T mice, without production of disease-specific GCIs, or with replication of MSA prions in primary astrocyte cultures from transgenic mice expressing human αSyn. Whereas recent intrastriatal injection of Lewy body-derived or synthetic human αSyn fibrils induced PD-like pathology including neuronal αSyn aggregates in macaques, no such transmission of αSyn pathology in non-human primates by MSA brain lysate has been reported until now. Given the similarities between αSyn and prions, there is a considerable debate whether they should be referred to as "prions", "prion-like", "prionoids", or something else. Here, the findings supporting the proposed nature of αSyn as a prion and its self-propagation through seeding as well as the transmissibility of neurodegenerative disorders are discussed. The proof of disease causation rests on the concordance of scientific evidence, none of which has provided convincing evidence for the classification of MSA as a prion disease or its human transmission until now.

The neuromicrobiology of Parkinson's disease: A unifying theory

Recent evidence confirms that PD is indeed a multifactorial disease with different aetiologies and prodromal symptomatology that likely depend on the initial trigger. New players with important roles as triggers, facilitators and aggravators of the PD neurodegenerative process have re-emerged in the last few years, the microbes. Having evolved in association with humans for ages, microbes and their products are now seen as fundamental regulators of human physiology with disturbances in their balance being increasingly accepted to have a relevant impact on the progression of disease in general and on PD in particular. In this review, we comprehensively address early studies that have directly or indirectly linked bacteria or other infectious agents to the onset and progression of PD, from the earliest suspects to the most recent culprits, the gut microbiota. The quest for effective treatments to arrest PD progression must inevitably address the different interactions between microbiota and human cells, and naturally consider the gut-brain axis. The comprehensive characterization of such mechanisms will help design innovative bacteriotherapeutic approaches to selectively shape the gut microbiota profile ultimately to halt PD progression. The present review describes our current understanding of the role of microorganisms and their endosymbiotic relatives, the mitochondria, in inducing, facilitating, or aggravating PD pathogenesis.

Ageing promotes pathological alpha-synuclein propagation and autonomic dysfunction in wild-type rats

Neuronal aggregates of misfolded alpha-synuclein protein are found in the brain and periphery of patients with Parkinson's disease. Braak and colleagues have hypothesized that the initial formation of misfolded alpha-synuclein may start in the gut, and then spread to the brain via peripheral autonomic nerves hereby affecting several organs, including the heart and intestine. Age is considered the greatest risk factor for Parkinson's disease, but the effect of age on the formation of pathology and its propagation has not been studied in detail. We aimed to investigate whether propagation of alpha-synuclein pathology from the gut to the brain is more efficient in old versus young wild-type rats, upon gastrointestinal injection of aggregated alpha-synuclein. Our results demonstrate a robust age-dependent gut-to-brain and brain-to-gut spread of alpha-synuclein pathology along the sympathetic and parasympathetic nerves, resulting in age-dependent dysfunction of the heart and stomach, as observed in patients with Parkinson's disease. Moreover, alpha-synuclein pathology is more densely packed and resistant to enzymatic digestion in old rats, indicating an age-dependent maturation of alpha-synuclein aggregates. Our study is the first to provide a detailed investigation of alpha-synuclein pathology in several organs within one animal model, including the brain, skin, heart, intestine, spinal cord and autonomic ganglia. Taken together, our findings suggest that age is a crucial factor for alpha-synuclein aggregation and complete propagation to heart, stomach and skin, similar to patients. Given that age is the greatest risk factor for human Parkinson's disease, it seems likely that older experimental animals will yield the most relevant and reliable findings. These results have important implications for future research to optimize diagnostics and therapeutics in Parkinson's disease and other age-associated synucleinopathies. Increased emphasis should be placed on using aged animals in preclinical studies and to elucidate the nature of age-dependent interactions.

Parkinson's Disease: A Prionopathy?

The principal pathogenic event in Parkinson's disease is characterized by the conformational change of α-synuclein, which form pathological aggregates of misfolded proteins, and then accumulate in intraneuronal inclusions causing dopaminergic neuronal loss in specific brain regions. Over the last few years, a revolutionary theory has correlated Parkinson's disease and other neurological disorders with a shared mechanism, which determines α-synuclein aggregates and progresses in the host in a prion-like manner. In this review, the main characteristics shared between α-synuclein and prion protein are compared and the cofactors that influence the remodeling of native protein structures and pathogenetic mechanisms underlying neurodegeneration are discussed.

Peripheral administration of SOD1 aggregates does not transmit pathogenic aggregation to the CNS of SOD1 transgenic mice

The deposition of aggregated proteins is a common neuropathological denominator for neurodegenerative disorders. Experimental evidence suggests that disease propagation involves prion-like mechanisms that cause the spreading of template-directed aggregation of specific disease-associated proteins. In transgenic (Tg) mouse models of superoxide dismutase-1 (SOD1)-linked amyotrophic lateral sclerosis (ALS), inoculation of minute amounts of human SOD1 (hSOD1) aggregates into the spinal cord or peripheral nerves induces premature ALS-like disease and template-directed hSOD1 aggregation that spreads along the neuroaxis. This infectious nature of spreading pathogenic aggregates might have implications for the safety of laboratory and medical staff, recipients of donated blood or tissue, or possibly close relatives and caregivers. Here we investigate whether transmission of ALS-like disease is unique to the spinal cord and peripheral nerve inoculations or if hSOD1 aggregation might spread from the periphery into the central nervous system (CNS). We inoculated hSOD1 aggregate seeds into the peritoneal cavity, hindlimb skeletal muscle or spinal cord of adult Tg mice expressing mutant hSOD1. Although we used up to 8000 times higher dose—compared to the lowest dose transmitting disease in spinal cord inoculations—the peripheral inoculations did not transmit seeded aggregation to the CNS or premature ALS-like disease in hSOD1 Tg mice. Nor was any hSOD1 aggregation detected in the liver, kidney, skeletal muscle or sciatic nerve. To explore potential reasons for the lack of disease transmission, we examined the stability of hSOD1 aggregates and found them to be highly vulnerable to both proteases and detergent. Our findings suggest that exposed individuals and personnel handling samples from ALS patients are at low risk of any potential transmission of seeded hSOD1 aggregation.

Differential seeding and propagating efficiency of α-synuclein strains generated in different conditions

Background

Accumulation of alpha-synuclein (α-syn) is a main pathological hallmark of Parkinson’s and related diseases, which are collectively known as synucleinopathies. Growing evidence has supported that the same protein can induce remarkably distinct pathological progresses and disease phenotypes, suggesting the existence of strain difference among α-syn fibrils. Previous studies have shown that α-syn pathology can propagate from the peripheral nervous system (PNS) to the central nervous system (CNS) in a “prion-like” manner. However, the difference of the propagation potency from the periphery to CNS among different α-syn strains remains unknown and the effect of different generation processes of these strains on the potency of seeding and propagation remains to be revealed in more detail.

Methods

Three strains of preformed α-syn fibrils (PFFs) were generated in different buffer conditions which varied in pH and ionic concentrations. The α-syn PFFs were intramuscularly (IM) injected into a novel bacterial artificial chromosome (BAC) transgenic mouse line that expresses wild-type human α-syn, and the efficiency of seeding and propagation of these PFFs from the PNS to the CNS was evaluated.

Results

The three strains of α-syn PFFs triggered distinct propagation patterns. The fibrils generated in mildly acidic buffer led to the most severe α-syn pathology, degeneration of motor neurons and microgliosis in the spinal cord.

Conclusions

The different α-syn conformers generated in different conditions exhibited strain-specific pathology and propagation patterns from the periphery to the CNS, which further supports the view that α-syn strains may be responsible for the heterogeneity of pathological features and disease progresses among synucleinopathies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40035-021-00242-5.

Activated microglia facilitate the transmission of α-synuclein in Parkinson's disease

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta and abnormal aggregates of α-synuclein protein called Lewy bodies. To date, there is no drug that can definitely slow down or stop the progression of this disease. The discovery of the cell-to-cell transmission of pathologic α-synuclein seeds offers the possibility to explore novel treatment strategies to prevent the spread of α-synuclein, with the purpose of slowing down the progression of PD in its tracks. Although recent studies have made tremendous progress in understanding how α-synuclein spreads throughout the brain, neuroinflammation seems to play a crucial role in the development of α-synuclein pathology in PD. The activation of microglia, one of the hallmarks of the neuroinflammatory process, is suggested to influence the neuron-to-neuron transmission of α-synuclein. This review summarizes how activated microglia facilitate this process, and focuses on the following mechanisms including the activation of microglia in PD, the reduced ability of activated microglia to clear α-synuclein and increased migratory capacity of microglia in PD, as well as the cooperation between microglia and exosomes in mediating α-synuclein release and propagation. In conclusion, this article help collate information on microglia in-relation to PD.

Evaluation of blood flow as a route for propagation in experimental synucleinopathy

In Parkinson's disease, synucleinopathy is hypothesized to spread from the enteric nervous system, via the vagus nerve, to the central nervous system. Recent evidences collected in non-human primates challenge however the hypothesis of a transmission of α-synuclein (α-syn) pathology through the vagus nerve. Would the hypothesis whereby the bloodstream acts as a route for long-distance transmission of pathological α-syn hold true, an inter-individual transmission of synucleinopathy could occur via blood contact. Here, we used a parabiosis approach to join the circulatory systems of wild type and GFP transgenic C57BL/6 J mice, for which one of the partners parabiont received a stereotaxic intranigral injection of patient-derived α-syn aggregates. While the Lewy Body-receiving mice exhibited a loss of dopamine neurons and an increase in nigral S129 phosphorylated α-syn immunoreactivity, their parabiotic bloodstream-sharing partners did not show any trend for a lesion or change in S129 phosphorylated-α-syn levels. Altogether, our study suggests that, in the patient-derived α-synuclein aggregates-injected mouse model and within the selected time frame, the disease is not "transmitted" through the bloodstream.

The food additive fast green FCF inhibits α-synuclein aggregation, disassembles mature fibrils and protects against amyloid-induced neurotoxicity

α-Synuclein (α-syn) aggregates into cytotoxic amyloid fibrils, which are recognized as the defining neuropathological feature of Parkinson's disease (PD). Therefore, inhibiting α-syn fibrillogenesis and disrupting the preformed fibrils are both considered attractive strategies to cure PD. We discovered that a safe food additive, fast green FCF, is capable of inhibiting α-synuclein fibrillogenesis and reducing the related cytotoxicity. Thioflavin T fluorescence assays demonstrated that fast green FCF could inhibit the fibrillogenesis α-synuclein. In the presence of 100 μM fast green FCF, amorphous aggregates were formed and observed by atomic force microscopy. Toxicity assays in cell cultures revealed that fast green FCF significantly reduced the cytotoxicity of α-syn. Molecular dynamics simulations revealed the potential mechanism of the interactions between fast green FCF and α-synuclein. Fast green FCF greatly disrupted the α-synuclein pentamer and reduced the β-sheet content by reducing both nonpolar and polar interactions. Furthermore, two binding sites were identified, named region I (Y39-K45) and region II (H50-Q62). Our data reveal that electrostatic interactions, hydrogen bonds, and π-π interactions synergistically contribute to the binding of fast green FCF to the α-synuclein pentamer. These results indicate that fast green FCF is a candidate prototype for the development of drugs against the aggregation of amyloid fibrils in PD.

Crucial Role of FABP3 in αSyn-Induced Reduction of Septal GABAergic Neurons and Cognitive Decline in Mice

In synucleinopathies, while motor symptoms are thought to be attributed to the accumulation of misfolded α-synuclein (αSyn) in nigral dopaminergic neurons, it remains to be elucidated how cognitive decline arises. Here, we investigated the effects of distinct αSyn strains on cognition and the related neuropathology in the medial septum/diagonal band (MS/DB), a key region for cognitive processing. Bilateral injection of αSyn fibrils into the dorsal striatum potently impaired cognition in mice. The cognitive decline was accompanied by accumulation of phosphorylated αSyn at Ser129 and reduction of gamma-aminobutyric acid (GABA)-ergic but not cholinergic neurons in the MS/DB. Since we have demonstrated that fatty acid-binding protein 3 (FABP3) is critical for αSyn neurotoxicity in nigral dopaminergic neurons, we investigated whether FABP3 also participates in αSyn pathology in the MS/DB and cognitive decline. FABP3 was highly expressed in GABAergic but rarely in cholinergic neurons in the MS/DB. Notably, Fabp3 deletion antagonized the accumulation of phosphorylated αSyn, decrease in GABAergic neurons, and cognitive impairment caused by αSyn fibrils. Overall, the present study indicates that FABP3 mediates αSyn neurotoxicity in septal GABAergic neurons and the resultant cognitive impairment, and that FABP3 in this subpopulation could be a therapeutic target for dementia in synucleinopathies.

Proposing a minimal set of metrics and methods to predict probabilities of amyloidosis disease and onset age in individuals

Many amyloid-driven pathologies have both genetic and stochastic components where assessing risk of disease development requires a multifactorial assessment where many of the variables are poorly understood. Risk of transthyretin-mediated amyloidosis is enhanced by age and mutation of the transthyretin (TTR) gene, but amyloidosis is not directly initiated by mutated TTR proteins. Nearly all of the 150+ known mutations increase dissociation of the homotetrameric protein structure and increase the probability of an individual developing a TTR amyloid disease late in life. TTR amyloidosis is caused by dissociated monomers that are destabilized and refold into an amyloidogenic form. Therefore, monomer concentration, monomer proteolysis rate, and structural stability are key variables that may determine the rate of development of amyloidosis. Here we develop a unifying biophysical model that quantifies the relationships among these variables in plasma and suggest the probability of an individual developing a TTR amyloid disease can be estimated. This may allow quantification of risk for amyloidosis and provide the information necessary for development of methods for early diagnosis and prevention. Given the similar observation of genetic and sporadic amyloidoses for other diseases, this model and the measurements to assess risk may be applicable to more proteins than just TTR.

Inhibition of copper transporter 1 prevents α-synuclein pathology and alleviates nigrostriatal degeneration in AAV-based mouse model of Parkinson's disease

The formation of α-synuclein aggregates is a major pathological hallmark of Parkinson's disease. Copper promotes α-synuclein aggregation and toxicity in vitro. The level of copper and copper transporter 1, which is the only known high-affinity copper importer in the brain, decreases in the substantia nigra of Parkinson's disease patients. However, the relationship between copper, copper transporter 1 and α-synuclein pathology remains elusive. Here, we aim to decipher the molecular mechanisms of copper and copper transporter 1 underlying Parkinson's disease pathology. We employed yeast and mammalian cell models expressing human α-synuclein, where exogenous copper accelerated intracellular α-synuclein inclusions and silencing copper transporter 1 reduced α-synuclein aggregates in vitro, suggesting that copper transporter 1 might inhibit α-synuclein pathology. To study our hypothesis in vivo, we generated a new transgenic mouse model with copper transporter 1 conditional knocked-out specifically in dopaminergic neuron. Meanwhile, we unilaterally injected adeno-associated viral human-α-synuclein into the substantia nigra of these mice. Importantly, we found that copper transporter 1 deficiency significantly reduced S129-phosphorylation of α-synuclein, prevented dopaminergic neuronal loss, and alleviated motor dysfunction caused by α-synuclein overexpression in vivo. Overall, our data indicated that inhibition of copper transporter 1 alleviated α-synuclein mediated pathologies and provided a novel therapeutic strategy for Parkinson's disease and other synucleinopathies.

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Ctr1 deficiency reduces α-synuclein aggregates in vitro.

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Ctr1 deficiency inhibits the level of pathological α-synuclein in vivo.

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Ctr1 deficiency prevents nigrostriatal neurodegeneration in vivo.

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Ctr1 deficiency alleviates motor dysfunction caused by α-synuclein in vivo.

Invited Review: The role of prion-like mechanisms in neurodegenerative diseases

The prototype of transmissible neurodegenerative proteinopathies is prion diseases, characterized by aggregation of abnormally folded conformers of the native prion protein. A wealth of mechanisms has been proposed to explain the conformational conversion from physiological protein into misfolded, pathological form, mode of toxicity, propagation from cell-to-cell and regional spread. There is increasing evidence that other neurodegenerative diseases, most notably Alzheimer's disease (Aβ and tau), Parkinson's disease (α-synuclein), frontotemporal dementia (TDP43, tau or FUS) and motor neurone disease (TDP43), exhibit at least some of the misfolded prion protein properties. In this review, we will discuss to what extent each of the properties of misfolded prion protein is known to occur for Aβ, tau, α-synuclein and TDP43, with particular focus on self-propagation through seeding, conformational strains, selective cellular and regional vulnerability, stability and resistance to inactivation, oligomers, toxicity and summarize the most recent literature on transmissibility of neurodegenerative disorders.

Modulation of the Microbiome in Parkinson's Disease: Diet, Drug, Stool Transplant, and Beyond

The gastrointestinal microbiome is altered in Parkinson's disease and likely plays a key role in its pathophysiology, affecting symptoms and response to therapy and perhaps modifying progression or even disease initiation. Gut dysbiosis therefore has a significant potential as a therapeutic target in Parkinson's disease, a condition elusive to disease-modifying therapy thus far. The gastrointestinal environment hosts a complex ecology, and efforts to modulate the relative abundance or function of established microorganisms are still in their infancy. Still, these techniques are being rapidly developed and have important implications for our understanding of Parkinson's disease. Currently, modulation of the microbiome can be achieved through non-pharmacologic means such as diet, pharmacologically through probiotic, prebiotic, or antibiotic use and procedurally through fecal transplant. Novel techniques being explored include the use of small molecules or genetically engineered organisms, with vast potential. Here, we review how some of these approaches have been used to date, important areas of ongoing research, and how microbiome modulation may play a role in the clinical management of Parkinson's disease in the future.

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

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

Brain-First versus Gut-First Parkinson's Disease: A Hypothesis

Parkinson’s disease (PD) is a highly heterogeneous disorder, which probably consists of multiple subtypes. Aggregation of misfolded alpha-synuclein and propagation of these proteinacious aggregates through interconnected neural networks is believed to be a crucial pathogenetic factor. It has been hypothesized that the initial pathological alpha-synuclein aggregates originate in the enteric or peripheral nervous system (PNS) and invade the central nervous system (CNS) via retrograde vagal transport. However, evidence from neuropathological studies suggests that not all PD patients can be reconciled with this hypothesis. Importantly, a small fraction of patients do not show pathology in the dorsal motor nucleus of the vagus. Here, it is hypothesized that PD can be divided into a PNS-first and a CNS-first subtype. The former is tightly associated with REM sleep behavior disorder (RBD) during the prodromal phase and is characterized by marked autonomic damage before involvement of the dopaminergic system. In contrast, the CNS-first phenotype is most often RBD-negative during the prodromal phase and characterized by nigrostriatal dopaminergic dysfunction prior to involvement of the autonomic PNS. The existence of these subtypes is supported by in vivo imaging studies of RBD-positive and RBD-negative patient groups and by histological evidence— reviewed herein. The present proposal provides a fresh hypothesis-generating framework for future studies into the etiopathogenesis of PD and seems capable of explaining a number of discrepant findings in the neuropathological literature.

Initiation and propagation of α-synuclein aggregation in the nervous system

The two main pathological hallmarks of Parkinson’s disease are loss of dopamine neurons in the substantia nigra pars compacta and proteinaceous amyloid fibrils composed mostly of α-synuclein, called Lewy pathology. Levodopa to enhance dopaminergic transmission remains one of the most effective treatment for alleviating the motor symptoms of Parkinson’s disease (Olanow, Mov Disord 34:812–815, 2019). In addition, deep brain stimulation (Bronstein et al., Arch Neurol 68:165, 2011) to modulate basal ganglia circuit activity successfully alleviates some motor symptoms. MRI guided focused ultrasound in the subthalamic nucleus is a promising therapeutic strategy as well (Martinez-Fernandez et al., Lancet Neurol 17:54–63, 2018). However, to date, there exists no treatment that stops the progression of this disease. The findings that α-synuclein can be released from neurons and inherited through interconnected neural networks opened the door for discovering novel treatment strategies to prevent the formation and spread of Lewy pathology with the goal of halting PD in its tracks. This hypothesis is based on discoveries that pathologic aggregates of α-synuclein induce the endogenous α-synuclein protein to adopt a similar pathologic conformation, and is thus self-propagating. Phase I clinical trials are currently ongoing to test treatments such as immunotherapy to prevent the neuron to neuron spread of extracellular aggregates. Although tremendous progress has been made in understanding how Lewy pathology forms and spreads throughout the brain, cell intrinsic factors also play a critical role in the formation of pathologic α-synuclein, such as mechanisms that increase endogenous α-synuclein levels, selective expression profiles in distinct neuron subtypes, mutations and altered function of proteins involved in α-synuclein synthesis and degradation, and oxidative stress. Strategies that prevent the formation of pathologic α-synuclein should consider extracellular release and propagation, as well as neuron intrinsic mechanisms.

Autophagy Activation Is Involved in Acidic Fibroblast Growth Factor Ameliorating Parkinson's Disease via Regulating Tribbles Homologue 3

Parkinson's disease (PD) is a degenerative disorder of the central nervous system, resulting in loss of dopamine neurons. Excessive endoplasmic reticulum (ER) stress and autophagy dysfunction play a crucial role on Parkinson's disease (PD) development. It has been showed that acidic fibroblast growth factor (aFGF) alleviates the development of PD by inhibiting ER stress. But the role of autophagy and its relationship with ER stress during aFGF treatment for PD has not been elucidated. We found that both aFGF and rapamycin (Rapa) improved 6-Hydroxy Dopamine (6-OHDA)-induced PD development as shown with histomorphology results in striatum and substantia nigra (SNpc). Additionally, aFGF promoted autophagy with increasing mTOR and decreasing p62 expressions, and then exerts its neuroprotective role in 6-OHDA-treated PC12 cells, which were abolished by chloroquine (CQ) treatment. Moreover, 4-phenylbutyric acid (4-PBA) administration inhibited the expressions of autophagy markers during 6-OHDA-treated PC12 cells, which was similar with aFGF treating PC12 cells under 6-OHDA condition. Furthermore, we had detected the expressions of CHOP and its downstream factor, tribbles homologue 3 (TRB3), a pro-apoptotic protein. We found that TRB3 and CHOP expressions were significantly downregulated after treating with aFGF and 4-PBA in 6-OHDA-treated PC12 cells and PD model. Taken together, this study has demonstrated that aFGF treatment ameliorates 6-OHDA-induced elevated ER stress and subsequently suppression of autophagy via inhibiting TRB3 activation, and consequently ameliorates 6-OHDA-induced neurotoxicity.