Neuroinvasion of α-Synuclein Prionoids after Intraperitoneal and Intraglossal Inoculation

α-Synuclein strain propagation is independent of cellular prion protein expression in a transgenic synucleinopathy mouse model

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.

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.

LRP10 and α-synuclein transmission in Lewy body diseases

Autosomal dominant variants in LRP10 have been identified in patients with Lewy body diseases (LBDs), including Parkinson's disease (PD), Parkinson's disease-dementia (PDD), and dementia with Lewy bodies (DLB). Nevertheless, there is little mechanistic insight into the role of LRP10 in disease pathogenesis. In the brains of control individuals, LRP10 is typically expressed in non-neuronal cells like astrocytes and neurovasculature, but in idiopathic and genetic cases of PD, PDD, and DLB, it is also present in α-synuclein-positive neuronal Lewy bodies. These observations raise the questions of what leads to the accumulation of LRP10 in Lewy bodies and whether a possible interaction between LRP10 and α-synuclein plays a role in disease pathogenesis. Here, we demonstrate that wild-type LRP10 is secreted via extracellular vesicles (EVs) and can be internalised via clathrin-dependent endocytosis. Additionally, we show that LRP10 secretion is highly sensitive to autophagy inhibition, which induces the formation of atypical LRP10 vesicular structures in neurons in human-induced pluripotent stem cells (iPSC)-derived brain organoids. Furthermore, we show that LRP10 overexpression leads to a strong induction of monomeric α-synuclein secretion, together with time-dependent, stress-sensitive changes in intracellular α-synuclein levels. Interestingly, patient-derived astrocytes carrying the c.1424 + 5G > A LRP10 variant secrete aberrant high-molecular-weight species of LRP10 in EV-free media fractions. Finally, we show that this truncated patient-derived LRP10 protein species (LRP10splice) binds to wild-type LRP10, reduces LRP10 wild-type levels, and antagonises the effect of LRP10 on α-synuclein levels and distribution. Together, this work provides initial evidence for a possible functional role of LRP10 in LBDs by modulating intra- and extracellular α-synuclein levels, and pathogenic mechanisms linked to the disease-associated c.1424 + 5G > A LRP10 variant, pointing towards potentially important disease mechanisms in LBDs.

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.

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.

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.

The unique neuropathological vulnerability of the human brain to aging

Alzheimer's disease (AD)-related neurofibrillary tangles (NFT), argyrophilic grain disease (AGD), aging-related tau astrogliopathy (ARTAG), limbic predominant TDP-43 proteinopathy (LATE), and amygdala-predominant Lewy body disease (LBD) are proteinopathies that, together with hippocampal sclerosis, progressively appear in the elderly affecting from 50% to 99% of individuals aged 80 years, depending on the disease. These disorders usually converge on the same subject and associate with additive cognitive impairment. Abnormal Tau, TDP-43, and α-synuclein pathologies progress following a pattern consistent with an active cell-to-cell transmission and abnormal protein processing in the host cell. However, cell vulnerability and transmission pathways are specific for each disorder, albeit abnormal proteins may co-localize in particular neurons. All these alterations are unique or highly prevalent in humans. They all affect, at first, the archicortex and paleocortex to extend at later stages to the neocortex and other regions of the telencephalon. These observations show that the phylogenetically oldest areas of the human cerebral cortex and amygdala are not designed to cope with the lifespan of actual humans. New strategies aimed at reducing the functional overload of the human telencephalon, including optimization of dream repair mechanisms and implementation of artificial circuit devices to surrogate specific brain functions, appear promising.

Disease mechanisms as subtypes: Microbiome

Abnormalities in gut microbiota have been suggested to be involved in the pathophysiology and progression of Parkinson's disease (PD). Gastrointestinal nonmotor symptoms often precede the onset of motor features in PD, suggesting a role for gut dysbiosis in neuroinflammation and α-synuclein (α-syn) aggregation. In the first part of this chapter, we analyze critical features of healthy gut microbiota and factors (environmental and genetic) that modify its composition. In the second part, we focus on the mechanisms underlying the gut dysbiosis and how it alters anatomically and functionally the mucosal barrier, triggering neuroinflammation and subsequently α-syn aggregation. In the third part, we describe the most common alterations in the gut microbiota of PD patients, dividing the gastrointestinal system in higher and lower tract to examine the association between microbiota abnormalities and clinical features. In the final section, we report on current and future therapeutic approaches to gut dysbiosis aiming to either reduce the risk for PD, modify the disease course, or improve the pharmacokinetic profile of dopaminergic therapies. We also suggest that further studies will be needed to clarify the role of the microbiome in PD subtyping and of pharmacological and nonpharmacological interventions in modifying specific microbiota profiles in individualizing disease-modifying treatments in PD.

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.

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.

Unique seeding profiles and prion-like propagation of synucleinopathies are highly dependent on the host in human α-synuclein transgenic mice

α-synuclein (αSyn) is an intrinsically disordered protein which can undergo structural transformations, resulting in the formation of stable, insoluble fibrils. αSyn amyloid-type nucleation can be induced by misfolded 'seeds' serving as a conformational template, tantamount to the prion-like mechanism. Accumulation of αSyn inclusions is a key feature of dementia with Lewy bodies (DLB) and multiple system atrophy (MSA), and are found as additional pathology in Alzheimer's disease (AD) such as AD with amygdala predominant Lewy bodies (AD/ALB). While these disorders accumulate the same pathological protein, they exhibit heterogeneity in clinical and histological features; however, the mechanism(s) underlying this variability remains elusive. Accruing data from human autopsy studies, animal inoculation modeling, and in vitro characterization experiments, have lent credence to the hypothesis that conformational polymorphism of the αSyn amyloid-type fibril structure results in distinct "strains" with categorical infectivity traits. Herein, we directly compare the seeding abilities and outcome of human brain lysates from these diseases, as well as recombinant preformed human αSyn fibrils by the intracerebral inoculation of transgenic mice overexpressing either human wild-type αSyn or human αSyn with the familial A53T mutation. Our study has revealed that the initiating inoculum heavily dictates the phenotypic and pathological course of disease. Interestingly, we have also established relevant host-dependent distinctions between propagation profiles, including burden and spread of inclusion pathology throughout the neuroaxis, as well as severity of neurological symptoms. These findings provide compelling evidence supporting the hypothesis that diverse prion-type conformers may explain the variability seen in synucleinopathies.

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.

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.

Altered gut microbiota in Parkinson's disease patients with motor complications

INTRODUCTION

Parkinson's disease (PD) is associated with gut dysbiosis. However, whether gut dysbiosis can cause motor complications is unclear.

METHODS

Subjects were enrolled from four independent movement disorder centers in Japan. We performed 16S ribosomal RNA gene sequence analysis of gut microbiota. Relative abundance of gut microbiota and relationships between them and clinical characteristics were statistically analyzed. Analysis of co-variance (ANCOVA) was used to assess altered gut microbiota associated with wearing-off or dyskinesia.

RESULTS

We enrolled 223 patients with PD. Wearing-off was noted in 47.5% of patients and dyskinesia in 21.9%. We detected 98 genera of bacteria. Some changes in the gut microbiota were observed in patients with PD and motor complications. After Bonferroni correction, patients with wearing-off showed decreased relative abundance of Lachnospiraceae Blautia (p < 0.0001) and increased relative abundance of Lactobacillaceae Lactobacillus (p < 0.0001), but patients with dyskinesia no longer showed significant changes in the gut microbiota. Adjustment with two models of confounding factors followed by ANCOVA revealed that age (p < 0.0001), disease duration (p = 0.01), and wearing-off (p = 0.0004) were independent risks for the decreased relative abundance of Lachnospiraceae Blautia, and wearing-off (p = 0.009) was the only independent risk factor for the increased relative abundance of Lachnospiraceae Lactobacillus.

CONCLUSION

Relative abundance of Lachnospiraceae Blautia and Lactobacillaceae Lactobacillus was significantly decreased and increased, respectively, in the gut microbiota of PD patients with motor complications. This indicates that an altered gut microbiota is associated with the development of motor complications in patients with advanced PD.

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.

Neuropathological evidence of body-first vs. brain-first Lewy body disease

Aggregation of alpha-synuclein into inclusion bodies, termed Lewy pathology, is a defining feature of Parkinson's disease (PD) and Dementia with Lewy bodies (DLB). In the majority of post mortem cases, the distribution of Lewy pathology seems to follow two overarching patterns: a caudo-rostral pattern with relatively more pathology in the brainstem than in the telencephalon, and an amygdala-centered pattern with the most abundant pathology in the "center of the brain", including the amygdala, entorhinal cortex, and substantia nigra, and relatively less pathology in the lower brainstem and spinal autonomic nuclei. The recent body-first versus brain-first model of Lewy Body Disorders proposes that the initial pathogenic alpha-synuclein in some patients originates in the enteric nervous system with secondary spreading to the brain; and in other patients originates inside the CNS with secondary spreading to the lower brainstem and peripheral autonomic nervous system. Here, we use two existing post mortem datasets to explore the possibility that clinical body-first and brain-first subtypes are equivalent to the caudo-rostral and amygdala-centered patterns of Lewy pathology seen at post mortem.

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

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

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.

Combining Automated Organoid Workflows With Artificial Intelligence-Based Analyses: Opportunities to Build a New Generation of Interdisciplinary High-Throughput Screens for Parkinson's Disease and Beyond

Parkinson's disease (PD) is the second most common neurodegenerative disease and primarily characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta of the midbrain. Despite decades of research and the development of various disease model systems, there is no curative treatment. This could be due to current model systems, including cell culture and animal models, not adequately recapitulating human PD etiology. More complex human disease models, including human midbrain organoids, are maturing technologies that increasingly enable the strategic incorporation of the missing components needed to model PD in vitro. The resulting organoid-based biological complexity provides new opportunities and challenges in data analysis of rich multimodal data sets. Emerging artificial intelligence (AI) capabilities can take advantage of large, broad data sets and even correlate results across disciplines. Current organoid technologies no longer lack the prerequisites for large-scale high-throughput screening (HTS) and can generate complex yet reproducible data suitable for AI-based data mining. We have recently developed a fully scalable and HTS-compatible workflow for the generation, maintenance, and analysis of three-dimensional (3D) microtissues mimicking key characteristics of the human midbrain (called "automated midbrain organoids," AMOs). AMOs build a reproducible, scalable foundation for creating next-generation 3D models of human neural disease that can fuel mechanism-agnostic phenotypic drug discovery in human in vitro PD models and beyond. Here, we explore the opportunities and challenges resulting from the convergence of organoid HTS and AI-driven data analytics and outline potential future avenues toward the discovery of novel mechanisms and drugs in PD research. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

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.

What Is Our Understanding of the Influence of Gut Microbiota on the Pathophysiology of Parkinson's Disease?

Microbiota have increasingly become implicated in predisposition to human diseases, including neurodegenerative disorders such as Parkinson's disease (PD). Traditionally, a central nervous system (CNS)-centric approach to understanding PD has predominated; however, an association of the gut with PD has existed since Parkinson himself reported the disease. The gut-brain axis refers to the bidirectional communication between the gastrointestinal tract (GIT) and the brain. Gut microbiota dysbiosis, reported in PD patients, may extend this to a microbiota-gut-brain axis. To date, mainly the bacteriome has been investigated. The change in abundance of bacterial products which accompanies dysbiosis is hypothesised to influence PD pathophysiology via multiple mechanisms which broadly centre on inflammation, a cause of alpha-synuclein (a-syn) misfolding. Two main routes are hypothesised by which gut microbiota can influence PD pathophysiology, the neural and humoral routes. The neural route involves a-syn misfolding peripherally in the enteric nerves which can then be transported to the brain via the vagus nerve. The humoral route involves transportation of bacterial products and proinflammatory cytokines from the gut via the circulation which can cause central a-syn misfolding by inducing neuroinflammation. This article will assess whether the current literature supports gut bacteria influencing PD pathophysiology via both routes.

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.

Evidence of distinct α-synuclein strains underlying disease heterogeneity

Synucleinopathies are a group of neurodegenerative disorders caused by the misfolding and self-templating of the protein α-synuclein, or the formation of α-synuclein prions. Each disorder differs by age of onset, presenting clinical symptoms, α-synuclein inclusion morphology, and neuropathological distribution. Explaining this disease-specific variability, the strain hypothesis postulates that each prion disease is encoded by a distinct conformation of the misfolded protein, and therefore, each synucleinopathy is caused by a unique α-synuclein structure. This review discusses the current data supporting the role of α-synuclein strains in disease heterogeneity. Several in vitro and in vivo models exist for evaluating strain behavior, however, as the focus of this article is to compare strains across synucleinopathy patients, our discussion predominantly focuses on the two models most commonly used for this purpose: the α-syn140*A53T-YFP cell line and the TgM83^+/- mouse model. Here we define each strain based on biochemical stability, ability to propagate in α-syn140-YFP cell lines, and incubation period, inclusion morphology and distribution, and neurological signs in TgM83^+/- mice.

Movement of prion-like α-synuclein along the gut-brain axis in Parkinson's disease: A potential target of curcumin treatment

A pathological hallmark of the neurodegenerative disorder, Parkinson's disease (PD), is aggregation of toxic forms of the presynaptic protein, α-synuclein in structures known as Lewy bodies. α-Synuclein pathology is found in both the brain and gastrointestinal tracts of affected individuals, possibly due to the movement of this protein along the vagus nerve that connects the brain to the gut. In this review, we discuss current insights into the spread of α-synuclein pathology along the gut-brain axis, which could be targeted for therapeutic interventions. The prion-like propagation of α-synuclein, and the clinical manifestations of gastrointestinal dysfunction in individuals living with PD, are discussed. There is currently insufficient evidence that surgical alteration of the vagus nerve, or removal of gut-associated lymphoid tissues, such as the appendix and tonsils, are protective against PD. Furthermore, we propose curcumin as a potential candidate to prevent the spread of α-synuclein pathology in the body by curcumin binding to α-synuclein's non-amyloid β-component (NAC) domain. Curcumin is an active component of the food spice turmeric and is known for its antioxidant, anti-inflammatory, and potentially neuroprotective properties. We hypothesize that once α-synuclein is bound to curcumin, both molecules are subsequently excreted from the body. Therefore, dietary supplementation with curcumin over one's lifetime has potential as a novel approach to complement existing PD treatment and/or prevention strategies. Future studies are required to validate this hypothesis, but if successful, this could represent a significant step towards improved nutrient-based therapeutic interventions and preventative strategies for this debilitating and currently incurable disorder.

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.

The α-Synuclein Origin and Connectome Model (SOC Model) of Parkinson's Disease: Explaining Motor Asymmetry, Non-Motor Phenotypes, and Cognitive Decline

A new model of Parkinson's disease (PD) pathogenesis is proposed, the α-Synuclein Origin site and Connectome (SOC) model, incorporating two aspects of α-synuclein pathobiology that impact the disease course for each patient: the anatomical location of the initial α-synuclein inclusion, and α-synuclein propagation dependent on the ipsilateral connections that dominate connectivity of the human brain. In some patients, initial α-synuclein pathology occurs within the CNS, leading to a brain-first subtype of PD. In others, pathology begins in the peripheral autonomic nervous system, leading to a body-first subtype. In brain-first cases, it is proposed that the first pathology appears unilaterally, often in the amygdala. If α-synuclein propagation depends on connection strength, a unilateral focus of pathology will disseminate more to the ipsilateral hemisphere. Thus, α-synuclein spreads mainly to ipsilateral structures including the substantia nigra. The asymmetric distribution of pathology leads to asymmetric dopaminergic degeneration and motor asymmetry. In body-first cases, the α-synuclein pathology ascends via the vagus to both the left and right dorsal motor nuclei of the vagus owing to the overlapping parasympathetic innervation of the gut. Consequently, the initial α-synuclein pathology inside the CNS is more symmetric, which promotes more symmetric propagation in the brainstem, leading to more symmetric dopaminergic degeneration and less motor asymmetry. At diagnosis, body-first patients already have a larger, more symmetric burden of α-synuclein pathology, which in turn promotes faster disease progression and accelerated cognitive decline. The SOC model is supported by a considerable body of existing evidence and may have improved explanatory power.

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.

Meta-Analysis of Gut Dysbiosis in Parkinson's Disease

BACKGROUND

PD may begin with the intestinal accumulation of α-synuclein fibrils, which can be causally associated with gut dysbiosis. The variability of gut microbiota across countries prevented us from identifying shared gut dysbiosis in PD.

OBJECTIVES

To identify gut dysbiosis in PD across countries.

METHODS

We performed 16S ribosomal RNA gene sequencing analysis of gut microbiota in 223 patients with PD and 137 controls, and meta-analyzed gut dysbiosis by combining our dataset with four previously reported data sets from the United States, Finland, Russia, and Germany. We excluded uncommon taxa from our analyses. For pathway analysis, we developed the Kyoto Encyclopedia of Genes and Genomes orthology set enrichment analysis method.

RESULTS

After adjusting for confounding factors (body mass index, constipation, sex, age, and catechol-O-methyl transferase inhibitor), genera Akkermansia and Catabacter, as well as families Akkermansiaceae, were increased, whereas genera Roseburia, Faecalibacterium, and Lachnospiraceae ND3007 group were decreased in PD. Catechol-O-methyl transferase inhibitor intake markedly increased family Lactobacillaceae. Inspection of these bacteria in 12 datasets that were not included in the meta-analysis revealed that increased genus Akkermansia and decreased genera Roseburia and Faecalibacterium were frequently observed across countries. Kyoto Encyclopedia of Genes and Genomes orthology set enrichment analysis revealed changes in short-chain fatty acid metabolisms in our dataset.

CONCLUSIONS

We report that intestinal mucin layer-degrading Akkermansia is increased and that short-chain fatty acid-producing Roseburia and Faecalibacterium are decreased in PD across countries. © 2020 International Parkinson and Movement Disorder Society.

Parkinson's Disease and the Gut: Future Perspectives for Early Diagnosis

Parkinson's disease (PD) is a neurodegenerative disease characterized by progressive degeneration of dopaminergic neurons, and at the cellular level by the formation of Lewy bodies in the central nervous system (CNS). However, the onset of the disease is believed to be localized to peripheral organs, particularly the gastrointestinal tract (GIT) and the olfactory bulb sooner before neuropathological changes occur in the CNS. Patients already in the pre-motor stage of PD suffer from various digestive problems and/or due to significant changes in the composition of the intestinal microbiome in this early stage of the disease. Detailed analyses of patient biopsies and autopsies as well as animal models of neuropathological changes characteristic of PD provided important information on the pathology or treatment of PD symptoms. However, presently is not clarified (i) the specific tissue in the GIT where the pathological processes associated with PD is initiated; (ii) the mechanism by which these processes are disseminated to the CNS or other tissues within the GIT; and (iii) which neuropathological changes could also serve as a reliable diagnostic marker of the premotor stages of PD, or (iv) which type of GIT tissue would be the most appropriate choice for routine examination of patient biopsies.

Propagation of Pathological α-Synuclein from the Urogenital Tract to the Brain Initiates MSA-like Syndrome

The neuropathological feature of multiple system atrophy (MSA), a fatal adult-onset disorder without effective therapy, is the accumulation of pathological α-synuclein (α-Syn) in the central nervous system (CNS). Here we show that pathological α-Syn exists in nerve terminals in detrusor and external urethral sphincter (EUS) of patients with MSA. Furthermore, α-Syn-preformed fibrils (PFFs) injected in the EUS or detrusor in TgM83^+/− mice initiated the transmission of pathological α-Syn from the urogenital tract to brain via micturition reflex pathways, and these mice developed widespread phosphorylated α-Syn inclusion pathology together with phenotypes. In addition, urinary dysfunction and denervation-reinnervation of external anal sphincter were detected earlier in the mouse models with α-Syn PFFs inoculation before the behavioral manifestations. These results suggest that pathological α-Syn spreading through the micturition reflex pathways retrogradely from the urogenital tract to CNS may lead to urinary dysfunction in patients with MSA, which is different from the etiology of idiopathic Parkinson disease.

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Pathological α-Syn exhibits in nerve terminals in DET and EUS of patients with MSA

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Propagation of pathological α-Syn from urinary tract to CNS causes MSA-like syndrome

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The mouse models show urinary dysfunction and abnormal EAS EMG before motor deficits

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Lower urinary tract injection of α-Syn PFFs induces autonomic and motor dysfunctions

α-Synuclein filaments from transgenic mouse and human synucleinopathy-containing brains are major seed-competent species

Assembled α-synuclein in nerve cells and glial cells is the defining pathological feature of neurodegenerative diseases called synucleinopathies. Seeds of α-synuclein can induce the assembly of monomeric protein. Here, we used sucrose gradient centrifugation and transiently transfected HEK 293T cells to identify the species of α-synuclein from the brains of homozygous, symptomatic mice transgenic for human mutant A53T α-synuclein (line M83) that seed aggregation. The most potent fractions contained Sarkosyl-insoluble assemblies enriched in filaments. We also analyzed six cases of idiopathic Parkinson's disease (PD), one case of familial PD, and six cases of multiple system atrophy (MSA) for their ability to induce α-synuclein aggregation. The MSA samples were more potent than those of idiopathic PD in seeding aggregation. We found that following sucrose gradient centrifugation, the most seed-competent fractions from PD and MSA brains are those that contain Sarkosyl-insoluble α-synuclein. The fractions differed between PD and MSA, consistent with the presence of distinct conformers of assembled α-synuclein in these different samples. We conclude that α-synuclein filaments are the main driving force for amplification and propagation of pathology in synucleinopathies.

Protein transmission in neurodegenerative disease

Most neurodegenerative diseases are characterized by the intracellular or extracellular aggregation of misfolded proteins such as amyloid-β and tau in Alzheimer disease, α-synuclein in Parkinson disease, and TAR DNA-binding protein 43 in amyotrophic lateral sclerosis. Accumulating evidence from both human studies and disease models indicates that intercellular transmission and the subsequent templated amplification of these misfolded proteins are involved in the onset and progression of various neurodegenerative diseases. The misfolded proteins that are transferred between cells are referred to as 'pathological seeds'. Recent studies have made exciting progress in identifying the characteristics of different pathological seeds, particularly those isolated from diseased brains. Advances have also been made in our understanding of the molecular mechanisms that regulate the transmission process, and the influence of the host cell on the conformation and properties of pathological seeds. The aim of this Review is to summarize our current knowledge of the cell-to-cell transmission of pathological proteins and to identify key questions for future investigation.

Autonomic ganglionic injection of α-synuclein fibrils as a model of pure autonomic failure α-synucleinopathy

α-Synucleinopathies are characterized by autonomic dysfunction and motor impairments. In the pure autonomic failure (PAF), α-synuclein (α-Syn) pathology is confined within the autonomic nervous system with no motor features, but mouse models recapitulating PAF without motor dysfunction are lacking. Here, we show that in TgM83+/- mice, inoculation of α-Syn preformed fibrils (PFFs) into the stellate and celiac ganglia induces spreading of α-Syn pathology only through the autonomic pathway to both the central nervous system (CNS) and the autonomic innervation of peripheral organs bidirectionally. In parallel, the mice develop autonomic dysfunction, featured by orthostatic hypotension, constipation, hypohidrosis and hyposmia, without motor dysfunction. Thus, we have generated a mouse model of pure autonomic dysfunction caused by α-Syn pathology. This model may help define the mechanistic link between transmission of pathological α-Syn and the cardinal features of autonomic dysfunction in α-synucleinopathy.

Bioluminescence Imaging of Neuroinflammation in a Mouse Model of Parkinson's Disease

In Parkinson's disease (PD) and related disorders pathological alpha-synuclein has been discussed to propagate via a prion-like mechanism in the CNS. The application of exogenous alpha-synuclein fibrils via injection to animal models of PD has been shown to be a useful method to study prion-like propagation of pathological alpha-synuclein and of transmission pathways that play a critical role in recapitulating characteristics of synucleinopathies. Using bigenic mice expressing mutant human alpha-synuclein in neurons and firefly luciferase in astrocytes we showed that transmission via the tongue and the peritoneum represent entrance points for pathological alpha-synuclein to invade the CNS. Here we present a method to quantify astrogliosis by bioluminescence imaging in an animal model of PD. This method allows noninvasive tracking of the neuroinflammatory process that often precedes neurological signs of disease and represents an alternative to behavioral or histological and biochemical analysis to detect disease.

Mechanisms of alpha-synuclein toxicity: An update and outlook

Alpha-synuclein (aSyn) was identified as the main component of inclusions that define synucleinopathies more than 20 years ago. Since then, aSyn has been extensively studied in an attempt to unravel its roles in both physiology and pathology. Today, studying the mechanisms of aSyn toxicity remains in the limelight, leading to the identification of novel pathways involved in pathogenesis. In this chapter, we address the molecular mechanisms involved in synucleinopathies, from aSyn misfolding and aggregation to the various cellular effects and pathologies associated. In particular, we review our current understanding of the mechanisms involved in the spreading of aSyn between different cells, from the periphery to the brain, and back. Finally, we also review recent studies on the contribution of inflammation and the gut microbiota to pathology in synucleinopathies. Despite significant advances in our understanding of the molecular mechanisms involved, we still lack an integrated understanding of the pathways leading to neurodegeneration in PD and other synucleinopathies, compromising our ability to develop novel therapeutic strategies.

The Gut and Parkinson's Disease: Hype or Hope?

In the last two decades it has become clear that Parkinson’s disease (PD) is associated with a plethora of gastrointestinal symptoms originating from functional and structural changes in the gut and its associated neural structures. This is of particular interest not only because such symptoms have a major impact on the quality of life of PD patients, but also since accumulating evidence suggests that in at least a subgroup of patients, these disturbances precede the motor symptoms and diagnosis of PD by years and may thus give important insights into the origin and pathogenesis of the disease. In this mini-review we attempt to concisely summarize the current knowledge after two decades of research on the gut-brain axis in PD. We focus on alpha-synuclein pathology, biomarkers, and the gut microbiota and envision the development and impact of these research areas for the two decades to come.

Evidence for bidirectional and trans-synaptic parasympathetic and sympathetic propagation of alpha-synuclein in rats

The conversion of endogenous alpha-synuclein (asyn) to pathological asyn-enriched aggregates is a hallmark of Parkinson’s disease (PD). These inclusions can be detected in the central and enteric nervous system (ENS). Moreover, gastrointestinal symptoms can appear up to 20 years before the diagnosis of PD. The dual-hit hypothesis posits that pathological asyn aggregation starts in the ENS, and retrogradely spreads to the brain. In this study, we tested this hypothesis by directly injecting preformed asyn fibrils into the duodenum wall of wild-type rats and transgenic rats with excess levels of human asyn. We provide a meticulous characterization of the bacterial artificial chromosome (BAC) transgenic rat model with respect to initial propagation of pathological asyn along the parasympathetic and sympathetic pathways to the brainstem, by performing immunohistochemistry at early time points post-injection. Induced pathology was observed in all key structures along the sympathetic and parasympathetic pathways (ENS, autonomic ganglia, intermediolateral nucleus of the spinal cord (IML), heart, dorsal motor nucleus of the vagus, and locus coeruleus (LC)) and persisted for at least 4 months post-injection. In contrast, asyn propagation was not detected in wild-type rats, nor in vehicle-injected BAC rats. The presence of pathology in the IML, LC, and heart indicate trans-synaptic spread of the pathology. Additionally, the observed asyn inclusions in the stomach and heart may indicate secondary anterograde propagation after initial retrograde spreading. In summary, trans-synaptic propagation of asyn in the BAC rat model is fully compatible with the “body-first hypothesis” of PD etiopathogenesis. To our knowledge, this is the first animal model evidence of asyn propagation to the heart, and the first indication of bidirectional asyn propagation via the vagus nerve, i.e., duodenum-to-brainstem-to-stomach. The BAC rat model could be very valuable for detailed mechanistic studies of the dual-hit hypothesis, and for studies of disease modifying therapies targeting early pathology in the gastrointestinal tract.

Oral and intravenous transmission of α-synuclein fibrils to mice

Parkinson’s disease and related disorders are neuropathologically characterized by cellular deposits of misfolded and aggregated α-synuclein in the CNS. Disease-associated α-synuclein adopts a conformation that causes it to form oligomers and fibrils, which have reduced solubility, become hyperphosphorylated, and contribute to the spatiotemporal spreading of pathology in the CNS. The infectious properties of disease-associated α-synuclein, e.g., by which peripheral route and with which efficiency it can be transmitted, are not fully understood. Here, we investigated the potential of α-synuclein fibrils to induce neurological disease in TgM83^+/− mice expressing the A53T mutant of human α-synuclein after oral or intravenous challenge and compared it to intraperitoneal and intracerebral challenge. Oral challenge with 50 µg of α-synuclein fibrils caused neurological disease in two out of eight mice in 220 days and 350 days, and challenge with 500 µg in four out of eight mice in 384 ± 131 days, respectively. Intravenous challenge with 50 µg of α-synuclein fibrils led to disease in 208 ± 20 days in 10 out of 10 mice and was in duration comparable to intraperitoneal challenge with 50 µg of α-synuclein fibrils, which caused disease in 10 out of 10 mice in 202 ± 35 days. Ten out of 10 mice that were each intracerebrally challenged with 10 µg or 50 µg of α-synuclein fibrils developed disease in 156 ± 20 days and 133 ± 4 days, respectively. The CNS of diseased mice displayed aggregates of sarkosyl-insoluble and phosphorylated α-synuclein, which colocalized with ubiquitin and p62 and were accompanied by gliosis indicative of neuroinflammation. In contrast, none of the control mice that were challenged with bovine serum albumin via the same routes developed any neurological disease or neuropathology. These findings are important, because they show that α-synuclein fibrils can neuroinvade the CNS after a single oral or intravenous challenge and cause neuropathology and disease.

Modeling α-Synuclein Propagation with Preformed Fibril Injections

The aggregation of α-synuclein (α-syn) has been implicated in the pathogenesis of many neurodegenerative disorders, including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). Postmortem analyses of α-syn pathology, especially that of PD, have suggested that aggregates progressively spread from a few discrete locations to wider brain regions. The neuron-to-neuron propagation of α-syn has been suggested to be the underlying mechanism by which aggregates spread throughout the brain. Many cellular and animal models has been created to study cell-to-cell propagation. Recently, it has been shown that a single injection of preformed fibrils (PFFs) made of recombinant α-syn proteins into various tissues and organs of many different animal species results in widespread α-syn pathology in the central nervous system (CNS). These PFF models have been extensively used to study the mechanism by which aggregates spread throughout the brain. Here, we review what we have learned from PFF models, describe the nature of PFFs and the neuropathological features, neurophysiological characteristics, and behavioral outcomes of the models.

Unique α-synuclein pathology within the amygdala in Lewy body dementia: implications for disease initiation and progression

The protein α-synuclein (αsyn) forms pathologic aggregates in a number of neurodegenerative diseases including Lewy body dementia (LBD) and Parkinson's disease (PD). It is unclear why diseases such as LBD may develop widespread αsyn pathology, while in Alzheimer's disease with amygdala restricted Lewy bodies (AD/ALB) the αsyn aggregates remain localized. The amygdala contains αsyn aggregates in both LBD and in AD/ALB; to understand why αsyn pathology continues to progress in LBD but not in AD/ALB, tissue from the amygdala and other regions were obtained from 14 cases of LBD, 9 cases of AD/ALB, and 4 controls for immunohistochemical and biochemical characterization. Utilizing a panel of previously characterized αsyn antibodies, numerous unique pathologies differentiating LBD and AD/ALB were revealed; particularly the presence of dense neuropil αsyn aggregates, astrocytic αsyn, and αsyn-containing dystrophic neurites within senile plaques. Within LBD, these unique pathologies were predominantly present within the amygdala. Biochemically, the amygdala in LBD prominently contained specific carboxy-truncated forms of αsyn which are highly prone to aggregate, suggesting that the amygdala may be prone to initiate development of αsyn pathology. Similar to carboxy-truncated αsyn, it was demonstrated herein that the presence of aggregation prone A53T αsyn is sufficient to drive misfolding of wild-type αsyn in human disease. Overall, this study identifies within the amygdala in LBD the presence of unique strain-like variation in αsyn pathology that may be a determinant of disease progression.

Seeded propagation of α-synuclein aggregation in mouse brain using protein misfolding cyclic amplification

α-Synuclein (α-syn) protein aggregation is associated with several neurodegenerative disorders collectively referred to as synucleinopathies, including Parkinson's disease. We used protein misfolding cyclic amplification (PMCA) to study α-syn aggregation in brain homogenates of wild-type or transgenic mice expressing normal (D line) or A53T mutant (M83 line) human α-syn. We found that sonication-incubation cycles of M83 mouse brain gradually produce large quantities of SDS-resistant α-syn aggregates, involving both human and mouse proteins. These PMCA products, containing partially proteinase K-resistant α-syn species, are competent to accelerate the onset of neurologic symptoms after intracerebral inoculation to young M83 mice and to seed aggregate formation of α-syn following PMCA, including in D and wild-type mouse brain substrates. PMCA seeding activity in the M83 diseased brain correlates positively with regions mostly targeted by the α-syn pathology in this model. Our data indicate that similar to prions, PMCA can reproduce some characteristics of α-syn aggregation and seeded propagation in vitro in a complex milieu. This opens new opportunities for the molecular study of synucleinopathies.-Nicot, S., Verchère, J., Bélondrade, M., Mayran, C., Bétemps, D., Bougard, D., Baron, T. Seeded propagation of α-synuclein aggregation in mouse brain using protein misfolding cyclic amplification.

Prion-like propagation of α-synuclein in neurodegenerative diseases

Prions are defined as proteinaceous infectious particles that do not contain nucleic acids. Neuropathological investigations of post-mortem brains and recent studies of experimental transmission have suggested that amyloid-like abnormal protein aggregates, which are the defining feature of many neurodegenerative diseases, behave like prions and propagate throughout the brain. This prion-like propagation may be the underlying mechanism of onset and progression of neurodegenerative diseases, although the precise molecular mechanisms involved remain unclear. However, in vitro and in vivo experimental models of prion-like propagation using pathogenic protein seeds are well established and are extremely valuable for the exploration and evaluation of novel drugs and therapies for neurodegenerative diseases for which there is no effective treatment. In this chapter, we introduce the experimental models of prion-like propagation of α-synuclein, which is accumulated in Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, and we describe their applications for the development of new diagnostic and therapeutic modalities. We also introduce the concept of "α-syn strains," which may underlie the pathological and clinical diversity of α-synucleinopathies.