A critical review of the prion hypothesis of human synucleinopathies

Alpha-synuclein pathology in isolated rapid eye movement sleep behaviour disorder: a meta-analysis

Accumulating evidence indicates that patients with isolated rapid eye movement sleep behaviour disorder (iRBD), a prodromal stage of synucleinopathies, show abnormal deposition of misfolded alpha-synuclein (a-Syn) in peripheral tissues. The clinical utility of testing for a-Syn in iRBD is unclear. This meta-analysis focused on the utility of testing for the abnormal a-Syn phosphorylated at Ser129 (p-syn) and a-Syn seeding activity (a-Syn seed amplification assays [aSyn-SAA]). Following an electronic database search, 15 studies were included that provided at a minimum data on test positivity in participants with iRBD. Test positivity from cerebrospinal fluid (CSF) was 80% (95% confidence interval [CI] 68-88%, I2 = 71%) and for skin was 74.8% (95% CI 53.2-88.5%, I2 = 64%) for aSyn-SAA and 78.5% (95% CI 70.4-84.9%, I2 = 14%) for p-syn. The phenoconversion rate ratio of biopsy-positive versus biopsy-negative iRBD was 1.28 (95% CI 0.68-2.41, I2 = 0%). Skin as a source had a specificity of 99% (95% CI 95-100%, I2 = 0%; p = 0.01 compared to CSF). As a test, p-syn, had a specificity of 100% (95% CI 93-100%, I2 = 0%; p < 0.001) compared to aSyn-SAA. The odds ratio of a-Syn test positivity in iRBD versus other RBDs was 112 (95% CI 20-629, I2 = 0%). These results demonstrate clinically significant test positivity in iRBD and favour skin over CSF as the source of a-Syn pathological analysis, and p-syn over aSyn-SAA as the testing method. Overall, these findings indicate that testing for a-Syn could help in differentiating iRBD from RBD secondary to other conditions.

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.

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.

Disease-Modifying Therapies for Multiple System Atrophy: Where Are We in 2022?

Multiple system atrophy is a rapidly progressive and fatal neurodegenerative disorder. While numerous preclinical studies suggested efficacy of potentially disease modifying agents, none of those were proven to be effective in large-scale clinical trials. Three major strategies are currently pursued in preclinical and clinical studies attempting to slow down disease progression. These target α-synuclein, neuroinflammation, and restoration of neurotrophic support. This review provides a comprehensive overview on ongoing preclinical and clinical developments of disease modifying therapies. Furthermore, we will focus on potential shortcomings of previous studies that can be avoided to improve data quality in future studies of this rare disease.

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.

Infectious Agents as Potential Drivers of α-Synucleinopathies

α-synucleinopathies, encompassing Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy, are devastating neurodegenerative diseases for which available therapeutic options are scarce, mostly because of our limited understanding of their pathophysiology. Although these pathologies are attributed to an intracellular accumulation of the α-synuclein protein in the nervous system with subsequent neuronal loss, the trigger(s) of this accumulation is/are not clearly identified. Among the existing hypotheses, interest in the hypothesis advocating the involvement of infectious agents in the onset of these diseases is renewed. In this article, we aimed to review the ongoing relevant factors favoring and opposing this hypothesis, focusing on (1) the potential antimicrobial role of α-synuclein, (2) potential entry points of pathogens in regard to early symptoms of diverse α-synucleinopathies, (3) pre-existing literature reviews assessing potential associations between infectious agents and Parkinson's disease, (4) original studies assessing these associations for dementia with Lewy bodies and multiple system atrophy (identified through a systematic literature review), and finally (5) potential susceptibility factors modulating the effects of infectious agents on the nervous system. © 2022 International Parkinson and Movement Disorder Society.

Autophagy in α-Synucleinopathies-An Overstrained System

Alpha-synucleinopathies comprise progressive neurodegenerative diseases, including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). They all exhibit the same pathological hallmark, which is the formation of α-synuclein positive deposits in neuronal or glial cells. The aggregation of α-synuclein in the cell body of neurons, giving rise to the so-called Lewy bodies (LBs), is the major characteristic for PD and DLB, whereas the accumulation of α-synuclein in oligodendroglial cells, so-called glial cytoplasmic inclusions (GCIs), is the hallmark for MSA. The mechanisms involved in the intracytoplasmic inclusion formation in neuronal and oligodendroglial cells are not fully understood to date. A possible mechanism could be an impaired autophagic machinery that cannot cope with the high intracellular amount of α-synuclein. In fact, different studies showed that reduced autophagy is involved in α-synuclein aggregation. Furthermore, altered levels of different autophagy markers were reported in PD, DLB, and MSA brains. To date, the trigger point in disease initiation is not entirely clear; that is, whether autophagy dysfunction alone suffices to increase α-synuclein or whether α-synuclein is the pathogenic driver. In the current review, we discuss the involvement of defective autophagy machinery in the formation of α-synuclein aggregates, propagation of α-synuclein, and the resulting neurodegenerative processes in α-synucleinopathies.

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.

Mechanisms of Neurodegeneration in Various Forms of Parkinsonism-Similarities and Differences

Parkinson’s disease (PD), dementia with Lewy body (DLB), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD) and multiple system atrophy (MSA) belong to a group of neurodegenerative diseases called parkinsonian syndromes. They share several clinical, neuropathological and genetic features. Neurodegenerative diseases are characterized by the progressive dysfunction of specific populations of neurons, determining clinical presentation. Neuronal loss is associated with extra- and intracellular accumulation of misfolded proteins. The parkinsonian diseases affect distinct areas of the brain. PD and MSA belong to a group of synucleinopathies that are characterized by the presence of fibrillary aggregates of α-synuclein protein in the cytoplasm of selected populations of neurons and glial cells. PSP is a tauopathy associated with the pathological aggregation of the microtubule associated tau protein. Although PD is common in the world’s aging population and has been extensively studied, the exact mechanisms of the neurodegeneration are still not fully understood. Growing evidence indicates that parkinsonian disorders to some extent share a genetic background, with two key components identified so far: the microtubule associated tau protein gene (MAPT) and the α-synuclein gene (SNCA). The main pathways of parkinsonian neurodegeneration described in the literature are the protein and mitochondrial pathways. The factors that lead to neurodegeneration are primarily environmental toxins, inflammatory factors, oxidative stress and traumatic brain injury.

Abnormal Gray Matter Volume and Functional Connectivity in Parkinson's Disease with Rapid Eye Movement Sleep Behavior Disorder

Objective

Rapid eye movement (REM) sleep behavior disorder (RBD) is a common symptom in Parkinson's disease (PD), and patients with PD-RBD tend to have an increased risk of cognitive decline and have the tendency to be akinetic/rigidity predominant. At the same time, the mechanisms of RBD in patients with PD remain unclear. Therefore, this study aimed to detect the structural and functional differences in patients with PD-RBD and PD without RBD (PD-nRBD).

Methods

Twenty-four polysomnography-confirmed patients with PD-RBD, 26 patients with PD-nRBD, and 26 healthy controls were enrolled. Structural and functional patterns were analyzed based on voxel-based morphometry and seed-based functional connectivity (FC). Correlations between altered gray matter volume (GMV)/FC values and cognitive scores and motor impairment scores in PD subgroups were assessed.

Results

Compared with patients with PD-nRBD, patients with PD-RBD showed relatively high GMV in the cerebellar vermis IV/V and low GMV in the right superior occipital gyrus (SOG). For the FC, patients with PD-RBD displayed lower FC between the right SOG and the posterior regions (left fusiform gyrus, left calcarine sulcus, and left superior parietal gyrus) compared with the patients with PD-nRBD. The GMV values in the right SOG were negatively correlated with the Unified PD Rating Scale-III scores in patients with PD-RBD but positively correlated with delayed memory scores. The GMV values in the cerebellar vermis IV/V were positively correlated with the tonic chin EMG density scores. There were positive correlations between the FC values in the right SOG-left superior parietal gyrus and MoCA and visuospatial skills/executive function scores and in the right SOG-left calcarine sulcus and delayed memory scores.

Conclusion

Higher GMV in the cerebellum may be linked with the abnormal motor behaviors during REM sleep in patients with PD-RBD, and lower GMV and FC in the posterior regions may indicate that PD-RBD correspond to more serious neurodegeneration, especially the visuospatial–executive function impairment and delayed memory impairment. These findings provided new insights to learn more about the complicated characteristics in patients with PD-RBD.

Alpha-Synuclein: Mechanisms of Release and Pathology Progression in Synucleinopathies

The accumulation of misfolded alpha-synuclein (aSyn) throughout the brain, as Lewy pathology, is a phenomenon central to Parkinson’s disease (PD) pathogenesis. The stereotypical distribution and evolution of the pathology during disease is often attributed to the cell-to-cell transmission of aSyn between interconnected brain regions. The spreading of conformationally distinct aSyn protein assemblies, commonly referred as strains, is thought to result in a variety of clinically and pathologically heterogenous diseases known as synucleinopathies. Although tremendous progress has been made in the field, the mechanisms involved in the transfer of these assemblies between interconnected neural networks and their role in driving PD progression are still unclear. Here, we present an update of the relevant discoveries supporting or challenging the prion-like spreading hypothesis. We also discuss the importance of aSyn strains in pathology progression and the various putative molecular mechanisms involved in cell-to-cell protein release. Understanding the pathways underlying aSyn propagation will contribute to determining the etiology of PD and related synucleinopathies but also assist in the development of new therapeutic strategies.

Synuclein in neurodegeneration

Neurodegenerative diseases are a heterogeneous group of disorders characterized by gradual progressive neuronal loss in the central nervous system. Unfortunately, the pathogenesis of many of these diseases remains unknown. Synucleins are a family of small, highly charged proteins expressed predominantly in neurons. Following their discovery, much has been learned about their structure, function, interaction with other proteins and role in neurodegenerative disease over the last two decades. One of these proteins, α-Synuclein (α-Syn), appears to be involved in many neurodegenerative disorders. These include Parkinson's disease (PD), dementia with Lewy bodies (DLB), Rapid Eye Movement Sleep Behavior Disorder (RBD) and Pure Autonomic Failure (PAF), i.e., collectively termed α-synucleinopathies. This review focuses on α-Syn dysfunction in neurodegeneration and assesses its role in synucleinopathies from a biochemical, genetic and neuroimaging perspective.

α-Synuclein in Parkinson's Disease: Does a Prion-Like Mechanism of Propagation from Periphery to the Brain Play a Role?

Parkinson's disease (PD) is one of the most common neurodegenerative diseases, defined as motor and non-motor symptoms associated with the loss of dopaminergic neurons and a decreased release of dopamine (DA). Currently, PD patients are believed to have a neuropathological basis denoted by the presence of Lewy bodies (LBs) or Lewy neurites (LNs), which mostly comprise α-synuclein (α-syn) inclusions. Remarkably, there is a growing body of evidence indicating that the inclusions undergo template-directed aggregation and propagation via template-directed among the brain and peripheral organs, mainly in a prion-like manner. Interestingly, some studies reported that an integral loop was reminiscent of the mechanism of Parkinson's disease, denoting that α-syn as prionoid was transmitted from the periphery to the brain via specific pathways. Also the systematic life cycle of α-syn in the cellular level is illustrated. In this review, we critically assess landmark evidence in the field of Parkinson's disease with a focus on the genesis and prion-like propagation of the α-syn pathology. The anatomical and cell-to-cell evidences are discussed to depict the theory behind the propagation and transferred pathways. Furthermore, we highlight effective therapeutic perspectives and clinical trials targeting prion-like mechanisms. Major controversies surrounding this topic are also discussed.

Alternative Structures of α-Synuclein

The object of our analysis is the structure of alpha-synuclein (ASyn), which, under in vivo conditions, associates with presynaptic vesicles. Misfolding of ASyn is known to be implicated in Parkinson's disease. The availability of structural information for both the micelle-bound and amyloid form of ASyn enables us to speculate on the specific mechanism of amyloid transformation. This analysis is all the more interesting given the fact that-Unlike in Aβ(1-42) amyloids-only the central fragment (30-100) of ASyn has a fibrillar structure, whereas, its N- and C-terminal fragments (1-30 and 100-140, respectively) are described as random coils. Our work addresses the following question: Can the ASyn chain-as well as the aforementioned individual fragments-adopt globular conformations? In order to provide an answer, we subjected the corresponding sequences to simulations carried out using Robetta and I-Tasser, both of which are regarded as accurate protein structure predictors. In addition, we also applied the fuzzy oil drop (FOD) model, which, in addition to optimizing the protein's internal free energy, acknowledges the presence of an external force field contributed by the aqueous solvent. This field directs hydrophobic residues to congregate near the center of the protein body while exposing hydrophilic residues on its surface. Comparative analysis of the obtained models suggests that fragments which do not participate in forming the amyloid fibril (i.e., 1-30 and 100-140) can indeed attain globular conformations. We also explain the influence of mutations observed in vivo upon the susceptibility of ASyn to undergo amyloid transformation. In particular, the 30-100 fragment (which adopts a fibrillar structure in PDB) is not predicted to produce a centralized hydrophobic core by any of the applied toolkits (Robetta, I-Tasser, and FOD). This means that in order to minimize the entropically disadvantageous contact between hydrophobic residues and the polar solvent, ASyn adopts the form of a ribbonlike micelle (rather than a spherical one). In other words, the ribbonlike micelle represents a synergy between the conformational preferences of the protein chain and the influence of its environment.

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.

Analysis of the Relationship between Type II Diabetes Mellitus and Parkinson's Disease: A Systematic Review

In the early sixties, a discussion started regarding the association between Parkinson's disease (PD) and type II diabetes mellitus (T2DM). Today, this potential relationship is still a matter of debate. This review aims to analyze both diseases concerning causal relationships and treatments. A total of 104 articles were found, and studies on animal and “in vitro” models showed that T2DM causes neurological alterations that may be associated with PD, such as deregulation of the dopaminergic system, a decrease in the expression of peroxisome proliferator-activated receptor-gamma coactivator-1α (PGC-1α), an increase in the expression of phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes 15 (PED/PEA-15), and neuroinflammation, as well as acceleration of the formation of alpha-synuclein amyloid fibrils. In addition, clinical studies described that Parkinson's symptoms were notably worse after the onset of T2DM, and seven deregulated genes were identified in the DNA of T2DM and PD patients. Regarding treatment, the action of antidiabetic drugs, especially incretin mimetic agents, seems to confer certain degree of neuroprotection to PD patients. In conclusion, the available evidence on the interaction between T2DM and PD justifies more robust clinical trials exploring this interaction especially the clinical management of patients with both conditions.

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.

Prion-Like Mechanisms in Parkinson's Disease

Formation and aggregation of misfolded proteins in the central nervous system (CNS) is a key hallmark of several age-related neurodegenerative diseases, including Parkinson’s disease (PD), Alzheimer’s disease (AD), and amyotrophic lateral sclerosis (ALS). These diseases share key biophysical and biochemical characteristics with prion diseases. It is believed that PD is characterized by abnormal protein aggregation, mainly that of α-synuclein (α-syn). Of particular importance, there is growing evidence indicating that abnormal α-syn can spread to neighboring brain regions and cause aggregation of endogenous α-syn in these regions as seeds, in a “prion-like” manner. Abundant studies in vitro and in vivo have shown that α-syn goes through a templated conformational change, propagates from the original region to neighboring regions, and eventually cause neuron degeneration in the substantia nigra and striatum. The objective of this review is to summarize the mechanisms involved in the aggregation of abnormal intracellular α-syn and its subsequent cell-to-cell transmission. According to these findings, we look forward to effective therapeutic perspectives that can block the progression of neurodegenerative diseases.

Neuropathology and pathogenesis of extrapyramidal movement disorders: a critical update-I. Hypokinetic-rigid movement disorders

Extrapyramidal movement disorders include hypokinetic rigid and hyperkinetic or mixed forms, most of them originating from dysfunction of the basal ganglia (BG) and their information circuits. The functional anatomy of the BG, the cortico-BG-thalamocortical, and BG-cerebellar circuit connections are briefly reviewed. Pathophysiologic classification of extrapyramidal movement disorder mechanisms distinguish (1) parkinsonian syndromes, (2) chorea and related syndromes, (3) dystonias, (4) myoclonic syndromes, (5) ballism, (6) tics, and (7) tremor syndromes. Recent genetic and molecular-biologic classifications distinguish (1) synucleinopathies (Parkinson's disease, dementia with Lewy bodies, Parkinson's disease-dementia, and multiple system atrophy); (2) tauopathies (progressive supranuclear palsy, corticobasal degeneration, FTLD-17; Guamian Parkinson-dementia; Pick's disease, and others); (3) polyglutamine disorders (Huntington's disease and related disorders); (4) pantothenate kinase-associated neurodegeneration; (5) Wilson's disease; and (6) other hereditary neurodegenerations without hitherto detected genetic or specific markers. The diversity of phenotypes is related to the deposition of pathologic proteins in distinct cell populations, causing neurodegeneration due to genetic and environmental factors, but there is frequent overlap between various disorders. Their etiopathogenesis is still poorly understood, but is suggested to result from an interaction between genetic and environmental factors. Multiple etiologies and noxious factors (protein mishandling, mitochondrial dysfunction, oxidative stress, excitotoxicity, energy failure, and chronic neuroinflammation) are more likely than a single factor. Current clinical consensus criteria have increased the diagnostic accuracy of most neurodegenerative movement disorders, but for their definite diagnosis, histopathological confirmation is required. We present a timely overview of the neuropathology and pathogenesis of the major extrapyramidal movement disorders in two parts, the first one dedicated to hypokinetic-rigid forms and the second to hyperkinetic disorders.

Prion and Prion-Like Protein Strains: Deciphering the Molecular Basis of Heterogeneity in Neurodegeneration

Increasing evidence suggests that neurodegenerative disorders share a common pathogenic feature: the presence of deposits of misfolded proteins with altered physicochemical properties in the Central Nervous System. Despite a lack of infectivity, experimental data show that the replication and propagation of neurodegenerative disease-related proteins including amyloid-β (Aβ), tau, α-synuclein and the transactive response DNA-binding protein of 43 kDa (TDP-43) share a similar pathological mechanism with prions. These observations have led to the terminology of "prion-like" to distinguish between conditions with noninfectious characteristics but similarities with the prion replication and propagation process. Prions are considered to adapt their conformation to changes in the context of the environment of replication. This process is known as either prion selection or adaptation, where a distinct conformer present in the initial prion population with higher propensity to propagate in the new environment is able to prevail over the others during the replication process. In the last years, many studies have shown that prion-like proteins share not only the prion replication paradigm but also the specific ability to aggregate in different conformations, i.e., strains, with relevant clinical, diagnostic and therapeutic implications. This review focuses on the molecular basis of the strain phenomenon in prion and prion-like proteins.

Are We Ready for Detecting α-Synuclein Prone to Aggregation in Patients? The Case of "Protein-Misfolding Cyclic Amplification" and "Real-Time Quaking-Induced Conversion" as Diagnostic Tools

The accumulation and deposition of α-synuclein aggregates in brain tissue is the main event in the pathogenesis of different neurodegenerative disorders grouped under the term of synucleinopathies. They include Parkinson's disease, dementia with Lewy bodies and multiple system atrophy. To date, the diagnosis of any of these disorders mainly relies on the recognition of clinical symptoms, when the neurodegeneration is already in an advanced phase. In the last years, several efforts have been carried out to develop new diagnostic tools for early diagnosis of synucleinopathies, with special interest to Parkinson's disease. The Protein-Misfolding Cyclic Amplification (PMCA) and the Real-Time Quaking-Induced Conversion (RT-QuIC) are ultrasensitive protein amplification assays for the detection of misfolded protein aggregates. Starting from the successful application in the diagnosis of human prion diseases, these techniques were recently tested for the detection of misfolded α-synuclein in brain homogenates and cerebrospinal fluid samples of patients affected by synucleinopathies. So far, only a few studies on a limited number of samples have been performed to test PMCA and RT-QuIC diagnostic reliability. Neverthless, these assays have shown very high sensitivity and specificity in detecting synucleinopathies even at the pre-clinical stage. Despite the application of PMCA and RT-QuIC for α-synuclein detection in biological fluids is very recent, these techniques seem to have the potential for identifying subjects that will be likely to develop synucleinopathies.

ɑ-Synuclein strains and seeding in Parkinson's disease, incidental Lewy body disease, dementia with Lewy bodies and multiple system atrophy: similarities and differences

Several age-related neurodegenerative disorders are characterized by the deposition of aberrantly folded endogenous proteins. These proteins have prion-like propagation and amplification properties but so far appear nontransmissible between individuals. Because of the features they share with the prion protein, PrP, the characteristics of pathogenic protein aggregates in several progressive brain disorders, including different types of Lewy body diseases (LBDs), such as Parkinson's disease (PD), multiple system atrophy (MSA) and dementia with Lewy bodies (DLB), have been actively investigated. Even though the pleomorphic nature of these syndromes might suggest different underlying causes, ɑ-synuclein (ɑSyn) appears to play an important role in this heterogeneous group of diseases (the synucleinopathies). An attractive hypothesis is that different types of ɑSyn protein assemblies have a unique and causative role in distinct synucleinopathies. We will discuss the recent research progress on ɑSyn assemblies involved in PD, MSA and DLB; their behavior as strains; current spreading hypotheses; their ability to seed centrally and peripherally; and their implication for disease pathogenesis.

The concept of alpha-synuclein as a prion-like protein: ten years after

Parkinson's disease is characterized by the loss of nigrostriatal dopaminergic signaling and the presence of alpha-synuclein aggregates (also called Lewy bodies and neurites) throughout the brain. In 2003, Braak and colleagues created a staging system for Parkinson's disease describing the connection between the alpha-synuclein pathology and disease severity. Later, they suggested that the pathology might initially be triggered by exogenous insults targeting the gut and olfactory system. In 2008, we and other groups documented Lewy pathology in grafted neurons in people with Parkinson's disease who had been transplanted over a decade prior to autopsy. We proposed that the Lewy pathology in the grafted neurons was the result of permissive templating or prion-like spread of alpha-synuclein pathology from neurons in the host to those in the grafts. During the following ten years, several studies described the transmission of alpha-synuclein pathology between neurons, both in cell culture and in experimental animals. Recent research has also begun to identify underlying molecular mechanisms. Collectively, these experimental studies tentatively support the idea that the progression from one Braak stage to the next is the consequence of prion-like propagation of Lewy pathology. However, definitive proof that intercellular propagation of alpha-synuclein pathology occurs in Parkinson's disease cases has proven difficult to secure. In this review, we highlight several open questions that currently prevent us from concluding with certainty that prion-like transfer of alpha-synuclein contributes to the progression of Parkinson's disease.