An Efficient Procedure for Removal and Inactivation of Alpha-Synuclein Assemblies from Laboratory Materials

Sensitive detection of pathological seeds of α-synuclein, tau and prion protein on solid surfaces

Prions or prion-like aggregates such as those composed of PrP, α-synuclein, and tau are key features of proteinopathies such as prion, Parkinson's and Alzheimer's diseases, respectively. Their presence on solid surfaces may be biohazardous under some circumstances. PrP prions bound to solids are detectable by ultrasensitive real-time quaking-induced conversion (RT-QuIC) assays if the solids can be immersed in assay wells or the prions transferred to pads. Here we show that prion-like seeds can remain detectable on steel wires for at least a year, or even after enzymatic cleaning and sterilization. We also show that contamination of larger objects with pathological seeds of α-synuclein, tau, and PrP can be detected by simply assaying a sampling medium that has been transiently applied to the surface. Human α-synuclein seeds in dementia with Lewy bodies brain tissue were detected by α-synuclein RT-QuIC after drying of tissue dilutions with concentrations as low as 10-6 onto stainless steel. Tau RT-QuIC detected tau seeding activity on steel exposed to Alzheimer's disease brain tissue diluted as much as a billion fold. Prion RT-QuIC assays detected seeding activity on plates exposed to brain dilutions as extreme as 10-5-10-8 from prion-affected humans, sheep, cattle and cervids. Sampling medium collected from surgical instruments used in necropsies of sporadic Creutzfeldt-Jakob disease-infected transgenic mice was positive down to 10-6 dilution. Sensitivity for prion detection was not sacrificed by omitting the recombinant PrP substrate from the sampling medium during its application to a surface and subsequent storage as long as the substrate was added prior to performing the assay reaction. Our findings demonstrate practical prototypic surface RT-QuIC protocols for the highly sensitive detection of pathologic seeds of α-synuclein, tau, and PrP on solid objects.

Colonoscopy and Subsequent Risk of Parkinson's Disease

Background

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

Objective

This study examines a possible association between colonoscopy and PD.

Methods

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

Results

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

Conclusions

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

Single-dose intranasal administration of α-syn PFFs induce lewy neurite-like pathology in olfactory bulbs

INTRODUCTION

Pathological α-synuclein (α-Syn) propagation may cause Parkinson's disease progression. We aimed to verify whether single-dose intranasal administration of α-Syn preformed fibrils (PFFs) induces α-Syn pathology in the olfactory bulb (OB).

METHODS

A single dose of α-Syn PFFs was administered to the left nasal cavity of wild-type mice. The untreated right side served as a control. The α-Syn pathology of the OBs was examined up to 12 months after the injection.

RESULTS

Lewy neurite-like aggregates were observed in the OB 6 and 12 months after the treatment.

CONCLUSIONS

These findings suggest that pathological α-Syn can propagate from the olfactory mucosa to the OB and reveal the potential dangers of α-Syn PFFs inhalation.

Identifying Biological and Biophysical Features of Different Maturation States of α-Synuclein Amyloid Fibrils

Protein aggregates, hereunder amyloid fibrils, can undergo a maturation process, whereby early formed aggregates undergo a structural and physicochemical transition leading to more mature species. In the case of amyloid-related diseases, such maturation confers distinctive biological properties of the aggregates, which may account for a range of diverse pathological subtypes. Here, we present a protocol for the preparation of α-synuclein amyloid fibrils differing in the level of their maturation. We utilize widely accessible biophysical techniques to characterize the structure and morphology and a simple thermal treatment procedure to test their thermodynamic stability. Their biological properties are probed by means of binding to native plasma membrane sheets originating from mammalian cell lines.

Solid-State NMR Structure of Amyloid-β Fibrils

Amyloid fibrils are involved in a number of diseases and notably play a role in neurodegeneration, where they are present in plaques in the brain. Their structure determination might help in finding ways to interfere with their formation, and ultimately prevent disease, by revealing the structure-function relationship and helping to design molecules targeting initial assembly steps and further propagation. Here, we describe the different steps in NMR protocols which allowed the 3D structure determination of amyloid-β fibrils.

Aggregation of alpha-synuclein in enteric neurons does not impact function in vitro

Recent evidence implicates a gut-first pathogenesis in the enteric nervous system (ENS) within a portion of PD patients, yet in vitro investigations have primarily focused on the central nervous system. Here, the preformed fibril (PFF) PD model is applied with co-administered groups of butyrate and lipopolysaccharide to model the effects of the local gut microbiome. Significant PFF uptake and retention occur in isolated rat enteric neurons compared to untreated controls resulting in increasing immunostained aggregate conformation-specific, alpha-synuclein (a-Syn) average intensity between 6 µg PFF and untreated controls. Cortical neurons significantly retain PFFs with an increase in aggregated a-Syn average intensity within all dosages. Differences in growth cone morphology but not dynamics in PFF-treated ENS cultures occur. Electrophysiological recordings via a microelectrode array (MEA) indicate no overall difference in spontaneous spike rate. However, only untreated controls respond to PD-relevant dopamine stimulus, while 1 µg PFF and control populations respond to stimulus with ENS-abundant acetylcholine. Finally, no differences in substance P levels-correlated with PD and neurodegeneration-are observed. Overall, these findings suggest the ENS retains PFF dosage absent acute loss in function, however, does experience changes in growth cone morphology and dopamine-stimulated activity.

Neuronal haemoglobin induces loss of dopaminergic neurons in mouse Substantia nigra, cognitive deficits and cleavage of endogenous α-synuclein

Parkinson's disease (PD) presents the selective loss of A9 dopaminergic (DA) neurons of Substantia Nigra pars compacta (SNpc) and the presence of intracellular aggregates called Lewy bodies. α-synuclein (α-syn) species truncated at the carboxy-terminal (C-terminal) accumulate in pathological inclusions and promote α-syn aggregation and toxicity. Haemoglobin (Hb) is the major oxygen carrier protein in erythrocytes. In addition, Hb is expressed in A9 DA neurons where it influences mitochondrial activity. Hb overexpression increases cells' vulnerability in a neurochemical model of PD in vitro and forms cytoplasmic and nucleolar aggregates upon short-term overexpression in mouse SNpc. In this study, α and β-globin chains were co-expressed in DA cells of SNpc in vivo upon stereotaxic injections of an Adeno-Associated Virus isotype 9 (AAV9) and in DA iMN9D cells in vitro. Long-term Hb over-expression in SNpc induced the loss of about 50% of DA neurons, mild motor impairments, and deficits in recognition and spatial working memory. Hb triggered the formation of endogenous α-syn C-terminal truncated species. Similar α-syn fragments were found in vitro in DA iMN9D cells over-expressing α and β- globins when treated with pre-formed α-syn fibrils. Our study positions Hb as a relevant player in PD pathogenesis for its ability to trigger DA cells' loss in vivo and the formation of C-terminal α-syn fragments.

Single Molecule Fingerprinting Reveals Different Amplification Properties of α-Synuclein Oligomers and Preformed Fibrils in Seeding Assay

The quantification of α-synuclein aggregates has emerged as a promising biomarker for synucleinopathies. Assays that amplify and detect such aggregates have revealed the presence of seeding-competent species in biosamples of patients diagnosed with Parkinson's disease. However, multiple species, such as oligomers and amyloid fibrils, are formed during the aggregation of α-synuclein; these species are likely to coexist in biological samples, and thus it remains unclear which species(s) are contributing to the signal detected in seeding assays. To identify individual contributions to the amplification process, recombinant oligomers and preformed fibrils were produced and purified to characterize their individual biochemical and seeding potential. Here, we used single molecule spectroscopy to track the formation and purification of oligomers and fibrils at the single particle level and compare their respective seeding potential in an amplification assay. Single molecule detection validates that size-exclusion chromatography efficiently separates oligomers from fibrils. Oligomers were found to be seeding-competent, but our results reveal that their seeding behavior is very different compared to that of preformed fibrils, in our amplification assay. Overall, our data suggest that even a low number of preformed fibrils present in biosamples is likely to dominate the response in seeding assays.

Cortical alpha-synuclein preformed fibrils do not affect interval timing in mice

One hallmark feature of Parkinson's disease (PD) is Lewy body pathology associated with misfolded alpha-synuclein. Previous studies have shown that striatal injection of alpha-synuclein preformed fibrils (PFF) can induce misfolding and aggregation of native alpha-synuclein in a prion-like manner, leading to cell death and motor dysfunction in mouse models. Here, we tested whether alpha-synuclein PFFs injected into the medial prefrontal cortex results in deficits in interval timing, a cognitive task which is disrupted in human PD patients and in rodent models of PD. We injected PFF or monomers of human alpha-synuclein into the medial prefrontal cortex of mice pre-injected with adeno-associated virus (AAV) coding for overexpression of human alpha-synuclein or control protein. Despite notable medial prefrontal cortical synucleinopathy, we did not observe consistent deficits in fixed-interval timing. These results suggest that cortical alpha-synuclein does not reliably disrupt fixed-interval timing.

Strong acids induce amyloid fibril formation of β2-microglobulin via an anion-binding mechanism

Amyloid fibrils, crystal-like fibrillar aggregates of proteins associated with various amyloidoses, have the potential to propagate via a prion-like mechanism. Among known methodologies to dissolve preformed amyloid fibrils, acid treatment has been used with the expectation that the acids will degrade amyloid fibrils similar to acid inactivation of protein functions. Contrary to our expectation, treatment with strong acids, such as HCl or H₂SO₄, of β₂-microglobulin (β2m) or insulin actually promoted amyloid fibril formation, proportionally to the concentration of acid used. A similar promotion was observed at pH 2.0 upon the addition of salts, such as NaCl or Na₂SO₄. Although trichloroacetic acid, another strong acid, promoted amyloid fibril formation of β2m, formic acid, a weak acid, did not, suggesting the dominant role of anions in promoting fibril formation of this protein. Comparison of the effects of acids and salts confirmed the critical role of anions, indicating that strong acids likely induce amyloid fibril formation via an anion-binding mechanism. The results suggest that although the addition of strong acids decreases pH, it is not useful for degrading amyloid fibrils, but rather induces or stabilizes amyloid fibrils via an anion-binding mechanism.

α-Synuclein Propagation Mouse Models of Parkinson's Disease

Parkinson's disease (PD) is pathologically characterized by intraneuronal α-synuclein (α-Syn) inclusions called Lewy bodies (LBs) and the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc). Autopsy studies have suggested that Lewy pathology initially occurs in the olfactory bulb and enteric nervous system, subsequently spreading in the brain stereotypically. Recent studies have demonstrated that templated fibrillization and intercellular dissemination of misfolded α-Syn underlie this pathological progression. Injection of animals with α-Syn preformed fibrils (PFFs) can recapitulate LB-like inclusions and the subsequent intercellular transmission of α-Syn pathology. Moreover, targeting specific brain regions or body parts enables the generation of unique models depending on the injection sites. These features of α-Syn PFF-injected animal models provide a platform to explore disease mechanisms and to test disease modifying therapies in PD research. Here, we describe a methodology for the generation of α-Syn PFFs and the surgery on mice.

Mechanisms of Cell-to-Cell Transmission of Pathological Tau: A Review

Importance

Intracellular tau protein aggregates are a pathological hallmark of neurodegenerative tauopathies, including Alzheimer disease (AD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and Pick disease. Emerging evidence supports a model of cell-to-cell transmission of proteinaceous pathological tau seeds, which leads to recruitment and templated fibrillization of endogenous cellular tau followed by the spread of abnormal tau throughout the brain. These findings lead to the strain hypothesis, which predicts that distinct conformational strains or polymorphs of tau may underlie the clinical and neuropathological heterogeneity and cell-type specificity of tauopathies. In this review, we describe the evidence for propagation of distinct tau strains in cell culture and animal models of AD and mechanistic insights into cell-to-cell transmission of pathological tau.

Observations

Intracranial injections of synthetic tau-preformed fibrils and human brain-derived pathological tau into nontransgenic wild-type mice and transgenic mouse models of AD expressing β-amyloid and tau-amyloid deposits yield widespread pathological tau aggregates observed in neuroanatomically connected brain regions distant from the site of injection. Furthermore, when human brain-derived pathological tau obtained from distinct tauopathies (ie, brains with AD, PSP, and CBD) were injected into the brains of wild-type mice, they seeded tau pathology and faithfully recapitulated cell-type specific tau inclusions characteristic of each tauopathy in a time-dependent, dose-dependent, and injection site-dependent spread reflective of the connectome of the injection site.

Conclusions and Relevance

These findings provide compelling evidence that misfolded or pathological conformers of tau undergo cell-to-cell spread in a tauopathy strain-specific manner. Importantly, evidence to date supports that pathological tau strains do not behave like infectious agents, despite growing evidence that these tau strains undergo templated propagation and spread linked to the neuroanatomical connectome of the injection site.

Best Practices for Generating and Using Alpha-Synuclein Pre-Formed Fibrils to Model Parkinson's Disease in Rodents

Parkinson’s disease (PD) is the second most common neurodegenerative disease, affecting approximately one-percent of the population over the age of sixty. Although many animal models have been developed to study this disease, each model presents its own advantages and caveats. A unique model has arisen to study the role of alpha-synuclein (aSyn) in the pathogenesis of PD. This model involves the conversion of recombinant monomeric aSyn protein to a fibrillar form—the aSyn pre-formed fibril (aSyn PFF)—which is then injected into the brain or introduced to the media in culture. Although many groups have successfully adopted and replicated the aSyn PFF model, issues with generating consistent pathology have been reported by investigators. To improve the replicability of this model and diminish these issues, The Michael J. Fox Foundation for Parkinson’s Research (MJFF) has enlisted the help of field leaders who performed key experiments to establish the aSyn PFF model to provide the research community with guidelines and practical tips for improving the robustness and success of this model. Specifically, we identify key pitfalls and suggestions for avoiding these mistakes as they relate to generating the aSyn PFFs from monomeric protein, validating the formation of pathogenic aSyn PFFs, and using the aSyn PFFs in vivo or in vitro to model PD. With this additional information, adoption and use of the aSyn PFF model should present fewer challenges, resulting in a robust and widely available model of PD.

Assays for Neuronal Defects Caused by Early Formation of α-Synuclein Inclusions in Primary Cultured Neurons

Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are characterized by intracellular inclusions composed mostly of α-synuclein (Baba et al., Am J Pathol 152:879-884, 1998). How inclusion formation impacts neuronal function prior to death is key to understanding disease progression and identifying therapeutic windows. In the α-synuclein fibril model, exposure of primary neurons to α-synuclein fibrils induces endogenously expressed α-synuclein to form inclusions which closely resembles pathologic mechanisms in humans with PD and DLB (Volpicelli-Daley et al., Neuron 72, 57-71, 2011). In this model, at 7 days after exposure of neurons to fibrils, when there is no neuron death, inclusions in the axon selectively impair axonal transport of endosomes carrying the TrkB receptor and LC3-positive autophagosomes (Volpicelli-Daley et al., Mol Biol Cell 25:4010-4023, 2014). In addition, the frequency and amplitude of spontaneous Ca²⁺ transients are reduced in neurons 7 days after fibril exposure. Here we discuss protocols for plating primary hippocampal neurons, generating fibrils and measuring axonal transport and Ca²⁺ transients. These assays provide additional assays of neurotoxicity allowing researchers to determine if a therapeutic intervention can prevent neuronal defects before intractable neurodegeneration.

Stereotaxic Targeting of Alpha-Synuclein Pathology in Mouse Brain Using Preformed Fibrils

The accumulation of intraneuronal inclusions containing misfolded alpha-synuclein (aSyn) within the central nervous system (CNS) is a common feature found in several neurodegenerative disorders including Parkinson's disease (PD). Emerging evidence indicates that aSyn amyloid fibrils, a configuration that is present within these characteristic inclusions, are capable of self-replicating by templating the conversion of endogenously expressed aSyn in neurons. Stereotaxic administration of synthetic α-synuclein preformed fibrils (PFFs) into the mouse brain has been shown to seed the formation of intracellular aSyn pathology reminiscent of Lewy body (LB) inclusions present in human PD and related synucleinopathies. Moreover, pathology can be targeted to specific CNS regions. This experimental approach provides a versatile platform for investigating PD-like LB pathology in vivo. We focus here on procedures for initiating aSyn inclusion formation at various regions of the mouse brain using computer-assisted motorized stereotaxic microinjection of aSyn PFFs and discuss appropriate strategies for controls and analysis.

CONFINE-MAS: a magic-angle spinning NMR probe that confines the sample in case of a rotor explosion

Magic-angle spinning (MAS) is mandatory in solid-state NMR experiments to achieve resolved spectra. In rare cases, instabilities in the rotation or damage of either the rotor or the rotor cap can lead to a so called "rotor crash" involving a disintegration of the sample container and possibly the release of an aerosol or of dust. We present a modified design of a 3.2 mm probe with a confining chamber which in case of a rotor crash prevents the release of aerosols and possibly hazardous materials. 1D and 2D NMR experiments show that such a hazardous material-confining MAS probe ("CONFINE-MAS" probe) has a similar sensitivity compared to a standard probe and performs equally well in terms of spinning stability. We illustrate the CONFINE-MAS probe properties and performance by application to a fungal amyloid.

Assessment of the efficacy of different procedures that remove and disassemble alpha-synuclein, tau and A-beta fibrils from laboratory material and surfaces

α-synuclein fibrillar polymorphs, Tau and Aß 1–42 fibrillar assemblies have been shown to propagate, amplify and trigger the formation of protein deposits reminiscent of those present within the central nervous system of patients developing synucleinopathies, tauopathies and amyloid plaques after injection intracerebrally, intramuscularly, intraperitoneally or within the blood stream of model animals. They are thus hazardous and there is need for decontamination and inactivation procedures for laboratory surfaces and non-disposable material. We assessed the effectiveness of different reagents to clean and disassemble potentially pathogenic assemblies adsorbed on non-disposable materials in laboratories. We show that commercial detergents and SDS are way more suited to detach α-synuclein fibrillar polymorphs, Tau and Aß 1–42 fibrillar assemblies from contaminated surfaces and disassemble the fibrils than methods designed to decrease PrP prion infectivity. Our observations reveal that the choice of the most adapted cleaning procedure for one given protein assembly or fibrillar polymorph should integrate detergent’s cleaning efficiency, material compatibility and capacity to dismantle assemblies. We provide an integrated representation where desorption and neutralization efficacy and surface compatibility are combined to facilitate the choice of the most adapted decontamination procedure. This representation, together with good laboratory practices, contributes to reducing potential health hazards associated to manipulating protein assemblies with prion-like properties.

Potent prion-like behaviors of pathogenic α-synuclein and evaluation of inactivation methods

The concept that abnormal protein aggregates show prion-like propagation between cells has been considered to explain the onset and progression of many neurodegenerative diseases. Indeed, both synthetic amyloid-like fibrils and pathogenic proteins extracted from patients’ brains induce self-templated amplification and cell-to-cell transmission in vitro and in vivo. However, it is unclear whether exposure to exogenous prion-like proteins can potentially cause these diseases in humans. Here, we investigated in detail the prion-like seeding activities of several kinds of pathogenic α-synuclein (α-syn), including synthetic fibrils and detergent-insoluble fractions extracted from brains of patients with α-synucleinopathies. Exposure to synthetic α-syn fibrils at concentrations above 100 pg/mL caused seeded aggregation of α-syn in SH-SY5Y cells, and seeded aggregation was also observed in C57BL/6 J mice after intracerebral inoculation of at least 0.1 μg/animal. α-Syn aggregates extracted from brains of multiple system atrophy (MSA) patients showed higher seeding activity than those extracted from patients with dementia with Lewy bodies (DLB), and their potency was similar to that of synthetic α-syn fibrils. We also examined the effects of various methods that have been reported to inactivate abnormal prion proteins (PrP^Sc), including autoclaving at various temperatures, exposure to sodium dodecyl sulfate (SDS), and combined treatments. The combination of autoclaving and 1% SDS substantially reduced the seeding activities of synthetic α-syn fibrils and α-syn aggregates extracted from MSA brains. However, single treatment with 1% SDS or generally used sterilization conditions proved insufficient to prevent accumulation of pathological α-syn. In conclusion, α-syn aggregates derived from MSA patients showed a potent prion-like seeding activity, which could be efficiently reduced by combined use of SDS and autoclaving.

MSA prions exhibit remarkable stability and resistance to inactivation

In multiple system atrophy (MSA), progressive neurodegeneration results from the protein α-synuclein misfolding into a self-templating prion conformation that spreads throughout the brain. MSA prions are transmissible to transgenic (Tg) mice expressing mutated human α-synuclein (TgM83^+/-), inducing neurological disease following intracranial inoculation with brain homogenate from deceased patient samples. Noting the similarities between α-synuclein prions and PrP scrapie (PrP^Sc) prions responsible for Creutzfeldt-Jakob disease (CJD), we investigated MSA transmission under conditions known to result in PrP^Sc transmission. When peripherally exposed to MSA via the peritoneal cavity, hind leg muscle, and tongue, TgM83^+/- mice developed neurological signs accompanied by α-synuclein prions in the brain. Iatrogenic CJD, resulting from PrP^Sc prion adherence to surgical steel instruments, has been investigated by incubating steel sutures in contaminated brain homogenate before implantation into mouse brain. Mice studied using this model for MSA developed disease, whereas wire incubated in control homogenate had no effect on the animals. Notably, formalin fixation did not inactivate α-synuclein prions. Formalin-fixed MSA patient samples also transmitted disease to TgM83^+/- mice, even after incubating in fixative for 244 months. Finally, at least 10% sarkosyl was found to be the concentration necessary to partially inactivate MSA prions. These results demonstrate the robustness of α-synuclein prions to denaturation. Moreover, they establish the parallel characteristics between PrP^Sc and α-synuclein prions, arguing that clinicians should exercise caution when working with materials that might contain α-synuclein prions to prevent disease.

Measurement of Chaperone-Mediated Effects on Polyglutamine Protein Aggregation by the Filter Trap Assay

The formation of aggregates by polyglutamine-containing (polyQ) proteins in neurons is a key to the pathogenesis of several progressive neurodegenerative diseases such as Huntington's disease (HD) spinocerebellar ataxias (SCAs), and spinal and bulbar muscular atrophy (SBMA). In order to study whether the members of the heat shock protein (HSP) families, by virtue of their molecular chaperone activity, can inhibit the formation of polyQ aggregates, we developed a cell culture model expressing the GFP tagged fragment of exon1 of the huntingtin gene with an expanded polyQ chain and tetracycline inducible chaperones. Expression of mutated Huntington's protein leads to the formation of 2% SDS insoluble high molecular weight polyQ aggregates that are retarded on a cellulose acetate membrane in the so-called filter trap assay (FTA). This chapter explains in detail the protocols of the FTA and how it can be a useful tool to study the effect of HSPs or their functional mutants on aggregation of polyglutamine proteins. Moreover, the assay is useful to investigate how externally added polyQ peptides can act as nucleation seeds for internally expressed polyQ proteins.

α-Synuclein fibrils recruit peripheral immune cells in the rat brain prior to neurodegeneration

Genetic variation in a major histocompatibility complex II (MHCII)-encoding gene (HLA-DR) increases risk for Parkinson disease (PD), and the accumulation of MHCII-expressing immune cells in the brain correlates with α-synuclein inclusions. However, the timing of MHCII-cell recruitment with respect to ongoing neurodegeneration, and the types of cells that express MHCII in the PD brain, has been difficult to understand. Recent studies show that the injection of short α-synuclein fibrils into the rat substantia nigra pars compacta (SNpc) induces progressive inclusion formation in SNpc neurons that eventually spread to spiny projection neurons in the striatum. Herein, we find that α-synuclein fibrils rapidly provoke a persistent MHCII response in the brain. In contrast, equivalent amounts of monomeric α-synuclein fail to induce MHCII or persistent microglial activation, consistent with our results in primary microglia. Flow cytometry and immunohistochemical analyses reveal that MHCII-expressing cells are composed of both resident microglia as well as cells from the periphery that include monocytes, macrophages, and lymphocytes. Over time, α-Synuclein fibril exposures in the SNpc causes both axon loss as well as monocyte recruitment in the striatum. While these monocytes in the striatum initially lack MHCII expression, α-synuclein inclusions later form in nearby spiny projection neurons and MHCII expression becomes robust. In summary, in the rat α-synuclein fibril model, peripheral immune cell recruitment occurs prior to neurodegeneration and microglia, monocytes and macrophages all contribute to MHCII expression.

Emerging Structural Understanding of Amyloid Fibrils by Solid-State NMR

Amyloid structures at atomic resolution have remained elusive mainly because of their extensive polymorphism and because their polymeric properties have hampered structural studies by classical approaches. Progress in sample preparation, as well as solid-state NMR methods, recently enabled the determination of high-resolution 3D structures of fibrils such as the amyloid-β fibril, which is involved in Alzheimer's disease. Notably, the simultaneous but independent structure determination of Aβ1-42, a peptide that forms fibrillar deposits in the brain of Alzheimer patients, by two independent laboratories, which yielded virtually identical results, has highlighted how structures can be obtained that allow further functional investigation.

Bioassays and Inactivation of Prions

The experimental study of prions requires a model for their propagation. However, because prions lack nucleic acids, the simple techniques used to replicate bacteria and viruses are not applicable. For much of the history of prion research, time-consuming bioassays in animals were the only option for measuring infectivity. Although cell models and other in vitro tools for the propagation of prions have been developed, they all suffer limitations, and animal bioassays remain the gold standard for measuring infectivity. A wealth of recent data argues that both β-amyloid (Aβ) and tau proteins form prions that cause Alzheimer's disease, and α-synuclein forms prions that cause multiple system atrophy and Parkinson's disease. Cell and animal models that recapitulate some of the key features of cell-to-cell spreading and distinct strains of prions can now be measured.

α-Synuclein fibril-induced inclusion spread in rats and mice correlates with dopaminergic Neurodegeneration

Proteinaceous inclusions in neurons, composed primarily of α-synuclein, define the pathology in several neurodegenerative disorders. Neurons can internalize α-synuclein fibrils that can seed new inclusions from endogenously expressed α-synuclein. The factors contributing to the spread of pathology and subsequent neurodegeneration are not fully understood, and different compositions and concentrations of fibrils have been used in different hosts. Here, we systematically vary the concentration and length of well-characterized α-synuclein fibrils and determine their relative ability to induce inclusions and neurodegeneration in different hosts (primary neurons, C57BL/6J and C3H/HeJ mice, and Sprague Dawley rats). Using dynamic-light scattering profiles and other measurements to determine fibril length and concentration, we find that femptomolar concentrations of fibrils are sufficient to induce robust inclusions in primary neurons. However, a narrow and non-linear dynamic range characterizes fibril-mediated inclusion induction in axons and the soma. In mice, the C3H/HeJ strain is more sensitive to fibril exposures than C57BL/6J counterparts, with more inclusions and dopaminergic neurodegeneration. In rats, injection of fibrils into the substantia nigra pars compacta (SNpc) results in similar inclusion spread and dopaminergic neurodegeneration as injection of the fibrils into the dorsal striatum, with prominent inclusion spread to the amygdala and several other brain areas. Inclusion spread, particularly from the SNpc to the striatum, positively correlates with dopaminergic neurodegeneration. These results define biophysical characteristics of α-synuclein fibrils that induce inclusions and neurodegeneration both in vitro and in vivo, and suggest that inclusion spread in the brain may be promoted by a loss of neurons.

Pathogenic mechanisms of prion protein, amyloid-β and α-synuclein misfolding: the prion concept and neurotoxicity of protein oligomers

Proteinopathies represent a group of diseases characterized by the unregulated misfolding and aggregation of proteins. Accumulation of misfolded protein in the central nervous system (CNS) is associated with neurodegenerative diseases, such as the transmissible spongiform encephalopathies (or prion diseases), Alzheimer's disease, and the synucleinopathies (the most common of which is Parkinson's disease). Of these, the pathogenic mechanisms of prion diseases are particularly striking where the transmissible, causative agent of disease is the prion, or proteinaceous infectious particle. Prions are composed almost exclusively of PrP^Sc ; a misfolded isoform of the normal cellular protein, PrP^C , which is found accumulated in the CNS in disease. Today, mounting evidence suggests other aggregating proteins, such as amyloid-β (Aβ) and α-synuclein (α-syn), proteins associated with Alzheimer's disease and synucleinopathies, respectively, share similar biophysical and biochemical properties with PrP^Sc that influences how they misfold, aggregate, and propagate in disease. In this regard, the definition of a 'prion' may ultimately expand to include other pathogenic proteins. Unifying knowledge of folded proteins may also reveal common mechanisms associated with other features of disease that are less understood, such as neurotoxicity. This review discusses the common features Aβ and α-syn share with PrP and neurotoxic mechanisms associated with these misfolded proteins. Several proteins are known to misfold and accumulate in the central nervous system causing a range of neurodegenerative diseases, such as Alzheimer's, Parkinson's, and the prion diseases. Prions are transmissible misfolded conformers of the prion protein, PrP, which seed further generation of infectious proteins. Similar effects have recently been observed in proteins associated with Alzheimer's disease and the synucleinopathies, leading to the proposition that the definition of a 'prion' may ultimately expand to include other pathogenic proteins. Unifying knowledge of misfolded proteins may also reveal common mechanisms associated with other features of disease that are less understood, such as neurotoxicity.