α-Synuclein propagates from mouse brain to grafted dopaminergic neurons and seeds aggregation in cultured human cells

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.

Advantages of Vasoactive Intestinal Peptide for the Future Treatment of Parkinson's Disease

Parkinson's disease is the second most common neurodegenerative disorder in adults over the age of 65. The characteristic symptoms of Parkinson's disease, such as resting tremor, muscular rigidity, bradykinesia, postural instability and gait imbalance, are thought to be a result of the progressive degeneration of the dopaminergic neurons of the substantia nigra compacta, resulting in insufficient dopamine integrated signalling on GABAergic medium spiny neurons in the striatum. Despite tremendous research, the molecular mechanisms underlying the pathogenesis of neurodegeneration in Parkinson's disease have remained largely unknown. Although a variety of possible pathogenic mechanisms have been proposed over the years, including excessive release of oxygen free radicals, impairment of mitochondrial function, loss of trophic support, abnormal kinase activity, disruption of calcium homeostasis, dysfunction of protein degradation and neuroinflammation, the pathogenesis is still largely uncertain, and there is currently no effective cure for Parkinson's disease. To develop potential therapies for Parkinson's disease, inflammatory processes, mitochondrial dynamics, oxidative stress, production of reactive aldehydes, excitotoxicity and synucleinopathies are to be targeted. In this respect, vasoactive intestinal peptide has beneficial effects that provide an advantage for the treatment of Parkinson's disease. Vasoactive intestinal peptide is a major neuropeptide-neurotransmitter having antioxidant, anti-inflammatory, neurotropic, neuromodulator, and anti-apoptotic properties. In addition to its direct neuroprotective actions regulating the activity of astrocytes, microglia and brain mast cells, it also plays important roles for neuronal adaptation, maintenance and survival.

Exosome release and neuropathology induced by α-synuclein: new insights into protective mechanisms of Drp1 inhibition

Targeting alpha-synuclein (α-syn) as a therapeutic strategy for Parkinson’s disease (PD) has been intensively pursued largely due to its well-recognized pathogenic role. Since its discovery as the first familial link to PD over two decades ago, this protein has been associated with multiple neurotoxic mechanisms, such as mitochondrial dysfunction and impaired autophagic flux. We report here that blocking dynamin-related protein 1 (Drp1) improved both mitochondrial function and autophagic flux in experimental models of α-syn. Using rat dopaminergic neuronal cells with inducible wild-type human α-syn, we observed excessive mitochondrial fragmentation and increased Drp1 levels 48 h after gene induction. Functionally, these cells exhibited lower mitochondrial membrane potential, reduced ATP production rate and mitochondrial spare respiratory capacity, as well as increased levels of mitochondrial reactive oxygen species. To evaluate the protective role of Drp1 inhibition, we used three complementary approaches: gene silencing mediated by siRNA, overexpression of Drp1-dominant negative and the small molecule mitochondrial division inhibitor-1 (mdivi-1). Both morphological and functional defects induced by α-syn were attenuated by these strategies. Importantly, Drp1 inhibition reduced proteinase K-resistant α-syn aggregates. Based on that observation, we investigated the involvement of autophagy. Through a combination of stable autophagy reporter cells and immunoreactivity for LC3 and p62 in neuronal cells with either α-syn overexpression or treatment of human α-syn preformed fibrils (PFF), we observed that Drp1 inhibition abolished autophagic impairment induced by α-syn. Consistent with its role in improving autophagy function, Drp1 inhibition reduced exosome release and spread of α-syn pathology from neurons to neurons and from microglia to neurons. In summary, this study highlights new insights that Drp1 inhibition confers neuroprotection through both mitochondrial and autophagy-lysosomal pathways, further strengthening the therapeutic potential of targeting Drp1.

Pharmacological Targeting of Microglial Activation: New Therapeutic Approach

Mounting evidence suggests that neuroinflammation is not just a consequence but a vital contributor to the development and progression of Parkinson’s disease (PD). Microglia in particular, may contribute to the induction and modulation of inflammation in PD. Upon stimulation, microglia convert into activated phenotypes, which exist along a dynamic continuum and bear different immune properties depending on the disease stage and severity. Activated microglia release various factors involved in neuroinflammation, such as cytokines, chemokines, growth factors, reactive oxygen species (ROS), reactive nitrogen species (RNS), and prostaglandins (PGs). Further, activated microglia interact with other cell types (e.g., neurons, astrocytes and mast cells) and are closely associated with α-synuclein (α-syn) pathophysiology and iron homeostasis disturbance. Taken together, microglial activation and microglia-mediated inflammatory responses play essential roles in the pathogenesis of PD and elucidation of the complexity and imbalance of microglial activation may shed light on novel therapeutic approaches for PD.

Multiple System Atrophy: Recent Developments and Future Perspectives

Multiple system atrophy (MSA) is a rare and fatal neurodegenerative disorder characterized by a variable combination of parkinsonism, cerebellar impairment, and autonomic dysfunction. The pathologic hallmark is the accumulation of aggregated α-synuclein in oligodendrocytes, forming glial cytoplasmic inclusions, which qualifies MSA as a synucleinopathy together with Parkinson's disease and dementia with Lewy bodies. The underlying pathogenesis is still not well understood. Some symptomatic treatments are available, whereas neuroprotection remains an urgent unmet treatment need. In this review, we critically appraise significant developments of the past decade with emphasis on pathogenesis, diagnosis, prognosis, and treatment development. We further discuss unsolved questions and highlight some perspectives. © 2019 International Parkinson and Movement Disorder Society.

Different Heat Shock Proteins Bind α-Synuclein With Distinct Mechanisms and Synergistically Prevent Its Amyloid Aggregation

α-Synuclein (α-Syn) forms pathological amyloid aggregates deposited in Lewy bodies and Lewy neurites in the brain of Parkinson's disease (PD) patients. Heat shock proteins (Hsps) are the major components of the cellular chaperone network, which are responsible for preventing proteins from amyloid aggregation. Different Hsps were reported to interact with α-syn. However, the underlying mechanism of the interplay between α-syn and different Hsps remains unclear. Here, by combing NMR spectroscopy, electron microscope and other biochemical approaches, we systemically investigated the interaction between α-syn and three Hsps from different families including Hsp27, HDJ1, and Hsp104. We found that all three Hsps can weakly bind to α-syn and inhibit it from amyloid aggregation. Intriguingly, different Hsps recognize distinct regions of α-syn monomer, and act synergistically in chaperoning α-syn from fibril formation in sub-stoichiometry. Our results revealed the diverse binding mechanisms employed by different Hsps to tackle α-syn, and suggested that different Hsps form a network for cooperatively chaperoning α-syn from pathological aggregation.

Targeting Microglial and Neuronal Toll-like Receptor 2 in Synucleinopathies

Synucleinopathies are neurodegenerative disorders characterized by the progressive accumulation of α-synuclein (α-syn) in neurons and glia and include Parkinson's disease (PD) and dementia with Lewy bodies (DLB). In this review, we consolidate our key findings and recent studies concerning the role of Toll-like receptor 2 (TLR2), a pattern recognition innate immune receptor, in the pathogenesis of synucleinopathies. First, we address the pathological interaction of α-syn with microglial TLR2 and its neurotoxic inflammatory effects. Then, we show that neuronal TLR2 activation not only induces abnormal α-syn accumulation by impairing autophagy, but also modulates α-syn transmission. Finally, we demonstrate that administration of a TLR2 functional inhibitor improves the neuropathology and behavioral deficits of a synucleinopathy mouse model. Altogether, we present TLR2 modulation as a promising immunotherapy for synucleinopathies.

Converging Patterns of α-Synuclein Pathology in Multiple System Atrophy

We aimed to determine patterns of α-synuclein (α-syn) pathology in multiple system atrophy (MSA) using 70-µm-thick sections of 20 regions of the central nervous system of 37 cases with striato-nigral degeneration (SND) and 10 cases with olivo-ponto-cerebellar atrophy (OPCA). In SND cases with the shortest disease duration (phase 1), α-syn pathology was observed in striatum, lentiform nucleus, substantia nigra, brainstem white matter tracts, cerebellar subcortical white matter as well as motor cortex, midfrontal cortex, and sensory cortex. SND with increasing duration of disease (phase 2) was characterized by involvement of spinal cord and thalamus, while phase 3 was characterized by involvement of hippocampus and amygdala. Cases with the longest disease duration (phase 4) showed involvement of the visual cortex. We observed an increasing overlap of α-syn pathology with increasing duration of disease between SND and OPCA, and noted increasingly similar regional distribution patterns of α-syn pathology. The GBA variant, p.Thr408Met, was found to have an allele frequency of 6.94% in SND cases which was significantly higher compared with normal (0%) and other neurodegenerative disease pathologies (0.74%), suggesting that it is associated with MSA. Our findings indicate that SND and OPCA show distinct early foci of α-syn aggregations, but increasingly converge with longer disease duration to show overlapping patterns of α-syn pathology.

Noradrenaline is crucial for the substantia nigra dopaminergic cell maintenance

In Parkinson's disease, degeneration of substantia nigra dopaminergic neurons is accompanied by damage on other neuronal systems. A severe denervation is for example seen in the locus coerulean noradrenergic system. Little is known about the relation between noradrenergic and dopaminergic degeneration, and the effects of noradrenergic denervation on the function of the dopaminergic neurons of substantia nigra are not fully understood. In this study, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) was injected in rats, whereafter behavior, striatal KCl-evoked dopamine and glutamate releases, and immunohistochemistry were monitored at 3 days, 3 months, and 6 months. Quantification of dopamine-beta-hydroxylase-immunoreactive nerve fiber density in the cortex revealed a tendency towards nerve fiber regeneration at 6 months. To sustain a stable noradrenergic denervation throughout the experimental timeline, the animals in the 6-month time point received an additional DSP4 injection (2 months after the first injection). Behavioral examinations utilizing rotarod revealed that DSP4 reduced the time spent on the rotarod at 3 but not at 6 months. KCl-evoked dopamine release was significantly increased at 3 days and 3 months, while the concentrations were normalized at 6 months. DSP4 treatment prolonged both time for onset and reuptake of dopamine release over time. The dopamine degeneration was confirmed by unbiased stereology, demonstrating significant loss of tyrosine hydroxylase-immunoreactive neurons in the substantia nigra. Furthermore, striatal glutamate release was decreased after DSP4. In regards of neuroinflammation, reactive microglia were found over the substantia nigra after DSP4 treatment. In conclusion, long-term noradrenergic denervation reduces the number of dopaminergic neurons in the substantia nigra and affects the functionality of the nigrostriatal system. Thus, locus coeruleus is important for maintenance of nigral dopaminergic neurons.

Small Heat Shock Proteins, Big Impact on Protein Aggregation in Neurodegenerative Disease

Misfolding, aggregation, and aberrant accumulation of proteins are central components in the progression of neurodegenerative disease. Cellular molecular chaperone systems modulate proteostasis, and, therefore, are primed to influence aberrant protein-induced neurotoxicity and disease progression. Molecular chaperones have a wide range of functions from facilitating proper nascent folding and refolding to degradation or sequestration of misfolded substrates. In disease states, molecular chaperones can display protective or aberrant effects, including the promotion and stabilization of toxic protein aggregates. This seems to be dependent on the aggregating protein and discrete chaperone interaction. Small heat shock proteins (sHsps) are a class of molecular chaperones that typically associate early with misfolded proteins. These interactions hold proteins in a reversible state that helps facilitate refolding or degradation by other chaperones and co-factors. These sHsp interactions require dynamic oligomerization state changes in response to diverse cellular triggers and, unlike later steps in the chaperone cascade of events, are ATP-independent. Here, we review evidence for modulation of neurodegenerative disease-relevant protein aggregation by sHsps. This includes data supporting direct physical interactions and potential roles of sHsps in the stewardship of pathological protein aggregates in brain. A greater understanding of the mechanisms of sHsp chaperone activity may help in the development of novel therapeutic strategies to modulate the aggregation of pathological, amyloidogenic proteins. sHsps-targeting strategies including modulators of expression or post-translational modification of endogenous sHsps, small molecules targeted to sHsp domains, and delivery of engineered molecular chaperones, are also discussed.

Mechanistic insights into the protective roles of polyphosphate against amyloid cytotoxicity

This study provides novel insights into the mechanisms by which presence of polyP alters the formation, structural properties, and cytotoxic effects of α-synuclein fibers.

The universally abundant polyphosphate (polyP) accelerates fibril formation of disease-related amyloids and protects against amyloid cytotoxicity. To gain insights into the mechanism(s) by which polyP exerts these effects, we focused on α-synuclein, a well-studied amyloid protein, which constitutes the major component of Lewy bodies found in Parkinson’s disease. Here, we demonstrate that polyP is unable to accelerate the rate-limiting step of α-synuclein fibril formation but effectively nucleates fibril assembly once α-synuclein oligomers are formed. Binding of polyP to α-synuclein either during fibril formation or upon fibril maturation substantially alters fibril morphology and effectively reduces the ability of α-synuclein fibrils to interact with cell membranes. The effect of polyP appears to be α-synuclein fibril specific and successfully prevents the uptake of fibrils into neuronal cells. These results suggest that altering the polyP levels in the extracellular space might be a potential therapeutic strategy to prevent the spreading of the disease.

Molecular mechanisms of proteinopathies across neurodegenerative disease: a review

Background

Although there is a range of different symptoms across neurodegenerative diseases, they have been noted to have common pathogenic features. An archetypal feature shared between these diseases is protein misfolding; however, the mechanism behind the proteins abnormalities is still under investigation. There is an emerging hypothesis in the literature that the mechanisms that lead to protein misfolding may be shared across neurodegenerative processes, suggesting a common underlying pathology.

Main body

This review discusses the literature to date of the shared features of protein misfolding, failures in proteostasis, and potential propagation pathways across the main neurodegenerative disorders.

Conclusion

The current data suggests, despite overarching processes being shared, that the molecular events implicated in protein pathology are distinct across common neurodegenerative disorders.

Human NPCs can degrade α-syn fibrils and transfer them preferentially in a cell contact-dependent manner possibly through TNT-like structures

Parkinson's disease (PD) is the second most common neurodegenerative disorder whereby loss of midbrain dopaminergic neurons results in motor dysfunction. Transplantation of human induced pluripotent stem cells (iPSCs) into the brain of patients affected by PD is one of the therapeutic approaches that has gained interest to compensate for the degeneration of neurons and improve disease symptoms. However, only a part of transplanted cells can differentiate into mature neurons while the majority remains in undifferentiated state. Here we investigated whether human neuronal precursor cells (hNPCs) derived from iPSCs have an active role in α-synuclein (α-syn) pathology. Our findings demonstrate that α-syn fibrils are taken up by hNPCs and are preferentially localized in lysosomes where they can be degraded. However, α-syn fibrils are also transferred between hNPCs in a cell-to-cell contact dependent manner, and are found in tunneling nanotube (TNT)-like structures. Thus, NPCs can have a dual role in the progression of α-syn pathology, which should be considered in human transplants.

Establishing a framework for neuropathological correlates and glymphatic system functioning in Parkinson's disease

Recent evidence has advanced our understanding of the function of sleep to include removal of neurotoxic protein aggregates via the glymphatic system. However, most research on the glymphatic system utilizes animal models, and the function of waste clearance processes in humans remains unclear. Understanding glymphatic function offers new insight into the development of neurodegenerative diseases that result from toxic protein inclusions, particularly those characterized by neuropathological sleep dysfunction, like Parkinson's disease (PD). In PD, we propose that glymphatic flow may be compromised due to the combined neurotoxic effects of alpha-synuclein protein aggregates and deteriorated dopaminergic neurons that are linked to altered REM sleep, circadian rhythms, and clock gene dysfunction. This review highlights the importance of understanding the functional role of glymphatic system disturbance in neurodegenerative disorders and the subsequent clinical and neuropathological effects on disease progression. Future research initiatives utilizing noninvasive brain imaging methods in human subjects with PD are warranted, as in vivo identification of functional biomarkers in glymphatic system functioning may improve clinical diagnosis and treatment of PD.

Understanding the pathogenesis of multiple system atrophy: state of the art and future perspectives

Multiple System Atrophy (MSA) is a severe neurodegenerative disease clinically characterized by parkinsonism, cerebellar ataxia, dysautonomia and other motor and non-motor symptoms.

Although several efforts have been dedicated to understanding the causative mechanisms of the disease, MSA pathogenesis remains widely unknown.

The aim of the present review is to describe the state of the art about MSA pathogenesis, with a particular focus on alpha-synuclein accumulation and mitochondrial dysfunction, and to highlight future possible perspectives in this field.

In particular, this review describes the most widely investigated hypotheses explaining alpha-synuclein accumulation in oligodendrocytes, including SNCA expression, neuron-oligodendrocyte protein transfer, impaired protein degradation and alpha-synuclein spread mechanisms.

Afterwards, several recent achievements in MSA research involving mitochondrial biology are described, including the role of COQ2 mutations, Coenzyme Q10 reduction, respiratory chain dysfunction and altered mitochondrial mass.

Some hints are provided about alternative pathogenic mechanisms, including inflammation and impaired autophagy.

Finally, all these findings are discussed from a comprehensive point of view, putative explanations are provided and new research perspectives are suggested.

Overall, the present review provides a comprehensive and up-to-date overview of the mechanisms underlying MSA pathogenesis.

α-Synuclein Trafficking in Parkinson's Disease: Insights From Fly and Mouse Models

Protein aggregation and accumulation are common pathological hallmarks in neurodegenerative diseases. To efficiently clear and eliminate such aggregation becomes an important cellular strategy for cell survival. Lewy bodies inclusion and aggregation of α-Synuclein (α-Syn) during the pathogenesis of Parkinson's disease (PD) serve as a good example and are potentially linked to other pathological PD features such as progressive dopaminergic neuron cell death, behavioral defects, and nonmotor symptoms like anosmia, cognitive impairment, and depression. Years of research have revealed a variety of mechanisms underlying α-Syn aggregation, clearance, and spread. Particularly, vesicular routes associated with the trafficking of α-Syn, leading to its aggregation and accumulation, have been shown to play vital roles in PD pathogenesis. How α-Syn proteins propagate among cells in a prion-like manner, either from or to neurons and glia, via means of uptake or secretion, are questions under active investigation and have been of central interest in the field of PD study. This review covers components and pathways of possible vesicular routes involved in α-Syn trafficking. Events including but not limited to exocytosis and endocytosis will be discussed within the context of an overall cellular trafficking theme. Recent advances on α-Syn trafficking mechanisms and their significance in mediating PD pathogenesis will be thoroughly reviewed, ending with a discussion on the advantages and limitations of different animal PD models.

Can infections trigger alpha-synucleinopathies?

As synucleinopathies, Parkinson's disease (PD) and multiple system atrophy (MSA) are neurodegenerative diseases that involve the spread of pathogenic alpha-synuclein (αSyn) throughout the brain. Recent studies have suggested a role for αSyn as an antimicrobial peptide in response to PD- and MSA-related infections of peripheral tissues, including those in the respiratory, gastrointestinal, and urogenital systems. In this chapter, we examine epidemiological and experimental evidence for a role of peripheral microbial infections in triggering alpha-synucleinopathies. We propose a model of how infectious triggers, in conjunction with inflammatory, environmental, and genetic facilitators, may result in transfer of pathogenic αSyn strains from the periphery to the brain, where they propagate and spread. Finally, we discuss future research challenges and programs necessary to clarify the role of infections as triggers of PD and MSA and, ultimately, to prevent the onset of these diseases by infectious triggers.

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.

Studying Parkinson’s disease using Caenorhabditis elegans models in microfluidic devices

Parkinson’s disease (PD) is a progressive neurological disorder associated with the loss of dopaminergic neurons (DNs) in the substantia nigra and the widespread accumulation of α-synuclein (α-syn) protein, leading to motor impairments and eventual cognitive dysfunction. In-vitro cell cultures and in-vivo animal models have provided the opportunity to investigate the PD pathological hallmarks and identify different therapeutic compounds. However, PD pathogenesis and causes are still not well understood, and effective inhibitory drugs for PD are yet to be discovered. Biologically simple but pathologically relevant disease models and advanced screening technologies are needed to reveal the mechanisms underpinning protein aggregation and PD progression. For instance, Caenorhabditis elegans (C. elegans) offers many advantages for fundamental PD neurobehavioral studies including a simple, well-mapped, and accessible neuronal system, genetic homology to humans, body transparency and amenability to genetic manipulation. Several transgenic worm strains that exhibit multiple PD-related phenotypes have been developed to perform neuronal and behavioral assays and drug screening. However, in conventional worm-based assays, the commonly used techniques are equipment-intensive, slow and low in throughput. Over the past two decades, microfluidics technology has contributed significantly to automation and control of C. elegans assays. In this review, we focus on C. elegans PD models and the recent advancements in microfluidic platforms used for manipulation, handling and neurobehavioral screening of these models. Moreover, we highlight the potential of C. elegans to elucidate the in-vivo mechanisms of neuron-to-neuron protein transfer that may underlie spreading Lewy pathology in PD, and its suitability for in-vitro studies. Given the advantages of C. elegans and microfluidics technology, their integration has the potential to facilitate the investigation of disease pathology and discovery of potential chemical leads for PD.

α-Synuclein pathology in Parkinson's disease and related α-synucleinopathies

Over 20 years ago, the synaptic protein α-synuclein was identified as the primary component of the Lewy bodies (LBs) that are a sine qua non of Parkinson's disease (PD). Since that time, extensive research has demonstrated that α-synuclein pathology is not only a hallmark of PD, but can also cause neuronal dysfunction and death. Detailed staging of α-synuclein pathology in the brains of patients has revealed a progressive pattern of pathology that correlates with the symptoms of disease. Early in the disease course, PD patients exhibit motor dysfunction, and α-synuclein pathology at this stage is primarily found in regions controlling motor function. At later stages of disease as patients' cognitive function deteriorates, α-synuclein pathology can be found in cortical structures responsible for higher cognitive processing. The stereotypical progression of α-synuclein pathology through the brain over time suggests that there may be a physical transmission of pathological α-synuclein from one area of the brain to another. The transmission hypothesis posits that an initial seed of pathological α-synuclein in one neuron may be released and taken up by another vulnerable neuron and thereby initiate pathological misfolding of α-synuclein in the recipient neuron. In recent years, convergent evidence from various studies has indicated that pathological protein transmission can occur in the human brain. Cell and animal models based on the transmission hypothesis have shown not only that pathological α-synuclein can be transmitted from cell-to-cell, but that this pathology can lead to neuronal dysfunction and degeneration. The α-synuclein transmission hypothesis has profound implications for treatment of what is currently an intractable neurodegenerative disease. In this review, we explore the evidence for cell-to-cell transmission of pathological α-synuclein, the current understanding of how pathological α-synuclein can move to a new cell and template misfolding, and the therapeutic implications of α-synuclein transmission.

Demonstration of prion-like properties of mutant huntingtin fibrils in both in vitro and in vivo paradigms

In recent years, evidence has accumulated to suggest that mutant huntingtin protein (mHTT) can spread into healthy tissue in a prion-like fashion. This theory, however, remains controversial. To fully address this concept and to understand the possible consequences of mHTT spreading to Huntington’s disease pathology, we investigated the effects of exogenous human fibrillar mHTT (Q48) and huntingtin (HTT) (Q25) N-terminal fragments in three cellular models and three distinct animal paradigms. For in vitro experiments, human neuronal cells [induced pluripotent stem cell-derived GABA neurons (iGABA) and (SH-SY5Y)] as well as human THP1-derived macrophages, were incubated with recombinant mHTT fibrils. Recombinant mHTT and HTT fibrils were taken up by all cell types, inducing cell morphology changes and death. Variations in HTT aggregation were further observed following incubation with fibrils in both THP1 and SH-SY5Y cells. For in vivo experiments, adult wild-type (WT) mice received a unilateral intracerebral cortical injection and R6/2 and WT pups were administered fibrils via bilateral intraventricular injections. In both protocols, the injection of Q48 fibrils resulted in cognitive deficits and increased anxiety-like behavior. Post-mortem analysis of adult WT mice indicated that most fibrils had been degraded/cleared from the brain by 14 months post-surgery. Despite the absence of fibrils at these later time points, a change in the staining pattern of endogenous HTT was detected. A similar change was revealed in post-mortem analysis of the R6/2 mice. These effects were specific to central administration of fibrils, as mice receiving intravenous injections were not characterized by behavioral changes. In fact, peripheral administration resulted in an immune response mounting against the fibrils. Together, the in vitro and in vivo data indicate that exogenously administered mHTT is capable of both causing and exacerbating disease pathology.

Salidroside protects SH‑SY5Y from pathogenic α‑synuclein by promoting cell autophagy via mediation of mTOR/p70S6K signaling

The abnormal aggregation of α‑synuclein (α‑syn), which is an important pathological feature of Parkinson's disease (PD), is cytotoxic to dopaminergic neurons and causes cellular damage and apoptosis. Salidroside (SAL) is the main active component of the traditional Chinese medicine Rhodiola rosea. Previous research has demonstrated that SAL exerts cellular protection against cell senescence and neurodegeneration. However, the role and mechanism of action of SAL in PD remain unclear. The present study used overexpression of the wild‑type and the A53T mutation of α‑syn to induce a neuronal model of PD in SH‑SY5Y cells, which led to neuronal toxicity and a reduced cell proliferation index. SAL increased the cell proliferation index of both PD model groups in a dose‑dependent manner. Additionally, SAL alleviated pathogenic phosphorylated (Ser129) α‑syn expression as well as the ratio of microtubule‑associated proteins 1A/1B light chain 3 (LC3)‑I to LC3‑II expression, which is related to autophagic function. Furthermore, the results suggested that the underlying mechanism for the SAL‑induced protection of PD model neurons may involve the preservation of autophagy, which attenuates the phosphorylation of α‑syn in neurons predominantly via mTOR/p70S6K, and is independent of the PI3K/Akt signaling pathway.

Spreading of α-Synuclein and Tau: A Systematic Comparison of the Mechanisms Involved

Alzheimer's disease (AD) and Parkinson's disease (PD) are age-associated neurodegenerative disorders characterized by the misfolding and aggregation of alpha-synuclein (aSyn) and tau, respectively. The coexistence of aSyn and tau aggregates suggests a strong overlap between tauopathies and synucleinopathies. Interestingly, misfolded forms of aSyn and tau can propagate from cell to cell, and throughout the brain, thereby templating the misfolding of native forms of the proteins. The exact mechanisms involved in the propagation of the two proteins show similarities, and are reminiscent of the spreading characteristic of prion diseases. Recently, several models were developed to study the spreading of aSyn and tau. Here, we discuss the mechanisms involved, the similarities and differences between the spreading of the two proteins and that of the prion protein, and the different cell and animal models used for studying these processes. Ultimately, a deeper understanding of the molecular mechanisms involved may lead to the identification of novel targets for therapeutic intervention in a variety of devastating neurodegenerative diseases.

The Roles of Post-translational Modifications on α-Synuclein in the Pathogenesis of Parkinson's Diseases

Parkinson’s disease is the second most common neurodegenerative disorder. Although the pathogenesis of Parkinson’s disease is not entirely clear, the aberrant aggregation of α-synuclein has long been considered as an important risk factor. Elucidating the mechanisms that influence the aggregation of α-synuclein is essential for developing an effective diagnostic, preventative and therapeutic strategy to treat this devastating disease. The aggregation of α-synuclein is influenced by several post-translational modifications. Here, we summarized the major post-translational modifications (phosphorylation, ubiquitination, truncation, nitration, O-GlcNAcylation) of α-synuclein and the effect of these modifications on α-synuclein aggregation, which may provide potential targets for future therapeutics.

Found in Translation: The Utility of C. elegans Alpha-Synuclein Models of Parkinson's Disease

Parkinson's Disease (PD) is the second-most common neurodegenerative disease in the world, yet the fundamental and underlying causes of the disease are largely unknown, and treatments remain sparse and impotent. Several biological systems have been employed to model the disease but the nematode roundworm Caenorhabditis elegans (C. elegans) shows unique promise among these to disinter the elusive factors that may prevent, halt, and/or reverse PD phenotypes. Some of the most salient of these C. elegans models of PD are those that position the misfolding-prone protein alpha-synuclein (α-syn), a hallmark pathological component of PD, as the primary target for scientific interrogation. By transgenic expression of human α-syn in different tissues, including dopamine neurons and muscle cells, the primary cellular phenotypes of PD in humans have been recapitulated in these C. elegans models and have already uncovered multifarious genetic factors and chemical compounds that attenuate dopaminergic neurodegeneration. This review describes the paramount discoveries obtained through the application of different α-syn models of PD in C. elegans and highlights their established utility and respective promise to successfully uncover new conserved genetic modifiers, functional mechanisms, therapeutic targets and molecular leads for PD with the potential to translate to humans.

Propagation of Protein Aggregation in Neurodegenerative Diseases

Most common neurodegenerative diseases feature deposition of protein amyloids and degeneration of brain networks. Amyloids are ordered protein assemblies that can act as templates for their own replication through monomer addition. Evidence suggests that this characteristic may underlie the progression of pathology in neurodegenerative diseases. Many different amyloid proteins, including Aβ, tau, and α-synuclein, exhibit properties similar to those of infectious prion protein in experimental systems: discrete and self-replicating amyloid structures, transcellular propagation of aggregation, and transmissible neuropathology. This review discusses the contribution of prion phenomena and transcellular propagation to the progression of pathology in common neurodegenerative diseases such as Alzheimer's and Parkinson's. It reviews fundamental events such as cell entry, amplification, and transcellular movement. It also discusses amyloid strains, which produce distinct patterns of neuropathology and spread through the nervous system. These concepts may impact the development of new diagnostic and therapeutic strategies.

Trehalose as a promising therapeutic candidate for the treatment of Parkinson's disease

Parkinson's disease (PD) is a progressive movement disorder resulting primarily from loss of nigrostriatal dopaminergic neurons. PD is characterized by the accumulation of protein aggregates, and evidence suggests that aberrant protein deposition in dopaminergic neurons could be related to the dysregulation of the lysosomal autophagy pathway. The therapeutic potential of autophagy modulators has been reported in experimental models of PD. Trehalose is a natural disaccharide that has been considered as a new candidate for the treatment of neurodegenerative diseases. It has a chaperone-like activity, prevents protein misfolding or aggregation, and by promoting autophagy, contributes to the removal of accumulated proteins. In this review, we briefly summarize the role of aberrant autophagy in PD and the underlying mechanisms that lead to the development of this disease. We also discuss reports that used trehalose to counteract the neurotoxicity in PD, focusing particularly on the autophagy promoting, protein stabilization, and anti-neuroinflammatory effects of trehalose.

Astrocyte Heterogeneity: Impact to Brain Aging and Disease

Astrocytes, one of the largest glial cell population in the central nervous system (CNS), play a key function in several events of brain development and function, such as synapse formation and function, control of neurotransmitters release and uptake, production of trophic factors and control of neuronal survival. Initially described as a homogenous population, several evidences have pointed that astrocytes are highly heterogeneous, both morphologically and functionally, within the same region, and across different brain regions. Recent findings suggest that the heterogeneity in the expression profile of proteins involved in astrocyte function may predict the selective vulnerability of brain regions to specific diseases, as well as to the age-related cognitive decline. However, the molecular mechanisms underlying these changes, either in aging as well as in brain disease are scarce. Neuroinflammation, a hallmark of several neurodegenerative diseases and aging, is reported to have a dubious impact on glial activation, as these cells release pro- and anti-inflammatory cytokines and chemokines, anti-oxidants, free radicals, and neurotrophic factors. Despite the emerging evidences supporting that reactive astrocytes have a duality in their phenotype, neurotoxic or neuroprotective properties, depending on the age and stimuli, the underlying mechanisms of their activation, cellular interplays and the impact of regional astrocyte heterogeneity are still a matter of discussion. In this review article, we will summarize recent findings on astrocyte heterogeneity and phenotypes, as well as their likely impact for the brain function during aging and neural diseases. We will focus on the molecules and mechanisms triggered by astrocyte to control synapse formation in different brain regions. Finally, we will discuss new evidences on how the modulation of astrocyte phenotype and function could impact the synaptic deficits and glial dysfunction present in aging and pathological states.

Manganese promotes the aggregation and prion-like cell-to-cell exosomal transmission of α-synuclein

The aggregation of α-synuclein (αSyn) is considered a key pathophysiological feature of certain neurodegenerative disorders, collectively termed synucleinopathies. Given that a prion-like, cell-to-cell transfer of misfolded αSyn has been recognized in the spreading of αSyn pathology in synucleinopathies, we investigated the biological mechanisms underlying the propagation of the disease with respect to environmental neurotoxic stress. Considering the potential role of the divalent metal manganese (Mn2+) in protein aggregation, we characterized its effect on αSyn misfolding and transmission in experimental models of Parkinson's disease. In cultured dopaminergic neuronal cells stably expressing wild-type human αSyn, misfolded αSyn was secreted through exosomes into the extracellular medium upon Mn2+ exposure. These exosomes were endocytosed through caveolae into primary microglial cells, thereby mounting neuroinflammatory responses. Furthermore, Mn2+-elicited exosomes exerted a neurotoxic effect in a human dopaminergic neuronal model (LUHMES cells). Moreover, bimolecular fluorescence complementation (BiFC) analysis revealed that Mn2+ accelerated the cell-to-cell transmission of αSyn, resulting in dopaminergic neurotoxicity in a mouse model of Mn2+ exposure. Welders exposed to Mn2+ had increased misfolded αSyn content in their serum exosomes. Stereotaxically delivering αSyn-containing exosomes, isolated from Mn2+-treated αSyn-expressing cells, into the striatum initiated Parkinsonian-like pathological features in mice. Together, these results indicate that Mn2+ exposure promotes αSyn secretion in exosomal vesicles, which subsequently evokes proinflammatory and neurodegenerative responses in both cell culture and animal models.

α-Synuclein misfolding and aggregation: Implications in Parkinson's disease pathogenesis

α-Synuclein (α-Syn) has been extensively studied for its structural and biophysical properties owing to its pathophysiological role in Parkinson's disease (PD). Lewy bodies and Lewy neurites are the pathological hallmarks of PD and contain α-Syn aggregates as their major component. It was therefore hypothesized that α-Syn aggregation is actively associated with PD pathogenesis. The central role of α-Syn aggregation in PD is further supported by the identification of point mutations in α-Syn protein associated with rare familial forms of PD. However, the correlation between aggregation propensities of α-Syn mutants and their association with PD phenotype is not straightforward. Recent evidence suggested that oligomers, formed during the initial stages of aggregation, are the potent neurotoxic species causing cell death in PD. However, the heterogeneous and unstable nature of these oligomers limit their detailed characterization. α-Syn fibrils, on the contrary, are shown to be the infectious agents and propagate in a prion-like manner. Although α-Syn is an intrinsically disordered protein, it exhibits remarkable conformational plasticity by adopting a range of structural conformations under different environmental conditions. In this review, we focus on the structural and functional aspects of α-Syn and role of potential factors that may contribute to the underlying mechanism of synucleinopathies. This information will help to identify novel targets and develop specific therapeutic strategies to combat Parkinson's and other protein aggregation related neurodegenerative diseases.

The prion-like spreading of α-synuclein: From in vitro to in vivo models of Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease. PD is characterized by the loss of dopaminergic neurons, primarily in brain regions that control motor functions, thereby leading to motor impairments in the patients. Pathological aggregated forms of the synaptic protein, α-synuclein (α-syn), are involved in the generation and progression of PD. In PD brains, α-syn accumulates inside neurons and propagates from cell-to-cell in a prion-like manner. In this review, we discuss the in vitro and in vivo models used to study the prion-like properties of α-syn and related findings. In particular, we focus on the different mechanisms of α-syn spreading, which could be relevant for the development of alternative therapeutic approaches for PD treatment.

Microglia as modulators of exosomal alpha-synuclein transmission

Recent researches regarding to exosomal involvement in alpha-synuclein (α-syn) transmission relating to the pathological process of Parkinson's disease (PD) have attracted considerable attention. It is highly desirable to make clear the diffusion process and cellular uptake of α-syn-associated exosomes and the underlying mechanism of exosomes-involved communication in the synucleinopathy pathogenesis. To determine the contribution of α-syn-associated exosomes to the initiation and progression of PD, plasma exosomes derived from PD patients were stereotaxically injected into the striatum of mice brains. Exosomes extracted from plasma diagnosed with PD contained monomeric and oligomeric α-syn. Here, we found that microglia display a high potency for uptake of plasma exosomes derived from PD patients, and therefore could be activated by exogenous exosomes in vitro and in vivo. In addition, immunofluorescent double staining verified the transfer of exogenous human exosomal α-syn to neurons. The release of human exosomal α-syn from microglia may facilitate this propagation. Finally, we described a mechanism underlying this potential role of microglia in the transmission of exosomal α-syn. Specifically, exogenous exosomes were found to dysregulate autophagy of the BV2 mouse microglia cell line with presentation of increased accumulation of intracellular α-syn and accelerated secretion of α-syn into extracellular space. These results suggest that microglia play a crucial role in the transmission of α-syn via exosomal pathways, in additional to idea that the progression of PD may be altered by the modulation of exosome secretion and/or microglial states.

The primary neuronal cells are more resistant than PC12 cells to α-synuclein toxic aggregates

BACKGROUND

Alpha-synuclein (αSN) is an abundant presynaptic brain protein that its aggregated species believed to play pivotal roles in the development of neurodegenerative diseases, especially Parkinson's disease (PD). In this study, we compared the response of primary neuronal cells with a well-known cell line model, PC12, against the toxic aggregates of αSN.

METHODS

Primary hippocampal neurons (PHNs) were isolated from 17 to 18 days old rat embryos. Fibrillization was induced in recombinant αSN and monitored by standard methods. The toxicity of different aggregates of αSN on the treated cells was then studied. Furthermore, changes in the intracellular reactive oxygen species (ROS) and Ca²⁺ levels were also compared in two kinds of treated cells. We also studied the gene expression profile of certain Ca²⁺ channels and carriers using the GEO2 database.

RESULTS

The viability rate was significantly lower in PC12 versus PHNs, in response to αSN. This is while the intracellular ROS and Ca²⁺ levels were significantly increased in both cell types. Analysis of microarray data indicated that some factors involved in Ca²⁺ hemostasis may face significant changes in the PD condition.

CONCLUSION

By putting these data together, it is clear that PHN is more resistant than PC12 toward αSN cytotoxicity even in the presence of rising cytoplasmic ROS and Ca²⁺ levels. Exploring the supporting mechanisms which PHN uses to be more resistant to αSN cytotoxicity can help to open a roadmap toward therapeutic plans in PD and other synucleinopathy disorders.

Identification of Differentially Expressed Genes and Long Noncoding RNAs Associated with Parkinson's Disease

Objectives

This study aims to determine differentially expressed genes (DEGs) and long noncoding RNAs (lncRNAs) associated with Parkinson's disease (PD) using a microarray.

Methods

We downloaded the microarray data GSE6613 from the Gene Expression Omnibus, which included 105 samples. We selected 72 samples comprising 22 healthy control blood samples and 50 PD blood samples for further analysis. Later, we used Limma to screen DEGs and differentially expressed lncRNAs (DElncRNAs) and estimated their functions by the Gene Ontology (GO). Besides, the competing endogenous RNA (ceRNA) network, including microRNAs, lncRNAs, and mRNAs, was constructed to elucidate the regulatory mechanism. Furthermore, we performed the KEGG pathway enrichment with mRNAs in the ceRNA regulatory network and constructed a final network, including pathways, mRNAs, microRNAs, and lncRNAs.

Results

Overall, we obtained 394 DEGs, including 207 upregulated DEGs and 187 downregulated DEGs, and 7 DElncRNAs, including 2 upregulated DElncRNAs and 5 downregulated DElncRNAs. Insulin-like growth factor-1 receptor (IGF1R) was considerably enriched in the endocytosis pathway. In the ceRNA regulation network, IGF1R was the target of hsa-miR-133b and lncRNAs of XIST, and PART1 could also be the target of hsa-miR-133b. While the upregulated DEGs were enriched in the GO terms of the cytoskeleton, cytoskeletal part, and microtubule cytoskeleton, the downregulated DEGs were enriched in the immune response. PRKACA was markedly enriched in numerous pathways, including the MAPK and insulin signaling pathways.

Conclusions

IGF1R, PRKACA, and lncRNA-XIST could be potentially involved in PD, and these diverse molecular mechanisms could support the development of the similar treatment for PD.

Engineering synucleinopathy-resistant human dopaminergic neurons by CRISPR-mediated deletion of the SNCA gene

An emerging treatment for Parkinson's disease (PD) is cell replacement therapy. Authentic midbrain dopaminergic (mDA) neuronal precursors can be differentiated from human embryonic stem cells (hESCs) and human induced pluripotent stem cells (iPSCs). These laboratory-generated mDA cells have been demonstrated to mature into functional dopaminergic neurons upon transplantation into preclinical models of PD. However, clinical trials with human fetal mesenchephalic cells have shown that cell replacement grafts in PD are susceptible to Lewy body formation suggesting host-to-graft transfer of α-synuclein pathology. Here, we have used CRISPR/Cas9n technology to delete the endogenous SNCA gene, encoding for α-synuclein, in a clinical-grade hESC line to generate SNCA+/- and SNCA-/- cell lines. These hESC lines were first differentiated into mDA neurons, and then challenged with recombinant α-synuclein preformed fibrils (PFFs) to seed the formation for Lewy-like pathology as measured by phosphorylation of serine-129 of α-synuclein (pS129-αSyn). Wild-type neurons were fully susceptible to the formation of protein aggregates positive for pS129-αSyn, while SNCA+/- and SNCA-/- neurons exhibited significant resistance to the formation of this pathological mark. This work demonstrates that reducing or completely removing SNCA alleles by CRISPR/Cas9n-mediated gene editing confers a measure of resistance to Lewy pathology.

Degeneration of dopaminergic circuitry influences depressive symptoms in Lewy body disorders

Aims

Depression is commonly observed even in prodromal stages of Lewy body disorders (LBD), and is associated with cognitive impairment and a faster rate of cognitive decline. Given the role of dopamine in the development of movement disorders, but also in motivation and reward, we investigated neurodegenerative pathology in dopaminergic circuitry in Parkinson's disease (PD), PD with dementia (PDD) and dementia with Lewy bodies (DLB) patients in relation to depressive symptoms.

Methods

α‐synuclein, hyperphosphorylated tau and amyloid‐beta pathology was assessed in 17 DLB, 14 PDD and 8 PD cases within striatal and midbrain subregions, with neuronal cell density assessed in substantia nigra and ventral tegmental area. Additionally, we used a structural equation modeling (SEM) approach to investigate the extent to which brain connectivity might influence the deposition of pathological proteins within dopaminergic pathways.

Results

A significantly higher α‐synuclein burden was observed in the substantia nigra (P = 0.006), ventral tegmental area (P = 0.011) and nucleus accumbens (P = 0.031) in LBD patients with depression. Significant negative correlations were observed between cell density in substantia nigra with Lewy body (LB) Braak stage (P = 0.013), whereas cell density in ventral tegmental area showed negative correlations with LB Braak stage (P = 0.026) and neurofibrillary tangle Braak stage (P = 0.007).

Conclusions

Dopaminergic α‐synuclein pathology appears to drive depression. Selective targeting of dopaminergic pathways may therefore provide symptomatic relief for depressive symptoms in LBD patients.

Propagation of α-Synuclein Strains within Human Reconstructed Neuronal Network

Reappraisal of neuropathological studies suggests that pathological hallmarks of Alzheimer’s disease and Parkinson’s disease (PD) spread progressively along predictable neuronal pathways in the human brain through unknown mechanisms. Although there is much evidence supporting the prion-like propagation and amplification of α-synuclein (α-Syn) in vitro and in rodent models, whether this scenario occurs in the human brain remains to be substantiated. Here we reconstructed in microfluidic devices corticocortical neuronal networks using human induced pluripotent stem cells derived from a healthy donor. We provide unique experimental evidence that different strains of human α-Syn disseminate in “wild-type” human neuronal networks in a prion-like manner. We show that two distinct α-Syn strains we named fibrils and ribbons are transported, traffic between neurons, and trigger to different extents, in a dose- and structure-dependent manner, the progressive accumulation of PD-like pathological hallmarks. We further demonstrate that seeded aggregation of endogenous soluble α-Syn affects synaptic integrity and mitochondria morphology.

•

Different α-Syn strains propagate within WT human iPSC-derived cortical neuronal networks

•

α-Syn strains differentially seed endogenous WT α-Syn forming LB/LN-like structures

•

Phospho-α-Syn endogenous aggregates resist degradation and accumulate in cytoplasm

•

Accumulation of phospho-α-Syn induces early neuronal dysfunctions

Emerging Developments in Targeting Proteotoxicity in Neurodegenerative Diseases

The most common neurodegenerative diseases are Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease, frontotemporal lobar degeneration, and the motor neuron diseases, with AD affecting approximately 6% of people aged 65 years and older, and PD affecting approximately 1% of people aged over 60 years. Specific proteins are associated with these neurodegenerative diseases, as determined by both immunohistochemical studies on post-mortem tissue and genetic screening, where protein misfolding and aggregation are key hallmarks. Many of these proteins are shown to misfold and aggregate into soluble non-native oligomers and large insoluble protein deposits (fibrils and plaques), both of which may exert a toxic gain of function. Proteotoxicity has been examined intensively in cell culture and in in vivo models, and clinical trials of methods to attenuate proteotoxicity are relatively new. Therapies to enhance cellular protein quality control mechanisms such as upregulation of chaperones and clearance/degradation pathways, as well as immunotherapies against toxic protein conformations, are being actively pursued. In this article, we summarize the common pathophysiology of neurodegenerative disease, and review therapies in early-phase clinical trials that target the proteotoxic component of several neurodegenerative diseases.

Alpha-synuclein is highly prone to distribution in the hippocampus and midbrain in tree shrews, and its fibrils seed Lewy body-like pathology in primary neurons

The Chinese tree shrew (TS) has many unique advantages that make it suitable for use as an experimental animal model for human disease including moderate body size, low cost of feeding, short reproductive cycle and lifespan, and close phylogenetic relationship to primates. Our previous studies have shown that TS treated with the mitochondrial inhibitor MPTP displayed classic Parkinsonian symptoms. Additionally, the structure of TS alpha-synuclein (α-syn) is highly homologous to that found in humans. Previous studies have concluded that misfolded, fibrillar α-syn is a hallmark of α-synucleinopathies. In this study, we examined the distribution and expression levels of α-syn in different TS brain regions. We also obtained recombinant TS α-syn protein to study its aggregation and cytotoxic properties in primary neurons. Our results showed that α-syn was expressed in numerous different brain regions in TS but was most abundant in the hippocampus and midbrain. The recombinant α-syn of TS displayed straight fibrils when incubated for 72 h in vitro, which is very similar to human α-syn. When exposed to primary neurons, the TS and human α-syn fibrils led to cytotoxicity and Lewy-like pathology. Our findings indicated that TS could be a potential animal model to study the pathology of α-synucleinopathies.

Parkinson's disease and multiple system atrophy have distinct α-synuclein seed characteristics

Parkinson's disease (PD) and multiple system atrophy (MSA) are distinct clinical syndromes characterized by the pathological accumulation of α-synuclein (α-syn) protein fibrils in neurons and glial cells. These disorders and other neurodegenerative diseases may progress via prion-like mechanisms. The prion model of propagation predicts the existence of "strains" that link pathological aggregate structure and neuropathology. Prion strains are aggregated conformers that stably propagate in vivo and cause disease with defined incubation times and patterns of neuropathology. Indeed, tau prions have been well defined, and research suggests that both α-syn and β-amyloid may also form strains. However, there is a lack of studies characterizing PD- versus MSA-derived α-syn strains or demonstrating stable propagation of these unique conformers between cells or animals. To fill this gap, we used an assay based on FRET that exploits a HEK293T "biosensor" cell line stably expressing α-syn (A53T)-CFP/YFP fusion proteins to detect α-syn seeds in brain extracts from PD and MSA patients. Both soluble and insoluble fractions of MSA extracts had robust seeding activity, whereas only the insoluble fractions of PD extracts displayed seeding activity. The morphology of MSA-seeded inclusions differed from PD-seeded inclusions. These differences persisted upon propagation of aggregation to second-generation biosensor cells. We conclude that PD and MSA feature α-syn conformers with very distinct biochemical properties that can be transmitted to α-syn monomers in a cell system. These findings are consistent with the idea that distinct α-syn strains underlie PD and MSA and offer possible directions for synucleinopathy diagnosis.

New Perspectives on Roles of Alpha-Synuclein in Parkinson's Disease

Parkinson’s disease (PD) is one of the synucleinopathies spectrum of disorders typified by the presence of intraneuronal protein inclusions. It is primarily composed of misfolded and aggregated forms of alpha-synuclein (α-syn), the toxicity of which has been attributed to the transition from an α-helical conformation to a β-sheetrich structure that polymerizes to form toxic oligomers. This could spread and initiate the formation of “LB-like aggregates,” by transcellular mechanisms with seeding and subsequent permissive templating. This hypothesis postulates that α-syn is a prion-like pathological agent and responsible for the progression of Parkinson’s pathology. Moreover, the involvement of the inflammatory response in PD pathogenesis has been reported on the excessive microglial activation and production of pro-inflammatory cytokines. At last, we describe several treatment approaches that target the pathogenic α-syn protein, especially the oligomers, which are currently being tested in advanced animal experiments or are already in clinical trials. However, there are current challenges with therapies that target α-syn, for example, difficulties in identifying varying α-syn conformations within different individuals as well as both the cost and need of long-duration large trials.

Patient-Derived Induced Pluripotent Stem Cells and Organoids for Modeling Alpha Synuclein Propagation in Parkinson's Disease

Parkinson's disease (PD) is an age-associated, progressive neurodegenerative disorder characterized by motor impairment and in some cases cognitive decline. Central to the disease pathogenesis of PD is a small, presynaptic neuronal protein known as alpha synuclein (a-syn), which tends to accumulate and aggregate in PD brains as Lewy bodies or Lewy neurites. Numerous in vitro and in vivo studies confirm that a-syn aggregates can be propagated from diseased to healthy cells, and it has been suggested that preventing the spread of pathogenic a-syn species can slow PD progression. In this review, we summarize the works of recent literature elucidating mechanisms of a-syn propagation, and discussed the advantages in using patient-derived induced pluripotent stem cells (iPSCs) and/or induced neurons to study a-syn transmission.

Converging Patterns of α-Synuclein Pathology in Multiple System Atrophy

We aimed to determine patterns of α-synuclein (α-syn) pathology in multiple system atrophy (MSA) using 70-µm-thick sections of 20 regions of the central nervous system of 37 cases with striato-nigral degeneration (SND) and 10 cases with olivo-ponto-cerebellar atrophy (OPCA). In SND cases with the shortest disease duration (phase 1), α-syn pathology was observed in striatum, lentiform nucleus, substantia nigra, brainstem white matter tracts, cerebellar subcortical white matter as well as motor cortex, midfrontal cortex, and sensory cortex. SND with increasing duration of disease (phase 2) was characterized by involvement of spinal cord and thalamus, while phase 3 was characterized by involvement of hippocampus and amygdala. Cases with the longest disease duration (phase 4) showed involvement of the visual cortex. We observed an increasing overlap of α-syn pathology with increasing duration of disease between SND and OPCA, and noted increasingly similar regional distribution patterns of α-syn pathology. The GBA variant, p.Thr408Met, was found to have an allele frequency of 6.94% in SND cases which was significantly higher compared with normal (0%) and other neurodegenerative disease pathologies (0.74%), suggesting that it is associated with MSA. Our findings indicate that SND and OPCA show distinct early foci of α-syn aggregations, but increasingly converge with longer disease duration to show overlapping patterns of α-syn pathology.