Parkinson's disease (PD), an adult neurodegenerative disorder, has been clinically linked to the lysosomal storage disorder Gaucher disease (GD), but the mechanistic connection is not known. Here, we ...show that functional loss of GD-linked glucocerebrosidase (GCase) in primary cultures or human iPS neurons compromises lysosomal protein degradation, causes accumulation of α-synuclein (α-syn), and results in neurotoxicity through aggregation-dependent mechanisms. Glucosylceramide (GlcCer), the GCase substrate, directly influenced amyloid formation of purified α-syn by stabilizing soluble oligomeric intermediates. We further demonstrate that α-syn inhibits the lysosomal activity of normal GCase in neurons and idiopathic PD brain, suggesting that GCase depletion contributes to the pathogenesis of sporadic synucleinopathies. These findings suggest that the bidirectional effect of α-syn and GCase forms a positive feedback loop that may lead to a self-propagating disease. Therefore, improved targeting of GCase to lysosomes may represent a specific therapeutic approach for PD and other synucleinopathies.
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► Mechanistic link between Gaucher disease and synucleinopathies such as Parkinson's ► Lysosomal glucosylceramide accumulation in Gaucher stabilizes α-synuclein oligomers ► α-synuclein inhibits lysosomal trafficking of glucocerebrosidase in synucleinopathies ► Glucocerebrosidase may be a therapeutic target in synucleinopathies
Aggregation of alpha-synuclein (αsyn) and resulting cytotoxicity is a hallmark of sporadic and familial Parkinson's disease (PD) as well as dementia with Lewy bodies, with recent evidence implicating ...oligomeric and pre-fibrillar forms of αsyn as the pathogenic species. Recent in vitro studies support the idea of transcellular spread of extracellular, secreted αsyn across membranes. The aim of this study is to characterize the transcellular spread of αsyn oligomers and determine their extracellular location.
Using a novel protein fragment complementation assay where αsyn is fused to non-bioluminescent amino-or carboxy-terminus fragments of humanized Gaussia Luciferase we demonstrate here that αsyn oligomers can be found in at least two extracellular fractions: either associated with exosomes or free. Exosome-associated αsyn oligomers are more likely to be taken up by recipient cells and can induce more toxicity compared to free αsyn oligomers. Specifically, we determine that αsyn oligomers are present on both the outside as well as inside of exosomes. Notably, the pathway of secretion of αsyn oligomers is strongly influenced by autophagic activity.
Our data suggest that αsyn may be secreted via different secretory pathways. We hypothesize that exosome-mediated release of αsyn oligomers is a mechanism whereby cells clear toxic αsyn oligomers when autophagic mechanisms fail to be sufficient. Preventing the early events in αsyn exosomal release and uptake by inducing autophagy may be a novel approach to halt disease spreading in PD and other synucleinopathies.
The normal functions and pathologic facets of the small presynaptic protein α-synuclein (α-syn) are of exceptional interest. In previous studies, we found that α-syn attenuates synaptic ...exo/endocytosis 1, 2; however, underlying mechanisms remain unknown. More recent evidence suggests that α-syn exists as metastable multimers and not solely as a natively unfolded monomer 3–8. However, conformations of α-syn at synapses—its physiologic locale—are unclear, and potential implications of such higher-order conformations to synaptic function are unknown. Exploring α-syn conformations and synaptic function in neurons, we found that α-syn promptly organizes into physiological multimers at synapses. Furthermore, our experiments indicate that α-syn multimers cluster synaptic vesicles and restrict their motility, suggesting a novel role for these higher-order structures. Supporting this, α-syn mutations that disrupt multimerization also fail to restrict synaptic vesicle motility or attenuate exo/endocytosis. We propose a model in which α-syn multimers cluster synaptic vesicles, restricting their trafficking and recycling, and consequently attenuate neurotransmitter release.
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•Presynaptic α-synuclein assembles into dynamic multimers that bind synaptic vesicles•Multimers restrict synaptic vesicle trafficking and attenuate neurotransmission•Subtle disruptions of N terminus α-synuclein helices attenuate multimeric assemblies•Such disruptions also abrogate effects on vesicle trafficking and neurotransmission
While α-synuclein suppresses neurotransmitter release, underlying mechanisms are unclear. Wang et al. show that α-synuclein organizes into multimers that bind to and cluster synaptic vesicles (SVs), restricting SV trafficking. They propose a model wherein α-synuclein multimers cluster SVs, inhibiting SV recycling and attenuating neurotransmission.
Protein misfolding, aggregation and deposition are common disease mechanisms in many neurodegenerative diseases including Parkinson’s disease (PD). Accumulation of damaged or abnormally modified ...proteins may lead to perturbed cellular function and eventually to cell death. Thus, neurons rely on elaborated pathways of protein quality control and removal to maintain intracellular protein homeostasis. Molecular chaperones, the ubiquitin–proteasome system (UPS) and the autophagy–lysosomal pathway (ALP) are critical pathways that mediate the refolding or removal of abnormal proteins. The successive failure of these protein degradation pathways, as a cause or consequence of early pathological alterations in vulnerable neurons at risk, may present a key step in the pathological cascade that leads to spreading neurodegeneration. A growing number of studies in disease models and patients have implicated dysfunction of the UPS and ALP in the pathogenesis of Parkinson’s disease and related disorders. Deciphering the exact mechanism by which the different proteolytic systems contribute to the elimination of pathogenic proteins, like α-synuclein, is therefore of paramount importance. We herein review the role of protein degradation pathways in Parkinson’s disease and elaborate on the different contributions of the UPS and the ALP to the clearance of altered proteins. We examine the interplay between different degradation pathways and provide a model for the role of the UPS and ALP in the evolution and progression of α-synuclein pathology. With regards to exciting recent studies we also discuss the putative potential of using protein degradation pathways as novel therapeutic targets in Parkinson’s disease.
Misfolding and aggregation of alpha‐synuclein (α‐synuclein) with concomitant cytotoxicity is a hallmark of Lewy body related disorders such as Parkinson’s disease, dementia with Lewy bodies, and ...multiple system atrophy. Although it plays a pivotal role in pathogenesis and disease progression, the function of α‐synuclein and the molecular mechanisms underlying α‐synuclein‐induced neurotoxicity in these diseases are still elusive. Many in vitro and in vivo experimental models mimicking α‐synuclein pathology such as oligomerization, toxicity and more recently neuronal propagation have been generated over the years. In particular, cellular models have been crucial for our comprehension of the pathogenic process of the disease and are beneficial for screening of molecules capable of modulating α‐synuclein toxicity. Here, we review α‐synuclein based cell culture models that reproduce some features of the neuronal populations affected in patients, from basic unicellular organisms to mammalian cell lines and primary neurons, to the cutting edge models of patient‐specific cell lines. These reprogrammed cells known as induced pluripotent stem cells (iPSCs) have garnered attention because they closely reproduce the characteristics of neurons found in patients and provide a valuable tool for mechanistic studies. We also discuss how different cell models may constitute powerful tools for high‐throughput screening of molecules capable of modulating α‐synuclein toxicity and prevention of its propagation.
This article is part of the Special Issue “Synuclein”.
The ability to model cellular characteristics of synucleinopathies in a petri dish can offer myriad opportunities for studying pathogenic mechanisms and aid the development and validation of future pharmacological interventions. Here, we review available cellular models to study α‐synuclein toxicity from basic unicellular organisms to mammalian cell linesand primary neurons, to more recent cellular systems such as human induced pluripotent stem cells. We describe the strategies employed to monitor and measure α‐synuclein–induced dysfunction and discuss how these models constitute powerful tools for high‐throughput screening of molecules capable of modulating α‐synuclein toxicity and preventing propagation.
This article is part of the Special Issue “Synuclein”.
Objective
Excessive inflammation in the central nervous system (CNS) and the periphery can result in neurodegeneration and parkinsonism. Recent evidence suggests that immune responses in Parkinson ...disease patients are dysregulated, leading to an increased inflammatory reaction to unspecific triggers. Although α‐synuclein pathology is the hallmark of Parkinson disease, it has not been investigated whether pathologic α‐synuclein is a specific trigger for excessive inflammatory responses in Parkinson disease.
Methods
We investigated the immune response of primary human monocytes and a microglial cell line to pathologic forms of α‐synuclein by assessing cytokine release upon exposure.
Results
We show that pathologic α‐synuclein (mutations, aggregation) results in a robust inflammatory activation of human monocytes and microglial BV2 cells. The activation is conformation‐ dependent, with increasing fibrillation and early onset mutations having the strongest effect on immune activation. We also found that activation of immune cells by extracellular α‐synuclein is potentiated by extracellular vesicles, possibly by facilitating the uptake of α‐synuclein. Blood extracellular vesicles from Parkinson disease patients induce a stronger activation of monocytes than blood extracellular vesicles from healthy controls. Most importantly, monocytes from Parkinson disease patients are dysregulated and hyperactive in response to stimulation with pathologic α‐synuclein. Furthermore, we demonstrate that α‐synuclein pathology in the CNS is sufficient to induce the monocyte dysregulation in the periphery of a mouse model.
Interpretation
Taken together, our data suggest that α‐synuclein pathology and dysregulation of monocytes in Parkinson disease can act together to induce excessive inflammatory responses to α‐synuclein. ANN NEUROL 2019;86:593–606
Misfolding, oligomerization, and fibrillization of alpha-synuclein are thought to be central events in the onset and progression of Parkinson's disease (PD) and related disorders. Although fibrillar ...alpha-synuclein is a major component of Lewy bodies (LBs), recent data implicate prefibrillar, oligomeric intermediates as the toxic species. However, to date, oligomeric species have not been identified in living cells.
Here we used bimolecular fluorescence complementation (BiFC) to directly visualize alpha-synuclein oligomerization in living cells, allowing us to study the initial events leading to alpha-synuclein oligomerization, the precursor to aggregate formation. This novel assay provides us with a tool with which to investigate how manipulations affecting alpha-synuclein aggregation affect the process over time. Stabilization of alpha-synuclein oligomers via BiFC results in increased cytotoxicity, which can be rescued by Hsp70 in a process that reduces the formation of alpha-synuclein oligomers. Introduction of PD-associated mutations in alpha-synuclein did not affect oligomer formation but the biochemical properties of the mutant alpha-synuclein oligomers differ from those of wild type alpha-synuclein.
This novel application of the BiFC assay to the study of the molecular basis of neurodegenerative disorders enabled the direct visualization of alpha-synuclein oligomeric species in living cells and its modulation by Hsp70, constituting a novel important tool in the search for therapeutics for synucleinopathies.
Misfolding and aggregation of the presynaptic protein alpha-synuclein (αsyn) is a hallmark of Parkinson's disease (PD) and related synucleinopathies. Although predominantly localized in the cytosol, ...a body of evidence has shown that αsyn localizes to mitochondria and contributes to the disruption of key mitochondrial processes. Mitochondrial dysfunction is central to the progression of PD and mutations in mitochondrial-associated proteins are found in familial cases of PD. The sirtuins are highly conserved nicotinamide adenine dinucleotide (NAD
)-dependent enzymes that play a broad role in cellular metabolism and aging. Interestingly, mitochondrial sirtuin 3 (SIRT3) plays a major role in maintaining mitochondrial function and preventing oxidative stress, and is downregulated in aging and age-associated diseases such as neurodegenerative disorders. Herein, we hypothesize that αsyn is associated with decreased SIRT3 levels contributing to impaired mitochondrial dynamics and biogenesis in PD.
The level of mitochondrial SIRT3 was assessed in cells expressing oligomeric αsyn within the cytosolic and mitochondrial-enriched fractions. Mitochondrial integrity, respiration, and health were examined using several markers of mitochondrial dynamics and stress response and by measuring the rate of oxygen consumption (OCR). Our findings were validated in a rodent model of PD as well as in human post-mortem Lewy body disease (LBD) brain tissue.
Here, we demonstrate that αsyn associates with mitochondria and induces a decrease in mitochondrial SIRT3 levels and mitochondrial biogenesis. We show that SIRT3 downregulation is accompanied by decreased phosphorylation of AMPK and cAMP-response element binding protein (CREB), as well as increased phosphorylation of dynamin-related protein 1 (DRP1), indicative of impaired mitochondrial dynamics. OCR was significantly decreased suggesting a mitochondria respiratory deficit. Interestingly treatment with AMPK agonist 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) restores SIRT3 expression, improves mitochondrial function, and decreases αsyn oligomer formation in a SIRT3-dependent manner.
Together, our findings suggest that pharmacologically increasing SIRT3 levels can counteract αsyn-induced mitochondrial dysfunction by reducing αsyn oligomers and normalizing mitochondrial bioenergetics. These data support a protective role for SIRT3 in PD-associated pathways and contribute significant mechanistic insight into the interplay of SIRT3 and αsyn.
α-Synuclein is a key molecule in the pathogenesis of synucleinopathy including dementia with Lewy bodies, Parkinson's disease, and multiple system atrophy. Sirtuins are NAD(+)-dependent protein ...deacetylases that are highly conserved and counter aging in lower organisms. We show that the life span of a mouse model with A53T α-synuclein mutation is increased by overexpressing SIRT1 and decreased by knocking out SIRT1 in brain. Furthermore, α-synuclein aggregates are reduced in the brains of mice with SIRT1 overexpression and increased by SIRT1 deletion. We show that SIRT1 deacetylates HSF1 (heat shock factor 1) and increases HSP70 RNA and protein levels, but only in the brains of mice with A53T and SIRT1 expression. Thus, SIRT1 responds to α-synuclein aggregation-induced stress by activating molecular chaperones to protect against disease.
Abstract Parkinson's disease (PD) is a neurodegenerative disorder characterized by progressive motor disturbances and affects more than 1% of the worldwide population. Despite considerable progress ...in understanding PD pathophysiology, including genetic and biochemical causes, diagnostic approaches lack accuracy and interventions are restricted to symptomatic treatments. PD is a complex syndrome with different clinical subtypes and a wide variability in disorder course. In order to deliver better clinical management of PD patients and discovery of novel therapies, there is an urgent need to find sensitive, specific, and reliable biomarkers. The development of biomarkers will not only help the scientific community to identify populations at risk, but also facilitate clinical diagnosis. Furthermore, these tools could monitor progression, which could ultimately deliver personalized therapeutic strategies. The field of biomarker discovery in PD has attracted significant attention and there have been numerous contributions in recent years. Although none of the parameters have been validated for clinical practice, some candidates hold promise. This review summarizes recent advances in the development of PD biomarkers and discusses new strategies for their utilization.
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