Conformational templating of α-synuclein aggregates in neuronal-glial cultures

Alpha synuclein and inflammaging

The α-synuclein protein is an established molecule in Lewy body pathology, especially Parkinson's disease (PD). While the pathological role of α-synuclein (α-syn) in PD has been well described, novel evidence may suggest that α-syn interacts with inflammasomes in response to aging. As age is an inevitable physiological state and is also considered the greatest risk factor for PD, this calls for investigation into how α-syn, aging, and PD could be linked. There is a growing amount of data regarding α-syn normal function in the body that includes involvement in cellular transport such as protein complexes assembly, vesicular trafficking, neurotransmitter release, as well as immune cell maturation. Regarding abnormal α-syn, a number of autosomal dominant mutations have been identified as causes of familial PD, however, symptomatology may not become apparent until later in life due to compensatory mechanisms in the dopaminergic response. This potentially links age-related physiological changes not only as a risk factor for PD, but for the concept of "inflammaging ". This is defined as chronic inflammation that accompanies aging observed in many neurodegenerative pathologies, that include α-syn's ability to form oligomers and toxic fibrils seen in PD. This oligomeric α-syn stimulates pro-inflammatory signals, which may worsen PD symptoms and propagate chronic inflammation. Thus, this review will explore a potential link between α-syn's role in the immune system, inflammaging, and PD.

The Hidden Cell-to-Cell Trail of α-Synuclein Aggregates

The progressive accumulation of insoluble aggregates of the presynaptic protein alpha-synuclein (α-Syn) is a hallmark of neurodegenerative disorders including Parkinson's disease (PD), Multiple System Atrophy, and Dementia with Lewy Bodies, commonly referred to as synucleinopathies. Despite considerable progress on the structural biology of these aggregates, the molecular mechanisms mediating their cell-to-cell transmission, propagation, and neurotoxicity remain only partially understood. Numerous studies have highlighted the stereotypical spatiotemporal spreading of pathological α-Syn aggregates across different tissues and anatomically connected brain regions over time. Experimental evidence from various cellular and animal models indicate that α-Syn transfer occurs in two defined steps: the release of pathogenic α-Syn species from infected cells, and their uptake via passive or active endocytic pathways. Once α-Syn aggregates have been internalized, little is known about what drives their toxicity or how they interact with the endogenous protein to promote its misfolding and subsequent aggregation. Similarly, unknown genetic factors modulate different cellular responses to the aggregation and accumulation of pathogenic α-Syn species. Here we discuss the current understanding of the molecular phenomena associated with the intercellular spreading of pathogenic α-Syn seeds and summarize the evidence supporting the transmission hypothesis. Understanding the molecular mechanisms involved in α-Syn aggregates transmission is essential to develop novel targeted therapeutics against PD and related synucleinopathies.

Optical pulse labeling studies reveal exogenous seeding slows α-synuclein clearance

The accumulation of α-synuclein (α-syn) in intracellular formations known as Lewy bodies (LBs) is associated with several neurodegenerative diseases including Parkinson's disease and Lewy Body Dementia. There is still limited understanding of how α-syn and LB formation is associated with cellular dysfunction and degeneration in these diseases. To examine the clearance and production dynamics of α-syn we transduced organotypic murine brain slice cultures (BSCs) with recombinant adeno-associated viruses (rAAVs) to express Dendra2-tagged human wild-type (WT) and mutant A53T α-syn, with and without the addition of exogenous α-syn fibrillar seeds and tracked them over several weeks in culture using optical pulse labeling. We found that neurons expressing WT or mutant A53T human α-syn show similar rates of α-syn turnover even when insoluble, phosphorylated Ser129 α-syn has accumulated. Taken together, this data reveals α-syn aggregation and overexpression, pSer129 α-syn, nor the A53T mutation affect α-syn dynamics in this system. Prion-type seeding with exogenous α-syn fibrils significantly slows α-syn turnover, in the absence of toxicity but is associated with the accumulation of anti-p62 immunoreactivity and Thiazin Red positivity. Prion-type induction of α-syn aggregation points towards a potential protein clearance deficit in the presence of fibrillar seeds and the ease of this system to explore precise mechanisms underlying these processes. This system facilitates the exploration of α-syn protein dynamics over long-term culture periods. This platform can further be exploited to provide mechanistic insight on what drives this slowing of α-syn turnover and how therapeutics, other genes or different α-syn mutations may affect α-syn protein dynamics.

Diagnostic and therapeutic agents that target alpha-synuclein in Parkinson's disease

The development of disease-modifying drugs and differential diagnostic agents is an urgent medical need in Parkinson’s disease. Despite the complex pathophysiological pathway, the misfolding of alpha-synuclein has been identified as a putative biomarker for detecting the onset and progression of the neurodegeneration associated with Parkinson’s disease. Identifying the most appropriate alpha-synuclein-based diagnostic modality with clinical translation will revolutionize the diagnosis of Parkinson’s. Likewise, molecules that target alpha-synuclein could alter the disease pathway that leads to Parkinson’s and may serve as first-in class therapeutics compared to existing treatment options such as levodopa and dopamine agonist that do not necessarily modify the disease pathway. Notwithstanding the promising benefits that alpha-synuclein presents to therapeutics and diagnostics development for Parkinson’s disease, finding ways to address potential challenges such as inadequate preclinical models, safety and efficacy will be paramount to achieving clinical translation. In this comprehensive review paper, we described the role of alpha-synuclein in the pathogenesis of Parkinson’s disease, as well as how its structure and function relationship delineate disease onset and progression. We further discussed different alpha-synuclein-based diagnostic modalities including biomolecular assays and molecular imaging. Finally, we presented current small molecules and biologics that are being developed as disease-modifying drugs or positron emission tomography imaging probes for Parkinson’s disease.

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

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

Neurons with Cat's Eyes: A Synthetic Strain of α-Synuclein Fibrils Seeding Neuronal Intranuclear Inclusions

The distinct neuropathological features of the different α-Synucleinopathies, as well as the diversity of the α-Synuclein (α-Syn) intracellular inclusion bodies observed in post mortem brain sections, are thought to reflect the strain diversity characterizing invasive α-Syn amyloids. However, this "one strain, one disease" view is still hypothetical, and to date, a possible disease-specific contribution of non-amyloid factors has not been ruled out. In Multiple System Atrophy (MSA), the buildup of α-Syn inclusions in oligodendrocytes seems to result from the terminal storage of α-Syn amyloid aggregates first pre-assembled in neurons. This assembly occurs at the level of neuronal cytoplasmic inclusions, and even earlier, within neuronal intranuclear inclusions (NIIs). Intriguingly, α-Syn NIIs are never observed in α-Synucleinopathies other than MSA, suggesting that these inclusions originate (i) from the unique molecular properties of the α-Syn fibril strains encountered in this disease, or alternatively, (ii) from other factors specifically dysregulated in MSA and driving the intranuclear fibrillization of α-Syn. We report the isolation and structural characterization of a synthetic human α-Syn fibril strain uniquely capable of seeding α-Syn fibrillization inside the nuclear compartment. In primary mouse cortical neurons, this strain provokes the buildup of NIIs with a remarkable morphology reminiscent of cat's eye marbles (see video abstract). These α-Syn inclusions form giant patterns made of one, two, or three lentiform beams that span the whole intranuclear volume, pushing apart the chromatin. The input fibrils are no longer detectable inside the NIIs, where they become dominated by the aggregation of endogenous α-Syn. In addition to its phosphorylation at S129, α-Syn forming the NIIs acquires an epitope antibody reactivity profile that indicates its organization into fibrils, and is associated with the classical markers of α-Syn pathology p62 and ubiquitin. NIIs are also observed in vivo after intracerebral injection of the fibril strain in mice. Our data thus show that the ability to seed NIIs is a strain property that is integrally encoded in the fibril supramolecular architecture. Upstream alterations of cellular mechanisms are not required. In contrast to the lentiform TDP-43 NIIs, which are observed in certain frontotemporal dementias and which are conditional upon GRN or VCP mutations, our data support the hypothesis that the presence of α-Syn NIIs in MSA is instead purely amyloid-strain-dependent.

Neurons and Glia Interplay in α-Synucleinopathies

Accumulation of the neuronal presynaptic protein alpha-synuclein within proteinaceous inclusions represents the key histophathological hallmark of a spectrum of neurodegenerative disorders, referred to by the umbrella term a-synucleinopathies. Even though alpha-synuclein is expressed predominantly in neurons, pathological aggregates of the protein are also found in the glial cells of the brain. In Parkinson's disease and dementia with Lewy bodies, alpha-synuclein accumulates mainly in neurons forming the Lewy bodies and Lewy neurites, whereas in multiple system atrophy, the protein aggregates mostly in the glial cytoplasmic inclusions within oligodendrocytes. In addition, astrogliosis and microgliosis are found in the synucleinopathy brains, whereas both astrocytes and microglia internalize alpha-synuclein and contribute to the spread of pathology. The mechanisms underlying the pathological accumulation of alpha-synuclein in glial cells that under physiological conditions express low to non-detectable levels of the protein are an area of intense research. Undoubtedly, the presence of aggregated alpha-synuclein can disrupt glial function in general and can contribute to neurodegeneration through numerous pathways. Herein, we summarize the current knowledge on the role of alpha-synuclein in both neurons and glia, highlighting the contribution of the neuron-glia connectome in the disease initiation and progression, which may represent potential therapeutic target for a-synucleinopathies.

Comparative Studies in the A30P and A53T α-Synuclein C. elegans Strains to Investigate the Molecular Origins of Parkinson's Disease

The aggregation of α-synuclein is a hallmark of Parkinson's disease (PD) and a variety of related neurological disorders. A number of mutations in this protein, including A30P and A53T, are associated with familial forms of the disease. Patients carrying the A30P mutation typically exhibit a similar age of onset and symptoms as sporadic PD, while those carrying the A53T mutation generally have an earlier age of onset and an accelerated progression. We report two C. elegans models of PD (PD_A30P and PD_A53T), which express these mutational variants in the muscle cells, and probed their behavior relative to animals expressing the wild-type protein (PD_WT). PD_A30P worms showed a reduced speed of movement and an increased paralysis rate, control worms, but no change in the frequency of body bends. By contrast, in PD_A53T worms both speed and frequency of body bends were significantly decreased, and paralysis rate was increased. α-Synuclein was also observed to be less well localized into aggregates in PD_A30P worms compared to PD_A53T and PD_WT worms, and amyloid-like features were evident later in the life of the animals, despite comparable levels of expression of α-synuclein. Furthermore, squalamine, a natural product currently in clinical trials for treating symptomatic aspects of PD, was found to reduce significantly the aggregation of α-synuclein and its associated toxicity in PD_A53T and PD_WT worms, but had less marked effects in PD_A30P. In addition, using an antibody that targets the N-terminal region of α-synuclein, we observed a suppression of toxicity in PD_A30P, PD_A53T and PD_WT worms. These results illustrate the use of these two C. elegans models in fundamental and applied PD research.

Bee venom attenuates neurodegeneration and motor impairment and modulates the response to L-dopa or rasagiline in a mice model of Parkinson's disease

Objectives

This study aimed to investigate the effect of bee venom, a form of alternative therapy, on rotenone-induced Parkinson's disease (PD) in mice. Moreover, the possible modulation by bee venom of the effect of L-dopa/carbidopa or rasagiline was examined.

Materials and Methods

Rotenone (1.5 mg/kg, subcutaneously; SC) was administered every other day for two weeks and at the same time mice received the vehicle (DMSO, SC), bee venom (0.065, 0.13, and 0.26 mg/kg; intradermal; ID), L-dopa/carbidopa (25 mg/kg, intraperitoneal; IP), L-dopa/carbidopa+bee venom (0.13 mg/kg, ID), rasagiline (1 mg/kg, IP) or rasagiline+bee venom (0.13 mg/kg, ID). Then, wire hanging and staircase tests were performed and mice were euthanized and brains' striata separated. Oxidative stress biomarkers namely, malondialdehyde (MDA), nitric oxide (NO), reduced glutathione (GSH), paraoxonase-1 (PON-1), and total antioxidant capacity (TAC) were measured. Additionally, butyrylcholinesterase (BuChE), monocyte chemoattractant protein-1 (MCP-1), tumor necrosis factor-alpha (TNF-α), and dopamine (DA) were evaluated. Brain histopathological changes and caspase-3- expression were done.

Results

Bee venom significantly enhanced motor performance and inhibited rotenone-induced oxidative/nitrosative stress, observed as a reduction in both MDA and NO along with increasing GSH, PON-1, and TAC. Besides, bee venom decreased MCP-1, TNF-α, and caspase-3 expression together with an increase in BuChE activity and DA content.

Conclusion

Bee venom alone or in combination with L-dopa/carbidopa or rasagiline alleviated neuronal degeneration compared with L-dopa/carbidopa or rasagiline treatment only. Bee venom via its antioxidant and cytokine reducing potentials might be of value either alone or as adjunctive therapy in the management of PD.

In Search of Effective Treatments Targeting α-Synuclein Toxicity in Synucleinopathies: Pros and Cons

Parkinson's disease (PD), multiple system atrophy (MSA) and Dementia with Lewy bodies (DLB) represent pathologically similar, progressive neurodegenerative disorders characterized by the pathological aggregation of the neuronal protein α-synuclein. PD and DLB are characterized by the abnormal accumulation and aggregation of α-synuclein in proteinaceous inclusions within neurons named Lewy bodies (LBs) and Lewy neurites (LNs), whereas in MSA α-synuclein inclusions are mainly detected within oligodendrocytes named glial cytoplasmic inclusions (GCIs). The presence of pathologically aggregated α-synuclein along with components of the protein degradation machinery, such as ubiquitin and p62, in LBs and GCIs is considered to underlie the pathogenic cascade that eventually leads to the severe neurodegeneration and neuroinflammation that characterizes these diseases. Importantly, α-synuclein is proposed to undergo pathogenic misfolding and oligomerization into higher-order structures, revealing self-templating conformations, and to exert the ability of "prion-like" spreading between cells. Therefore, the manner in which the protein is produced, is modified within neural cells and is degraded, represents a major focus of current research efforts in the field. Given that α-synuclein protein load is critical to disease pathogenesis, the identification of means to limit intracellular protein burden and halt α-synuclein propagation represents an obvious therapeutic approach in synucleinopathies. However, up to date the development of effective therapeutic strategies to prevent degeneration in synucleinopathies is limited, due to the lack of knowledge regarding the precise mechanisms underlying the observed pathology. This review critically summarizes the recent developed strategies to counteract α-synuclein toxicity, including those aimed to increase protein degradation, to prevent protein aggregation and cell-to-cell propagation, or to engage antibodies against α-synuclein and discuss open questions and unknowns for future therapeutic approaches.

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

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

The emerging role of α-synuclein truncation in aggregation and disease

α-Synuclein (αsyn) is an abundant brain neuronal protein that can misfold and polymerize to form toxic fibrils coalescing into pathologic inclusions in neurodegenerative diseases, including Parkinson's disease, Lewy body dementia, and multiple system atrophy. These fibrils may induce further αsyn misfolding and propagation of pathologic fibrils in a prion-like process. It is unclear why αsyn initially misfolds, but a growing body of literature suggests a critical role of partial proteolytic processing resulting in various truncations of the highly charged and flexible carboxyl-terminal region. This review aims to 1) summarize recent evidence that disease-specific proteolytic truncations of αsyn occur in Parkinson's disease, Lewy body dementia, and multiple system atrophy and animal disease models; 2) provide mechanistic insights on how truncation of the amino and carboxyl regions of αsyn may modulate the propensity of αsyn to pathologically misfold; 3) compare experiments evaluating the prion-like properties of truncated forms of αsyn in various models with implications for disease progression; 4) assess uniquely toxic properties imparted to αsyn upon truncation; and 5) discuss pathways through which truncated αsyn forms and therapies targeted to interrupt them. Cumulatively, it is evident that truncation of αsyn, particularly carboxyl truncation that can be augmented by dysfunctional proteostasis, dramatically potentiates the propensity of αsyn to pathologically misfold into uniquely toxic fibrils with modulated prion-like seeding activity. Therapeutic strategies and experimental paradigms should operate under the assumption that truncation of αsyn is likely occurring in both initial and progressive disease stages, and preventing truncation may be an effective preventative strategy against pathologic inclusion formation.

Carboxy-terminal truncation and phosphorylation of α-synuclein elongates survival in a prion-like seeding mouse model of synucleinopathy

Pathologic intracellular inclusions formed from polymers of misfolded α-synuclein (αsyn) protein define a group of neurodegenerative diseases termed synucleinopathies which includes Parkinson's disease (PD). Prion-like recruitment of endogenous cellular αsyn has been demonstrated to occur in animal models of synucleinopathy, whereby misfolded αsyn can induce further pathologic αsyn inclusions to form through a prion-like mechanism. It has been suggested that misfolded αsyn may assume differing conformations which lead to varied clinical and pathological manifestations of disease; this phenomenon bears similarities to that of prion strains whereby the same misfolded protein can produce unique diseases. It is unclear what factors influence the development of unique αsyn strains, however post-translational modifications (PTMs) such as phosphorylation and truncation that are present in misfolded αsyn in disease may play a role due to their modulation of biochemical and structural αsyn properties. Herein, we investigate the prion-like properties of misfolded αsyn polymers containing either phosphomimetic (S129E) αsyn, 5 different major carboxy (C)-truncated forms of αsyn (1-115, 1-119, 1-122, 1-125, and 1-129 αsyn), or a mixture of these PTM containing αsyn forms compared to full-length (FL) αsyn in HEK293T cells and M83 transgenic mice overexpressing A53T αsyn. It is demonstrated that upon peripheral intramuscular injection of these C-truncated or S129E αsyn polymers into M83 mice, prion-like progression and time to disease onset in this mouse model is elongated when any of these PTMs are present, demonstrating that common modifications to the C-terminus of αsyn present in disease modulates the prion-like seeding properties of αsyn.

Advancement in the modelling and therapeutics of Parkinson's disease

Since the discovery of L-dopa in the middle of the 20th century (1960s), there is not any neuroprotective therapy available although significant development has been made in the treatment of symptomatic Parkinson's disease (PD). Neurological disorders like PD can be modelled in animals so as to recapitulates most of the symptoms seen in PD patients. In aging population, PD is the second most common neurodegenerative disease after Alzheimer's disease, even though significant outcomes have been achieved in PD research yet it still is a mystery to solve the treatments for PD. In the last two decades, PD models have provided enhanced precision into the understanding of the process of PD disease, its etiology, pathology, and molecular mechanisms behind it. Furthermore, at the same time as cellular models have helped to recognize specific events, animal models, both toxic and genetic, have replicated almost all of the hallmarks of PD and are very helpful for testing and finding new strategies for neuroprotection. Recently, in both classical and newer models, major advances have been done in the modelling of supplementary PD features have come into the light. In this review, we have try to provide an updated summary of the characteristics of these models related to in vitro and in vivo models, animal models for PD, stem cell model for PD, newer 3D model as well as the strengths and limitations of these most popular PD models.

Endogenous oligodendroglial alpha-synuclein and TPPP/p25α orchestrate alpha-synuclein pathology in experimental multiple system atrophy models

Multiple system atrophy (MSA) is characterized by the presence of distinctive glial cytoplasmic inclusions (GCIs) within oligodendrocytes that contain the neuronal protein alpha-synuclein (aSyn) and the oligodendroglia-specific phosphoprotein TPPP/p25α. However, the role of oligodendroglial aSyn and p25α in the formation of aSyn-rich GCIs remains unclear. To address this conundrum, we have applied human aSyn (haSyn) pre-formed fibrils (PFFs) to rat wild-type (WT)-, haSyn-, or p25α-overexpressing oligodendroglial cells and to primary differentiated oligodendrocytes derived from WT, knockout (KO)-aSyn, and PLP-haSyn-transgenic mice. HaSyn PFFs are readily taken up by oligodendroglial cells and can recruit minute amounts of endogenous aSyn into the formation of insoluble, highly aggregated, pathological assemblies. The overexpression of haSyn or p25α accelerates the recruitment of endogenous protein and the generation of such aberrant species. In haSyn PFF-treated primary oligodendrocytes, the microtubule and myelin networks are disrupted, thus recapitulating a pathological hallmark of MSA, in a manner totally dependent upon the seeding of endogenous aSyn. Furthermore, using oligodendroglial and primary cortical cultures, we demonstrated that pathology-related S129 aSyn phosphorylation depends on aSyn and p25α protein load and may involve different aSyn “strains” present in oligodendroglial and neuronal synucleinopathies. Importantly, this hypothesis was further supported by data obtained from human post-mortem brain material derived from patients with MSA and dementia with Lewy bodies. Finally, delivery of haSyn PFFs into the mouse brain led to the formation of aberrant aSyn forms, including the endogenous protein, within oligodendroglia and evoked myelin decompaction in WT mice, but not in KO-aSyn mice. This line of research highlights the role of endogenous aSyn and p25α in the formation of pathological aSyn assemblies in oligodendrocytes and provides in vivo evidence of the contribution of oligodendroglial aSyn in the establishment of aSyn pathology in MSA.

α-Synuclein and astrocytes: tracing the pathways from homeostasis to neurodegeneration in Lewy body disease

α-Synuclein is a soluble protein that is present in abundance in the brain, though its normal function in the healthy brain is poorly defined. Intraneuronal inclusions of α-synuclein, commonly referred to as Lewy pathology, are pathological hallmarks of a spectrum of neurodegenerative disorders referred to as α-synucleinopathies. Though α-synuclein is expressed predominantly in neurons, α-synuclein aggregates in astrocytes are a common feature in these neurodegenerative diseases. How and why α-synuclein ends up in the astrocytes and the consequences of this dysfunctional proteostasis in immune cells is a major area of research that can have far-reaching implications for future immunobiotherapies in α-synucleinopathies. Accumulation of aggregated α-synuclein can disrupt astrocyte function in general and, more importantly, can contribute to neurodegeneration in α-synucleinopathies through various pathways. Here, we summarize our current knowledge on how astrocytic α-synucleinopathy affects CNS function in health and disease and propose a model of neuroglial connectome altered by α-synuclein proteostasis that might be amenable to immune-based therapies.

Recapitulating Parkinson's disease pathology in a three-dimensional human neural cell culture model

Extensive loss of dopaminergic neurons and aggregation of the protein α-synuclein into ubiquitin-positive Lewy bodies represents a major neuropathological hallmark of Parkinson's disease (PD). At present, the generation of large nuclear-associated Lewy bodies from endogenous wild-type α-synuclein, translationally regulated under its own promoter in human cell culture models, requires costly and time-consuming protocols. Here, we demonstrate that fully differentiated human SH-SY5Y neuroblastoma cells grown in three-dimensional cell culture develop Lewy-body-like pathology upon exposure to exogenous α-synuclein species. In contrast to most cell- and rodent-based PD models, which exhibit multiple diffuse α-synuclein aggregates throughout the cytoplasm, a single large nuclear inclusion that is immunopositive for α-synuclein and ubiquitin is rapidly obtained in our model. This was achieved without the need for overexpression of α-synuclein or genetic modification of the cell line. However, phosphorylation of α-synuclein within these inclusions was not observed. The system described here provides an ideal tool to screen compounds to therapeutically intervene in Lewy body formation, and to investigate the mechanisms involved in disease progression in synucleinopathies.

rAAV-based brain slice culture models of Alzheimer's and Parkinson's disease inclusion pathologies

It has been challenging to produce ex vivo models of the inclusion pathologies that are hallmark pathologies of many neurodegenerative diseases. Using three-dimensional mouse brain slice cultures (BSCs), we have developed a paradigm that rapidly and robustly recapitulates mature neurofibrillary inclusion and Lewy body formation found in Alzheimer's and Parkinson's disease, respectively. This was achieved by transducing the BSCs with recombinant adeno-associated viruses (rAAVs) that express α-synuclein or variants of tau. Notably, the tauopathy BSC model enables screening of small molecule therapeutics and tracking of neurodegeneration. More generally, the rAAV BSC "toolkit" enables efficient transduction and transgene expression from neurons, microglia, astrocytes, and oligodendrocytes, alone or in combination, with transgene expression lasting for many months. These rAAV-based BSC models provide a cost-effective and facile alternative to in vivo studies, and in the future can become a widely adopted methodology to explore physiological and pathological mechanisms related to brain function and dysfunction.

Epitope determines efficacy of therapeutic anti-Tau antibodies in a functional assay with human Alzheimer Tau

In Alzheimer's disease (AD) and other tauopathies, the cytosolic protein Tau misfolds and forms intracellular aggregates which accumulate within the brain leading to neurodegeneration. Clinical progression is tightly linked to the progressive spread of Tau pathology throughout the brain, and several lines of evidence suggest that Tau aggregates or "seeds" may propagate pathology by spreading from cell to cell in a "prion like" manner. Accordingly, blocking the spread of extracellular seeds with an antibody could be a viable therapeutic approach. However, as the structure of Tau seeds is unknown, it is only possible to rationally design therapeutic Tau antibodies by making a priori assumptions. To avoid this, we developed a robust and quantitative cell based assay and employed an unbiased screening approach to identify the antibody with the highest activity against human Tau seeds. The selected antibody (D), directed to the mid-region of Tau (amino acids 235-250), potently blocked the seeding of human AD Tau and was also fully efficacious against seeds from progressive supranuclear palsy. When we compared this antibody with previously described reference antibodies, we were surprised to find that none of these antibodies showed comparable efficacy against human pathological seeds. Our data highlight the difficulty of predicting antibody accessible epitopes on pathological Tau seeds and question the potential efficacy of some of the Tau antibodies that are currently in clinical development.

Localized Induction of Wild-Type and Mutant Alpha-Synuclein Aggregation Reveals Propagation along Neuroanatomical Tracts

Misfolded alpha-synuclein (αS) may exhibit a number of characteristics similar to those of the prion protein, including the apparent ability to spread along neuroanatomical connections. The demonstration for this mechanism of spread is largely based on the intracerebral injections of preaggregated αS seeds in mice, in which it cannot be excluded that diffuse, surgical perturbations and hematogenous spread also contribute to the propagation of pathology. For this reason, we have utilized the sciatic nerve as a route of injection to force the inoculum into the lumbar spinal cord and induce a localized site for the onset of αS inclusion pathology. Our results demonstrate that mouse αS fibrils (fibs) injected unilaterally in the sciatic nerve are efficient in inducing pathology and the onset of paralytic symptoms in both the M83 and M20 lines of αS transgenic mice. In addition, a spatiotemporal study of these injections revealed a predictable spread of pathology to brain regions whose axons synapse directly on ventral motor neurons in the spinal cord, strongly supporting axonal transport as a mechanism of spread of the αS inducing, or seeding, factor. We also revealed a relatively decreased efficiency for human αS fibs containing the E46K mutation to induce disease via this injection paradigm, supportive of recent studies demonstrating a diminished ability of this mutant αS to undergo aggregate induction. These results further demonstrate prion-like properties for αS by the ability for a progression and spread of αS inclusion pathology along neuroanatomical connections.IMPORTANCE The accumulation of alpha-synuclein (αS) inclusions is a hallmark feature of Parkinson's disease (PD) and PD-related diseases. Recently, a number of studies have demonstrated similarities between the prion protein and αS, including its ability to spread along neuroanatomical tracts throughout the central nervous system (CNS). However, there are caveats in each of these studies in which the injection routes used had the potential to result in a widespread dissemination of the αS-containing inocula, making it difficult to precisely define the mechanisms of spread. In this study, we assessed the spread of pathology following a localized induction of αS inclusions in the lumbar spinal cord following a unilateral injection in the sciatic nerve. Using this paradigm, we demonstrated the ability for αS inclusion spread and/or induction along neuroanatomical tracts within the CNS of two αS-overexpressing mouse models.

Increased brain hemopexin levels improve outcomes after intracerebral hemorrhage

Following intracerebral hemorrhage (ICH), extracellular heme precipitates secondary brain injury, which results in irreversible brain damage and enduring neurological deficits. Hemopexin (Hpx) is an endogenous protein responsible for scavenging heme, thereby modulating its intrinsic proxidant/proinflammatory properties. Although Hpx is present in the brain, the endogenous levels are insufficient to combat the massive heme overload following ICH. We hypothesized that increasing brain Hpx levels would improve ICH outcomes. Unique recombinant adeno-associated viral vectors were designed to specifically overexpress Hpx within the mouse brain. Western blotting, ELISA, and immunohistochemistry of brain homogenates/sections, CSF, and serum were performed. As compared to controls, Hpx mice have increased Hpx protein levels in all three types of biospecimens evaluated, which results in 45.6 ± 6.9% smaller lesions and improved functional recovery after ICH (n=14-19/group, p < 0.05). Local mechanistic analyses show significantly less tissue injury, trends toward smaller hematoma volumes, unchanged heme oxygenase 1 and iron levels, and significantly increased microgliosis and decreased astrogliosis and lipid peroxidation. Peripheral levels of heme-related markers indicate a positive modulation of iron-binding capacity. These findings reveal that high local Hpx levels improve ICH outcomes, likely through both central and peripheral clearance mechanisms, and establish the potential for therapeutically administering clinical-grade Hpx for ICH.

Prion-like mechanisms in amyotrophic lateral sclerosis

The prion hypothesis - a protein conformation capable of replicating without a nucleic acid genome - was heretical at the time of its discovery. However, the characteristics of the disease-misfolded prion protein and its ability to transmit disease, replicate, and spread are now widely accepted throughout the scientific community. In fact, in the last decade a wealth of evidence has emerged supporting similar properties observed for many of the misfolded proteins implicated in other neurodegenerative diseases, such as Alzheimer disease, Parkinson disease, tauopathies, and as described in this chapter, amyotrophic lateral sclerosis (ALS). Multiple studies have now demonstrated the ability for superoxide dismutase-1, 43-kDa transactive response (TAR) DNA-binding protein, fused-in sarcoma, and most recently, C9orf72-encoded polypeptides to display properties similar to those of prions. The majority of these are cell-free and in vitro assays, while superoxide dismutase-1 remains the only ALS-linked protein to demonstrate several prion-like properties in vivo. In this chapter, we provide an introduction to ALS and review the recent literature linking several proteins implicated in the familial forms of the disease to properties of the prion protein.

Vertebrate food products as a potential source of prion-like α-synuclein

The aberrant aggregation of the protein α-synuclein is thought to be involved in Parkinson’s disease (PD). However, the factors that lead to initiation and propagation of α-synuclein aggregation are not clearly understood. Recently, the hypothesis that α-synuclein aggregation spreads via a prion-like mechanism originating in the gut has gained much scientific attention. If α-synuclein spreads via a prion-like mechanism, then an important question becomes, what are the origins of this prion-like species? Here we review the possibility that α-synuclein aggregation could be seeded via the ingestion of a prion-like α-synuclein species contained within food products originating from vertebrates. To do this, we highlight current evidence for the gut-to-brain hypothesis of PD, and put this in context of available routes of α-synuclein prion infectivity via the gastrointestinal (GI) tract. We then discuss meat as a ready exogenous source of α-synuclein and how certain risk factors, including inflammation, may allow for dietary α-synuclein to pass from the GI lumen into the host to induce pathology. Lastly, we review epidemiological evidence that dietary factors may be involved in PD. Overall, research to date has yet to directly test the contribution of dietary α-synuclein to the mechanism of initiation and progression of the disease. However, numerous experimental findings, including the potent seeding and spreading behavior of α-synuclein fibrils, seem to support, at least in part, the feasibility of an infection with a prion α-synuclein particle via the GI tract. Further studies are required to determine whether dietary α-synuclein contributes to seeding pathology in the gut.

α-Synuclein transfer between neurons and astrocytes indicates that astrocytes play a role in degradation rather than in spreading

Recent evidence suggests that disease progression in Parkinson's disease (PD) could occur by the spreading of α-synuclein (α-syn) aggregates between neurons. Here we studied the role of astrocytes in the intercellular transfer and fate of α-syn fibrils, using in vitro and ex vivo models. α-Syn fibrils can be transferred to neighboring cells; however, the transfer efficiency changes depending on the cell types. We found that α-syn is efficiently transferred from astrocytes to astrocytes and from neurons to astrocytes, but less efficiently from astrocytes to neurons. Interestingly, α-syn puncta are mainly found inside the lysosomal compartments of the recipient cells. However, differently from neurons, astrocytes are able to efficiently degrade fibrillar α-syn, suggesting an active role for these cells in clearing α-syn deposits. Astrocytes co-cultured with organotypic brain slices are able to take up α-syn fibrils from the slices. Altogether our data support a role for astrocytes in trapping and clearing α-syn pathological deposits in PD.

Computational investigation on the effects of H50Q and G51D mutations on the α-Synuclein aggregation propensity

The aggregation of α-synuclein is linked directly to the histopathology of Parkinson's disease (PD). However, several missense mutations present in the α-synuclein gene (SNCA) have been known to be associated with PD. Several studies have highlighted the effect of SNCA mutations on the α-synuclein aggregation, but their pathological roles are not completely established. In this study, we have focused on the effects of the recently discovered α-synuclein missense mutants (H50Q and G51D) on the aggregation using computational approaches. We performed all atom molecular dynamics (MD) simulation on these mutants and compared their conformational dynamics with Wild-Type (WT) α-synuclein. We noticed the solvent accessible surface area (SASA), radius of gyration, atomic fluctuations, and beta strand content to be higher in H50Q than G51D and WT. Using PDBSum online server; we analyzed the inter-molecular interactions that drive the association of monomeric units of H50Q, WT, and G51D in forming the respective homo-dimer. We noticed the interface area, number of interacting residues and binding free energy to be higher for H50Q homo-dimer than the WT and G51D homo-dimers. Our findings in this study suggest that in comparison to WT and G51D, H50Q mutation to have a positive effect on increasing the α-synuclein aggregation propensity. Hence, we see that H50Q and G51D mutation show conflicting effect on the aggregation propensity of α-synuclein.

Intrastriatal injection of α-synuclein can lead to widespread synucleinopathy independent of neuroanatomic connectivity

Background

Prionoid transmission of α-synuclein (αSyn) aggregates along neuroanatomically connected projections is posited to underlie disease progression in α-synucleinopathies. Here, we specifically wanted to study whether this prionoid progression occurs via direct inter-neuronal transfer and, if so, would intrastriatal injection of αSyn aggregates lead to nigral degeneration.

Methods

To test prionoid transmission of αSyn aggregates along the nigro-striatal pathway, we injected amyloidogenic αSyn aggregates into two different regions of the striatum of adult human wild type αSyn transgenic mice (Line M20) or non-transgenic (NTG) mice and aged for 4 months.

Results

M20 mice injected in internal capsule (IC) or caudate putamen (CPu) regions of the striatum showed florid αSyn inclusion pathology distributed throughout the neuraxis, irrespective of anatomic connectivity. These αSyn inclusions were found in different cell types including neurons, astrocytes and even ependymal cells. On the other hand, intra-striatal injection of αSyn fibrils into NTG mice resulted in sparse αSyn pathology, mostly localized in the striatum and entorhinal cortex. Interestingly, NTG mice injected with preformed human αSyn fibrils showed no induction of αSyn inclusion pathology, suggesting the presence of a species barrier for αSyn fibrillar seeds. Modest levels of nigral dopaminergic (DA) neuronal loss was observed exclusively in substantia nigra (SN) of M20 cohorts injected in the IC, even in the absence of frank αSyn inclusions in DA neurons. None of the NTG mice or CPu-injected M20 mice showed DA neurodegeneration. Interestingly, the pattern and distribution of induced αSyn pathology corresponded with neuroinflammation especially in the SN of M20 cohorts. Hypermorphic reactive astrocytes laden with αSyn inclusions were abundantly present in the brains of M20 mice.

Conclusions

Overall, our findings show that the pattern and extent of dissemination of αSyn pathology does not necessarily follow expected neuroanatomic connectivity. Further, the presence of intra-astrocytic αSyn pathology implies that glial cells participate in αSyn transmission and possibly have a role in non-cell autonomous disease modification.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-017-0182-z) contains supplementary material, which is available to authorized users.

Robust Central Nervous System Pathology in Transgenic Mice following Peripheral Injection of α-Synuclein Fibrils

Misfolded α-synuclein (αS) is hypothesized to spread throughout the central nervous system (CNS) by neuronal connectivity leading to widespread pathology. Increasing evidence indicates that it also has the potential to invade the CNS via peripheral nerves in a prion-like manner. On the basis of the effectiveness following peripheral routes of prion administration, we extend our previous studies of CNS neuroinvasion in M83 αS transgenic mice following hind limb muscle (intramuscular [i.m.]) injection of αS fibrils by comparing various peripheral sites of inoculations with different αS protein preparations. Following intravenous injection in the tail veins of homozygous M83 transgenic (M83^+/+) mice, robust αS pathology was observed in the CNS without the development of motor impairments within the time frame examined. Intraperitoneal (i.p.) injections of αS fibrils in hemizygous M83 transgenic (M83^+/-) mice resulted in CNS αS pathology associated with paralysis. Interestingly, injection with soluble, nonaggregated αS resulted in paralysis and pathology in only a subset of mice, whereas soluble Δ71-82 αS, human βS, and keyhole limpet hemocyanin (KLH) control proteins induced no symptoms or pathology. Intraperitoneal injection of αS fibrils also induced CNS αS pathology in another αS transgenic mouse line (M20), albeit less robustly in these mice. In comparison, i.m. injection of αS fibrils was more efficient in inducing CNS αS pathology in M83 mice than i.p. or tail vein injections. Furthermore, i.m. injection of soluble, nonaggregated αS in M83^+/- mice also induced paralysis and CNS αS pathology, although less efficiently. These results further demonstrate the prion-like characteristics of αS and reveal its efficiency to invade the CNS via multiple routes of peripheral administration.

IMPORTANCE

The misfolding and accumulation of α-synuclein (αS) inclusions are found in a number of neurodegenerative disorders and is a hallmark feature of Parkinson's disease (PD) and PD-related diseases. Similar characteristics have been observed between the infectious prion protein and αS, including its ability to spread from the peripheral nervous system and along neuroanatomical tracts within the central nervous system. In this study, we extend our previous results and investigate the efficiency of intravenous (i.v.), intraperitoneal (i.p.), and intramuscular (i.m.) routes of injection of αS fibrils and other protein controls. Our data reveal that injection of αS fibrils via these peripheral routes in αS-overexpressing mice are capable of inducing a robust αS pathology and in some cases cause paralysis. Furthermore, soluble, nonaggregated αS also induced αS pathology, albeit with much less efficiency. These findings further support and extend the idea of αS neuroinvasion from peripheral exposures.

Inflammatory pre-conditioning restricts the seeded induction of α-synuclein pathology in wild type mice

Background

Cell-to-cell transmission of α-synuclein (αSyn) is hypothesized to play an important role in disease progression in synucleinopathies. This process involves cellular uptake of extracellular amyloidogenic αSyn seeds followed by seeding of endogenous αSyn. Though it is well known that αSyn is an immunogenic protein that can interact with immune receptors, the role of innate immunity in regulating induction of αSyn pathology in vivo is unknown. Herein, we explored whether altering innate immune activation affects induction of αSyn pathology in wild type mice.

Methods

We have previously demonstrated that recombinant adeno-associated virus (AAV) mediated expression of the inflammatory cytokine, Interleukin (IL)-6, in neonatal wild type mice brains leads to widespread immune activation in the brain without overt neurodegeneration. To investigate how IL-6 expression affects induction of αSyn pathology, we injected mouse wild type αSyn fibrils in the hippocampus of AAV-IL-6 expressing mice. Control mice received AAV containing an Empty vector (EV) construct. Two separate cohorts of AAV-IL-6 and AAV-EV mice were analyzed in this study: 4 months or 2 months following intrahippocampal αSyn seeding.

Results

Here, we show that IL-6 expression resulted in widespread gliosis and concurrently reduced αSyn inclusion pathology induced by a single intra-hippocampal injection of exogenous amyloidogenic αSyn. The reduction in αSyn inclusion pathology in IL-6 expressing mice was time-dependent. Suppression of αSyn pathology was accompanied by reductions in both argyrophilic and p62 immunoreactive inclusions.

Conclusions

Our data supports a beneficial role of inflammatory priming of the CNS in wild type mice challenged with exogenous αSyn. A likely mechanism is efficient astroglial scavenging of exogenous αSyn, at least early in the disease process, and in the absence of human αSyn transgene overexpression. Given evidence that a pro-inflammatory environment may restrict seeding of αSyn pathology, this can be used to design anti-αSyn immunobiotherapies by harnessing innate immune function.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0142-z) contains supplementary material, which is available to authorized users.

Environmental and genetic factors support the dissociation between α-synuclein aggregation and toxicity

Synucleinopathies are a group of progressive disorders characterized by the abnormal aggregation and accumulation of α-synuclein (aSyn), an abundant neuronal protein that can adopt different conformations and biological properties. Recently, aSyn pathology was shown to spread between neurons in a prion-like manner. Proteins like aSyn that exhibit self-propagating capacity appear to be able to adopt different stable conformational states, known as protein strains, which can be modulated both by environmental and by protein-intrinsic factors. Here, we analyzed these factors and found that the unique combination of the neurodegeneration-related metal copper and the pathological H50Q aSyn mutation induces a significant alteration in the aggregation properties of aSyn. We compared the aggregation of WT and H50Q aSyn with and without copper, and assessed the effects of the resultant protein species when applied to primary neuronal cultures. The presence of copper induces the formation of structurally different and less-damaging aSyn aggregates. Interestingly, these aggregates exhibit a stronger capacity to induce aSyn inclusion formation in recipient cells, which demonstrates that the structural features of aSyn species determine their effect in neuronal cells and supports a lack of correlation between toxicity and inclusion formation. In total, our study provides strong support in favor of the hypothesis that protein aggregation is not a primary cause of cytotoxicity.

Prion-like disorders and Transmissible Spongiform Encephalopathies: An overview of the mechanistic features that are shared by the various disease-related misfolded proteins

Prion diseases or Transmissible Spongiform Encephalopathies (TSEs) are a group of fatal neurodegenerative disorders affecting several mammalian species. Its causative agent, disease-associated prion protein (PrP^d), is a self-propagating β-sheet rich aberrant conformation of the cellular prion protein (PrP^C) with neurotoxic and aggregation-prone properties, capable of inducing misfolding of PrP^C molecules. PrP^d is the major constituent of prions and, most importantly, is the first known example of a protein with infectious attributes. It has been suggested that similar molecular mechanisms could be shared by other proteins implicated in diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis or systemic amyloidoses. Accordingly, several terms have been proposed to collectively group all these disorders. Through the stringent evaluation of those aspects that characterise TSE-causing prions, in particular propagation and spread, strain variability or transmissibility, we will discuss whether terms such as "prion", "prion-like", "prionoid" or "propagon" can be used when referring to the aetiological agents of the above other disorders. Moreover, it will also be discussed whether the term "infectious", which defines a prion essential trait, is currently misused when referring to the other misfolded proteins.

Proteolysis of α-synuclein fibrils in the lysosomal pathway limits induction of inclusion pathology

Progression of α-synuclein inclusion pathology may occur through cycles of release and uptake of α-synuclein aggregates, which induce additional intracellular α-synuclein inclusion pathology. This process may explain (i) the presence of α-synuclein inclusion pathology in grafted cells in human brains, and (ii) the slowly progressive nature of most human α-synucleinopathies. It also provides a rationale for therapeutic targeting of extracellular aggregates to limit pathology spread. We investigated the cellular mechanisms underlying intraneuronal α-synuclein aggregation following exposure to exogenous preformed α-synuclein amyloid fibrils. Exogenous α-synuclein fibrils efficiently attached to cell membranes and were subsequently internalized and degraded within the endosomal/lysosomal system. However, internalized α-synuclein amyloid fibrils can apparently overwhelm the endosomal/lysosomal machinery leading to the induction of intraneuronal α-synuclein inclusions comprised of endogenous α-synuclein. Furthermore, the efficiency of inclusion formation was relatively low in these studies compared to studies using primary neuronal-glial cultures over-expressing α-synuclein. Our study indicates that under physiologic conditions, endosomal/lysosomal function acts as an endogenous barrier to the induction of α-synuclein inclusion pathology, but when compromised, it may lower the threshold for pathology induction/transmission. Cover Image for this issue: doi: 10.1111/jnc.13787.

Novel antibodies to phosphorylated α-synuclein serine 129 and NFL serine 473 demonstrate the close molecular homology of these epitopes

Pathological inclusions containing aggregated, highly phosphorylated (at serine129) α-synuclein (αS pSer129) are characteristic of a group of neurodegenerative diseases termed synucleinopathies. Antibodies to the pSer129 epitope can be highly sensitive in detecting αS inclusions in human tissue and experimental models of synucleinopathies. However, the generation of extensively specific pSer129 antibodies has been problematic, in some cases leading to the misinterpretation of αS inclusion pathology. One common issue is cross-reactivity to the low molecular mass neurofilament subunit (NFL) phosphorylated at Ser473. Here, we generated a series of monoclonal antibodies to the pSer129 αS and pSer473 NFL epitopes. We determined the relative abilities of the known αS kinases, polo-like kinases (PLK) 1, 2 and 3 and casein kinase (CK) II in phosphorylating NFL and αS, while using this information to characterize the specificity of the new antibodies. NFL can be phosphorylated by PLK1, 2 and 3 at Ser473; however CKII shows the highest phosphorylation efficiency and specificity for this site. Conversely, PLK3 is the most efficient kinase at phosphorylating αS at Ser129, but there is overlay in the ability of these kinases to phosphorylate both epitopes. Antibody 4F8, generated to the pSer473 NFL epitope, was relatively specific for phosphorylated NFL, however it could uniquely cross-react with pSer129 αS when highly phosphorylated, further showing the structural similarity between these phospho-epitopes. All of the new pSer129 antibodies detected pathological αS inclusions in human brains and mouse and cultured cell experimental models of induced synucleinopathies. Several of these pSer129 αS antibodies reacted with the pSer473 NFL epitope, but 2 clones (LS3-2C2 and LS4-2G12) did not. However, LS3-2C2 demonstrated cross-reactivity with other proteins. Our findings further demonstrate the difficulties in generating specific pSer129 αS antibodies, but highlights that the use of multiple antibodies, such as those generated here, can provide a sensitive and accurate assessment of αS pathology.

Bee venom phospholipase A2 ameliorates motor dysfunction and modulates microglia activation in Parkinson's disease alpha-synuclein transgenic mice

α-Synuclein (α-Syn) has a critical role in microglia-mediated neuroinflammation, which leads to the development of Parkinson's disease (PD). Recent studies have shown that bee venom (BV) has beneficial effects on PD symptoms in human patients or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxin-induced PD mice. This study investigated whether treatment with BV-derived phospholipase A2 (bvPLA2) would improve the motor dysfunction and pathological features of PD in human A53T α-Syn mutant transgenic (A53T Tg) mice. The motor dysfunction of A53T Tg mice was assessed using the pole test. The levels of α-Syn, microglia and the M1/M2 phenotype in the spinal cord were evaluated by immunofluorescence. bvPLA2 treatment significantly ameliorated motor dysfunction in A53T Tg mice. In addition, bvPLA2 significantly reduced the expression of α-Syn, the activation and numbers of microglia, and the ratio of M1/M2 in A53T Tg mice. These results suggest that bvPLA2 could be a promising treatment option for PD.

The Effect of Fragmented Pathogenic α-Synuclein Seeds on Prion-like Propagation

Aggregates of abnormal proteins are widely observed in neuronal and glial cells of patients with various neurodegenerative diseases, and it has been proposed that prion-like behavior of these proteins can account for not only the onset but also the progression of these diseases. However, it is not yet clear which abnormal protein structures function most efficiently as seeds for prion-like propagation. In this study, we aimed to identify the most pathogenic species of α-synuclein (α-syn), the main component of the Lewy bodies and Lewy neurites that are observed in α-synucleinopathies. We prepared various forms of α-syn protein and examined their seeding properties in vitro in cells and in mouse experimental models. We also characterized these α-syn species by means of electron microscopy and thioflavin fluorescence assays and found that fragmented β sheet-rich fibrous structures of α-syn with a length of 50 nm or less are the most efficient promoters of accumulation of phosphorylated α-syn, which is the hallmark of α-synucleinopathies. These results indicate that fragmented amyloid-like aggregates of short α-syn fibrils are the key pathogenic seeds that trigger prion-like conversion.

Prion-like propagation of mutant SOD1 misfolding and motor neuron disease spread along neuroanatomical pathways

A hallmark feature of amyotrophic lateral sclerosis (ALS) is that symptoms appear to spread along neuroanatomical pathways to engulf the motor nervous system, suggesting a propagative toxic entity could be involved in disease pathogenesis. Evidence for such a propagative entity emerged recently in studies using mice that express G85R-SOD1 mutant protein fused to YFP (G85R-SOD1:YFP). Heterozygous G85R-SOD1:YFP transgenic mice do not develop ALS symptoms out to 20 months of age. However, when newborns are injected with spinal homogenates from paralyzed mutant SOD1 mice, the G85R-SOD1:YFP mice develop paralysis as early as 6 months of age. We now demonstrate that injecting spinal homogenates from paralyzed mutant SOD1 mice into the sciatic nerves of adult G85R-SOD1:YFP mice produces a spreading motor neuron disease within 3.0 ± 0.2 months of injection. The formation of G85R-SOD1:YFP inclusion pathology spreads slowly in this model system; first appearing in the ipsilateral DRG, then lumbar spinal cord, before spreading rostrally up to the cervical cord by the time mice develop paralysis. Reactive astrogliosis mirrors the spread of inclusion pathology and motor neuron loss is most severe in lumbar cord. G85R-SOD1:YFP inclusion pathology quickly spreads to discrete neurons in the brainstem and midbrain that are synaptically connected to spinal neurons, suggesting a trans-synaptic propagation of misfolded protein. Taken together, the data presented here describe the first animal model that recapitulates the spreading phenotype observed in patients with ALS, and implicates the propagation of misfolded protein as a potential mechanism for the spreading of motor neuron disease.

Propagation of alpha-synuclein pathology: hypotheses, discoveries, and yet unresolved questions from experimental and human brain studies

Progressive aggregation of alpha-synuclein (αS) through formation of amorphous pale bodies to mature Lewy bodies or in neuronal processes as Lewy neurites may be the consequence of conformational protein changes and accumulations, which structurally represents "molecular template". Focal initiation and subsequent spread along anatomically connected structures embody "structural template". To investigate the hypothesis that both processes might be closely associated and involved in the progression of αS pathology, which can be observed in human brains, αS amyloidogenic precursors termed "seeds" were experimentally injected into the brain or peripheral nervous system of animals. Although these studies showed that αS amyloidogenic seeds can induce αS pathology, which can spread in the nervous system, the findings are still not unequivocal in demonstrating predominant transsynaptic or intraneuronal spreads either in anterograde or retrograde directions. Interpretation of some of these studies is further complicated by other concurrent aberrant processes including neuroimmune activation, injury responses and/or general perturbation of proteostasis. In human brain, αS deposition and neuronal degeneration are accentuated in distal axon/synapse. Hyperbranching of axons is an anatomical commonality of Lewy-prone systems, providing a structural basis for abundance in distal axons and synaptic terminals. This neuroanatomical feature also can contribute to such distal accentuation of vulnerability in neuronal demise and the formation of αS inclusion pathology. Although retrograde progression of αS aggregation in hyperbranching axons may be a consistent feature of Lewy pathology, the regional distribution and gradient of Lewy pathology are not necessarily compatible with a predictable pattern such as upward progression from lower brainstem to cerebral cortex. Furthermore, "focal Lewy body disease" with the specific isolated involvement of autonomic, olfactory or cardiac systems suggests that spread of αS pathology is not always consistent. In many instances, the regional variability of Lewy pathology in human brain cannot be explained by a unified hypothesis such as transsynaptic spread. Thus, the distribution of Lewy pathology in human brain may be better explained by variable combinations of independent focal Lewy pathology to generate "multifocal Lewy body disease" that could be coupled with selective but variable neuroanatomical spread of αS pathology. More flexible models are warranted to take into account the relative propensity to develop Lewy pathology in different Lewy-prone systems, even without interconnections, compatible with the expanding clinicopathological spectra of Lewy-related disorders. These revised models are useful to better understand the mechanisms underlying the variable progression of Lewy body diseases so that diagnostic and therapeutic strategies are improved.

The prion hypothesis of Parkinson's disease

The discovery of alpha-synuclein's prion-like behaviors in mammals, as well as a non-Mendelian type of inheritance, has led to a new concept in biology, the "prion hypothesis" of Parkinson's disease. The misfolding and aggregation of alpha-synuclein (α-syn) within the nervous system occur in many neurodegenerative diseases including Parkinson's disease (PD), Lewy body dementia (LBD), and multiple system atrophy (MSA). The molecular basis of synucleinopathies appears to be tightly coupled to α-syn's conformational conversion and fibril formation. The pathological form of α-syn consists of oligomers and fibrils with rich in β-sheets. The conversion of its α-helical structure to the β-sheet rich fibril is a defining pathologic feature of α-syn. These kinds of disorders have been classified as protein misfolding diseases or proteopathies which share key biophysical and biochemical characteristics with prion diseases. In this review, we highlight α-syn's prion-like activities in PD and PD models, describe the idea of a prion-like mechanism contributing to PD pathology, and discuss several key molecules that can modulate the α-syn accumulation and propagation.

Studies of lipopolysaccharide effects on the induction of α-synuclein pathology by exogenous fibrils in transgenic mice

BACKGROUND

Parkinson's disease (PD) is a progressive neurodegenerative disorder that is pathologically characterized by loss of dopaminergic neurons from the substantia nigra, the presence of aggregated α-synuclein (αS) and evidence of neuroinflammation. Experimental studies have shown that the cerebral injection of recombinant fibrillar αS, especially in αS transgenic mouse models, can induce the formation and spread of αS inclusion pathology. However, studies reporting this phenomenon did not consider the presence of lipopolysaccharide (LPS) in the injected αS, produced in E. coli, as a potential confound. The objectives of this study are to develop a method to remove the LPS contamination and investigate the differences in pathologies induced by αS containing LPS or αS highly purified of LPS.

RESULTS AND CONCLUSIONS

We were able to remove >99.5% of the LPS contamination from the αS preparations through the addition of a cation exchange step during purification. The αS pathology induced by injection of fibrils produced from αS containing LPS or purified of LPS, showed a similar distribution pattern; however, there was less spread into the cortex of the mice injected with αS containing higher levels of LPS. As previously reported, injection of αS fibrils could induce astrogliosis, and αS inclusions were present within astrocytes in mice injected with fibrils comprised of αS with or without cation exchange purification. Furthermore, we identified the presence of αS pathology in ependymal cells in both groups of mice, which suggests the involvement of a novel mechanism for spread in this model of αS pathology.

Viral expression of ALS-linked ubiquilin-2 mutants causes inclusion pathology and behavioral deficits in mice

Background

UBQLN2 mutations have recently been associated with familial forms of amyotrophic lateral sclerosis (ALS) and ALS-dementia. UBQLN2 encodes for ubiquilin-2, a member of the ubiquitin-like protein family which facilitates delivery of ubiquitinated proteins to the proteasome for degradation. To study the potential role of ubiquilin-2 in ALS, we used recombinant adeno-associated viral (rAAV) vectors to express UBQLN2 and three of the identified ALS-linked mutants (P497H, P497S, and P506T) in primary neuroglial cultures and in developing neonatal mouse brains.

Results

In primary cultures rAAV2/8-mediated expression of UBQLN2 mutants resulted in inclusion bodies and insoluble aggregates. Intracerebroventricular injection of FVB mice at post-natal day 0 with rAAV2/8 expressing wild type or mutant UBQLN2 resulted in widespread, sustained expression of ubiquilin-2 in brain. In contrast to wild type, mutant UBQLN2 expression induced significant pathology with large neuronal, cytoplasmic inclusions and ubiquilin-2-positive aggregates in surrounding neuropil. Ubiquilin-2 inclusions co-localized with ubiquitin, p62/SQSTM, optineurin, and occasionally TDP-43, but were negative for α-synuclein, neurofilament, tau, and FUS. Mutant UBLQN2 expression also resulted in Thioflavin-S-positive inclusions/aggregates. Mice expressing mutant forms of UBQLN2 variably developed a motor phenotype at 3–4 months, including nonspecific clasping and rotarod deficits.

Conclusions

These findings demonstrate that UBQLN2 mutants (P497H, P497S, and P506T) induce proteinopathy and cause behavioral deficits, supporting a “toxic” gain-of-function, which may contribute to ALS pathology. These data establish also that our rAAV model can be used to rapidly assess the pathological consequences of various UBQLN2 mutations and provides an agile system to further interrogate the molecular mechanisms of ubiquilins in neurodegeneration.

Distinct higher-order α-synuclein oligomers induce intracellular aggregation

The cell-to-cell transmission of pathology in Parkinson's disease has been linked to soluble amyloid oligomers. Ion mobility spectrometry (IMS)–MS has been used to show that these soluble oligomers have a compact ring-like conformation.

The stress response neuropeptide CRF increases amyloid-β production by regulating γ-secretase activity

The biological underpinnings linking stress to Alzheimer's disease (AD) risk are poorly understood. We investigated how corticotrophin releasing factor (CRF), a critical stress response mediator, influences amyloid-β (Aβ) production. In cells, CRF treatment increases Aβ production and triggers CRF receptor 1 (CRFR1) and γ-secretase internalization. Co-immunoprecipitation studies establish that γ-secretase associates with CRFR1; this is mediated by β-arrestin binding motifs. Additionally, CRFR1 and γ-secretase co-localize in lipid raft fractions, with increased γ-secretase accumulation upon CRF treatment. CRF treatment also increases γ-secretase activity in vitro, revealing a second, receptor-independent mechanism of action. CRF is the first endogenous neuropeptide that can be shown to directly modulate γ-secretase activity. Unexpectedly, CRFR1 antagonists also increased Aβ. These data collectively link CRF to increased Aβ through γ-secretase and provide mechanistic insight into how stress may increase AD risk. They also suggest that direct targeting of CRF might be necessary to effectively modulate this pathway for therapeutic benefit in AD, as CRFR1 antagonists increase Aβ and in some cases preferentially increase Aβ42 via complex effects on γ-secretase.

Review: Prion-like mechanisms of transactive response DNA binding protein of 43 kDa (TDP-43) in amyotrophic lateral sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a fatal devastating neurodegenerative disorder which predominantly affects the motor neurons in the brain and spinal cord. The death of the motor neurons in ALS causes subsequent muscle atrophy, paralysis and eventual death. Clinical and biological evidence now demonstrates that ALS has many similarities to prion disease in terms of disease onset, phenotype variability and progressive spread. The pathognomonic ubiquitinated inclusions deposited in the neurons and glial cells in brains and spinal cords of patients with ALS and fronto-temporal lobar degeneration with ubiquitinated inclusions contain aggregated transactive response DNA binding protein of 43 kDa (TDP-43), and evidence now suggests that TDP-43 has cellular prion-like properties. The cellular mechanisms of prion protein misfolding and aggregation are thought to be responsible for the characteristics of prion disease. Therefore, there is a strong mechanistic basis for a prion-like behaviour of the TDP-43 protein being responsible for some characteristics of ALS. In this review, we compare the prion-like mechanisms of TDP-43 to the clinical and biological nature of ALS in order to investigate how this protein could be responsible for some of the characteristic properties of the disease.

In vitro aggregation assays for the characterization of α-synuclein prion-like properties

Aggregation of α-synuclein plays a crucial role in the pathogenesis of synucleinopathies, a group of neurodegenerative diseases including Parkinson disease (PD), dementia with Lewy bodies (DLB), diffuse Lewy body disease (DLBD) and multiple system atrophy (MSA). The common feature of these diseases is a pathological deposition of protein aggregates, known as Lewy bodies (LBs) in the central nervous system. The major component of these aggregates is α-synuclein, a natively unfolded protein, which may undergo dramatic structural changes resulting in the formation of β-sheet rich assemblies. In vitro studies have shown that recombinant α-synuclein protein may polymerize into amyloidogenic fibrils resembling those found in LBs. These aggregates may be uptaken and propagated between cells in a prion-like manner. Here we present the mechanisms and kinetics of α-synuclein aggregation in vitro, as well as crucial factors affecting this process. We also describe how PD-linked α-synuclein mutations and some exogenous factors modulate in vitro aggregation. Furthermore, we present a current knowledge on the mechanisms by which extracellular aggregates may be internalized and propagated between cells, as well as the mechanisms of their toxicity.

Dynamic droplets: the role of cytoplasmic inclusions in stress, function, and disease

Neurodegenerative diseases and other proteinopathies constitute a class of several dozen illnesses etiologically linked to pathological protein misfolding and aggregation. Because of this strong association with disease pathology, cell death, and aging, accumulation of proteins in aggregates or aggregation-associated structures (inclusions) has come to be regarded by many as a deleterious process, to be avoided if possible. Recent work has led us to see inclusion structures and disordered aggregate-like protein mixtures (which we call dynamic droplets) in a new light: not necessarily as a result of a pathological breakdown of cellular order, but as an elaborate cellular architecture regulating function and stress response. In this review, we discuss what is currently known about the role of inclusion structures in cellular homeostasis, stress response, toxicity, and disease. We will focus on possible mechanisms of aggregate toxicity, in contrast to the homeostatic function of several inclusion structures.

Brain injection of α-synuclein induces multiple proteinopathies, gliosis, and a neuronal injury marker

Intracerebral injection of amyloidogenic α-synuclein (αS) has been shown to induce αS pathology in the CNS of nontransgenic mice and αS transgenic mice, albeit with varying efficiencies. In this study, using wild-type human αS transgenic mice (line M20), we demonstrate that intracerebral injection of recombinant amyloidogenic or soluble αS induces extensive αS intracellular inclusion pathology that is associated with robust gliosis. Near the injection site, a significant portion of αS inclusions are detected in neurons but also in astrocytes and microglia. Aberrant induction of expression of the intermediate filament protein peripherin, which is associated with CNS neuronal injury, was also observed predominantly near the site of injection. In addition, many pSer129 αS-induced inclusions colocalize with the low-molecular-mass neurofilament subunit (NFL) or peripherin staining. αS inclusion pathology was also induced in brain regions distal from the injection site, predominantly in neurons. Unexpectedly, we also find prominent p62-immunoreactive, αS-, NFL-, and peripherin-negative inclusions. These findings provide evidence that exogenous αS challenge induces αS pathology but also results in the following: (1) a broader disruption of proteostasis; (2) glial activation; and (3) a marker of a neuronal injury response. Such data suggest that induction of αS pathology after exogenous seeding may involve multiple interdependent mechanisms.

Protein Transmission, Seeding and Degradation: Key Steps for α-Synuclein Prion-Like Propagation

Converging lines of evidence suggest that cell-to-cell transmission and the self-propagation of pathogenic amyloidogenic proteins play a central role in the initiation and the progression of several neurodegenerative disorders. This "prion-like" hypothesis has been recently reported for α-synuclein, a presynaptic protein implicated in the pathogenesis of Parkinson's disease (PD) and related disorders. This review summarizes recent findings on α-synuclein prion-like propagation, focusing on its transmission, seeding and degradation and discusses some key questions that remain to be explored. Understanding how α-synuclein exits cells and propagates from one brain region to another will lead to the development of new therapeutic strategies for the treatment of PD, aiming at slowing or stopping the disease progression.

Parkinson's disease as a member of Prion-like disorders

Parkinson's disease is one of several neurodegenerative diseases associated with a misfolded, aggregated and pathological protein. In Parkinson's disease this protein is alpha-synuclein and its neuronal deposits in the form of Lewy bodies are considered a hallmark of the disease. In this review we describe the clinical and experimental data that have led to think of alpha-synuclein as a prion-like protein and we summarize data from in vitro, cellular and animal models supporting this view.

New α- and γ-synuclein immunopathological lesions in human brain

Introduction

Several neurodegenerative diseases are classified as proteopathies as they are associated with the aggregation of misfolded proteins. Synucleinopathies are a group of neurodegenerative disorders associated with abnormal deposition of synucleins. α-Synucleinopathies include Parkinson’s disease, dementia with Lewy bodies, and multiple system atrophy. Recently accumulation of another member of the synuclein family- γ−synuclein in neurodegenerative diseases compelled the introduction of the term γ−synucleinopathy. The formation of aggregates and deposits of γ−synuclein is facilitated after its oxidation at methionine 38 (Met³⁸).

Results

Several types of intracytoplasmic inclusions containing post-translationally modified α- and γ−synucleins are detected. Oxidized Met³⁸-γ-synuclein forms aberrant inclusions in amygdala and substantia nigra. Double staining revealed colocalization of oxidized-γ-synuclein with α-synuclein in the cytoplasm of neurons. Another type of synuclein positive inclusions in the amygdala of dementia with Lewy bodies patients has the appearance of Lewy bodies. These inclusions are immunoreactive when analyzed with antibodies to α-synuclein phosphorylated on serine 129, as well as with antibodies to oxidized-γ-synuclein. Some of these Lewy bodies have doughnut-like shape with round or elongated shape. The separate immunofluorescent images obtained with individual antibodies specific to oxidized-γ-synuclein and phospho-α-synuclein clearly shows the colocalization of these synuclein isoforms in substantia nigra inclusions. Phospho-α-synuclein is present almost exclusively at the periphery of these structures, whereas oxidized-γ-syn immunoreactivity is also located in the internal parts forming dot-like pattern of staining.

We also identified several types of oxidized-γ-syn positive astrocytes with different morphology and examined their immunohistochemical phenotypes. Some of them are compact cells with short processes, others have longer processes. Oxidized-γ-synuclein positive astrocytes may also display mixed morphological and immunocytochemical phenotypes between protoplasmic and fibrous astrocytes.

Conclusions

These results reveal new γ−synuclein positive lesions in human brain. Oxidized-γ-synuclein is colocalized with phospho-α-synuclein in doughnut-like inclusions. Several types of astrocytes with different morphology are immunopositive for oxidized-γ-synuclein.