Amyloidogenic α-synuclein seeds do not invariably induce rapid, widespread pathology in mice

Limitations and Applications of Rodent Models in Tauopathy and Synucleinopathy Research

Rodent models that accurately recapitulate key aspects of human disease have long been fundamental to the successful development of clinical interventions. This is greatly underscored in the neurodegenerative disease field, where preclinical testing of anti-prion therapeutics against rodent-adapted prions resulted in the development of small molecules effective against rodent-adapted prions but not against human prions. These findings provided critical lessons for ongoing efforts to develop treatments for patients with neurodegenerative diseases caused by misfolding and accumulation of the proteins tau and α-synuclein, or tauopathies and synucleinopathies, respectively. To avoid the potential pitfalls previously identified in the prion field, this review focuses on rodent models currently available to study tau and α-synuclein disease pathogenesis, emphasizing the strengths and limitations of each with the particular goal of better supporting preclinical research.

Single-domain antibody-based protein degrader for synucleinopathies

Synucleinopathies are a group of neurodegenerative diseases characterized by the accumulation of α-synuclein (α-syn) in the brain, leading to motor and neuropsychiatric symptoms. Currently, there are no known cures for synucleinopathies, and treatments mainly focus on symptom management. In this study, we developed a single-domain antibody (sdAb)-based protein degrader with features designed to enhance proteasomal degradation of α-syn. This sdAb derivative targets both α-syn and Cereblon (CRBN), a substrate-receptor for the E3-ubiquitin ligase CRL4CRBN, and thereby induces α-syn ubiquitination and proteasomal degradation. Our results indicate that this therapeutic candidate enhances proteasomal degradation of α-syn, in addition to the endogenous lysosomal degradation machinery. By promoting proteasomal degradation of α-syn, we improved clearance of α-syn in primary culture and mouse models of synucleinopathy. These findings indicate that our sdAb-based protein degrader is a promising therapeutic candidate for synucleinopathies. Considering that only a small percentage of antibodies enter the brain, more potent sdAbs with greater brain entry than whole antibodies could enhance clinical benefits of antibody-based therapies.

Cell-to-cell transmitted alpha-synuclein recapitulates experimental Parkinson's disease

Parkinson's disease is characterized by a progressive accumulation of alpha-Synuclein (αSyn) neuronal inclusions called Lewy bodies in the nervous system. Lewy bodies can arise from the cell-to-cell propagation of αSyn, which can occur via sequential steps of secretion and uptake. Here, by fusing a removable short signal peptide to the N-terminus of αSyn, we developed a novel mouse model with enhanced αSyn secretion and cell-to-cell transmission. Expression of the secreted αSyn in the mouse brain was under the control of a novel hybrid promoter in combination with adeno-associated virus serotype 9 (AAV9). This combination of promoter and viral vector induced a robust expression in neurons but not in the glia of injected mice. Biochemical characterization of the secreted αSyn revealed that, in cultured cells, this protein is released to the extracellular milieu via conventional secretion. The released αSyn is then internalized and processed by acceptor cells via the endosome-lysosome pathway indicating that the secreted αSyn is cell-to-cell transmitted. The secreted αSyn is aggregation-prone and amyloidogenic, and when expressed in the brain of wild-type non-transgenic mice, it induces a Parkinson's disease-like phenotype that includes a robust αSyn pathology in the substantia nigra, neuronal loss, neuroinflammation, and motor deficits, all the key features of experimental animal models of Parkinson's disease. In summary, a novel animal model of Parkinson's disease based on enhanced cell-to-cell transmission of αSyn was developed. The neuron-produced cell-to-cell transmitted αSyn triggers all phenotypic features of experimental Parkinson's disease in mice.

Cellular processing of α-synuclein fibrils results in distinct physiological C-terminal truncations with a major cleavage site at residue Glu 114

α-synuclein (αS) is an abundant, neuronal protein that assembles into fibrillar pathological inclusions in a spectrum of neurodegenerative diseases that include Lewy body diseases (LBD) and Multiple System Atrophy (MSA). The cellular and regional distributions of pathological inclusions vary widely between different synucleinopathies contributing to the spectrum of clinical presentations. Extensive cleavage within the carboxy (C)-terminal region of αS is associated with inclusion formation, although the events leading to these modifications and the implications for pathobiology are of ongoing study. αS preformed fibrils can induce prion-like spread of αS pathology in both in vitro and animal models of disease. Using C truncation-specific antibodies, we demonstrated here that prion-like cellular uptake and processing of αS preformed fibrils resulted in two major cleavages at residues 103 and 114. A third cleavage product (122 αS) accumulated upon application of lysosomal protease inhibitors. In vitro, both 1-103 and 1-114 αS polymerized rapidly and extensively in isolation and in the presence of full-length αS. 1-103 αS also demonstrated more extensive aggregation when expressed in cultured cells. Furthermore, we used novel antibodies to αS cleaved at residue Glu114, to assess x-114 αS pathology in postmortem brain tissue from patients with LBD and MSA, as well as three different transgenic αS mouse models of prion-like induction. The distribution of x-114 αS pathology was distinct from that of overall αS pathology. These studies reveal the cellular formation and behavior of αS C-truncated at residues 114 and 103 as well as the disease dependent distribution of x-114 αS pathology.

The Synucleins and the Astrocyte

Synucleins consist of three proteins exclusively expressed in vertebrates. α-Synuclein (αS) has been identified as the main proteinaceous aggregate in Lewy bodies, a pathological hallmark of many neurodegenerative diseases. Less is understood about β-synuclein (βS) and γ-synuclein (γS), although it is known βS can interact with αS in vivo to inhibit aggregation. Likewise, both γS and βS can inhibit αS's propensity to aggregate in vitro. In the central nervous system, βS and αS, and to a lesser extent γS, are highly expressed in the neural presynaptic terminal, although they are not strictly located there, and emerging data have shown a more complex expression profile. Synapse loss and astrocyte atrophy are early aspects of degenerative diseases of the brain and correlate with disease progression. Synucleins appear to be involved in synaptic transmission, and astrocytes coordinate and organize synaptic function, with excess αS degraded by astrocytes and microglia adjacent to the synapse. βS and γS have also been observed in the astrocyte and may provide beneficial roles. The astrocytic responsibility for degradation of αS as well as emerging evidence on possible astrocytic functions of βS and γS, warrant closer inspection on astrocyte-synuclein interactions at the synapse.

Neuroinflammation, immune response and α-synuclein pathology: how animal models are helping us to connect dots

INTRODUCTION

A key pathological event occurring in Parkinson's disease (PD) is the transneuronal spreading of alpha-synuclein (α-syn). Other hallmarks of PD include neurodegeneration, glial activation, and immune cell infiltration in susceptible brain regions. Although preclinical models can mimic most of the key characteristics of PD, it is crucial to know the biological bases of individual differences between them when choosing one over another, to ensure proper interpretation of the results and to positively influence the outcome of the experiments.

AREAS COVERED

This review provides an overview of current preclinical models actively used to study the interplay between α-syn pathology, neuroinflammation and immune response in PD but also to explore new potential preclinical models or emerging therapeutic strategies intended to fulfill the unmet medical needs in this disease. Lastly, this review also considers the current state of the ongoing clinical trials of new drugs designed to target these processes and delay the initiation or progression of the disease.

EXPERT OPINION

Anti-inflammatory and immunomodulatory agents have been demonstrated to be very promising candidates for reducing disease progression; however, more efforts are needed to reduce the enormous gap between these and dopaminergic drugs, which have dominated the therapeutic market for the last sixty years.

Specific Detection of Physiological S129 Phosphorylated α-Synuclein in Tissue Using Proximity Ligation Assay

BACKGROUND

Synucleinopathies are a group of neurodegenerative disorders that are pathologically characterized by intracellular aggregates called Lewy bodies. Lewy bodies are primarily composed of α-synuclein (asyn) protein, which is mostly phosphorylated at serine 129 (pS129) when aggregated and therefore used as a marker for pathology. Currently commercial antibodies against pS129 asyn stain aggregates well but in healthy brains cross react with other proteins, thus making it difficult to specifically detect physiological pS129 asyn.

OBJECTIVE

To develop a staining procedure that detects endogenous and physiological relevant pS129 asyn with high specificity and low background.

METHODS

We used the fluorescent and brightfield in situ proximity ligation assay (PLA) to specifically detect pS129 asyn in cell culture, mouse, and human brain sections.

RESULTS

The pS129 asyn PLA specifically stained physiological and soluble pS129 asyn in cell culture, mouse brain sections, and human brain tissue without significant cross-reactivity or background signal. However, this technique was not successful in detecting Lewy bodies in human brain tissue.

CONCLUSION

We successfully developed a novel PLA method that can, in the future, be used on in vitro and in vivo samples as a tool to explore and better understand the cellular localization and function of pS129 asyn in health and disease.

The E46K mutation modulates α-synuclein prion replication in transgenic mice

In multiple system atrophy (MSA), the α-synuclein protein misfolds into a self-templating prion conformation that spreads throughout the brain, leading to progressive neurodegeneration. While the E46K mutation in α-synuclein causes familial Parkinson's disease (PD), we previously discovered that this mutation blocks in vitro propagation of MSA prions. Recent studies by others indicate that α-synuclein adopts a misfolded conformation in MSA in which a Greek key motif is stabilized by an intramolecular salt bridge between residues E46 and K80. Hypothesizing that the E46K mutation impedes salt bridge formation and, therefore, exerts a selective pressure that can modulate α-synuclein strain propagation, we asked whether three distinct α-synuclein prion strains could propagate in TgM47+/- mice, which express human α-synuclein with the E46K mutation. Following intracranial injection of these strains, TgM47+/- mice were resistant to MSA prion transmission, whereas recombinant E46K preformed fibrils (PFFs) transmitted neurological disease to mice and induced the formation of phosphorylated α-synuclein neuropathology. In contrast, heterotypic seeding following wild-type (WT) PFF-inoculation resulted in preclinical α-synuclein prion propagation. Moreover, when we inoculated TgM20+/- mice, which express WT human α-synuclein, with E46K PFFs, we observed delayed transmission kinetics with an incomplete attack rate. These findings suggest that the E46K mutation constrains the number of α-synuclein prion conformations that can propagate in TgM47+/- mice, expanding our understanding of the selective pressures that impact α-synuclein prion replication.

Revisiting the specificity and ability of phospho-S129 antibodies to capture alpha-synuclein biochemical and pathological diversity

Antibodies against phosphorylated alpha-synuclein (aSyn) at S129 have emerged as the primary tools to investigate, monitor, and quantify aSyn pathology in the brain and peripheral tissues of patients with Parkinson's disease and other neurodegenerative diseases. Herein, we demonstrate that the co-occurrence of multiple pathology-associated C-terminal post-translational modifications (PTMs) (e.g., phosphorylation at Tyrosine 125 or truncation at residue 133 or 135) differentially influences the detection of pS129-aSyn species by pS129-aSyn antibodies. These observations prompted us to systematically reassess the specificity of the most commonly used pS129 antibodies against monomeric and aggregated forms of pS129-aSyn in mouse brain slices, primary neurons, mammalian cells and seeding models of aSyn pathology formation. We identified two antibodies that are insensitive to pS129 neighboring PTMs. Although most pS129 antibodies showed good performance in detecting aSyn aggregates in cells, neurons and mouse brain tissue containing abundant aSyn pathology, they also showed cross-reactivity towards other proteins and often detected non-specific low and high molecular weight bands in aSyn knock-out samples that could be easily mistaken for monomeric or high molecular weight aSyn species. Our observations suggest that not all pS129 antibodies capture the biochemical and morphological diversity of aSyn pathology, and all should be used with the appropriate protein standards and controls when investigating aSyn under physiological conditions. Finally, our work underscores the need for more pS129 antibodies that are not sensitive to neighboring PTMs and more thorough characterization and validation of existing and new antibodies.

The Quest for Anti-α-Synuclein Antibody Specificity-Lessons Learnt From Flow Cytometry Analysis

The accumulation of alpha-synuclein (aSyn) is the hallmark of a group of neurodegenerative conditions termed synucleopathies. Physiological functions of aSyn, including those outside of the CNS, remain elusive. However, a reliable and reproducible evaluation of aSyn protein expression in different cell types and especially in low-expressing cells is impeded by the existence of a huge variety of poorly characterized anti-aSyn antibodies and a lack of a routinely used sensitive detection methods. Here, we developed a robust flow cytometry-based workflow for aSyn detection and antibody validation. We test our workflow using three commercially available antibodies (MJFR1, LB509, and 2A7) in a variety of human cell types, including induced pluripotent stem cells, T lymphocytes, and fibroblasts, and provide a cell- and antibody-specific map for aSyn expression. Strikingly, we demonstrate a previously unobserved unspecificity of the LB509 antibody, while the MJFR1 clone revealed specific aSyn binding however with low sensitivity. On the other hand, we identified an aSyn-specific antibody clone 2A7 with an optimal sensitivity for detecting aSyn in a range of cell types, including those with low aSyn expression. We further utilize our workflow to demonstrate the ability of the 2A7 antibody to distinguish between physiological differences in aSyn expression in neuronal and non-neuronal cells from the cortical organoids, and in neural progenitors and midbrain dopaminergic neurons from healthy controls and in patients with Parkinson's disease who have aSyn gene locus duplication. Our results provide a proof of principle for the use of high-throughput flow cytometry-based analysis of aSyn and highlight the necessity of rigorous aSyn antibody validation to facilitate the research of aSyn physiology and pathology.

Consequences of variability in α-synuclein fibril structure on strain biology

Synucleinopathies are a group of clinically and neuropathologically distinct protein misfolding diseases caused by unique α-synuclein conformations, or strains. While multiple atomic resolution cryo-electron microscopy structures of α-synuclein fibrils are now deposited in Protein Data Bank, significant gaps in the biological consequences arising from each conformation have yet to be unraveled. Mutations in the α-synuclein gene (SNCA), cofactors, and the solvation environment contribute to the formation and maintenance of each disease-causing strain. This review highlights the impact of each of these factors on α-synuclein misfolding and discusses the implications of the resulting structural variability on therapeutic development.

Ischemic stroke causes Parkinson's disease-like pathology and symptoms in transgenic mice overexpressing alpha-synuclein

The etiology of Parkinson's disease is poorly understood and is most commonly associated with advancing age, genetic predisposition, or environmental toxins. Epidemiological findings suggest that patients have a higher risk of developing Parkinson's disease after ischemic stroke, but this potential causality lacks mechanistic evidence. We investigated the long-term effects of ischemic stroke on pathogenesis in hemizygous TgM83 mice, which express human α-synuclein with the familial A53T mutation without developing any neuropathology or signs of neurologic disease for more than 600 days. We induced transient focal ischemia by middle cerebral artery occlusion in 2-month-old TgM83+/- mice and monitored their behavior and health status for up to 360 days post surgery. Groups of mice were sacrificed at 14, 30, 90, 180, and 360 days after surgery for neuropathological analysis of their brains. Motor deficits first appeared 6 months after focal ischemia and worsened until 12 months afterward. Immunohistochemical analysis revealed ischemia-induced neuronal loss in the infarct region and astrogliosis and microgliosis indicative of an inflammatory response, which was most pronounced at 14 days post surgery. Infarct volume and inflammation gradually decreased in size and severity until 180 days post surgery. Surprisingly, neuronal loss and inflammation were increased again by 360 days post surgery. These changes were accompanied by a continuous increase in α-synuclein aggregation, its neuronal deposition, and a late loss of dopaminergic neurons in the substantia nigra, which we detected at 360 days post surgery. Control animals that underwent sham surgery without middle cerebral artery occlusion showed no signs of disease or neuropathology. Our results establish a mechanistic link between ischemic stroke and Parkinson's disease and provide an animal model for studying possible interventions.

Characterization of a Novel Monoclonal Antibody for Serine-129 Phosphorylated α-Synuclein: A Potential Application for Clinical and Basic Research

The Lewy bodies (LBs) are the pathological hallmark of Parkinson's disease (PD). More than 90% of α-synuclein (α-syn) within LBs is phosphorylated at the serine-129 residue [pSer129 α-syn (p-α-syn)]. Although various studies have revealed that this abnormally elevated p-α-syn acts as a pathological biomarker and is involved in the pathogenic process of PD, the exact pathophysiological mechanisms of p-α-syn are still not fully understood. Therefore, the development of specific and reliable tools for p-α-syn detection is important. In this study, we generated a novel p-α-syn mouse monoclonal antibody (C140S) using hybridoma technology. To further identify the characteristics of C140S, we performed several in vitro assays using recombinant proteins, along with ex vivo assays utilizing the brains of Thy1-SNCA transgenic (Tg) mice, the preformed fibril (PFF)-treated neurons, and the brain sections of patients with PD. Our C140S specifically recognized human and mouse p-α-syn proteins both in vitro and ex vivo, and similar to commercial p-α-syn antibodies, the C140S detected higher levels of p-α-syn in the midbrain of the Tg mice. Using immunogold electron microscopy, these p-α-syn particles were partly deposited in the cytoplasm and colocalized with the outer mitochondrial membrane. In addition, the C140S recognized p-α-syn pathologies in the PFF-treated neurons and the amygdala of patients with PD. Overall, the C140S antibody was a specific and potential research tool in the detection and mechanistic studies of pathogenic p-α-syn in PD and related synucleinopathies.

Different α-synuclein prion strains cause dementia with Lewy bodies and multiple system atrophy

Dementia with Lewy bodies (DLB) and multiple system atrophy (MSA) are caused by α-synuclein prions that differ from each other and from those causing Parkinson’s disease (PD). DLB prions differ in their infectivity from those causing MSA or PD. The wild-type, normal version of the α-synuclein protein has the acidic amino acid glutamate (E) at residue 46, while in cases of inherited PD, it is mutated to the basic amino acid lysine (K). Using genetically engineered α-synuclein, we identified unique conditions for propagating MSA and DLB prions. Being able to distinguish among strains of naturally occurring α-synuclein prions may make it possible to develop strain-specific therapeutics for MSA, DLB, and PD.

The α-synuclein protein can adopt several different conformations that cause neurodegeneration. Different α-synuclein conformers cause at least three distinct α-synucleinopathies: multiple system atrophy (MSA), dementia with Lewy bodies (DLB), and Parkinson’s disease (PD). In earlier studies, we transmitted MSA to transgenic (Tg) mice and cultured HEK cells both expressing mutant α-synuclein (A53T) but not to cells expressing α-synuclein (E46K). Now, we report that DLB is caused by a strain of α-synuclein prions that is distinct from MSA. Using cultured HEK cells expressing mutant α-synuclein (E46K), we found that DLB prions could be transmitted to these HEK cells. Our results argue that a third strain of α-synuclein prions likely causes PD, but further studies are needed to identify cells and/or Tg mice that express a mutant α-synuclein protein that is permissive for PD prion replication. Our findings suggest that other α-synuclein mutants should give further insights into α-synuclein prion replication, strain formation, and disease pathogenesis, all of which are likely required to discover effective drugs for the treatment of PD as well as the other α-synucleinopathies.

Rapid Induction of Dopaminergic Neuron Loss Accompanied by Lewy Body-Like Inclusions in A53T BAC-SNCA Transgenic Mice

Parkinson's disease (PD), the most common neurodegenerative movement disorder, is characterized by dopaminergic neuron loss in the substantia nigra pars compacta (SNpc) and intraneuronal α-synuclein (α-syn) inclusions. It is highly needed to establish a rodent model that recapitulates the clinicopathological features of PD within a short period to efficiently investigate the pathological mechanisms and test disease-modifying therapies. To this end, we analyzed three mouse lines, i.e., wild-type mice, wild-type human α-syn bacterial artificial chromosome (BAC) transgenic (BAC-SNCA Tg) mice, and A53T human α-syn BAC transgenic (A53T BAC-SNCA Tg) mice, receiving dorsal striatum injections of human and mouse α-syn preformed fibrils (hPFFs and mPFFs, respectively). mPFF injections induced more severe α-syn pathology in most brain regions, including the ipsilateral SNpc, than hPFF injections in all genotypes at 1-month post-injection. Although these Tg mouse lines expressed a comparable amount of α-syn in the brains, the mPFF-injected A53T BAC-SNCA Tg mice exhibited the most severe α-syn pathology as early as 0.5-month post-injection. The mPFF-injected A53T BAC-SNCA Tg mice showed a 38% reduction in tyrosine hydroxylase (TH)-positive neurons in the ipsilateral SNpc, apomorphine-induced rotational behavior, and motor dysfunction at 2 months post-injection. These data indicate that the extent of α-syn pathology induced by α-syn PFF injection depends on the types of α-syn PFFs and exogenously expressed α-syn in Tg mice. The mPFF-injected A53T BAC-SNCA Tg mice recapitulate the key features of PD more rapidly than previously reported mouse models, suggesting their usefulness for testing disease-modifying therapies as well as analyzing the pathological mechanisms.

Structural and Functional Insights into α-Synuclein Fibril Polymorphism

Abnormal accumulation of aggregated α-synuclein (α-Syn) is seen in a variety of neurodegenerative diseases, including Parkinson's disease (PD), multiple system atrophy (MSA), dementia with Lewy body (DLB), Parkinson's disease dementia (PDD), and even subsets of Alzheimer's disease (AD) showing Lewy-body-like pathology. These synucleinopathies exhibit differences in their clinical and pathological representations, reminiscent of prion disorders. Emerging evidence suggests that α-Syn self-assembles and polymerizes into conformationally diverse polymorphs in vitro and in vivo, similar to prions. These α-Syn polymorphs arising from the same precursor protein may exhibit strain-specific biochemical properties and the ability to induce distinct pathological phenotypes upon their inoculation in animal models. In this review, we discuss clinical and pathological variability in synucleinopathies and several aspects of α-Syn fibril polymorphism, including the existence of high-resolution molecular structures and brain-derived strains. The current review sheds light on the recent advances in delineating the structure-pathogenic relationship of α-Syn and how diverse α-Syn molecular polymorphs contribute to the existing clinical heterogeneity in synucleinopathies.

Dose-related biphasic effect of the Parkinson's disease neurotoxin MPTP, on the spread, accumulation, and toxicity of α-synuclein

BACKGROUND

Parkinson's disease (PD), the second most common progressive neurodegenerative disorder, is characterized by the abnormal accumulation of intraneuronal inclusions enriched in aggregated α-synuclein (α-syn), known as Lewy bodies (LBs) and Lewy neurites (LNs), and significant loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) of the brain. Recent evidence suggests that the intrastriatal inoculation of α-syn preformed fibrils (PFF) in mice brain triggers endogenous α-syn in interconnected brain regions. 1-methyl, 4-phenyl, 1,2,3,6 tetrahydropyridine (MPTP), a mitochondrial neurotoxin, has been used previously to generate a PD mouse model. However, the common methods of MPTP exposure do not induce LB or α-syn aggregation in mice. In the present study, we evaluated the effect of different doses of MPTP (10 mg/kg.b.wt and/or 25 mg/kg.b.wt) on the spread, accumulation, and toxicity of endogenous α-syn in mice administered an intrastriatal injection of human α-syn PFF.

METHODS

We inoculated human WT α-syn PFF in mouse striatum. At 6 weeks post PFF injection, we challenged the animal with two different doses of MPTP (10 mg/kg.b.wt and 25 mg/kg.b.wt) once daily for five consecutive days. At 2 weeks from the start of the MPTP regimen, we collected the mice brain and performed immunohistochemical analysis, and Rotarod test to assess motor coordination and muscle strength before and after MPTP injection.

RESULTS

A single injection of human WT α-syn PFF in the mice striatum induced the propagation of α-syn, occurring as phosphorylated α-synuclein (pS129), towards the SNpc, within a very short time. Injection of a low dose of MPTP (10 mg/kg.b.wt) at 6 weeks post α-syn PFF inoculation further enhanced the spread, whereas a high dose of MPTP (25 mg/kg.b.wt.) reduced the spread. Majority of the accumulated α-syn were proteinase K resistant, as recognized using a conformation-specific α-syn antibody. Injection of α-syn PFF alone caused 12 % reduction in the number of tyrosine hydroxylase positive neurons while α-syn PFF + a low dose of MPTP caused 33 % reduction (loss), compared to the control mice injected with saline. This combination also reduced the motor coordination. Interestingly, a low dose of MPTP alone did not cause any significant reduction in the number of tyrosine hydroxylase positive neurons compared to saline treatment. Animals that received α-syn PFF and a high dose of MPTP showed massive activation of glial cells and decreased spread of α-syn, majority of which were detected in the nucleus.

CONCLUSION

Our results suggest that a combination of human WT α-syn PFF and a low dose of MPTP increases the pathological conversion and propagation of endogenous α-syn, and neurodegeneration, within a very short time. Our model can be used to study the mechanisms of α-syn propagation and screen for potential drugs against PD.

Generation and Characterization of Novel Monoclonal Antibodies Targeting p62/sequestosome-1 Across Human Neurodegenerative Diseases

Human neurodegenerative diseases can be characterized as disorders of protein aggregation. As a key player in cellular autophagy and the ubiquitin proteasome system, p62 may represent an effective immunohistochemical target, as well as mechanistic operator, across neurodegenerative proteinopathies. In this study, 2 novel mouse-derived monoclonal antibodies 5G3 and 2A5 raised against residues 360-380 of human p62/sequestosome-1 were characterized via immunohistochemical application upon human tissues derived from cases of C9orf72-expansion spectrum diseases, Alzheimer disease, progressive supranuclear palsy, Lewy body disease, and multiple system atrophy. 5G3 and 2A5 reliably highlighted neuronal dipeptide repeat, tau, and α-synuclein inclusions in a distribution similar to a polyclonal antibody to p62, phospho-tau antibodies 7F2 and AT8, and phospho-α-synuclein antibody 81A. However, antibodies 5G3 and 2A5 consistently stained less neuropil structures, such as tau neuropil threads and Lewy neurites, while 2A5 marked fewer glial inclusions in progressive supranuclear palsy. Both 5G3 and 2A5 revealed incidental astrocytic tau immunoreactivity in cases of Alzheimer disease and Lewy body disease with resolution superior to 7F2. Through their unique ability to highlight specific types of pathological deposits in neurodegenerative brain tissue, these novel monoclonal p62 antibodies may provide utility in both research and diagnostic efforts.

Distinct alpha-Synuclein species induced by seeding are selectively cleared by the Lysosome or the Proteasome in neuronally differentiated SH-SY5Y cells

A major pathological feature of Parkinson's disease (PD) is the aberrant accumulation of misfolded assemblies of alpha-synuclein (α-Syn). Protein clearance appears as a regulator of the 'α-Syn burden' underlying PD pathogenesis. The picture emerging is that a combination of pathways with complementary roles, including the Proteasome System and the Autophagy-Lysosome Pathway, contributes to the intracellular degradation of α-Syn. This study addresses the mechanisms governing the degradation of α-Syn species seeded by exogenous fibrils in neuronally differentiated SH-SY5Y neuroblastoma cells with inducible expression of α-Syn. Using human α-Syn recombinant fibrils (pre-formed fibrils, PFFs), seeding and aggregation of endogenous Proteinase K (PK)-resistant α-Syn species occurs within a time frame of 6 days, and is still prominent after 12 days of PFF addition. Clearance of α-Syn assemblies in this inducible model was enhanced after switching off α-Syn expression with doxycycline. Lysosomal inhibition led to accumulation of SDS-soluble α-Syn aggregates 6 days after PFF-addition or when switching off α-Syn expression. Additionally, the autophagic enhancer, rapamycin, induced the clearance of α-Syn aggregates 13 days post-PFF addition, indicating that autophagy is the major pathway for aggregated α-Syn clearance. SDS-soluble phosphorylated α-Syn at S129 was only apparent at 7 days of incubation with a higher amount of PFFs. Proteasomal inhibition resulted in further accumulation of SDS-soluble phosphorylated α-Syn at S129, with limited PK resistance. Our data suggest that in this inducible model autophagy is mainly responsible for the degradation of fibrillar α-Syn, whereas the proteasome system is responsible, at least in part, for the selective clearance of phosphorylated α-Syn oligomers.

Locus coeruleus neurons are most sensitive to chronic neuroinflammation-induced neurodegeneration

Parkinson's disease (PD) develops over decades through spatiotemporal stages that ascend from the brainstem to the forebrain. The mechanism behind this caudo-rostral neurodegeneration remains largely undefined. In unraveling this phenomenon, we recently developed a lipopolysaccharide (LPS)-elicited chronic neuroinflammatory mouse model that displays sequential losses of neurons in brainstem, substantia nigra, hippocampus and cortex. In this study, we aimed to investigate the mechanisms of caudo-rostral neurodegeneration and focused our efforts on the earliest neurodegeneration of vulnerable noradrenergic locus coeruleus (NE-LC) neurons in the brainstem. We found that compared with neurons in other brain regions, NE-LC neurons in untreated mice displayed high levels of mitochondrial oxidative stress that was severely exacerbated in the presence of LPS-elicited chronic neuroinflammation. In agreement, NE-LC neurons in LPS-treated mice displayed early reduction of complex IV expression and mitochondrial swelling and loss of cristae. Mechanistically, the activation of the superoxide-generating enzyme NADPH oxidase (NOX2) on NE-LC neurons was essential for their heightened vulnerability during chronic neuroinflammation. LPS induced early and high expressions of NOX2 in NE-LC neurons. Genetic or pharmacological inactivation of NOX2 markedly reduced mitochondrial oxidative stress and dysfunction in LPS-treated mice. Furthermore, inhibition of NOX2 significantly ameliorated LPS-induced NE-LC neurodegeneration. More importantly, post-treatment with NOX2 inhibitor diphenyleneiodonium when NE-LC neurodegeneration had already begun, still showed high efficacy in protecting NE-LC neurons from degeneration in LPS-treated mice. This study strongly supports that chronic neuroinflammation and NOX2 expression among vulnerable neuronal populations contribute to caudo-rostral degeneration in PD.

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.

Propagation of Pathological α-Synuclein from the Urogenital Tract to the Brain Initiates MSA-like Syndrome

The neuropathological feature of multiple system atrophy (MSA), a fatal adult-onset disorder without effective therapy, is the accumulation of pathological α-synuclein (α-Syn) in the central nervous system (CNS). Here we show that pathological α-Syn exists in nerve terminals in detrusor and external urethral sphincter (EUS) of patients with MSA. Furthermore, α-Syn-preformed fibrils (PFFs) injected in the EUS or detrusor in TgM83^+/− mice initiated the transmission of pathological α-Syn from the urogenital tract to brain via micturition reflex pathways, and these mice developed widespread phosphorylated α-Syn inclusion pathology together with phenotypes. In addition, urinary dysfunction and denervation-reinnervation of external anal sphincter were detected earlier in the mouse models with α-Syn PFFs inoculation before the behavioral manifestations. These results suggest that pathological α-Syn spreading through the micturition reflex pathways retrogradely from the urogenital tract to CNS may lead to urinary dysfunction in patients with MSA, which is different from the etiology of idiopathic Parkinson disease.

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Pathological α-Syn exhibits in nerve terminals in DET and EUS of patients with MSA

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Propagation of pathological α-Syn from urinary tract to CNS causes MSA-like syndrome

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The mouse models show urinary dysfunction and abnormal EAS EMG before motor deficits

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Lower urinary tract injection of α-Syn PFFs induces autonomic and motor dysfunctions

Modelling cognitive deficits in Parkinson's disease: Is CA2 a gateway for hippocampal synucleinopathy?

BACKGROUND

Cognitive dysfunction is one of the most disabling non-motor symptoms of Parkinson's disease (PD), though its pathological correlates still remain elusive. Hippocampal Lewy pathology has recently been correlated by compelling evidence from post-mortem and imaging studies. Animal models recapitulating cognitive impairment in PD are essential to better understand the underlying pathophysiology. To investigate the hippocampal involvement in cognitive dysfunction of PD, we generated an experimental model by inducing midbrain and hippocampal α-synuclein pathology simultaneously.

METHODS

Rats were injected either with human α-synuclein or green fluorescent protein (GFP) expressing adeno-associated viral vectors (AAV), or saline bilaterally into substantia nigra (SN) and dentate gyrus (DG). A group of untreated animals were used as naïve controls. Cognitive and behavioral changes were evaluated with tests probing for spatial learning, short-term memory, anxiety and hedonistic behavior. Immunohistochemical staining, immunoblotting and stereological analysis were performed for pathological characterization.

RESULTS

Bilateral α-synuclein overexpression in SN and DG led to mild but significant motor impairment as well as dysfunctions in short-term memory and spatial learning. There was no hedonistic deficit, whereas a hypo-anxious state was induced. While stereological analysis revealed no significant neuronal loss in any sectors of cornu ammonis, there was considerable decrease (43%) in TH⁺-neurons in SN pars compacta supporting the well-known vulnerability of nigral dopaminergic neurons to α-synuclein mediated neurodegeneration. On the other hand, synaptophysin levels decreased in similar amounts both in striatum and hippocampus, suggesting comparable synaptic loss in target areas. Interestingly, phosphorylated-S129-α-synuclein staining revealed significant expression in CA2 characterized by more mature and dense cellular accumulations compared to CA1-CA3 sub-regions displaying more diffuse grain-like aggregates, suggesting preferential susceptibility of CA2 to produce α-synuclein induced pathology.

CONCLUSION

Bilateral α-synuclein overexpression in DG and SN reproduced partial motor and hippocampus related cognitive deficits. Using this model, we showed a predisposition of CA2 for pathological α-synuclein accumulation, which may provide further insights for future experimental and clinical studies.

The dual role of c-src in cell-to-cell transmission of α-synuclein

Parkinson's disease (PD) is characterized by the loss of dopaminergic neurons located in the substantia nigra pars compacta and the presence of proteinaceous inclusions called Lewy bodies and Lewy neurites in numerous brain regions. Increasing evidence indicates that Lewy pathology progressively involves additional regions of the nervous system as the disease advances, and the prion-like propagation of α-synuclein (α-syn) pathology promotes PD progression. Accordingly, the modulation of α-syn transmission may be important for the development of disease-modifying therapies in patients with PD. Here, we demonstrate that α-syn fibrils induce c-src activation in neurons, which depends on the FcγRIIb-SHP-1/-2-c-src pathway and enhances signals for the uptake of α-syn into neurons. Blockade of c-src activation inhibits the uptake of α-syn and the formation of Lewy body-like inclusions. Furthermore, the blockade of c-src activation also inhibits the release of α-syn via activation of autophagy. The brain-permeable c-src inhibitor, saracatinib, efficiently reduces α-syn propagation into neighboring regions in an in vivo model system. These results suggest a new therapeutic target against progressive PD.

Glucocerebrosidase Activity Modulates Neuronal Susceptibility to Pathological α-Synuclein Insult

Mutations in the GBA1 gene are the most common genetic risk factor for Parkinson's disease (PD) and dementia with Lewy bodies (DLB). GBA1 encodes the lysosomal lipid hydrolase glucocerebrosidase (GCase), and its activity has been linked to accumulation of α-synuclein. The current study systematically examines the relationship between GCase activity and both pathogenic and non-pathogenic forms of α-synuclein in primary hippocampal, cortical, and midbrain neuron and astrocyte cultures, as well as in transgenic mice and a non-transgenic mouse model of PD. We find that reduced GCase activity does not result in aggregation of α-synuclein. However, in the context of extant misfolded α-synuclein, GCase activity modulates neuronal susceptibility to pathology. Furthermore, this modulation does not depend on neuron type but rather is driven by the level of pathological α-synuclein seeds. This study has implications for understanding how GBA1 mutations influence PD pathogenesis and provides a platform for testing novel therapeutics.

Targeting Alpha-Synuclein as a Therapy for Parkinson's Disease

Parkinson's disease (PD) is one of the most common neurodegenerative disorders with a global burden of approximately 6.1 million patients. Alpha-synuclein has been linked to both the sporadic and familial forms of the disease. Moreover, alpha-synuclein is present in Lewy-bodies, the neuropathological hallmark of PD, and the protein and its aggregation have been widely linked to neurotoxic pathways that ultimately lead to neurodegeneration. Such pathways include autophagy/lysosomal dysregulation, synaptic dysfunction, mitochondrial disruption, and endoplasmic reticulum (ER) and oxidative stress. Alpha-synuclein has not only been shown to alter cellular pathways but also to spread between cells, causing aggregation in host cells. Therapeutic approaches will need to address several, if not all, of these angles of alpha-synuclein toxicity. Here we review the current advances in therapeutic efforts for PD that aim to produce a disease-modifying therapy by targeting the spread, production, aggregation, and degradation of alpha-synuclein. These include: receptor blocking strategies whereby putative alpha-synuclein receptors could be blocked inhibiting alpha-synuclein spread, an alpha-synuclein reduction which will decrease the amount alpha-synuclein available for aggregation and pathway disruption, the use of small molecules in order to target alpha-synuclein aggregation, immunotherapy and the increase of alpha-synuclein degradation by increasing autophagy/lysosomal flux. The research discussed here may lead to a disease-modifying therapy that tackles disease onset and progression in the future.

Mechanisms of alpha-synuclein toxicity: An update and outlook

Alpha-synuclein (aSyn) was identified as the main component of inclusions that define synucleinopathies more than 20 years ago. Since then, aSyn has been extensively studied in an attempt to unravel its roles in both physiology and pathology. Today, studying the mechanisms of aSyn toxicity remains in the limelight, leading to the identification of novel pathways involved in pathogenesis. In this chapter, we address the molecular mechanisms involved in synucleinopathies, from aSyn misfolding and aggregation to the various cellular effects and pathologies associated. In particular, we review our current understanding of the mechanisms involved in the spreading of aSyn between different cells, from the periphery to the brain, and back. Finally, we also review recent studies on the contribution of inflammation and the gut microbiota to pathology in synucleinopathies. Despite significant advances in our understanding of the molecular mechanisms involved, we still lack an integrated understanding of the pathways leading to neurodegeneration in PD and other synucleinopathies, compromising our ability to develop novel therapeutic strategies.

Evidence for bidirectional and trans-synaptic parasympathetic and sympathetic propagation of alpha-synuclein in rats

The conversion of endogenous alpha-synuclein (asyn) to pathological asyn-enriched aggregates is a hallmark of Parkinson’s disease (PD). These inclusions can be detected in the central and enteric nervous system (ENS). Moreover, gastrointestinal symptoms can appear up to 20 years before the diagnosis of PD. The dual-hit hypothesis posits that pathological asyn aggregation starts in the ENS, and retrogradely spreads to the brain. In this study, we tested this hypothesis by directly injecting preformed asyn fibrils into the duodenum wall of wild-type rats and transgenic rats with excess levels of human asyn. We provide a meticulous characterization of the bacterial artificial chromosome (BAC) transgenic rat model with respect to initial propagation of pathological asyn along the parasympathetic and sympathetic pathways to the brainstem, by performing immunohistochemistry at early time points post-injection. Induced pathology was observed in all key structures along the sympathetic and parasympathetic pathways (ENS, autonomic ganglia, intermediolateral nucleus of the spinal cord (IML), heart, dorsal motor nucleus of the vagus, and locus coeruleus (LC)) and persisted for at least 4 months post-injection. In contrast, asyn propagation was not detected in wild-type rats, nor in vehicle-injected BAC rats. The presence of pathology in the IML, LC, and heart indicate trans-synaptic spread of the pathology. Additionally, the observed asyn inclusions in the stomach and heart may indicate secondary anterograde propagation after initial retrograde spreading. In summary, trans-synaptic propagation of asyn in the BAC rat model is fully compatible with the “body-first hypothesis” of PD etiopathogenesis. To our knowledge, this is the first animal model evidence of asyn propagation to the heart, and the first indication of bidirectional asyn propagation via the vagus nerve, i.e., duodenum-to-brainstem-to-stomach. The BAC rat model could be very valuable for detailed mechanistic studies of the dual-hit hypothesis, and for studies of disease modifying therapies targeting early pathology in the gastrointestinal tract.

Neuromelanin, aging, and neuronal vulnerability in Parkinson's disease

Neuromelanin, a dark brown intracellular pigment, has long been associated with Parkinson's disease (PD). In PD, neuromelanin-containing neurons preferentially degenerate, tell-tale neuropathological inclusions form in close association with this pigment, and neuroinflammation is restricted to neuromelanin-containing areas. In humans, neuromelanin accumulates with age, which in turn is the main risk factor for PD. The potential contribution of neuromelanin to PD pathogenesis remains unknown because, in contrast to humans, common laboratory animals lack neuromelanin. The recent introduction of a rodent model exhibiting an age-dependent production of human-like neuromelanin has allowed, for the first time, for the consequences of progressive neuromelanin accumulation-up to levels reached in elderly human brains-to be assessed in vivo. In these animals, intracellular neuromelanin accumulation above a specific threshold compromises neuronal function and triggers a PD-like pathology. As neuromelanin levels reach this threshold in PD patients and presymptomatic PD patients, the modulation of neuromelanin accumulation could provide a therapeutic benefit for PD patients and delay brain aging. © 2019 The Author. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Seeded propagation of α-synuclein aggregation in mouse brain using protein misfolding cyclic amplification

α-Synuclein (α-syn) protein aggregation is associated with several neurodegenerative disorders collectively referred to as synucleinopathies, including Parkinson's disease. We used protein misfolding cyclic amplification (PMCA) to study α-syn aggregation in brain homogenates of wild-type or transgenic mice expressing normal (D line) or A53T mutant (M83 line) human α-syn. We found that sonication-incubation cycles of M83 mouse brain gradually produce large quantities of SDS-resistant α-syn aggregates, involving both human and mouse proteins. These PMCA products, containing partially proteinase K-resistant α-syn species, are competent to accelerate the onset of neurologic symptoms after intracerebral inoculation to young M83 mice and to seed aggregate formation of α-syn following PMCA, including in D and wild-type mouse brain substrates. PMCA seeding activity in the M83 diseased brain correlates positively with regions mostly targeted by the α-syn pathology in this model. Our data indicate that similar to prions, PMCA can reproduce some characteristics of α-syn aggregation and seeded propagation in vitro in a complex milieu. This opens new opportunities for the molecular study of synucleinopathies.-Nicot, S., Verchère, J., Bélondrade, M., Mayran, C., Bétemps, D., Bougard, D., Baron, T. Seeded propagation of α-synuclein aggregation in mouse brain using protein misfolding cyclic amplification.

Spreading of alpha-synuclein - relevant or epiphenomenon?

The intracellular accumulation of misfolded alpha-synuclein pathology, termed Lewy pathology, throughout the brain is a phenomenon central to Parkinson's disease pathogenesis. In recent years it has become apparent that Lewy pathology can spread from neuron-to-neuron and between interconnected brain regions. Understanding the phenomenon of Lewy pathology propagation holds great promise in its explanatory power to determine the etiology of Parkinson's disease and related synucleinopathies. However, it remains to be seen if the spread of Lewy pathology is critical for driving this disease. Here we discuss the spreading of Lewy pathology while highlighting some important concepts and experimental observations. We conclude that further studies are required to determine if, and how, the spreading behavior of Lewy pathology is involved in Parkinson's disease. "This article is part of the Special Issue Synuclein".

Dissecting α-synuclein inclusion pathology diversity in multiple system atrophy: implications for the prion-like transmission hypothesis

Synucleinopathies are a group of neurodegenerative diseases characterized by the accumulation of insoluble, aggregated α-synuclein (αS) pathological inclusions. Multiple system atrophy (MSA) presents with extensive oligodendroglial αS pathology and additional more limited neuronal inclusions while most of the other synucleinopathies, such as Parkinson's disease and dementia with Lewy bodies (DLB), develop αS pathology primarily in neuronal cell populations. αS biochemical alterations specific to MSA have been described but thorough examination of these unique and disease-specific protein deposits is further warranted especially given recent findings implicating the prion-like nature of synucleinopathies perhaps with distinct strain-like properties. Taking advantage of an extensive panel of antibodies that target a wide range of epitopes within αS, we investigated the distinct properties of the various types of αS inclusion present in MSA brains with comparison to DLB. Brain biochemical fractionation followed by immunoblotting revealed that the immunoreactive profiles were significantly more consistent for DLB than for MSA. Furthermore, epitope-specific immunohistochemistry varied greatly between different types of MSA αS inclusions and even within different brain regions of individual MSA brains. These studies highlight the importance of using a battery of antibodies for adequate appreciation of the various pathology in this distinct synucleinopathy. In addition, it can be posited that if the spread of pathology in MSA undergoes prion-like mechanisms, "strains" of αS aggregated conformers must be inherently unstable and readily mutable, perhaps resulting in a more stochastic progression process.

α-synuclein in the pathophysiology of Alzheimer's disease

The Alzheimer’s disease (AD) afflicted brain is neuropathologically defined by extracellular amyloid-β (Aβ) plaques and intraneuronal neurofibrillary tangles composed of hyperphosphorylated tau protein. However, accumulating evidence suggests that the presynaptic protein α-synuclein (αSyn), mainly associated with synucleinopathies like Parkinson’s disease (PD), dementia with Lewy bodies (DLB) and multiple system atrophy (MSA), is involved in the pathophysiology of AD. Lewy-related pathology (LRP), primarily comprised of αSyn, is present in a majority of autopsied AD brains, and higher levels of αSyn in the cerebrospinal fluid (CSF) of patients with mild cognitive impairment (MCI) and AD have been linked to cognitive decline. Recent studies also suggest that the asymptomatic accumulation of Aβ plaques is associated with higher CSF αSyn levels in subjects at risk of sporadic AD and in individuals carrying autosomal dominant AD mutations. Experimental evidence has further linked αSyn mainly to tau hyperphosphorylation, but also to the pathological actions of Aβ and the APOEε4 allele, the latter being a major genetic risk factor for both AD and DLB. In this review, we provide a summary of the current evidence proposing an involvement of αSyn either as an active or passive player in the pathophysiological ensemble of AD, and furthermore describe in detail the current knowledge of αSyn structure and inferred function.

A cullin-RING ubiquitin ligase targets exogenous α-synuclein and inhibits Lewy body-like pathology

Parkinson's disease (PD) is a neurological disorder characterized by the progressive accumulation of neuronal α-synuclein (αSyn) inclusions called Lewy bodies. It is believed that Lewy bodies spread throughout the nervous system due to the cell-to-cell propagation of αSyn via cycles of secretion and uptake. Here, we investigated the internalization and intracellular accumulation of exogenous αSyn, two key steps of Lewy body pathogenesis, amplification and spreading. We found that stable αSyn fibrils substantially accumulate in different cell lines upon internalization, whereas αSyn monomers, oligomers, and dissociable fibrils do not. Our data indicate that the uptake-mediated accumulation of αSyn in a human-derived neuroblastoma cell line triggered an adaptive response that involved proteins linked to ubiquitin ligases of the S-phase kinase-associated protein 1 (SKP1), cullin-1 (Cul1), and F-box domain-containing protein (SCF) family. We found that SKP1, Cul1, and the F-box/LRR repeat protein 5 (FBXL5) colocalized and physically interacted with internalized αSyn in cultured cells. Moreover, the SCF containing the F-box protein FBXL5 (SCFFBXL5) catalyzed αSyn ubiquitination in reconstitution experiments in vitro using recombinant proteins and in cultured cells. In the human brain, SKP1 and Cul1 were recruited into Lewy bodies from brainstem and neocortex of patients with PD and related neurological disorders. In both transgenic and nontransgenic mice, intracerebral administration of exogenous αSyn fibrils triggered a Lewy body-like pathology, which was amplified by SKP1 or FBXL5 loss of function. Our data thus indicate that SCFFXBL5 regulates αSyn in vivo and that SCF ligases may constitute targets for the treatment of PD and other α-synucleinopathies.

Comparative analyses of the in vivo induction and transmission of α-synuclein pathology in transgenic mice by MSA brain lysate and recombinant α-synuclein fibrils

α-synuclein (αS) is the major component of several types of brain pathological inclusions that define neurodegenerative diseases termed synucleinopathies. Central nervous system (CNS) inoculation studies using either in vitro polymerized αS fibrils or in vivo derived lysates containing αS aggregates to induce the progressive spread of αS inclusion pathology in animal disease models have supported the notion that αS mediated progressive neurodegeneration can occur by a prion-like mechanism. We have previously shown that neonatal brain inoculation with preformed αS fibrils in hemizygous M20^+/− transgenic mice expressing wild type human αS and to a lesser extent in non-transgenic mice can result in a concentration-dependent progressive induction of CNS αS pathology. Recent studies using brain lysates from patients with multiple system atrophy (MSA), characterized by αS inclusion pathology in oligodendrocytes, indicate that these may be uniquely potent at inducing αS pathology with prion-like strain specificity. We demonstrate here that brain lysates from MSA patients, but not control individuals, can induce αS pathology following neonatal brain inoculation in transgenic mice expressing A53T human αS (M83 line), but not in transgenic expressing wild type human αS (M20 line) or non-transgenic mice within the timeframe of the study design. Further, we show that neuroanatomical and immunohistochemical properties of the pathology induced by MSA brain lysates is very similar to what is produced by the neonatal brain injection of preformed human αS fibrils in hemizygous M83^+/− transgenic mice. Collectively, these findings reinforce the idea that the intrinsic traits of the M83 mouse model dominates over any putative prion-like strain properties of MSA αS seeds that can induce pathology.

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

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

The center of olfactory bulb-seeded α-synucleinopathy is the limbic system and the ensuing pathology is higher in male than in female mice

At early disease stages, Lewy body disorders are characterized by limbic vs. brainstem α-synucleinopathy, but most preclinical studies have focused solely on the nigrostriatal pathway. Furthermore, male gender and advanced age are two major risk factors for this family of conditions, but their influence on the topographical extents of α-synucleinopathy and the degree of cell loss are uncertain. To fill these gaps, we infused α-synuclein fibrils in the olfactory bulb/anterior olfactory nucleus complex-one of the earliest and most frequently affected brain regions in Lewy body disorders-in 3-month-old female and male mice and in 11-month-old male mice. After 6 months, we observed that α-synucleinopathy did not expand significantly beyond the limbic connectome in the 9-month-old male and female mice or in the 17-month-old male mice. However, the 9-month-old male mice had developed greater α-synucleinopathy, smell impairment and cell loss than age-matched females. By 10.5 months post-infusion, fibril treatment hastened mortality in the 21.5-month-old males, but the inclusions remained centered in the limbic system in the survivors. Although fibril infusions reduced the number of cells expressing tyrosine hydroxylase in the substantia nigra of young males at 6 months post-infusion, this was not attributable to true cell death. Furthermore, mesencephalic α-synucleinopathy, if present, was centered in mesolimbic circuits (ventral tegmental area/accumbens) rather than within strict boundaries of the nigral pars compacta, which were defined here by tyrosine hydroxylase immunolabel. Nonprimate models cannot be expected to faithfully recapitulate human Lewy body disorders, but our murine model seems reasonably suited to (i) capture some aspects of Stage IIb of Lewy body disorders, which displays a heavier limbic than brainstem component compared to incipient Parkinson's disease; and (ii) leverage sex differences and the acceleration of mortality following induction of olfactory α-synucleinopathy.

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.

Propagation of α-Synuclein Strains within Human Reconstructed Neuronal Network

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

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

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

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

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Accumulation of phospho-α-Syn induces early neuronal dysfunctions

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

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

Physiological C-terminal truncation of α-synuclein potentiates the prion-like formation of pathological inclusions

α-Synuclein (αsyn) aggregates into toxic fibrils in multiple neurodegenerative diseases where these fibrils form characteristic pathological inclusions such as Lewy bodies (LBs). The mechanisms initiating αsyn aggregation into fibrils are unclear, but ubiquitous post-translational modifications of αsyn present in LBs may play a role. Specific C-terminally (C)-truncated forms of αsyn are present within human pathological inclusions and form under physiological conditions likely in lysosome-associated pathways, but the roles for these C-truncated forms of αsyn in inclusion formation and disease are not well understood. Herein, we characterized the in vitro aggregation properties, amyloid fibril structures, and ability to induce full-length (FL) αsyn aggregation through prion-like mechanisms for eight of the most common physiological C-truncated forms of αsyn (1-115, 1-119, 1-122, 1-124, 1-125, 1-129, 1-133, and 1-135). In vitro, C-truncated αsyn aggregated more readily than FL αsyn and formed fibrils with unique morphologies. The presence of C-truncated αsyn potentiated aggregation of FL αsyn in vitro through co-polymerization. Specific C-truncated forms of αsyn in cells also exacerbated seeded aggregation of αsyn. Furthermore, in primary neuronal cultures, co-polymers of C-truncated and FL αsyn were potent prion-like seeds, but polymers composed solely of the C-truncated protein were not. These experiments indicated that specific physiological C-truncated forms of αsyn have distinct aggregation properties, including the ability to modulate the prion-like aggregation and seeding activity of FL αsyn. Proteolytic formation of these C-truncated species may have an important role in both the initiation of αsyn pathological inclusions and further progression of disease with strain-like properties.

Loss of Brain Norepinephrine Elicits Neuroinflammation-Mediated Oxidative Injury and Selective Caudo-Rostral Neurodegeneration

Environmental toxicant exposure has been strongly implicated in the pathogenesis of Parkinson's disease (PD). Clinical manifestations of non-motor and motor symptoms in PD stem from decades of progressive neurodegeneration selectively afflicting discrete neuronal populations along a caudo-rostral axis. However, recapitulating this spatiotemporal neurodegenerative pattern in rodents has been unsuccessful. The purpose of this study was to generate such animal PD models and delineate mechanism underlying the ascending neurodegeneration. Neuroinflammation, oxidative stress, and neuronal death in mice brains were measured at different times following a single systemic injection of lipopolysaccharide (LPS). We demonstrate that LPS produced an ascending neurodegeneration that temporally afflicted neurons initially in the locus coeruleus (LC), followed by substantia nigra, and lastly the primary motor cortex and hippocampus. To test the hypothesis that LPS-elicited early loss of noradrenergic LC neurons may underlie this ascending pattern, we used a neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) to deplete brain norepinephrine. DSP-4 injection resulted in a time-dependent ascending degenerative pattern similar to that generated by the LPS model. Mechanistic studies revealed that increase in nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-2 (NOX2)-dependent superoxide/reactive oxygen species (ROS) production plays a key role in both LPS- and DSP-4-elicited neurotoxicity. We found that toxin-elicited chronic neuroinflammation, oxidative neuronal injuries, and neurodegeneration were greatly suppressed in mice deficient in NOX2 gene or treated with NOX2-specific inhibitor. Our studies document the first rodent PD model recapturing the ascending neurodegenerative pattern of PD patients and provide convincing evidence that the loss of brain norepinephrine is critical in initiating and maintaining chronic neuroinflammation and the discrete neurodegeneration in PD.

Comparison of the in vivo induction and transmission of α-synuclein pathology by mutant α-synuclein fibril seeds in transgenic mice

Parkinson's disease (PD) is one of many neurodegenerative diseases termed synucleinopathies, neuropathologically defined by inclusions containing aggregated α-synuclein (αS). αS gene (SNCA) mutations can directly cause autosomal dominant PD. In vitro studies demonstrated that SNCA missense mutations may either enhance or diminish αS aggregation but cross-seeding of mutant and wild-type αS proteins appear to reduce aggregation efficiency. Here, we extended these studies by assessing the effects of seeded αS aggregation in αS transgenic mice through intracerebral or peripheral injection of various mutant αS fibrils. We observed modestly decreased time to paralysis in mice transgenic for human A53T αS (line M83) intramuscularly injected with H50Q, G51D or A53E αS fibrils relative to wild-type αS fibrils. Conversely, E46K αS fibril seeding was significantly delayed and less efficient in the same experimental paradigm. However, the amount and distribution of αS inclusions in the central nervous system were similar for all αS fibril muscle injected mice that developed paralysis. Mice transgenic for human αS (line M20) injected in the hippocampus with wild-type, H50Q, G51D or A53E αS fibrils displayed induction of αS inclusion pathology that increased and spread over time. By comparison, induction of αS aggregation following the intrahippocampal injection of E46K αS fibrils in M20 mice was much less efficient. These findings show that H50Q, G51D or A53E can efficiently cross-seed and induce αS pathology in vivo. In contrast, E46K αS fibrils are intrinsically inefficient at seeding αS inclusion pathology. Consistent with previous in vitro studies, E46K αS polymers are likely distinct aggregated conformers that may represent a unique prion-like strain of αS.

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

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

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

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

Critical appraisal of pathology transmission in the α-synuclein fibril model of Lewy body disorders

Lewy body disorders are characterized by the emergence of α-synucleinopathy in many parts of the central and peripheral nervous systems, including in the telencephalon. Dense α-synuclein+ pathology appears in regio inferior of the hippocampus in both Parkinson's disease and dementia with Lewy bodies and may disturb cognitive function. The preformed α-synuclein fibril model of Parkinson's disease is growing in use, given its potential for seeding the self-propagating spread of α-synucleinopathy throughout the mammalian brain. Although it is often assumed that the spread occurs through neuroanatomical connections, this is generally not examined vis-à-vis the uptake and transport of tract-tracers infused at precisely the same stereotaxic coordinates. As the neuronal connections of the hippocampus are historically well defined, we examined the first-order spread of α-synucleinopathy three months following fibril infusions centered in the mouse regio inferior (CA2+CA3), and contrasted this to retrograde and anterograde transport of the established tract-tracers FluoroGold and biotinylated dextran amines (BDA). Massive hippocampal α-synucleinopathy was insufficient to elicit memory deficits or loss of cells and synaptic markers in this model of early disease processes. However, dense α-synuclein+ inclusions in the fascia dentata were negatively correlated with memory capacity. A modest compensatory increase in synaptophysin was evident in the stratum radiatum of cornu Ammonis in fibril-infused animals, and synaptophysin expression correlated inversely with memory function in fibril but not PBS-infused mice. No changes in synapsin I/II expression were observed. The spread of α-synucleinopathy was somewhat, but not entirely consistent with FluoroGold and BDA axonal transport, suggesting that variables other than innervation density also contribute to the materialization of α-synucleinopathy. For example, layer II entorhinal neurons of the perforant pathway exhibited somal α-synuclein+ inclusions as well as retrogradely labeled FluoroGold+ somata. However, some afferent brain regions displayed dense retrograde FluoroGold label and no α-synuclein+ inclusions (e.g. medial septum/diagonal band), supporting the selective vulnerability hypothesis. The pattern of inclusions on the contralateral side was consistent with specific spread through commissural connections (e.g. stratum pyramidale of CA3), but again, not all commissural projections exhibited α-synucleinopathy (e.g. hilar mossy cells). The topographical extent of inclusions is displayed here in high-resolution images that afford viewers a rich opportunity to dissect the potential spread of pathology through neural circuitry. Finally, the results of this expository study were leveraged to highlight the challenges and limitations of working with preformed α-synuclein fibrils.

Spread of aggregates after olfactory bulb injection of α-synuclein fibrils is associated with early neuronal loss and is reduced long term

Parkinson’s disease is characterized by degeneration of substantia nigra dopamine neurons and by intraneuronal aggregates, primarily composed of misfolded α-synuclein. The α-synuclein aggregates in Parkinson’s patients are suggested to first appear in the olfactory bulb and enteric nerves and then propagate, following a stereotypic pattern, via neural pathways to numerous regions across the brain. We recently demonstrated that after injection of either mouse or human α-synuclein fibrils into the olfactory bulb of wild-type mice, α-synuclein fibrils recruited endogenous α-synuclein into pathological aggregates that spread transneuronally to over 40 other brain regions and subregions, over 12 months. We previously reported the progressive spreading of α-synuclein aggregates, between 1 and 12 months following α-synuclein fibril injections, and now report how far the pathology has spread 18- and 23-month post-injection in this model. Our data show that between 12 and 18 months, there is a further increase in the number of brain regions exhibiting pathology after human, and to a lesser extent mouse, α-synuclein fibril injections. At both 18 and 23 months after injection of mouse and human α-synuclein fibrils, we observed a reduction in the density of α-synuclein aggregates in some brain regions compared to others at 12 months. At 23 months, no additional brain regions exhibited α-synuclein aggregates compared to earlier time points. In addition, we also demonstrate that the induced α-synucleinopathy triggered a significant early neuron loss in the anterior olfactory nucleus. By contrast, there was no loss of mitral neurons in the olfactory bulb, even at 18 month post-injection. We speculate that the lack of continued progression of α-synuclein pathology is due to compromise of the neural circuitry, consequential to neuron loss and possibly to the activation of proteolytic mechanisms in resilient neurons of wild-type mice that counterbalances the spread and seeding by degrading pathogenic α-synuclein.

Are rodent models of Parkinson's disease behaving as they should?

In recent years our understanding of Parkinson's disease has expanded both in terms of pathological hallmarks as well as relevant genetic influences. In parallel with the aetiological discoveries a multitude of PD animal models have been established. The vast majority of these are rodent models based on environmental, genetic and mechanistic insight. A major challenge in many of these models is their ability to only recapitulate some of the complex disease features seen in humans. Although symptom alleviation and clinical signs are of utmost importance in therapeutic research many of these models lack comprehensive behavioural testing. While non-motor symptoms become increasingly important as early diagnostic markers in PD, they are poorly characterized in rodents. In this review we look at well-established and more recent animal models of PD in terms of behavioural characterization and discuss how they can best contribute to progression in Parkinson's research.

Prion-like transmission of α-synuclein pathology in the context of an NFL null background

Neurofilaments are a major component of the axonal cytoskeleton in neurons and have been implicated in a number of neurodegenerative diseases due to their presence within characteristic pathological inclusions. Their contributions to these diseases are not yet fully understood, but previous studies investigated the effects of ablating the obligate subunit of neurofilaments, low molecular mass neurofilament subunit (NFL), on disease phenotypes in transgenic mouse models of Alzheimer's disease and tauopathy. Here, we tested the effects of ablating NFL in α-synuclein M83 transgenic mice expressing the human pathogenic A53T mutation, by breeding them onto an NFL null background. The induction and spread of α-synuclein inclusion pathology was triggered by the injection of preformed α-synuclein fibrils into the gastrocnemius muscle or hippocampus in M83 versus M83/NFL null mice. We observed no difference in the post-injection time to motor-impairment and paralysis endpoint or amount and distribution of α-synuclein inclusion pathology in the muscle injected M83 and M83/NFL null mice. Hippocampal injected M83/NFL null mice displayed subtle region-specific differences in the amount of α-synuclein inclusions however, pathology was observed in the same regions as the M83 mice. Overall, we observed only minor differences in the induction and transmission of α-synuclein pathology in these induced models of synucleinopathy in the presence or absence of NFL. This suggests that NFL and neurofilaments do not play a major role in influencing the induction and transmission of α-synuclein aggregation.

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

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