Assembly of α-synuclein and neurodegeneration in the central nervous system of heterozygous M83 mice following the peripheral administration of α-synuclein seeds

α-Synuclein pathology from the body to the brain: so many seeds so close to the central soil

ABSTRACT

α-Synuclein is a protein that mainly exists in the presynaptic terminals. Abnormal folding and accumulation of α-synuclein are found in several neurodegenerative diseases, including Parkinson's disease. Aggregated and highly phosphorylated α-synuclein constitutes the main component of Lewy bodies in the brain, the pathological hallmark of Parkinson's disease. For decades, much attention has been focused on the accumulation of α-synuclein in the brain parenchyma rather than considering Parkinson's disease as a systemic disease. Recent evidence demonstrates that, at least in some patients, the initial α-synuclein pathology originates in the peripheral organs and spreads to the brain. Injection of α-synuclein preformed fibrils into the gastrointestinal tract triggers the gut-to-brain propagation of α-synuclein pathology. However, whether α-synuclein pathology can occur spontaneously in peripheral organs independent of exogenous α-synuclein preformed fibrils or pathological α-synuclein leakage from the central nervous system remains under investigation. In this review, we aimed to summarize the role of peripheral α-synuclein pathology in the pathogenesis of Parkinson's disease. We also discuss the pathways by which α-synuclein pathology spreads from the body to the brain.

In vitro modulation of mTOR and mGlur5 influence α-synuclein accumulation

One of the main hallmarks of Parkinson's disease (PD) is abnormal alpha-synuclein (α-syn) aggregation which forms the main component of intracellular Lewy body inclusions. This short report used preformed α-syn fibrils, as well as an A53T mutant α-syn adenovirus to mimic conditions of pathological protein aggregation in dopaminergic human derived SH-SY5Y neural cells. Since there is evidence that the mTOR pathway and glutamatergic signaling each influence protein aggregation, we also assessed the impact of the mTOR inhibitor, rapamycin and the mGluR5 allosteric modulator, CTEP. We found that both rapamycin and CTEP induced a significant reduction of α-syn fibrils in SH-SY5Y cells and this effect was associated with a reduction in mTOR signaling and enhancement in autophagic pathway factors. These data support the possibility that CTEP (or rapamycin) might be a useful pharmacological approach to target abnormal α-syn accumulation by promoting intracellular degradation or enhanced clearance.

Ultrastructures of α-Synuclein Filaments in Synucleinopathy Brains and Experimental Models

Intracellular α-synuclein (α-syn) inclusions are a neuropathological hallmark of Lewy body disease (LBD) and multiple system atrophy (MSA), both of which are termed synucleinopathies. LBD is defined by Lewy bodies and Lewy neurites in neurons, while MSA displays glial cytoplasmic inclusions in oligodendrocytes. Pathological α-syn adopts an ordered filamentous structure with a 5-10 nm filament diameter, and this conformational change has been suggested to be involved in the disease onset and progression. Synucleinopathies also exhibit characteristic ultrastructural and biochemical properties of α-syn filaments, and α-syn strains with distinct conformations have been identified. Numerous experimental studies have supported the idea that pathological α-syn self-amplifies and spreads throughout the brain, during which processes the conformation of α-syn filaments may drive the disease specificity. In this review, we summarize the ultrastructural features and heterogeneity of α-syn filaments in the brains of patients with synucleinopathy and in experimental models of seeded α-syn aggregation.

Experimental models of gut-first Parkinson's disease: A systematic review

BACKGROUND

There is strong support from studies in humans and in animal models that Parkinson's disease (PD) may begin in the gut. This brings about a unique opportunity for researchers in the field of neurogastroenterology to contribute to advancing the field and making contributions that could lead to the ability to diagnose and treat PD in the premotor stages. Lack of familiarity with some of the aspects of the experimental approaches used in these studies may present a barrier for neurogastroenterology researchers to enter the field. Much remains to be understood about intestinal-specific components of gut-first PD pathogenesis and the field would benefit from contributions of enteric and central nervous system neuroscientists.

PURPOSE

To address these issues, we have conducted a systematic review of the two most frequently used experimental models of gut-first PD: transneuronal propagation of α-synuclein preformed fibrils and oral exposure to environmental toxins. We have reviewed the details of these studies and present methodological considerations for the use of these models. Our aim is that this review will serve as a framework and useful reference for neuroscientists, gastroenterologists, and neurologists interested in applying their expertise to advancing our understanding of gut-first PD.

Animal models of Parkinson's disease: bridging the gap between disease hallmarks and research questions

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by motor and non-motor symptoms. More than 200 years after its first clinical description, PD remains a serious affliction that affects a growing proportion of the population. Prevailing treatments only alleviate symptoms; there is still neither a cure that targets the neurodegenerative processes nor therapies that modify the course of the disease. Over the past decades, several animal models have been developed to study PD. Although no model precisely recapitulates the pathology, they still provide valuable information that contributes to our understanding of the disease and the limitations of our treatment options. This review comprehensively summarizes the different animal models available for Parkinson's research, with a focus on those induced by drugs, neurotoxins, pesticides, genetic alterations, α-synuclein inoculation, and viral vector injections. We highlight their characteristics and ability to reproduce PD-like phenotypes. It is essential to realize that the strengths and weaknesses of each model and the induction technique at our disposal are determined by the research question being asked. Our review, therefore, seeks to better aid researchers by ensuring a concrete discernment of classical and novel animal models in PD research.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Gut-to-Brain α-Synuclein Transmission in Parkinson's Disease: Evidence for Prion-like Mechanisms

Parkinson's disease (PD) is a multifactorial disorder involving both motor and non-motor symptoms caused by the progressive death of distinct neuronal populations, including dopaminergic neurons in the substantia nigra. The deposition of aggregated α-synuclein protein into Lewy body inclusions is a hallmark of the disorder, and α-synuclein pathology has been found in the enteric nervous system (ENS) of PD patients up to two decades prior to diagnosis. In combination with the high occurrence of gastrointestinal dysfunction in early stages of PD, current evidence strongly suggests that some forms of PD may originate in the gut. In this review, we discuss human studies that support ENS Lewy pathology as a characteristic feature of PD, and present evidence from humans and animal model systems that α-synuclein aggregation may follow a prion-like spreading cascade from enteric neurons, through the vagal nerve, and into the brain. Given the accessibility of the human gut to pharmacologic and dietary interventions, therapeutic strategies aimed at reducing pathological α-synuclein in the gastrointestinal tract hold significant promise for PD treatment.

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.