Intraneuronal α-synuclein (α-Syn) aggregates known as Lewy bodies (LBs) and the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) are the pathological hallmarks of Parkinson’s disease (PD). Braak’s hypothesis based on autopsy studies suggests that Lewy pathology initially occurs in the enteric nervous system (ENS) and then travels retrogradely to the dorsal motor nucleus of the vagus nerve (dmX), proceeding from there in a caudo-rostral direction. Recent evidence that α-Syn aggregates propagate between interconnected neurons supports this hypothesis. However, there is no direct evidence demonstrating this transmission from the ENS to the dmX and then to the SNpc. We inoculated α-Syn preformed fibrils (PFFs) or phosphate-buffered saline (PBS) into the mouse gastric wall and analyzed the progression of the pathology. The mice inoculated with α-Syn PFFs, but not with PBS, developed phosphorylated α-Syn (p-α-Syn)–positive LB-like aggregates in the dmX at 45 days postinoculation. This aggregate formation was completely abolished when vagotomy was performed prior to inoculation of α-Syn PFFs, suggesting that the aggregates in the dmX were retrogradely induced via the vagus nerve. Unexpectedly, the number of neurons containing p-α-Syn–positive aggregates in the dmX decreased over time, and no further caudo-rostral propagation beyond the dmX was observed up to 12 months postinoculation. P-α-Syn–positive aggregates were also present in the myenteric plexus at 12 months postinoculation. However, unlike in patients with PD, there was no cell-type specificity in neurons containing those aggregates in this model. Conclusions: These results indicate that α-Syn PFF inoculation into the mouse gastrointestinal tract can induce α-Syn pathology resembling that of very early PD, but other factors are apparently required if further progression of PD pathology is to be replicated in this animal model.

Synucleinopathies of the aging population are an heterogeneous group of neurological disorders that includes Parkinson’s disease (PD) and dementia with Lewy bodies (DLB) and are characterized by the progressive accumulation of α-synuclein in neuronal and glial cells. Toll-like receptor 2 (TLR2), a pattern recognition immune receptor, has been implicated in the pathogenesis of synucleinopathies because TLR2 is elevated in the brains of patients with PD and TLR2 is a mediator of the neurotoxic and pro-inflammatory effects of extracellular α-synuclein aggregates. Therefore, blocking TLR2 might alleviate α-synuclein pathological and functional effects. For this purpose, herein, we targeted TLR2 using a functional inhibitory antibody (anti-TLR2). Two different human α-synuclein overexpressing transgenic mice were used in this study. α-synuclein low expresser mouse (α-syn-tg, under the PDGFβ promoter, D line) was stereotaxically injected with TLR2 overexpressing lentivirus to demonstrate that increment of TLR2 expression triggers neurotoxicity and neuroinflammation. α-synuclein high expresser mouse (α-Syn-tg; under mThy1 promoter, Line 61) was administrated with anti-TLR2 to examine that functional inhibition of TLR2 ameliorates neuropathology and behavioral defect in the synucleinopathy animal model. In vitro α-synuclein transmission live cell monitoring system was used to evaluate the role of TLR2 in α-synuclein cell-to-cell transmission. We demonstrated that administration of anti-TLR2 alleviated α-synuclein accumulation in neuronal and astroglial cells, neuroinflammation, neurodegeneration, and behavioral deficits in an α-synuclein tg mouse model of PD/DLB. Moreover, in vitro studies with neuronal and astroglial cells showed that the neuroprotective effects of anti-TLR2 antibody were mediated by blocking the neuron-to-neuron and neuron-to-astrocyte α-synuclein transmission which otherwise promotes NFκB dependent pro-inflammatory responses. This study proposes TLR2 immunotherapy as a novel therapeutic strategy for synucleinopathies of the aging population.

Parkinson’s disease (PD) is characterized by the loss of dopaminergic neurons in the substantia nigra and subsequent motor symptoms, but various non-motor symptoms (NMS) often precede motor symptoms. Recently, NMS have attracted much attention as a clue for identifying patients in a prodromal stage of PD, which is an excellent point at which to administer disease-modifying therapies (DMTs). These prodromal symptoms include olfactory loss, constipation, and sleep disorders, especially rapid eye movement sleep behavior disorder (RBD), all of which are also important for elucidating the mechanisms of the initiation and progression of the disease. For the development of DMTs, an animal model that reproduces the prodromal stage of PD is also needed. There have been various mammalian models reported, including toxin-based, genetic, and alpha synuclein propagation models. In this article, we review the animal models that exhibit NMS as prodromal symptoms and also discuss an appropriate prodromal model and its importance for the development of DMT of PD.

Multiple system atrophy (MSA) is a debilitating and fatal neurodegenerative disorder. The disease severity warrants urgent development of disease-modifying therapy, but the disease pathogenesis is still enigmatic. Neurodegeneration in MSA brains is preceded by the emergence of glial cytoplasmic inclusions (GCIs), which are insoluble α-synuclein accumulations within oligodendrocytes (OLGs). Thus, preventive strategies against GCI formation may suppress disease progression. However, although numerous studies have tried to elucidate the molecular pathogenesis of GCI formation, difficulty remains in understanding the pathological interaction between the two pivotal aspects of GCIs; α-synuclein and OLGs. The difficulty originates from several enigmas: 1) what triggers the initial generation and possible propagation of pathogenic α-synuclein species? 2) what contributes to OLG-specific accumulation of α-synuclein, which is abundantly expressed in neurons but not in OLGs? and 3) how are OLGs and other glial cells affected and contribute to neurodegeneration? The primary pathogenesis of GCIs may involve myelin dysfunction and dyshomeostasis of the oligodendroglial cellular environment such as autophagy and iron metabolism. We have previously reported that oligodendrocyte precursor cells are more prone to develop intracellular inclusions in the presence of extracellular fibrillary α-synuclein. This finding implies a possibility that the propagation of GCI pathology in MSA brains is mediated through the internalization of pathological α-synuclein into oligodendrocyte precursor cells. In this review, in order to discuss the pathogenesis of GCIs, we will focus on the composition of neuronal and oligodendroglial inclusions in synucleinopathies. Furthermore, we will introduce some hypotheses on how α-synuclein pathology spreads among OLGs in MSA brains, in the light of our data from the experiments with primary oligodendrocyte lineage cell culture. While various reports have focused on the mysterious source of α-synuclein in GCIs, insights into the mechanism which regulates the uptake of pathological α-synuclein into oligodendroglial cells may yield the development of the disease-modifying therapy for MSA. The interaction between glial cells and α-synuclein is also highlighted with previous studies of post-mortem human brains, cultured cells, and animal models, which provide comprehensive insight into GCIs and the MSA pathomechanisms.

Parkinson's disease (PD) is a common neurodegenerative disorder characterized by selective dopaminergic cell loss in the substantia nigra, but its pathogenesis remains unclear. The recessively inherited familial PD genes PARK2 and PARK6 have been attributed to mutations in the Parkin and PTEN-induced kinase 1 (PINK1) genes, respectively. Recent reports suggest that PINK1 works upstream of Parkin in the same pathway to regulate mitochondrial dynamics and/or conduct autophagic clearance of damaged mitochondria. This phenomenon is preserved from Drosophila to human cell lines but has not been demonstrated in a vertebrate animal model in vivo. Here, we developed a medaka fish (Oryzias latipes) model that is deficient in Pink1 and Parkin. We found that despite the lack of a conspicuous phenotype in single mutants for Pink1 or Parkin, medaka that are deficient in both genes developed phenotypes similar to that of human PD: late-onset locomotor dysfunction, a decrease in dopamine levels and a selective degeneration of dopaminergic neurons. Further analysis also revealed defects in mitochondrial enzymatic activity as well as cell death. Consistently, PINK1 and Parkin double-deficient MEF showed a further decrease in mitochondrial membrane potential and mitochondrial complex I activity as well as apoptosis compared with single-deficient MEF. Interestingly, these mitochondrial abnormalities in Parkin-deficient MEF were compensated by exogenous PINK1, but not by disease-related mutants. These results suggest that PINK1 and Parkin work in a complementary way to protect dopaminergic neurons by maintaining mitochondrial function in vertebrates.

Background: In autosomal recessive early-onset Parkinsonism (PARK2), the pathogenetic process from the loss of function of a ubiquitin ligase parkin to the death of dopamine neurons remains unclear. A dominant hypothesis attributes the neurotoxicity to accumulated substrates that are exempt from parkin-mediated degradation. Parkin substrates include two septins; SEPT4/CDCrel-2 which coaggregates with α-synuclein as Lewy bodies in Parkinson’s disease, and its closest homolog SEPT5/CDCrel-1/PNUTL1 whose overload with viral vector can rapidly eliminate dopamine neurons in rats. However, chronic effects of pan-neural overload of septins have never been examined in mammals. To address this, we established a line of transgenic mice that express the largest gene product SEPT4^54kDa via the prion promoter in the entire brain. Results: Histological examination and biochemical quantification of SEPT4-associated proteins including α-synuclein and the dopamine transporter in the nigrostriatal dopamine neurons found no significant difference between Sept4 ^Tg/+ and wild-type littermates. Thus, the hypothetical pathogenicity by the chronic overload of SEPT4 alone, if any, is insufficient to trigger neurodegenerative process in the mouse brain. Intriguingly, however, a systematic battery of behavioral tests revealed unexpected abnormalities in Sept4 ^Tg/+ mice that include consistent attenuation of voluntary activities in distinct behavioral paradigms and altered social behaviors. Conclusions: Together, these data indicate that septin dysregulations commonly found in postmortem human brains with Parkinson’s disease, schizophrenia and bipolar disorders may be responsible for a subset of behavioral abnormalities in the patients.