Intranasal Rotenone Induces Alpha-Synuclein Accumulation, Neuroinflammation and Dopaminergic Neurodegeneration in Middle-Aged Mice

Intranasal α-Synuclein induces progressive behavioral impairments in mice

α-Synuclein (α-Syn) is implicated in the progression of Parkinson's disease, yet the disease's etiology remains unclear. This study aims to explore how α-Syn affects olfactory, motor, mood and cognitive functions if it initiates from the olfactory bulb. Mice were administered intranasal human AAV-α-Syn and subsequently evaluated for olfactory, motor, mood, and cognitive functions. Immunofluorescence was performed to assess dopaminergic neuronal damage. Results shown that olfactory dysfunction was evident as AAV-α-Syn-treated mice took longer to find buried pellets compared to controls at 3, 9, and 12 months post-instillation. Motor activity remained normal at 6 months but significantly declined at 9 months. Reduced tyrosine hydroxylase expression but increased amount of human α-Syn were observed in the substantia nigra at end of behavioral measurements. AAV-α-Syn mice showed reduced sucrose intake and decreased time in the center zone of the open field at 9 months. Cognitive deficits were observed in recognition function and social memory at 6 and 9 months, with impaired working memory at 12 months. Thus, intranasal AAV-α-Syn instillation in mice leads to progressive olfactory, motor, anxiety, depression-like, and cognitive dysfunctions, reflecting α-Syn pathology propagation.

Chronic DSS-Induced Colitis Exacerbates Parkinson's Disease Phenotype and Its Pathological Features Following Intragastric Rotenone Exposure

Background: Parkinson's disease (PD) is intricately linked to gastrointestinal inflammation and the presence of neurotoxins in the gut, integrating α-syn pathologic alterations and subsequent neurodegeneration into the brain. Objectives: This study aimed to explore the enduring impact of dextran sodium sulfate (DSS)-mediated colitis on the vulnerability of central dopaminergic neurons to subsequent rotenone exposure. Methods: To induce chronic colitis, 10-month-old C57BL/6 mice were pre-exposed to 3 cycles of 1 week of 1% (w/v) DSS administration in drinking water followed by 2 weeks of regular drinking water. After colitis induction, animals received a low dose of intragastric rotenone for the next 8 weeks, followed by testing for Parkinsonian behavior and GI phenotypes of inflammation. At the end of the 17th week, colon, brain stem, and midbrain tissue were isolated and analyzed for α-syn, inflammatory markers, and dopaminergic neuronal loss. Gut microbial composition was assessed by 16S rRNA sequencing analysis. Results: We found that chronic rotenone administration in the presence of preexisting colitis led to a further increase in colonic pro-inflammatory mediator expressions, α-syn expression, and reduced colonic tight junction protein expressions. We also found early impairment of GI functions and worsened grip strength in rotenone-exposed colitic mice. Furthermore, α-syn pathology specific to the colitic mice exposed to rotenone showed dopaminergic neurons degeneration and astroglial activation in substantia nigra and striatum, including regions of the brain stem, i.e., dorsal motor of the vagus and locus coeruleus. Finally, the result of 16S rRNA gene sequencing analysis indicated that colitic mice, after being exposed to rotenone, exhibited a discernible trend in their microbiota composition (Catenibacterium, Turicibactor, and clostridium sensue stricto 1), linking it to the development of PD. Conclusions: These findings indicate that prolonged low-dose rotenone exposure, combined with an early inflammatory intestinal milieu, provides a preconditioning effect on α-syn pathology and exerts neurodegeneration in the intragastric rotenone PD mouse model.

Alpha synuclein and inflammaging

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

Hyperglycemia-Driven Insulin Signaling Defects Promote Parkinson's Disease-like Pathology in Mice

This study aims to determine the effect of chronic hyperglycemia, induced by a high-fat diet and STZ-induced diabetes, on the development of Parkinson's disease-like characteristics. Understanding this relationship is crucial in pharmacology, neurology, and diabetes, as it could potentially lead to developing new therapeutic strategies for Parkinson's disease. Our study employed a comprehensive approach to investigate the effect of hyperglycemia on Parkinson's disease-like characteristics. Hyperglycemia was induced by a high-fat diet for 6- and 9-week duration with a single intraperitoneal STZ (100 mg/kg) injection at week 5 in C57/BL6 mice. Rotenone (10 mg/kg p.o.) was administered to C57/BL6 mice for 6 and 9 weeks. Time-dependent behavioral studies (wire-hang tests, pole tests, Y-maze tests, and round beam walk tests) were carried out to monitor pathology progression and deficits. Molecular protein levels (GLP1, PI3K, AKT, GSK-3β, NF-κB, and α-syn), oxidative stress (GSH and MDA) parameters, and histopathological alterations (H&E and Nissl staining) were determined after 6 weeks as well as 9 weeks. After 9 weeks of study, molecular protein expression (p-AKT and p-α-syn) was determined. Hyperglycemia induced by HFD and STZ induced significant motor impairment in mice, correlated with the rotenone group. Insulin receptor signaling (GLP1/PI3K/AKT) was found to be disrupted in the HFD+STZ group and also in rotenone-treated mice, which further enhanced phosphorylation of α-syn, suggesting its role in α-syn accumulation. Histopathological alterations indicating neuroinflammation and neurodegeneration were quite evident in the HFD+STZ and rotenone groups. Exposure to hyperglycemia induced by HFD+STZ administration exhibits PD-like characteristics after 9 weeks of duration, which was correlative with rotenone-induced PD-like symptoms.

Research Priorities on the Role of α-Synuclein in Parkinson's Disease Pathogenesis

Various forms of Parkinson's disease, including its common sporadic form, are characterized by prominent α-synuclein (αSyn) aggregation in affected brain regions. However, the role of αSyn in the pathogenesis and evolution of the disease remains unclear, despite vast research efforts of more than a quarter century. A better understanding of the role of αSyn, either primary or secondary, is critical for developing disease-modifying therapies. Previous attempts to hone this research have been challenged by experimental limitations, but recent technological advances may facilitate progress. The Scientific Issues Committee of the International Parkinson and Movement Disorder Society (MDS) charged a panel of experts in the field to discuss current scientific priorities and identify research strategies with potential for a breakthrough. © 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Targets to Search for New Pharmacological Treatment in Idiopathic Parkinson's Disease According to the Single-Neuron Degeneration Model

One of the biggest problems in the treatment of idiopathic Parkinson's disease is the lack of new drugs that slow its progression. L-Dopa remains the star drug in the treatment of this disease, although it induces severe side effects. The failure of clinical studies with new drugs depends on the use of preclinical models based on neurotoxins that do not represent what happens in the disease since they induce rapid and expansive neurodegeneration. We have recently proposed a single-neuron degeneration model for idiopathic Parkinson's disease that requires years to accumulate enough lost neurons for the onset of motor symptoms. This single-neuron degeneration model is based on the excessive formation of aminochrome during neuromelanin synthesis that surpass the neuroprotective action of the enzymes DT-diaphorase and glutathione transferase M2-2, which prevent the neurotoxic effects of aminochrome. Although the neurotoxic effects of aminochrome do not have an expansive effect, a stereotaxic injection of this endogenous neurotoxin cannot be used to generate a preclinical model in an animal. Therefore, the aim of this review is to evaluate the strategies for pharmacologically increasing the expression of DT diaphorase and GSTM2-2 and molecules that induce the expression of vesicular monoamine transporter 2, such as pramipexole.

Potential convergence of olfactory dysfunction in Parkinson's disease and COVID-19: The role of neuroinflammation

Parkinson's disease (PD) is a prevalent neurodegenerative disorder that affects 7-10 million individuals worldwide. A common early symptom of PD is olfactory dysfunction (OD), and more than 90% of PD patients suffer from OD. Recent studies have highlighted a high incidence of OD in patients with SARS-CoV-2 infection. This review investigates the potential convergence of OD in PD and COVID-19, particularly focusing on the mechanisms by which neuroinflammation contributes to OD and neurological events. Starting from our fundamental understanding of the olfactory bulb, we summarize the clinical features of OD and pathological features of the olfactory bulb from clinical cases and autopsy reports in PD patients. We then examine SARS-CoV-2-induced olfactory bulb neuropathology and OD and emphasize the SARS-CoV-2-induced neuroinflammatory cascades potentially leading to PD manifestations. By activating microglia and astrocytes, as well as facilitating the aggregation of α-synuclein, SARS-CoV-2 could contribute to the onset or exacerbation of PD. We also discuss the possible contributions of NF-κB, the NLRP3 inflammasome, and the JAK/STAT, p38 MAPK, TLR4, IL-6/JAK2/STAT3 and cGAS-STING signaling pathways. Although olfactory dysfunction in patients with COVID-19 may be reversible, it is challenging to restore OD in patients with PD. With the emergence of new SARS-CoV-2 variants and the recurrence of infections, we call for continued attention to the intersection between PD and SARS-CoV-2 infection, especially from the perspective of OD.

Inhibition of GluN2D-Containing NMDA Receptors Protects Dopaminergic Neurons against 6-OHDA-Induced Neurotoxicity via Activating ERK/NRF2/HO-1 Signaling

Abnormal glutamate signaling is implicated in the heightened vulnerability of dopaminergic neurons in Parkinson's disease (PD). NMDA receptors are ion-gated glutamate receptors with high calcium permeability, and their GluN2D subunits are prominently distributed in the basal ganglia and brainstem nuclei. Previous studies have reported that dopamine depletion led to the dysfunctions of GluN2D-containing NMDA receptors in PD animal models. However, it remains unknown whether selective modulation of GluN2D could protect dopaminergic neurons against neurotoxicity in PD. In this study, we found that allosteric activation of GluN2D-containing NMDA receptors decreased the cell viability of MES23.5 dopaminergic cells and the GluN2D inhibitor, QNZ46, showed antioxidant effects and significantly relieved apoptosis in 6-OHDA-treated cells. Meanwhile, we demonstrated that QNZ46 might act via activation of the ERK/NRF2/HO-1 pathway. We also verified that QNZ46 could rescue abnormal behaviors and attenuate dopaminergic cell loss in a 6-OHDA-lesioned rat model of PD. Although the precise mechanisms underlying the efficacy of QNZ46 in vivo remain elusive, the inhibition of the GluN2D subunit should be a considerable way to treat PD. More GluN2D-selective drugs, which present minimal side effects and broad therapeutic windows, need to be developed for PD treatment in future studies.