Activation of α7-nAChRs Promotes the Clearance of α-Synuclein and Protects Against Apoptotic Cell Death Induced by Exogenous α-Synuclein Fibrils

Nicotinic Acetylcholine Receptors in Glial Cells as Molecular Target for Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disease characterized by resting tremor, bradykinesia, rigidity, and postural instability that also includes non-motor symptoms such as mood dysregulation. Dopamine (DA) is the primary neurotransmitter involved in this disease, but cholinergic imbalance has also been implicated. Current intervention in PD is focused on replenishing central DA, which provides remarkable temporary symptomatic relief but does not address neuronal loss and the progression of the disease. It has been well established that neuronal nicotinic cholinergic receptors (nAChRs) can regulate DA release and that nicotine itself may have neuroprotective effects. Recent studies identified nAChRs in nonneuronal cell types, including glial cells, where they may regulate inflammatory responses. Given the crucial role of neuroinflammation in dopaminergic degeneration and the involvement of microglia and astrocytes in this response, glial nAChRs may provide a novel therapeutic target in the prevention and/or treatment of PD. In this review, following a brief discussion of PD, we focus on the role of glial cells and, specifically, their nAChRs in PD pathology and/or treatment.

Unprecedented effect of vitamin D3 on T-cell receptor beta subunit and alpha7 nicotinic acetylcholine receptor expression in a 3-nitropropionic acid induced mouse model of Huntington's disease

Introduction

3-NP induction in rodent models has been shown to induce selective neurodegeneration in the striatum followed by the cortex (Brouillet, 2014). However, it remains unclear whether, under such a neurotoxic condition, characterized by neuroinflammation and oxidative stress, the gene expression of the immune resident protein, T-cell receptor beta subunit (TCR-β), α7 nicotinic acetylcholine receptor (α7 nAChRs), the nuclear factor kappa B (NF-κB), inflammatory cytokines (TNF-α and IL-6), and antioxidants (Cat and GpX4) get modulated on Vitamin D3 (VD) supplementation in the central nervous system.

Methods

In the present study, real-time polymerase chain reaction (RT-PCR) was performed to study the expression of respective genes. Male C57BL/6 mice (8-12 weeks) were divided into four groups namely, Group I: Control (saline); Group II: 3-NP induction via i.p (HD); Group III: Vitamin D3 (VD) and Group IV: (HD + VD) (Manjari et al., 2022).

Results

On administration of 500IU/kg/day of VD, HD mice showed a significant reduction in the gene expression of the immune receptor, TCR-β subunit, nuclear factor kappa B (NF-κB), inflammatory cytokines, and key antioxidants, followed by a decrease in the acetylcholinesterase activity.

Conclusion

A novel neuroprotective effect of VD in HD is demonstrated by combating the immune receptor, TCR-β gene expression, antioxidant markers, and inflammatory cytokines. In addition, HD mice on VD administration for 0-15 days showed an enhancement in cholinergic signaling with restoration in α7 nAChRs mRNA and protein expression in the striatum and cortex.

Apoptosis in the neuroprotective effect of α7 nicotinic receptor in neurodegenerative models

The α7 subtype of nicotinic receptors (α7 nAChRs) is one of the most abundant nicotinic receptor subtypes in the central nervous system (CNS) and both neurons and nonneuronal cells express α7 nAChRs. When activated, α7 nAChRs become permeable to cations and promote cellular responses such as anti-apoptotic signaling by modulating the caspases and proteins of the Bcl-2 family. Neuroprotection is an important function of these receptors, promoting neuronal survival under pathological conditions, including situations of stress and neuronal degeneration. Studies have demonstrated the relationship between the activation of these receptors and the reduction of neuronal or glial cell injury, by controlling apoptotic processes in different models, including neurodegenerative diseases such as Alzheimer's disease. Therefore, one of the most important signaling pathways activated by α7 nAChRs is the PI3K/Akt signaling cascade, which promotes the stimulation of anti-apoptotic molecules of the Bcl-2 family, Bcl-2 and Bcl-xl, and reduces the expression of caspases and proapoptotic molecules, resulting in cell survival. In Alzheimer's models, the literature shows that α7 nAChR activation attenuates Aβ-induced neurotoxicity through modulation of different intrinsic apoptotic pathways via PI3K/Akt and mitogen-activated protein kinase (MAPK). In this review, we provide an up-to-date summary of the current evidence on the relationship between the activation of α7 nAChRs, a subtype of nicotinic acetylcholine receptor, and its role in neuroprotection by modulating apoptotic pathways.

Neurocytotoxicity of imidacloprid- and acetamiprid-based comercial insecticides over the differentiation of SH-SY5Y neuroblastoma cells

Neonicotinoids are effective insecticides with specificity for invertebrate nicotinic acetylcholine receptors. Neonicotinoids are chemically stable and tend to remain in the environment for long so concerns about their neurotoxicity in humans do nothing but increase. Herein, we evaluated the chronic toxic effects of acetamiprid- and imidacloprid-based insecticides over the differentiation of human neuroblastoma SH-SY5Y cells, which were exposed to these insecticides at a concentration range similar to that applied to crop fields (0.01-0.5 mM). Both insecticides did not have acute cytotoxic effects in both non-differentiated and in staurosporine-differentiated SH-SY5Y cells cytotoxicity as measured by the MTT and vital-dye exclusion tests. However, after a chronic (7-day) treatment, only imidacloprid dose-dependently decreased the viability of SH-SY5Y cells (F(4,39) = 43.05, P < 0.001), largely when administered-during cell differentiation (F(4,39) = 51.86, P < 0.001). A well-defined dose-response curve was constructed for imidacloprid on day 4 (R² = 0.945, EC₅₀ = 0.14 mM). During differentiation, either imidacloprid or acetamiprid dose-dependently caused neurite branch retraction on day 3, likely because of oxidative stress, to the extent that cells turned into spheres without neurites after 7-day treatment. Despite their apparent safety, the neurodevelopmental vulnerability of SH-SY5Y neurons to the chronic exposure to imidacloprid and to a lesser extent to acetamiprid points to a neurotoxic risk for humans.

Modulation of cytotoxic amyloid fibrillation and mitochondrial damage of α-synuclein by catechols mediated conformational changes

The interplay between α-synuclein (α-syn) and catechols plays a central role in Parkinson's disease. This may be related to the modulating effects of catechols on the various aspects of α-syn fibrillization. Some of these effects may be attributed to the membrane-binding properties of the protein. In this work, we compare the effect of some catechols, including dopamine, epinephrine, DOPAL, and levodopa in micromolar concentrations, on the in vitro cytotoxicity of α-syn fibrils on human neuroblastoma SH-SY5Y cells. The study was followed by comparing the interactions of resulting structures with rat brain mitochondria used as an in vitro biological model. The obtained results demonstrate that catechols-induced structures have lost their cytotoxicity mimicking apoptotic cell death mediated by α-syn aggregates in different proportions. Moreover, α-syn fibrils-induced mitochondrial dysfunction, evaluated by a range of biochemical assays, was modulated by catechols-modified α-syn oligomers in different manners, as levodopa and DOPAL demonstrated the maximal and minimal effects, respectively. The plausible mechanism causing the inhibition of α-syn cytotoxic fibrillization and mitochondrial dysfunction by catechols is discussed. Taken together, we propose that catechols can prevent the cytotoxic assembly of α-syn and its destructive effects on mitochondria at various stages, suggesting that decreased levels of catechols in dopaminergic neurons might accelerate the α-syn cytotoxicity and mitochondrial dysfunction implicating Parkinson's disease.

Nicotine alleviates MPTP-induced nigrostriatal damage through modulation of JNK and ERK signaling pathways in the mice model of Parkinson's disease

Introduction: Nicotine (Nic) has previously been proven to reduce neurodegeneration in the models of Parkinson's disease (PD). The present study is intended to investigate the detailed mechanisms related to the potential neuroprotective effects of Nic in vivo. Methods: We established a PD model using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced C57BL6 mice (25 mg/kg/d, 5 d, i.p.) to investigate the neuropharmacological modulation of Nic pretreatment (2.5 mg/kg/d, 5 d, i.p., 30 min before MPTP injection) from the perspectives of neurobehavioral assessment, the pathological alterations, microglial cell inflammation and MAPK signaling pathways in specific brain regions. Results: The open field test, elevated plus maze, rotarod and traction test suggested that Nic pretreatment could significantly improve MPTP-induced motor impairment and had an anxiolytic effect. Nic was found to improve neuroapoptosis, enhance tyrosine hydroxylase activity, and reduce the accumulation of the phosphorylated α-synuclein in the substantia nigra and striatal regions of PD mice by TUNEL and immunohistochemical assays. Immuno-fluorescent method for labeling Iba1 and CD68 indicated that Nic remarkably alleviates the activation of microglia which represents the M1 polarization state in the mice brain under MPTP stimulation. No significant difference in the expression of p38/MAPK pathway was found in the nigrostriatal regions, while Nic could significantly inhibit the elevated p-JNK/JNK ratio and increase the declined p-ERK/ERK ratio in the substantia nigra of MPTP-exposed brains, which was further confirmed by the pretreatment of CYP2A5 inhibitor to decline the metabolic activity of Nic. Discussion: The molecular signaling mechanism by which Nic exerts its neuroprotective effects against PD may be achieved by regulating the JNK and ERK signaling pathways in the nigra-striatum related brain regions.

Role of the Cholinergic Anti-Inflammatory Reflex in Central Nervous System Diseases

In several central nervous system diseases, it has been reported that inflammation may be related to the etiologic process, therefore, therapeutic strategies are being implemented to control inflammation. As the nervous system and the immune system maintain close bidirectional communication in physiological and pathological conditions, the modulation of inflammation through the cholinergic anti-inflammatory reflex has been proposed. In this review, we summarized the evidence supporting chemical stimulation with cholinergic agonists and vagus nerve stimulation as therapeutic strategies in the treatment of various central nervous system pathologies, and their effect on inflammation.