The potential role of nitrogen dioxide inhalation in Parkinson’s disease

Parkinson’s disease (PD) is a complex neurodegenerative disease in the elderly with motor and non-motor characteristics. PD is characterized by a unique set of clinical manifestations, including resting tremor, bradykinesia, and postural gait disorders. Patients may also experience non-motor symptoms such as depression, constipation and sleep disorders.1 If the disease progresses, gradual loss of brain function and premature death may occur. PD is initially associated with the loss of nigrostriatal dopaminergic neurons, with Lewy bodies found in intact nigrostriatal cells. PD affects 1–2% of adults over the age of 65 years and 4% of adults over the age of 80 years. Approximately 60,000 Americans are diagnosed with PD each year and there are currently more than one million Americans suffering from the disease. Given the increase in life expectancy, the amount of people living with PD is expected to increase by over 50% by 2030.2

In the Braak phase of PD, α-synuclein aggregates pathologically in the olfactory bulb and gut, and then spreads to the central nervous system. In addition, risk factors for PD have been identified, such as exposure to air pollution and environmental pollutants which contains metals, pesticides and the microbiome. A previous animal study has proven that exposure to environmental particles leads to neuropathological damages to dopaminergic neurons, which is a feature of PD.3 Reducing the burden of PD can be achieved by a two-pronged strategy: implementing measures to decrease changeable factors such as behavioral or environmental risk factors and development of drugs that target the mechanisms of pathological genes or environmental exposure related to PD.4 Therefore, understanding the relationship between environmental risk factors and PD pathogenesis can promote relevant prevention and treatment strategies.

Due to the widespread distribution of natural and man-made sources, nitrogen dioxide (NO₂) is a ubiquitous atmospheric pollutant that can cause respiratory irritation when inhaled at high level concentrations. Natural sources of NO₂ include volcanism, hill fire, lightning, and the stratosphere. The association between NO₂ exposures and sanitation is often attributed to the exposure to ultrafine particles from transport-related emissions. Jo’s study3 found that the increase of the risk of PD was highly associated with exposure to NO₂ (Hazard ratio for highest compared with lowest quartile, 1.41; 95% confidence interval, 1.02 to 1.95; P value of tendency = 0.045), so did another study5 that evaluated the risk of PD based on the concentration of NO₂. Meanwhile, several studies have demonstrated the direct toxic effects of NO₂ on the central nerve system. Studies have indicated that NO₂ inhalation exacerbates amyloid β42 (Aβ₄₂) accumulation and causes cognitive impairment through the metabolism of prostaglandin E2,6 causes synaptic dysfunction accompanied by auopathy,7 and increases the concentrations of pro-inflammatory markers in the brain8 (Table 1). Besides, oxidative stress and systemic inflammation following the inhalation of NO₂ have been reported.9 The direct toxic effects of NO₂ are consistent with a number of epidemiological studies suggesting associations between neurological disorders and NO₂, such as stroke, PD, and amyotrophic lateral sclerosis.

Table 1.

The potential role of nitrogen dioxide (NO₂) in neurodegenerative diseases (including Parkinson’s disease)

Disease	Animal	Injury type	Result
Parkinson’s disease	Rats	Mitochondrial dysfunction	Exposure to NO2 is involved in the significant damage of mitochondrial energy metabolism and the impairment of biogenesis.
	Mice	Tau pathology	The impaired taupathy and insulin resistance are associated with NO2 inhalation.
Neurodegenerative disease	Mice	Amyloid β42 accumulation aggravated	NO2 inhalation contributes to the amyloid β42 accumulation and neuroinflammation which mediated by arachidonic acid metabolism pathway
		Oxidative stress and inflammation	NO2 inhalation may induce mild inflammation occurred in the whole body and marked oxidative stress.

We reviewed the related articles systematically in the online databases PubMed, Embase and Cochrane on March 3, 2022, using the following search strategies: nitrogen dioxide or nitrogen peroxide, and Parkinson’s disease or primary parkinsonism, in various combinations as needed. In this manuscript, we explore the potential role of NO₂ inhalation in PD to provide insight into the pathogenic mechanisms involved and to guide possible future treatment and prevention methods.

The potential role of NO₂ inhalation in mitochondrial dysfunction and excessive reactive oxygen species (ROS) generation:

NO₂ is reported to be a highly concentrated pollutant that may be involved in the pathogenesis of PD and plays an important role in neuronal cell death.10 The impairment of mitochondrial has been implicated in a number of neurological disorders, and mitochondrial production of ROS is associated with these physiological signaling cascades. The interaction of mitochondrial membrane damage with compounds disrupts the efficiency of the coupling between oxidation and phosphorylation, resulting in a large bioenergetic deficit that is essential for the survival of cells and organisms. Mitochondria generate a membrane potential in the form of a proton gradient through the inner mitochondrial membrane by using oxidizable substrates.8 NO₂ exposure may induce a reduction in mitochondrial intima potential. The membrane potential provides the impetus for adenosine triphosphate synthesis, and a drop in membrane potential affects adenosine triphosphate production in cells, ultimately leading to cell death.11 Meanwhile, after NO₂ inhalation, thiazolyl blue tetrazolium bromide metabolism, cytochrome C oxidase activity and the expression levels of four respiratory complex subunits are decreased, which means that mitochondrial respiratory function is affected after NO₂ inhalation, and mitochondrial respiratory function and neural activity are closely linked.12 The main source of ROS production is the mitochondrial respiratory enzyme complexes. The activities of these complexes are decreased after NO₂ inhalation, which is an indirect indication of oxidative damage and results in loss of dopamine release from striatal axons, reduction of dendritic structures and dopamine release in the substantia nigra. After NO₂ inhalation, the increased malonaldehyde level and overproduction of ROS in the cortex has been confirmed in the study by Li et al.10 These results confirm the close association between mitochondrial dysfunction or excessive ROS and NO₂ inhalation, which may induce neurotoxicity and finally result in PD (Figure 1).

Figure 1.

Nitrogen dioxide (NO₂) inhalation may disrupt the efficiency of the coupling between oxidation and phosphorylation, induce a decrease in mitochondrial intima potential which contributes to the decrease of adenosine triphosphate (ATP) generation.

Note: NO₂ can decrease the activities of mitochondrial respiratory enzyme complexes that results in excessive reactive oxygen species (ROS) generation. The decrease of ATP generation and excessive ROS generation finally result in neuron death. Created with Pathway Builder 2.0.

The potential role of NO₂ inhalation in tau pathology and insulin resistance: Hyperphosphorylated tau protein is the main source of pathological tau inclusions that accumulate in neurogenic fibers and eventually lead to several neurodegenerative diseases such as PD.13 Tau phosphorylation increased in the cerebral cortex and hippocampus in a dose-dependent manner after inhalation of NO₂, clearly indicating that the inhalation of NO₂ induces taupathy by enhancing tau phosphorylation. There is evidence that accumulation of highly phosphorylated tau in the brain deletions related to tau significantly impairs the function of synapses by altering glutamate receptor expression and function. Glutamate receptor expression was reduced in the cerebral cortex and hippocampus after exposure to NO₂. Based on these findings, NO₂ exposure shows a great association with synaptic transmission dysfunction. Tau can influence the activity of synapses by directly interacting with post-synaptic signaling complexes in addition to regulating glutamate receptor content.14 The hyperphosphorylated tau protein, following exposure to NO₂, initiates the reduction cascade of the synaptic protein. Insulin is a toxicological target of air pollutants such as NO₂ which can impair insulin sensitivity and induce insulin resistance, based on previous epidemiological and experimental studies.15 Findings focusing on the function of insulin and its related receptor in the central nervous system have shown that the neuronal insulin signaling can directly regulate the tau function and disrupt intracellular insulin signaling molecules, leading to tauopathy.16 Physiological functions of insulin are performed in various tissues targeted by insulin through several intracellular signaling cascades. The insulin receptor substrate-1/phosphoinositide-3-kinase/protein kinase B (Akt) pathway is the most remarkable cascades. Tyrosine phosphorylation of insulin receptor substrate-1 and Akt was significantly reduced after exposure to NO₂, indicating the impairment of insulin signaling function. In response to inhaled NO₂, abnormal tau phosphorylation helped increase glycogen synthase kinase 3β by Akt inhibition activity.7 As a result, NO₂ inhalation contributed to the tauopathy and disturbed insulin signaling, which is also involved in mediating the insulin receptor substrate-1/Akt/glycogen synthase kinase 3β signaling pathway (Figure 2).

Figure 2.

The potential mechanism of nitrogen dioxide (NO₂) causing taupathy.

Note: NO₂ inhalation decreases tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and protein kinase B (AKT) and increase the level of glycogen synthase kinase 3β (GSK-3β) which finally results in taupathy and insulin signaling impairment. Created with Pathway Builder 2.0.

The potential role of NO₂ inhalation in Αβ₄₂ accumulation aggravated: Aβ₄₂ is one of the isoforms of Aβ that accumulates when the gene encoding amyloid precursor protein is mutated or otherwise causes an abnormal increase in β-secretase activity.17 The typical pathological feature of PD is the degenerative absence of dopaminergic neurons in the substantia nigra of the midbrain.18 A study found that Aβ₄₂ could cause progressive degeneration of dopaminergic neurons, significantly altering the morphology of dopaminergic neurons, damaging proteins and membrane structures, and leading to neuronal necrosis or apoptosis.19 NO₂ inhalation was shown to selectively cause Aβ₄₂ deposition and dose-dependent impairment of mouse memory and cognitive ability. These findings suggest that inhaling NO₂ may enhance the production of Aβ₄₂ and lead to deterioration of spatial learning and memory. Through the Kyoto Encyclopedia of Genes and Genomes and Biocarta pathways, the significantly differentially expressed genes were mainly involved in the function of synapse and the abilities of learning and memory, including long-term potentiation/depression, the pathway of NO signaling, glutamatergic synapses, neurotrophin signaling, and the pathway of calcium signaling.6 The arachidonic acid metabolism pathway was also involved in the Kyoto Encyclopedia of Genes and Genomes analysis. Arachidonic acid-derived prostaglandin E2, mainly derived from the cyclooxygenase-2 reaction, plays a critical role in stimulating Aβ formation, promoting neuroinflammatory responses, and regulating synaptic events (Figure 3).20 Thus, cyclooxygenase-2-mediated arachidonic acid metabolism seems to be a potential mechanism to promote neurodegenerative disease progression. Furthermore, improving the endogenous 2-arachidonoylglycerol levels by suppressing monoacylglycerol lipase effectively inhibits the neuroinflammation and accumulation of Aβ induced by the excessive release of prostaglandin E2,21 thereby reducing the damage to spatial learning and memory caused by inhalation of NO₂, which provides a mechanistic basis for treating PD-related diseases in contaminated areas.

Figure 3.

After nitrogen dioxide (NO₂) inhalation, arachidonic acid metabolism pathway may be involved in amyloid β₄₂ (Aβ₄₂) accumulation and neuroinflammation by increase the level of cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2).

Note: COX-2-mediated AA metabolism seems to be a potential mechanism to promote necrosis and Apoptosis of neuron. Created with Pathway Builder 2.0.

Limitations: First of all, the real air environment is a complex system that contains various components. Therefore, previous evidence may not be sufficient to confirm the exposure response correlation between NO₂ and PD progression because of the different components in air pollutant mixtures. In addition, the studies included in this review investigated the potential mechanisms of NO₂ in PD using animal models. Further studies should focus on the relationship between pathophysiological changes of populations exposed to high-level NO₂ and the morbidity of PD in the future.

Conclusion: With an aging population, PD urgently needs more attention. Many studies have proven that NO₂ can increase the incidence of PD. We need to continuously study the role of NO₂ in the pathophysiology of PD, so as to provide guidance for the further prevention and treatment of PD.

Gang Chen is an Editorial Board member of Medical Gas Research. He is blindedfrom reviewing or making decisions on the manuscript. The article was subject to the journal ’s standard procedures, with peer review handled independently of this Editorial Board member and his research group.