Environmental Pollutants and Epilepsy: Is It Nrf or Nothing?

Abstract

PM2.5 Exposure Exacerbates Seizure Symptoms and Cognitive Dysfunction by Disrupting Iron Metabolism and the Nrf2-Mediated Ferroptosis Pathway

Mei H, Wu D, Yong Z, Cao Y, Chang Y, Liang J, Jiang X, Xu H, Yang J, Shi X, Xie R, Zhao W, Wu Y, Liu Y. Sci Total Environ 2024;910:168578.

In recent years, air pollution has garnered global attention due to its ability to traverse borders and regions, thereby impacting areas far removed from the emission sources. While prior studies predominantly focused on the deleterious effects of PM2.5 on the respiratory and cardiovascular systems, emerging evidence has highlighted the potential risks of PM2.5 exposure to the central nervous system. Nonetheless, research elucidating the potential influences of PM2.5 exposure on seizures, specifically in relation to neuronal ferroptosis, remains limited. In this study, we investigated the potential effects of PM2.5 exposure on seizure symptoms and seizures-induced hippocampal neuronal ferroptosis. Our findings suggest that seizure patients residing in regions with high PM2.5 levels are more likely to disturb iron homeostasis and the Nrf2-dependent ferroptosis pathway compared to those living in areas with lower PM2.5 levels. The Morris Water Maze test, Racine scores, and EEG recordings in epileptic mice suggest that PM2.5 exposure can exacerbate seizure symptoms and cognitive dysfunction. Neurotoxic effects of PM2.5 exposure were demonstrated via Nissl staining and CCK-8 assays. Direct evidence of PM2.5-induced hippocampal neuronal ferroptosis was provided through TEM images. Additionally, increased Fe²⁺ and lipid ROS levels indirectly supported the notion of PM2.5-induced hippocampal ferroptosis. Therefore, our study underscores the necessity of preventing and controlling PM2.5 levels, particularly for patients with seizures.

Commentary

Environmental pollutants can have a significant impact on overall health, but increasingly have garnered attention for their impact on neurological diseases. Fine inhalable particulate matter (diameter 2.5 µm and smaller, PM2.5) is a pollutant that is a heterogeneous mixture of chemicals (including organic compounds and metals) and contributes to a number of neurological sequelae, but less is known of its effects on epilepsy morbidity and mortality. In this study by Mei et al, ¹ PM2.5 exposure corresponded to reduced expression of several genes in the ferroptosis pathway in humans, including those with newly diagnosed seizures, as well as in a mouse model of epilepsy. Ferroptosis is an iron-dependent and distinct form of programmed cell death driven by excess lipid peroxidation of polyunsaturated fatty acids (PUFAs) in membrane lipids, accumulation of lipid peroxides, and decreased glutathione (GSH)-dependent antioxidant capacity. Ferroptosis is regulated at the epigenetic, transcriptional, posttranscriptional, and posttranslational levels. Key proteins in the ferroptosis pathway include the cystine transporter solute carrier family 7 member 11 (SLC7A11 or xCT), iron-storing ferritin heavy and light chains (FTH, FTL), nuclear factor E2-related factor 2 (Nrf2), and glutathione peroxidase (GPX4). Ferroptosis has garnered much attention in recent years as a potential mechanism contributing to neurologic diseases and worsening disease outcomes. The brain is uniquely vulnerable to oxidative stress and ferroptosis due to its high metabolic rate, limited antioxidant capacity, and limited regenerative capacity of neurons. Extensive evidence from animal models and humans with epilepsy shows that recurrent seizures can cause oxidative distress, mitochondrial dysfunction, membrane disruption, decreased antioxidant activity, and neuroinflammation. In addition, genes within the ferroptosis signaling pathway are upregulated in the hippocampus following seizures in rodents.^2,3 Moreover, iron overload and transferrin saturation have been reported in epilepsy patients. Therefore oxidative stress, decreased cellular antioxidant capacity, lipid peroxidation, and ferroptosis may facilitate cell loss in the hippocampus and contribute to epileptogenesis.

Nrf2 plays a pivotal role in the interplay between seizures, ROS generation, neuroinflammation, and ferroptosis. The activation of Nrf2 and its subsequent translocation to the nucleus promotes the expression of several antioxidant and inflammatory genes (including FHT1, GPX4, and SLC7A11). Several key players in the Nrf2 pathway are decreased in temporal lobe epilepsy and following traumatic brain injury whereas increasing Nrf2 activity alleviates seizures, reduces microglial activation, prevents neuronal loss, and reduces learning and memory deficits.^4,5 Therefore, the regulation of Nrf2 signaling in response to stressors may play an important role in epilepsy pathology. Furthermore, Nrf2 activation also occurs following exposure to environmental toxicants like PM2.5, making it a likely point of convergence wherein PM2.5 may contribute to epilepsy morbidity and mortality.

Mei et al evaluated a comprehensive gene expression data set, containing total RNA from individuals exposed to heavy air pollution (vs a control population) and found that areas with greater levels of PM2.5 are associated with decreases in proteins in the Nrf2 pathway. ¹ Similar changes occurred in individuals with newly diagnosed epilepsy, which suggests the potential for convergence around Nrf2 signaling that could contribute to epileptogenesis. To this point, transcriptomic analysis in these studies revealed considerable overlap between epilepsy- and PM2.5-related targets, including those within inflammatory, oxidative stress, and ferroptosis pathways.

Several important questions arise following this work. While the studies described point to a directionality between seizure-induced oxidative stress, neuroinflammation, and Nrf2-mediated ferroptosis, it is important to know that this relationship can be multifaceted. It is unclear, for example, whether PM2.5 has a direct effect on brain cells (e.g., glial populations), that may subsequently promote ferroptosis-mediated neuronal death. PM2.5 may also act systemically to promote widespread inflammation that may subsequently affect the brain. Further, both seizures and PM2.5 exposure can contribute to blood–brain–barrier dysfunction, affect iron homeostasis, and promote ferroptosis. However, while ferroptosis may be a contributing mechanism to epileptogenesis, it is unclear the extent to which PM2.5 may be exacerbating symptoms. Using the mouse kainic acid model, the authors showed that PM2.5 exposure modestly elevated seizure scores and cell loss in the hippocampus. This effect was partially reversed by co-administration of Ferrostatin-1 (Fer-1). Notably, previous work demonstrates that Fer-1 in KA-treated mice (in the absence of PM2.5) reduces seizures, and hippocampal cell loss, and prevents learning and memory deficits. ⁵ However, there was no treatment condition in the present study that demonstrated the reversal of ferroptosis in the presence of KA alone. Taken together, results from the previous study and the current study suggest that at the very least ferroptosis plays an important role in both PM2.5 exposure and the onset of seizures.

A few studies have linked PM2.5 with seizure and seizure-related hospitalization risk^6,7 For instance, a nationwide retrospective population study was conducted in Taiwan to investigate the association between weather changes, air pollution, and a number of epilepsy-related hospitalizations. Surprisingly, six air pollutants including PM2.5 had a significant positive correlation with seizure-related hospital visits. ⁷ A daily time-series analysis in Chile revealed a significant correlation between days of greater air pollution (gases and particulate matter), and increased risk of hospitalization for epilepsy. ⁸ Two studies from China also showed an increase in epilepsy-related hospitalizations on high pollution days, particularly where markedly higher concentrations of particulate matter (PM2.5, and <PM10) were reported.^9,10 Much of this work has been performed by comparing patient hospitalizations with local and historical environmental data. To date, there have not been any prospective clinical studies tracking seizure history and differential PM2.5 exposures. Such work may lead to a causal link between seizure risk and environmental pollutants.

While this is the first study of its kind to investigate the complex interplay between fine particulate matter, Nrf2-mediated ferroptosis, and epilepsy, there are still several mechanistic aspects that remain unclear. Future studies that investigate the following are warranted: (i) distribution and uptake of PM2.5 in various tissues/target organs based on route of exposure; (ii) how PM2.5 disrupts the blood-brain barrier, influences epileptogenesis, and worsens cognitive outcomes; (iii) how PM2.5 interacts with different cell types in the brain and influences the ferroptotic cascade; (iv) the potential connection between PM2.5-induced cardio-respiratory issues, and seizure outcomes; and finally, (v) if and how PM2.5 exposure would influence the efficacy of anti-seizure medications (ASMs) in the epilepsy population.