Prince Kumar Singh, Tawfeeq Shekh-Ahmad

Epilepsy is a prevalent chronic brain disorder that is characterized by a persistent predisposition to recurrently generate epileptic seizures and is often associated with cognitive and psychological consequences.Epilepsy affects approximately 65 million individuals, including both males and females of all ages worldwide, and poses a significant burden on patients, their families, and the health system (Vezzani et al., 2019).Although a substantial number of anti-seizure medications have been approved by the FDA for the treatment of epilepsy, these therapies fail to prevent the development of seizures or permanently halt the occurrence of chronic disease (Glauser et al., 2013), and more than 30% of patients with epilepsy remain resistant to these medications and endure recurrent spontaneous seizures.Therefore, the search for alternative therapeutic targets with anti-epileptogenic and anti-seizure potential could offer a promising avenue for addressing the challenges related to drugresistant epilepsy treatment.Drug resistance is a particular challenge in patients with temporal lobe epilepsy, in which seizures originate mostly in the hippocampus, and many of these cases are acquired and arise following brain insults, such as prolonged seizures, traumatic brain injuries,or strokes.Several studies have confirmed that oxidative stress (OS) and elevated levels of reactive oxygen species (ROS) play crucial roles in the development and progression of epilepsy following such brain injuries.

Oxidative stress and epilepsy: OS arises when ROS production overcomes the cellular antioxidant defense system, either due to an increase in ROS levels or a decrease in cellular antioxidant capacity.The brain is a major target for OS-induced damage due to its high level of oxygen consumption and elevated metabolic rate.During a seizure, the energy demand increases promoting mitochondrial dysfunction to fulfill energy demands and lead to elevated ROS production.Several reports have demonstrated elevated ROS production in different epilepsy models, including evidence of oxidative insult and disrupted redox balance (Fabisiak and Patel, 2022).Cock et al.(2002) demonstrated that redox regulated tricarboxylic acid cycle enzyme aconitase and α-ketoglutarate dehydrogenase have been inactivated following status epilepticus(SE).Jarrett et al.(2008) observed elevated level of H2O2in mitochondria and oxidative damage of mitochondrial DNA following the kainate model of temporal lobe epilepsy.In addition,Malinska et al.(2010) confirmed elevated levels of oxidative phosphorylation and complex IIImediated ROS production in mitochondria.Other studies in animal models and human epilepsies have also reported reduced levels of glutathione(GSH) coupled with elevated levels of oxidized glutathione in the hippocampus and neocortex,suggesting the presence of OS.

OS imposes a variety of irreversible deleterious effects on cells, including DNA damage, which leads to cell necrosis, apoptosis, and consequent cellular and tissue injury.ROS also contributes to mitochondrial permeability transition pore opening by increasing Ca2+load, which is considered the no returning point of cell death.Furthermore,ROS can react with cell membrane lipids resulting in lipid peroxidation by reducing mutagenic and toxic products, such as malondialdehyde and 4-hydroxynonenal, leading to cell membrane instability and cell death.

The crosstalk between epilepsy and OS is complex and multifaceted (Figure 1), and the exact mechanism underlying the coupling of OS with epilepsy requires further elucidation.

Keap1-Nrf2 pathway as an attractive therapeutic target for epilepsy: The nuclear factor erythroid 2-related factor 2 (Nrf2), a master transcription factor that regulates cellular redox balance, has recently emerged as a prominent therapeutic target in a wide range of diseases, including neurological disorders.Under physiological conditions, Nrf2 is retained within the cytosol by its repressor Kelch-like ECH associating protein 1 (Keap1) and is degraded by proteasomal degradation.Following OS or other alterations in cellular homeostasis, Nrf2 translocates to the nucleus and binds to the antioxidant response element to promote the expression of antioxidant enzymes and cytoprotective proteins, leading to an orchestrated protective response (Figure 1).

Previous reports have shown that Nrf2 knockout increases susceptibility to kainate toxicity, resulting in a high frequency and prolonged duration of seizures, hippocampal neuronal damage, and elevated mortality (Kraft et al., 2004).Mazzuferi et al.(2013) demonstrated the overexpression of Nrf2 in the hippocampal tissues of temporal lobe epilepsy patients and in pilocarpineinduced epileptic mice.Moreover, we recently demonstrated the spatiotemporal and cell type-specific expression of Nrf2 in the rat brain following acute seizure and kainic acid induces SE (Sandouka et al., 2023a, b).Following acute seizures the expression of Nrf2 and its downstream target was transient in the hippocampus and restricted to neuronal populations (Sandouka et al., 2023a).Interestingly, after kainic acid induced SE we observed significant Nrf2 activation in the hippocampus and transient expression in the cortex (Sandouka et al., 2023b).Interestingly, Nrf2 expression was predominant in neurons and only transient expression was detected in astrocytes(Sandouka et al., 2023b).However, the specific physiological mechanisms of Nrf2 signaling on neuronal excitability and synaptic transmission remain poorly understood.The increased Nrf2 activation following seizures suggested a potential role of OS in the development of epilepsy after brain injury.

Therapeutic strategies targeting Keap1-Nrf2 pathway: Activation of the Nrf2 pathway has demonstrated neuroprotective potential in various animal models, strengthening the idea of exploring Nrf2 activators as a potential therapeutic strategy for neurological disorders (Patel, 2015).

Figure 1 ｜Schematic illustration showing crosstalk between Nrf2, oxidative stress, and epilepsy.The illustration shows the downstream effects of the Nrf2 activators (blue arrows).Fragmented arrows indicate distant effects.Created with BioRender.com.ARE: Antioxidant response element; GCLC-1: glutamate-cysteine ligase catalytic subunit; HO-1: Heme oxygenase 1; Keap1: Kelch-like ECH associating protein 1; NQO1: NAD(P)H: quinone oxidoreductase-1; Nrf2: nuclear factor erythroid 2-related factor 2; ROS: reactive oxygen species.

Although several mechanisms have been proposed underlying the activation of the Keap1-Nrf2 system in response to electrophiles, the precise mechanism remains unclear.Importantly, two main models have been proposed underlying Nrf2 activation via cysteine modifications in Keap1: the first suggests modification in Keap1 disrupts its interaction with Nrf2 preventing Nrf2’s polyubiquitinylation, and the second suggests thiol modification leads to the dissociation of Cul3 from Keap1.

The Nrf2 agonist sulforaphane (SFN) exhibits antioxidant and protective effects in epileptic models.Pretreatment with SFN efficiently inhibits the progression of amygdala kindling in adult rats, protects against brain insult, and improves cognitive impairments (Wang et al., 2014).Pretreatment with SFN significantly prevented the elevation of OS-induced markers as well as mitochondrial dysfunction in both the acute phase of SE and the early phase of epilepsy induced by Li-pilocarpine.Even though SFN does not exhibit an anti-seizure effect, it exerts a protective effect via Nrf2 activation (Folbergrová et al., 2023).

Previous antioxidant-based therapies targeting OS show promising efficacy up to a certain extent in limiting neuronal damage following brain injury.For instance, in a post-traumatic epilepsy animal model, the antioxidant N-acetylcysteine efficiently protects against elevated OS-associated markers and reduces seizures (Silva et al., 2011).However,such therapies were reportedly ineffective in preventing the development of epilepsy, possibly because such antioxidant therapies are either too short-lived or have a limited permeability through the BBB.We recently showed that combination therapy of N-acetylcysteine with SFN significantly inhibited the progression of spontaneous seizures,neuronal cell loss, and rescue comorbidities by reducing OS in an acquired epilepsy model.Remarkably, this combinatorial therapeutic regimen exhibits a prolonged therapeutic impact even after a brief duration of treatment following the onset of SE.

Moreover, we observed that the Nrf2 pathway is endogenously activated following SE and significantly induces GSH levels in the cortex and hippocampus of epileptic animals.Interestingly,treatment with the Nrf2 activator omaveloxolone shortly after SE further increased the level of GSH in a dose-dependent manner, particularly in the hippocampus.Our findings suggest that the functional capacity of the Keap1-Nrf2 pathway is much higher than that of the endogenous response to SE and that exogenous activation of Nrf2 effectively enhances cellular antioxidant capacity.This seems paradoxical, in that Nrf2 was activated to a greater extent in the hippocampus than in the cortex.However, GSH synthesis via the Nrf2 pathway was more easily induced in the hippocampus.This finding implies that the brain has a fine-tuned system that exhibits regional differences.Altogether, these observations indicate that the antioxidant defense system is not uniformly activated throughout the brain,indicating a region-specific antioxidant defense.It is well established that the hippocampus is highly susceptible to seizures, and we observed significant neuronal cell loss in the CA1 and CA3 regions after kainic acid induced SE (Shekh-Ahmad et al., 2018).To overcome OS-induced neuronal cell death, neurons rely on extrinsic antioxidant support from neighboring astrocytes.

Conclusion and remarks: In conclusion, the vulnerability of the brain to OS holds a pivotal significance in acute neurological injuries as well as in the initiation and progression of epilepsy conditions.Seizures can induce OS, while chronic OS can contribute to neuronal excitability,inflammation, and neuronal cell loss, all of which contribute to epilepsy.However, the current evidence on the use of antioxidants for epilepsy treatment is limited and has produced mixed results.Clinical trials of vitamin E, N-acetylcysteine,ebselen, and tirilazad have primarily focused on symptomatic treatment of epilepsy rather than investigating their potential for modifying the disease.To achieve significant clinical benefits in chronic and progressive neurological conditions, such as epilepsy, it is necessary to administer antioxidant therapy at an early stage for a prolonged duration.Nonetheless, another significant challenge in utilizing antioxidants efficiently is their short life and limited BBB permeability.Our previous report suggested that the antioxidant defense system is not homogeneously activated throughout the brain,but possibly has a region-specific antioxidant defense, suggesting that most antioxidant-based therapies failed to exhibit prolonged effects in preclinical and clinical investigations.Thus,targeting the Keap1-Nrf2 pathway, a master regulator of antioxidant and anti-inflammatory responses, is a promising therapeutic approach,as revealed in various preclinical investigations.The activation of Nrf2 enhances antioxidant and anti-inflammatory responses and provides neuroprotection.Collectively, these therapeutic strategies for targeting OS via activation of Nrf2 to orchestrate antioxidant and anti-inflammatory responses in epilepsy show exciting avenues;however, further research and strategic clinical trials are required to fully comprehend the relationship between OS and epilepsy to understand and mitigate disease development and progression by validating efficacy and safety to achieve maximum clinical benefits.

This work was supported by The Israel Science Foundation, No.1976/20 (to TSA).

*Correspondence to: Tawfeeq Shekh-Ahmad, PhD,Tawfeeq.Shekh-Ahmad@mail.huji.ac.il.https://orcid.org/0000-0002-6165-2219(Tawfeeq Shekh-Ahmad)

Date of submission: July 31, 2023

Date of decision: October 9, 2023

Date of acceptance: October 22, 2023

Date of web publication: December 15, 2023

https://doi.org/10.4103/1673-5374.390975 How to cite this article:Singh PK, Shekh-Ahmad T(2024) Nrf2 as a potential target for the treatment of epilepsy.Neural Regen Res 19(9):1865-1866.

Open access statement:This is an open access journal, and articles are distributed under the terms of the Creative Commons AttributionNonCommercial-ShareAlike 4.0 License,which allows others to remix, tweak, and build upon the work non-commercially, as long as appropriate credit is given and the new creations are licensed under the identical terms.