Multitarget Effects of Nrf2 Signalling in the Brain: Common and Specific Functions in Different Cell Types

Abstract

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a crucial regulator of cellular defence mechanisms, essential for maintaining the brain's health. Nrf2 supports mitochondrial function and protects against oxidative damage, which is vital for meeting the brain's substantial energy and antioxidant demands. Furthermore, Nrf2 modulates glial inflammatory responses, playing a pivotal role in preventing neuroinflammation. This review explores these multifaceted functions of Nrf2 within the central nervous system, focusing on its activity across various brain cell types, including neurons, astrocytes, microglia, and oligodendrocytes. Due to the brain's vulnerability to oxidative stress and metabolic challenges, Nrf2 is emerging as a key therapeutic target to enhance resilience against oxidative stress, inflammation, mitochondrial dysfunction, and demyelination, which are central to many neurodegenerative diseases.

Keywords: Nrf2; antioxidants; astrocytes; brain; inflammation; microglia; mitochondria; neurodegeneration; neurons.

Figure 1

Molecular regulation of Nrf2. The transcription factor Nrf2 is a master regulator of multiple cytoprotective pathways by inducing the expression of antioxidant, detoxification, and intermediary metabolic enzymes. Under basal conditions, Nrf2 is targeted for continuous proteasomal degradation by different ubiquitin-ligase systems. The cullin 3 (Cul3) RING-box 1 (RBX1) ligase complex mediated degradation employs the Kelch-like ECH-associated protein 1, Keap1, as a substrate adaptor. Keap1 is a cysteine-rich adaptor protein that binds Nrf2, facilitating its ubiquitination and targeting it for degradation under basal conditions. Keap1 acts as an oxidative stress sensor, and in the presence of oxidative signals or electrophiles, Keap1’s highly reactive Cys residues become oxidised, leading to a conformational change that prevents Nrf2 binding, allowing its stabilisation and translocation to the nucleus. Nrf2 stabilisation can also be achieved by several molecules, such as p62 or pharmacological inhibitors, which prevent Nrf2 binding to Keap1. The β-TrCP/Cul 1 ubiquitin ligase complex mediates the degradation of Nrf2 in a mechanism involving the glycogen synthase kinase 3 (GSK3β). In basal conditions, GSK3β-mediated phosphorylation of Nrf2 targets the protein for degradation through this system. When GSK3β is inhibited, such as after its phosphorylation by the PI3K/Akt survival pathway, Nrf2 degradation does not occur, allowing its stabilisation and translocation to the nucleus. By either way, once in the nucleus, Nrf2 binds to the antioxidant response elements (ARE) present in the promoter of its target genes, inducing their transcriptional activation. Created in Biorender.

Figure 2

Main cytoprotective pathways of Nrf2 in the brain. Nrf2 orchestrates several essential cellular functions in the brain, including mitochondrial function and metabolism, antioxidant defence, and the modulation of the inflammatory response. Left panel: The brain primarily relies on glucose as an energy source. Glucose is taken up by cells via glucose transporters and metabolised through glycolysis, generating pyruvate, ATP, and NADH. Alternatively, glucose can be processed via the pentose phosphate pathway, which is critical to produce the cofactor NADPH. Pyruvate can either be converted into lactate during anaerobic respiration or enter the mitochondria to fuel the Krebs cycle. The Krebs cycle, a series of enzymatic reactions, produces NADH and FADH2, which donate electrons to the mitochondrial electron transport chain for ATP generation through oxidative phosphorylation. Nrf2 modulates glucose metabolism by increasing glucose uptake, enhancing the activity of enzymes involved in its metabolism and improving the efficiency of the Krebs cycle, thereby increasing the availability of the substrates NADH and FADH₂ for mitochondrial respiration and, subsequently, ATP production. Middle panel: Enzymatic sources, such as xanthine oxidase and NADPH oxidase, along with non-enzymatic sources like electron leakage from the mitochondria, trigger the production of reactive oxygen species (ROS). This occurs primarily through the conversion of O₂ into the superoxide anion (O₂^•−) which is rapidly converted into hydrogen peroxide (H₂O₂), either spontaneously or by the action of superoxide dismutase (SOD). In the presence of iron, H₂O₂ can generate the hydroxyl radical (•OH) through the Fenton reaction. The hydroxyl radical (•OH) is extremely reactive and can initiate lipid peroxidation in cellular membranes, ultimately triggering ferroptosis. To limit the damage caused by ROS, H₂O₂ is detoxified into water (H₂O) by various antioxidant pathways, such as catalase and glutathione peroxidases (GPx), which catalyse the reduction of H₂O₂ (and other peroxides) using glutathione (GSH) as a co-substrate. GSH is synthesised in the cytosol from its constituent amino acids by the sequential actions of glutamate-cysteine ligase (GCL) and glutathione synthetase (GS). Another crucial player in antioxidant defence is the thioredoxin (Trx) system, which reduces disulfide bonds in oxidised proteins (Prot-SS). Cellular redox reactions are ultimately driven by NADPH, which is used to reduce oxidised thioredoxin (Trx-SS) and oxidised glutathione (GSSG) through thioredoxin reductase (TrxR) and glutathione reductase (GSR), respectively, regenerating reduced thioredoxin (Trx-SH) and GSH. Among other mechanisms, Nrf2 modulates the cellular antioxidant defence by upregulating the expression of key enzymes involved in the synthesis and regeneration of GSH (GCL, GS, GR) and the thioredoxin (TrxR) systems, while also enhancing the production of NADPH through the activation of the pentose phosphate pathway. Right panel: Various pro-inflammatory signals, including lipopolysaccharide (LPS), cytokines, damage-associated patterns, and reactive oxygen species (ROS), can activate specific receptors on microglial cells, thereby initiating the NF-κB and inflammasome pathways. Within the NF-κB pathway, IKK (in the canonical pathway) or NIK (in the non-canonical pathway) phosphorylate IKBα, leading to its degradation by the proteasome. This degradation results in the release of RelA/p50 or RelB/p52, which then translocate to the nucleus to activate the transcription of inflammatory mediators such as cytokines, chemokines, and adhesion molecules. Moreover, it also permits the expression of NLRP3 and pro-IL-1β, coordinating the priming step in the NLRP3 (NOD-like receptor protein 3) inflammasome pathway. Afterwards, in the activation step, the NLRP3 protein, the adaptor protein ASC, and pro-caspase-1 assemble the inflammasome complex and activate caspase-1, IL-1β, and IL-18. The crosstalk between Nrf2 and NF-κB pathways underscores the intricate balance between antioxidant defences and inflammatory responses in maintaining brain homeostasis. Created in Biorender.