Disruption of nuclear factor (erythroid-derived-2)-like 2 antioxidant signaling: a mechanism for impaired activation of stem cells and delayed regeneration of skeletal muscle

Orientin Promotes Antioxidant Capacity, Mitochondrial Biogenesis, and Fiber Transformation in Skeletal Muscles through the AMPK Pathway

The sleep-breathing condition obstructive sleep apnea (OSA) is characterized by repetitive upper airway collapse, which can exacerbate oxidative stress and free radical generation, thereby detrimentally impacting both motor and sensory nerve function and inducing muscular damage. OSA development is promoted by increasing proportions of fast-twitch muscle fibers in the genioglossus. Orientin, a water-soluble dietary C-glycosyl flavonoid with antioxidant properties, increased the expression of slow myosin heavy chain (MyHC) and signaling factors associated with AMP-activated protein kinase (AMPK) activation both in vivo and in vitro. Inhibiting AMPK signaling diminished the effects of orientin on slow MyHC, fast MyHC, and Sirt1 expression. Overall, orientin enhanced type I muscle fibers in the genioglossus, enhanced antioxidant capacity, increased mitochondrial biogenesis through AMPK signaling, and ultimately improved fatigue resistance in C2C12 myotubes and mouse genioglossus. These findings suggest that orientin may contribute to upper airway stability in patients with OSA, potentially preventing airway collapse.

Roles of Oxidative Stress and Nrf2 Signaling in Pathogenic and Non-Pathogenic Cells: A Possible General Mechanism of Resistance to Therapy

The coordinating role of nuclear factor erythroid-2-related factor 2 (Nrf2) in cellular function is undeniable. Evidence indicates that this transcription factor exerts massive regulatory functions in multiple signaling pathways concerning redox homeostasis and xenobiotics, macromolecules, and iron metabolism. Being the master regulator of antioxidant system, Nrf2 controls cellular fate, influencing cell proliferation, differentiation, apoptosis, resistance to therapy, and senescence processes, as well as infection disease success. Because Nrf2 is the key coordinator of cell defence mechanisms, dysregulation of its signaling has been associated with carcinogenic phenomena and infectious and age-related diseases. Deregulation of this cytoprotective system may also interfere with immune response. Oxidative burst, one of the main microbicidal mechanisms, could be impaired during the initial phagocytosis of pathogens, which could lead to the successful establishment of infection and promote susceptibility to infectious diseases. There is still a knowledge gap to fill regarding the molecular mechanisms by which Nrf2 orchestrates such complex networks involving multiple pathways. This review describes the role of Nrf2 in non-pathogenic and pathogenic cells.

Mitochondrial Oxidative Stress and Mitophagy Activation Contribute to TNF-Dependent Impairment of Myogenesis

Many muscular pathologies are associated with oxidative stress and elevated levels of the tumor necrosis factor (TNF) that cause muscle protein catabolism and impair myogenesis. Myogenesis defects caused by TNF are mediated in part by reactive oxygen species (ROS), including those produced by mitochondria (mitoROS), but the mechanism of their pathological action is not fully understood. We hypothesized that mitoROS act by triggering and enhancing mitophagy, an important tool for remodelling the mitochondrial reticulum during myogenesis. We used three recently developed probes—MitoTracker Orange CM-H2TMRos, mito-QC, and MitoCLox—to study myogenesis in human myoblasts. Induction of myogenesis resulted in a significant increase in mitoROS generation and phospholipid peroxidation in the inner mitochondrial membrane, as well as mitophagy enhancement. Treatment of myoblasts with TNF 24 h before induction of myogenesis resulted in a significant decrease in the myoblast fusion index and myosin heavy chain (MYH2) synthesis. TNF increased the levels of mitoROS, phospholipid peroxidation in the inner mitochondrial membrane and mitophagy at an early stage of differentiation. Trolox and SkQ1 antioxidants partially restored TNF-impaired myogenesis. The general autophagy inducers rapamycin and AICAR, which also stimulate mitophagy, completely blocked myogenesis. The autophagy suppression by the ULK1 inhibitor SBI-0206965 partially restored myogenesis impaired by TNF. Thus, suppression of myogenesis by TNF is associated with a mitoROS-dependent increase in general autophagy and mitophagy.

Effects of feeding a Saccharomyces cerevisiae fermentation product and ractopamine hydrochloride to finishing beef steers on growth performance, immune system, and muscle gene expression

The objective of this study was to determine impacts on immune parameters, anti-oxidant capacity, and growth of finishing steers fed a Saccharomyces cerevisiae fermentation product (SCFP; NaturSafe; Diamond V, Cedar Rapids, IA) and ractopamine hydrochloride (RAC; Optaflexx; Elanco Animal Health, Greenfield, IN). Angus-crossbred steers (N = 288) from two sources were utilized in this 90-d study. Steers were blocked by source, stratified by initial body weight to pens of six steers, and pens randomly assigned to treatments (16 pens per treatment). Three treatments compared feeding no supplemental SCFP (control; CON) and supplemental SCFP for 57 d (SCFP57), and 29 d (SCFP29) before harvest. Supplementation of SCFP was 12 g per steer per d, and all steers were fed RAC at 300 mg per steer per d for 29 d before harvest. Blood samples were collected from3 steers per pen, and muscle samples were collected from 1 steer per pen at 57, 29 (start of RAC), and 13 (midRAC) days before harvest. Blood was analyzed from 2 steers per pen for ferric reducing anti-oxidant power (FRAP). Muscle gene expression of myokines, markers of anti-oxidant and growth signaling were assessed. Individual animal BW were also collected on 57, 29, 13, and 1 d before being harvested at a commercial facility (National Beef, Tama, IA). Data were analyzed using the Mixed procedure of SAS 9.4 (Cary, NC) with pen as the experimental unit. The model included fixed effects of treatment and group. Increased BW compared to CON was observed days -29, -13, and -1 in SCFP57 steers (P ≤ 0.05), with SCFP29 being intermediate days -13 and -1. Overall G:F was improved in SCFP29 and SCFP57 (P = 0.01). On day -29, FRAP was greater in SCFP57 than CON (P = 0.02). The percent of gamma delta T cells and natural killer cells in both SCFP29 and SCFP57 was greater than CON on day -13 (P = 0.02). There were no treatment × day effects for muscle gene expression measured (P ≥ 0.25). Interleukin 6 tended to decrease in SCFP29 and SCFP57 on day -13 (P = 0.10). No other treatment effects were observed for muscle gene expression. Muscle gene expression of interleukin 15 was increased (P = 0.01), and expression of interleukin 8 was decreased (P = 0.03) due to RAC feeding. Increased growth in SCFP-fed cattle may be related to changes in anti-oxidant capacity and the immune system.

Skeletal Muscle Regeneration in Cardiotoxin-Induced Muscle Injury Models

Skeletal muscle injuries occur frequently in daily life and exercise. Understanding the mechanisms of regeneration is critical for accelerating the repair and regeneration of muscle. Therefore, this article reviews knowledge on the mechanisms of skeletal muscle regeneration after cardiotoxin-induced injury. The process of regeneration is similar in different mouse strains and is inhibited by aging, obesity, and diabetes. Exercise, microcurrent electrical neuromuscular stimulation, and mechanical loading improve regeneration. The mechanisms of regeneration are complex and strain-dependent, and changes in functional proteins involved in the processes of necrotic fiber debris clearance, M1 to M2 macrophage conversion, SC activation, myoblast proliferation, differentiation and fusion, and fibrosis and calcification influence the final outcome of the regenerative activity.

NRF2 Regulates Viability, Proliferation, Resistance to Oxidative Stress, and Differentiation of Murine Myoblasts and Muscle Satellite Cells

Increased oxidative stress can slow down the regeneration of skeletal muscle and affect the activity of muscle satellite cells (mSCs). Therefore, we evaluated the role of the NRF2 transcription factor (encoded by the Nfe2l2 gene), the main regulator of the antioxidant response, in muscle cell biology. We used (i) an immortalized murine myoblast cell line (C2C12) with stable overexpression of NRF2 and (ii) primary mSCs isolated from wild-type and Nfe2l2 (transcriptionally)-deficient mice (Nfe2l2^tKO). NRF2 promoted myoblast proliferation and viability under oxidative stress conditions and decreased the production of reactive oxygen species. Furthermore, NRF2 overexpression inhibited C2C12 cell differentiation by down-regulating the expression of myogenic regulatory factors (MRFs) and muscle-specific microRNAs. We also showed that NRF2 is indispensable for the viability of mSCs since the lack of its transcriptional activity caused high mortality of cells cultured in vitro under normoxic conditions. Concomitantly, Nfe2l2^tKO mSCs grown and differentiated under hypoxic conditions were viable and much more differentiated compared to cells isolated from wild-type mice. Taken together, NRF2 significantly influences the properties of myoblasts and muscle satellite cells. This effect might be modulated by the muscle microenvironment.

Balenine, Imidazole Dipeptide Promotes Skeletal Muscle Regeneration by Regulating Phagocytosis Properties of Immune Cells

Balenine is one of the endogenous imidazole dipeptides derived from marine products. It is composed of beta-alanine and 3-methyl-L-histidine, which exist mainly in the muscles of marine organisms. The physiological functions of dietary balenine are not well-known. In this study, we investigated whether the supplementation of dietary balenine was associated with muscle function in a cardiotoxin-indued muscle degeneration/regeneration model. Through morphological observation, we found that the supplementation of balenine-enriched extract promoted the regeneration stage. In addition, the expression of regeneration-related myogenic marker genes, such as paired box protein 7, MyoD1, myogenin, and Myh3, in a group of mice fed a balenine-enriched extract diet was higher than that in a group fed a normal diet. Moreover, the supplementation of balenine-enriched extract promoted the expression of anti-inflammatory cytokines as well as pro-inflammatory cytokines at the degeneration stage. Interestingly, phagocytic activity in the balenine group was significantly higher than that in the control group in vitro. These results suggest that balenine may promote the progress of muscle regeneration by increasing the phagocytic activity of macrophages.

Acid Sphingomyelinase Controls Early Phases of Skeletal Muscle Regeneration by Shaping the Macrophage Phenotype

Skeletal muscle regeneration is a complex process involving crosstalk between immune cells and myogenic precursor cells, i.e., satellite cells. In this scenario, macrophage recruitment in damaged muscles is a mandatory step for tissue repair since pro-inflammatory M1 macrophages promote the activation of satellite cells, stimulating their proliferation and then, after switching into anti-inflammatory M2 macrophages, they prompt satellite cells' differentiation into myotubes and resolve inflammation. Here, we show that acid sphingomyelinase (ASMase), a key enzyme in sphingolipid metabolism, is activated after skeletal muscle injury induced in vivo by the injection of cardiotoxin. ASMase ablation shortens the early phases of skeletal muscle regeneration without affecting satellite cell behavior. Of interest, ASMase regulates the balance between M1 and M2 macrophages in the injured muscles so that the absence of the enzyme reduces inflammation. The analysis of macrophage populations indicates that these events depend on the altered polarization of M1 macrophages towards an M2 phenotype. Our results unravel a novel role of ASMase in regulating immune response during muscle regeneration/repair and suggest ASMase as a supplemental therapeutic target in conditions of redundant inflammation that impairs muscle recovery.

Skeletal muscle-specific Keap1 disruption modulates fatty acid utilization and enhances exercise capacity in female mice

Skeletal muscle health is important for the prevention of various age-related diseases. The loss of skeletal muscle mass, which is known as sarcopenia, underlies physical disability, poor quality of life and chronic diseases in elderly people. The transcription factor NRF2 plays important roles in the regulation of the cellular defense against oxidative stress, as well as the metabolism and mitochondrial activity. To determine the contribution of skeletal muscle NRF2 to exercise capacity, we conducted skeletal muscle-specific inhibition of KEAP1, which is a negative regulator of NRF2, and examined the cell-autonomous and non-cell-autonomous effects of NRF2 pathway activation in skeletal muscles. We found that NRF2 activation in skeletal muscles increased slow oxidative muscle fiber type and improved exercise endurance capacity in female mice. We also observed that female mice with NRF2 pathway activation in their skeletal muscles exhibited enhanced exercise-induced mobilization and β-oxidation of fatty acids. These results indicate that NRF2 activation in skeletal muscles promotes communication with adipose tissues via humoral and/or neuronal signaling and facilitates the utilization of fatty acids as an energy source, resulting in increased mitochondrial activity and efficient energy production during exercise, which leads to improved exercise endurance.

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Systemic Keap1 knockdown enhances exercise endurance capacity in mice.

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Keap1 deficiency in skeletal muscle activates NRF2 pathway.

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Keap1 deficiency in skeletal muscle enhances endurance capacity in female mice.

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Keap1 deficiency in skeletal muscle promotes exercise-induced fatty acid utilization.

KAT6A regulates stemness of aging bone marrow-derived mesenchymal stem cells through Nrf2/ARE signaling pathway

Background

This study aimed to explore the effect of KAT6A on the decreased stemness of aging bone marrow-derived mesenchymal stem cells (BMSCs) and its potential mechanism.

Methods

The acetylation level and KAT6A expression of BMSCs from the young (YBMSCs) and the old (OBMSCs) were examined. Gain- and loss-of-function experiments were performed to determine the effect of KAT6A on BMSC proliferation, colony formation, and osteogenic differentiation. The effect of KAT6A on Nrf2/ARE signaling pathway was investigated after KAT6A inhibition in YBMSCs or overexpression in OBMSCs, and the role of Nrf2/ARE signaling pathway on stemness was examined by investigating proliferation, colony formation, and osteogenic differentiation. Further in vivo study was performed to explore osteogenesis ability of OBMSCs after modulation of KAT6A and Nrf2/ARE pathway through cell sheet technology.

Results

The acetylation level and KAT6A expression of OBMSCs were decreased, and KAT6A downregulation resulted in decreased proliferation, colony formation, and osteogenic differentiation of OBMSCs. Mechanically, KAT6A was found to regulate Nrf2/ARE signaling pathway and inhibit ROS accumulation in OBMSCs, thus promoting proliferation, colony formation, and osteogenic differentiation of OBMSCs. Further study demonstrated that KAT6A could promote osteogenesis of OBMSCs by regulating Nrf2/ARE signaling pathway.

Conclusions

Downregulation of KAT6A resulted in the decreased stemness of OBMSCs by inhibiting the Nrf2/ARE signaling pathway.

Graphical abstract

KAT6A was downregulated in aging bone marrow-derived mesenchymal stem cells (BMSCs), and downregulation of KAT6A resulted in Nrf2/ARE signaling pathway inhibition and ROS accumulation, thus leading to decreased stemness of aging BMSCs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02164-5.

The role of Nrf2 in acute and chronic muscle injury

The nuclear factor erythroid 2-related factor 2 (Nrf2) is considered as a master cytoprotective factor regulating the expression of genes encoding anti-oxidant, anti-inflammatory, and detoxifying proteins. The role of Nrf2 in the pathophysiology of skeletal muscles has been evaluated in different experimental models, however, due to inconsistent data, we aimed to investigate how Nrf2 transcriptional deficiency (Nrf2tKO) affects muscle functions both in an acute and chronic injury. The acute muscle damage was induced in mice of two genotypes-WT and Nrf2tKO mice by cardiotoxin (CTX) injection. To investigate the role of Nrf2 in chronic muscle pathology, mdx mice that share genetic, biochemical, and histopathological features with Duchenne muscular dystrophy (DMD) were crossed with mice lacking transcriptionally active Nrf2 and double knockouts (mdx/Nrf2tKO) were generated. To worsen the dystrophic phenotype, the analysis of disease pathology was also performed in aggravated conditions, by applying a long-term treadmill test. We have observed slightly increased muscle damage in Nrf2tKO mice after CTX injection. Nevertheless, transcriptional ablation of Nrf2 in mdx mice did not significantly aggravate the most deleterious, pathological hallmarks of DMD related to degeneration, inflammation, fibrotic scar formation, angiogenesis, and the number and proliferation of satellite cells in non-exercised conditions. On the other hand, upon chronic exercises, the degeneration and inflammatory infiltration of the gastrocnemius muscle, but not the diaphragm, turned to be increased in Nrf2tKOmdx in comparison to mdx mice. In conclusion, the lack of transcriptionally active Nrf2 influences moderately muscle pathology in acute CTX-induced muscle injury and chronic DMD mouse model, without affecting muscle functionality. Hence, in general, we demonstrated that the deficiency of Nrf2 transcriptional activity has no profound impact on muscle pathology in various models of muscle injury.

Arsenic Directs Stem Cell Fate by Imparting Notch Signaling Into the Extracellular Matrix Niche

Compromise of skeletal muscle metabolism and composition may underlie the etiology of cardiovascular and metabolic disease risk from environmental arsenic exposures. We reported that arsenic impairs muscle maintenance and regeneration by inducing maladaptive mitochondrial phenotypes in muscle stem cells (MuSC), connective tissue fibroblasts (CTF), and myofibers. We also found that arsenic imparts a dysfunctional memory in the extracellular matrix (ECM) that disrupts the MuSC niche and is sufficient to favor the expansion and differentiation of fibrogenic MuSC subpopulations. To investigate the signaling mechanisms involved in imparting a dysfunctional ECM, we isolated skeletal muscle tissue and CTF from mice exposed to 0 or 100 μg/l arsenic in their drinking water for 5 weeks. ECM elaborated by arsenic-exposed CTF decreased myogenesis and increased fibrogenic/adipogenic MuSC subpopulations and differentiation. However, treating arsenic-exposed mice with SS-31, a mitochondrially targeted peptide that repairs the respiratory chain, reversed the arsenic-promoted CTF phenotype to one that elaborated an ECM supporting normal myogenic differentiation. SS-31 treatment also reversed arsenic-induced Notch1 expression, resulting in an improved muscle regeneration after injury. We found that persistent arsenic-induced CTF Notch1 expression caused the elaboration of dysfunctional ECM with increased expression of the Notch ligand DLL4. This DLL4 in the ECM was responsible for misdirecting MuSC myogenic differentiation. These data indicate that arsenic impairs muscle maintenance and regenerative capacity by targeting CTF mitochondria and mitochondrially directed expression of dysfunctional regulators in the stem cell niche. Therapies that restore muscle cell mitochondria may effectively treat arsenic-induced skeletal muscle dysfunction and compositional decline.

Reductive Stress Causes Pathological Cardiac Remodeling and Diastolic Dysfunction

Aims: Redox homeostasis is tightly controlled and regulates key cellular signaling pathways. The cell's antioxidant response provides a natural defense against oxidative stress, but excessive antioxidant generation leads to reductive stress (RS). This study elucidated how chronic RS, caused by constitutive activation of nuclear erythroid related factor-2 (caNrf2)-dependent antioxidant system, drives pathological myocardial remodeling. Results: Upregulation of antioxidant transcripts and proteins in caNrf2-TG hearts (TGL and TGH; transgenic-low and -high) dose dependently increased glutathione (GSH) redox potential and resulted in RS, which over time caused pathological cardiac remodeling identified as hypertrophic cardiomyopathy (HCM) with abnormally increased ejection fraction and diastolic dysfunction in TGH mice at 6 months of age. While the TGH mice exhibited 60% mortality at 18 months of age, the rate of survival in TGL was comparable with nontransgenic (NTG) littermates. Moreover, TGH mice had severe cardiac remodeling at ∼6 months of age, while TGL mice did not develop comparable phenotypes until 15 months, suggesting that even moderate RS may lead to irreversible damages of the heart over time. Pharmacologically blocking GSH biosynthesis using BSO (l-buthionine-SR-sulfoximine) at an early age (∼1.5 months) prevented RS and rescued the TGH mice from pathological cardiac remodeling. Here we demonstrate that chronic RS causes pathological cardiomyopathy with diastolic dysfunction in mice due to sustained activation of antioxidant signaling. Innovation and Conclusion: Our findings demonstrate that chronic RS is intolerable and adequate to induce heart failure (HF). Antioxidant-based therapeutic approaches for human HF should consider a thorough evaluation of redox state before the treatment.

Reductive stress impairs myogenic differentiation

Myo-satellite cells regenerate and differentiate into skeletal muscle (SM) after acute or chronic injury. Changes in the redox milieu towards the oxidative arm at the wound site are known to compromise SM regeneration. Recently, we reported that abrogation of Nrf2/antioxidant signaling promotes oxidative stress and impairs SM regeneration in C57/Bl6 mice. Here, we investigated whether the activation of intracellular Nrf2 signaling favors antioxidant transcription and promotes myoblast differentiation. Satellite cell-like C2C12 myoblasts were treated with sulforaphane (SF; 1.0 & 5.0 μM) to activate Nrf2/antioxidant signaling during proliferation and differentiation (i.e. formation of myotubes/myofibers). SF-mediated Nrf2 activation resulted in increased expression of Nrf2-antioxidants (e.g. GCLC and G6PD) and augmented the production of reduced glutathione (GSH) leading to a reductive redox state. Surprisingly, this resulted in significant inhibition of myoblast differentiation, as observed from morphological changes and reduced expression of MyoD, Pax7, and Myh2, due to reductive stress (RS). Furthermore, supplementation of N-acetyl-cysteine (NAC) or GSH-ester or genetic knock-down of Keap1 (using siRNA) also resulted in RS-driven inhibition of differentiation. Interestingly, withdrawing Nrf2 activation rescued differentiation potential and formation of myotubes/myofibers from C2C12 myoblasts. Thus, abrogation of physiological ROS signaling through over-activation of Nrf2 (i.e. RS) and developing RS hampers differentiation of muscle satellite cells.

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Sulforaphane activates Nrf2 and establishes reductive stress (RS) in C2C12 myoblasts.

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RS abolishes basal ROS and significantly impede the differentiation of myoblasts.

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Augmentation of glutathione using pharmacological agents (NAC and GSH-ester) promotes RS and impairs differentiation.

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Precluding RS restores the myoblast differentiation process.

Glutathione and Nitric Oxide: Key Team Players in Use and Disuse of Skeletal Muscle

Glutathione (GSH) is the main non-enzymatic antioxidant playing an important role in detoxification, signal transduction by modulation of protein thiols redox status and direct scavenging of radicals. The latter function is not only performed against reactive oxygen species (ROS) but GSH also has a fundamental role in buffering nitric oxide (NO), a physiologically-produced molecule having-multifaceted functions. The efficient rate of GSH synthesis and high levels of GSH-dependent enzymes are characteristic features of healthy skeletal muscle where, besides the canonical functions, it is also involved in muscle contraction regulation. Moreover, NO production in skeletal muscle is a direct consequence of contractile activity and influences several metabolic myocyte pathways under both physiological and pathological conditions. In this review, we will consider the homeostasis and intersection of GSH with NO and then we will restrict the discussion on their role in processes related to skeletal muscle function and degeneration.

Activation of cannabinoid type 2 receptor protects skeletal muscle from ischemia-reperfusion injury partly via Nrf2 signaling

AIMS

Cannabinoid type 2 (CB₂) receptor activation has been shown to attenuate IRI in various organs. NF-E2-related factor (Nrf2) is an anti-oxidative factor that plays multiple roles in regulating cellular redox homeostasis and modulating cell proliferation and differentiation. The protective effects of CB₂ receptor activation on skeletal muscle IRI and the underlying mechanism that involves Nrf2 signaling remain unknown.

MAIN METHODS

We evaluated the in vivo effect of CB₂ receptor activation by the CB₂ receptor agonist AM1241 on IR-induced skeletal muscle damage and early myogenesis. We also assessed the effects of CB₂ receptor activation on C2C12 myoblasts differentiation and H₂O₂-induced C2C12 myoblasts damage in vitro, with a focus on the mechanism of Nrf2 signaling.

KEY FINDINGS

Our results showed that CB₂ receptor activation reduced IR-induced histopathological lesions, edema, and oxidative stress 1 day post-injury and accelerated early myogenesis 4 days post-injury in mice. Nrf2 knockout mice that were treated with AM1241 exhibited deteriorative skeletal muscle oxidative damage and myogenesis. In vitro, pretreatment with AM1241 significantly increased the expression of Nrf2 and its nuclear translocation, attenuated the decrease in H₂O₂-induced C2C12 cell viability, and decreased reactive oxygen species generation and apoptosis. CB₂ receptor activation also significantly enhanced C2C12 myoblasts differentiation, which was impaired by silencing Nrf2.

SIGNIFICANCE

Overall, CB₂ receptor activation protected skeletal muscle against IRI by ameliorating oxidative damage and promoting early skeletal muscle myogenesis, which was partly via Nrf2 signaling.

Role of Keap1-Nrf2 Signaling in Anhedonia Symptoms in a Rat Model of Chronic Neuropathic Pain: Improvement With Sulforaphane

Patients with chronic neuropathic pain frequently suffer from symptoms of anhedonia (loss of pleasure), which is a core clinical manifestation of depression. Accumulating studies have shown the beneficial effects of the natural compound sulforaphane (SFN), an activator of nuclear factor (erythroid-derived 2)-like 2 (Nrf2), on depression-like phenotype through a potent anti-inflammatory effect. However, it is unknown whether SFN confers beneficial effects in neuropathic pain-associated anhedonia. Spared nerve injury (SNI) is classical rodent model of chronic neuropathic pain. We here used a rat model of SNI. Hierarchical cluster analysis of sucrose preference test (SPT) results was used to classify the SNI rats with or without an anhedonia phenotype. Nrf2 protein expression was significantly decreased in the medial prefrontal cortex (mPFC), hippocampus, spinal cord, and skeletal muscle, but not in the nucleus accumbens, in anhedonia-susceptible rats compared with sham or anhedonia-resistant rats. The expression of Kelch-like erythroid cell-derived protein with CNC homology (ECH)-associated protein 1 (Keap1), a partner of Nrf2, in mPFC, hippocampus, and muscle of anhedonia-susceptible rats was also significantly lower than that in sham or anhedonia-resilient rats. Subsequent SFN administration after SNI surgery exerted therapeutic effects on reduced mechanical withdrawal threshold (MWT) scores, but not on sucrose preference, through the normalization of Keap1-Nrf2 signaling in the spinal cords of anhedonia-susceptible rats. Interestingly, treatment with SFN 30 min prior to SNI surgery significantly attenuated reduced MWT scores and sucrose preference, and restored tissue Keap1 and Nrf2 levels. In conclusion, this study suggests that decreased Keap1-Nrf2 signaling in mPFC, hippocampus, and muscle may contribute to anhedonia susceptibility post-SNI surgery, and that SFN exerts beneficial effects in SNI rats by normalization of decreased Keap1-Nrf2 signaling.

Implication of basal lamina dependency in survival of Nrf2-null muscle stem cells via an antioxidative-independent mechanism

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a master regulator for the induction of antioxidative genes and plays roles in diverse cellular functions. The roles of Nrf2 in muscle regeneration have been investigated, and both important and unimportant roles of Nrf2 for muscle regeneration have been reported. Here, using aged Nrf2-null and Nrf2-dystrophic double-null mice, we showed nonsignificant phenotypes in the muscle regeneration ability of Nrf2-null mice. In contrast with these results, strikingly, almost all Nrf2-null muscle stem cells (MuSCs) isolated by fluorescence-activated cell sorting died in vitro of apoptosis and were not rescued by antioxidative reagents. Although their proliferation was still impaired, the Nrf2-null MuSCs attached to myofibers activated and divided normally, at least in the first round. To elucidate these discrepancies of MuSCs behaviors, we focused on the basal lamina, because both in vivo and single myofiber culture allow MuSCs within the basal lamina to become activated. In a basal lamina-disrupted model, Nrf2-null mice exhibited remarkable regeneration defects without increased levels of reactive oxidative species in MuSCs, suggesting that the existence of the basal lamina affects the survival of Nrf2-null MuSCs. Taken together, these results suggest that the basal lamina compensates for the loss of Nrf2, independent of the antioxidative roles of Nrf2. In addition, experimental conditions might explain the discrepant results of Nrf2-null regenerative ability.

Nrf2-Keap1 signaling in oxidative and reductive stress

Nrf2 and its endogenous inhibitor, Keap1, function as a ubiquitous, evolutionarily conserved intracellular defense mechanism to counteract oxidative stress. Sequestered by cytoplasmic Keap1 and targeted to proteasomal degradation in basal conditions, in case of oxidative stress Nrf2 detaches from Keap1 and translocates to the nucleus, where it heterodimerizes with one of the small Maf proteins. The heterodimers recognize the AREs, that are enhancer sequences present in the regulatory regions of Nrf2 target genes, essential for the recruitment of key factors for transcription. In the present review we briefly introduce the Nrf2-Keap1 system and describe Nrf2 functions, illustrate the Nrf2-NF-κB cross-talk, and highlight the effects of the Nrf2-Keap1 system in the physiology and pathophysiology of striated muscle tissue taking into account its role(s) in oxidative stress and reductive stress.

Nutritional Ketosis and Mitohormesis: Potential Implications for Mitochondrial Function and Human Health

Impaired mitochondrial function often results in excessive production of reactive oxygen species (ROS) and is involved in the etiology of many chronic diseases, including cardiovascular disease, diabetes, neurodegenerative disorders, and cancer. Moderate levels of mitochondrial ROS, however, can protect against chronic disease by inducing upregulation of mitochondrial capacity and endogenous antioxidant defense. This phenomenon, referred to as mitohormesis, is induced through increased reliance on mitochondrial respiration, which can occur through diet or exercise. Nutritional ketosis is a safe and physiological metabolic state induced through a ketogenic diet low in carbohydrate and moderate in protein. Such a diet increases reliance on mitochondrial respiration and may, therefore, induce mitohormesis. Furthermore, the ketone β-hydroxybutyrate (BHB), which is elevated during nutritional ketosis to levels no greater than those resulting from fasting, acts as a signaling molecule in addition to its traditionally known role as an energy substrate. BHB signaling induces adaptations similar to mitohormesis, thereby expanding the potential benefit of nutritional ketosis beyond carbohydrate restriction. This review describes the evidence supporting enhancement of mitochondrial function and endogenous antioxidant defense in response to nutritional ketosis, as well as the potential mechanisms leading to these adaptations.

Partial involvement of Nrf2 in skeletal muscle mitohormesis as an adaptive response to mitochondrial uncoupling

Mitochondrial dysfunction is usually associated with various metabolic disorders and ageing. However, salutary effects in response to mild mitochondrial perturbations have been reported in multiple organisms, whereas molecular regulators of cell-autonomous stress responses remain elusive. We addressed this question by asking whether the nuclear factor erythroid-derived-like 2 (Nrf2), a transcription factor and master regulator of cellular redox status is involved in adaptive physiological responses including muscle mitohormesis. Using a transgenic mouse model with skeletal muscle-specific mitochondrial uncoupling and oxidative phosphorylation (OXPHOS) inefficiency (UCP1-transgenic, TG) we show that additional genetic ablation of Nrf2 abolishes an adaptive muscle NAD(P)H quinone dehydrogenase 1 (NQO1) and catalase induction. Deficiency of Nrf2 also leads to decreased mitochondrial respiratory performance although muscle functional integrity, fiber-type profile and mitochondrial biogenesis were not significantly altered. Importantly, Nrf2 ablation did not abolish the induction of key genes and proteins of muscle integrated stress response including the serine, one-carbon cycle, and glycine synthesis (SOG) pathway in TG mice while further increasing glutathione peroxidase (GPX) activity linked to increased GPX1 protein levels. Conclusively, our results tune down the functions controlled by Nrf2 in muscle mitohormesis and oxidative stress defense during mitochondrial OXPHOS inefficiency.

Nrf2 deficiency does not affect denervation-induced alterations in mitochondrial fission and fusion proteins in skeletal muscle

Oxidative stress‐induced mitochondrial dysfunction is associated with age‐related and disuse‐induced skeletal muscle atrophy. However, the role of nuclear factor erythroid 2‐related factor 2 (Nrf2) during muscle fiber atrophy remains to be elucidated. In this study, we examined whether deficiency of Nrf2, a master regulator of antioxidant transcription, promotes denervation‐induced mitochondrial fragmentation and muscle atrophy. We found that the expression of Nrf2 and its target antioxidant genes was upregulated at 2 weeks after denervation in wild‐type (WT) mice. The response of these antioxidant genes was attenuated in Nrf2 knockout (KO) mice. Nrf2 KO mice exhibited elevated levels of 4‐hydroxynonenal in the skeletal muscle, whereas the protein levels of the mitochondrial oxidative phosphorylation complex IV was declined in the denervated muscle of these mice. Increased in mitochondrial fission regulatory proteins and decreased fusion proteins in response to denervation were observed in both WT and KO mice; however, no difference was observed between the two groups. These findings suggest that Nrf2 deficiency aggravates denervation‐induced oxidative stress, but does not affect the alterations in mitochondrial morphology proteins and the loss of skeletal muscle mass.

Redox Control of Skeletal Muscle Regeneration

Skeletal muscle shows high plasticity in response to external demand. Moreover, adult skeletal muscle is capable of complete regeneration after injury, due to the properties of muscle stem cells (MuSCs), the satellite cells, which follow a tightly regulated myogenic program to generate both new myofibers and new MuSCs for further needs. Although reactive oxygen species (ROS) and reactive nitrogen species (RNS) have long been associated with skeletal muscle physiology, their implication in the cell and molecular processes at work during muscle regeneration is more recent. This review focuses on redox regulation during skeletal muscle regeneration. An overview of the basics of ROS/RNS and antioxidant chemistry and biology occurring in skeletal muscle is first provided. Then, the comprehensive knowledge on redox regulation of MuSCs and their surrounding cell partners (macrophages, endothelial cells) during skeletal muscle regeneration is presented in normal muscle and in specific physiological (exercise-induced muscle damage, aging) and pathological (muscular dystrophies) contexts. Recent advances in the comprehension of these processes has led to the development of therapeutic assays using antioxidant supplementation, which result in inconsistent efficiency, underlying the need for new tools that are aimed at precisely deciphering and targeting ROS networks. This review should provide an overall insight of the redox regulation of skeletal muscle regeneration while highlighting the limits of the use of nonspecific antioxidants to improve muscle function. Antioxid. Redox Signal. 27, 276-310.

Activation of the Keap1/Nrf2 stress response pathway in autophagic vacuolar myopathies

Nrf2 (nuclear factor [erythroid-derived 2]-like 2; the transcriptional master regulator of the antioxidant stress response) is regulated through interaction with its cytoplasmic inhibitor Keap1 (Kelch-like ECH-associated protein 1), which under basal conditions targets Nrf2 for proteasomal degradation. Sequestosome 1 (SQSTM1)/p62–a multifunctional adapter protein that accumulates following autophagy inhibition and can serve as a diagnostic marker for human autophagic vacuolar myopathies (AVMs)–was recently shown to compete with Nrf2 for Keap1 binding, resulting in activation of the Nrf2 pathway. In this study, we used 55 human muscle biopsies divided into five groups [normal control, hydroxychloroquine- or colchicine-treated non-AVM control, hydroxychloroquine- or colchicine-induced toxic AVM, polymyositis, and inclusion body myositis (IBM)] to evaluate whether Keap1-SQSTM1 interaction led to increased Nrf2 signaling in human AVMs. In toxic AVMs and IBM, but not in control muscle groups or polymyositis, Keap1 antibody labeled sarcoplasmic protein aggregates that can be used as an alternate diagnostic marker for both AVM types; these Keap1-positive aggregates were co-labeled with the antibody against SQSTM1 but not with the antibody against autophagosome marker LC3 (microtubule-associated protein 1 light chain 3). In human AVM muscle, sequestration of Keap1 into the SQSTM1-positive protein aggregates was accompanied by an increase in mRNA and protein levels of Nrf2 target genes; similarly, treatment of differentiated C2C12 myotubes with autophagy inhibitor chloroquine led to an increase in the nuclear Nrf2 protein level and an increase in expression of the Nrf2-regulated genes. Taken together, our findings demonstrate that Nrf2 signaling is upregulated in autophagic muscle disorders and raise the possibility that autophagy disruption in skeletal muscle leads to dysregulation of cellular redox homeostasis.