Overexpression of Skeletal Muscle Nrf2 Protects Against Aging‐Associated Dysfunction in Skeletal Muscle and Heart
Authors: Gao Lie , Wang Lucas , Hackfort Bryan T. , Zucker Irving H. 
Published: May 2022

IntroductionThe nuclear factor erythroid 2–related factor 2 (Nrf2) is a master transcription factor governing hundreds of genes coding proteins involved in anti‐oxidation, anti‐inflammation, detoxification, and metabolism. Using a model of tissue‐specific deletion of the gene for Kelch Like ECH Associated Protein 1 (Keap1), an endogenous inhibitor of Nrf2, we previously found that Nrf2 overexpression upregulates over 100 proteins in skeletal muscle (SkM), which results in activation of endogenous antioxidant defenses and the enhancement of exercise capacity in adult mice. In addition, by employing a SkM‐reporter mouse, where SkM exclusively expresses GFP while non‐muscle tissues express td‐Tomato, we demonstrated that SkM‐derived extracellular vesicles (EVs) can be transferred from SkM to remote non‐SkM tissues, including cardiomyocytes. Given the critical role of oxidative stress in the pathogenesis of sarcopenia and cardiac aging, we hypothesized that Keap1 KO can protect against aging‐associated myopathy not only in SkM but also in myocardium via EV‐mediated inter‐organ antioxidant protein communication.Materials & MethodsExperiments were carried out in 43 male iMS‐Keap1flox/flox (i.e. Keap1 SkM specific) mice assigned to 4 groups: young‐WT (11), young‐Keap1‐KO (12), aged‐WT (11), and aged‐Keap1‐KO (9). Keap1 KO was induced by intraperitoneal injection of tamoxifen at 3 months while the WT group received sunflower oil (vehicle) at the same age. At the age of 6 months in young groups and 27 months in aged groups, mice a maximal exercise tolerance test (treadmill) and echocardiography to evaluate cardiac function.ResultsWe found that, aged‐WT displayed significantly shorter running distance than young‐WT (280.2 ± 35.1 vs 531.6 ± 38.4 m, p < 0.001). This was improved in aged‐Keap1‐KO mice (508.2 ± 66.8 m, p < 0.01 vs aged WT). Compared with young‐WT, aged‐WT displayed significantly reduced SkM weights of soleus (0.0128 ± 0.0011 vs 0.0182 ± 0.0021 g, p < 0.05) and tibialis anterior (0.0627 ± 0.0093 vs 0.0937 ± 0.0101 g, p < 0.005), which were ameliorated in aged‐Keap1‐KO mice. Echocardiography indicated that aged‐WT mice have lower ejection fraction (EF), longer isovolumic relaxation time (IVRT), and higher myocardial performance index (MPI) than young‐WT (EF: 60.3 ± 3.1 vs 68.1 ± 1.3 %, p < 0.05; IVRT: 22.6 ± 1.5 vs 16.9 ± 0.5 ms, p < 0.01; MPI: 0.71 ± 0.03 vs 0.55 ± 0.02, p < 0.001). These aging‐associated cardiac dysfunctions were partially alleviated in aged‐Keap1‐KO (EF: 65.6 ± 0.65 %, p = 0.073; IVRT: 16.4 ± 1.5 ms, p < 0.01; MPI: 0.57 ± 0.03, p < 0.001 vs aged WT). In addition, aged‐WT mice exhibited attenuated responses to intraperitoneal injection of the ß‐1 agonist, dobutamine. EF, cardiac output, stroke volume, and diastolic volume, were significantly improved in aged‐Keap1‐KO mice (p < 0.05 vs aged‐WT).ConclusionsOur data suggest that a life‐long activation of SkM Nrf2 not only attenuates aging‐associated SkM dysfunction but also improves cardiac aging parameters. The later effect, we propose is mediated by transference of Nrf2‐dependent upregulated antioxidant enzymes in the Keap1‐deficient SkM to the myocardium through SkM‐derived EVs.