Redox Homeostasis and Metabolic Reprogramming in Skeletal Muscle: Exploring the Role of the Cystine/Glutamate Antiporter

Abstract

Skeletal muscle regeneration is a highly regulated process that primarily relies on the activity of muscle satellite cells (MuSCs). The functions and fates of MuCSs are highly sensitive to alterations in cellular redox, as different phases of the myogenic program are redox-sensitive. Glutathione (GSH), a major antioxidant in muscle cells, plays a key role in myogenesis by maintaining redox homeostasis. Redox homeostasis is often disrupted in muscle injury, aging, and diseases such as muscular dystrophies. Cysteine is the rate-controlling amino acid in the synthesis of GSH in the cytosol. Cystine, the predominant form of free cysteine in blood, is imported into cells by the plasma membrane antiporter, xCT, which imports cystine and exports glutamate. As such, this antiporter plays a central role in controlling intracellular GSH levels. xCT can also regulate cellular metabolism by modulating the intracellular availability of cysteine and glutamate, which impact a number of metabolic pathways. Therefore, xCT, through its dual redox and metabolic roles, can profoundly impact skeletal muscle health and its regenerative capacity. The first project of my doctoral research aimed to elucidate the role of xCT in regulating MuSC myogenic programming processes. It also explored the implications of xCT in the context of exercise training. Given the well-recognized redox role of xCT, we hypothesized that xCT controls skeletal muscle regeneration by modulating GSH redox. To test this hypothesis, we employed a multifaceted approach, including bioinformatic analyses, in vitro studies using the murine C2C12 muscle cell line and mouse primary muscle cells, as well as in vivo experiments with xCT-mutant mice (Slc7a11sut/sut). Our findings revealed that xCT is indispensable for MuSC proliferation and self-renewal processes, while its expression is downregulated during myotube differentiation. Strikingly, xCT deficiency enhanced myogenic differentiation in vitro and in vivo. Furthermore, xCT-mutant mice displayed improved insulin sensitivity pre- and post-exercise training and had blunted muscle mitochondrial biogenesis and respiration in response to exercise training. These findings highlight the role of xCT in skeletal muscle regenerative capacity and metabolism, which may help inform novel therapeutic approaches for muscle wasting and dysfunction observed in metabolic diseases and aging. MuSC proliferation is an essential myogenic phase for stem cell pool maintenance and successful muscle repair. Disruptions in metabolic processes during MuSC proliferation, particularly a decreased supply of amino acids, can impair MuSC growth and function and compromise muscle health. Thus, my second project investigated the metabolic consequences of xCT deficiency in proliferating MuSCs. We hypothesized that loss of xCT perturbs redox balance, disrupts mitochondrial structure, and alters metabolic pathways in proliferating MuSCs. By leveraging complementary bioinformatic, metabolomic, and mechanistic approaches, our findings revealed that xCT-deficient MuSCs have impaired GSH redox and lower glycolytic and oxidative capacities associated with DRP1-mediated mitochondrial fragmentation. To compensate for dysregulated xCT transport activity, MuSCs underwent metabolic reprogramming, directing glycolytic intermediates toward serine and cysteine biosynthesis. Additionally, xCT-deficient MuSCs channeled excess glutamate toward proline biosynthesis to maintain cellular redox and balance between oxidative and reductive pathways. These findings emphasize the role of xCT in driving metabolic adaptations in MuSCs, further confirming the link between redox regulation and cellular metabolism. In conclusion, these projects provide novel insights into the dual role of xCT in skeletal muscle redox and metabolism during various myogenic stages. A better understanding of xCT function in skeletal muscle may pave the way for therapeutic interventions aimed at restoring/improving muscle health in conditions characterized by dysregulated redox and metabolism.