Activated mTOR Signaling Pathway in Myofibers with Inherited Metabolic Defect Might Be an Evidence for mTOR Inhibition Therapies
Inherited metabolic myopathies (IMMs) represent a diverse group of rare genetic disorders characterized by defects in cellular metabolism, leading to muscle dysfunction ranging from exercise intolerance to muscle weakness and atrophy. These disorders are generally categorized into three major types: glycogen metabolism disorders, lipid metabolism disorders, and mitochondrial function disorders. Despite their clinical significance, effective treatments for IMMs remain limited, with current therapies primarily focusing on palliative care, dietary modifications, and exercise regimens.
The mechanistic target of rapamycin (mTOR) pathway has emerged as a critical regulator of cell metabolism and energy homeostasis. mTOR is a highly conserved serine/threonine kinase that forms two multi-protein complexes: mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2). mTORC1, in particular, plays a central role in shifting cellular metabolism from catabolic to anabolic processes, promoting the synthesis of proteins, nucleotides, and lipids while inhibiting autophagy. Given its pivotal role in cellular metabolism, the mTOR pathway has been implicated in various diseases, including cancer, transplant medicine, and metabolic disorders.
Recent studies have reported abnormal activation of the mTOR signaling pathway in animal models of IMMs, suggesting that mTOR inhibition could be a potential therapeutic strategy. However, the activity of the mTOR pathway in human skeletal muscles affected by IMMs has not been thoroughly investigated. This study aimed to address this gap by analyzing the activity of the mTOR pathway in muscle samples from patients with mitochondrial myopathy (MM), lipid storage disease (LSD), and Pompe disease (PD). The findings provide valuable insights into the role of mTOR signaling in IMMs and its potential as a therapeutic target.
Methods
The study was conducted with ethical approval from the Human Ethics Board of Qilu Hospital, Shandong University, and written informed consent was obtained from all participants. Muscle specimens were collected from 25 patients diagnosed with MM, LSD, or PD, along with disease controls (DC) and normal controls (NC). The diagnoses were confirmed based on clinical features, electrophysiological studies, muscle biopsies, blood acyl-carnitine/urine organic acid analysis, acid alpha-glucosidase (GAA) activity assays, and genetic analysis.
Muscle samples were frozen in isopentane precooled in liquid nitrogen and stored at -80°C. Western blotting was performed to evaluate the activity of the mTOR pathway by analyzing the phosphorylation of S6 ribosomal protein (p-S6) and p70S6 kinase (p-p70S6K), which are downstream targets of mTOR. Muscle pathology was assessed using hematoxylin and eosin (HE), succinate reductase (SDH), cytochrome C oxidase (COX), oil red O (ORO), and periodic acid Schiff (PAS) staining. Immunohistochemical studies were conducted to detect p-S6 expression in muscle fibers.
Results
The study revealed significant upregulation of the mTOR signaling pathway in muscle samples from patients with IMMs compared to normal and disease controls. Western blotting analysis showed that the phosphorylation of p70S6K was significantly increased in muscles from patients with LSD and MM (NC vs. LSD, U=2.000, P=0.024; NC vs. MM: U=6.000, P=0.043). Similarly, the phosphorylation of S6 ribosomal protein was also upregulated in muscles from all three subgroups of IMMs (NC vs. LSD, U=0.000, P=0.006; NC vs. PD, U=0.000, P=0.006; NC vs. MM, U=1.000, P=0.007).
Immunohistochemical analysis further demonstrated that p-S6 was predominantly expressed in muscle fibers with metabolic defects. In MM muscles, most p-S6 positive fibers exhibited cytochrome C oxidase (COX) deficiency (U=5.000, P=0.001). In LSD and PD muscles, p-S6 was mainly overexpressed in fibers with intramuscular vacuoles containing lipid droplets (U=0.000, P=0.002) or basophilic materials (U=0.000, P=0.002). These findings indicate that the activation of the mTOR pathway is closely associated with metabolic defects in affected muscle fibers.
Discussion
The mTOR pathway is a critical regulator of cellular metabolism, energy homeostasis, and stress responses. Its activation promotes anabolic processes while suppressing autophagy, making it a key player in maintaining cellular function under various physiological and pathological conditions. The findings of this study suggest that the mTOR pathway is abnormally activated in muscle fibers with inherited metabolic defects, providing a potential rationale for mTOR inhibition as a therapeutic strategy in IMMs.
The upregulation of mTOR signaling observed in this study aligns with previous reports in animal models of IMMs. For instance, increased mTOR activity has been documented in the brains and muscles of mice with mitochondrial encephalopathy and myopathy. Similarly, activated mTOR signaling has been observed in the brains, livers, and kidneys of zebrafish models of lipid storage disease. These findings collectively suggest that mTOR pathway activation may be a common response to metabolic stress across different species and tissues.
Interestingly, the study also found that the mTOR pathway was upregulated in muscle samples from patients with Pompe disease, a glycogen storage disorder. This finding contrasts with some previous studies, which reported suppressed mTOR signaling in primary skin fibroblasts from Pompe disease patients and myotubes from GAA knockout mice. The discrepancy may be attributed to differences in disease stages, tissue types, or experimental conditions. Nonetheless, the results highlight the complexity of mTOR signaling in glycogen storage disorders and underscore the need for further investigation.
The activation of the mTOR pathway in IMMs may be a compensatory response to severe or persistent oxidative stress. Increased reactive oxygen species (ROS) levels have been well-documented in cells from patients with mitochondrial encephalomyopathy, lipid storage disease, and Pompe disease. Accumulation of ROS can induce metabolic reprogramming, shifting cellular metabolism from oxidative phosphorylation to glycolysis, a phenomenon known as the Warburg effect. The mTOR pathway plays a crucial role in modulating this metabolic switch by activating hypoxia-inducible factor-1α (HIF-1α). Therefore, the upregulation of mTOR signaling in IMMs may represent an attempt to mitigate oxidative stress and restore metabolic homeostasis.
In addition to its role in metabolic reprogramming, the mTOR pathway may also be involved in other stress responses, such as the transcription of metabolic cytokines and the mitochondrial unfolded protein response. These mechanisms could further contribute to the activation of mTOR signaling in IMMs and provide additional targets for therapeutic intervention.
Conclusion
This study provides the first evidence of mTOR pathway activation in human skeletal muscles affected by inherited metabolic myopathies. The findings suggest that enhanced mTOR signaling may be a compensatory response to metabolic stress, offering a potential rationale for mTOR inhibition as a therapeutic strategy in IMMs. Future studies should explore the efficacy and safety of mTOR inhibitors in clinical trials, with the ultimate goal of developing effective treatments for these debilitating disorders.
doi.org/10.1097/CM9.0000000000000144
Was this helpful?
0 / 0