Deciphering additional mechanisms of mTORC1 signaling in skeletal muscle

Skeletal muscle comprises about 40% of body mass and plays an essential role in metabolism and movement. The gradual age-associated loss of muscle mass and function, known as sarcopenia, results in immobility, increased morbidity and a greatly decreased quality of life. The causes of sarcopenia are multifactorial, including both genetic and environmental factors. An important regulator of muscle mass is the mammalian target of rapamycin complex 1 (mTORC1). mTORC1 promotes translation and regulates cell growth and cell size. Additionally, it down-regulates autophagy and is therefore crucial for maintaining cell homeostasis, balancing protein synthesis and degradation. Thus, it is postulated that activation of mTORC1 may be a promising therapeutic strategy to delay and reduce the signs of sarcopenia. However, in muscle-specific TSC1 knockout (TSCmKO) mice, sustained activation of mTORC1 in skeletal muscle tissue has been shown to lead to the development of a severe late-onset myopathy and premature death. Some of the characteristics of this myopathy are similar to the symptoms of sarcopenia.
The aim of this study was to characterize the development of the myopathy of the TSCmKO mice by focusing on mechanisms directly or indirectly regulated by mTORC1. This PhD thesis compromises of three chapters; the first identifies the specific mitochondrial phenotype of TSCmKO mice associated with the myopathy; the second evaluates whether calorie restriction (CR) ameliorates myopathic phenotype features in the skeletal muscle of TSCmKO mice; and the third focuses on the role of mTORC1 in skeletal muscle calcium homeostasis.
In specifying the mitochondrial phenotype, only the formation of mitochondrial herds was found in young TSCmKO mice. These minor pathological signs of a mitochondrial myopathy occur without any changes in mitochondrial number, size or metabolic changes. In contrast, an impaired mitophagy in both young and old transgenic mice was observed. This is in line with previously observed impaired autophagy. Additionally, an increase in mitochondrial dynamics and density was exclusively detected in old TSCmKO mice. In combination, the observed mitophagy and the increase in mitochondrial dynamics represent a mitochondrial myopathy-like phenotype (increases in both mitochondrial density and size), which coincides with the development of other myopathic features (e.g. loss of muscle force).
The impact of CR, without malnutirion, was tested as it is a promising non-invasive intervention, proven to increase health and lifespan in which mTORC1 inhibition is thought to have a mediating effect. A 30% CR regimen was implemented in TSCmKO mice (and controls) for a 6 months period. In both control and transgenic mice, CR decreased endoplasmic reticulum stress and decreased antioxidative stress response in the skeletal muscle while increasing cellular stress resistance by shifting skeletal muscle fiber type towards a slow phenotype. In TSCmKO mice, improved skeletal muscle health through the reduction of muscle damage was demonstrated by a lower percentage of centro-nucleated fibers and normalization of plasma creatine kinase levels. This was accompanied by decreased amounts of fibers with aggregates of the ubiquitin-binding protein p62, and additionally reduced ubiquitinated proteins marked for degradation. Hence, CR clearly ameliorated the myopathic phenotype in the transgenic mice. In addition, as there was no change in the activation state of mTORC1 with CR in TSCmKO mice, it can be concluded that mTORC1-independent pathways contribute to the amelioration of the myopathy observed in TSCmKO mice upon long-term CR.
Lastly, I analyzed the transcriptome and proteome of TSCmKO muscles to reveal new pathways involved in the metabolic alteration of skeletal muscle upon sustained activation of mTORC1. The results show that TSCmKO mice exhibit a slower contractile profile, is in agreement with the previously observed metabolic shift toward slower muscle properties. The slow contractile properties could be verified by the decreased expression of calcium storage proteins and reduction in the calcium ATPase SERCA1. Analyses of the calcineurin/nuclear factor of activated T-cell signaling pathway points toward an increased activity of the slow fiber gene expression program in TSCmKO mice. Additionally, ex-vivo measurements revealed an increase in half-relaxation time and time-to-peak tension in the transgenic animals. These results point to perturbations in the calcium homeostasis of the TSCmKO glycolytic skeletal muscle and are confirmatory of the involvement of mTORC1 in the regulation of skeletal muscle calcium signaling.
Overall, these results demonstrate the multifaceted effects of sustained mTORC1 activation on skeletal muscle, on muscle metabolism and within the development of skeletal muscle myopathies, which share common signs with sarcopenia. Thus highlighting new putative roles of mTORC1 in mitochondrial maintenance, muscle force generation and calcium signaling. They also confirm that altering mTORC1 signaling could help to reduce the loss of muscle strength and muscle mass associated with aging, thereby increasing health and quality of life.