The role of SIRT1 in skeletal muscle function and repair of older mice

Human skeletal muscle is a highly metabolic tissue necessary for mobility and coordination. Responsible for approximately one-fifth of the resting human metabolism, skeletal muscle is also an important regulator of metabolites like glucose and contributes to the regulation of body temperature. Although there is a gradual decline in muscle mass associated with aging, a certain percentage of the population suffer from severe muscle mass and strength deterioration, classified as sarcopenia (5-13% for people aged 60-70 years old, 11-50% for those 80 or older). Sarcopenia is linked to increased morbidity and mortality rates in the elderly population, while annual healthcare costs related to sarcopenia total in the millions of dollars. Because the prevalence of sarcopenia is expected to increase as a larger percentage of the population transitions into old age, it becomes imperative to understand the mechanisms of aging and longevity so that more effective interventions can be taken against age-related muscle deterioration. Our laboratory has previously demonstrated that resveratrol, a known activator of the protein sirtuin 1 (SIRT1), was effective in enhancing human muscle adaptations to exercise in elderly populations. A rich body of literature has long supported the association of SIRT1 with longevity, but there are still gaps in our knowledge of how SIRT1 expression affects the functionality and performance of muscles in aging skeletal muscle. Furthermore, SIRT1 has been shown to be important in the function of muscle satellite cells—which are muscle stem cells that are responsible for the majority of muscle regeneration. However, there is little knowledge about how SIRT1 expression affects muscle regeneration and performance after injuries. To investigate the role of SIRT1 in the performance of aging and injured skeletal muscle, we have employed the use of several transgenic mouse models with differential expression of SIRT1. Using these models, we performed a series of functional muscle tests, before and after cardiotoxin (CTX) induced muscle injuries, to identify and compare muscle aptitude and recovery capability. Skeletal muscle sections from each model were also taken to identify differences in muscle fiber size and type distribution. Additionally, both mitochondria and satellite cells were isolated from these models to assess whether SIRT1 expression contributed to differences in metabolic or regenerative capacities. We found that there was little functional difference between young wild-type (YWT, aged 20-30 weeks) and aged (80+ weeks old) wild-type (WT-80), SIRT1 overexpressor (OE-80), and SIRT1 muscle-knockout (MKO-80) mice in either force production or fatigability in the absence of intervention. Mice lacking SIRT1 expression in their satellite cells (SKO-80), however, did show a reduction in force production. Interestingly, both the OE-80 and MKO-80 mice showed significant (P \u3c 0.05) increases for p53 expression and reduced fatigability after recovering from injury, with the SIRT1 overexpressor model showing some signs of muscle potentiation. MKO-80 mice showed a significant increase in satellite cell regeneration (P \u3c 0.05) in vitro when analyzed with EdU, but no difference in proliferation when compared in vivo with BrdU, indicating that SIRT1 expression in adult skeletal muscle may be an early factor in limiting the proliferation of satellite cells. The mitochondrial and structural profiles of each model were found to have minimal differences. Overall, our data indicate that although SIRT1 expression in skeletal muscle does not seem to be necessary for normal muscle function after injury, it does exert some influence in muscle repair. Altering SIRT1 expression either positively or negatively in skeletal muscle improves muscle fatigability in injury-recovered muscles, indicating a potential regulatory role for SIRT1 in skeletal muscle, but not an essential requirement for its deacetylation activity. Interestingly, these alterations of SIRT1 expression in aged skeletal muscle also resulted in a significant increase of p53 expression, indicating a potential benefit for p53 expression to muscle recovery. SIRT1 expression in satellite cells was shown to be necessary to achieve normal contractile force, but did not affect fatigability in those muscle. Our work has indicated a complex role for SIRT1 in skeletal muscle regeneration. We have shown for the first time that SIRT1 is required in satellite cells for proper function, but is not essential for muscles to recover their functionality after injury. We have also provided evidence for a potentially new target for muscle recovery, the protein p53, and new insights into the role of SIRT1 in muscle recovery