[Background] Autophagy in the skeletal muscle maintains muscle mass. The NAD⁺-dependent deacetylase SIRT1 positively regulates autophagy. Here, we examined whether SIRT1 in the skeletal muscle maintains muscle mass via promoting autophagy.
[Methods and Results] We obtained tibialis anterior muscle (TA) from 60-weeks old wild-type mice (WT) and muscle-specific SIRT1 knockout mice (SIRT1MKO). Western blotting showed that acetylated lysine levels were increased in SIRT1MKO compared with WT, suggesting suppressed SIRT1 activity in SIRT1MKO. Histological analysis by HE staining showed that the myofiber diameter was 9% smaller in MKO than WT. The percentage of central nuclei, an indicator of muscle regeneration, was 19% higher in SIRT1MKO than WT. To test autophagic activity in the muscle, we treated mice with colchicine, an inhibitor of autophagosome degradation, and measured markers of autophagosomes including LC3-II/LC3-I ratio (Western blotting) and the level of LC3 dots (Immunostaining). At baseline, LC3-II/LC3-I ratio and LC3 dot levels were unchanged in MKO. However, colchicine treatment increased the LC3-II/LC3-I ratio and the LC3 dot level in WT but not in SIRT1MKO, suggesting suppression of autophagic flux in SIRT1MKO. Levels ubiquitinated proteins, which are degraded by autophagy, was also increased in SIRT1MKO.
[Conclusion] These results suggest that SIRT1 plays a critical role in maintenance of skeletal muscle mass via positive regulation of autophagy.