Muscle protection during hibernation of Daurian ground squirrels (Spermophilus dauricus): role of atrogin-1, MuRF1, and fiber-type transition

We investigated the mechanism of protection from skeletal muscle atrophy in the hind limb extensor digitorum longus (EDL) muscle of hibernating Daurian ground squirrels (Spermophilus dauricus Brandt, 1843). The effects of unrestrained hibernation and 14 day hind limb unloading (HLU) on EDL were studied in three seasons (summer, autumn, and winter). Atrogin-1 and MuRF1 mRNA skeletal muscle expression, wet muscle mass, and muscle to body mass ratios were unchanged during hibernation in all three seasons. EDL mass measurements decreased following HLU and atrogin-1 and MuRF1 mRNA expression increased. In summer, atrogin-1 and MuRF1 mRNA expression increased by 85% and 75%, respectively; in autumn, by 95% and 69%, respectively; and in winter, by 91% and 65%, respectively (P < 0.05). In the HLU group, microscopic skeletal muscle changes were present, including a reduction in the percentage of type-I skeletal muscle fibers. Fat storage in Daurian ground squirrels and a shorter photoperiod during hibernation did not affect the protective mechanisms that prevented skeletal muscle atrophy. The results of this study suggest that the stable expression of atrogin-1 and MuRF1 and the transition from fast glycolytic fibers to slow oxidative fibers are associated with a lack of skeletal muscle atrophy in the hibernating Daurian ground squirrel.