Proteomic identification of seasonally expressed proteins contributing to heart function and the avoidance of skeletal muscle disuse atrophy in a hibernating mammal

Hibernation in mammals such as the thirteen-lined ground squirrel (Ictidomys tridecemlineatus) takes place over 4 to 6 months and is characterized by multi-day bouts of hypothermic torpor (5-7 °C core body temperature) that are regularly interrupted every 1-2 weeks by brief (12-24 hour) normothermic arousal periods called interbout arousals. The goal of our work was to identify the molecular mechanisms which underlie the hibernator’s ability to preserve heart function and avoid the deleterious effects of skeletal muscle disuse atrophy over prolonged periods of inactivity, starvation, and near-freezing body temperatures. To achieve this goal, we performed organelle enrichment of heart and skeletal muscle at five seasonal time points followed by LC-MS-based label-free quantitative proteomics. In both organs we saw a dramatic increase in levels of many proteins as ground squirrels transition from an active state in September to a pre-hibernation state in October. Interestingly, seasonal abundance patterns also identified new candidates for mitigating disuse atrophy in skeletal muscle which include higher levels of DHRS7C, SRL, and RTN2 in December-January torpid, IBA, and March active animals; and higher March active levels of TRIM72 and MPZ. Elevation of these proteins prompted us to perform an ex vivo contractile mechanics analysis of both type 1 (soleus) and type 2 (extensor digitorum longus) muscles in fall active, torpid, and spring active animals. Consistent with our hypotheses, both muscle types showed no deleterious effects despite several months of sedentary activity that would normally result in reduced muscle performance in non-hibernating mammals. An increased understanding of protein expression in natural hibernators may enable novel therapeutic strategies to treat human health concerns such as muscle disuse atrophy and heart disease.