Expression of myocyte enhancer factor-2 and downstream genes in ground squirrel skeletal muscle during hibernation

Myocyte enhancer factor-2 (MEF2) transcription factors regulate the expression of a variety of genes encoding contractile proteins and other proteins associated with muscle performance. We proposed that changes in MEF2 levels and expression of selected downstream targets would aid the skeletal muscle of thirteen-lined ground squirrels (Spermophilus tridecemlineatus) in meeting metabolic challenges associated with winter hibernation; e.g., cycles of torpor-arousal, body temperature that can fall to near 0A degrees C, long periods of inactivity that could lead to atrophy. MEF2A protein levels were significantly elevated when animals were in torpor (maximally 2.8-fold higher than in active squirrels) and the amount of phosphorylated active MEF2A Thr312 increased during entrance into torpor. MEF2C levels also rose significantly during entrance and torpor as did the amount of phosphorylated MEF2C Ser387. Furthermore, both MEF2 members showed elevated amounts in the nuclear fraction during torpor as well as enhanced binding to DNA indicating that MEF2-mediated gene expression was up-regulated in torpid animals. Indeed, the protein products of two MEF2 downstream gene targets increased in muscle during torpor (glucose transporter isoforms 4; GLUT4) or early arousal (myogenic differentiation; MyoD). Significant increases in Glut4 and MyoD mRNA transcript levels correlated with the rise in protein product levels and provided further support for the activation of MEF2-mediated gene expression in the hibernator. Transcript levels of Mef2a and Mef2c also showed time-dependent patterns with levels of both being highest during arousal from torpor. The data suggest a significant role for MEF2-mediated gene transcription in the selective adjustment of muscle protein complement over the course of torpor-arousal cycles.