Role of differential gene and protein regulation in hibernating mammals
Winter survival for many mammals involves hibernation. By strongly suppressing metabolic rate, animals conserve energy and endure long months of subzero environmental temperatures and lack of food. The little brown bat, Myotis lucifugus, and thirteen-lined ground squirrel, Spermophilus tridecemlineatus, are two such hibernating mammals. Hibernation consists of long periods (1-2 weeks) of cold torpor with body temperature near ambient interspersed with brief periods of arousal to euthermia. Precise control over gene and protein expression is needed to prevent costly overuse of essential fuel reserves while ensuring that selected specific changes are made that aid survival in the cold, torpid state. The present studies evaluated signal transduction pathways and changes in gene expression in skeletal and cardiac muscle during hibernation. The p38 MAP kinase signal transduction pathway in skeletal muscle of both species appears triggered with increased amounts of active phosphorylated p38 found during hibernation along with activation of various downstream targets of p38 including ATF-2, CREB, HSP27 and IκB. The Akt-mediated insulin-signalling pathway was, by contrast, apparently suppressed in hibernator muscle; this may help restrict the use of carbohydrate fuels.cDNA arrays were used to compare gene expression in euthermic versus hibernating states; selected genes (0.5-2%) were up-regulated during hibernation but most were unaffected. A general suppression of protein synthesis is likely during hibernation and this was supported by elevated levels of phosphorylated initiation (eIF2α) and elongation (eEF2) ribosomal proteins. Despite the probable suppression of transcription and translation, selected genes and proteins were up-regulated during hibernation including two involved in fatty acid transport and metabolism: fatty acid-binding protein and carnitine palmitoyl transferase-1. Furthermore, two-dimensional polyacrylamide gel electrophoresis coupled with mass spectrometry revealed the up-regulation of a thioredoxin peroxidase type enzyme in heart of M. lucifugus; both mRNA and protein levels rose during hibernation. This, along with elevated amounts of the active forms of oxidative stress markers, HSP27 and IκB, demonstrates that oxidative stress has a role in hibernating tissues. The data enhance our knowledge of the molecular mechanisms of hibernation with novel contributions to the understanding of the roles of fatty acid metabolism, oxidative stress, and muscle atrophy.