Comparative functional genomics of adaptation to muscular disuse in hibernating mammals

Abstract

Hibernation is an energy-saving adaptation that involves a profound suppression of physical activity that can continue for 6-8 months in highly seasonal environments. While immobility and disuse generate muscle loss in most mammalian species, in contrast, hibernating bears and ground squirrels demonstrate limited muscle atrophy over the prolonged periods of physical inactivity during winter, suggesting that hibernating mammals have adaptive mechanisms to prevent disuse muscle atrophy. To identify common transcriptional programmes that underlie molecular mechanisms preventing muscle loss, we conducted a large-scale gene expression screen in hind limb muscles comparing hibernating and summer-active black bears and arctic ground squirrels using custom 9600 probe cDNA microarrays. A molecular pathway analysis showed an elevated proportion of overexpressed genes involved in all stages of protein biosynthesis and ribosome biogenesis in muscle of both species during torpor of hibernation that suggests induction of translation at different hibernation states. The induction of protein biosynthesis probably contributes to attenuation of disuse muscle atrophy through the prolonged periods of immobility of hibernation. The lack of directional changes in genes of protein catabolic pathways does not support the importance of metabolic suppression for preserving muscle mass during winter. Coordinated reduction in multiple genes involved in oxidation-reduction and glucose metabolism detected in both species is consistent with metabolic suppression and lower energy demand in skeletal muscle during inactivity of hibernation.

Keywords: arctic ground squirrel; black bear; functional genomics; gene expression; hibernation; protein biosynthesis.

Figure 1

Differentially expressed genes in skeletal muscle tissue confirmed with real-time PCR: A – black bear, B - arctic ground squirrel. Solid and open bars with standard deviation bars show normalized expression values obtained in real-time PCR and microarray experiments, respectively. The values were normalized to the mean in summer active animals.

Figure 2

Clustering and heat maps of expression values of genes involved in translation, glucose metabolism and oxidation-reduction GO category in skeletal muscle in black bears (A) and arctic ground squirrels (B). On the top hibernating animals are in blue and summer active individuals in yellow. Over-expressed genes are in red, under-expressed genes are in blue.

Figure 3

Gene Ontology (GO) enrichment analysis of common genes differentially expressed in the black bear and arctic ground squirrel skeletal muscles during torpor of hibernation. A –translation GO category, enrichment is 5.625 (P<0.001), B – oxidation reduction GO category, enrichment is 2.904 (P<0.05). Numbers show a percent of genes in the correspondent GO category among the total number of common genes with significant hybridization signals on the microarrays for both species (left pie), and percent among the total differentially expressed genes shared between two species (right pie).