Stress Proteins in Mammalian Hibernation

Many organisms whose body temperatures (Tb) vary when they are exposed to a wide range of environmental temperatures exhibit differential expression of stress (heat shock) proteins, presumably to minimize protein damage during thermal stress. In contrast, we know relatively little about natural variation in stress proteins in homeotherms ( i.e. , birds and mammals), perhaps due to the relatively constant Tb that is the hallmark of this vertebrate group. The significant changes in Tb and metabolism characteristic of mammalian hibernators suggest they could provide insight into the ecological relevance of stress proteins in mammals. Here we examined differential expression of stress proteins in several tissues of active and torpid 13-lined ground squirrels. There were few significant differences in expression of inducible Hsp70 protein in liver, kidney, heart, intestine, and skeletal muscle between active and torpid squirrels, and neither fasting in active squirrels nor torpor bout length in hibernators significantly altered Hsp70 abundance in these tissues. However, Hsp70 protein was lower in brown adipose tissue (BAT) of torpid compared with active squirrels. In contrast, abundance of GRP75, the mitochondria] form of Hsp70, was greater in liver, skeletal muscle and intestine of torpid compared with active squirrels, with the greatest change in intestine. Because GRP75 has been shown to be induced by non-thermal stressors including glucose deprivation and oxidative stress, these results suggest that the ecological significance of stress proteins for hibernators may be more closely associated with the metabolic demands of heterothermy rather than thermal stress per se .