Detection of Differential Gene Expression in Brown Adipose Tissue of Hibernating Arctic Ground Squirrels

Hibernation is an energy-saving strategy adopted by a wide range of mammals to survive highly seasonal or unpredictable environments. Arctic ground squirrels living in Alaska provide an extreme example, with 6-9 months long hibernation seasons when body temperature alternates between levels near 0 C during torpor and 37 C during arousal episodes. Heat production during hibernation is provided, in part, by non-shivering thermogenesis that occurs in large deposits of brown adipose tissue (BAT). BAT is active at tissue temperatures from 0 to 37 C during rewarming and continuously at near 0 C during torpor in subfreezing conditions. Despite its crucial role in hibernation, the global gene expression patterns in BAT during hibernation compared to the non-hibernation season remain largely unknown. We report a large-scale study of differential gene expression in BAT between winter hibernating and summer active arctic ground squirrels using mouse microarrays. Selected differentially expressed genes identified on the arrays were validated by quantitative real-time PCR using ground squirrel specific primers. Our results show that the mRNA levels of the genes involved in nearly every step of the biochemical pathway leading to non-shivering thermogenesis are significantly increased in BAT during hibernation, whereas those of genes involved in protein biosynthesis are significantly decreased compared to the summer active animals in August. The differentially expressed genes also include those involved in adipose differentiation, substrate transport, and structure remodeling, which may enhance thermogenesis at low tissue temperatures in BAT. Keywords: hibernating animals vs. summer active animals Four Wistar mouse arrays (MA07, MA08, MA10, MA11) each carrying 9,600 probes were used to screen the differentially expressed genes in brown adipose tissue (BAT) between 5 winter hibernating (BAT_torpor_1, BAT_torpor_2, BAT_torpor_3, BAT_torpor_4, BAT_torpor_5) and 3 summer active (BAT_summer_1, BAT_summer_2, BAT_summer_3) arctic ground squirrels. There are technical replicas (samples split from the same total RNA extraction) for BAT_summer_1 (a, b), BAT_summer_2 (a, b), and BAT_torpor_5 (a, b).