Pier Morin, Ph.D. Chemistry, 2006

Transcriptional regulation in the hibernating thirteen-lined ground squirrel, Spermophilus tridecemlineatus.



Many small mammals enter hibernation to survive the winter. Metabolic rate during torpor can drop to just 1-5 % of the euthermic rate providing energy savings of ~90% compared to the costs of remaining euthermic. Hibernation consists cycles of torpor bouts interspersed by short arousal periods. Regulation of torpor-arousal requires tight overall control of energy-consuming metabolic processes as well as selective expression of genes to accomplish these transitions and readjust metabolism for long term viability in a hypometabolic, hypothermic state. Hence, despite overall metabolic rate depression in hibernation, certain processes must be activated to ensure survival. The present studies examined transcriptional control in the hibernating thirteen-lined ground squirrel, Spermophilus tridecemlineatus. Regulation of gene expression by the hypoxia-inducible transcription factor (HIF-1) pathway was shown to be important in hibernation; HIF-1α subunit levels rose by 60-70 % in skeletal muscle and BAT during hibernation and HIF-1 DNA-binding activity increased 6-fold in hibernating BAT.  By contrast, the overall transcriptional state in muscle was strongly suppressed during torpor.  Both activity and protein levels of histonedeacetylases, enzymes involved in transcriptional repression, were elevated during torpor whereas the activity of RNA Polymerase II, a key enzyme of gene transcription, was strongly reduced by 43 %.  To evaluate the involvement of oxidative stress and antioxidant defenses in hibernation, the responses of the Nrf2 transcription factor, involved in the oxidative stress response pathway, were evaluated as well as antioxidant genes/proteins under Nrf2 control. Nrf2 protein levels were elevated (by 1.4-fold) in all heterothermic parts of the hibernation cycle whereas the protein contents of three downstream gene targets of Nrf2 were only elevated during entrance into hibernation. This suggests aNrf2-mediated anticipatory enhancement of antioxidant defenses to deal with oxidative stress during torpor and/or arousal. Other antioxidant enzymes, the 2-Cys peroxiredoxins, also showed enhanced protein levels in torpor and increased enzyme activity (1.5-fold in heart and 3.4-fold in BAT) indicating a potential role in ROS detoxification during hibernation.  The data reported in this thesis provides new insights on the roles of selected transcription factors and on the importance of their tight regulation during all stages of hibernation.