Mammalian hibernation: Gene expression and transcription factor regulation of the unfolded protein response, apoptosis, and atrophy in ground squirrels

Various mammalian species hibernate as a way to survive extended winter periods of food scarcity and cold environmental conditions. Hibernation is an energy-conserving strategy, characterized by periods of torpor with extreme decreases in core body temperature and strong metabolic rate depression, interrupted by brief periods of arousal to euthermia. To endure the conditions of cold torpor, as well as the wide fluctuations over cycles of torpor-arousal, differential expression of genes and their tight regulation is required. The present studies examined changes in the expression and regulation of selected genes involved in the unfolded protein response, muscle atrophy and anti-apoptotic defense during hibernation in thirteen-lined ground squirrels, Spermophilus tridecemlineatus. Despite overall suppression of transcription and translation during torpor, selected genes and their products were up-regulated. The molecular chaperone GRP78 increased in BAT and brain of torpid animals, indicating endoplasmic reticulum stress and a role for GRP78 in alleviating stresses that cause protein misfolding during hibernation. Regulation of the grp78 gene by the activating transcription factor ATF4 via the PERK/eIF2alpha/ATF4 pathway was shown to be important in hibernation; ATF4 protein expression increased in BAT, brain and skeletal muscle of hibernating squirrels and ATF4 DNA-binding activity increased in hibernating brain. Subcellular localization of ATF4 showed that this transcription factor and its cofactor, pCREB-1, were translocated into the nucleus during hibernation where they could activate downstream genes. Another transcription factor, FoxO1a, and the downstream genes that it controls via the PI3K/AKT/FOXO pathway can induce muscle atrophy. Hibernators appear to counteract this by phosphorylating and inactivating FoxO1a in heart and skeletal muscle and strongly suppressing FoxO1a DNA binding activity by 76% in muscle during torpor. Finally, the anti-apoptotic proteins, Bcl-XL and Bcl-2, showed enhanced expression in tissues of ground squirrels whereas the pro-apoptotic protein, BAD, was suppressed via phosphorylation during torpor. These results show that anti-apoptotic defense is also important to cell survival in hibernation. The data in this thesis enhance our knowledge of the molecular mechanisms that govern hibernation and the role played by selected transcription factors in regulating subsets of genes that are physiologically relevant to the hibernation phenotype.