Molecular adaptations of mammalian hibernation: Roles of metabolic signaling regulation in the torpor-arousal cycle
For many small mammals, survival over the winter months is a serious challenge because of low environmental temperatures and limited food availability. The solution for thirteen-lined ground squirrels (Ictidomys tridecemlineatus) is hibernation, a metabolically re-programmed state that is characterized by seasonal heterothermy and entry into long periods of torpor. Although many studies have defined the physiological responses of hibernation, including drastic reductions in heart rate, respiration, and body temperature, the regulation of such phenotypic plasticity has yet to be fully characterized at the molecular level. As part of hibernation, metabolic rate is suppressed during torpor to achieve major energy savings through coordinated suppression of non-essential ATP-costly processes. The present thesis examined the role of cell signaling cascades in the regulation of energy dependent cellular processes over the torpor-arousal cycles of hibernation.
The insulin signaling pathway, which functions as the regulator of many pro-growth processes such as protein synthesis, was shown to be regulated during torpor. Significant inhibition of this pathway was most evident in skeletal muscle but not in cardiac muscle during torpor. This inhibition was characterized by reduced phosphorylation of mammalian target of rapamycin kinase (mTOR), which led to subsequent inhibition of various proteins involved in ribosome assembly that are required for protein translation. The cell cycle, another energy dependent metabolic process, was also strongly inhibited during torpor. Cell cycle arrest was evident in liver (proliferative capable) but not in skeletal muscle (terminally differentiated), through mechanisms similar to those observed in G1 and G1/S arrest. The observed cell cycle arrest was characterized by down-regulation of proteins cyclin D and cyclin E which function as positive regulators of cell cycle progression, and up-regulation of the cell cycle inhibitors (CKIs) p15INK4b and p21CIP1. Up-regulation of CKIs during torpor was linked to members of the Smad family of transcription factors, which were activated during torpor, including increased nuclear inclusion and DNA binding activity of Smad 3. Overall, the data presented in this thesis document molecular mechanisms that function to reduce cell-growth and proliferation during torpor, via inhibition of protein synthesis and cell cycle progression.