044 Linking signaling pathways to gene expression during mammalian hibernation

Abstract

Mammalian hibernators survive hypothermia (core body temperatures of ∼ 4–5 °C), ischemia-reperfusion, restricted nutritional resources, and demonstrate regulatory mechanisms which enhance cellular preservation and ensure seamless transitions to and from the hypometabolic state.Consequently, the induction of a hibernation-like state which is resistant to parallel stresses holds immense clinical promise in diverse fields including increasing the functional shelf-life of excised human organs for transplant.We proposed that signal transduction pathways, in coordination with downstream transcription factors and cytoprotective strategies, would aid cardiac and skeletal muscle of thirteen-lined ground squirrels ( Ictidomys tridecemlineatus ) in meeting challenges associated with hibernation.Gene and protein responses by ground squirrel muscles were compared over five experimental conditions (euthermic control, entrance into torpor, deep torpor, arousal, and interbout) spanning three central themes; (1) transmission of signals along highly ordered signal transduction pathways (e.g.MAPK cascades) which lead to changes in cell metabolism and gene/protein expression, (2) transcription factors (TFs) which alter gene expression resulting in changes in the amount and type of proteins synthesized to minimize stress (e.g.Nrf2, ATF2, p53), and (3) cellular defense strategies (e.g.antioxidants, unfolded protein response, heat shock proteins, pro-survival) which counteract the consequences of a depressed metabolic rate and hypothermia.While relative increases were seen in selected targets throughout the torpor-arousal cycle, the data show that activation is particularly important during arousal and relative increases in MAPKs (e.g.ERK1/2, p38, JNK) and transcription factors (e.g.Nrf2, ATF2) were detected.Since each transcription factor regulates the expression of a group of genes/proteins that serve common cell functions, identification of the regulatory mechanisms available to TFs holds immense insight into their important cell functions.In order to further understand the mechanisms of transcription factor regulation during hibernation, we focused on post-translational mechanisms, such as covalent modifications and protein-protein interactions.The data showed that Nrf2 is subject to differential phosphorylation, acetylation, and ubiquitination over the torpor-arousal cycle as well as changes in its physical interactions with its binding partner protein, KEAP1.These regulatory controls modulate Nrf2 activity by inhibiting nuclear retention when antioxidant defenses are sufficient and promoting DNA binding upon rapid re-introduction of oxygen during arousal.Acknowledgements:We would like to thank Dr.J.M. Hallenbeck and Dr.D.C. McMullen (NINDS, NIH, Bethesda) for providing the tissue samples for this study.Thanks also to J.M. Storey for editorial review. Source of funding:Research was supported by a discovery grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada to KBS and the Canada Research Chairs program.SNT held a NSERC PGSD scholarship. Conflict of interest:None declared. shannon_tessier@carleton.ca