Determination In Vivo of Newly Synthesized Gene Expression in Hamsters During Phases of the Hibernation Cycle

Abstract

Hibernation is a circannual adaptation and within the hibernation season, the hibernation cycle of a 'classic' hibernator, such as the Syrian hamster, can be divided into six distinct physiological phases. An entrance phase, when metabolism is inhibited and body temperature decreases [1]. A long period of torpor when metabolic rate (MR) is approximately 2.5% of resting metabolic rate (RMR) [2, 3]. This is followed by an arousal period of three phases: an early arousal phase during which the animal warms anterior crucial organs by non-shivering thermogenesis; a middle arousal phase when shivering thermogenesis is recruited to heat the anterior body to cenothermia (CEN) and a late arousal phase when posterior organs attain CEN and the animal rests or sleeps. An inter-bout arousal phase, of hours to days, during which time the hibernator maintains CEN body temperature before the initiation of another hibernation cycle. At present the onset of the hibernation cycle cannot be predicted. (Cenothermia is the IUPS term replacing euthermia [4]).Molecularly orientated studies have searched for up-or-down regulation of gene expression in a small number of organs from torpid animals in an attempt to demonstrate molecular changes that are consistent with the enormous physiological changes occurring across the hibernation cycle [5]. Early investigations used radioactively labeled precursors to measure in vivo transcription in brains [6,7], translation in liver [8,9], and more recently translation in brain and heart 14 C]-leucine. In torpor, RNA synthesis was 5-25% of CEN levels depending upon tissue. In brain and heart mRNA was not preferentially synthesized. Protein was synthesized at low, tissue specific levels during torpor. Initiation of arousal and the warming of anterior organs via non-shivering thermogenesis during the early arousal occurred without measurable synthesis of RNA or proteins. Tissue specific levels of RNA and protein synthesis occurred later after shivering thermogenesis had been recruited and was strongly influenced by thermal gradients in the body. In the middle arousal phase, protein synthesis is most active in the brain despite modest synthesis of RNA and mRNA. The majority of molecular processing required for the induction and maintenance of torpor and the arousal from torpor up until the onset of shivering thermogenesis occurs during the cenothermic period before the hamster initiates the hibernation cycle. [The Japanese Journal of Physiology 54: [295][296][297][298][299][300][301][302][303][304][305] 2004]

Mammalian hibernation: differential gene expression and novel application of epigenetic controls

“…For example, the rate of [ 3 H]-uridine incorporation into RNA was reduced by 8-fold in brain of torpid ground squirrels compared with euthermic animals (Bocharova et al, 1992). A similar result was seen when [ 3 H]-uridine incorporation into RNA was measured in multiple organs of hibernating Syrian hamsters (Mesocricetus auratus) (Osborne et al, 2004) and was also indicated from nuclear run-on assays performed on liver extracts of golden-mantled ground squirrels (van Breukelen and Martin, 2002). A reduction in energetically-costly transcription was also seen in Djungarian hamsters (Phodopus sungorus) undergoing daily torpor bouts (Berriel Diaz et al, 2004).…”

“…For example, plasma membrane Na + K + -ATPase activity was reduced by 60% during hibernation (MacDonald and Storey, 1999) and the activity of sarco(endo)plasmic reticulum Ca 2+ -ATPase as well as the levels of other proteins associated with calcium signaling were similarly reduced (Malysheva et al, 2001). Strong overall suppression of ATP-expensive protein synthesis has also been confirmed in hibernators in both in vivo and in vitro studies that monitored 14 C-leucine or 3 H-leucine incorporation into proteins (Frerichs et al, 1998;Hittel and Storey, 2002;Osborne et al, 2004). Finally, the overall rates of gene transcription are also greatly reduced during deep torpor bouts in hibernators.…”

A systems-level approach to understanding transcriptional regulation by p53 during mammalian hibernation

“…Use of nuclear run-on assays from liver allowed for mechanistic insight into how this transcriptional depression is effected; overall, transcriptional initiation is reduced by ~50% during torpor, but the cold temperatures typical of hibernation would essentially arrest elongation (van Breukelen and Martin, 2002). Data from nuclear run-on assays in hibernating hamsters are consistent with a significant role for depression of elongation as well (Osborne et al, 2004; Berriel Diaz et al, 2004). One might expect that employment of a hibernation phenotype, with the associated reduction in metabolism, might be accompanied by dramatic changes in gene expression.…”

“…Because organs such as the heart and brain must function during torpor, it seems likely that these tissues would experience higher transcriptional activities than tissues such as muscle. Indeed, such differences have been noted (Osborne et al, 2004). Use of nuclear run-on assays from liver allowed for mechanistic insight into how this transcriptional depression is effected; overall, transcriptional initiation is reduced by ~50% during torpor, but the cold temperatures typical of hibernation would essentially arrest elongation (van Breukelen and Martin, 2002).…”

“…When one considers that transcription accounts for as much as 10% of standard metabolic rate in mammalian cells (Rolfe and Brown, 1997), it follows that transcription must be severely depressed during torpor. Incorporation of radiolabeled precursors into RNA is severely reduced during torpor depending on tissue type (Osborne et al, 2004;Bocharova et al, 1992). Because organs such as the heart and brain must function during torpor, it seems likely that these tissues would experience higher transcriptional activities than tissues such as muscle.…”