AMPK and ACCchange with fasting and physiological condition in euthermic and hibernating golden-mantled ground squirrels (Callospermophilus lateralis)

Abstract

AMP-activated protein kinase (AMPK) is a cellular energy sensor that responds to low endogenous energy by stimulating fatty acid oxidation (through inactivation of acetyl-CoA carboxylase (ACC)) and food intake. Fasting generally stimulates phosphorylation of AMPK (pAMPK) and ACC (pACC), but it is unclear how AMPK and ACC react to a long-term fast (i.e. hibernation). We performed Western blots for total and pAMPK and pACC on tissues from a species of hibernator (Callospermophilus lateralis) after short-term summer fasting (1–5 days) and long-term winter fasting (3 months). Winter animals were sacrificed during hibernation at low body temperature (torpid, Tb~5°C) or at normal high Tb(euthermic, Tb~37°C). We found a general increase in pAMPK in most tissues (liver, muscle, white adipose tissue (WAT), but not hypothalamus) and pACC in all tissues after a short-term summer fast. Response of AMPK and ACC to a long-term winter fast differed by tissue—in liver, there was no difference in total or pAMPK or pACC between groups, but in muscle, WAT and BAT, euthermic GMGS had lower relative abundance of pAMPK and pACC than torpid animals. Therefore, AMPK may be an important energy sensor at all points in hibernator’s circannual cycles of food intake and Tb.

Skeletal muscle mass and composition during mammalian hibernation

“…The increase in muscle activity during arousal associated with shivering thermogenesis (Lee et al, 2010) may have an impact on both intracellular Ca 2+ and AMPK activity, similar to the effects that endurance training sessions have on intracellular Ca 2+ , AMPK activity and subsequent PGC-1α upregulation (Baar et al, 2002;Norrbom et al, 2004). Indeed, several studies have shown that relative expression of AMPK does increase during hibernation (Healy et al, 2011;Xu et al, 2013). Besides AMPK, other potential regulators of PGC-1α such as the NAD + -dependent protein acetylases (sirtuins) are also upregulated during hibernation (Xu et al, 2013).…”

Role of AMP-activated protein kinase in metabolic depression in animals

“…In 13-lined ground squirrels (Ictidomys tridecemlineatus) during deep torpor, we showed that AMPK was activated in white adipose tissue (WAT), but not in liver, skeletal muscle, brown adipose tissue (BAT) or brain (Horman et al 2005). In a later study on golden-mantled ground squirrels (Callospermophilus lateralis), increases in AMPK Thr172 phosphorylation were seen in skeletal muscle, liver and WAT, but not in hypothalamus (Healy et al 2011). The discrepancy might have been due to differences in protocol.…”

“…The discrepancy might have been due to differences in protocol. In the study of Horman et al (2005), squirrels were taken after pre-hibernation hyperphagia and euthermic controls were compared with animals in deep torpor (3-7 days in the dark at 5-6 °C), whereas in the study of Healy et al (2011), hibernating winterstarved animals in torpor were compared with euthermics during an interbout arousal. AMPK activation seen in WAT in both studies might have been due to a rise in AMP that would result from the fatty acyl-CoA ligase reaction during re-esterification of fatty acids released during lipolysis (Gauthier et al 2008).…”

The regulation of food intake in mammalian hibernators: a review

“…There is some evidence that AMPK is involved in seasonal regulation of food intake in hibernators as well, but that evidence is not conclusive (Healy et al 2011b;Horman et al 2005). In ground squirrels, AMPK appears to be differentially regulated by physiological condition: a short-term fast in summeracclimated animals resulted in increased relative abundance of the active form of the enzyme (pAMPK), and torpid winter-acclimated animals had higher relative abundance of pAMPK than did winter-acclimated euthermic animals (Healy et al 2011b). However, the cellular mechanisms behind these changes are unclear, and differences in cellular energy between torpor states (i.e., torpor entry, early torpor, late torpor, and arousal) remain to be elucidated.…”