Analysis of the hibernation cycle using LC-MS-based metabolomics in ground squirrel liver

Abstract

A hallmark of hibernation in mammals is metabolic flexibility, which is typified by reversible bouts of metabolic depression (torpor) and the seasonal shift from predominantly carbohydrate to lipid metabolism from summer to winter. To provide new insight into the control and consequences of hibernation, we used LC/MS-based metabolomics to measure differences in small molecules in ground squirrel liver in five activity states: summer, entering torpor, late torpor, arousing from torpor, and interbout arousal. There were significant alterations both seasonally and within torpor-arousal cycles in enzyme cofactor metabolism, amino acid catabolism, and purine and pyrimidine metabolism, with observed metabolites reduced during torpor and increased upon arousal. Multiple lipids also changed, including 1-oleoyllysophosphatidylcholine, cholesterol sulfate, and sphingosine, which tended to be lowest during torpor, and hexadecanedioic acid, which accumulated during a torpor bout. The results reveal the dramatic alterations that occur in several classes of metabolites, highlighting the value of metabolomic analyses in deciphering the hibernation phenotype.

Are long chain acyl CoAs responsible for suppression of mitochondrial metabolism in hibernating 13-lined ground squirrels?

“…I predicted that the addition of carnitine would facilitate fatty acyl CoAs transported to the matrix, relieving any accumulation of fatty acyl CoAs in the IMS that might inhibit the DCT and, as a result, succinate transport would resume and respiration would increase (figure 1.4). This prediction was further supported by the work of Nelson et al (2009) who found carnitine concentrations in liver tissue to be higher in arousing and IBE animals, but low in torpid hibernators. However as their work was performed in homogenized liver tissue (Nelson et al, 2009), I am unable to distinguish whether these higher carnitine concentrations in IBE occur in the mitochondria or in the cytosol.…”

“…This prediction was further supported by the work of Nelson et al (2009) who found carnitine concentrations in liver tissue to be higher in arousing and IBE animals, but low in torpid hibernators. However as their work was performed in homogenized liver tissue (Nelson et al, 2009), I am unable to distinguish whether these higher carnitine concentrations in IBE occur in the mitochondria or in the cytosol.…”

“…Further building on the idea that suppression in hibernators is due to an accumulation of fatty acyl CoAs in the absence of carnitine, is work on carnitine metabolomics by Nelson et al (2009), who found higher levels of carnitine in the livers of euthermic animals (IBE and summer), compared with torpid animals. This supports the idea that fatty acyl CoAs accumulate during torpor, inhibit mitochondrial transporters (such as the DCT), and are then removed by carnitine in periods of euthermia (figure 1.4).…”

Metabolic cycles in a circannual hibernator

“…Biliverdin and bilirubin are heme catabolites, and all three compounds have been implicated in redox regulation (35), riboflavin primarily in the context of fatty acid oxidation (20). It appears likely that they are taken up rapidly by other tissues, including liver which demonstrated a perfectly reciprocal pattern of both flavin and biliverdin levels in the same species (32). Similar to our results, bilirubin was found elevated during torpor in plasma in jerboas (30) and in bile of torpid goldenmantled ground squirrel (4).…”

“…Previous work to measure metabolite changes in hibernators documents a number of alterations in liver (2,32,36), brain (21,40), brown adipose tissue (13), bile (4), and blood (1,9,24,31) that begin to assess the metabolites cycling in association with hibernation. These data suggest a two-switch model for the circannual rhythm of hibernation whereby the torporarousal cycle is temporally segregated from the summer-winter cycle such that torpor-arousal occurs only in winter (36).…”