Prehibernation and hibernation effects on the D-3-hydroxybutyrate dehydrogenase of the heavy and light mitochondria from liver jerboa (Jaculus orientalis) and related metabolism

Integrated Redox-Metabolic Orchestration Sustains Life in Hibernating Ground Squirrels

Significance: The ultimate manifestations of life, birth, survival under various environmental pressures and death are based on bioenergetics. Hibernation is a unique survival strategy for many small mammals that is characterised by severe metabolic depression and transition from euthermia to hypothermia (torpor) at body temperatures close to 0°C. These manifestations of life were made possible by the remarkable "social" behavior of biomolecules during billions of years of evolution: the evolution of life with oxygen. Oxygen was necessary for energy production and the evolutionary explosion of aerobic organisms. Recent Advances: Nevertheless, reactive oxygen species, formed through oxidative metabolism, are dangerous-they can kill a cell and, on the other hand, play a plethora of fundamentally valuable roles. Therefore, the evolution of life depended on energy metabolism and redox-metabolic adaptations. The more extreme the conditions for survival are, the more sophisticated the adaptive responses of organisms become. Hibernation is a beautiful illustration of this principle. Hibernating animals use evolutionarily conserved molecular mechanisms to survive adverse environmental conditions, including reducing body temperature to ambient levels (often to ∼0°C) and severe metabolic depression. This long-built secret of life lies at the intersection of oxygen, metabolism, and bioenergetics, and hibernating organisms have learned to exploit all the underlying capacities of molecular pathways to survive. Critical Issues: Despite such drastic changes in phenotype, tissues and organs of hibernators sustain no metabolic or histological damage during hibernation or upon awakening from hibernation. This was made possible by the fascinating integration of redox-metabolic regulatory networks whose molecular mechanisms remain undisclosed to this day. Future Directions: Discovering these molecular mechanisms is not warranted only to understand hibernation in itself but to help explain complex medical conditions (hypoxia/reoxygenation, organ transplantation, diabetes, and cancer) and to even help overcome limitations associated with space travel. This is a review of integrated redox-metabolic orchestration in hibernation. Antioxid. Redox Signal. 40, 345-368.

Purification and characterization of aldose reductase from jerboa (Jaculus orientalis) and evaluation of its inhibitory activity by Euphorbia regis-jubae (Webb & Berth) extracts

This study aimed, for the first time, to assess the purification of aldose reductase (AR) in Jaculus orientalis (Dipodidae family) kidney and to evaluate the in vitro aldose reductase inhibitory (ARI) effects of Euphorbia regis-jubae (Euphorbiaceae family) aqueous and hydroethanolic extracts. Initial screening assay of the enzymatic AR activity in different jerboa states (euthermic, prehibernating and hibernating) and tissues (brain, brown adipose tissue, liver and kidneys) was assessed. Then, AR has been purified to homogeneity from the kidneys of prehibernating jerboas by a series of chromatographic technics. Furthermore, the in vitro and in silico ARI effects of E. regis-jubae (Webb & Berth) extracts, characterized by hight performance liquid chromatography (HPLC) on the purified enzyme were evaluated. Our results showed that the highest enzyme activity was detected in the kidneys, followed by white adipose tissue and the lungs of pre-hibernating jerboa. The enzyme was purified to homogeneity from jerboa kidneys during prehibernating state with a purification factor of 53.4-fold and a yield of about 6%. AR is monomeric, active in D(+)-glyceraldehyde substrate and in disodium phosphate buffer. The pH and temperature for AR were determined to be 6.5-7.5 and 35 °C, respectively. Results of the in vitro ARI activity was strongest with both the hydroethanolic extract (IC₅₀ = 96.45 μg/mL) and aqueous extract (IC₅₀ = 140 μg/mL). Molecular docking study indicated that catechin might be the main component in both aqueous and hydroethanolic extracts to inhibited AR. This study provides new evidence on the ARI effect of E. regis-jubae (Webb & Berth), which may be related to its phenolic constituents.

Cholesterol and lipoprotein dynamics in a hibernating mammal

Hibernating mammals cease feeding during the winter and rely primarily on stored lipids to fuel alternating periods of torpor and arousal. How hibernators manage large fluxes of lipids and sterols over the annual hibernation cycle is poorly understood. The aim of this study was to investigate lipid and cholesterol transport and storage in ground squirrels studied in spring, summer, and several hibernation states. Cholesterol levels in total plasma, HDL and LDL particles were elevated in hibernators compared with spring or summer squirrels. Hibernation increased plasma apolipoprotein A-I expression and HDL particle size. Expression of cholesterol 7 alpha-hydroxylase was 13-fold lower in hibernators than in active season squirrels. Plasma triglycerides were reduced by fasting in spring but not summer squirrels. In hibernators plasma β-hydroxybutyrate was elevated during torpor whereas triglycerides were low relative to normothermic states. We conclude that the switch to a lipid-based metabolism during winter, coupled with reduced capacity to excrete cholesterol creates a closed system in which efficient use of lipoproteins is essential for survival.

Metabolic cycles in a circannual hibernator

Hibernation as manifested in ground squirrels is arguably the most plastic and extreme of physiological phenotypes in mammals. Homeostasis is challenged by prolonged fasting accompanied by heterothermy, yet must be facilitated for survival. We performed LC and GC-MS metabolomic profiling of plasma samples taken reproducibly during seven natural stages of the hibernator's year, three in summer and four in winter (each n ≥ 5), employing a nontargeted approach to define the metabolite shifts associated with the phenotype. We quantified 231 named metabolites; 106 of these altered significantly, demarcating a cycle within a cycle where torpor-arousal cycles recur during the winter portion of the seasonal cycle. A number of robust hibernation biomarkers that alter with season and winter stage are identified, including specific free fatty acids, antioxidants, and previously unpublished modified amino acids that are likely to be associated with the fasting state. The major pattern in metabolite levels is one of either depletion or accrual during torpor, followed by reversal to an apparent homeostatic level by interbout arousal. This finding provides new data that strongly support the predictions of a long-standing hypothesis that periodic arousals are necessary to restore metabolic homeostasis.

Heme oxygenase expression and Nrf2 signaling during hibernation in ground squirrelsThis article is one of a selection of papers published in a Special Issue on Oxidative Stress in Health and Disease

Mammalian hibernation is composed of long periods of deep torpor interspersed with brief periods of arousal in which the animals, fueled by high rates of oxygen-based thermogenesis in brown adipose tissue and skeletal muscle, power themselves back to euthermic (~37 °C) body temperatures. Strong antioxidant defences are important both for long-term cytoprotection during torpor and for coping with high rates of reactive oxygen species generated during arousal. The present study shows that the antioxidant enzyme heme oxygenase 1 (HO1) is strongly upregulated in selected organs of thirteen-lined ground squirrels (Spermophilus tridecemlineatus) during hibernation. Compared with euthermic controls, HO1 mRNA transcript levels were 1.4- to 3.8-fold higher in 5 organs of hibernating squirrels, whereas levels of the constitutive isozyme HO2 were unchanged. Similarly, HO1 protein levels increased by 1.5- to 2.0-fold in liver, kidney, heart, and brain during torpor. Strong increases in the levels of the Nrf2 transcription factor and its heterodimeric partner protein, MafG, in several tissues indicated the mechanism of activation of hibernation-responsive HO1 gene expression. Furthermore, subcellular distribution studies with liver showed increased nuclear translocation of both Nrf2 and MafG in torpid animals. The data are consistent with the suggestion that Nrf2-mediated upregulation of HO1 expression provides enhanced antioxidant defence to counter oxidative stress in hibernating squirrels during torpor and (or) arousal.

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

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.

Immunoaffinity purification and characterization of mitochondrial membrane-bound D-3-hydroxybutyrate dehydrogenase from Jaculus orientalis

BACKGROUND

The interconversion of two important energy metabolites, 3-hydroxybutyrate and acetoacetate (the major ketone bodies), is catalyzed by D-3-hydroxybutyrate dehydrogenase (BDH1: EC 1.1.1.30), a NAD+-dependent enzyme. The eukaryotic enzyme is bound to the mitochondrial inner membrane and harbors a unique lecithin-dependent activity. Here, we report an advanced purification method of the mammalian BDH applied to the liver enzyme from jerboa (Jaculus orientalis), a hibernating rodent adapted to extreme diet and environmental conditions.

RESULTS

Purifying BDH from jerboa liver overcomes its low specific activity in mitochondria for further biochemical characterization of the enzyme. This new procedure is based on the use of polyclonal antibodies raised against BDH from bacterial Pseudomonas aeruginosa. This study improves the procedure for purification of both soluble microbial and mammalian membrane-bound BDH. Even though the Jaculus orientalis genome has not yet been sequenced, for the first time a D-3-hydroxybutyrate dehydrogenase cDNA from jerboa was cloned and sequenced.

CONCLUSION

This study applies immunoaffinity chromatography to purify BDH, the membrane-bound and lipid-dependent enzyme, as a 31 kDa single polypeptide chain. In addition, bacterial BDH isolation was achieved in a two-step purification procedure, improving the knowledge of an enzyme involved in the lipid metabolism of a unique hibernating mammal. Sequence alignment revealed conserved putative amino acids for possible NAD+ interaction.

Effects of dietary polyunsaturated fatty acids on mitochondrial metabolism in mammalian hibernation

Thirteen-lined ground squirrels (Spermophilus tridecemlineatus)were fed one of four isocaloric, isolipemic diets containing 16, 22, 35 or 55 mg linoleic acid (18:2n-6) per gram. Mitochondrial properties were compared between hibernating and summer active states, and between diet groups. As in other studies, state 3 respiration was significantly reduced in hibernation, but only in animals fed the 22 mg g–1 18:2 diet. In the other diet groups, there was no difference in state 3 respiration between the hibernating and summer active groups. In the 22 mg g–1 18:2 diet group, there was no difference in mitochondrial proton conductance between hibernating and summer active animals, again in agreement with earlier studies. However, for all other diet groups,mitochondrial proton conductance was significantly reduced during hibernation. Mitochondrial phospholipid fatty acids changed significantly with hibernation,including increases in unsaturation indices and n-6/n-3, but no differences were found among diet groups. Mitochondrial proton conductance in hibernation showed a positive correlation with the content of linoleic acid(18:2) and arachidonic acid (20:4) in mitochondrial phospholipids. Lipid peroxidation was higher in mitochondria from hibernating animals, probably due to higher unsaturation, but there was no effect of dietary 18:2 on this pattern. Despite the dietary effects on mitochondrial metabolism, all animals hibernated with no differences in bout durations, body temperatures or whole-animal metabolic rates among the diet groups. The reduced mitochondrial proton leak in the 15, 35 and 55 mg g–1 18:2 diet groups might compensate for the inability to suppress respiration, permitting whole-animal energy savings over the hibernation season.

Mitochondrial metabolism in hibernation and daily torpor: a review

Hibernation and daily torpor involve substantial decreases in body temperature and metabolic rate, allowing birds and mammals to cope with cold environments and/or limited food. Regulated suppression of mitochondrial metabolism probably contributes to energy savings: state 3 (phosphorylating) respiration is lower in liver mitochondria isolated from mammals in hibernation or daily torpor compared to normothermic controls, although data on state 4 (non-phosphorylating) respiration are equivocal. However, no suppression is seen in skeletal muscle, and there is little reliable data from other tissues. In both daily torpor and hibernation, liver state 3 substrate oxidation is suppressed, especially upstream of electron transport chain complex IV. In hibernation respiratory suppression is reversed quickly in arousal even when body temperature is very low, implying acute regulatory mechanisms, such as oxaloacetate inhibition of succinate dehydrogenase. Respiratory suppression depends on in vitro assay temperature (no suppression is evident below approximately 30 degrees C) and (at least in hibernation) dietary polyunsaturated fats, suggesting effects on inner mitochondrial membrane phospholipids. Proton leakiness of the inner mitochondrial membrane does not change in hibernation, but this also depends on dietary polyunsaturates. In contrast proton leak increases in daily torpor, perhaps limiting reactive oxygen species production.