Metabolic effects of cold storage on livers from euthermic and hibernating Columbian ground squirrels

Regulation of liver glutamate dehydrogenase by reversible phosphorylation in a hibernating mammal

Glutamate dehydrogenase (GDH) is a key enzyme that links amino acid and carbohydrate metabolism in cells. Regulation is likely most important when organisms are confronted with extreme stresses such as the low environmental temperatures and lack of food associated with winter. Many small mammals, such as Richardson's ground squirrels, Spermophilus richardsonii, cope with these conditions by hibernating. Animals enter long periods of profound torpor where metabolic rate is greatly suppressed, body temperature drops to near-ambient and all metabolic needs must be met from fixed internal body stores of fuels. To investigate how GDH is regulated under these conditions, kinetic properties of GDH were analyzed in liver from euthermic and torpid squirrels, revealing significant differences in V(max), K(m) glutamate, K(a) ADP and inhibition by urea between the two forms of GDH. These data suggested an activation of the glutamate-oxidizing activity of GDH in the hypometabolic state. Subsequent experiments suggested that the molecular basis of the kinetic differences was a change in the protein phosphorylation state of GDH between euthermia and torpor. Specifically, liver GDH appears to be dephosphorylated and activated when animals transition into torpor and this may serve to promote amino acid oxidation to contribute to energy production and gluconeogenesis. This is the first study to show that mammalian liver GDH can be regulated by reversible phosphorylation, providing an important new regulatory mechanism for GDH control.

Is human hibernation possible?

The induction of hypometabolism in cells and organs to reduce ischemia damage holds enormous clinical promise in diverse fields, including treatment of stroke and heart attack. However, the thought that humans can undergo a severe hypometabolic state analogous to hibernation borders on science fiction. Some mammals can enter a severe hypothermic state during hibernation in which metabolic activity is extremely low, and yet full viability is restored when the animal arouses from such a state. To date, the underlying mechanism for hibernation or similar behaviors remains an enigma. The beneficial effect of hypothermia, which reduces cellular metabolic demands, has many well-established clinical applications. However, severe hypothermia induced by clinical drugs is extremely difficult and is associated with dramatically increased rates of cardiac arrest for nonhibernators. The recent discovery of a biomolecule, 5'-AMP, which allows nonhibernating mammals to rapidly and safely enter severe hypothermia could remove this impediment and enable the wide adoption of hypothermia as a routine clinical tool.

Natural resistance to liver cold ischemia-reperfusion injury associated with the hibernation phenotype

The success of liver grafts is currently limited by the length of time organs are cold preserved before transplant. Novel insights to improve viability of cold-stored organs may emerge from studies with animals that naturally experience low body temperatures (T(b)) for extended periods. In this study, we tested whether livers from hibernating ground squirrels tolerate cold ischemia-warm reperfusion (cold I/R) for longer times and with better quality than livers from rats or summer squirrels. Hibernators were used when torpid (T(b) < 10 degrees C) or aroused (T(b) = 37 degrees C). Livers were stored at 4 degrees C in University of Wisconsin solution for 0-72 h and then reperfused with 37 degrees C buffer in vitro. Lactate dehydrogenase (LDH) release after 60 min was increased 37-fold in rat livers after 72 h cold I/R but only 10-fold in summer livers and approximately three- to sixfold in torpid and aroused hibernator livers, despite twofold higher total LDH content in livers from hibernators compared with rats or summer squirrels. Reperfusion for up to 240 min had the least effect on LDH release in livers from hibernators and the greatest effect in rats. Compared with rats or summer squirrels, livers from hibernators after 72 h cold I/R showed better maintenance of mitochondrial respiration, bile production, and sinusoidal lining cell viability, as well as lower vascular resistance and Kupffer cell phagocytosis. These results demonstrate that the hibernation phenotype in ground squirrels confers superior resistance to liver cold I/R injury compared with rats and summer squirrels. Because hibernation-induced protection is not dependent on animals being in the torpid state, the mechanisms responsible for this effect may provide new strategies for liver preservation in humans.

Mammalian hibernation: cellular and molecular responses to depressed metabolism and low temperature

Mammalian hibernators undergo a remarkable phenotypic switch that involves profound changes in physiology, morphology, and behavior in response to periods of unfavorable environmental conditions. The ability to hibernate is found throughout the class Mammalia and appears to involve differential expression of genes common to all mammals, rather than the induction of novel gene products unique to the hibernating state. The hibernation season is characterized by extended bouts of torpor, during which minimal body temperature (Tb) can fall as low as -2.9 degrees C and metabolism can be reduced to 1% of euthermic rates. Many global biochemical and physiological processes exploit low temperatures to lower reaction rates but retain the ability to resume full activity upon rewarming. Other critical functions must continue at physiologically relevant levels during torpor and be precisely regulated even at Tb values near 0 degrees C. Research using new tools of molecular and cellular biology is beginning to reveal how hibernators survive repeated cycles of torpor and arousal during the hibernation season. Comprehensive approaches that exploit advances in genomic and proteomic technologies are needed to further define the differentially expressed genes that distinguish the summer euthermic from winter hibernating states. Detailed understanding of hibernation from the molecular to organismal levels should enable the translation of this information to the development of a variety of hypothermic and hypometabolic strategies to improve outcomes for human and animal health.