An impressive array of organisms are capable of radically depressing basal metabolic rate and entering a hypometabolic state characterized by the suspension of many normal physiological functions. Metabolic rate depression is often used to elude harsh environmental conditions such as low oxygen, low temperature, or lack of water. Entry into a hypometabolic state involves the slow down or shut down of a variety of physical activities and physiological processes coordinated by molecular-level controls. At a biochemical level we have identified several mechanisms that contribute to metabolic rate depression including: 1) Reversible phosphorylation of key regulatory enzymes (e.g. phosphofructokinase, pyruvate kinase) to produce less active enzyme forms, 2) Dissociation of enzymes from complexes bound to the subcellular particulate fraction to reduce flux through metabolic pathways, and 3) Reduced levels of fructose-2,6-bisphosphate, a potent activator of phosphofructokinase, to limit the anabolic use of carbohydrate in the depressed state. In new studies we are moving in two different directions. Studies of signal transduction are analyzing the protein kinases and phosphatases, second messengers and triggers that are responsible for initiating and coordinating metabolic arrest. Studies of gene expression are identifying genes that are up-regulated during entry into or arousal from an arrested state, analyzing the patterns and influences on gene expression, and investigating the roles of the associated gene products. Studies show that these mechanisms form a common basis for the control of metabolic rate depression in diverse animal including anoxia tolerance in marine molluscs, goldfish, and turtles, hibernation in small mammals, and estivation in land snails.
Various species of amphibians and reptiles living in temperate and polar regions of the earth survive the freezing of extracellular body fluids during the winter. The strategy is also widespread amongst insects and other invertebrates. Adaptations supporting freeze tolerance include: 1) Protein: nucleating proteins induce and regulate extracellular freezing; thermal hysteresis proteins prevent recrystallization, 2) Cryoprotectants: high concentrations of polyols and sugars prevent excessive cell volume reduction, increase intracellular bound water content, and stabilize proteins; trehalose and proline stabilize membrane bilayer structure, and 3) Ischemia tolerance: cells and organs have adapted to tolerate the anoxia/ischemia imposed by extracellular freezing. Our studies with freeze tolerant frogs and turtles are targeting a variety of questions that we hope will lead to applied solutions that can be used to improve the cryopreservation of transplantable human and mammalian organs. New work includes a major focus on the role of gene expression in freezing survival including the identification of genes and their protein products that have never before been implicated in freeze endurance. Other studies are analyzing the roles of protein kinases and protein phosphatases in coordinating the metabolic changes that occur during freezing and other work is determining how frog cells can endure levels of glucose accumulation that would be lethal to human diabetics.
Many types of invertebrates have developed cold hardiness strategies that allow them to endure long exposures to subzero temperatures during overwintering. Two strategies are possible: freeze tolerance (an ability to endure the freezing of extracellular body fluids) and freeze avoidance (an ability to remain liquid in a supercooled state at temperatures far below the freezing point of body fluids). Many insects as well as various intertidal invertebrates are freeze tolerant (as also are various reptiles and amphibians) whereas many other types of insects and terrestrial arthropods use the freeze avoidance strategy. Adaptations supporting freeze tolerance and freeze avoidance include: 1) Protein: nucleating proteins induce and regulate extracellular freezing; antifreeze proteins inhibit ice formation, 2) Cryoprotectants: high concentrations of polyols and sugars prevent excessive cell volume reduction in freeze tolerant animals and push supercooling point to low levels in freeze avoiding animals, and 3) Metabolic arrest and ischemia tolerance: freeze tolerant animals have a well-developed tolerance for the anoxia/ischemia imposed by extracellular freezing, both types use metabolic rate suppression (e.g. diapause) for long winter survival using limited internal fuel reserves. Using both insect and marine mollusc models, current research is focusing on the role of gene expression in supporting cold hardiness, looking for genes that are induced or upregulated during cooling and/or freezing and determining the role of their protein products in cold survival.
An impressive array of organisms are capable of surviving extended periods without breathing oxygen. Turtles that hibernate underwater can live for 3-4 months without breathing; intertidal invertebrates undergo daily bouts of anaerobiosis with the rising and falling of the tides. Our studies of the biochemical adaptations that allow animals to live without oxygen have shown that these include the maintenance of high reserves of fermentative fuels, modified pathways of fermentative catabolism that generate a greater ATP output than glycolysis alone, mechanisms of buffering the accumulation of acidic end products, and entrance into a hypometabolic state in which energy requirements are reduced by >90%. Mechanisms of metabolic rate depression are conserved across phylogenetic lines and we have shown that the same mechanisms are called into play to allow animals to elude a variety of harsh environmental conditions including low oxygen, low temperature, or lack of water (see hibernation above). Our current studies of anoxia tolerance range over a variety of topics but are primarily focused on the role of anoxia-induced gene expression in animal adaptation to stress and the signal transduction pathways (signals, second messengers, protein kinases and phosphatases, nuclear transcription factors) that mediate cell responses to stress.