Many kinds of animals are capable of surviving extended periods without breathing oxygen. For example, turtles that hibernate underwater can live for 3-4 months without breathing whereas intertidal invertebrates that breathe with gills undergo daily bouts of oxygen deprivation 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, strong antioxidant defenses to resist damage to macromolecules by oxyradicals when oxygen is suddenly reintroduced, and entrance into a hypometabolic state in which animal energy requirements are reduced by >90%. 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. In the Storey lab we study a variety of animals that can live without oxygen for long periods of time.
Biochemical mechanisms of anoxia tolerance: Adaptations that support anoxia tolerance can include some or all of the following depending on the species and the circumstances.
(1) Fuels:High levels of carbohydrate fuels are stored in tissues because carbohydrates (glycogen, glucose) can be catabolized to produce energy (ATP) without the use of oxygen via an ancient pathway called glycolysis.
(2) Dealing with acidosis:The end product of glycolysis is normally lactic acid (also called lactate) and when it accumulates in high levels it causes a dangerous drop in cellular pH. Anoxia tolerant animals use mechanisms to prevent or buffer acid build-up. Turtles and mollusks release calcium carbonate from their bone and/or shell to buffer acid. Turtles also move a lot of lactate from their blood and store it in their shell. Carp and goldfish have a unique solution – instead of letting lactate build up, they convert it to ethanol and then excrete the alcohol from their bodies across the gills.
(3) Maximizing ATP yield: The energy yield from the conversion of glucose to lactate is low, a net of just 2 ATP produced per molecule of glucose processed. However, if glucose is fully burned to CO2and H2O using oxygen-dependent respiration, the yield is 36 molecules of ATP per glucose. Hence, biochemical mechanisms that can increase the ATP yield from the anaerobic fermentation of glucose are very important. Marine mollusks have this mastered. Instead of producing lactate, they make a range of other products (e.g. succinate, acetate, propionate) in reactions that have additional ATP-yielding steps so that they can double or triple the ATP output compared to making lactate. These animals also use some amino acids (e.g. aspartate, glutamate) as fuels during anoxia for additional bonus ATP production
(4) Metabolic rate depression: When body oxygen levels fall below a critical point, anoxia tolerant animals strongly reduce their metabolic rate, reducing their overall energy needs to a level that can be supported by the ATP output of glycolysis. Turtles and fish typically reduce their metabolic rate by 80-90% whereas many mollusks can lower their metabolism by >95%. Hypometabolism is achieved by turning down and turning off many cellular processes in a priority manner so that just the critical life-support processes are left.
(5) Antioxidant defenses: Lack of oxygen causes metabolic injuries but so does a too-rapid return of oxygen to normal levels. Indeed, much of the damage caused by blocked circulation during heart attack or stroke is caused not by the prevention of oxygen delivery to tissues but by a burst of oxygen-free radicals produced when oxygen floods back into the tissues. These reactive oxygen species can attack and damage cellular DNA, proteins and lipids. All organisms have antioxidant defenses that can destroy reactive oxygen species and repair damaged molecules but these are often overwhelmed in anoxia intolerant animals, including man. Anoxia tolerant animals correct this by maintaining extra high levels of antioxidants at all times and by triggering the synthesis of extra antioxidants whenever they are exposed to oxygen depletion.
(6) Activation of anoxia responsive genes: In response to low oxygen, anoxia tolerant animals shut down the expression of most genes and the synthesis of most proteins as part of metabolic rate depression. Uniquely, however, they also increase the transcription of a few selected special genes and greatly increase the synthesis of the protein products of these genes. These protein products have special protective actions for anoxia survival and they include antioxidant enzymes, iron-binding proteins, chaperone proteins that help to protect the structures of other cell proteins, and a variety of other proteins with actions that help to extend viability in the absence of oxygen.