Naturally Evolved Cellular Adaptations to Anoxia

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The Buck lab is investigating the cellular mechanisms that permit some vertebrate species, such as freshwater turtles and goldfish to survive without oxygen (anoxia) for days to months. We are interested in the fundamental mechanisms underlying this ability but our results will have clear clinical significance and application in the treatment of stroke, heart attack and organ storage for transplant. Both brain and liver cell models are used to study the cellular mechanisms of anoxia tolerance in tissue that is electrically excitable and one that is not. Key to surviving anoxia is the shutting off of energy utilizing cellular activities, such as the synthesis of new proteins and maintaining ion gradients across cell membranes. We have previously shown that protein synthesis decreases by about 90%, the pump that maintains ion gradients across cell membranes decreases by 75%, and ion channel activity (pathways for ions to cross membranes) decreases and in one special case increases. The ability to reduce the rate of these energy consuming reactions is something that mammals (humans) lack. However, basic biochemical pathways are common to most species, certainly amongst reptiles (turtles), fish, birds and mammals. Our goal is to determine the natural cellular pathways responsible for shutting off energy consuming processes in anoxia-tolerant species. We have already identified several possible pathways and discovered that brain excitatory signaling by pyramidal neurons involving glutamate is decreased, while inhibitory activity involving GABA (gamma-aminobutyric acid) and stellate neurons is increased during anoxic periods. In fact, GABA currents double in magnitude during anoxia and are larger than any GABA current observed in mammal brain. A major focus of this grant is to measure the electrical activity of GABA releasing stellate neurons during anoxia to understand what triggers this large release of neurotransmitter. Furthermore, using high-powered microscopy we will visualize the position of the stellate neurons in relation to the pyramidal neurons to determine if the stellate neurons are specialized for a recently discovered form of neurotransmission – volume transmission. Brain is an electrically excitable tissue and we are also investigating anoxic regulation of ion channels in non-excitable tissue - liver. In this grant cycle we will continue to explore the electrophysiological properties and second messenger pathways of brain and liver tissue to better understand which ion channels are regulated in a way that reduces energetic demands and permits anoxic survival.  Importantly, our work will demonstrate that different cell types, even within a single tissue, respond differently to an anoxic challenge; therefore, multiple different strategies are needed to protect tissues from anoxic injury.