Beyond anoxia: the physiology of metabolic downregulation and recovery in the anoxia-tolerant turtle

Abstract

The freshwater turtle Trachemys scripta is among the most anoxia-tolerant of vertebrates, a true facultative anaerobe able to survive without oxygen for days at room temperature to weeks or months during winter hibernation. Our good friend and colleague Peter Lutz devoted nearly 25 years to the study of the physiology of anoxia tolerance in these and other model organisms, promoting not just the basic science but also the idea that understanding the physiology and molecular mechanisms behind anoxia tolerance provides insights into critical survival pathways that may be applicable to the hypoxic/ischemic mammalian brain. Work by Peter and his colleagues focused on the factors which enable the turtle to enter a deep hypometabolic state, including decreases in ion flux ("channel arrest"), increases in inhibitory neuromodulators like adenosine and GABA, and the maintenance of low extracellular levels of excitatory compounds such as dopamine and glutamate. Our attention has recently turned to molecular mechanisms of anoxia tolerance, including the upregulation of such protective factors as heat shock proteins (Hsp72, Hsc73), the reversible downregulation of voltage gated potassium channels, and the modulation of MAP kinase pathways. In this review we discuss three phases of anoxia tolerance, including the initial metabolic downregulation over the first several hours, the long-term maintenance of neuronal function over days to weeks of anoxia, and finally recovery upon reoxygenation, with necessary defenses against reactive oxygen stress.

Figure 1

Potential sites where T. scripta may be adapted to reduce or prevent reactive oxygen stress upon reoxygenation following anoxia. Anoxia/reperfusion in mammals results in massive ROS increases that damage proteins, lipids, and nucleotides, resulting in cellular dysfunction and eventual death. Freshwater turtles are clearly able to survive repeated anoxia/reoxygenation, and may have increased resistance to ROS stress.

Figure 2

Representative digital images showing mitochondrial oxidation of rhodamine dye in neuronally-enriched turtle primary cultures utilizing Mitotracker Red®. Cultures were photographed using a BioRad Radiance 2000 Scanning Laser Confocal Microscopemicroscope at 10X magnification. a) normoxic controls b) 4 hr anoxia c) upon reoxygenation following 4 hr anoxia.