Forever young: mechanisms of natural anoxia tolerance and potential links to longevity

Abstract

While mammals cannot survive oxygen deprivation for more than a few minutes without sustaining severe organ damage, some animals have mastered anaerobic life. Freshwater turtles belonging to the Trachemys and Chrysemys genera are the champion facultative anaerobes of the vertebrate world, often surviving without oxygen for many weeks at a time. The physiological and biochemical mechanisms that underlie anoxia tolerance in turtles include profound metabolic rate depression, post-translational modification of proteins, strong antioxidant defenses, activation of specific stress-responsive transcription factors, and enhanced expression of cytoprotective proteins. Turtles are also known for their incredible longevity and display characteristics of "negligible senescence". We propose that the robust stress-tolerance mechanisms that permit long term anaerobiosis by turtles may also support the longevity of these animals. Many of the mechanisms involved in natural anoxia tolerance, such as hypometabolism or the induction of various protective proteins/pathways, have been shown to play important roles in mammalian oxygen-related diseases and improved understanding of how cells survive without oxygen could aid in the understanding and treatment of various pathological conditions that involve hypoxia or oxidative stress. In the present review we discuss the recent advances made in understanding the molecular nature of anoxia tolerance in turtles and the potential links between this tolerance and longevity.

Figure 1

Activation of the heat shock response. Various stresses can lead to unfolding of proteins, calling heat shock proteins (HSPs) into action to aid refolding. As a consequence, HSPs dissociate from association with the heat shock transcription factor (HSF). The freed HSF then becomes activated through trimerization and hyperphosphorylation, binds to the heat shock element (HSE) and mediates the upregulation of hsp genes. This results in the proliferation of cytoprotective HSPs in the cell.

Figure 2

Activation of the NFκB pathway. Under normal conditions, the NFκB dimer (composed of the proteins p50 and p65) is retained in the cytoplasm through interaction with its inhibitor protein, IκB. In response to various stimuli, the IκB kinase becomes activated and phosphorylates IκB, thereby targeting it for ubiquitination and subsequent degradation by the proteasome. This leaves NFκB free to move to the nucleus and activate transcription of various target genes.

Figure 3

Activation of NFκB in T. s. elegans liver. (A) Effect of 5 and 20 h of anoxic submergence on the nuclear content of the proteins making up NFκB dimer, p50 and p65 in T. s. elegans liver. Representative western blots bands of p50 and p65, as well as a histogram showing normalized protein levels are shown. (B) Effect of 5 and 20 h of anoxic submergence on the DNA-binding activity of NFκB in T. s. elegans liver, as assessed by a transcription factor ELISA. (a) Significantly different from the corresponding control (p < 0.05); (b) significantly different from the 5 h anoxic value (p < 0.05). Data are taken from ref. .

Figure 4

A generalized schematic for the mitochondrial apoptosis pathway. During apoptosis, the mitochondrial membrane swells and cytochrome c leaks out. This leads to binding with the pro-apoptotic protein Apaf-1, subsequent formation of the apoptosome and activation of caspase-9. Caspase-9 subsequently activates other caspases and results in cell death. Pro-survival Bcl-2 family members (such as Bcl-2 and Bcl-x_L) can inhibit apoptosis by binding to the mitochondrial membrane and blocking its swelling.

Figure 5

Effect of 5 and 20 h of anoxic submergence on transcript levels of prosurvival target genes of NFκB in T. s. elegans liver. Representative RT-PCR bands and a histogram showing normalized transcript levels under control and anoxic conditions are shown. Data were normalized against α-tubulin and are shown as means ± S.E.M., n = 3 independent trials. (a) Significantly different from the corresponding control (p < 0.05). Data are taken from ref. .