The brains of most vertebrates, mammals especially, are extremely sensitive to a lack of oxygen. Within a few minutes of oxygen deprivation induced by lack of blood flow (ischemia, stroke) mammalian neurons experience a massive loss of ATP, loss of mitochondrial integrity, depolarization of cellular membranes, and the release of calcium stores and a number of compounds that induce cell death. Cell death can be immediate due to massive failure of the cell to maintain homeostasis (necrosis) or delayed due to conditions that activate a variety of programmed cell death (apoptosis) pathways that take several hours to initiate and execute. However, a handful of vertebrates,including the freshwater turtle, Trachemys scripta elegans, have evolved the ability to survive without oxygen for extended periods of time with no apparent loss of neurons. Shailaja Kesaraju and colleagues at Florida Atlantic University set out to explore how the neurons of this turtle avoid cell death in the face of a complete lack of oxygen (anoxia).
Sampling the brains of turtles after 24 h of anoxia at room temperature(22–23°C) and after 1 and 3 days of recovery from anoxia, the team examined sections of the turtles' brains to see how they faired after oxygen deprivation. The team measured levels of overall cell survival and organization, and monitored changes in neurons and astrocytes in the brain sections and found no major changes in cell number or morphology in the turtles' brains 3 days after a 24 h exposure to anoxia.
In a separate set of experiments, the team examined the expression of a suite of proteins in turtle brains 1, 4 and 24 h after anoxia. They also assessed how the turtles recovered from anoxia by measuring the expression of these proteins after the turtles experienced 4 h of anoxia followed by a 4 h period of recovery in air. Proteins monitored included a number of heat shock proteins (Hsp), such as Hsp72, Hsp27, and proteins involved in programmed cell death such as Bcl-2, Bax, apoptosis-inducing factor (AIF) and caspase-3.
The levels of the stress proteins Hsp72, Hsp60, heme oxygenase-1, Hsp27 and the glucose-regulated protein Grp94 rose significantly after 1 h of anoxia and remained elevated after 24 h of anoxia. The increases in stress response proteins are likely to reduce cell death by stabilizing components of the cell(e.g. macromolecular complexes, cytoskeletal elements) that may be disrupted by changes in cell physiology associated with anoxia and by blocking induction of apoptosis. This is especially true of Hsp72 and Hsp27, both of which are potent molecular chaperones that act to stabilize proteins, membranes and DNA to protect the cell from stress-induced damage. They have also been shown to block apoptosis in mammalian cells. The team also found that the turtle brains block multiple pathways that induce apoptosis, including AIF, Bax and caspase-3 proteins.
The wealth of information on stress response and apoptosis-regulating proteins presented in this paper paint a very complex molecular response to anoxia that blocks the induction of apoptosis in turtle neurons and prevents tissue death. These data indicate that turtle brains are probably pre-adapted to block the molecular events that induce apoptosis in mammalian neurons during anoxia, in order to protect them from brain damage during oxygen deprivation.
