Integrative neurophysiology of extreme hypoxia tolerance in turtles

Oxygen is essential for all animals to thrive. Within the natural world, there is significant variation between different species’ ability to temporarily live in its complete absence, a condition that is also called anoxia. Most mammals, including humans, can survive for several minutes of anoxia before tissue injury accumulates.

The most vulnerable tissues are those with high rates of metabolism, especially the heart and brain. Indeed, brain injury caused by the cessation of oxygen delivery because of cardiac arrest or stroke is a major human health and societal problem. Many aquatic animals are also vulnerable to periods of anoxia because of human-induced releases of water or chemicals that can cause oxygen depletion.

Pond turtles, especially painted turtles, show an extreme tolerance to anoxia, surviving for many months while overwintering in ice-locked ponds. This research aims to understand how the brain of anoxia-tolerant turtles function without anoxia, with the overarching goals of understanding the basic requirements for neurological anoxia tolerance, and to identify novel therapeutic strategies to minimize cellular injury in anoxia-sensitive species, including humans.

The work includes studies of anoxia and temperature on the function of isolated brain cells and sensory function and on gene expression in different regions of the turtle brain. It will enhance the STEM workforce by training a postdoctoral fellow, a graduate student and multiple undergraduate students. It will also create a STEM learning experience for middle school children from economically disadvantaged backgrounds within the St. Louis metropolitan area.

The work will exploit a recent breakthrough isolating and culturing primary cortical neurons from painted turtles to test hypotheses concerning the control of ion and synaptic arrest, with the ultimate goal of determining if the anoxia-tolerant functional phenotype is intrinsic to isolated turtle neurons or requires extrinsic factors, such as low pH, lactate, GABA, and adenosine. The effects of these factors on NMDA and AMPA receptor and voltage-gated sodium channel function under normoxic and severely hypoxic conditions using intracellular calcium recordings and whole-cell voltage clamping will also be studied.

It will investigate how complex sensory functions formed by neuronal circuits are affected by anoxia and temperature in the visual and auditory systems using reduced brain preparations. It was previously discovered that evoked potentials in the brainstem during displacement of the tympanum are unaffected by severe hypoxia in turtles acclimated to 20oC, indicating that turtles may still hear while anoxic.

This differs from optic tectum and retina, which show suppression. These patterns will be studied under more ecologically relevant conditions, i.e., when normoxic or severely hypoxic while acclimated to 3oC. Finally, the project will use next-generation sequencing to understand spatial gene expression patterns in turtle brain, particularly in cerebrocortex, midbrain, and brainstem with cold-acclimation and anoxia.

The work should reveal factors responsible for the adaptive functional responses of single turtle neurons to anoxia, how sensory systems comprised of complex neural circuits with many neurons respond to anoxia and cold-acclimation, and the molecular signatures that underlie these functional responses.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.