Sitting at the bottom of a muddy puddle for months might not seem like the most thrilling way to pass winter, but it is the annual fate of most adult western painted turtles. ‘As their pond ices over, these turtles grab a breath, then settle to the bottom where there is no oxygen and wait for the spring thaw’, says Daniel Warren from Saint Louis University, USA. Most incredibly, the turtles emerge from their icy tombs unscathed, despite surviving for months with virtually no oxygen. ‘Most cells in an animal's body, including ours, die very quickly if they don't receive enough oxygen to make the energy they need’, explains Sarah Alderman from the University of Guelph, Canada; and our hearts are particularly vulnerable. Yet, the hearts of icy western painted turtles continue ticking, albeit slower, despite the disruption to their oxygen supply. Knowing that some reptiles become more resilient to oxygen deprivation – anoxia – as they age and when the temperature drops, Warren and Claire Riggs (also from Saint Louis University) decided to find out how hatchling and adult western painted turtles prepare their hearts for their lengthy winter hibernation.
As the freshwater turtles stop making proteins when their oxygen supply dwindles, Warren and Riggs decided to monitor the protein signature of the adult and hatchling turtle hearts to identify the key changes that keep adult hearts going during winter. First, the duo tricked the adult and hatchling western painted turtles into preparing for winter by gradually cooling the water in their tanks to 3°C – even though the youngsters don't naturally endure the first winter immersed in water; they weather the season in nests in sandy soil. After the turtles spent 5 weeks in the frigid water, Riggs collected portions of the hearts and investigated the protein signatures before striking up a collaboration with Alderman, Todd Gillis and Oliver Bullingham at the University of Guelph to analyse how the proteins produced in the heart differed between the hatchling and adult turtles and how they changed as the turtles were chilled.
After months of painstaking analysis, the team identified 1316 turtle heart proteins – from muscle proteins to proteins that contribute to energy production and proteins that store oxygen and fats. However, as the scientists delved into how the relative protein quantities altered after 5 weeks in cold water, they found that the fall in temperature appeared to have little impact, although the chilly adults slightly reduced the quantities of a few proteins that participate in protein production – to conserve energy and counter the effects as their oxygen stores vanished. In addition, the chilly hatchlings slightly increased production of some of the key proteins in the mitochondria powerhouses – which generate energy – although that wasn't surprising, given that the youngsters spend winter huddled out of water in a nest.
In contrast, the differences between the older and younger turtles’ hearts were more remarkable. The older turtles produced more of the oxygen storage protein – myoglobin – than the hatchlings and produced more of the proteins necessary for aerobic and anaerobic respiration, allowing them to spin out their oxygen reserves for longer before switching to anaerobic respiration to sustain them through to spring. ‘Development is likely a more powerful driver of anoxia tolerance than any temperature induction’, says Warren.
Western painted turtles build up their hearts as they grow to survive a season without oxygen, with a little additional fine tuning as the cold season approaches. And the team hopes that we could learn a thing or two from the turtles. ‘Human hearts fail without oxygen, but painted turtle hearts don't – understanding why this difference exists could save lives’, Alderman says.