They are probably the toughest creatures on Earth, and now we know how they manage to survive years of complete dehydration.

 

Water bears, or tardigrades, have been recorded surviving the vacuum of space, high doses of radiation and pressure. These water dwelling creatures can also survive dry environments in a shrivelled-up, dormant state for as long as a decade, reviving within an hour when exposed to water.

To pull off this remarkable trick, the animals rely on proteins unique to them, called tardigrade-specific intrinsically disordered proteins (TDPs).

 

When there is water around, these anti-dehydration proteins are jelly-like and don’t form into well-defined three-dimensional structures like most known proteins.

But when water bears start to dry out, these proteins turn into a kind of glassy sanctuary that cocoons all dehydration-sensitive materials in the animal from harm.

 

“When the animal completely desiccates, the TDPs vitrify, turning the cytoplasmic fluid of cells into glass,” says lead author Thomas Boothby of the University of North Carolina at Chapel Hill.

“We think this glassy mixture is trapping [other] desiccation-sensitive proteins and other biological molecules and locking them in place, physically preventing them from unfolding, breaking apart or aggregating together,” says Boothby.

 

Surviving a dry spell

Boothby and his colleagues discovered the proteins after monitoring gene activity as tardigrades dried out. They noticed a spike in activity of genes that turned out to make the TDPs.

And when they blocked the activity of these genes, tardigrades died after dehydration, demonstrating they were vital for surviving desiccation.

Moreover, Boothby showed that yeast and bacteria artificially equipped with the genes could also survive dehydration, suggesting that they could potentially be transferred into crops to help them survive drought.

 

The researchers also ruled out a long-held assumption that tardigrades survived dehydration thanks to a specialised sugar called trehalose, which is known to perform the task in tree frogs, for example. They found that tardigrades either don’t make trehalose at all, or only make tiny amounts.

Sweet trick

However, the researchers were surprised to discover that the proteins  protect their living cargo in the same way as trehalose, by forming glass like sanctuaries.

 

Boothby says the results provide a new example of convergent evolution, in which evolution comes up with a similar solution more than once. “It’s just that some animals have evolved to rely on trehalose, while tardigrades have also evolved the ability to vitrify, but using a completely different type of molecule – a protein,” he says.

“It is amazing to see that evolution has found several biochemical ways to obtain the same kind of mechanism for solving the problem of desiccation. Trehalose in nematodes and brine shrimps, and apparently TDPs in tardigrades,” says Ingemar Jönsson of Kristianstad University, in Sweden. “And what a wonderful adaptation to turn into a stable glassy state when the body dries out!”

 

The team is now investigating whether other organisms such as plant seeds rely on these proteins to survive desiccation, too.

Boothby is hoping the find can be translated into practical applications, for example as a way to store vaccines and pharmaceuticals at room temperature by dehydrating them instead of having to constantly refrigerate them.

“This could help us break dependence on the cold-chain, a huge economic and logistical hurdle for getting medicine to people in remote or developing parts of the world,” he says. “We are pursuing these applications.”

Journal reference: Molecular Cell, DOI: 10.1016/j.molcel.2017.02.018