16 May 2014

Tun-ing in on water bears' superpowers

Water bears, or tardigrades, are harmless microscopic animals. Yet, despite their endearing bear-like appearance, tardigrades are the hardest animals to kill on Earth. And boy, many have tried.

Tardigrades are chubby eight-legged animals, no longer than the head of a pin, related to velvet worms and also arthropodes, a large family including insects, spiders and crustaceans. They can be found anywhere where there’s water, but they prefer to live in damp moss and lichens. These tough creatures can survive boiling temperatures up to 125˚C* and freezing temperatures so extreme (-272˚C!) they can only be artificially created in a laboratory. They can also survive astonishing amounts of radiation with no apparent damage to their DNA, extremely high pressures, and, unlike any other earthly creature, tardigrades can hang out for a few minutes in the vacuum of space and come back alive to tell the story.

Tardigrades (Hypsibius dujardini) imaged with a scanning electron microscope.

So what’s their secret? Tardigrades have the amazing ability to reversibly slow down their metabolism to nearly a halt (less than 0,01% of their normal metabolic rate) in response to a change in their environment—a process called cryptobiosis. Other organisms can do it—nematodes, rotifers, brine shrimp—but not nearly as spectacularly as tardigrades. It is estimated that they can lose up to 99% of their water content, and enter a so-called ‘tun’ stage that protects them against harsh environmental conditions. Yet, if you rehydrate these tuns, the animals will quickly return to their normal selves—moving about, growing and having babies, as you do when you’re a tardigrade (watch movie below).

video

Scientists grow tardigrades in the lab (and sometimes in space) to study cryptobiosis. Understanding how tardigrades survive extreme dehydration during the tun stage could help developing better techniques for dry preservation of biological material, for example.

In a recent study, Marcus Frohme and colleagues from the Technical University of Applied Sciences in Wildau (Germany) compared differences in gene expression between happy, dehydrating, tun stage and rehydrated tardigrades. The idea was to search for the genes that are more, or less active in each of these metabolic states, which could give some clues as to how the tardigrades’ cells cope with severe dehydration. The researchers grew four groups of animals in the lab under different conditions (from moist to dry) and then smashed them up to chemically extract mRNA molecules (copies of DNA that will be translated into proteins) from their cells. They then sequenced and quantified these molecules, and finally analysed the huge amount of data using a powerful computer software.

The team found that in the dehydration stages, genes involved in cell division and growth were less active, but genes encoding for proteins that protect or repair cellular components, such as heat-shock proteins, were highly expressed. These results confirm previous research, but some preliminary data in Frohme's study also suggest that several genes involved in DNA repair are more active in the rehydration stage than in the dehydration stage. The authors propose that tardigrades adopt a dual strategy combining mechanisms of protection (during dehydrating stages) and recovery (during rehydration stages) to survive desiccation.

* In the original article it was written 151˚C, this has been corrected to 125˚C (reference: Doyère P.L.N. Memoires sur les Tardigrades. Sur le facilité possedent les tardigardes, les rotifers, les anguilleles des toit et quelques autres animacules, de renvenir à la vie après été complement désesschées. Ann. Sci. Nat. 18: 5, 1842.)

Reference:
Wang C., Grohme M.A., Mali B., Schill R.O., Frohme M. & Gibas C. (2014). Towards Decrypting Cryptobiosis—Analyzing Anhydrobiosis in the Tardigrade Milnesium tardigradum Using Transcriptome Sequencing, PLoS ONE, 9 (3) e92663. DOI:

A shorter version of this article was published in the print issue of Lab Times on the 13-05-2014.

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