Tardigrades are hardy little critters; there isn't a single niche they haven't made their own. And the secret of the microscopic creature's success? Cryptobiosis: when it's too dry to survive, the tiny animals curl up and dry out too. But instead of perishing under the stressful conditions, tardigrades fall into cryptobiosis, a state of suspended animation, ready to emerge as soon as water returns. Intrigued by the cellular mechanisms that protect the tiny creatures, Ralph Schill, Günther Steinbrück and Heinz Köhler began analysing the tardigrade's stress response by looking to see whether stress resistance proteins protect tardigrade's from death by dehydration (p. 1607).
Schill explains that there are probably over 600 species of tardigrade across the planet. But while some species survive the most extreme climates on earth, Milnesium tardigradum failed to thrive in the lab; at least until Schill discovered their taste for Volvic™ water. Content to live on agar made from the French mineral water, Schill finally had access to one of the few captive tardigrade colonies in the world, ready to analyse their response to dehydration stress.
Schill soon faced other technical problems. For a start, the 0.8 mm long creatures were too small to produce enough protein for detection with antibodies. Schill realised that he would have to use alternative methods relying on nucleic acids to identify whether tardigrades use heat shock proteins to protect them from stress. But tardigrade cells have the lowest nucleic acid levels of any cell known, making extraction tricky. Undeterred, he set about extracting DNA from 100 tardigrades, to find whether they carried the gene for Hsp 70, a well-known heat shock protein that protects proteins damaged by physiological stress from aggregating and causing further cellular damage.
Sure enough, the tiny creatures carried Hsp 70 genes; in fact, they carried three. But would the animals use the genes to defend themselves from hot and stressful conditions?
Warming individual tardigrades up to 37°C, Schill tested all three isoforms' expression patterns. Tracking the mRNA levels of each gene with real time PCR, Schill realised that all three Hsp 70 genes were activated to produce mRNA and proteins to protect the tardigrades from the sudden warm spell. But would the genes respond to other stresses too? Would desiccated tardigrades also show high levels of the Hsp 70 isoforms?
Drying out the tardigrades' Petri dish homes, Schill monitored the animals as they descended into cryptobiosis and later while they rehydrated. Extracting miniscule amounts of RNA from the animals, Schill used real time PCR to measure each isoform's mRNA level. While the first and third isoform levels fell during cryoptobiosis, the second isoform's levels more then doubled as the tiny creatures curled up and dried out. The second isoform was clearly involved in some way to protect the desiccated animals.
Delighted that he's beginning to understand the molecular basis of these robust little animals resilience, Schill knows there's still much left to learn. But he admits that he still finds cryptobiosis fascinating; `it's amazing that animals can dry out and still be alive afterwards' he says.
- © The Company of Biologists Limited 2004