High-definition rendering of an electron microscope image of a tardigrade.

If this is the first time you’re hearing about tardigrades, also called water bears or moss piglets, I’m very excited for this wild ride you’re embarking upon.
Here are the basics to whet your appetite: These microanimals belong to the same superphylum as insects, crustaceans, arachnids, and centipedes. They range between 0.3 and 1.2 mm in size, and are generally found in water (hence, water bears). However, they have also been found on mountaintops, in the deep sea and mud volcanoes, in tropical rainforests and even Antarctica [1].

Tardigrades are almost large enough to see with the naked eye. A simple $50 light microscope is enough to spot these creatures. Source: Reddit

A story of resilience

Tardigrades are one of the most resilient animals on Earth. There are virtually no environmental conditions on Earth or in space that Tardigrades can’t overcome. Here’s the highlight reel:
Temperature: Some can survive freezing at extremely cold temperatures down to −450 °F or −272 °C. That’s close to absolute zero, or the lowest temperature theoretically possible in this universe! Others can withstand being boiled in water, at temperatures as high as 300 °F or 150 °C [2].
Radiation: Tardigrades can survive gamma ray bursts, which is several hundred times the lethal dose for humans.
Atmospheric pressure: One of the reasons humans need space suits is that they will literally explode in the low pressure environment of space. Tardigrades, on the other hand, can survive being in a vacuum. Being deep underwater is the opposite – the world record deep dive for humans is 1,000 feet deep. Much more than that would crush humans (most organisms, actually) because any space occupied by air would collapse entirely. Tardigrades have been found living 13,000 feet below, putting us humans to shame.
Dehydration: These biological miracles can survive in a state of complete dehydration for over 10 years [2]. Meanwhile, I get cranky if I’ve had less than four cups of water a day.

Secrets revealed: Cryptobiosis

Tardigrades are able to bounce back from these extreme conditions because they can enter a metabolic state called cryptobiosis [3]. A word that sounds more like science fiction than reality, cryptobiosis is the ability of organisms to suspend their metabolism entirely, stopping processes like development, repair, and reproduction. Cryptobiotic tardigrades can slow their metabolism down 1,000 fold under adverse environmental conditions, and return to normal once favorable conditions resume.

That can’t be real!

Brace yourself. A lot of extreme experiments have been performed on tardigrades. The results and implications of these experiments will make you throw your hands in the air and say, “That can’t be real!”

Protection from radiation
Radiation destroys DNA. It causes mutations, double-strand breaks, and leads to genome instability and even lethality. Organisms capable of withstanding heavy irradiation often have DNA-binding proteins that are highly capable of protecting and repairing severely-damaged DNA. An investigation of these proteins in tardigrades revealed an impressive repertoire of stress-related proteins, including proteins that neutralize radiation-induced reactive oxygen species, and DNA protection and repair proteins. Scientists discovered one protein of particular interest: a DNA-associating protein called Dsup (Damage suppressor protein) [4].
It turns out that tardigrades do undergo severe DNA damage by irradiation, but this DNA is instantaneously patched up by Dsup.

Engineering the next Superman
To make a definitive conclusion about the Dsup protein, Hashimoto et al. used genetic engineering to transfer the gene coding for Dsup into a human cell line, HEK293. When they irradiated Dsup-HEK293 human cells, they found that these cells had 40% less X-ray-induced damage compared to control cells without Dsup! Below is an image modified from the original publication showing the quantification of DNA damage [4].

Adapted and edited from Hashimoto, et al. Nature Communications volume 7, Article number: 12808 (2016)
An alkaline comet tail assay was used wherein broken/damaged DNA content was analyzed by looking at the “comet tail”. A bigger tail represents more broken DNA. The Dsup cells show a much smaller tail, indicating significantly less DNA damage.

Ionizing radiation (like UV rays and X-rays) creates highly-reactive radicals. The radical’s unstable, unpaired electron acts as a warhead, and can damage biomolecules like proteins and DNA with ease. Recent work by Chavez et al. shows that Dsup protects DNA by blocking adverse effects of hydroxyl radicals created during ionizing radiation or hydrogen peroxide treatment [5].

Real world implications
Apart from engineering the next intergalactic indestructible Superman, these findings have real, immediate implications. DNA damage is at the core of many diseases like cancer and ageing. Protection and repair of DNA have been extensively studied, but Dsup might be a novel route to dealing with and preventing DNA damage.
Analysis might also reveal how to protect human cells during freezing, for applications such as cryonics. Further, if we can stabilize proteins without needing refrigeration, it might transform worldwide healthcare, as well. Vaccines can be inaccessible in remote parts of the world because they typically require refrigeration, and global health would greatly benefit if we can sidestep the issue.

Scientists have just started exploring the marvellous biology of tardigrades, and there’s so much more we have to learn from them. Until then, stay curious!

Questions or comments? Please post them below.

References:
1. Wikipedia. “Tardigrades”. Accessed Mar 27, 2020
2. Sloan, Batista, and Loeb. The Resilience of Life to Astrophysical Events. Science Reports. Vol. 7, 5419 (2017).
3. Møbjerg…Kristensen, et al. Survival in extreme environments – on the current knowledge of adaptations in tardigrades. Acta Physiologica. Vol. 2002, 409-420 (2011).
4. Hashimoto…Kunieda, et al. Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein. Nature Communications. Vol. 7, 12808 (2016).
5. Chavez…Kadonaga, et al. The tardigrade damage suppressor protein binds to nucleosomes and protects DNA from hydroxyl radicals. eLife. Vol. 8, e47682 (2019).

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