Dsup: What are water baers doing in space?

Whoever read the newspaper lately, probably came across this story: the BBC has written about it, but also The Guardian and the Washington Post: there might be life now on the moon! The questions: since when? from where? and what kind? are easy to answer: since April 2019, from Earth and it’s water bears or tardigrades. I still remember my biology undergrad classes: tardigrades were a highlight in them!

Unfortunately, since my specialization in molecular medicine, developmental biology and genetics I had nothing to do with these maximally cute microscopic creatures. That’s why I was all the happier to find more articles on tardigrades within the last couple of years. Above all, their incredible resistance to radiation was of interest. And this superpower is bestowed on tardigrades by this week’s gene Dsup, damage suppressor protein. Attention: the dsup gene should not be confused with the sup gene. Because sup encoded the SUPERMAN Protein of the thale cress, or arabidopsis thaliana, probably the most important plant model organism (more on that perhaps in another article).

Water bears, the perfect astronauts

Tardigrades are tiny. With the naked eye you cannot see them. However, they can be found almost everywhere. Well, at least everywhere where water is not too far away. So if you have access to a microscope anywhere, I would like to recommend you the following: get a piece of moss in the forest and put it in a little bit of water at home (not too much to dilute your potential tardigrade solution too much). After a few hours you search the water in the microscope. The chances are not bad that you will find tardigrades in it. Under the light microscope, they look something like this:

Video from https://upload.wikimedia.org/wikipedia/commons/1/15/Mikrofoto.de-baertierchen.ogv used under CC BY-SA 3.0 DE. Many thanks to Frank Fox (http://www.mikro-foto.de).

However, if there is no water around, tardigrades can outlast very long periods in an exceptionally dry state. This “cryptobiotic” condition, in which the metabolism of the animals is almost zero, has fascinated researchers for a long time and sometimes even initiates discussions on the definition of life and death. In addition, however, tardigrades can withstand extreme amounts of ionizing radiation. Ionizing radiation is so energetic that it is able to strike electrons from atoms or molecules; this makes it extremely dangerous for the genetic material of every living being. For radioactive gamma radiation, for example, approximately 5 Gray corresponds to a lethal dose in half of all exposed humans. Experiments have shown that this value is 1000 times higher in tardigrades, at around 5000 Gray. The combination of these two skills make tardigrades the ideal candidate for the job of an astronaut without (!) Spacesuit. In fact, Swedish and German scientists in the FOTON-M3 mission have been able to show that some of the accompanying tardigrades returned from the orbit relatively unimpressed by the lack of oxygen and the high radiation exposure and could indeed be “revived”. Already back then, in the artcile published in 2008, the authors speculated that probably either special configurations of DNA or exceptional DNA repair mechanisms are the basis for this.

The exceptional genetic makeup of water bears

There was considerable scientific dispute about the genome of the tardigrades: first, American researchers published a variant of the genome that suggested that tardigrades would have inherited an extremely high percentage of their genes via horizontal gene transfer from bacteria. Then a Japanese researcher suggested that this value came about because the tardigrade DNA of the American researchers were probably just contaminated with bacteria. Indeed, a British research team could not confirm the previous results and found no evidence for above-average horizontal gene transfer in the tardigrade genome.

Agnostic about this dispute, other Japanese scientists looked for genes in the tardigrade genome that looked somewhat new, i.e. of which not similar variants were found in other animals. They came across a protein that was localized directly on the DNA in both the tardigrades and also in human cells, if it were introduced there. Moreover, the human cells into which this protein was inserted, were significantly more resistant to gamma radiation; therefore, the gene got his name: damage suppressor (dsup).

Gamma radiation creates double-strand breaks in the DNA. We already know several genetic factors involved in the repair of DNA double-strand breaks (see, eg, BRCA), and therefore also factors that influence tolerance to this type of DNA damage. However, Dsup behaves differently: here researchers found much less DNA double-strand breaks in the cells with the tardigrade Dsup protein than in control cells immediately after irradiation. The time given to the cells was far too short, which makes it clear that Dsup is not involved in the repair of such DNA damage, but instead protects(!) the DNA directly from radiation-induced damage. Because Dsup’s structure is so unique that it can not be compared to already well-researched proteins, researchers still have no idea how Dsup is able to accomplish this amazing achievement.

Picture taken from Hashimoto, T. and Kunieda, T. (2017). DNA Protection Protein, a Novel Mechanism of Radiation Tolerance: Lessons from Tardigrades. Life 7. doi:10.3390/life7020026 under CC BY 4.0. Many thanks.

And why would water bears be on the moon now?

picture modified from Goldstein Lab via flickr

Well, in February 2019 again an unmanned probe was launched to the moon, this time it was an Israeli project. “Beresheet” had a kind of digital time capsule on board which contained lots of documentary material on human history and culture, including the entire English-language wikipedia, the Torah and testimonies of a Holocaust survivor. On 11 April, however, the landing of Beresheet went awry and the space capsule did not land, but crashed on the moon. It beacem known only recently, that at the last minute a few tardigrades were put between all the digital data media. Since then, humanity has been speculating as to whether these tardigrades might have survived the crash and if the moon could thus now host a “life form”. However, since tardigrades without water and oxygen enter the death-like dormant condition described above, one should not imagine their stay to be too lively.

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