Comparative transcriptomics reveal a novel tardigrade-specific DNA-binding protein induced in response to ionizing radiation
Tardigrades are microscopic creatures known for their unique ability to survive even the strongest of conditions. One of these conditions is an absurd amount of radiation, up to 4000+ Gy, an amount capable of shredding human DNA. Their ability to survive has kept them alive for millions of years and could help with future space travel and medicine, prompting scientists to dive deep into the secrets of the Tardigrade.
The researchers in this study were trying to find out how tardigrades survive extreme doses of ionizing radiation that can shred DNA. They wanted to know what genes made this possible and what type of DNA protection this was — shielding or repairing? This is significant because understanding the genes in tardigrades helps researchers figure out how they can survive harsh conditions. Learning how these genes work helps their understanding of how fast DNA repair is and how effective Dsup shields the DNA when radiation strikes a tardigrade. TDR1 is a novel gene that protects tardigrades from even the most extreme conditions. This could help humans in the future in risky situations such as space travel and cancer treatments.
In diagram A, the tardigrades were targeted with 100 and 1000 Gy of ionizing radiation (IR), with results from a group where DNA breakage is shown with phospho-H2AX (a biomarker) and from the control group, which is shown from the diagram with Tubulin. The more phospho-H2AX there is (the darker the bands are), the more DNA breakage there is. In the diagram with 100 Gy, we can see that the bands get slightly darker and then get more faint, which shows that there was DNA breakage when IR was introduced but then there was less breakage as time progressed. Similarly when 1000 Gy was used, the DNA breakage was very strong but then cleared after some time. In both diagrams the damage peaks 4 hours after IR is introduced and mostly improves after around 24 hours. Tubulin was used as a loading control to ensure equal protein concentration across all lanes, confirming the changes observed in phospho-H2AX is reliable and accurate.
In diagram B, they tested Dsup and TDR1 in human cancerous cells because they multiply very quickly, which makes testing quicker and more efficient. The labels for the rows are different dyes and biomarkers that make the features of the cells easier to see under a microscope. Hoechst is used just so we can see the shape of the cells, GFP shows how much Dsup (which acts as a shield against IR) or TDR1 (which binds broken DNA together, making it much easier to repair) is present (the more brightly colored the cells are, the more Dsup or TDR1 is expressed), and Phospho-H2AX shows how much DNA breakage there is. The first column is the control group, where there were no genes that were added. There was very little glowing color (GFP), meaning that there wasn’t much radio-resistance. In that column we can also see that there were a lot more red spots in the phospho-H2AX image, which means that there was high DNA breakage since there wasn’t any Dsup or TDR1. In the second column, the Dsup protein was added to the cells. In the third column, the TDR1 protein was added to the cells. In both of these there is much more GFP shown in the images because there is more neon color. Therefore, we can also see that for these columns there are less red spots (phospho-H2AX), indicating that there’s less DNA breakage because there was more Dsup and TDR1, which implies that Dsup was able to shield the radiation, and TDR1 was able to bind the broken DNA together for repair. This study demonstrates that Dsup and TDR1 can be used in humans and can have the same effect in humans as in tardigrades.
By measuring the γH2AX at 30 minutes, 4 hours, 8.5 hours, 24 hours and 73 hours, the scientists found out that tardigrades repair their cells quickly instead of protecting them from radiation. They continued measuring γH2AX over several days, and noticed that it slowly returned to almost-normal levels, and realized that, even after facing 1000 Gy of radiation, most of the DNA damage done to the tardigrade was repaired within 24–73 hours. This demonstrates the tardigrades’ unusually efficient DNA repair systems. By using whole-mount immunolabeling to visualize γH2AX inside different tissues and compare the tardigrade’s body cells with its gonad cells, the scientists figured that most of the tardigrades’ body cells recover, with the exception of its reproductive cells. The researchers reckon this explains why heavily irradiated tardigrades survive, but become sterile. They observed that γH2AX remained high in gonads 72 hours after 1000 Gy and compared this with previous observations that irradiated tardigrades become sterile. This means that cells trying to copy their DNA were more vulnerable to the radiation, because it can interfere with the replication process. Hence, DNA repair is powerful, but cannot restore every single cell.
This research is important because the protein, TDR1, repairs DNA damaged due to radiation. When TDR1 was put inside human cells, it had fewer DNA breaks. This can be beneficial to humans because it can be used to save them from dangerous amounts of radiation, too. It can help astronauts stay safer from radiation in outer space. Also, if tardigrade repair proteins were added to human cells, the healthy cells will be protected from radiation used to destroy cancer cells. This study further enhances research of biology and genes, and opens up new doors.
In this research paper, many new discoveries were made, such as the DNA shield Dsup and a new protein found only in tardigrades called TDR1, which helps in bunching up broken DNA until they can be repaired. When exposed to ionizing radiation, the tardigrade’s DNA repair genes went into overdrive, being overexpressed after being hit with the ionizing radiation. This seems interesting, as it establishes that repairing broken DNA is the key to radiation survival, even if it doesn’t completely block out all damage. Scientists also implemented these genes into human cells, which increased resistance to radiometric drugs, showing that implementing these proteins and genes into human cells is a valuable tool. It could also be studied and replicated into pharmaceutical drugs, but for any use in humans we must understand all effects that implementing these genes may have on the body. Surprisingly, a protein that destroys remaining DNA repair proteins after their job is done called RNF146 is found in tardigrades as well as humans. This shows that humans also have DNA repair genes, however they are nowhere near strong enough to give us efficient radiation survivability. Overall, humans have potential for DNA repair to maintain a damage control system for ionizing radiation, as seen with the few DNA repair genes we have, but genetic modification or specific drugs are necessary to harness the full potential of DNA repair.