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To introduce her children to the hidden marvels of the animal kingdom a few years ago, Anne De Cian stepped into her garden in Paris. Dr. De Cian, a molecular biologist, gathered bits of moss, then came back inside to soak them in water and place them under a microscope. Her children gazed into the eyepiece at strange, eight-legged creatures clambering over the moss.
“They were impressed,” Dr. De Cian said.
But she was not finished with the tiny beasts, known as tardigrades. She brought them to her laboratory at the French National Museum of Natural History, where she and her colleagues hit them with gamma rays. The blasts were hundreds of times greater than the radiation required to kill a human being. Yet the tardigrades survived, going on with their lives as if nothing had happened.
Scientists have long known that tardigrades are freakishly resistant to radiation, but only now are Dr. De Cian and other researchers uncovering the secrets of their survival. Tardigrades turn out to be masters of molecular repair, able to quickly reassemble piles of shattered DNA, according to a study published on Friday and another from earlier this year.
Scientists have been trying to breach the defenses of tardigrades for centuries. In 1776, Lazzaro Spallanzani, an Italian naturalist, described how the animals could dry out completely and then be resurrected with a splash of water. In the subsequent decades, scientists found that tardigrades could withstand crushing pressure, deep freezes and even a trip to outer space.
In 1963, a team of French researchers found that tardigrades could withstand massive blasts of X-rays. In more recent studies, researchers have found that some species of tardigrades can withstand a dose of radiation 1,400 times higher than what’s required to kill a person.
Radiation is deadly because it breaks apart DNA strands. A high-energy ray that hits a DNA molecule can cause direct damage; it can also wreak havoc by colliding with another molecule inside a cell. That altered molecule may then attack the DNA.
Scientists suspected that tardigrades could prevent or undo this damage. In 2016, researchers at the University of Tokyo discovered a protein called Dsup, which appeared to shield tardigrade genes from energy rays and errant molecules. The researchers tested their hypothesis by putting Dsup into human cells and pelting them with X-rays. The Dsup cells were less damaged than cells without the tardigrade protein.
That research prompted Dr. De Cian’s interest in tardigrades. She and her colleagues studied the animals she had gathered in her Paris garden, along with a species found in England and a third from Antarctica. As they reported in January, gamma rays shattered the DNA of the tardigrades, yet failed to kill them.
Courtney Clark-Hachtel, a biologist at the University of North Carolina Asheville, and her colleagues independently found that the tardigrades ended up with broken genes. Their study was published on Friday in the journal Current Biology.
These findings suggest that Dsup on its own does not prevent DNA damage, though it’s possible the proteins provide partial protection. It’s hard to know for sure because scientists are still figuring out how to run experiments with tardigrades. They cannot engineer the animals without the Dsup gene, for example, to see how they would handle radiation.
“We’d love to do this experiment,” Jean-Paul Concordet, Dr. De Cian’s collaborator at the museum, said. “But what we can do with tardigrades is still quite rudimentary.”
Both new studies revealed another trick of the tardigrades: They quickly fix their broken DNA.
After tardigrades are exposed to radiation, their cells use hundreds of genes to make a new batch of proteins. Many of these genes are familiar to biologists, because other species — ourselves included — use them to repair damaged DNA.
Our own cells are continually repairing genes. The strands of DNA in a typical human cell break about 40 times a day — and each time, our cells have to fix them.
The tardigrades make these standard repair proteins in astonishing large amounts. “I thought, ‘This is ridiculous’,” Dr. Clark-Hachtel recalled when she first measured their levels.
Dr. De Cian and her colleagues also discovered that radiation causes tardigrades to make a number of proteins not seen in other animals. For now, their functions remain mostly a mystery.
The scientists picked out a particularly abundant protein to study, called TRD1. When inserted in human cells, it seemed to help the cells withstand damage to their DNA. Dr. Concordet speculated that TRD1 may grab onto chromosomes and hold them in their correct shape, even as their strands start to fray.
Studying proteins like TRD1 won’t just reveal the powers of tardigrades, Dr. Concordet said, but could also lead to new ideas about how to treat medical disorders. DNA damage plays a part in many kinds of cancer, for example. “Any tricks they use we might benefit from,” Dr. Concordet said.
Dr. Concordet still finds it bizarre that tardigrades are so good at surviving radiation. After all, they don’t have to survive in nuclear power plants or uranium-lined caves.
“This is one of the big enigmas: Why are these organisms resistant to radiation in the first place?” he said.
Dr. Concordet said that this tardigrade superpower could just be an extraordinary coincidence. Dehydration can also break DNA, so tardigrades may use their shields and repair proteins to withstand drying out.
While a Paris garden may look to us like an easy place to live, Dr. Concordet said that it might pose a lot of challenges to a tardigrade. Even the disappearance of the dew each morning might be a catastrophe.
“We don’t know what life is like down there in the moss,” he said.
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Credit: NYTimes.com