The slow walkers continue to reveal their secrets

Genetic causes of invulnerability of slow walkers have been found

Oleg Lischuk, N+1

Japanese scientists have found out that slow walkers protect their DNA from the effects of ionizing radiation by stabilizing it with a special protein. In the experiment, this protein significantly reduced radiation damage to the genome of human cells. A number of other protective mechanisms were also found in the genome of slow walkers. The results of the work are published in the journal Nature Communications (Hashimoto et al., Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein).

Tardigrades (type Tardigrada) are microscopic (up to 1.5 millimeters in length, most are much smaller) eight–legged animals close to arthropods and onychophores. They are known for their exceptional survivability – being distributed all over the world, slow walkers prefer moist soils and moss, but can live both high in the mountains and on the ocean floor. If there is a lack of moisture or in other unfavorable conditions, the slow walkers lose up to 90 percent or more of the liquid and plunge into a state of anhydrobiosis, in which they are able to stay for years. In this state, they can withstand temperatures for a short time from almost absolute zero to 100 degrees Celsius, pressure from almost zero to 600 megapascals (about 6000 times more than atmospheric) and ionizing radiation of 5000 gray (a thousand times more than the lethal dose for most animals and humans). In addition, slow walkers are able to survive in organic solvents, under the influence of toxic gases and even in outer space. When favorable conditions occur, the tardigrades quickly return to life, retaining the ability to reproduce. At the same time, quite little is known about the reasons for such survivability.

The staff of the University of Tokyo with colleagues from other scientific centers conducted a complete decoding of the nuclear DNA of one of the most stable species of slow walkers (about a thousand of them are known in total) – Ramazzottius varieornatus. It turned out that 52.5 percent of genes have a common origin with other animals, 43.9 percent are unique or have minimal similarity, 2.4 percent are indeterminate and only 1.2 percent are borrowed by horizontal transfer from other realms of living beings. This refutes the conclusions of the American team that a large number of borrowed genes give vitality to slow walkers (these data have been questioned before).

R. varieornatus in states of activity (a) and anhydrobiosis (b),
and also the origin of his genes (c).

 Scientists have discovered a unique protein in the genome of the tardigrade, which they called Dsup (from English. damage suppressor – suppressing damage). It forms stable complexes with DNA, causing its aggregation in the nucleus. Researchers have suggested that this protein may have the function of protecting genetic material from radiation damage. To test this hypothesis, they introduced the Dsup gene into the culture of embryonic human kidney cells. Then transgenic and ordinary cells were irradiated with X-ray radiation at a dose of 10 gray (such a dose is fatal to humans). It turned out that in Dsup–expressing cells, single-stranded DNA fragmentation under the influence of radiation was 16 percent, and in ordinary cells - 33 percent. Thus, the slow-moving protein reduced the damage to the genetic material by more than half. Experiments with oxidants have shown that it also protects DNA from the action of reactive oxygen species (ROS). The researchers noted that previously no proteins with similar functions were found in any organisms.

In addition, a large number of protective genes were found in the genome of the tardigrade. In particular, it contains 16 enzymes from the family of superoxide dismutases that neutralize ROS. At the same time, in most animals their number does not exceed 10. Also, R. varieornatus turned out to have four copies of the MRE11 gene, which repairs double-stranded DNA breaks (most animals have only one copy of it).

The genetic material of tardigrades was selectively deprived of the peroxisomal fatty acid oxidation pathway, which under unfavorable conditions can contribute to the synthesis of ROS and oxidative stress. Also, these animals lack key components of the most important intracellular signaling pathway mTORC1, reacting to hypoxia, genotoxic and oxidative stress. At the same time, the components responsible for the recognition of amino acids and energy deficiency remained intact.

Lost (pink) and preserved (green) components of the mTORC1 signaling pathway

The revealed features of the genome largely explain the resistance of slow walkers to radiation and other damaging environmental factors. The researchers hope that the detection of Dsup will help to develop effective methods of protection against ionizing radiation and oxidative stress.

Earlier, scientists found out that the ability of slow walkers to fall into anhydrobiosis is associated with the presence of special "glassy" proteins that, in the absence of water, form a kind of coating that protects cell organelles and cell membranes.

Portal "Eternal youth" http://vechnayamolodost.ru  21.09.2016