How tardigrades survive dehydration

tardigrade

Electron micrograph of a tardigrade. © 2022 S Tanaka, H Sagara, T Kunieda

Whether in space or in the eternal ice: tardigrades can survive even in extreme environments. A key to this is their ability to allow themselves to be almost completely dried out. But how do their cells survive dehydration? Researchers have now discovered special proteins that allow the tardigrades to survive exceptionally well. During dehydration, these proteins form a protective gel that, without water, stiffens and supports cells. In this way, the cells escape otherwise lethal mechanical stress. Even in experiments with human cells, the tardigrade proteins retained at least some of their functionality.

Water is the basis of all life on earth. But some organisms are able to do without it completely at times. Tardigrades are true artists in this ability. The little animals, which are only about half a millimeter in size, can dry out almost completely in a waterless environment. They shrink in on themselves and resemble small barrels in this state. Their metabolism grinds to a halt, so they don't even need to breathe and can even survive in a vacuum. Some species can survive in this barrel state for decades. When water is available again, they come back to life within hours.

Trick against mechanical stress

But how do the tardigrades manage to protect their cells from the massive mechanical stress that accompanies dehydration? A team led by Akihiro Tanaka from the University of Tokyo in Japan has now discovered a new mechanism that enables the tardigrades to achieve their extraordinary survival skills. "The trick is how their cells deal with this stress during the dehydration process," explains Tanaka's colleague Takekazu Kunieda. "The assumption was that some type of protein must help the cell maintain its physical stability so that it doesn't collapse in on itself when the water leaves the cell."

Tanaka's team therefore tested various tardigrade proteins for the relevant abilities - and actually found what they were looking for: "After testing various species, we found out that so-called CAHS proteins, which only exist in tardigrades, are responsible for protecting their cells from drying out are,” says Kunieda. The abbreviation CAHS stands for "cytoplasmic-abundant heat-soluble proteins" - these are heat-soluble proteins that are abundantly present in the cell plasma of tardigrades.

Protective gel network

In several experiments, the researchers examined the functioning of the CAHS proteins. The result: During dehydration, they form a gel that wraps itself protectively around the inner structures of the cell. As the water disappears, the gel stiffens into cross-linked filaments that support the cell's shape. In this way, the mechanical stress is minimized. The process is completely reversible. As soon as the cells come into contact with water again, the filaments slowly dissolve and redistribute themselves in the cell plasma. The speed is adjusted so that the cell is not damaged.

For further experiments, the researchers tested to what extent the CAHS proteins can also fulfill their function in cells other than those of tardigrades. To do this, they inserted the proteins into human cell cultures. In order to make them visible under the microscope, they linked the CAHS proteins to a green fluorescent protein. And indeed: In human cells, too, the CAHS proteins are initially distributed in the cytoplasm in order to link to form filaments when there is a lack of water. When water was absorbed again, the filaments reformed again.

From the researchers' point of view, this result opens up new perspectives for the preservation of cell materials and biomolecules in the dry state. It would be conceivable to use the CAHS proteins to convert materials for research, but also medicines with a short expiry date, into a longer-lasting dry state and, if necessary, to mix them with water again before use.

Source: Akihiro Tanaka (University of Tokyo, Japan) et al., PLOS Biology, doi: 10.1371/journal.pbio.3001780

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