Tiny tardigrades can survive conditions that would kill most other forms of life. By expelling their body's water and transforming into a seemingly lifeless ball called a tun, they enter a state of dried-up suspended animation in which they can survive for decades without food and water and withstand extreme temperatures, pressures and even the vacuum of space. However, little is known about what drives this protective mechanism and what keeps tardigrades from succumbing to the stresses of prolonged desiccation.
Now, a new study reveals how tardigrades survive without any water at all: Unique proteins turn the insides of tardigrade cells into gel, thereby preventing the critters' cell membranes from crinkling and collapsing. This strategy is completely different from those seen in other types of animals that can survive dry periods.
In fact, "no such proteins have been reported in other desiccation-tolerant organisms," said Takekazu Kunieda, a biologist at the University of Tokyo who led the new research, published Sept. 6 in the journal PLOS Biology.
Tardigrades, also known as water bears or moss piglets, are a group of microscopic animals with plump bodies and eight legs ending in disproportionately delicate claws. They're famously resilient, able to survive exposure to space, freezing temperatures, and boiling for an hour (though they can be killed by longer exposure to hot water).
Scientists have long been interested in how tardigrades do this. Many animals that can survive long periods of desiccation, such as aquatic crustaceans known as brine shrimp, use sugars called trehalose to essentially freeze their cells in a glass-like state that protects their inner workings until the animals are exposed to water again.
But tardigrades don't have much trehalose. What they do have are numerous proteins not found in other animals. These proteins are hard to understand, because in a non-tun tardigrade, they appear disorganized and disordered, though a 2017 genetic study found that some of these disordered proteins seem to promote a glassy state in dried-out tardigrades, much like trehalose does in other animals.
The new research focused on a group of tardigrade-specific proteins known as cytoplasmic-abundant heat-soluble (CAHS) proteins. In tardigrades, these proteins float around the cytoplasm, or liquid filling the cells. Kunieda and his colleagues discovered these proteins a decade ago, and other research groups found that the proteins were involved in the survival of tardigrades during desiccation. But no one knew how.
Kunieda and his team ended up circling back to CAHS proteins while looking for tardigrade proteins that changed form upon stress. They identified more than 300, and CAHS proteins were among them.
To learn what CAHS proteins do to protect tardigrades under duress, the researchers dehydrated CAHS-carrying cells and analyzed how the proteins changed. They found that when the cells were threatened with desiccation, these proteins condensed, forming a network of filaments. These filaments shored up the cell, transforming the cytoplasm into a gel-like state and preventing the cell from collapsing as water leached out. This condensation happened in minutes and reversed just as quickly. Within six minutes of rehydration, a cell could be up and running normally again.
In their experiments, the researchers found that CAHS could make insect cells more resilient to desiccation, but those CAHS-enhanced cells still weren't as tough as tardigrade cells. That means CAHS wasn't working alone, Kunieda told Live Science.
"It seems obvious for me that other factors are needed to reproduce the tolerant ability of tardigrades," he said.
Fortunately, there are plenty of tardigrade proteins to study; the researchers identified more than 300 that react to stress. Future findings could have applications beyond tardigrades — for example, to help scientists develop better preservatives for improving the shelf life of vaccines and medication, Kunieda said.
Originally published on Live Science
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Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.