What makes tardigrades so tough?

No beast on Earth is more robust than the little tardigrade. It can survive being frozen at -272° Celsius, being exposed to the vacuum of space, and even being irradiated with 500 times the dose of X-rays that would kill a human.

In other words, the creature can withstand conditions that don’t even exist on Earth. This unearthly durability, combined with their alluring appearance, has made tardigrades a favorite among animal lovers. But beyond that, researchers are looking to microscopic mite-sized animals to learn how to prepare humans and cultures to cope with the rigors of space travel.

The tardigrade’s indestructibility stems from its adaptations to its environment – which may seem surprising since it lives in seemingly cozy places, such as the cool, wet clumps of moss that dot a garden wall. In homage to such habitats, along with the fluffy appearance, some people call tardigrades water bears or, delightfully, moss pigs.

But it turns out that the tardigrade’s moist, moss-covered home can dry out many times each year. Desiccation is quite catastrophic for most living things. It damages cells in some of the same ways as freezing, vacuum, and radiation.

On the one hand, drying leads to high levels of peroxides and other reactive oxygen species. These toxic molecules cut the cell’s DNA into short fragments – just as radiation does. Desiccation also causes cell membranes to wrinkle and crack. And it can cause delicate proteins to unfold, rendering them as useless as crumpled paper airplanes. Tardigrades have developed special strategies to deal with these types of damage.

As the tardigrade dries, its cells create long, cross-linked proteins (shown) that soften and protect cell membranes.M. Yagi-Utsumi et al/Scientific reports 2021

As the tardigrade dries, its cells secrete several strange proteins that are unlike anything found in other animals. In water, proteins are soft and shapeless. But when the water disappears, the proteins self-assemble into long, crisscrossing fibers that fill the interior of the cell. Like peanuts wrapped in Styrofoam, the fibers support cell membranes and proteins, keeping them from breaking or unfolding.

At least two species of tardigrades also produce another protein found in no other animal on Earth. This protein, called Dsup, short for “damage suppressor,” binds to DNA and can physically protect it from reactive oxygen species.

Emulating tardigrades could one day help humans colonize space. Food crops, yeast and insects can be engineered to produce tardigrade proteins, allowing these organisms to grow more efficiently in spacecraft where radiation levels are elevated compared to Earth.

Scientists have already inserted the gene for the Dsup protein into human cells in the laboratory. Many of these modified cells survive the levels of X-rays or peroxide chemicals that kill normal cells (SN: 11/9/19, p. 13). And when inserted into tobacco plants — an experimental model for food crops — the gene for Dsup appears to protect the plants from exposure to a DNA-damaging chemical called ethyl methanesulfonate. Plants with the extra gene grow faster than those without it. Plants with Dsup also suffer less DNA damage when exposed to UV radiation.

Microscopic tardigrades can withstand freezing cold, desiccation and extreme levels of radiation thanks to unique molecular adaptations.VIDEOLOGIA/ISTOCK/GETTY IMAGES PLUS

Tardigrade ‘peanut packaging’ proteins show early signs of being protective for humans. When modified to produce these proteins, human cells become resistant to camptothecin, a chemotherapy agent that kills cells, researchers reported March 18 ACS Synthetic Biology. The tardigrade proteins did this by inhibiting apoptosis, a cellular self-destruction program often triggered by exposure to harmful chemicals or radiation.

So if humans are ever to reach for the stars, they may accomplish the feat, in part, by standing on the shoulders of the little eight-legged endurance experts in your backyard.

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