Life Without Water
1Tardigrades, often called water bears, are microscopic animals rarely more than half a millimeter long. They have eight stubby legs tipped with claws, and they are found almost everywhere: in ocean sediments, in hot springs and, most famously, in the thin films of water that cling to mosses and lichens. That last habitat is an unstable one. A clump of moss on a sunny wall can be soaked in the morning and dry by afternoon, and most animals in such a place would simply die. Many tardigrade species instead enter a state called anhydrobiosis, literally “life without water,” in which they lose nearly all of their body water, show no measurable metabolism, and can remain for months or years before reviving within hours of being wetted again.
2The transition begins with a change in shape. As its surroundings dry, a tardigrade pulls in its head and legs and contracts into a compact, barrel-shaped form known as a tun. The tun has a much smaller surface area than the active animal, so it loses water more slowly. That slowness turns out to matter a great deal. In laboratory experiments, tardigrades dried gradually over many hours usually recover, whereas animals dried within minutes usually do not, even when both groups end up with the same final water content. The difference suggests that survival is not a passive consequence of becoming dry. Rather, the animal appears to need time to prepare, and researchers have spent several decades trying to work out what that preparation involves.
3To appreciate the problem, it helps to consider what water does inside a cell. Water is not simply a filler in which molecules float. Its hydrogen bonds help hold proteins in their folded, working shapes, and its interactions with the charged heads of membrane lipids keep membranes flexible and fluid. When most of the water is removed, proteins tend to unfold and stick to one another in useless clumps, membranes can become leaky or fuse together, and DNA strands can break. Any organism that survives drying must therefore protect its molecules against all of these forms of damage at once, and it must be able to undo whatever damage does occur once water returns.
4For many years the leading explanation centered on a sugar. Trehalose, a disaccharide made of two glucose units, accumulates to high levels in several organisms that tolerate drying, including the dormant cysts of brine shrimp, some nematode worms and baker’s yeast. Two complementary ideas were proposed to explain how trehalose protects cells. According to the water-replacement hypothesis, the sugar forms hydrogen bonds with proteins and membranes in place of the departing water, helping them keep their shapes. According to the vitrification hypothesis, the sugar turns the drying cytoplasm into a glass. A glass, in the physical sense, is a solid that keeps the disordered molecular arrangement of a liquid rather than the orderly lattice of a crystal. Because a glass is so viscous, molecular motion within it almost stops, and the chemical reactions that would ordinarily destroy unfolded proteins are slowed to a crawl.
5Tardigrades complicated this tidy picture. When researchers measured trehalose in a range of tardigrade species, they found that many produce only small amounts, and in some species it could not be detected at all. Whatever allows these animals to survive drying, then, trehalose could not be the whole story, and in many species it seemed to play little part. The search therefore shifted from measuring known protective molecules to asking a broader question: which genes do tardigrades switch on when they begin to dry out?
6The answer included a set of proteins found in no other group of animals. Most proteins fold into a characteristic three-dimensional shape, but these tardigrade-specific proteins are intrinsically disordered: in solution they have no fixed structure and behave more like flexible, shifting chains. They were grouped into families according to where they occur, for example cytosolic abundant heat soluble (CAHS) proteins in the cytoplasm and secreted abundant heat soluble (SAHS) proteins outside cells. Two kinds of experiment pointed to their importance. When the genes for CAHS proteins were silenced in tardigrades using RNA interference, fewer animals survived drying. And when the same genes were inserted into yeast and bacteria, which normally tolerate drying poorly, the modified microbes survived desiccation markedly better than unmodified ones. A single protein from a tardigrade, in other words, conferred part of the tardigrade’s resilience on an unrelated organism.
7How a protein without a fixed shape could protect other proteins is still being worked out. One proposal is that the disorder is itself the point. As water is lost and the proteins become more concentrated, CAHS proteins appear to gain structure and link together, forming a gel and eventually a glass-like solid that immobilizes the cell’s other contents. In this view, tardigrades achieve with proteins what brine shrimp achieve largely with sugar. Not all of the evidence fits neatly, however. Some studies have found that the degree of vitrification correlates well with how much protection a protein provides, while others have found protective proteins that form glasses no better than unprotective ones. Many researchers now suspect that the proteins work in several ways at once, perhaps also acting as molecular shields that keep unfolding proteins from clumping together.
8Drying is not the only insult tardigrades withstand. Some species survive doses of ionizing radiation hundreds of times greater than would kill a human. Part of the explanation may lie in another tardigrade-specific protein, named Dsup, for “damage suppressor.” Dsup binds to chromatin, the complex of DNA and proteins in the nucleus, and when it was produced in cultured human cells, those cells suffered roughly 40 percent fewer DNA breaks after exposure to X-rays than cells without it. Because drying itself breaks DNA, many biologists regard radiation tolerance not as an adaptation to radiation, which tardigrades rarely meet at such levels in nature, but as a by-product of the machinery that evolved to cope with desiccation. Notably, Dsup has been found in only some tardigrade lineages, a reminder that different species may have arrived at their toughness by different routes.
9These discoveries have attracted interest well beyond zoology. Many vaccines, enzymes and protein-based drugs must be kept refrigerated from factory to clinic, a requirement that is costly and difficult to meet in regions with unreliable electricity. If tardigrade proteins, alone or mixed with sugars, could stabilize such products in a dry state at room temperature, this “cold chain” might be shortened or avoided. Early laboratory studies have reported that CAHS proteins can help preserve the activity of a dried enzyme and of a blood-clotting protein. Even so, the step from a promising test-tube result to a reliable product is long. The tardigrade itself relies on a coordinated response involving many molecules, and it would be surprising if a single borrowed protein reproduced all of its protection.
10Perhaps the most lasting lesson of tardigrade research concerns proteins in general. For much of the twentieth century, biochemists assumed that a protein’s function depends on its folding into a precise shape, the way a key depends on its cut. Intrinsically disordered proteins, now known to be common in many organisms, challenge that assumption, and the tardigrade proteins offer an especially vivid example: molecules whose usefulness seems to emerge precisely as their surroundings change. A creature smaller than a grain of sand has thus helped to reshape a basic idea about how the molecules of life can work.