Tardigrades are able to tolerate almost complete dehydration by entering a reversible state called anhydrobiosis and resume their animation upon rehydration. To shed light on how tardigrades can endure extreme dehydration, researchers at the University of Tokyo explored proteins that form a gel during cellular dehydration.
This electron microscope image shows a tardigrade. Image credit: S. Tanaka / H. Sagara / T. Kunieda.
Water is an essential molecule for maintaining the metabolic activity and cellular integrity of living organisms.
Some organisms, however, can tolerate almost complete dehydration by entering a reversible state called anhydrobiosis.
Tardigrades, also known as water bears and moss piglets, are a prominent example of such organisms.
Under a drying environment, these small invertebrate animals gradually lose almost all body water and concurrently contract their bodies to a shrunken round form called a tun.
Dehydrated tardigrades are exceptionally stable and can withstand various physically extreme environments including exposure to space.
Even after exposure to extreme stressors, tardigrades can reanimate within a few dozen minutes after rehydration.
“Although water is essential to all life we know of, some tardigrades can live without it potentially for decades,” said Dr. Takekazu Kunieda, a researcher in the Department of Biological Sciences at the University of Tokyo.
“The trick is in how their cells deal with this stress during the process of dehydration.”
“It’s thought that as water leaves a cell, some kind of protein must help the cell maintain physical strength to avoid collapsing in on itself,” he added.
“After testing several different kinds, we’ve found that cytoplasmic-abundant heat soluble (CAHS) proteins, unique to tardigrades, are responsible for protecting their cells against dehydration.”
Recent research into CAHS proteins revealed that they can sense when the cell encapsulating them becomes dehydrated, and that’s when they kick into action.
CAHS proteins form gel-like filaments as they dry out. These form networks that support the shape of the cell as it loses its water.
The process is reversible, so as the tardigrade cells become rehydrated, the filaments recede at a rate that doesn’t cause undue stress on the cell.
Interestingly though, the proteins exhibited the same kind of action even when isolated from tardigrade cells.
“Trying to see how CAHS proteins behaved in insect and human cells presented some interesting challenges,” said Akihiro Tanaka, a graduate student in the Department of Biological Sciences at the University of Tokyo.
“For one thing, in order to visualize the proteins, we needed to stain them so they show up under our microscopes. However, the typical staining method requires solutions containing water, which obviously confounds any experiment where water concentration is a factor one seeks to control for. So we turned to a methanol-based solution to get around this problem.”
The team now plans to sift through more than 300 other kinds of proteins, some of which likely play a role in the incredible life-preserving ability of tardigrades.
“Everything about tardigrades is fascinating,” Dr. Kunieda said.
“The extreme range of environments some species can survive leads us to explore never-before-seen mechanisms and structures.”
“For a biologist, this field is a gold mine.”
The research is described in a paper in the journal PLoS Biology.
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A. Tanaka et al. 2022. Stress-dependent cell stiffening by tardigrade tolerance proteins that reversibly form a filamentous network and gel. PLoS Biol 20 (9): e3001780; doi: 10.1371/journal.pbio.3001780
