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UNC-Led Study Finds Proteins That Help Tardigrades Survive Drying Out

NEW YORK (GenomeWeb) – A new study has identified proteins that tardigrades have evolved to survive extreme dryness.

Tardigrades, also called water bears, are a phylum of microscopic animals renowned for their ability to survive in the most extreme conditions, even outer space. On earth, they can be revived after completely drying out for years. Now, a gene-expression study using RNA sequencing from Illumina has helped identify the proteins that allows tardigrades to survive desiccation.

Led by first author Thomas Boothby and senior author Bob Goldstein of the University of North Carolina at Chapel Hill, an international team of researchers identified "intrinsically disordered" proteins (IDPs) that are expressed in multiple tardigrade species during desiccation, the process of drying out. Included in the differentially expressed proteins were cytosolic abundant heat soluble (CAHS) proteins and secreted abundant heat soluble proteins. These proteins become glass-like inside cells, which helps protect the water bears. The team published its results today in Molecular Cell.

The tardigrade-specific disordered proteins (TDPs) may even be able to protect proteins prone to dryness-related damage in vitro, with potential applications in transporting pharmaceuticals.

Other species, including Caenorhabditis elegans larvae, can survive extremely dry conditions by producing a sugar, trehelose, which prevents protein aggregation or misfolding. Trehelose also helps these organisms survive by forming glass-like solids without an ordered solid state.

However, research suggests that tardigrades produce very low levels of trehelose, or none at all. Proteins, including heat-shock response proteins and anti-oxidant proteins, have been proposed as tardigrade protectors.

"The question has been, 'If tardigrades aren't relying on trehelose to survive desiccation, what do they use instead?'" Boothby said in a statement.

Special proteins had been identified that contribute to tardigrade hardiness, such as DNA-binding damage suppressing proteins identified by a Japanese team in September 2016.

The UNC-led researchers looked at gene expression of the tardigrade species Hypsibius dujardini under a variety of conditions. They extracted tardigrade RNA using the Epicenter MasterPure RNA purification kit; constructed transcriptome, assembly, and differential expression analysis sequencing libraries using the Illumina mRNA TruSeq v2 kit; and sequenced on the Illumina HiSeq 2000 platform.

After finding the upregulated TDPs in H. dujardini tardigrades that were in the process of drying out, they looked in two more species, Paramacrobiotus richtersi and Milnesium tardigradum,with the same results.

M. tardigradum even expresses CAHS proteins at a continuously high level, conferring it an even greater survival advantage in dry conditions. "We think it can do this because it has so many of these proteins around already and doesn't need time to make them," Boothby said.

IDPs have no fixed 3D structure, the authors wrote, arranging themselves into amorphous solids, like glass.

To validate their finding, the researchers transplanted the tardigrade-specific IDP genes into single-celled organisms and tested their resilience to drying out. They also used RNAi to disrupt IDP gene function, which "severely diminished desiccation tolerance," the authors wrote.

The researchers also tested whether purified tardigrade IDPs would protect proteins in vitro, using them to surround lactate dehydrogenase (LDH) enzyme prior to drying out and then rehydrating it. "When desiccated and rehydrated by itself, the activity of LDH is reduced to [less than 1 percent]," the authors wrote. But the more tardigrade protein added beforehand, the more enzyme activity was preserved. "Importantly, our experiments show that at a sufficient concentration, [tardigrade] proteins preserve 100 percent of lactate dehydrogenase activity and do so an order of magnitude better than trehelose and bovine serum albumin."

Boothby noted that the ability to stabilize sensitive pharmaceuticals in a dry state is of personal importance. "I grew up in Africa, where lack of refrigeration in remote areas is a huge problem. These real-world applications are one of the things that led me to study tardigrades."

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