How Do Tardigrades Withstand Drying?

by Elio
 

Recently, tardigrades or water bears made the news because they had been loaded on a space capsule headed for the moon. Alas, the spacecraft crashed on its way down and the fate of the ti­ny passengers is unknown. This gives us a reason for revisiting these fascinating creatures and to bring the topic up to date. They have been introduced here in a fine article by Christoph. For a re­view, see here (Open Access).
 

These small animals (they are about ¼ to ½ millimeters in length) are remarkable for their ability to withstand drying, extreme heat and cold, and even some ionizing radiation. Their ability to sur­vive in a dry state makes them especially suitable for space travel. They can go without water for years on end and happily swim away once water is provided. What mechanisms are involved has been subject of speculation and investigation for over 250 years. In some species, a disaccharide called trehalose, known to be essential for withstanding drying in some other organisms has been implicated here too. However, some species of tardigrades lack this compound, so the search for other compounds has been on. In a detailed study, Boothby and collaborators reported that a class of proteins known as 'Tardigrade Intrinsically Disordered proteins' (TDPs) become enriched in the desiccation process and are required for survival.
 

A Published data on the survival versus relative humi­dity for Hypsibius dujardini (red), Parama­cro­biotus richtersi (green), and Milnesium tardi­gradum (black). Ani­mals desiccated at lower re­la­tive humi­dity experi­ence increased rates of drying compared with those desiccated at hig­her relative humidity. B Survival of H. dujardini after slow drying (95% relative humidity), quick drying (70% RH), and slow followed by quick dry­ing. C MA plot showing en­richment (log2-fold change) versus ab­undance (log2 CPM) of expressed H. dujardini genes under hydrated and dry conditions. Colored cir­cles indicate CAHS (red), SAHS (blue), and MAHS (green) ge­nes encoding tardigrade-specific IDPs. Source. Frontispiece: Actinarctus doryphorus (ma­r­ine tardi­gra­de) autofluorescence of cuticle. Magn. 40×. Image by A. Schmidt-Rhaesa C. Schulze, R. Neves. Source

TDPs are expressed constitutively at high levels in many species of tardigrades. These proteins vitrify (become glass-like) upon drying, which may explain their protec­tive ability, as vitrification keeps harmful ice needles from forming. What are intrinsically disordered proteins? Unlike the typical globular proteins, these have no persistent tertiary structure but essentially flop around in the en­vi­ronment.
 

Several facts speak in favor of the role of these proteins in surviving drying. Thus, tardigrades upregulate the ex­pression of TDP genes in response to drying. Sure enough, transcription and translation are required for survival. Tolerance to drying diminishes when TDP genes are disrupted (using RNAi) In addition, heterologous ex­pression of TDP genes in several prokaryotic and eukar­yotic cells (E. coli, yeast, and human cells in culture) in­creases their tolerance to drying. Convincing, no?
 

One of the most telling findings has been that tardigra­des that are dried rapidly die faster than when dried slowly, suggesting that a protective substance needs to be made in order to allow protection. Boothby and col­laborators analyzed the gene expression in the transcrip­tomes of slowly dried specimens and controls, finding that 11 of 17 cytosolic abundant heat soluble (CAHS) protein transcripts were enriched 4- to 22-fold during desiccation. Three gene families, CAHS, SAHS, and MAHS, had been previously identified in a proteomic analysis of tardigrades by Yamaguchi et al. in 2012 and Tanaka et al. in 2015. That IDPs lack persistent secondary structure was confirmed using nuclear magnetic resonance spec­troscopy (NMR) heat solubility experiments, circular di­chroism spectropolarimetry, and a technique called back­bone proton-deuterium exchange. So, there is now good evidence for role of this special class of proteins in pro­tec­tion from desiccation.
 

The authors say: 'We anticipate that these findings will build a foundation for pursuing long-term goals of the desiccation tolerance field, including the engineering of desiccation-tolerant crops and the development of techno­logies for the dry preservation of pharmaceuticals, cells, and tissues.' It may seems a bit startling that the study of such lowly little animals has contributed to our understanding of such basic biological attributes as protein structure. But microbiologists are used to that.
 

To return for a moment to the lunar tardigrades, it is of course unknown if they survived, as their site won't be revisited for quite some time. If shielded from radiation by space craft debris, they might withstand the conditions of drying and extremes of temperature. For those of us concerned with issues of cosmic pollution, the NASA Office of Planetary Protection limits its concerns to the con­tamination of sites that may conceivably harbor life, the moon not being one of them (sure enough, on its surface there are bags containing pounds of human feces). Yet we must be thankful to the tardigrades for reminding us of the gravity of such issues in space exploration.