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.
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.