Cryptobiosis
Tardigrades, although microscopic and obscure, are becoming well known as an incredible “immortal” animal. If you search “tardigrade” on Google, a t-shirt with a tardigrade and the slogan “live tiny, die never” pops up. Among this result are curious discussions wondering if tardigrades can be killed and even articles about tardigrades being sent to space. Are tardigrades actually invincible superheroes? Tardigrades are part of a group of organisms, which also includes tiny worms and yeast, capable of a phenomenon called cryptobiosis (Koshland & Tapia, 2019). Cryptobiosis is a biological state where all biological processes come to almost a complete standstill, without stopping altogether (Figure 1). When something is in a cryptobiotic state, metabolism, reproduction, development, and repair all take a pause. In this state, organisms can survive extreme conditions, such as complete dehydration (desiccation), extreme temperatures and pressures, and a lack of oxygen. Essentially, cryptobiosis is an extreme version of hibernating animals sleeping through winter.
Figure 1. Tardigrades in an active state (left) and a cryptobiotic state (right).
This scanning electron microscope image shows cryptobiosis in tardigrade species Milnesium tardigradum. In cryptobiosis, the tardigrade is shrunken and lacks most body details. This tardigrade entered cryptobiosis by dehydration (Beisser et al., 2012).
Researchers do not know the complete story of how cryptobiosis works. Most studies have looked into the extreme survival capabilities of nematode C. elegans and yeast. Most species have to be prepared to enter cryptobiosis; either by starvation or the gradual changing of conditions (Koshland & Tapia, 2019). This is most likely because organisms need time to produce specialized molecules that allow them to survive extreme conditions (Figure 2). In the case of extreme dehydration, desiccation tolerance requires tolerance against a vast array of stresses. This is because water is needed in many biological processes and structures to support life. Two of these stresses are protein aggregation and loss of membrane integrity. In C. elegans and yeast, the sugar trehalose and small disordered proteins called hydrophilins are involved in the cryptobiotic response to desiccation. Trehalose and hydrophilins form a glassy substance in a process called vitrification, which may physically prevent protein aggregation and membrane breakdown by keeping everything from moving. Similar molecules are found in most organisms capable of cryptobiosis, including tardigrades (Koshland & Tapia, 2019).
Figure 2. Tardigrades need time to prepare for extreme conditions.
Tardigrades only survived desiccation when dried gradually because they were able to upregulate TDP expression. TDPs (tardigrade intrinsically disordered proteins) are proteins that help tardigrades enter cryptobiosis (Boothby et al., 2017).
Although not as easy to work with in the lab, some studies have specifically looked at the cryptobiotic response in tardigrades. Interestingly, trehalose is not found to increase significantly during desiccation in tardigrades despite its importance for desiccation tolerance in other species. So, tardigrades must have their own method. In 2012, a group of researchers found two protein families expressed in tardigrades during desiccation. Cytoplasmic Abundant Heat Soluble (CAHS) and Secretory Abundant Heat Soluble (SAHS) are intrinsically disordered proteins that form specific structures able to interact with many other proteins when dehydrated (Yamaguchi et al., 2012). In 2017, another researcher found another group of proteins called Tardigrade Intrinsically Disordered Proteins (TDPs) that increase in expression during desiccation. Like hydrophilins, TDPs protect tardigrades through vitrification and also hydrogen bond interactors in place of water. Interestingly, tardigrades without TDPs cannot survive desiccation, and non-cryptobiotic cells were able to survive desiccation when given TDPs (Boothby et al., 2017). There are likely many more factors that allow these species to survive extreme conditions that are yet to be discovered.
“Live tiny, die never” is an apt slogan for tardigrades, because they can survive some pretty extreme conditions. Three tardigrades were found in a frozen moss sample collected in Antarctica and recovered after being frozen for 30.5 years, and even went on to reproduce. Usually, long-term cryptobiotic survival is limited due to uncontrolled oxidative damage. But, at freezing conditions, oxidative damage may be slowed and reduced to allow for longer survival. These tardigrades had a fairly slow recovery after being rehydrated and warmed up, probably because their bodies were repairing damaged tissues and organs (Tsujimoto et al., 2016). Tardigrades can also survive extreme radiation, hot and cold temperatures, pressures, and treatment with noxious chemicals. To explore the idea of tardigrade survival in space, researchers in 2012 studied the survival of both tardigrade adults and eggs under these extremes. The cryptobiotic tardigrades survived intense radiation (gamma rays, x-rays, protons, and high-LET heavy ions), treatment with methyl bromide and organic solvents, temperatures ranging from -273C to +151C, and pressures up to 7.5 GPa (Horikawa et al., 2012). Maybe tardigrades will be the first astronauts that don’t need a suit!
References
Beisser, Daniela & Grohme, Markus & Kopka, Joachim & Frohme, Marcus & Schill, Ralph & Hengherr, Steffen & Dandekar, Thomas & Klau, Gunnar & Dittrich, Marcus & Müller, Tobias. (2012). Integrated pathway modules using time-course metabolic profiles and EST data from Milnesium tardigradum. BMC systems biology. 6. 72. 10.1186/1752-0509-6-72.
Boothby, Thomas C., Hugo Tapia, Alexandra H. Brozena, Samantha Piszkiewicz, Austin E. Smith, Ilaria Giovannini, Lorena Rebecchi, Gary J. Pielak, Doug Koshland, and Bob Goldstein. “Tardigrades Use Intrinsically Disordered Proteins to Survive Desiccation.” Molecular Cell 65, no. 6 (2017): 975-984.e5. https://doi.org/10.1016/j.molcel.2017.02.018.
Horikawa, Daiki D., Ayami Yamaguchi, Tetsuya Sakashita, Daisuke Tanaka, Nobuyuki Hamada, Fumiko Yukuhiro, Hirokazu Kuwahara, et al. “Tolerance of Anhydrobiotic Eggs of the Tardigrade Ramazzottius varieornatus to Extreme Environments.” Astrobiology 12, no. 4 (2012): 283–89. https://doi.org/10.1089/ast.2011.0669.
Koshland, Douglas, and Hugo Tapia. “Desiccation Tolerance: An Unusual Window into Stress Biology.” Edited by Doug Kellogg. Molecular Biology of the Cell 30, no. 6 (March 15, 2019): 737–41. https://doi.org/10.1091/mbc.E17-04-0257.
Tsujimoto, Megumu, Satoshi Imura, and Hiroshi Kanda. “Recovery and Reproduction of an Antarctic Tardigrade Retrieved from a Moss Sample Frozen for over 30 Years.” Cryobiology 72, no. 1 (2016): 78–81. https://doi.org/10.1016/j.cryobiol.2015.12.003.
Yamaguchi, Ayami, Sae Tanaka, Shiho Yamaguchi, Hirokazu Kuwahara, Chizuko Takamura, Shinobu Imajoh-Ohmi, Daiki D. Horikawa, et al. “Two Novel Heat-Soluble Protein Families Abundantly Expressed in an Anhydrobiotic Tardigrade.” Edited by John R. Battista. PLoS ONE 7, no. 8 (August 28, 2012): e44209. https://doi.org/10.1371/journal.pone.0044209.