Guide:
Why can tardigrades survive dehydration and then come back to life when exposed to water? On this question, the scientific community has clashed many times on experiments and ideas, but the answer is still in the balance.
Written by丨Wang Zhirong
Responsible editor丨Chen Xiaoxue
In the recent hit sci-fi TV series "The Three-Body Problem", the trisolarans who were tested in a harsh environment shouted "I'm dehydrated!" After that, the body trembled and burst out of the water, turning into a pair of leather bags that could be rolled up, which was convenient for companions to carry and convenient for centralized storage, which was surprising and shocking. When the environment was suitable in the Heng Era, the king gave an order, and the dry body poured into the water, and they were resurrected in the filling of the fountain of life.
Wang Miao witnessed the trisolarans on the ground dehydrated and turned into dry fibers.
The Trisolarans are revived after being immersed in water. Source: "The Three-Body Problem", Tencent Video
Is it biologically possible for a trisomy to be so dehydrated and then resurrected by immersion in water?
The answer is yes. In fact, there is a magical creature in nature that can also use dehydration to stop almost all metabolic activities of the body and survive the harsh environment - this is the famous water bear.
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Magical water bears
The tardigrade is a tiny creature of the tardigrade phylum with an adult length of only about 0.5 mm and is almost indistinguishable with the naked eye without the help of a microscope. There are currently 1380 known species of tardigrades, some living in seawater, some in fresh water, and some in moist moss on land, the key is that they are inseparable from water.
Source: References[5]
The picture above is a crawling tardigrade, transparent throughout. At first glance, the tardigrade looks like a Michelin-tire-tire mascot, or chubby bear, with four pairs of eight stocky feet and small paws at the ends. In 1773, German zoologist Johann August Ephraim Goeze named the animal "Little Water Bear".
Two water bears in normal condition. Source: References[4]
In extreme environments, tardigrades shed most of the water in their bodies and can survive extreme cold, heat, dryness, high pressure, lack of oxygen, and even vacuum cosmic rays. Scientists speculate whether there is a protective substance that is activated after tardigrades encounter extreme environments to protect their water-deprived cells.
The upper right is the water bears in normal state, and the lower left is the dehydrated water bears. Source: https://www.americanscientist.org/article/tardigrades
Unlike extremistic organisms such as thermophiles, tardigrades are not adapted to live stably in extreme environments, but use their resources to withstand extreme tests; It's like the trisolarans are dehydrating in the chaotic era just to survive, waiting for the development of Hengjian production. Therefore, a reasonable guess is that the substance that protects the tardigrade in the dehydrated state is usually not needed; It was only made when harsh conditions arrived—that is, the gene encoding the protein specifically increased expression at a critical moment.
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Discover the secrets of tardigrades?
In 2017, the laboratory of American scientist Bob Goldstein found that a class of CAHS proteins (cytosolic-abundant, heat-soluble proteins) that are specifically highly expressed during the drying and dehydration of tardigrades may be the answer.
They found that when the external environment deteriorated, the expression of CAHS genes in tardigrades increased, ensuring that tardigrades entered a protective state of dehydration. Of course, this does not indicate a causal relationship between environmental degradation and high CAHS gene expression, perhaps only temporally.
Fortunately, there are also some tools to verify the function of genes on tardigrades.
Verifying biological causality requires both "necessity" (no no, no, no) and "sufficiency" (added elsewhere). Goldstein's laboratory used RNA interference to reduce the expression of some CAHS genes, and found that the survival ability of tardigrades decreased significantly, which indicates that CAHS proteins are indeed indispensable; By adding these genes to cells such as yeast and bacteria that do not have CAHS genes, these single-celled organisms gain a stronger ability to survive drought. Lactate dehydroxylase reduces enzyme activity to 1% in dry environments, while mixing with CAHS protein protects enzyme activity from destruction. In other words, the CAHS protein puts a protective coat on other proteins or cells against drying and dehydration.
So, what is the process of putting on a "protective suit" after a tardigrade dehydrated?
The Goldstein lab believes that tardigrades form amorphous solids during the process of drying and dehydration, as if there is no formation of ice crystals during the rapid cooling of water, so as to achieve a transparent glass-like solid state, preserving the structure of protein and lipid membranes in cells, that is, "vitrification". During the dehydration of tardigrades, CAHS proteins are also vitrified and participate in protection. Of course, this protection is limited, if the ambient temperature is too high, exceeding the "glass transition temperature" (refers to the temperature at which a glassy substance is converted from a glass state to a highly elastic state), the vitrification is destroyed, similar to a crystalline solid crossing the melting point and becoming liquid. Whether in tardigrades or single-celled yeast, when the ambient temperature exceeds the glass transition temperature of the CAHS protein, the protective power is destroyed and the organism cannot survive.
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Is the answer reliable?
Does this mean that the secret weapon of the tardigrade's resurrection after dehydration has been revealed?
Four years later, two Japanese scientists wrote to the magazine questioning the 2017 article by Bob Goldstein's lab. The point of doubt is mainly in the "vitrification" theory at the end of the paper. They believe that the glass transition temperature measured by differential scanning calorimetry in the original text to reflect the nature of vitrification is not unique to CAHS protein, and many proteins in the body (such as human growth hormone or bovine serum albumin) or in vitro proteins (such as silk or spider silk) can also be, so it cannot be said that CAHS protein can be vitrified to protect dehydrated tardigrades.
The authors of the original paper responded that this refutation is somewhat untenable, because substances that exhibit similar physicochemical properties do not necessarily have to perform the same biological functions, and considering other components in the microenvironment, the variables will be greater, and CAHS proteins are probably only an important component of protection, not the only factor. That is, the glass transition temperature of dehydrated tardigrades is not necessarily a linear superposition of the glass transition temperature of CAHS proteins and other protective components.
In terms of experiments, the two Japanese scientists questioned two parts, one is to detect CAHS protein vitrification, and the other is to detect tardigrade vitrification, the focus is on the water content of the sample. As the temperature used during the measurement gradually rises to 200 degrees Celsius, the remaining water molecules in the solid sample evaporate, affecting the reading of the data curve. They repeated the experiment in the 2017 paper and added an indicator to monitor the change in quality during warming, and found that if the purified CAHS protein sample was thoroughly dried, the glass transition temperature would no longer exist. However, the data from the CAHS protein sample containing water were consistent with the Bob Goldstein lab paper. It makes sense to point out the parameter of water content, because this is a very different point in experimental conditions and needs to be tightly controlled. The usual weather forecast often says that the air humidity is a relative percentage, the common laboratory humidity is about 35%, and for the tardigrades living in extremely humid environments, 95% humidity is already a slow drying stimulus, 70% is a more deadly quick-drying stimulus.
For the test of tardigrade vitrification, Japanese scientists were unable to repeat the original results because no significant glass transition temperature was seen. After asking the original authors for the original data for analysis, they concluded that the glass transition temperature of 98 degrees Celsius reported in the original article was caused by a small noise in the reading curve and was not reliable.
The image on the left is the original report from the 2017 article, with gray shades indicating the glass transition temperature. The figure on the right shows the re-analysis of the original data by Japanese scientists in 2021, and the red arrow refers to the glass transition temperature reported in the original article. Pay attention to the scale of the vertical axis. Compared to the large changes in the higher temperature range that have been omitted, the temperature fluctuation around 98 degrees Celsius is somewhat insignificant. Source: References[2]
This is somewhat an indication that the original author is suspected of manipulating the data. Because under the experimental conditions in the original article, with specific tardigrade types, the survival rate of dried tardigrades will drop off a cliff when the outside temperature is heated to more than 90 degrees Celsius. The glass transition temperature of around 98 degrees Celsius seems to correspond to the temperature at which the survival rate plummets, thereby strengthening the correlation between vitrification and dehydration protection.
In the 2017 paper, the survival curve of tardigrades as a function of temperature after undehydration and dehydration, gray longitudinal shades indicate the measured tardigrade glass transition temperature. Source: References[1]
After receiving the challenge letter, the original authors responded-for-tat. For testing the vitrification of tardigrades, they believe that the Japanese scientists' repeated experiments were insufficiently sampled, using different drying conditions, and there were no controlled experiments to prove that dehydrated tardigrades could later be revived.
For the test of CAHS protein vitrification, the original author also came to the experimental data of Japanese scientists to re-analyze and map. By stretching the scale of the vertical axis to make the particle size smaller, it was shown that the glass transition temperature occurred in both the completely dehydrated and the protein sample containing a certain amount of moisture in the original experiment. Moreover, completely dry samples are known to shift to higher temperatures, so a peak shift of 80 degrees Celsius to 95 degrees Celsius, and a peak shift of 160 degrees Celsius out of the measurement range also makes sense. This refutes the Japanese scientists, who believe that completely dry samples do not have glass transition temperatures, and that the result of CAHS protein vitrification is the illusion of water content. Since there is no strict and uniform statistically significant standard for such measurement results, it is impossible to accurately define a glass transition temperature, so the two sides have different opinions and cannot reach an agreement.
The left figure shows the repeated experimental data conducted by Japanese scientists in 2021, and the right figure shows the reanalysis of the data on the left by American scientists in 2021. Pay attention to the scale of the vertical axis. Source: References[3]
Why can tardigrades survive dehydration and then come back to life when exposed to water? To sum up, scientists in both the United States and Japan agree that water plays a complex and important role in this, but whether water is cause or effect, and the vitrification of CAHS proteins to protect dehydrated tardigrades, requires careful experimentation and careful consideration. CAHS protein or vitrification is also obviously not the only solution, and whether it works with other protective components needs to be explored.
bibliography
1.Boothby, Thomas C et al. “Tardigrades Use Intrinsically Disordered Proteins to Survive Desiccation.” Molecular cell vol. 65,6 (2017): 975-984.e5.
2.Arakawa, Kazuharu, and Keiji Numata. “Reconsidering the "glass transition" hypothesis of intrinsically unstructured CAHS proteins in desiccation tolerance of tardigrades.” Molecular cell vol. 81,3 (2021): 409-410.
3.Boothby, Thomas C. “Water content influences the vitrified properties of CAHS proteins.” Molecular cell vol. 81,3 (2021): 411-413.
4.Gabriel, Willow N et al.“ The tardigrade Hypsibius dujardini, a new model for studying the evolution of development.” Developmental biology vol. 312,2 (2007): 545-59.
5.Goldstein, Bob. “Tardigrades and their emergence as model organisms.” Current topics in developmental biology vol. 147 (2022): 173-198.
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