Tardigrades, commonly known as ‘water bears,’ are eight-legged microscopic invertebrates renowned for their ability to withstand extreme stressors. Limb retraction and substantial decreases to their internal water store results in the tun state, greatly increasing their ability to survive. Emergence from the tun state and activity regain follows stress removal, where resumption of life cycle occurs as if stasis never occurred. However, the mechanisms through which tardigrades initiate tun formation are still poorly understood. New research demonstrates that tardigrade tun formation is mediated by reactive oxygen species.
Tardigrade observed using a confocal fluorescent microscope; the tardigrade was overexposed to 5-MF, a cysteine selective fluorescent probe, that allows for visualization of internal organs. Image credit: Smythers et al., doi: 10.1371/journal.pone.0295062.
Tardigrades are famous for their ability to withstand extreme conditions, and can survive freezing, radiation, and environments without oxygen or water.
They persist by going dormant and entering a tun state, in which their bodies become dehydrated, their eight legs retract and their metabolism slows to almost undetectable levels.
Previously, little was known about what signals tardigrades to enter and leave this state.
“Tardigrades are a phylum of eight-legged microscopic invertebrates renowned for their remarkable ability to survive extreme environmental stressors,” said Dr. Amanda Smythers from the University of North Carolina at Chapel Hill and colleagues.
“This survival is rooted in their ability to initiate cryptobiosis, a physiological state wherein metabolism slows to near undetectable conditions, enabling long-term survival despite inhospitable conditions.”
“Although some eukaryotes and bacteria are capable of cryptobiosis, no eukaryotes are able to do so across the entirety of their lifespan, including as eggs, juveniles, and adults, or in response to such a broad range of stressors as tardigrades.”
“Thus tardigrades’ ability to survive desiccation, freezing, oxygen starvation, fluctuating osmotic pressure, and ionizing radiation (via anhydrobiosis, non tun-forming cryobiosis, anoxybiosis, osmobiosis, and irradiation-induced dormancy, respectively) is matched by none.”
In their study, the authors exposed model tardigrade species, Hypsibius exemplaris, to freezing temperatures or high levels of hydrogen peroxide, salt or sugar to trigger dormancy.
In response to these harmful conditions, the animals’ cells produced damaging oxygen free radicals.
The researchers found that tardigrades use a molecular sensor — based on the amino acid cysteine — which signals the animals to enter the tun state when it is oxidized by oxygen free radicals.
Once conditions improve and the free radicals disappear, the sensor is no longer oxidized, and the tardigrades emerge from dormancy.
When the scientists applied chemicals that block cysteine, the tardigrades could not detect the free radicals and failed to go dormant.
Altogether, the new results indicate that cysteine is a key sensor for turning dormancy on and off in response to multiple stressors, including freezing temperatures, toxins and concentrated levels of salt or other compounds in the environment.
The findings suggest that cysteine oxidation is a vital regulatory mechanism that contributes to tardigrades’ remarkable hardiness and helps them survive in ever-changing environments.
“Our work reveals that tardigrade survival to stress conditions is dependent on reversible cysteine oxidation, through which reactive oxygen species serve as a sensor to enable tardigrades to respond to external changes,” the authors said.
The results were published in the journal PLoS ONE.
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A.L. Smythers et al. 2024. Chemobiosis reveals tardigrade tun formation is dependent on reversible cysteine oxidation. PLoS ONE 19 (1): e0295062; doi: 10.1371/journal.pone.0295062
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