Tardigrades, also known as water bears, are microscopic animals that survive in environments lethal to most other life forms. These eight-legged invertebrates can be found in diverse locations, from deep-sea trenches to high mountain peaks. They are capable of withstanding the vacuum of outer space, intense radiation, and extreme temperatures, which raises the question of how their individual cells endure such harsh conditions.
The State of Suspended Animation
When faced with inhospitable conditions, some tardigrade species enter a state of suspended animation called cryptobiosis, a reversible state where metabolic processes cease. While several forms exist, the most studied is anhydrobiosis, a response to extreme dehydration.
During anhydrobiosis, the tardigrade retracts its head and legs, contracting into a compact, barrel-shaped form called a “tun.” This transformation minimizes surface area and is accompanied by the loss of over 95% of its water, reducing metabolic activity to undetectable levels. This process must occur slowly, allowing time for the production of protective molecules within its cells.
Cellular Protection Against Dehydration
Cellular survival during dehydration relies on molecules known as Intrinsically Disordered Proteins (IDPs). Unlike typical proteins with a fixed structure, IDPs are flexible, allowing them to adapt their shape to changing cellular environments. In tardigrades, these are called tardigrade-specific disordered proteins (TDPs).
A prominent family of TDPs is the Cytoplasmic-Abundant Heat Soluble (CAHS) proteins. As a tardigrade dries out, its cells produce large quantities of CAHS proteins, which form a gel-like network throughout the cytoplasm when water is scarce. This process, known as vitrification, turns the cell’s interior into a non-crystalline solid, or a biological glass.
This glassy state supports the cellular machinery, preventing membranes from fusing and proteins from denaturing. The vitrified matrix locks cellular components in place, preserving their structure until water becomes available again, at which point the matrix dissolves. This understanding of a protein-based glass has updated an older hypothesis that relied on the sugar trehalose, which is found in only low levels in many tardigrade species.
Guarding the Genetic Blueprint
Tardigrades possess a separate mechanism to protect their genetic material from radiation. High doses of radiation create reactive particles called hydroxyl radicals inside cells, which can shatter DNA strands. Tardigrades withstand radiation doses thousands of times greater than humans.
This ability is attributed to a unique protein called Damage suppressor (or Dsup), discovered in the species Ramazzottius varieornatus. Unlike CAHS proteins, Dsup’s function is localized to the cell’s nucleus. Research shows Dsup binds directly to chromatin, the structure packaging DNA inside the nucleus. By associating with chromatin, Dsup forms a protective cloud that physically shields the DNA, acting as a barrier against damaging hydroxyl radicals.
Potential for Human Application
The molecular mechanisms behind the tardigrade’s resilience have potential applications in medicine and biotechnology. The proteins that protect tardigrades could be used to stabilize sensitive biological materials. For instance, CAHS proteins could preserve human cells, tissues, and organs for transplantation without the damage often caused by freezing, which could extend the viability of donor organs.
These proteins also show promise for the storage and distribution of pharmaceuticals. Many medicines, like vaccines and protein-based drugs, are unstable and require constant refrigeration, a challenge known as the “cold chain.” Incorporating tardigrade proteins like CAHS may create formulations that are stable in a dry state at room temperature, making life-saving treatments more accessible in remote or developing regions.
The Dsup protein also offers potential for protecting human cells from DNA damage. When introduced into cultured human cells, Dsup has been shown to reduce DNA damage from X-ray radiation by about 40%. This raises the possibility of using Dsup to shield healthy tissues in patients undergoing radiation therapy for cancer, potentially reducing side effects.