NEW YORK – A team from Japan has shown that several years of space radiation appeared to have manageable effects on freeze-dried mammalian sperm in terms of DNA damage and fertility, touting the results as evidence for the feasibility of a germ cell-based repository of animal DNA at a site in space.
In a study published in Science Advances on Friday, senior and co-corresponding author Teruhiko Wakayama, a researcher at the University of Yamanashi, and his colleagues called the early findings "essential and important for mankind to progress into the space age."
"To conserve genetic resources permanently … extraterrestrial preservation of germ cells is a new choice for our future," the authors wrote. "For this purpose, freeze-drying spermatozoa, oocytes, or embryos is a better solution than traditional cryopreservation."
The team reasoned that more work is needed to understand the effects that radiation and other environmental features of space can have on humans, pets, and livestock in advance of proposed missions to Mars and elsewhere, including potential consequences for sperm, eggs, and embryos transported into space.
"In the future, when the time comes to migrate to other planets, we will need to maintain the diversity of genetic resources, not only for humans but also for pets and domestic animals," the authors suggested.
"For cost and safety reasons, it is likely that stored germ cells will be transported by spaceships rather than by living animals," they added.
For their analyses, the researchers relied on DNA sequencing, immunostaining assays, and other methods to interrogate freeze-dried mouse sperm samples saved from dozens of male mice spanning four strains. They compared sperm samples that had taken a trip to the International Space Station for nine months, two years and nine months, or nearly six years with those that remained on Earth, as well as pups produced from the samples.
They also looked at the viability of mouse embryos generated from sperm samples that had been to space, along with the effects that X-ray radiation had on stored mouse sperm. Although embryos produced from sperm with long-term space radiation exposure seemed slightly less viable, there were no clear differences in mouse pups born after this process.
Compared to "ground-preserved" control samples on Earth, the team did not see a significant jump in DNA damage in freeze-dried sperm collected from 66 male mice after stints in space. Similarly, the fertilization potential appeared to be similar to the samples on Earth, though some chromosomal changes turned up as the time in space increased.
In addition, the researchers produced 168 mouse pups from freeze-dried sperm that was stored in space for five years and 10 months. Using RNA sequencing, they demonstrated that gene expression was similar in samples from eight pups produced from space sperm and eight control pups.
Those findings were bolstered by the team's X-ray radiation experiments on mouse sperm samples on Earth. While DNA damage did increase with longer radiation exposure times, for example, freeze-dried mouse sperm appeared far more resilient than radiation-exposed fresh sperm samples.
"Although there are differences between DNA damage from X-rays and space radiation, [we] can roughly predict that [freeze-dried] sperm can be preserved on the ISS over 200 years … even without any protection from space radiation," they wrote, though they cautioned that the space station "circles the Earth at an altitude of approximately 400 km [250 miles], which is inside the Earth's protective magnetic field."
The authors cautioned that additional research into radiation exposure at deep space sites will be needed in the future to further untangle the environment's effects on sperm, eggs, and embryos from mice and other animals. In a statement, they noted that ISS experiments focused on microgravity effects on embryos are anticipated later this year.
"When we can demonstrate the reliability and integrity of space-preserved [freeze-dried] sperm or germ cells, in the far future, underground storage on the Moon, such as in lava tubes, could be among the best places for prolonged or permanent preservation because of their very low temperatures, protection from space radiation by thick bedrock layers, and complete isolation from any disasters on Earth," the authors mused.