Just 13 days in space may be enough to cause profound changes in eye structure and gene expression, scientists have found.
Researchers from Houston Methodist Hospital, NASA Johnson Space Center, and other institutions looked at how low gravity and radiation and oxidative damage impacts mice.
"We found many changes in the expression of genes that help cells cope with oxidative stress in the retina, possibly caused by radiation exposure," said Houston Methodist pathologist Patricia Chevez-Barrios, the study's principal investigator.
"These changes were partially reversible upon return to Earth. We also saw optic nerve changes consistent with mechanical injury, but these changes did not resolve. And we saw changes in the expression of DNA damage repair genes and in apoptotic pathways, which help the body destroy cells that are irreparably damaged," she said.
To determine the impact of radiation exposure on eyes, Chevez-Barrios and lead author Susana Zanello, a space life scientist at NASA Johnson Space Center, examined mouse retinal gene expression on the 1st, 5th, and 7th days following a 13-day trip aboard space shuttle Discovery (STS-133), measuring indicators of oxidative and cellular stress.
Mice returning to Earth showed immediate evidence of oxidative stress in their retinas. But the increased expression of six oxidative stress response genes appeared to return to normal by the seventh day on Earth.
An indicator of oxidative stress in the cornea was also elevated one day after mice had returned from orbit, but returned to near-normal levels by the seventh day.
"This suggests oxidative stress in the retina and lens are at least partially reversible under the circumstances of the experiment," Chevez-Barrios said in the study published in the journal Gravitational and Space Research.
"This was after a relatively short time in orbit. We don't know if damage caused by longer periods of oxidative stress will be more severe," she said.
In the mice that had been to orbit, the researchers also found an increase in beta-amyloid in their optic nerves, and this increase persisted after seven days on Earth.
Beta-amyloid is tied to traumatic brain injury in humans, and was not detected in the mice that remained on Earth.
The researchers also found an increased number of glial cells - cells that respond to injury - in the optic nerves of mice that had been to space.
They also found orbiting mice were expressing elevated levels of caspase-3 in the retinal pigment epithelium.
Abnormalities of the epithelium are associated with development of age-related macular degeneration.
Researchers also found changes in cell and tissue shape and fluid balance similar to what has been reported from previous studies, specifically studies of astronauts who had experienced optic nerve changes.
