Mice were flown aboard shuttle flights STS-133 in March 2011 and STS-135 in July 2011 as part of the Commercial Biomedical Testing Module-2 (CBTM-2) and CBTM-3 investigations into how space affects muscle and bones.
These sets of mice found second life, contributing to other studies through a tissue-sharing program.
Two studies used eye tissue from the mice to provide the first direct evidence that spaceflight causes cellular-level damage that has the potential to cause long-term vision problems.
Susana Zanello, Universities Space Research Association scientist at NASA's Johnson Space Center in Houston, examined eye tissue for changes in gene expression in the retina-the sensory tissue at the back of the eye.
Results from a study of mice from the second flight were detailed by Xiao W. Mao, MD, a researcher in the Division of Radiation Research at Loma Linda University and Medical Center in California, and her colleagues in "Spaceflight Environment Induces Mitochondrial Oxidative Damage in Ocular Tissue.
Both studies implicated oxidative stress in eye damage. Spaceflight exposes astronauts - and mice - to radiation, hypothermia, hypoxia and variations in gravity, all of which may play roles in tissue injury, and, in particular, oxidative stress.
In the STS-133 samples, Zanello found increased expression of genes involved in response to oxidative stress in retina tissue. "We saw this effect immediately after landing, which means it was a defensive increase in response to oxidative stress," she said.
Also notable was that a week after spaceflight, the response had decreased, indicating that the damage may be reversible. "That opens the door to the possibility of countermeasures, such as nutritional anti-oxidants," Zanello said.
The studies have been published in journal Gravitational and Space Research and in journal Radiation Research.