According to researchers from Houston Methodist, NASA Johnson Space Center and two other institutions, two weeks in space might be enough to produce changes in eye structure and gene expression. The current study observed how low gravity and radiation and oxidative stress effects mice in eye-related gene expression and cell behaviors after space travel.

According to Patricia Chévez-Barrios, M.D., pathologist and principal investigator at Houston Methodist, “We found many changes in the expression of genes that help cells cope with oxidative stress in the retina, possibly caused by radiation exposure. 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.”

Studies as far back as 2001 have shown that astronauts are at risk of developing eye issues such as premature age-related macular degeneration. It is believed these issues are due to low gravity, heightened solar radiation exposure, or both.

An ophthalmology study of seven astronauts in November 2011 (a NASA-sponsored study), demonstrated that all seven astronauts presented with eye problems after spending at least six months in space. Physicians observed flattening of the back of the eyeball, folding of the choroid (capillaries), excess fluid around the optic nerve, as well as swelling

Oxidative stress can occur with high-energy radiation by generating harmful chemical reactions in cells. Under normal conditions, the Earth’s atmosphere reflects and/or absorbs harmful radiation and protects us from damage. Interestingly, metal hulls of space craft and the International Space station do not afford such protection.

Damage to the eyes of astronauts doesn’t just affect their long-term health but can pose problems for future missions. Any loss of visual acuity would make it difficult to complete long-term space travel such as the future Mars mission.

If low gravity and radiation are truly to blame, building a more protective haul would be beneficial as well as providing a spinning wheel that simulates gravity.

Chévez-Barrios and lead author Susana Zanello, Ph.D., a space life scientist at NASA Johnson Space Center, looked at mouse retinal gene expression on the 1st, 5th, and 7th days following a 13-day trip aboard space shuttle Discovery (STS-133) and measured indicators of oxidative and cellular stress. They also observed the eyes and surrounding tissues for changes in structure that might relate to low gravity damage. Two controls were established on Earth. In one situation, mice were maintained in the same condition as those on board the shuttle and the other control kept mice as they are normally kept on Earth.

The researchers report that mice coming back to Earth presented with evidence of oxidative stress in their retinas. Nevertheless, the increased expression of six stress response genes returned to normal by the seventh day. Another indicator of oxidative stress in the cornea was also increased one day after mice had returned, but returned to normal by the seventh day.

2001-human-eyeChévez-Barrios notes, “This suggests oxidative stress in the retina and lens are at least partially reversible under the circumstances of the experiment. 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. Only more studies with longer exposure times may help answer this question.”

The researchers report an increase in beta-amyloid in the optic nerve of mice who were in space and this increase continued after seven days on Earth. Beta-amyloid is linked to traumatic brain injury in humans. Mice that remained on Earth did not have an increase in beta-amyloid. Additionally, there was an increase in glial cells in the optic nerves of mice who went to space. It is uncertain whether the beta-amyloid and glial cell increases were due to sustained low gravity or occurred during flight to or from Earth orbit. Moreover, orbiting mice expressed increased levels of caspase-3 in retinal pigment epithelium. Caspase-3 is an enzyme precursor that aids the immune system to kill damaged cells (apoptosis). Damage to retinal pigment epithelium is linked to development of age-related macular degeneration.

Chévez-Barrios notes, that the current study had a small sample size so the study should be considered preliminary. “We think our results are plausible based on what we know from previous studies of structural changes and damage caused by oxidative stress and changes in the eyes of astronauts returning to Earth, but additional experiments are needed to confirm what we are reporting about gene expression, cellular behavior and mechanisms of damage.” Moreover, the particular strain of mice they used are particularly sensitive to light and that the oxidative stress would be less in healthy human eyes.

Sources:

1. Bio News:
   The effect of low gravity in space.