Years after exposure to space radiation,
many astronauts' vision becomes clouded by cataracts. Understanding
why may shed light on cataracts suffered by elderly people.
by Patrick Barry and Dr Tony
Gazing out of their
space capsules, Apollo astronauts witnessed sights that humans had
never before seen. They saw the breathtaking view of the Earth's
bright blue disc against the inky black of space. They saw the far
side of the Moon. They also saw strange flashes of light inside
Since then, astronauts
aboard Skylab, the Shuttle, Mir, and the International Space Station
have all reported seeing these flashes. No need to call Agents Mulder
and Scully of The X Files, though: what the astronauts are experiencing
is space radiation zipping through their eyes like subatomic bullets.
When a "bullet" strikes the retina, it triggers a false signal that
the brain interprets as a flash of light.
Needless to say, this
is not good for your eyes. Years after returning to Earth, many
of these astronauts developed cataracts - a clouding of the lens,
which focuses light onto the retina.
At least 39 former
astronauts have suffered some form of cataracts after flying in
space, according to a 2001 study by Francis Cucinotta of NASA's
Johnson Space Center. Of those 39 astronauts, 36 had flown on high-radiation
missions such as the Apollo Moon landings. Some cataracts appeared
as soon as 4 or 5 years after the mission, but others took 10 or
more years to manifest.
Joseph Tanner photographed during a 1997 spacewalk.
Scientists have long
known of this link between radiation and cataracts, but they've
never fully understood it. What exactly does radiation do to the
lens of the eye to make it cloudy? Are astronauts' genes involved?
Solving this puzzle
might help people on Earth. Without ever traveling through space,
more than half of people older than 65 get cataracts; cloudy lenses
seem to be a natural result of aging. These old-age cataracts, some
of them, resemble the cataracts astronauts get. If researchers can
figure out what's happening inside astronauts' eyes, they might
be able to develop medicines to stop the process.
That goal is many years
away, though. First, "we have to understand the details - the genes
and proteins and molecular pathways involved," says Eleanor Blakely,
a scientist at the Lawrence Berkeley National Laboratory (LBNL).
Supported by a NASA grant, she and her colleagues
are experimenting with human eye tissue to learn these details.
"For good vision, the
lens of the eye must be crystal clear," she says. The lens, shaped
like an M&M, sits in the opening of the eye and focuses incoming
light onto the retina. "At the center of the lens are transparent
cells called 'fiber cells.' Damage to the lens can lead to clouding
of the fiber cells, and this change in clarity is called a cataract."
of the human eye.
In a healthy human
eye, new fiber cells are constantly manufactured to replace old
ones. The process begins with "epithelial cells," a type of stem
cell that coats the front of the lens. When needed, epithelial cells
flatten out and shed their nucleuses and other internal structures
to become transparent fiber cells. It's an amazing metamorphosis.
"During the final stages," she says, "all of the cell's organelles
are disposed of by the cell in a carefully orchestrated process
that leaves the cell alive, but basically a bag of crystalline proteins."
Blakely's group has
shown that a radiation dose can upset the metamorphosis of epithelial
cells, interfering with the formation of fresh fiber cells that
make up the body of the lens.
They didn't deliberately
irradiate people's eyes to figure this out, of course. Instead,
they cultured human epithelial cells in petri dishes. As some of
the cells were beginning to turn into fiber cells, her team exposed
the cells to controlled doses of radiation. This work was done at
LBNL and at the NASA Space Radiation Laboratory at the Brookhaven
National Laboratory in Long Island, New York. Afterward, they used
modern genetics tools to find out how the cells' genes and proteins
They've found that
a particular gene, Fibroblast Growth Factor 2 (FGF-2), jumps in
activity eight-fold following a radiation dose. FGF-2 normally helps
cells respond to stress. In this case, it seems to boost the activity
of two other genes called "p21" and "p57." These genes control crucial
events in the life cycle of a cell - e.g., when a cell divides
to form two daughter cells, or when an epithelial cell transforms
into a fiber cell. Blakely suspects that an imbalance of p21 and
p57 leads to the formation of abnormal fiber cells and, thus, cataracts.
It takes a while for
abnormal fiber cells to accumulate and visibly fog the lens. Years
go by after high radiation missions before astronauts notice their
cataracts. The time delay complicates research. It's hard to pinpoint
the cause when you have to wait years for the effect.
simplified cross-section of the lens.
Early detection of
cataracts is a goal of Rafat Ansari, a physicist at NASA's Glenn
Research Center, working independently of Blakely. He has developed
a laser probe that can detect signs of cataracts in humans years
before they become visible. It's being clinically tested now at
the National Eye Institute. Eventually, astronauts might carry such
a probe with them on space missions, checking their eyes as they
Another team of researchers,
led by Leo Chylack, Jr., MD, of the Center for Ophthalmic Research
at Brigham and Women's Hospital in Boston, is comparing cataracts
in astronauts, test pilots and members of the ground crew at the
Johnson Space Center. They'll take stereoscopic pictures of the
subjects' lenses to investigate differences in the type of cataracts
these people get - another piece of the puzzle.
is going back to the lab to learn more about the molecular underpinnings
of it all. "We're still establishing the links between changes in
FGF-2 and [the other genes]," she says. And there are other questions.
For instance, cataracts usually develop slowly, but could sudden
big doses of radiation accelerate the process? Astronauts taking
off on a 6-month trip to Mars will want to know.
Her group recently
won a four-year extension of their NASA research grant. Stay tuned