Keeping in mind the importance of preventing astronauts from harmful effects of space radiation during extended missions to the Moon or Mars, scientists have shown that radiation targets stem cells in the hippocampus, a brain area important for learning and mood control.
The new findings are based on a study of mice conducted by researchers from the Cold Spring Harbor Laboratory, Brookhaven National Laboratory, NASA's Kennedy Space Center and the McKnight Brain Institute of the University of Florida.
According to the researchers, identifying medications or physical shielding to protect astronauts from cosmic and solar radiation will be important for the success of human space missions beyond low Earth orbit.
"Our discovery does not present any adverse issues for the astronaut program because the ground-based dose and application of radiation we used were not comparable to that seen for existing space travel," said Dr. Dennis A. Steindler, executive director of UF's McKnight Brain Institute, a professor of neuroscience at the UF College of Medicine and co-investigator in the study.
"But the exceptional sensitivity of these neural stem cells suggests that we are going to have to rethink our understanding of stem cell susceptibility to radiation, including cosmic radiation encountered during space travel, as well as radiation doses that accompany different medical procedures," he added.
During the course of study, Cold Spring Harbor Laboratory scientists developed mice that were genetically engineered with easily identifiable, fluorescent stem cells. The stem cells lose their fluorescence when they transform into neurons, which makes it easier to account for them.
Scientists at the NASA Space Radiation Laboratory at the Brookhaven National Laboratory in Upton, New York, administered a single dose of radiation to the mice about equal to the amount astronauts would receive after a three-year space voyage to Mars.
Dr. Grigori Enikolopov, a neurobiologist at Cold Spring Harbor Laboratory, said that the study revealed that a special type of stem cell is selectively killed in the hippocampus. The cell is described as quiescent, or quiet, because even though it is the wellspring that repopulates the brain with new cells, it exists in relative repose while its daughter cells divide and reproduce in great numbers.
"Our findings are surprising because it is assumed that dividing cells are the most vulnerable to radiation — that is why radiation is used in cancer therapy. These stem cells divide quite rarely and it was unexpected that they would be the most vulnerable to this type of radiation. But at least two thirds of these quiescent cells died. The challenge now is to find something to protect those cells," Enikolopov said.
The researchers believe that the information that certain brain cells are at risk more than others is vital for them because it will assist them in planning lengthy lunar expeditions or deep space missions more effectively.
"Space radiation has not been a serious problem for NASA human missions because they have been short in duration or have occurred in low Earth orbit, within the protective magnetic field of the Earth," said Dr. Philip Scarpa, a NASA flight surgeon at NASA's Kennedy Space Center in Florida and a study co-investigator.
"However, if we plan to leave low Earth orbit to go back to the moon for long durations or on to Mars, we need to better investigate this issue and assess the risk to the astronauts in order to know whether we need to develop countermeasures such as medications or improved shielding. We currently know very little about the effects of space radiation, especially heavy element cosmic radiation, which is expected on future space missions and was the type of radiation used in this study," Scarpa said.
The finding raises questions about the cognitive and emotional risks associated with radiation exposure during human space exploration missions.
"There is a growing body of evidence that the death of these types of cells is a potential adverse effect of radiation during cancer treatment, but it's not been discussed in terms of space travel," said Dr. Jack M. Parent, a neurologist at the University of Michigan who was not involved in the research.
"Radiation has been associated with adverse cognitive effects, which is a potential hazard during space missions. Shielding and other measures to block the effects of radiation have to be strongly considered. The subject certainly deserves more study," he added.