ARTICLE IN BRIEF
Transgenic Alzheimer's disease mice exposed to a dose of galactic cosmic radiation equivalent to what astronauts on a trip to Mars might experience showed an increase in amyloid-beta plaques and cognitive impairment.
With talk of a space trip to Mars, a team of scientists from the University of Rochester Medical Center (URMC) has simulated a scenario of radiation exposure in deep space. They report that animals exposed to a dose of galactic cosmic radiation equivalent to what astronauts on a trip to Mars might experience showed an increase in amyloid-beta (Abeta) plaques and cognitive impairment. The transgenic animals tested were destined to develop Alzheimer's disease with two mutations known to trigger the disease. But the researchers showed in the Dec. 31 online issue of PLoS One that the accumulation of Abeta plaques was greater than expected, as were the ensuing cognitive problems.
While the researchers did not study the effects on a non-Alzheimer's model, the finding raises the possibility that a long space trip — it would take three years to travel from Earth to Mars and back — might increase an astronaut's risk for cognitive problems decades down the road.
M. Kerry O'Banion, MD, PhD, a professor in the department of neurobiology and anatomy at URMC, who led the study, said that they are now doing similar studies with animals that are not going to develop Alzheimer's disease (AD) pathology.
In deep space, there is no shelter from an environment filled with low but continuous levels of galactic cosmic radiation (GCR), explained Dr. O'Banion. “GCR is made of high-energy, high-charged (HZE) particles that contain a variety of different elements, including 56Fe particles.”
THE STUDY METHODOLOGY
In the current study, the scientists examined the effects of 56 Fe particle irradiation in a double transgenic mouse model with amyloid precursor protein (APP) and presenilin 1 (PS1) mutations. They found six months after exposure to 10 and 100 cGy 56Fe radiation at 1GeV/μ that the mice had difficulty with behavioral studies testing fear-conditioned responses and novel-object recognition. [For context, 1 Gy equals 1 rem, and 1 SV equals a dose of 1GY; one head CT exposure is 2.34-3.78 cGY.]
In male animals that were sacrificed, they found an acceleration of Abeta deposition, which did not appear to arise from increased levels of APP. There was also no difference in microglia activation, suggesting that inflammation was not a trigger for their findings.
They observed endothelial activation after the male mice were exposed to 100 cGy radiation, which suggests alterations in Abeta trafficking through the blood-brain barrier.
Dr. O'Banion and his colleagues said that this is the first evidence “that HZE particle radiation can increase amyloid beta plaque pathology in an APP/PS1 mouse model of AD.” The study was supported by a grant from the NASA Space Radiobiology Research Program.
There are reasons to suspect radiation may have toxic effects on brain tissue. Other scientists have demonstrated that 56Fe particle radiation can trigger neuroinflammation. Radiation has been known to cause leaks in the blood-brain barrier and can alter DNA and RNA and cause genetic changes in the expression of proteins.
Dr. O'Banion and his colleagues wanted to test the effects of an acute low dose of HZE particle radiation to see whether it had other effects on brain tissue that could cause neurodegenerative changes. They chose an AD model as a specific approach to measure radiation's influence on disease progression. They could assess behavior and then look at pathology. And their findings could help them make some leap of research faith between what they see in animals and the men and women who will be exposed to GCR in deep space and Mars.
THE STUDY APPROACH
The work was carried out in collaboration with scientists at the NASA Space Radiation Laboratory at Brookhaven National Laboratory. Forty-nine male and female double transgenic mice bred at the Jackson Laboratory were sent to Brookhaven at approximately three months old. At about 3.5 months of age they were exposed to a beam of radiation at a dose equivalent to three years of chronic exposure in deep space. After the radiation, the animals were shipped to URMC, where cognitive and behavioral tests were done before they were sacrificed. The females were euthanized at seven months due to some deaths in the test subjects. Males were 9.5 months of age.
At the Brookhaven laboratory, the male mice received total doses of either 10 cGy or 100 cGy. Female mice received only the 100 cGy dose. Control AD mice were sham irradiated.
The URMC investigators conducted two types of memory tests. The first test, designed to capture hippocampal-dependent memory, was a contextual fear-conditioning paradigm that revealed significant differences in the sham versus irradiated animals. The male animals that were not exposed to radiation had significantly more freezing behavior than those that had been irradiated (p =.0118).
In the study, the mouse was acclimated to a cage with a wire grid in the floor. It then received a foot shock paired with a noise stimulus. Then, they put the mouse back in the same chamber 24 hours later and asked whether it “froze,” a behavioral response consistent with fear. This “contextual fear-conditioning” test asks if the mouse recognizes the place where it was shocked. As a control the mouse is placed into an entirely different kind of chamber and freezing is recorded. In this case, the mouse usually goes about its normal behaviors.
The irradiated females actually showed slightly more freezing than the control animals, though this was not statistically significant.
They also put the transgenic animals through a novel object recognition test. Animals tend to be more interested in a novel object when given a choice between something they have experienced and something they haven't. The irradiated animals were no more likely to explore a novel object than would have been expected by chance. In other words, they did not remember the object.
After the memory tests, their brains were harvested for study. The investigators used a variety of techniques to quantify the amyloid plaque load in the cortex and the hippocampus. They also measured glial activation.
They used two different markers to stain dense fibrillar plaques. At 9.5 months, the exposure of male mice to 100 cGy of radiation was sufficient to cause a significant increase of 38 percent in Congo red staining and a 54 percent increase in amyloid plaque burden, said Dr. O'Banion. By comparison, the 7-month old females did not show any significant difference in Congo red or plaque burden, he added.
They also looked for evidence of microglia that could explain the difference the increase in amyloid deposition following exposure to radiation. They measured the biomarker CD68, but found no increase of the biomarker in or near the plaques.
If it wasn't microglia, what was going on? They looked to the brain's vasculature for the answer. They identified increases in a marker, ICAM-1, of endothelial activation, which Dr. O'Banion, an expert on inflammation and AD, said might indicate central nervous system damage or inflammation.
Abeta is cleared from the brain through the blood-brain barrier and any alteration in the vascular system could compromise the clearance of amyloid beta. They looked at a trafficking protein called low-density lipoprotein receptor-related protein 1, but there was no evidence that the levels were different in the irradiated animals.
The scientists are now repeating the study in wild-type animals. Without this data it is impossible to tell whether the “cognitive impairment resulted from radiation alone or represented a synergy between radiation and mutant AD gene expression in these mice,” they wrote in the paper.
Also, it was hard to compare the differences between males and females as they were euthanized at different ages. Repeating the study may also help them figure out why several of the female animals died early.
“We don't think it was the radiation, rather it is known that mice harboring these transgenes are susceptible to early death. However, further studies will be required,” Dr. O'Banion said.
Probably one of the biggest problems of the study is that they delivered an acute dose of radiation rather than the low, constant levels that astronauts would be exposed to on the way to and from Mars. The doses are comparable to what astronauts would be exposed to over the three-year trip.
“While many of the pathological processes are believed to be similar, this model does not reflect the complete human condition,” the scientists added.
EXPERTS WEIGH IN
David Robertson, MD, professor of medicine, pharmacology and neurology and director of the Center for Space Physiology & Medicine at Vanderbilt University Medical Center in Nashville, TN, has spent much of his career studying autonomic diseases. His interest in blood pressure regulation prompted his study of astronauts in space. He worked on NASA's Neurolab project, in which four astronauts were successfully monitored for sympathetic activity during a space flight; it helped scientists on earth understand the nature of the changes in blood pressure regulation.
He worries about the effects of radiation on astronauts on a long mission. “This is a well-studied example of what can happen to the brain after exposure to space radiation,” Dr. Robertson said. “This is a big deal and we should worry about it.”
“These are the kinds of studies that help us appreciate the kinds of damage that radiation under certain circumstances can do,” he added.
Canadian neurologist Roberta Bondar, MD, PhD, agreed. Dr. Bondar was the first neurologist in space, part of the space shuttle Discovery mission in 1992 and for more than a decade she directed the international space medicine research team. She specializes in neuro-ophthalmology. On her mission, she was able to conduct studies to measure acute cognitive changes.
“The long-term effects are difficult to measure,” she said. “There is so much that we don't know about the long-term effects of how biological tissue responds to radiation exposure. People haven't been flying that long but NASA has longitudinal studies in place to look for any signals of increased cancers and other medical conditions among astronauts.”
She said that this study “raises the flag and should be replicated. But these clues give us ways to approach future studies.”
“We need to understand as much as we can about the long-term effects of radiation before we send people into space for a long period of time,” Dr. Bondar added.
LISTEN UP, TUNE IN: Michael K. O'Banion, MD, PhD, a professor in the department of neurobiology and anatomy at the University of Rochester Medical Center, discusses the methodology and findings of a study in which investigators exposed transgenic Alzheimer's disease mice to levels of radiation similar to that experienced by astronauts in space; they reported an increase in amyloid-beta plaques and cognitive impairment. What are the implications for human space travel? Listen on for Dr. O'Banion's data: http://bit.ly/X2LUtG.