What is Space Radiation and Why Do We Study It?

Radiation is a form of energy carried by particles through space. There are many kinds of radiation. The light you can see and the signals that make Wi-Fi work are both types of radiation. Different kinds of radiation have different energies, and types with very high energy can be dangerous because they can stop the cells of your body from working properly [1]. Outer space is filled with exotic kinds of very-high-energy radiation that we do not have here on Earth [2]. Astronauts very close to earth, like on the International Space Station, are protected from most of this radiation by the Earth’s magnetic field. But on a long trip to the moon or to Mars, astronauts will be exposed to much more space radiation than anyone ever has experienced before. Space radiation is especially good at damaging cells, so we want to learn more about it to help keep astronauts safe.

What is Alzheimer’s Disease and Why Does It Matter For Astronauts?

Alzheimer’s disease is a brain disease that kills nerve cells in the brain and damages memory, and it is very common in older people (Figure 1). In fact, it is so common that it is likely some astronauts will develop it when they get older. We do not know if the radiation from space travel will make Alzheimer’s worse, but we think it is possible. The kinds of damage that radiation does to cells can last for a long time, and similar damage can also be found in the brains of people with Alzheimer’s disease. Because of this link, we suspect that space radiation might make brains more likely to develop Alzheimer’s disease later in life [3]. It might also make the disease worse or make it develop faster. In our experiments, we studied a special type of mouse that gets Alzheimer’s disease as it ages so that we could learn more about how the Alzheimer’s disease process is affected by radiation.

How Did We Do This Experiment?

To study the effects of radiation without putting people in danger, we tested male and female mice that were exposed to kinds of radiation similar to the radiation in space. This radiation is made in a particle accelerator, which is a large machine that pushes individual atoms with magnets until they are traveling at high speeds and have a large amount of energy. Remember that radiation is just energy carried by small particles like atoms. We exposed mice to this particle accelerator radiation to approximate the radiation exposure an astronaut would get over multiple years in outer space. We did this when the mice were young and then waited for them to get old. It only takes mice a little over a year to get very old, which makes studying aging easier in mice than in humans. Mice do not normally get Alzheimer’s disease, so we used a special strain of mice genetically engineered to get the disease. When the mice were old, we studied their behavior with multiple tests, like solving mazes, and we also looked at their brains for signs of Alzheimer’s disease. As a comparison, we also looked at the behavior and brains of mice that were not exposed to radiation. Figure 2 diagrams the structure of the experiment.

We measured the amount of Alzheimer’s disease by looking at thin slices of the mouse brains under a microscope. The thin slices made it easier to detect the clumping of a particular kind of protein that builds up in Alzheimer’s disease. Proteins, in addition to fats and sugars, are a very common type of molecule made by all cells. However, in Alzheimer’s disease, very specific kinds of protein that brain cells make tend to clump together outside of the cells. This clumping can interrupt the normal function of cells and eventually cause them to die. Measuring the amount of bad clumps of protein can tell us how far the disease has progressed in the mice.

How Does Space Radiation Affect Alzheimer’s Disease?

When we looked at the brains of these mice with Alzheimer’s disease, we noticed a couple different patterns. First, we observed that the female mice had worse Alzheimer’s disease than the male mice, regardless of whether they were exposed to radiation. Second, we observed that the radiation did make Alzheimer’s disease worse in some mice but not in others. It turns out that radiation made the disease worse in male mice but not in female mice (Figure 3). Even though the female mice had worse Alzheimer’s disease, they did not appear to get worse with radiation like the male mice did. We do not know why this difference between males and females happens, but we are excited to keep doing our research to find out.

How Does Space Radiation Change Mouse Behavior?

The clumping of protein is one part of Alzheimer’s disease, but what we really care about is whether the memory and other behaviors of the mice got worse after radiation. We tested the mice on 10 different behaviors, including various kinds of memory. We saw changes in mice exposed to radiation. In general, the behaviors of male mice were more likely to be affected by radiation exposure than those of female mice. In particular, radiation appeared to make the male mice move around less and do worse in tests of their memory. This matches with the observation that the radiation made Alzheimer’s disease worse in the male mice brains but not in the female mice brains. The good news is that these changes were small, and in many tests, we did not see any difference between radiation-exposed mice and normal mice.

Summary

Space radiation can threaten the health of astronauts traveling in outer space. We want to understand if this radiation could worsen Alzheimer’s disease later in life. To test this, we exposed mice to radiation when they were young and looked at their behavior and brains much later, when they were old. The long-term changes caused by radiation were small but still observable. To begin with, female mice had worse Alzheimer’s disease than male mice regardless of radiation. We saw that radiation worsened Alzheimer’s disease in the brains of only the male mice and not in the female mice. We also saw that the radiation changed the behaviors of the male mice more than it did to the female mice. We still do not know how space radiation will affect humans, but this experiment with mice can give us clues. We can now guess that space radiation could worsen Alzheimer’s disease as astronauts get older. We also suspect that these changes might vary greatly from person to person, depending on things like whether they are male or female. As astronauts return to the moon and then venture out toward Mars, scientists and doctors will have to watch their health carefully, even after they get back to Earth. Space is a dangerous place! In addition to radiation, altered gravity levels, confined spaces, high carbon dioxide levels, and poor sleep all add stress to the brain during spaceflight. But by studying and understanding the risks of traveling in space, we can help make it safer for people to explore.

Glossary

Radiation: ↑ Energy carried through space by particles like light and atoms

Alzheimer’s Disease: ↑ A brain disease that causes problems with memory

Particle Accelerator: ↑ A very specialized machine that uses electricity and magnets to shoot small particles like atoms at very high speeds

Genetically Engineered: ↑ Artificial changes were made to the DNA

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We would like to thank NASA for funding this research (grant numbers NNX14AI07G and 80NSSC18K0810). We would also like to thank the team at the NASA Space Radiation Laboratory who helped irradiate the mice, and we would like to thank General Electric for gifting important materials for the experiment.