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The Biological Effects of Space Radiation During Long Stays in Space

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The Biological Effects of Space Radiation During Long Stays in Space Biological Sciences in Space, Vol.18 No.4 (2004):Ohnishi, 201-205 K. and Ohnishi, T. © 2004 Jpn. Soc. Biol. Sci. Space The Biological Effects of Space Radiation during Long Stays in Space Ken Ohnishi and Takeo Ohnishi Department of Biology, Nara Medical University School of Medicine,Shijo-cho 840, Kashihara, Nara 634-8521, Japan. Abstract Many space experiments are scheduled for the International Space Station (ISS). Completion of the ISS will soon become a reality. Astronauts will be exposed to low-level background components from space radiation including heavy ions and other high-linear energy transfer (LET) radiation. For long-term stay in space, we have to protect human health from space radiation. At the same time, we should recognize the maximum permissible doses of space radiation. In recent years, physical monitoring of space radiation has detected about 1 mSv per day. This value is almost 150 times higher than that on the surface of the Earth. However, the direct effects of space radiation on human health are currently unknown. Therefore, it is important to measure biological dosimetry to calculate relative biological effectiveness (RBE) for human health during long-term flight. The RBE is possibly modified by microgravity. In order to understand the exact RBE and any interaction with microgravity, the ISS centrifugation system will be a critical tool, and it is hoped that this system will be in operation as soon as possible. Key words; space, space radiation, biological effect, high-LET, ISS Characteristics of space and the composition of space radiation the radiation being composed of α-particles and heavy For several years, human beings have been particles. The sun changes its level of activity during an residing for long periods in the ISS where many space 11-year-cycle. Solar flares and solar winds, indicators of experiments are scheduled to be performed. In addition, solar activity, emit large quantities of charged particles, there are proposals for human to travel to Mars. electrons, protons, α-particles, X-rays, He, H and Fe Important characteristics of the environment in space which have much higher energies than galactic cosmic are the presence of microgravity and space radiation. rays. When the sun is producing solar flares and solar Radiation present in the space environment contains winds, astronauts are exposed to the particles emitted many components including low dose radiation, low from the solar flares and winds at a dose rate at more dose-rate radiation, and high-LET particles (Table 1). than an order of magnitude greater than normal. In The space environment produces microgravity- and/or addition, since the exposure to these particles outside of space radiation-induced physiological changes in the the space craft is several times higher than that inside of human body (e.g. calcium release from bone into the the space craft, astronauts can be seriously affected by urine, loss of muscular power, body fluid shifts, space or solar particles while working outside of the space craft. airsickness, reduced immunoreactivity and eye flashes). Geomagnetically trapped particles are particles which During a long-term stay in space, astronauts consist primarily of protons and electrons and which are will be constantly exposed to space radiation which trapped in the geomagnetic field layer. The high flux area contains various types of low dose-rate radiation. Space of geomagnetically trapped particles is called the Van radiation consists of galactic cosmic rays, solar particles Allen Belt and this is located over the Earthʼs equator. and geomagnetically trapped particles (Table 2). A An area of the Van Allen Belt over the South Atlantis is characteristic of galactic cosmic rays is their containing called the South Atlantic Anomaly (SAA) and this region high energy particles with energies over 10 GeV: contains an especially high flux of geomagnetically protons (90%), α-particles (9%) and heavy particles trapped particles. (1%). The origin of galactic cosmic rays is considered to be the explosion of supernovas. Solar particles are Biological effects of space radiation primarily protons and electrons with several percent of Space radiation contains high-LET radiation (α-particles, heavy particles and neutrons) derived from Table 1 Characteristics of space radiation Review Received: January 4, 2005 Low dose Address for correspondence: Dr. Takeo Ohnishi Low dose-rate Department of Biology, Long-term exposure Nara Medical University School of Medicine, High-LET radiation Shijo-cho 840, Kashihara, Nara 634-8521, Japan. E-mail; [email protected] Interactions with microgravity 201 Biological Effects of Space Radiation Table 2 Composition of space radiation 2003), and about 2.0 for 100 keV/µm (Takahashi 1) Galactic cosmic rays: protons (90%), α-particles (9%), et al., 2001). In accepting these high RBE heavy particles (1%) values for mutation and cell lethality induced by 2) Solar particles: primarily protons and electrons, α-particles (several %), high-LET radiation, it is assumed that this high- heavy particles (several %) LET radiation could cause DNA damage which 3) Geomagnetically trapped particles: protons, electrons can be repaired by the non-homologous end- joining repair processes. In recent experiments, galactic cosmic rays, solar particles and geomagnetically the indirect biological effects of radiation were studied: trapped particles. The RBE of the high-LET radiation is these indirect effects are called the “bystander effect”: higher than the RBE of X-rays or γ-rays. The RBE value non-irradiated cells are affected by their neighboring depends on the LET values, the size of the dose, and irradiated cells through cell to cell junctions and by the biological indicator analyzed, and thus it is difficult radicals generated by radiation. In the case of a space to estimate a value for the RBE based on a simple environment, the bystander effect probably should not physical parameter such as the absorbed dose. High- be neglected, because heavy particles such as α-particles LET radiation causes a larger amount of damage to DNA and carbon-ion beams were able to induce bystander than X-rays or γ-rays. DNA strand breaks induced by effect in chromatin damage through cell to cell junctions space radiation were detected as grains in fixed human (Suzuki et al., 2004) and in experiments observing cell cultured cells in Mir Space Station- and Space Shuttle- proliferation and micronuclei mediated by NO (Shao et experiments (Ohnishi et al., 2002). The number of grains al., 2002). visualized with in situ enzymatic post-labeling method Some space experiments reported that there increased with the increase of the length of the stay in appeared to be interactions between space radiation and space. DNA damage induced by radiation is known to microgravity. Abnormal differentiation was observed cause cell death, mutations, chromosomal aberrations, more frequently in one kind of insect, Carausius developmental abnormalities and senescence. In the morosus, under microgravity conditions than when central nervous systems of mice irradiated with high subjected to one gravity force generated by a centrifuge energy heavy particles using the Heavy Ion Medical in space (Bucker et al., 1986). Recessive lethal mutations Accelerator in Chiba (HIMAC), cell death was induced were induced by space radiation at low, supposedly non- in microglia but not in astrocytes (Nojima et al., 2000). effective doses in the progeny of fruit flies taken into Such selective cell death in the brain may also be induced space (Ikenaga, et al., 1997). These space experiments by space radiation. The death of an individual has not suggested the possibility that microgravity may have been reported after acute exposure to space radiation. In elevated the frequency of mutations induced by space contrast to an acute exposure, chronic exposure to space radiation. Some possible suggestions to explain the effect radiation may induce late effects such as central nervous of microgravity have been offered. One suggestion is system damage, cataracts and carcinogenesis. Effects of that a microgravity environment may inhibit the repair of heavy particles on the functioning of the central nervous DNA damage induced by space radiation. Another is that system (learning and memory) have been examined in metabolic changes induced by microgravity indirectly ground-based experiments using water maze analysis in modify biological processes. For example, microgravity mice. Mutations induced by space radiation could lead may lead to the accumulation of stress-related proteins to carcinogenesis. The types of chromosomal aberrations which then might modify cellular sensitivity to space induced by space radiation were translocations and radiation. This possibility is supported by investigations complex exchanges seen in the cytogenetic analysis of observing accumulated heat shock protein 72 or tumor lymphocytes of astronauts (Durante et al., 2003, 2004; suppressor gene product p53 in the muscle and spleen George et al., 2002). Large individual differences in the of goldfish, and in the skin and muscle of rats examined frequency of these types of chromosomal aberrations after space flight (Ohnishi et al., 1996, 1998). were observed among astronauts. Thus, individual In contrast, some space experiments have shown that differences in sensitivity to space radiation must be mutation frequencies and DNA repair activity are not expected. Since space radiation consists of different affected by microgravity in E. coli (Harada et al., 1998; types of particles having different energies and dose- Horneck et al., 1996), B. subtillis (Yatagai et. al., 2000), D. rates, it is difficult to determine an exact RBE value for discoideum (Takahashi et al., 1997), S. cerevisiae (Pross space radiation. Several studies on high-LET radiation et al., 1999; Fukuda, et al., 2000), human cells (Horneck have reported an RBE for the mutation frequencies of et al., 1996) and with in vitro assays (Takahashi et al., α-particles of about 20 (Kiefer, 2002) and those for 2000). carbon-ion beams were 3.6 and 7.3 for 68 keV/µm and 120 keV/µm beams, respectively (Suzuki et al., 2003).
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