The radiation environment in space remains a major concern for manned space exploration, as there is currently no shielding material capable of fully protecting flight crews. Additionally, there is growing concern for the social and cognitive welfare of astronauts, due to prolonged radiation exposure and confinement they will experience on a mission to Mars. In this artice, we report on the late effects of 16 O-particle radiation on social and cognitive behavior and neuronal morphology in the hippocampus of adult female mice. Six-month-old mice received 16 O-particle whole-body irradiation at doses of either 0.25 or 0.1 Gy (600 MeV/n; 18–33 cGy/min) at the NASA's Space Radiation Laboratory in Upton, NY. At nine months postirradiation, the animals underwent behavioral testing in the three-chamber sociability, novel object recognition and Y-maze paradigms. Exposure to 0.1 or 0.25 Gy 16 O significantly impaired object memory, a 0.25 Gy dose impaired social novelty learning, but neither dosage impaired short-term spatial memory. We observed significant decreases in mushroom spine density and dendrite morphology in the dentate gyrus, cornu ammonis 3, 2 and 1 of the hippocampus, which are critical areas for object novelty and sociability processing. Our data suggest exposure to 16 O modulates hippocampal pyramidal and granular neurons and induces behavioral deficits at a time point of nine months after exposure in females.

Chemotherapy has been successfully used to reduce radiation dose and volume for most pediatric patients. However, because of the failure of chemotherapeutic agents to cross the blood-brain barrier and the lack of response of some brain tumors to these agents, radiation therapy is still used to treat many childhood cancers with CNS involvement. In this study, we investigated the radiation effects on cognition and dendritic structure in the hippocampus in juvenile male mice. Twenty-one-day-old male C57BL/6 mice were irradiated using the small animal radiation research platform (SARRP). Animals were exposed to either a 10 Gy single dose or 10 Gy × 2 fractionated doses of X-ray cranial radiation. Five weeks after irradiation, animals were tested for hippocampus-dependent cognitive performance in the Morris water maze. Significant impairment in spatial memory retention was observed in the probe trial after the first day of hidden-platform training (first probe trial) in animals that received either 10 Gy single-dose or 10 Gy × 2 fractionated doses. However, by day 5, mice that received a 10 Gy single dose showed spatial memory retention in the probe trials, whereas mice that received the 20 Gy fractionated doses remained impaired. During Y-maze testing, animals exposed to radiation were impaired; the irradiated mice were not able to distinguish among the three Y-maze arms and spent approximately the same amount of time in all three arms during the retention trial. Radiation significantly compromised the dendritic architecture and reduced spine density throughout the hippocampal trisynaptic network.

Radiation from galactic cosmic rays (GCR) poses a significant health risk for deep-space flight crews. GCR are unique in their extremely high-energy particles. With current spacecraft shielding technology, some of the predominant particles astronauts would be exposed to are 1 H + 16 O. Radiation has been shown to cause cognitive deficits in mice. The hippocampus plays a key role in memory and cognitive tasks; it receives information from the cortex, undergoes dendritic-dependent processing and then relays information back to the cortex. In this study, we investigated the effects of combined 1 H + 16 O irradiation on cognition and dendritic structures in the hippocampus of adult male mice three months postirradiation. Six-month-old male C57BL/6 mice were irradiated first with 1 H (0.5 Gy, 150 MeV/n) and 1 h later with 16 O (0.1 Gy, 600 MeV/n) at the NASA Space Radiation Laboratory (Upton, NY). Three months after irradiation, animals were tested for hippocampus-dependent cognitive performance using the Y-maze. Upon sacrifice, molecular and morphological assessments were performed on hippocampal tissues. During Y-maze testing, the irradiated mice failed to distinguish the novel arm, spending approximately the same amount of time in all three arms during the retention trial relative to sham-treated controls. Irradiated animals also showed changes in expression of glutamate receptor subunits and synaptic density-associated proteins. 1 H + 16 O radiation compromised dendritic morphology in the cornu ammonis 1 and dentate gyrus within the hippocampus. These data indicate cognitive injuries due to 1 H + 16 O at three months postirradiation.

The space radiation environment contains protons and 56 Fe, which could pose a significant hazard to space flight crews during and after missions. The space environment involves complex radiation exposures, thus, the effects of a dose of protons might be modulated by a dose of heavy-ion radiation. The brain, and particularly the hippocampus, may be susceptible to space radiation-induced changes. In this study, we first determined the dose-response effect of proton radiation (150 MeV) on hippocampus-dependent cognition 1 and 3 months after exposure. Based on those results, we subsequently exposed mice to protons alone (150 MeV, 0.1 Gy), 56 Fe alone (600 MeV/n, 0.5 Gy) or combined proton and 56 Fe radiations (protons first) with the two exposures separated by 24 h. At one month postirradiation, all animal groups showed novel object recognition. However, at three months postirradiation, mice exposed to either protons or combined proton and 56 Fe radiations showed impaired novel object recognition, which was not observed in mice irradiated with 56 Fe alone. The mechanisms in these impairments might involve inflammation. In mice irradiated with protons alone or 56 Fe alone three months earlier, there was a negative correlation between a measure of novel object recognition and the number of newly born activated microglia in the dentate gyrus. Next, cytokine and chemokine levels were assessed in the hippocampus. At one month after exposure the levels of IL-12 were higher in mice exposed to combined radiations compared with sham-irradiated mice, while the levels of IFN-γ were lower in mice exposed to 56 Fe radiation alone or combined radiations. In addition, IL-4 levels were lower in 56 Fe-irradiated mice compared with proton-irradiated mice and TNF-α levels were lower in proton-irradiated mice than in mice receiving combined radiations. At three months after exposure, macrophage-derived chemokine (MDC) and eotaxin levels were lower in mice receiving combined radiations. The levels of MDC and eotaxin correlated and the levels of MDC, but not eotaxin, correlated with the percentage of newly born activated microglia in the blades of the dentate gyrus. Finally, hippocampal IL-6 levels were higher in mice receiving combined radiations compared with mice receiving 56 Fe radiation alone. These data demonstrate the sensitivity of novel object recognition for detecting cognitive injury three months after exposure to proton radiation alone, and combined exposure to proton and 56 Fe radiations, and that newly-born activated microglia and inflammation might be involved in this injury.

A unique feature of the space radiation environment is the presence of high-energy charged particles, which can be significantly hazardous to space flight crews who are exposed during a mission. Health risks associated with high-LET radiation exposure include cognitive injury. The pathogenesis of this injury is unknown but may involve modifications to dendritic structure and/or alterations in dendritic spine density and morphology. In this study, 24 two-month-old C57BL6/J male mice were either whole-body irradiated with 0.5 Gy 56 Fe (600 MeV/n; n = 12) or sham irradiated (n = 12). Three months postirradiation animals were tested for locomotor activity and habituation. After behavioral testing, animals were euthanized and the brains were flash frozen. Compared to sham-irradiated mice, irradiated mice moved less when first introduced to the environment, although they did recognize the environment when re-exposed to it one day later. Exposure to 56 Fe radiation significantly compromised the dendritic architecture and reduced spine density throughout the hippocampal tri-synaptic network. To our knowledge, these data represents the first reported evidence that high-LET radiation has deleterious effects on mature neurons associated with hippocampal learning and memory.

Space flight poses certain health risks to astronauts, including exposure to space radiation, with protons accounting for more than 80% of deep-space radiation. Proton radiation is also now being used with increasing frequency in the clinical setting to treat cancer. For these reasons, there is an urgent need to better understand the biological effects of proton radiation on the body. Such improved understanding could also lead to more accurate assessment of the potential health risks of proton radiation, as well as the development of improved strategies to prevent and mitigate its adverse effects. Previous studies have shown that exposure to low doses of protons is detrimental to mature leukocyte populations in peripheral blood, however, the underlying mechanisms are not known. Some of these detriments may be attributable to damage to hematopoietic stem cells (HSCs) that have the ability to self-renew, proliferate and differentiate into different lineages of blood cells through hematopoietic progenitor cells (HPCs). The goal of this study was to investigate the long-term effects of low-dose proton irradiation on HSCs. We exposed C57BL/6J mice to 1.0 Gy whole-body proton irradiation (150 MeV) and then studied the effects of proton radiation on HSCs and HPCs in the bone marrow (BM) 22 weeks after the exposure. The results showed that mice exposed to 1.0 Gy whole-body proton irradiation had a significant and persistent reduction of BM HSCs compared to unirradiated controls. In contrast, no significant changes were observed in BM HPCs after proton irradiation. Furthermore, irradiated HSCs and their progeny exhibited a significant impairment in clonogenic function, as revealed by the cobblestone area-forming cell (CAFC) and colony-forming cell assays, respectively. These long-term effects of proton irradiation on HSCs may be attributable to the induction of chronic oxidative stress in HSCs, because HSCs from irradiated mice exhibited a significant increase in NADPH oxidase 4 (NOX4) mRNA expression and reactive oxygen species (ROS) production. In addition, the increased production of ROS in HSCs was associated with a significant reduction in HSC quiescence and an increase in DNA damage. These findings indicate that exposure to proton radiation can lead to long-term HSC injury, probably in part by radiation-induced oxidative stress.

Radiation exposure due to radiological terrorism and military circumstances are a continuing threat for the civilian population. In an uncontrolled radiation event, it is likely that there will be other types of injury involved, including trauma. While radiation combined injury is recognized as an area of great significance, overall there is a paucity of information regarding the mechanisms underlying the interactions between irradiation and other forms of injury, or what countermeasures might be effective in ameliorating such changes. The objective of this study was to determine if difluoromethylornithine (DFMO) could reduce the adverse effects of single or combined injury if administered beginning 24 h after exposure. Eight-week-old C57BL/J6 young-adult male mice received whole-body cesium-137 ( 137 Cs) irradiation with 4 Gy. Immediately after irradiation, unilateral traumatic brain injury was induced using a controlled cortical impact system. Forty-four days postirradiation, animals were tested for hippocampus-dependent cognitive performance in the Morris water maze. After cognitive testing, animals were euthanized and their brains snap frozen for immunohistochemical assessment of neuroinflammation (activated microglia) and neurogenesis in the hippocampal dentate gyrus. Our data show that single and combined injuries induced variable degrees of hippocampus-dependent cognitive dysfunction, and when given 24 h post trauma, DFMO treatment ameliorated those effects. Cellular changes including neurogenesis and numbers of activated microglia were generally not associated with the cognitive changes. Further analyses also revealed that DFMO increased hippocampal protein levels of the antioxidants thioredoxin 1 and peroxiredoxin 3 compared to vehicle treated animals. While the mechanisms responsible for the improvement in cognition after DFMO treatment are not yet clear, these results constitute a basis for further development of DFMO as a countermeasure for ameliorating the of risks for cognitive dysfunction in individuals subjected to trauma and radiation combined injury.

There is an increasing need to better understand the long-term health effects of high-linear energy transfer (LET) radiation due to exposure during space missions, as well as its increasing use in clinical treatments. Previous studies have indicated that exposure to 56 Fe heavy ions increases the incidence of acute myeloid leukemia (AML) in mice but the underlying molecular mechanisms remain elusive. Epigenetic alterations play a role in radiation-induced genomic instability and the initiation and progression of AML. In this study, we assessed the effects of low-dose 56 Fe-ion irradiation on epigenetic alterations in bone marrow mononuclear cells (BM-MNCs) and hematopoietic progenitor and stem cells (HPSCs). Exposure to 56 Fe ions (600 MeV, 0.1, 0.2 and 0.4 Gy) resulted in significant epigenetic alterations involving methylation of DNA, the DNA methylation machinery and expression of repetitive elements. Four weeks after irradiation, these changes were primarily confined to HPSCs and were exhibited as dose-dependent hypermethylation of LINE1 and SINE B1 repetitive elements [4.2-fold increase in LINE1 ( P < 0.001) and 7.6-fold increase in SINE B1 ( P < 0.01) after exposure to 0.4 Gy; n = 5]. Epigenetic alterations were persistent and detectable for at least 22 weeks after exposure, when significant loss of global DNA hypomethylation (1.9-fold, P < 0.05), decreased expression of Dnmt1 (1.9-fold, P < 0.01), and increased expression of LINE1 and SINE B1 repetitive elements (2.8-fold, P < 0.001 for LINE1 and 1.9-fold, P < 0.05 for SINE B1; n = 5) were observed after exposure to 0.4 Gy. In contrast, exposure to 56 Fe ions did not result in accumulation of increased production of reactive oxygen species (ROS) and DNA damage, exhibited as DNA strand breaks. Furthermore, no significant alterations in cellular senescence and apoptosis were detected in HPSCs after exposure to 56 Fe-ion radiation. These findings suggest that epigenetic reprogramming is possibly involved in the development of radiation-induced genomic instability and thus, may have a causative role in the development of AML.

The space radiation environment consists of multiple species of high-energy charge particles (HZE), including 56 Fe and 28 Si nuclei, that may impact neuronal cells, but their damaging effects on the central nervous system (CNS) have been poorly defined. Hippocampus-dependent memory functions have been shown to be highly sensitive to 56 Fe HZE particles, which poses a significant risk to the cognitive performance of astronauts during space missions. While low doses of 56 Fe radiation do not induce cell death of mature neurons, they affect synaptic plasticity in the CA1 region, the principal neuronal output of the hippocampal formation involved in memory formation. The effects of 28 Si on the CNS have not been defined. Compared to behaviorally naïve mice, cognitive testing might affect synaptic plasticity and the effects of 28 Si radiation on synaptic plasticity might be modulated by prior cognitive testing. Therefore, in the current study, we quantified the effects of whole-body 28 Si radiation (600 MeV/n, 0.25 and 1 Gy) on hippocampus-dependent contextual freezing and synaptic plasticity in the CA1 region of animals not exposed (behaviorally naïve mice) and animals exposed to the contextual freezing test (cognitively tested mice). In behaviorally naïve mice exposed to 0.25 and 1 Gy of 28 Si radiation, the magnitude of long-term potentiation (LTP) was enhanced. However, in mice irradiated with 0.25 Gy contextual fear conditioning was enhanced and was associated with a further enhancement of the LTP magnitude. Such increase in synaptic plasticity was not seen in cognitively tested mice irradiated with 1 Gy. Thus, low dose 28 Si radiation has effects on synaptic plasticity in the CA1 region of the hippocampus and these effects are modulated by cognitive testing in a contextual fear-conditioning test.

Chemokines and their receptors play a crucial role in normal brain function as well as in pathological conditions such as injury and disease-associated neuroinflammation. Chemokine receptor-2 (CCR2), which mediates the recruitment of infiltrating and resident microglia to sites of central nervous system (CNS) inflammation, is upregulated by ionizing irradiation and traumatic brain injury. Our objective was to determine if a deficiency in CCR2 and subsequent effects on brain microglia affect neurogenesis and cognitive function after radiation combined injury (RCI). CCR2 knock-out (–/–) and wild-type (WT) mice received 4 Gy of whole body 137 Cs irradiation. Immediately after irradiation, unilateral traumatic brain injury was induced using a controlled cortical impact system. Forty-four days postirradiation, animals were tested for hippocampus-dependent cognitive performance in the Morris water-maze. After cognitive testing, animals were euthanized and their brains snap frozen for immunohistochemical assessment of neuroinflammation (activated microglia) and neurogenesis in the hippocampal dentate gyrus. All animals were able to locate the visible and hidden platform locations in the water maze; however, treatment effects were seen when spatial memory retention was assessed in the probe trials (no platform). In WT animals that received combined injury, a significant impairment in spatial memory retention was observed in the probe trial after the first day of hidden platform training (first probe trial). This impairment was associated with increased neurogenesis in the ipsilateral hemisphere of the dentate gyrus. In contrast, CCR2 –/– mice, independent of insult showed significant memory retention in the first probe trial and there were no differences in the numbers of newly born neurons in the animals receiving irradiation, trauma or combined injury. Although the mechanisms involved are not clear, our data suggests that CCR2 deficiency can exert a protective effect preventing the impairment of cognitive function after combined injury.