ApoE isoform modulates effects of cranial ⁵⁶Fe irradiation on spatial learning and memory in the water maze

Cognitive Effects of Simulated Galactic Cosmic Radiation Are Mediated by ApoE Status, Sex, and Environment in APP Knock-In Mice

Cosmic radiation experienced during space travel may increase the risk of cognitive impairment. While simulated galactic cosmic radiation (GCRsim) has led to memory deficits in wildtype (WT) mice, it has not been investigated whether GCRsim in combination with genetic risk factors for Alzheimer's disease (AD) worsens memory further in aging mice. Here, we investigated the central nervous system (CNS) effects of 0 Gy (sham) or 0.75 Gy five-ion GCRsim or 2 Gy gamma radiation (IRR) in 14-month-old female and male APPNL-F/NL-F knock-in (KI) mice bearing humanized ApoE3 or ApoE4 (APP;E3F and APP;E4F). As travel to a specialized facility was required for irradiation, both traveled sham-irradiated C57BL/6J WT and KI mice and non-traveled (NT) KI mice acted as controls for potential effects of travel. Mice underwent four behavioral tests at 20 months of age and were euthanized for pathological and biochemical analyses 1 month later. Fecal samples were collected pre- and post-irradiation at four different time points. GCRsim seemed to impair memory in male APP;E3F mice compared to their sham counterparts. Travel tended to improve cognition in male APP;E3F mice and lowered total Aβ in female and male APP;E3F mice compared to their non-traveled counterparts. Sham-irradiated male APP;E4F mice accumulated more fibrillar amyloid than their APP;E3F counterparts. Radiation exposure had only modest effects on behavior and brain changes, but travel-, sex-, and genotype-specific effects were seen. Irradiated mice had immediate and long-term differences in their gut bacterial composition that correlated to Alzheimer's disease phenotypes.

Cognitive deficits in human ApoE4 knock-in mice: A systematic review and meta-analysis

Apolipoprotein-E4 (ApoE4) is an important genetic risk factor for Alzheimer's disease. The development of targeted-replacement human ApoE knock-in mice facilitates research into mechanisms by which ApoE4 affects the brain. We performed meta-analyses and meta-regression analyses to examine differences in cognitive performance between ApoE4 and ApoE3 mice. We included 61 studies in which at least one of the following tests was assessed: Morris Water Maze (MWM), novel object location (NL), novel object recognition (NO) and Fear Conditioning (FC) test. ApoE4 vs. ApoE3 mice performed significantly worse on the MWM (several outcomes, 0.17 ≤ g ≤ 0.60), NO (exploration, g=0.33; index, g=0.44) and FC (contextual, g=0.49). ApoE4 vs. ApoE3 differences were not systematically related to sex or age. We conclude that ApoE4 knock-in mice in a non-AD condition show some, but limited cognitive deficits, regardless of sex and age. These effects suggest an intrinsic vulnerability in ApoE4 mice that may become more pronounced under additional brain load, as seen in neurodegenerative diseases.

The Effects of Whole Body Gamma Irradiation on Mice, Age-Related Behavioral, and Pathophysiological Changes

We designed a study with the objective to determine the long-term radiation effects of gamma rays, originating from a single shot of Co60 at a dose of 2 Gy on the 7-month-old male mice of the ICR line in 30 days after the irradiation. The aim of this study was to characterize the behavior of animals using the Open Field test, immuno-hematological status, and morpho-functional changes in the central nervous system of mice. Irradiated animals displayed significantly different behavior in the OF in comparison with the control group. The radiation damage was confirmed by assessing the ratio of leukocytes in the peripheral blood of mice at a later date after exposure to Co60. After irradiation, a decrease in the glioneuronal complex was observed in the irritated group as well as histological changes of brain cells. To sum up, not only was the hematological status of mice altered upon the total gamma irradiation, but also their behavior, which was most probably due to significant alterations in the CNS. Study of influence of ionizing radiation on female mice, comparison between different age groups. Open Field test on the 30 days after 2 Gy of γ-rays and histological analysis indicated changes in behavioral patterns, leucocytes, and brain tissue.

A multi-hit hypothesis for an APOE4-dependent pathophysiological state

The APOE gene encoding the Apolipoprotein E protein is the single most significant genetic risk factor for late-onset Alzheimer's disease. The APOE4 genotype confers a significantly increased risk relative to the other two common genotypes APOE3 and APOE2. Intriguingly, APOE4 has been associated with neuropathological and cognitive deficits in the absence of Alzheimer's disease-related amyloid or tau pathology. Here, we review the extensive literature surrounding the impact of APOE genotype on central nervous system dysfunction, focussing on preclinical model systems and comparison of APOE3 and APOE4, given the low global prevalence of APOE2. A multi-hit hypothesis is proposed to explain how APOE4 shifts cerebral physiology towards pathophysiology through interconnected hits. These hits include the following: neurodegeneration, neurovascular dysfunction, neuroinflammation, oxidative stress, endosomal trafficking impairments, lipid and cellular metabolism disruption, impaired calcium homeostasis and altered transcriptional regulation. The hits, individually and in combination, leave the APOE4 brain in a vulnerable state where further cumulative insults will exacerbate degeneration and lead to cognitive deficits in the absence of Alzheimer's disease pathology and also a state in which such pathology may more easily take hold. We conclude that current evidence supports an APOE4 multi-hit hypothesis, which contributes to an APOE4 pathophysiological state. We highlight key areas where further study is required to elucidate the complex interplay between these individual mechanisms and downstream consequences, helping to frame the current landscape of existing APOE-centric literature.

Long-Term Sex- and Genotype-Specific Effects of 56Fe Irradiation on Wild-Type and APPswe/PS1dE9 Transgenic Mice

Space radiation presents a substantial threat to travel beyond Earth. Relatively low doses of high-energy particle radiation cause physiological and behavioral impairments in rodents and may pose risks to human spaceflight. There is evidence that ⁵⁶Fe irradiation, a significant component of space radiation, may be more harmful to males than to females and worsen Alzheimer's disease pathology in genetically vulnerable models. Yet, research on the long-term, sex- and genotype-specific effects of ⁵⁶Fe irradiation is lacking. Here, we irradiated 4-month-old male and female, wild-type and Alzheimer's-like APP/PS1 mice with 0, 0.10, or 0.50 Gy of ⁵⁶Fe ions (1GeV/u). Mice underwent microPET scans before and 7.5 months after irradiation, a battery of behavioral tests at 11 months of age and were sacrificed for pathological and biochemical analyses at 12 months of age. ⁵⁶Fe irradiation worsened amyloid-beta (Aβ) pathology, gliosis, neuroinflammation and spatial memory, but improved motor coordination, in male transgenic mice and worsened fear memory in wild-type males. Although sham-irradiated female APP/PS1 mice had more cerebral Aβ and gliosis than sham-irradiated male transgenics, female mice of both genotypes were relatively spared from radiation effects 8 months later. These results provide evidence for sex-specific, long-term CNS effects of space radiation.

A review of astronaut mental health in manned missions: Potential interventions for cognitive and mental health challenges

Space is an isolated, confined environment for humans. These conditions can have numerous effects on astronaut mental health and safety. Psychological and social issues affect space crew due to the isolation, confinement, and prolonged separation from family and friends. This area of research is particularly crucial given the space sector's plans for Martian colonies and space tourism, as well as to aid astronauts when under high stress. Therefore, this paper reviews the effects of isolation/confinement on psychological and cognitive health; impact of radiation and microgravity on cognitive health; and implications of disturbances to the circadian rhythm and sleep in space. Possible solutions to relevant mentioned cognitive and mental health challenges are also discussed.

Consequences of space radiation on the brain and cardiovascular system

Staying longer in outer space will inevitably increase the health risks of astronauts due to the exposures to galactic cosmic rays and solar particle events. Exposure may pose a significant hazard to space flight crews not only during the mission but also later, when slow-developing adverse effects could finally become apparent. The body of literature examining ground-based outcomes in response to high-energy charged-particle radiation suggests differential effects in response to different particles and energies. Numerous animal and cellular models have repeatedly demonstrated the negative effects of high-energy charged-particle on the brain and cognitive function. However, research on the role of space radiation in potentiating cardiovascular dysfunction is still in its early stages. This review summarizes the available data from studies using ground-based animal models to evaluate the response of the brain and heart to the high-energy charged particles of GCR and SPE, addresses potential sex differences in these effects, and aims to highlight gaps in the current literature for future study.

Space-like 56Fe irradiation manifests mild, early sex-specific behavioral and neuropathological changes in wildtype and Alzheimer's-like transgenic mice

Space travel will expose people to high-energy, heavy particle radiation, and the cognitive deficits induced by this exposure are not well understood. To investigate the short-term effects of space radiation, we irradiated 4-month-old Alzheimer’s disease (AD)-like transgenic (Tg) mice and wildtype (WT) littermates with a single, whole-body dose of 10 or 50 cGy ⁵⁶Fe ions (1 GeV/u) at Brookhaven National Laboratory. At ~1.5 months post irradiation, behavioural testing showed sex-, genotype-, and dose-dependent changes in locomotor activity, contextual fear conditioning, grip strength, and motor learning, mainly in Tg but not WT mice. There was little change in general health, depression, or anxiety. Two months post irradiation, microPET imaging of the stable binding of a translocator protein ligand suggested no radiation-specific change in neuroinflammation, although initial uptake was reduced in female mice independently of cerebral blood flow. Biochemical and immunohistochemical analyses revealed that radiation reduced cerebral amyloid-β levels and microglia activation in female Tg mice, modestly increased microhemorrhages in 50 cGy irradiated male WT mice, and did not affect synaptic marker levels compared to sham controls. Taken together, we show specific short-term changes in neuropathology and behaviour induced by ⁵⁶Fe irradiation, possibly having implications for long-term space travel.

Behavioral effects of space radiation: A comprehensive review of animal studies

As NASA prepares for the first manned mission to Mars in the next 20 years, close attention has been placed on the cognitive welfare of astronauts, who will likely endure extended durations in confinement and microgravity and be subjected to the radioactive charged particles travelling at relativistic speeds in interplanetary space. The future of long-duration manned spaceflight, thus, depends on understanding the individual hazards associated with the environment beyond Earth's protective magnetosphere. Ground-based single-particle studies of exposed mice and rats have, in the last 30 years, overwhelmingly reported deficits in their cognitive behaviors. However, as particle-accelerator technologies at NASA's Space Radiation Laboratory continue to progress, more realistic representations of space radiation are materializing, including multiple-particle exposures and, eventually, at multiple energy distributions. These advancements help determine how to best mitigate possible hazards due to space radiation. However, risk models will depend on delineating which particles are most responsible for specific behavioral outcomes and whether multiple-particle exposures produce synergistic effects. Here, we review the literature on animal exposures by particle, energy, and behavioral assay to inform future mixed-field radiation studies of possible behavioral outcomes.

The Disparity of Impairment of Neurogenesis and Cognition After Acute or Fractionated Radiation Exposure in Adolescent BALB/c Mice

The effect of acute X-ray irradiation with 2 Gy or fractionated exposure with 0.2 Gy continuously for 10 days (0.2 Gy × 10 = 2 Gy) was evaluated in the postnatal day 21 (P21) BALB/c mouse model. Both acute and fractionated irradiation induced impairment of cell proliferation and neurogenesis in the subgranular zone of the dentate gyrus labeled by Ki67 and doublecortin, respectively. Parvalbumin immunopositive interneurons in the subgranular zone were also reduced significantly. However, the 2 patterns of irradiation did not affect animal weight gain when measured at ages of P90 and P180 or 69 and 159 days after irradiation. Behavioral tests indicated that neither acute nor fractionated irradiation with a total dose of 2 Gy induced deficits in the contextual fear or spatial memory and memory for novel object recognition. Animal motor activity was also not affected in the open-field test. The disparity of the impairment of neurogenesis and unaffected cognition suggests that the severity of impairment of neurogenesis induced by acute or fractionated irradiation with a total dose of 2 Gy at P21 may not be worse enough to induce the deficit of cognition.

Bi-directional and shared epigenomic signatures following proton and 56Fe irradiation

The brain’s response to radiation exposure is an important concern for patients undergoing cancer therapy and astronauts on long missions in deep space. We assessed whether this response is specific and prolonged and is linked to epigenetic mechanisms. We focused on the response of the hippocampus at early (2-weeks) and late (20-week) time points following whole body proton irradiation. We examined two forms of DNA methylation, cytosine methylation (5mC) and hydroxymethylation (5hmC). Impairments in object recognition, spatial memory retention, and network stability following proton irradiation were observed at the two-week time point and correlated with altered gene expression and 5hmC profiles that mapped to specific gene ontology pathways. Significant overlap was observed between DNA methylation changes at the 2 and 20-week time points demonstrating specificity and retention of changes in response to radiation. Moreover, a novel class of DNA methylation change was observed following an environmental challenge (i.e. space irradiation), characterized by both increased and decreased 5hmC levels along the entire gene body. These changes were mapped to genes encoding neuronal functions including postsynaptic gene ontology categories. Thus, the brain’s response to proton irradiation is both specific and prolonged and involves novel remodeling of non-random regions of the epigenome.

Short- and long-term effects of 56Fe irradiation on cognition and hippocampal DNA methylation and gene expression

Background

Astronauts are exposed to ⁵⁶Fe ions that may pose a significant health hazard during and following prolonged missions in deep space. We showed previously that object recognition requiring the hippocampus, a structure critical for cognitive function, is affected in 2-month-old mice irradiated with ⁵⁶Fe ions. Here we examined object recognition in 6-month-old mice irradiated with ⁵⁶Fe ions, a biological age more relevant to the typical ages of astronauts. Moreover, because the mechanisms mediating the detrimental effects of ⁵⁶Fe ions on hippocampal function are unclear, we examined changes in hippocampal networks involved in synaptic plasticity and memory, gene expression, and epigenetic changes in cytosine methylation (5mC) and hydroxymethylation (5hmC) that could accompany changes in gene expression. We assessed the effects of whole body ⁵⁶Fe ion irradiation at early (2 weeks) and late (20 weeks) time points on hippocampus-dependent memory and hippocampal network stability, and whether these effects are associated with epigenetic changes in hippocampal DNA methylation (both 5mC and 5hmC) and gene expression.

Results

At the two-week time point, object recognition and network stability were impaired following irradiation at the 0.1 and 0.4 Gy dose, but not following irradiation at the 0.2 Gy dose. No impairments in object recognition or network stability were seen at the 20-week time point at any irradiation dose used. Consistent with this pattern, the significance of pathways for gene categories for 5hmC was lower, though not eliminated, at the 20-week time point compared to the 2-week time point. Similarly, significant changes were observed for 5mC gene pathways at the 2-week time point, but no significant gene categories were observed at the 20-week time point. Only the 5hmC changes tracked with gene expression changes.

Conclusions

Dose- and time-dependent epigenomic remodeling in the hippocampus following ⁵⁶Fe ion exposure correlates with behavioral changes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3110-7) contains supplementary material, which is available to authorized users.

Genotype differences in anxiety and fear learning and memory of WT and ApoE4 mice associated with enhanced generation of hippocampal reactive oxygen species

Apolipoprotein E (apoE), involved in cholesterol and lipid metabolism, also influences cognitive function and injury repair. In humans, apoE is expressed in three isoforms. E4 is a risk factor for age-related cognitive decline and Alzheimer's disease, particularly in women. E4 might also be a risk factor for developing behavioral and cognitive changes following (56) Fe irradiation, a component of the space environment astronauts are exposed to during missions. These changes might be related to enhanced generation of reactive oxygen species (ROS). In this study, we compared the behavioral and cognitive performance of sham-irradiated and irradiated wild-type (WT) mice and mice expressing the human E3 or E4 isoforms, and assessed the generation of ROS in hippocampal slices from these mice. E4 mice had greater anxiety-like and conditioned fear behaviors than WT mice, and these genotype differences were associated with greater levels of ROS in E4 than WT mice. The greater generation of ROS in the hippocampus of E4 than WT mice might contribute to their higher anxiety levels and enhanced fear conditioning. In E4, but not WT, mice, phorbol-12-myristate-13-acetate-treated hippocampal slices showed more dihydroxy ethidium oxidation in sham-irradiated than irradiated mice and hippocampal heme oxygenase-1 levels were higher in irradiated than sham-irradiated E4 mice. Mice with apolipoprotein E4 (E4), a risk factor for Alzheimer's disease, have greater anxiety-like and conditioned fear behaviors than wild-type (WT) mice. Generation of reactive oxygen species (ROS, in red) 3 months following (56) Fe irradiation, a component of the space environment astronauts are exposed to, is more pronounced in the hippocampus of E4 than WT mice. In E4, but not WT, mice, hippocampal levels of the oxidative stress-relevant marker heme oxygenase-1 are higher in irradiated than sham-irradiated E4 mice.

Individual variations in dose response for spatial memory learning among outbred wistar rats exposed from 5 to 20 cGy of (56) Fe particles

Exposures of brain tissue to ionizing radiation can lead to persistent deficits in cognitive functions and behaviors. However, little is known about the quantitative relationships between exposure dose and neurological risks, especially for lower doses and among genetically diverse individuals. We investigated the dose relationship for spatial memory learning among genetically outbred male Wistar rats exposed to graded doses of (56) Fe particles (sham, 5, 10, 15, and 20 cGy; 1 GeV/n). Spatial memory learning was assessed on a Barnes maze using REL3 ratios measured at three months after exposure. Irradiated animals showed dose-dependent declines in spatial memory learning that were fit by a linear regression (P for slope <0.0002). The irradiated animals showed significantly impaired learning at 10 cGy exposures, no detectable learning between 10 and 15 cGy, and worsened performances between 15 and 20 cGy. The proportions of poor learners and the magnitude of their impairment were fit by linear regressions with doubling doses of ∼10 cGy. In contrast, there were no detectable deficits in learning among the good learners in this dose range. Our findings suggest that genetically diverse individuals can vary substantially in their spatial memory learning, and that exposures at low doses appear to preferentially impact poor learners. This hypothesis invites future investigations of the genetic and physiological mechanisms of inter-individual variations in brain function related to spatial memory learning after low-dose HZE radiation exposures and to determine whether it also applies to physical trauma to brain tissue and exposures to chemical neurotoxicants. Environ. Mol. Mutagen. 57:331-340, 2016. © 2016 Wiley Periodicals, Inc.

Effect of behavioral testing on spine density of basal dendrites in the CA1 region of the hippocampus modulated by (56)Fe irradiation

A unique feature of the space radiation environment is the presence of high-energy charged particles, including (56)Fe ions, which can present a significant hazard to space flight crews during and following a mission. (56)Fe irradiation-induced cognitive changes often involve alterations in hippocampal function. These alterations might involve changes in spine morphology and density. In addition to irradiation, performing a cognitive task can also affect spine morphology. Therefore, it is often hard to determine whether changes in spine morphology and density are due to an environmental challenge or group differences in performance on cognitive tests. In this study, we tested the hypothesis that the ability of exploratory behavior to increase specific measures of hippocampal spine morphology and density is affected by (56)Fe irradiation. In sham-irradiated mice, exploratory behavior increased basal spine density in the CA1 region of the hippocampus and the enclosed blade of the dentate gyrus. These effects were not seen in irradiated mice. In addition, following exploratory behavior, there was a trend toward a decrease in the percent stubby spines on apical dendrites in the CA3 region of the hippocampus in (56)Fe-irradiated, but not sham-irradiated, mice. Other hippocampal regions and spine measures affected by (56)Fe irradiation showed comparable radiation effects in behaviorally naïve and cognitively tested mice. Thus, the ability of exploratory behavior to alter spine density and morphology in specific hippocampal regions is affected by (56)Fe irradiation.

Post-training gamma irradiation-enhanced contextual fear memory associated with reduced neuronal activation of the infralimbic cortex

The brain might be exposed to irradiation under a variety of situations, including clinical treatments, nuclear accidents, dirty bomb scenarios, and military and space missions. Correctly recalling tasks learned prior to irradiation is important but little is known about post-learning effects of irradiation. It is not clear whether exposure to X-ray irradiation during memory consolidation, a few hours following training, is associated with altered contextual fear conditioning 24h after irradiation and which brain region(s) might be involved in these effects. Brain immunoreactivity patterns of the immediately early gene c-Fos, a marker of cellular activity was used to determine which brain areas might be altered in post-training irradiation memory retention tasks. In this study, we show that post-training gamma irradiation exposure (1 Gy) enhanced contextual fear memory 24h later and is associated with reduced cellular activation in the infralimbic cortex. Reduced GABA-ergic neurotransmission in parvalbumin-positive cells in the infralimbic cortex might play a role in this post-training radiation-enhanced contextual fear memory.

Impairment in extinction of contextual and cued fear following post-training whole-body irradiation

Because of the use of radiation in cancer therapy, the risk of nuclear contamination from power plants, military conflicts, and terrorism, there is a compelling scientific and public health interest in the effects of environmental radiation exposure on brain function, in particular hippocampal function and learning and memory. Previous studies have emphasized changes in learning and memory following radiation exposure. These approaches have ignored the question of how radiation exposure might impact recently acquired memories, which might be acquired under traumatic circumstances (cancer treatment, nuclear disaster, etc.). To address the question of how radiation exposure might affect the processing and recall of recently acquired memories, we employed a fear conditioning paradigm wherein animals were trained, and subsequently irradiated (whole-body X-ray irradiation) 24 h later. Animals were given 2 weeks to recover, and were tested for retention and extinction of hippocampus-dependent contextual fear conditioning or hippocampus-independent cued fear conditioning. Exposure to irradiation following training was associated with reduced daily increases in body weights over the 22-days of the study and resulted in greater freezing levels and aberrant extinction 2 weeks later. This was also observed when the intensity of the training protocol was increased. Cued freezing levels and measures of anxiety 2 weeks after training were also higher in irradiated than sham-irradiated mice. In contrast to contextual freezing levels, cued freezing levels were even higher in irradiated mice receiving 5 shocks during training than sham-irradiated mice receiving 10 shocks during training. In addition, the effects of radiation on extinction of contextual fear were more profound than those on the extinction of cued fear. Thus, whole-body irradiation elevates contextual and cued fear memory recall.

Lipopolysaccharide sensitized male and female juvenile brains to ionizing radiation

Radiotherapy is an effective tool in the treatment of pediatric malignancies but it is associated with adverse side effects, both short- and long-term. One common long-term side effect after cranial radiotherapy is cognitive impairment and this is, at least partly, thought to be caused by reduced hippocampal neurogenesis. Neuroinflammation and a perturbed microenvironment are thought to be important in the dysregulation of neurogenesis seen after irradiation (IR). We investigated the effects of a pre-existing, lipopolysaccharide (LPS)-induced systemic inflammation at the time of IR in both males and females. A single dose of 8 Gy to the brain of postnatal day 14 mice caused an upregulation of cytokines/chemokines (IL-1β, MIP-1β, IL-12, GM-CSF, MIP-1α, IL-17, CCL2 and KC) 6 h after IR, more so in females. Caspase-3 activity, reflecting apoptosis and possibly microglia activation, was elevated 6 h after IR. Females treated with LPS before IR showed a higher caspase-3 activity compared with males. During the chronic phase (3 months post IR), we found that LPS-induced inflammation at the time of IR aggravated the IR-induced injury in both male and female mice, as judged by reduced bromodeoxyuridine incorporation and neurogenesis (doublecortin-positive cells) in the hippocampus. At this late time point, the microglia density was increased by IR, more so in females, indicating long-term effects on the microenvironment. IR increased anxiety-related behavior in vehicle-, but not LPS-, treated animals. However, exploratory behavior was affected by IR in both vehicle- and LPS-treated mice. In conclusion, we found that LPS administration before IR of the young mouse brain aggravated the injury, as judged by reduced hippocampal neurogenesis. This supports the clinical practice to postpone radiotherapy if the patient shows signs of infection. Systemic inflammation is not always obvious, though, for example because of concurrent corticosteroid treatment, so careful monitoring of inflammation is warranted.