Space-Like Irradiation Exacerbated Cognitive Deficits and Amyloid Pathology in CRND8 Mouse Model of Alzheimer's Disease

Background

Space radiation was linked to neurological damage and behavioral deficits which raised concerns of increased degenerative risk on the brain and development of Alzheimer's disease following space travel.

Objective

In this study, we investigated the effects of irradiation by 56Fe and 28Si in CRND8 mice, an Alzheimer's disease mouse model.

Methods

Six-month-old CRND8 mice were exposed to whole body irradiation by 56Fe and 28Si at 0.5 Gy and 2 Gy doses. Behavior tests were administered 1-month to 3-months post-irradiation. Amyloid deposition and other pathological changes were analyzed 3-months and/or 6-months post-irradiatio.

Results

The Novel Object Recognition test showed some decline in 8-month-old mice compared to non-irradiated CRND8 mice. Male mice also showed a loss of freezing behavior in the fear conditioning contextual test following irradiation. Golgi staining revealed a loss of spines in hippocampal neurons after irradiation. Total amyloid immunohistochemistry showed a robust increase in 3-months post-irradiation 56Fe groups which became normalized to non-irradiated group by 6-months post-irradiation. However, 2 Gy 28Si caused a trend towards increased plaque load at 3-months post-irradiation which became significant at 6-months post irradiation only in male CRND8 mice. While 0.5 Gy Fe did not induce obvious changes in the total number of iba-1 positive microglia, more hippocampal microglia were found to express PCNA after 0.5 Gy Fe treatment, suggesting potential involvement of microglial dysfunction.

Conclusions

Overall, our study provides new evidence of gender-specific and ion-dependent effects of space radiation on cognition and amyloid pathology in AD models.

Fine motor deficits exhibited in rat string-pulling behavior following exposure to sleep fragmentation and deep space radiation

Deep space flight missions will expose astronauts to multiple stressors, including sleep fragmentation and space radiation. There is debate over whether sleep disruptions are an issue in deep space. While these stressors independently impair sensorimotor function, the combined effects on performance are currently unknown. String-pulling behavior involves highly organized bimanual reach-to-grasp and withdraw movements. This behavior was examined under rested wakeful conditions and immediately following one session of sleep fragmentation in Sham and irradiated rats 3 months after exposure (10 cGy ⁴Helium or 5-ion simulated Galactic Cosmic Radiation). Sleep fragmentation disrupted several aspects of string-pulling behavior, such that rats' ability to grasp the string was reduced, reach endpoint concentration was more variable, and distance traveled by the nose increased in the Y-range compared to rested wakeful performance. Overall, irradiated rats missed the string more than Sham rats 3 months post-exposure. Irradiated rats also exhibited differential impairments at 3 months, with additional deficits unveiled after sleep fragmentation. ⁴Helium-exposed rats took longer to approach the string after sleep fragmentation. Further, rats exposed to ⁴Helium traveled shorter withdraw distances 3 months after irradiation, while this only emerged in the other irradiated group after sleep fragmentation. These findings identify sleep fragmentation as a risk for fine motor dysfunction in Sham and irradiated conditions, in addition to radiation exposure. There may be complex temporal alterations in performance that are stressor- and ion-dependent. Thus, it is critical to implement appropriate models of multi-flight stressors and performance assessments in preparation for future deep space flight missions.

Targeting hippocampal neurogenesis to protect astronauts' cognition and mood from decline due to space radiation effects

Neurogenesis is an essential, lifelong process during which neural stem cells generate new neurons within the hippocampus, a center for learning, memory, and mood control. Neural stem cells are vulnerable to environmental insults spanning from chronic stress to radiation. These insults reduce their numbers and diminish neurogenesis, leading to memory decline, anxiety, and depression. Preserving neural stem cells could thus help prevent these neurogenesis-associated pathologies, an outcome particularly important for long-term space missions where environmental exposure to radiation is significantly higher than on Earth. Multiple developments, from mechanistic discoveries of radiation injury on hippocampal neurogenesis to new platforms for the development of selective, specific, effective, and safe small molecules as neurogenesis-protective agents hold great promise to minimize radiation damage on neurogenesis. In this review, we summarize the effects of space-like radiation on hippocampal neurogenesis. We then focus on current advances in drug discovery and development and discuss the nuclear receptor TLX/NR2E1 (oleic acid receptor) as an example of a neurogenic target that might rescue neurogenesis following radiation.

Quantitative proteomic analytic approaches to identify metabolic changes in the medial prefrontal cortex of rats exposed to space radiation

NASA’s planned mission to Mars will result in astronauts being exposed to ∼350 mSv/yr of Galactic Cosmic Radiation (GCR). A growing body of data from ground-based experiments indicates that exposure to space radiation doses (approximating those that astronauts will be exposed to on a mission to Mars) impairs a variety of cognitive processes, including cognitive flexibility tasks. Some studies report that 33% of individuals may experience severe cognitive impairment. Translating the results from ground-based rodent studies into tangible risk estimates for astronauts is an enormous challenge, but it would be germane for NASA to use the vast body of data from the rodent studies to start developing appropriate countermeasures, in the expectation that some level of space radiation (SR) -induced cognitive impairment could occur in astronauts. While some targeted studies have reported radiation-induced changes in the neurotransmission properties and/or increased neuroinflammation within space radiation exposed brains, there remains little information that can be used to start the development of a mechanism-based countermeasure strategy. In this study, we have employed a robust label-free mass spectrometry (MS) -based untargeted quantitative proteomic profiling approach to characterize the composition of the medial prefrontal cortex (mPFC) proteome in rats that have been exposed to 15 cGy of 600 MeV/n28Si ions. A variety of analytical techniques were used to mine the generated expression data, which in such studies is typically hampered by low and variable sample size. We have identified several pathways and proteins whose expression alters as a result of space radiation exposure, including decreased mitochondrial function, and a further subset of proteins differs in rats that have a high level of cognitive performance after SR exposure in comparison with those that have low performance levels. While this study has provided further insight into how SR impacts upon neurophysiology, and what adaptive responses can be invoked to prevent the emergence of SR-induced cognitive impairment, the main objective of this paper is to outline strategies that can be used by others to analyze sub-optimal data sets and to identify new information.