Ionizing radiation, cerebrovascular disease, and consequent dementia: A review and proposed framework relevant to space radiation exposure

Brains in space: impact of microgravity and cosmic radiation on the CNS during space exploration

Solar system exploration is a grand endeavour of humankind. Space agencies have been planning crewed missions to the Moon and Mars for several decades. However, several environmental stress factors in space, such as microgravity and cosmic radiation, confer health risks for human explorers. This Review examines the effects of microgravity and exposure to cosmic radiation on the CNS. Microgravity presents challenges for the brain, necessitating the development of adaptive movement and orientation strategies to cope with alterations in sensory information. Exposure to microgravity also affects cognitive function to a certain extent. Recent MRI results show that microgravity affects brain structure and function. Post-flight recovery from these changes is gradual, with some lasting up to a year. Regarding cosmic radiation, animal experiments suggest that the brain could be much more sensitive to this stressor than may be expected from experiences on Earth. This may be due to the presence of energetic heavy ions in space that have an impact on cognitive function, even at low doses. However, all data about space radiation risk stem from rodent experiments, and extrapolation of these data to humans carries a high degree of uncertainty. Here, after presenting an overview of current knowledge in the above areas, we provide a concise description of possible counter-measures to protect the brain against microgravity and cosmic radiation during future space missions.

Effects of Space Flight on Inflammasome Activation in the Brain of Mice

Space flight exposes astronauts to stressors that alter the immune response, rendering them vulnerable to infections and diseases. In this study, we aimed to determine the levels of inflammasome activation in the brains of mice that were housed in the International Space Station (ISS) for 37 days. C57BL/6 mice were launched to the ISS as part of NASA's Rodent Research 1 Mission on SpaceX-4 CRS-4 Dragon cargo spacecraft from 21 September 2014 to 25 October 2014. Dissected mouse brains from that mission were analyzed by immunoblotting of inflammasome signaling proteins and Electrochemiluminescence Immunoassay (ECLIA) for inflammatory cytokine levels. Our data indicate decreased inflammasome activation in the brains of mice that were housed in the ISS for 37 days when compared to the brains of mice that were maintained on the ground, and in mice corresponding to the baseline group that were sacrificed at the time of launching of SpaceX-4. Moreover, we did not detect any significant changes in the expression levels of the pro-inflammatory cytokines TNF-α, IL-2, IFN-γ, IL-5, IL-6, IL-12p70 and IL-10 between the ground control and the flight groups. Together, these studies suggest that spaceflight results in a decrease in the levels of innate immune signaling molecules that govern inflammasome signaling in the brain of mice.

Long-lasting sex-specific alteration in left ventricular cardiac transcriptome following gamma and simGCRsim radiation

Space irradiation (IR) is an important health risk for deep-space missions. We reported heart failure with preserved ejection fraction like cardiac phenotype 660-days following exposure to a single-dose of a simplified galactic cosmic ray simulation (simGCRsim) only in males with functional and structural impairment in left ventricular (LV) function. This sex-based dichotomy prompted us to investigate sex-specific changes in the LV transcriptome in three-month-old male and female mice exposed to 137Cs-γ- or simGCRsim-IR. Non-IR male and female (10 each) mice served as controls. LVs were collected at 440/660- and 440/550-days post-IR, male and female, respectively. RNA sequencing, differential gene expression, and functional annotation were performed on tissues from 5 mice/group. Sex and post-IR time points had the greatest influence on gene expression, surpassing the IR-type effects. SimGCRsim-IR showed more persistent transcriptome changes than γ-IR. We suggest that the single IR effects can persist up to 550-660 days, with overwhelmingly sex-biased responses at individual gene expression level.

Molecular mechanisms of neurofilament alterations and its application in assessing neurodegenerative disorders

Neurofilaments are intermediate filaments present in neurons. These provide structural support and maintain the size and shape of the neurons. Dysregulation, mutation, and aggregation of neurofilaments raise the levels of these proteins in the blood and cerebrospinal fluid (CSF), which are characteristic features of axonal damage and certain rare neurological diseases, such as Giant Axonal Neuropathy and Charcot-Mare-Tooth disease. Understanding the structure, dynamics, and function of neurofilaments has been greatly enhanced by a diverse range of biochemical and preclinical investigations conducted over more than four decades. Recently, there has been a resurgence of interest in post-translational modifications of neurofilaments, such as phosphorylation, aggregation, mutation, oxidation, etc. Over the past twenty years, several rare disorders have been studied from structural alterations of neurofilaments. These disorders are monitored by fluid biomarkers such as neurofilament light chains. Currently, there are many tools, such as Enzyme-Linked Immunosorbent Assay, Electrochemiluminescence Assay, Single-Molecule Array, Western/immunoblotting, etc., in use to assess the neurofilament proteins in Blood and CSF. However, all these techniques utilize expensive, non-specific, or antibody-based methods, which make them unsuitable for routine screening of neurodegenerative disorders. This provides room to search for newer sensitive, cost-effective, point-of-care tools for rapid screening of the disease. For a long time, the molecular mechanisms of neurofilaments have been poorly understood due to insufficient research attempts, and a deeper understanding of them remains elusive. Therefore, this review aims to highlight the available literature on molecular mechanisms of neurofilaments and the function of neurofilaments in axonal transport, axonal conduction, axonal growth, and neurofilament aggregation, respectively. Further, this review discusses the role of neurofilaments as potential biomarkers for the identification of several neurodegenerative diseases in clinical laboratory practice.

Oxidative stress, neuroinflammation, and the blood-brain barrier biomarkers on the brain response to spaceflight

Prolonged spaceflight can induce physiologic and pathologic abnormalities in the central nervous system (CNS). Our knowledge of the adaptive and/or detrimental effects of spaceflight on the structure and function of the nervous system is limited. Substantial effort has been devoted to identifying and developing reliable indicators to characterize and predict CNS injury and dysfunction associated with prolonged exposure to major components of the space environment including microgravity, physiological/psychological stress, and radiation from galactic cosmic rays (GCR) and solar particle events (SPEs) outside of low earth orbit (LEO). The blood-brain barrier (BBB) is a semi-permeable membrane that is essential to maintain homeostasis of the brain microenvironment. Oxidative stress or other environmental stressors may disrupt BBB integrity and increase permeability leading to immune cell infiltration and undesirable neuroinflammation. The focus of this review article is on BBB damage associated with spaceflight and space radiation in rodent and human studies. We will highlight potential biomarkers for this damage, including site-specific and circulating neuroinflammatory factors, BBB structural and brain parenchyma proteins, and neuroimaging tools for BBB damage evaluation. These knowledge will help to understand the risks associated with space travel and are also critical for novel countermeasure development to mitigate the space flight risk to astronaut performances.

AOP report: Development of an adverse outcome pathway for deposition of energy leading to learning and memory impairment

Understanding radiation-induced non-cancer effects on the central nervous system (CNS) is essential for the risk assessment of medical (e.g., radiotherapy) and occupational (e.g., nuclear workers and astronauts) exposures. Herein, the adverse outcome pathway (AOP) approach was used to consolidate relevant studies in the area of cognitive decline for identification of research gaps, countermeasure development, and for eventual use in risk assessments. AOPs are an analytical construct describing critical events to an adverse outcome (AO) in a simplified form beginning with a molecular initiating event (MIE). An AOP was constructed utilizing mechanistic information to build empirical support for the key event relationships (KERs) between the MIE of deposition of energy to the AO of learning and memory impairment through multiple key events (KEs). The evidence for the AOP was acquired through a documented scoping review of the literature. In this AOP, the MIE is connected to the AO via six KEs: increased oxidative stress, increased deoxyribonucleic acid (DNA) strand breaks, altered stress response signaling, tissue resident cell activation, increased pro-inflammatory mediators, and abnormal neural remodeling that encompasses atypical structural and functional alterations of neural cells and surrounding environment. Deposition of energy directly leads to oxidative stress, increased DNA strand breaks, an increase of pro-inflammatory mediators and tissue resident cell activation. These KEs, which are themselves interconnected, can lead to abnormal neural remodeling impacting learning and memory processes. Identified knowledge gaps include improving quantitative understanding of the AOP across several KERs and additional testing of proposed modulating factors through experimental work. Broadly, it is envisioned that the outcome of these efforts could be extended to other cognitive disorders and complement ongoing work by international radiation governing bodies in their review of the system of radiological protection.

APOE-ε4 Alleles Modify the Decline of MMSE Scores Associated With Time-Dependent PM2.5 Exposure: Findings From a Community-Based Longitudinal Cohort Study

OBJECTIVE

Limited research has explored the long-term effect of reduced PM2.5 exposure on cognitive function. This study aimed to investigate the effects of time-dependent PM2.5 exposure and the interactions of PM2.5 and aging on declines in Mini-Mental State Examination (MMSE) scores, in carriers and non-carriers of the APOE-ε4 allele.

METHODS

Participants aged over 60 were recruited for this cohort study, undergoing MMSE tests twice from the Taiwan Biobank Program from 2008 to 2020. Participants with dementia or baseline MMSE scores <24 were excluded. Annual PM2.5 levels were estimated using a hybrid kriging/land use regression model with extreme gradient boosting, treated as a time-dependent variable. Generalized estimating equations were used to assess the impacts of repeated PM2.5 on MMSE decline, further stratified by the presence of APOE-ε4 alleles.

RESULTS

After follow-up, 290 participants out of the overall 7,000 community residents in the Biobank dataset demonstrated incidences of MMSE declines (<24), with an average MMSE score decline of 1.11 per year. Participants with ε4/ε4 alleles in the APOE gene had significantly 3.68-fold risks of MMSE decline. High levels of PM2.5 across all visits were significantly associated with worsening of scores on the overall MMSE. As annual levels of PM2.5 decreased over time, the impact of PM2.5 on MMSE decline also slowly diminished.

CONCLUSION

Long-term PM2.5 exposure may be associated with increased risk of MMSE decline, despite improvements in ambient PM2.5 levels over time. Validation of these results necessitates a large-scale prospective cohort study with more concise cognitive screening tools.

Preliminary evidence of high prevalence of cerebral microbleeds in astronauts with spaceflight experience

Long-duration spaceflight poses a variety of health risks to astronauts, largely resulting from extended exposure to microgravity and radiation. Here, we assessed the prevalence and incidence of cerebral microbleeds in sixteen astronauts before and after a typical 6-month mission on board the International Space Station Cerebral microbleeds are microhemorrhages in the brain, which are typically interpreted as early evidence of small vessel disease and have been associated with cognitive impairment. We identified evidence of higher-than-expected microbleed prevalence in astronauts with prior spaceflight experience. However, we did not identify a statistically significant increase in microbleed burden up to 7 months after spaceflight. Altogether, these preliminary findings suggest that spaceflight exposure may increase microbleed burden, but this influence may be indirect or occur over time courses that exceed 1 year. For health monitoring purposes, it may be valuable to acquire neuroimaging data that are able to detect the occurrence of microbleeds in astronauts following their spaceflight missions.

Nourishing the brain on deep space missions: nutritional psychiatry in promoting resilience

The grueling psychological demands of a journey into deep space coupled with ever-increasing distances away from home pose a unique problem: how can we best take advantage of the benefits of fresh foods in a place that has none? Here, we consider the biggest challenges associated with our current spaceflight food system, highlight the importance of supporting optimal brain health on missions into deep space, and discuss evidence about food components that impact brain health. We propose a future food system that leverages the gut microbiota that can be individually tailored to best support the brain and mental health of crews on deep space long-duration missions. Working toward this goal, we will also be making investments in sustainable means to nourish the crew that remains here on spaceship Earth.