Cardiovascular risks associated with low dose ionizing particle radiation

Cosmic Radiations and the Cardiovascular System: A Narrative Review

In recent times, space flights receive continued interest. Humankind's next two goals are to return to the Moon and, a few years later, to land on the surface of Mars. Although technology will improve enough to enable long voyages, there are still some unresolved questions about the effects of the space environment on human health, including the effects of such long voyages on organs. Specifically, there is no information on the effects of radiation in space on the human cardiovascular system. To better understand the adaptation of the cardiovascular system to radiation exposure, the physical properties of radiation and the cellular and molecular mechanisms underlying tissue changes are essential. To this end, this article aims to provide an overview of the effects of radiation on the cardiovascular system by analyzing the physical properties of radiation and their relationship to cellular and molecular mechanisms and potential changes. Each type of radiation triggers different responses in the cardiovascular system. Radiation plays a relevant role in altering endothelial function and arterial wall stiffness by inducing vascular changes that accelerate atherosclerosis and affect endothelial adhesiveness. Clinical studies have shown that vascular changes due to radiation depend on the delayed manifestations of early radiation damage. To reduce the effects of radiation in space, some pharmacological treatments that seem to be able to counteract oxidative stress during flight are being used. At the same time, new shielding systems that can reduce or eliminate radiation exposure must be developed. Future studies should aim to replicate flights in the deep space environment to study in more detail the harmful effects of radiation on the whole cardiovascular system.

Simulated galactic cosmic radiation-induced cancer progression in mice

Prolonged manned space flight exposure risks to galactic comic radiation, has led to uncertainties in a variety of health risks. Our previous work, utilizing either single ion or multiple ion radiation exposure conducted at the NSRL (NASA Space Radiation Laboratory, Brookhaven, NY) demonstrated that HZE ion components of the GCR result in persistent inflammatory signaling, increased mutations, and higher rates of cancer initiation and progression. With the development of the 33-beam galactic cosmic radiation simulations (GCRsim) at the NSRL, we can more closely test on earth the radiation environment found in space. With a previously used lung cancer susceptible mouse model (K-rasLA-1), we performed acute exposure experiments lasting 1-2 h, and chronic exposure experiments lasting 2-6 weeks with a total dose of 50 cGy and 75 cGy. We obtained histological samples from a subset of mice 100 days post-irradiation, and the remaining mice were monitored for overall survival up to 1-year post-irradiation. When we compared acute exposures (1-2 hrs.) and chronic exposure (2-6 weeks), we found a trend in the increase of lung adenocarcinoma respectively for a total dose of 50 cGy and 75 cGy. Furthermore, when we added neutron exposure to the 75 cGy of GCRsim, we saw a further increase in the incidence of adenocarcinoma. We interpret these findings to suggest that the risks of carcinogenesis are heightened with doses anticipated during a round trip to Mars, and this risk is magnified when coupled with extra neutron exposure that are expected on the Martian surface. We also observed that risks are reduced when the NASA official 33-beam GCR simulations are provided at high dose rates compared to low dose rates.

Therapeutic strategies based on non-ionizing radiation to prevent venous neointimal hyperplasia: the relevance for stenosed arteriovenous fistula, and the role of vascular compliance

We have reviewed the development and current status of therapies based on exposure to non-ionizing radiation (with a photon energy less than 10 eV) aimed at suppressing the venous neointimal hyperplasia, and consequentially at avoiding stenosis in arteriovenous grafts. Due to the drawbacks associated with the medical use of ionizing radiation, prominently the radiation-induced cardiovascular disease, the availability of procedures using non-ionizing radiation is becoming a noteworthy objective for the current research. Further, the focus of the review was the use of such procedures for improving the vascular access function and assuring the clinical success of arteriovenous fistulae in hemodialysis patients. Following a brief discussion of the physical principles underlying radiotherapy, the current methods based on non-ionizing radiation, either in use or under development, were described in detail. There are currently five such techniques, including photodynamic therapy (PDT), far-infrared therapy, photochemical tissue passivation (PTP), Alucent vascular scaffolding, and adventitial photocrosslinking. The last three are contingent on the mechanical stiffening achievable by the exogenous photochemical crosslinking of tissular collagen, a process that leads to the decrease of venous compliance. As there are conflicting opinions on the role of compliance mismatch between arterial and venous conduits in a graft, this aspect was also considered in our review.

Chromosomal positioning and epigenetic architecture influence DNA methylation patterns triggered by galactic cosmic radiation

Despite surging interest in space travel in recent decades, the impacts of prolonged, elevated exposure to galactic cosmic radiation (GCR) on human health remain poorly understood. This form of ionizing radiation causes significant changes to biological systems including damage to DNA structure by altering epigenetic phenotype with emphasis on DNA methylation. Building on previous work by Kennedy et al. (Sci Rep 8(1): 6709. 10.1038/S41598-018-24755-8), we evaluated spatial DNA methylation patterns triggered by high-LET (56Fe, 28Si) and low-LET (X-ray) radiation and the influence of chromosome positioning and epigenetic architecture in distinct radial layers of cell nucleus. Next, we validated our results using gene expression data of mice irradiated with simulated GCR and JAXA astronauts. We showed that primarily 56Fe induces a persistent DNA methylation increase whereas 28Si and X-ray induce a decrease DNA methylation which is not persistent with time. Moreover, we highlighted the role of nuclear chromatin architecture in cell response to external radiation. In summary, our study provides novel insights towards epigenetic and transcriptomic response as well as chromatin multidimensional structure influence on galactic cosmic radiation damage.

Lifetime evaluation of left ventricular structure and function in male ApoE null mice after gamma and space-type radiation exposure

The space radiation (IR) environment contains high charge and energy (HZE) nuclei emitted from galactic cosmic rays with the ability to overcome current shielding strategies, posing increased IR-induced cardiovascular disease risks for astronauts on prolonged space missions. Little is known about the effect of 5-ion simplified galactic cosmic ray simulation (simGCRsim) exposure on left ventricular (LV) function. Three-month-old, age-matched male Apolipoprotein E (ApoE) null mice were irradiated with 137Cs gamma (γ; 100, 200, and 400 cGy) and simGCRsim (50, 100, 150 cGy all at 500 MeV/nucleon (n)). LV function was assessed using transthoracic echocardiography at early/acute (14 and 28 days) and late/degenerative (365, 440, and 660 days) times post-irradiation. As early as 14 and 28-days post IR, LV systolic function was reduced in both IR groups across all doses. At 14 days post-IR, 150 cGy simGCRsim-IR mice had decreased diastolic wall strain (DWS), suggesting increased myocardial stiffness. This was also observed later in 100 cGy γ-IR mice at 28 days. At later stages, a significant decrease in LV systolic function was observed in the 400 cGy γ-IR mice. Otherwise, there was no difference in the LV systolic function or structure at the remaining time points across the IR groups. We evaluated the expression of genes involved in hemodynamic stress, cardiac remodeling, inflammation, and calcium handling in LVs harvested 28 days post-IR. At 28 days post-IR, there is increased expression of Bnp and Ncx in both IR groups at the lowest doses, suggesting impaired function contributes to hemodynamic stress and altered calcium handling. The expression of Gals3 and β-Mhc were increased in simGCRsim and γ-IR mice respectively, suggesting there may be IR-specific cardiac remodeling. IR groups were modeled to calculate the Relative Biological Effectiveness (RBE) and Radiation Effects Ratio (RER). No lower threshold was determined using the observed dose-response curves. These findings do not exclude the possibility of the existence of a lower IR threshold or the presence of IR-induced cardiovascular disease (CVD) when combined with additional space travel stressors, e.g., microgravity.

Cardiovascular disease in space: A systematic review

BACKGROUND

With expanding commercial space programs, uncertainty remains about the cardiovascular effects of space environmental exposures including microgravity, confinement, isolation, space radiation, and altered bacterial virulence. Current limited data suggests additional health threats compared to Earth.

METHODS

We systematically reviewed PubMed, CENTRAL, Web of Science, EMBASE and Cochrane databases for prospective studies on spaceflight and cardiovascular outcomes. Search terms combined cardiovascular disease topics with spaceflight concepts. No date or language restrictions were imposed.

RESULTS

35 studies representing 2696 space travelers met inclusion criteria. Studies were grouped into spaceflight associations with: atherosclerosis, mortality, cardiac function, orthostatic intolerance, and arrhythmias. Atherosclerosis evidence was limited, with animal studies linking space radiation to endothelial damage, oxidative stress, and inflammation. However, human data showed no significantly increased atherosclerotic disease in astronauts. Mortality studies demonstrated lower cardiovascular mortality in astronauts compared to the general population however there was conflicting data. Cardiac function studies revealed physiologic ventricular atrophy, increased arterial stiffness, and altered blood flow distribution attributed to microgravity exposure. Effects appeared transient and reversible post-flight. Orthostatic intolerance studies found astronauts experienced altered heart rate variability, baroreflex response, and blood pressure changes post-flight. Arrhythmia studies showed increased ventricular ectopy during spaceflight, but limited data on long term flights.

CONCLUSIONS

Environmental space hazards impact the cardiovascular system through multiple mechanisms. Microgravity causes cardiac atrophy and orthostatic intolerance while space radiation may potentially accelerate atherosclerosis. Further research is needed, especially regarding long-term spaceflights.

Impact of Microgravity and Other Spaceflight Factors on Retina of Vertebrates and Humans In Vivo and In Vitro

Spaceflight (SF) increases the risk of developmental, regenerative, and physiological disorders in animals and humans. Astronauts, besides bone loss, muscle atrophy, and cardiovascular and immune system alterations, undergo ocular disorders affecting posterior eye tissues, including the retina. Few studies revealed abnormalities in the development and changes in the regeneration of eye tissues in lower vertebrates after SF and simulated microgravity. Under microgravity conditions, mammals show disturbances in the retinal vascular system and increased risk of oxidative stress that can lead to cell death in the retina. Animal studies provided evidence of gene expression changes associated with cellular stress, inflammation, and aberrant signaling pathways. Experiments using retinal cells in microgravity-modeling systems in vitro additionally indicated micro-g-induced changes at the molecular level. Here, we provide an overview of the literature and the authors' own data to assess the predictive value of structural and functional alterations for developing countermeasures and mitigating the SF effects on the human retina. Further emphasis is given to the importance of animal studies on the retina and other eye tissues in vivo and retinal cells in vitro aboard spacecraft for understanding alterations in the vertebrate visual system in response to stress caused by gravity variations.

Effects of proton and oxygen ion irradiation on cardiovascular function and structure in a rabbit model

PURPOSE

Astronauts on missions beyond low Earth orbit will be exposed to galactic cosmic radiation, and there is concern about potential adverse cardiovascular effects. Most of the research to identify cardiovascular risk of space radiation has been performed in rodent models. To aid in the translation of research results to humans, the current study identified long-term effects of high-energy charged particle irradiation on cardiovascular function and structure in a larger non-rodent animal model.

MATERIALS AND METHODS

At the age of 12 months, male New Zealand white rabbits were exposed to whole-body protons (250 MeV) or oxygen ions (16O, 600 MeV/n) at a dose of 0 or 0.5 Gy and were followed for 12 months after irradiation. Ultrasonography was used to measure in vivo cardiac function and blood flow parameters at 10- and 12-months post-irradiation. At 12 months after irradiation, blood cell counts and blood chemistry values were assessed, and cardiac tissue and aorta were collected for histological as well as molecular and biochemical analyses. Plasma was used for metabolomic analysis and to quantify common markers of cardiac injury.

RESULTS

A small but significant decrease in the percentage of circulating lymphocytes and an increase in neutrophil percentage was seen 12 months after 0.5 Gy protons, while 16O exposure resulted in an increase in monocyte percentage. Markers of cardiac injury, cardiac troponin I (cTnI) and N-Terminal pro-B-type Natriuretic Peptide were modestly increased in the proton group, and cTnI was also increased after 16O. On the other hand, metabolomics on plasma at 12 months revealed no changes. Both types of irradiation demonstrated alterations in cardiac mitochondrial morphology and an increase in left ventricular protein levels of inflammatory cell marker CD68. However, changes in cardiac function were only mild.

CONCLUSION

Low dose charged particle irradiation caused mild long-term changes in inflammatory markers, cardiac function, and structure in the rabbit heart, in line with previous studies in mouse and rat models.

Effects of Simulated 5-Ion Galactic Cosmic Radiation on Function and Structure of the Mouse Heart

Missions into deep space will expose astronauts to the harsh space environment, and the degenerative tissue effects of space radiation are largely unknown. To assess the risks, in this study, male BALB/c mice were exposed to 500 mGy 5-ion simulated GCR (GCRsim) at the NASA Space Radiation Laboratory. In addition, male and female CD1 mice were exposed to GCRsim and administered a diet containing Transforming Growth Factor-beta (TGF-β)RI kinase (ALK5) inhibitor IPW-5371 as a potential countermeasure. An ultrasound was performed to investigate cardiac function. Cardiac tissue was collected to determine collagen deposition, the density of the capillary network, and the expression of the immune mediator toll-like receptor 4 (TLR4) and immune cell markers CD2, CD4, and CD45. In male BALB/c mice, the only significant effects of GCRsim were an increase in the CD2 and TLR4 markers. In male CD1 mice, GCRsim caused a significant increase in total collagens and a decrease in the expression of TLR4, both of which were mitigated by the TGF-β inhibitor diet. In female CD1 mice, GCRsim caused an increase in the number of capillaries per tissue area in the ventricles, which may be explained by the decrease in the left ventricular mass. However, this increase was not mitigated by TGF-β inhibition. In both male and female CD1 mice, the combination of GCRsim and TGF-β inhibition caused changes in left ventricular immune cell markers that were not seen with GCRsim alone. These data suggest that GCRsim results in minor changes to cardiac tissue in both an inbred and outbred mouse strain. While there were few GCRsim effects to be mitigated, results from the combination of GCRsim and the TGF-β inhibitor do point to a role for TGF-β in maintaining markers of immune cells in the heart after exposure to GCR.

Lifetime Evaluation of Left Ventricular Structure and Function in Male C57BL/6J Mice after Gamma and Space-Type Radiation Exposure

The lifetime effects of space irradiation (IR) on left ventricular (LV) function are unknown. The cardiac effects induced by space-type IR, specifically 5-ion simplified galactic cosmic ray simulation (simGCRsim), are yet to be discovered. Three-month-old, age-matched, male C57BL/6J mice were irradiated with 137Cs gamma (γ; 100, 200 cGy) and simGCRsim (50 and 100 cGy). LV function was assessed via transthoracic echocardiography at 14 and 28 days (early), and at 365, 440, and 660 (late) days post IR. We measured the endothelial function marker brain natriuretic peptide in plasma at three late timepoints. We assessed the mRNA expression of the genes involved in cardiac remodeling, fibrosis, inflammation, and calcium handling in LVs harvested at 660 days post IR. All IR groups had impaired global LV systolic function at 14, 28, and 365 days. At 660 days, 50 cGy simGCRsim-IR mice exhibited preserved LV systolic function with altered LV size and mass. At this timepoint, the simGCRsim-IR mice had elevated levels of cardiac fibrosis, inflammation, and hypertrophy markers Tgfβ1, Mcp1, Mmp9, and βmhc, suggesting that space-type IR may induce the cardiac remodeling processes that are commonly associated with diastolic dysfunction. IR groups showing statistical significance were modeled to calculate the Relative Biological Effectiveness (RBE) and Radiation Effects Ratio (RER). The observed dose-response shape did not indicate a lower threshold at these IR doses. A single full-body IR at doses of 100-200 cGy for γ-IR, and 50-100 cGy for simGCRsim-IR decreases the global LV systolic function in WT mice as early as 14 and 28 days after exposure, and at 660 days post IR. Interestingly, there is an intermediate time point (365 days) where the impairment in LV function is observed. These findings do not exclude the possibility of increased acute or degenerative cardiovascular disease risks at lower doses of space-type IR, and/or when combined with other space travel-associated stressors such as microgravity.

Human Health during Space Travel: State-of-the-Art Review

The field of human space travel is in the midst of a dramatic revolution. Upcoming missions are looking to push the boundaries of space travel, with plans to travel for longer distances and durations than ever before. Both the National Aeronautics and Space Administration (NASA) and several commercial space companies (e.g., Blue Origin, SpaceX, Virgin Galactic) have already started the process of preparing for long-distance, long-duration space exploration and currently plan to explore inner solar planets (e.g., Mars) by the 2030s. With the emergence of space tourism, space travel has materialized as a potential new, exciting frontier of business, hospitality, medicine, and technology in the coming years. However, current evidence regarding human health in space is very limited, particularly pertaining to short-term and long-term space travel. This review synthesizes developments across the continuum of space health including prior studies and unpublished data from NASA related to each individual organ system, and medical screening prior to space travel. We categorized the extraterrestrial environment into exogenous (e.g., space radiation and microgravity) and endogenous processes (e.g., alteration of humans' natural circadian rhythm and mental health due to confinement, isolation, immobilization, and lack of social interaction) and their various effects on human health. The aim of this review is to explore the potential health challenges associated with space travel and how they may be overcome in order to enable new paradigms for space health, as well as the use of emerging Artificial Intelligence based (AI) technology to propel future space health research.

Murine models of radiation cardiotoxicity: A systematic review and recommendations for future studies

BACKGROUND AND PURPOSE

The effects of radiation on the heart are dependent on dose, fractionation, overall treatment time, and pre-existing cardiovascular pathology. Murine models have played a central role in improving our understanding of the radiation response of the heart yet a wide range of exposure parameters have been used. We evaluated the study design of published murine cardiac irradiation experiments to assess gaps in the literature and to suggest guidance for the harmonisation of future study reporting.

METHODS AND MATERIALS

A systematic review of mouse/rat studies published 1981-2021 that examined the effect of radiation on the heart was performed. The protocol was published on PROSPERO (CRD42021238921) and the findings were reported in accordance with the PRISMA guidance. Risk of bias was assessed using the SYRCLE checklist.

RESULTS

159 relevant full-text original articles were reviewed. The heart only was the target volume in 67% of the studies and simulation details were unavailable for 44% studies. Dosimetry methods were reported in 31% studies. The pulmonary effects of whole and partial heart irradiation were reported in 13% studies. Seventy-eight unique dose-fractionation schedules were evaluated. Large heterogeneity was observed in the endpoints measured, and the reporting standards were highly variable.

CONCLUSIONS

Current murine models of radiation cardiotoxicity cover a wide range of irradiation configurations and latency periods. There is a lack of evidence describing clinically relevant dose-fractionations, circulating biomarkers and radioprotectants. Recommendations for the consistent reporting of methods and results of in vivo cardiac irradiation studies are made to increase their suitability for informing the design of clinical studies.

MiRNA-Mediated Fibrosis in the Out-of-Target Heart following Partial-Body Irradiation

Recent reports have shown a link between radiation exposure and non-cancer diseases such as radiation-induced heart disease (RIHD). Radiation exposures are often inhomogeneous, and out-of-target effects have been studied in terms of cancer risk, but very few studies have been carried out for non-cancer diseases. Here, the role of miRNAs in the pathogenesis of RIHD was investigated. C57Bl/6J female mice were whole- (WBI) or partial-body-irradiated (PBI) with 2 Gy of X-rays or sham-irradiated (SI). In PBI exposure, the lower third of the mouse body was irradiated, while the upper two-thirds were shielded. From all groups, hearts were collected 15 days or 6 months post-irradiation. The MiRNome analysis at 15 days post-irradiation showed that miRNAs, belonging to the myomiR family, were highly differentially expressed in WBI and PBI mouse hearts compared with SI hearts. Raman spectral data collected 15 days and 6 months post-irradiation showed biochemical differences among SI, WBI and PBI mouse hearts. Fibrosis in WBI and PBI mouse hearts, indicated by the increased deposition of collagen and the overexpression of genes involved in myofibroblast activation, was found 6 months post-irradiation. Using an in vitro co-culture system, involving directly irradiated skeletal muscle and unirradiated ventricular cardiac human cells, we propose the role of miR-1/133a as mediators of the abscopal response, suggesting that miRNA-based strategies could be relevant for limiting tissue-dependent reactions in non-directly irradiated tissues.

Cardiovascular Disease Risk Modeling for Astronauts: Making the Leap From Earth to Space

NASA has recently completed several long-duration missions to the International Space Station and is solidifying plans to return to the Moon, with an eye toward Mars and beyond. As NASA pushes the boundaries of human space exploration, the hazards of spaceflight, including space radiation, levy an increasing burden on astronaut health and performance. The cardiovascular system may be especially vulnerable due to the combined impacts of space radiation exposure, lack of gravity, and other spaceflight hazards. On Earth, the risk for cardiovascular disease (CVD) following moderate to high radiation doses is well-established from clinical, environmental, and occupational exposures (largely from gamma- and x-rays). Less is known about CVD risks associated with high-energy charged ions found in space and increasingly used in radiotherapy applications on Earth, making this a critical area of investigation for occupational radiation protection. Assessing CVD risk is complicated by its multifactorial nature, where an individual's risk is strongly influenced by factors such as family history, blood pressure, and lipid profiles. These known risk factors provide the basis for development of a variety of clinical risk prediction models (CPMs) that inform the likelihood of medical outcomes over a defined period. These tools improve clinical decision-making, personalize care, and support primary prevention of CVD. They may also be useful for individualizing risk estimates for CVD following radiation exposure both in the clinic and in space. In this review, we summarize unique aspects of radiation risk assessment for astronauts, and we evaluate the most widely used CVD CPMs for their use in NASA radiation risk assessment applications. We describe a comprehensive dual-use risk assessment framework that supports both clinical care and operational management of space radiation health risks using quantitative metrics. This approach is a first step in using personalized medicine for radiation risk assessment to support safe and productive spaceflight and long-term quality of life for NASA astronauts.

Late onset cardiovascular dysfunction in adult mice resulting from galactic cosmic ray exposure

The complex and inaccessible space radiation environment poses an unresolved risk to astronaut cardiovascular health during long-term space exploration missions. To model this risk, healthy male c57BL/6 mice aged six months (corresponding to an astronaut of 34 years) were exposed to simplified galactic cosmic ray (GCR5-ion; 5-ion sim) irradiation at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratories (BNL). Multi-modal cardiovascular functional assessments performed longitudinally and terminally revealed significant impairment in cardiac function in mice exposed to GCR5-ion compared to unirradiated controls, gamma irradiation, or single mono-energetic ions (56Fe or 16O). GCR5-ion-treated mice exhibited increased arterial elastance likely mediated by disruption of elastin fibers. This study suggests that a single exposure to GCR5-ion is associated with deterioration in cardiac structure and function that becomes apparent long after exposure, likely associated with increased morbidity and mortality. These findings represent important health considerations when preparing for successful space exploration.

Enhanced Effects of Chronic Restraint-Induced Psychological Stress on Total Body Fe-Irradiation-Induced Hematopoietic Toxicity in Trp53-Heterozygous Mice

Humans are exposed to both psychological stress (PS) and radiation in some scenarios such as manned deep-space missions. It is of great concern to verify possible enhanced deleterious effects from such concurrent exposure. Pioneer studies showed that chronic restraint-induced PS (CRIPS) could attenuate Trp53 functions and increase gamma-ray-induced carcinogenesis in Trp53-heterozygous mice while CRIPS did not significantly modify the effects on X-ray-induced hematopoietic toxicity in Trp53 wild-type mice. As high-linear energy transfer (LET) radiation is the most important component of space radiation in causing biological effects, we further investigated the effects of CRIPS on high-LET iron-particle radiation (Fe)-induced hematopoietic toxicity in Trp53-heterozygous mice. The results showed that CRIPS alone could hardly induce significant alteration in hematological parameters (peripheral hemogram and micronucleated erythrocytes in bone marrow) while concurrent exposure caused elevated genotoxicity measured as micronucleus incidence in erythrocytes. Particularly, exposure to either CRISP or Fe-particle radiation at a low dose (0.1 Gy) did not induce a marked increase in the micronucleus incidence; however, concurrent exposure caused a significantly higher increase in the micronucleus incidence. These findings indicated that CRIPS could enhance the deleterious effects of high-LET radiation, particularly at a low dose, on the hematopoietic toxicity in Trp53-heterozygous mice.

Leveraging Spaceflight to Advance Cardiovascular Research on Earth

The direct (eg, radiation, microgravity) and indirect (eg, lifestyle perturbations) effects of spaceflight extend across multiple systems resulting in whole-organism cardiovascular deconditioning. For over 50 years, National Aeronautics and Space Administration has continually enhanced a countermeasures program designed to characterize and offset the adverse cardiovascular consequences of spaceflight. In this review, we provide a historical overview of research evaluating the effects of spaceflight on cardiovascular health in astronauts and outline mechanisms underpinning spaceflight-related cardiovascular alterations. We also discuss how spaceflight could be leveraged for aging, industry, and model systems such as human induced pluripotent stem cell-derived cardiomyocytes, organoid, and organ-on-a-chip technologies. Finally, we outline the increasing opportunities for scientists and clinicians to engage in cardiovascular research in space and on Earth.

Effects of low-dose oxygen ions on cardiac function and structure in female C57BL/6J mice

PURPOSE

Astronauts in space vehicles beyond low-Earth orbit will be exposed to high charge and energy (HZE) ions, and there is concern about potential adverse effects on the cardiovascular system. Thus far, most animal studies that assess cardiac effects of HZE particles have included only males. This study assessed the effects of oxygen ions (¹⁶O) as a representative ion of the intravehicular radiation environment on the heart of female mice.

MATERIALS AND METHODS

Female C57BL/6 J mice at 6 months of age were exposed to ¹⁶O (600 MeV/n) at 0.25-0.26 Gy/min to a total dose of 0, 0.1, or 0.25 Gy. Cardiac function and abdominal aorta blood velocity were measured with ultrasonography at 3, 5, 7, and 9 months after irradiation. At 2 weeks, 3 months, and 9 months, cardiac tissue was collected to assess collagen deposition and markers of immune cells.

RESULTS

Ultrasonography revealed increased left ventricle mass, diastolic volume and diameter but there was no change in the abdominal aorta. There was no indication of cardiac fibrosis however, a 75 kDa peptide of left ventricular collagen type III and α-smooth muscle cell actin were increased suggesting some remodeling had occurred. Left ventricular protein levels of the T-cell marker CD2 was significantly increased at all time points, while the neutrophil marker myeloperoxidase was decreased at 2 weeks and 9 months.

CONCLUSIONS

These results taken together suggest ¹⁶O ion exposure did not result in cardiac fibrosis or cardiac dysfunction in female mice. However, it does appear mild cardiac remodeling occurs in response to HZE radiation.

Mitigation of late cardiovascular effects of oxygen ion radiation by γ-tocotrienol in a mouse model

PURPOSE

While there is concern about degenerative tissue effects of exposure to space radiation during deep-space missions, there are no pharmacological countermeasures against these adverse effects. γ-Tocotrienol (GT3) is a natural form of vitamin E that has anti-oxidant properties, modifies cholesterol metabolism, and has anti-inflammatory and endothelial cell protective properties. The purpose of this study was to test whether GT3 could mitigate cardiovascular effects of oxygen ion (¹⁶O) irradiation in a mouse model.

MATERIALS AND METHODS

Male C57BL/6 J mice were exposed to whole-body ¹⁶O (600 MeV/n) irradiation (0.26-0.33 Gy/min) at doses of 0 or 0.25 Gy at 6 months of age and were followed up to 9 months after irradiation. Animals were administered GT3 (50 mg/kg/day s.c.) or vehicle, on Monday - Friday starting on day 3 after irradiation for a total of 16 administrations. Ultrasonography was used to measure in vivo cardiac function and blood flow parameters. Cardiac tissue remodeling and inflammatory infiltration were assessed with histology and immunoblot analysis at 2 weeks, 3 and 9 months after radiation.

RESULTS

GT3 mitigated the effects of ¹⁶O radiation on cardiac function, the expression of a collagen type III peptide, and markers of mast cells, T-cells and monocytes/macrophages in the left ventricle.

CONCLUSIONS

GT3 may be a potential countermeasure against late degenerative tissue effects of high-linear energy transfer radiation in the heart.

Fundamental Biological Features of Spaceflight: Advancing the Field to Enable Deep-Space Exploration

Research on astronaut health and model organisms have revealed six features of spaceflight biology that guide our current understanding of fundamental molecular changes that occur during space travel. The features include oxidative stress, DNA damage, mitochondrial dysregulation, epigenetic changes (including gene regulation), telomere length alterations, and microbiome shifts. Here we review the known hazards of human spaceflight, how spaceflight affects living systems through these six fundamental features, and the associated health risks of space exploration. We also discuss the essential issues related to the health and safety of astronauts involved in future missions, especially planned long-duration and Martian missions.

Myocardial Disease and Long-Distance Space Travel: Solving the Radiation Problem

Radiation-induced cardiovascular disease is a well-known complication of radiation exposure. Over the last few years, planning for deep space missions has increased interest in the effects of space radiation on the cardiovascular system, as an increasing number of astronauts will be exposed to space radiation for longer periods of time. Research has shown that exposure to different types of particles found in space radiation can lead to the development of diverse cardiovascular disease via fibrotic myocardial remodeling, accelerated atherosclerosis and microvascular damage. Several underlying mechanisms for radiation-induced cardiovascular disease have been identified, but many aspects of the pathophysiology remain unclear. Existing pharmacological compounds have been evaluated to protect the cardiovascular system from space radiation-induced damage, but currently no radioprotective compounds have been approved. This review critically analyzes the effects of space radiation on the cardiovascular system, the underlying mechanisms and potential countermeasures to space radiation-induced cardiovascular disease.

Ionizing radiation-induced circulatory and metabolic diseases

Risks to health are the prime consideration in all human situations of ionizing radiation exposure and therefore of relevance to radiation protection in all occupational, medical, and public exposure situations. Over the past few decades, advances in therapeutic strategies have led to significant improvements in cancer survival rates. However, a wide range of long-term complications have been reported in cancer survivors, in particular circulatory diseases and their major risk factors, metabolic diseases. However, at lower levels of exposure, the evidence is less clear. Under real-life exposure scenarios, including radiotherapy, radiation effects in the whole organism will be determined mainly by the response of normal tissues receiving relatively low doses, and will be mediated and moderated by systemic effects. Therefore, there is an urgent need for further research on the impact of low-dose radiation. In this article, we review radiation-associated risks of circulatory and metabolic diseases in clinical, occupational or environmental exposure situations, addressing epidemiological, biological, risk modelling, and systems biology aspects, highlight the gaps in knowledge and discuss future directions to address these gaps.

Fate of Hematopoiesis During Aging. What Do We Really Know, and What are its Implications?

There is an ongoing shift in demographics such that older persons will outnumber young persons in the coming years, and with it age-associated tissue attrition and increased diseases and disorders. There has been increased information on the association of the aging process with dysregulation of hematopoietic stem (HSC) and progenitor (HPC) cells, and hematopoiesis. This review provides an extensive up-to date summary on the literature of aged hematopoiesis and HSCs placed in context of potential artifacts of the collection and processing procedure, that may not be totally representative of the status of HSCs in their in vivo bone marrow microenvironment, and what the implications of this are for understanding aged hematopoiesis. This review covers a number of interactive areas, many of which have not been adequately explored. There are still many unknowns and mechanistic insights to be elucidated to better understand effects of aging on the hematopoietic system, efforts that will take multidisciplinary approaches, and that could lead to means to ameliorate at least some of the dysregulation of HSCs and HPCs associated with the aging process. Graphical Abstract.

Simultaneous Exposure of Cultured Human Lymphoblastic Cells to Simulated Microgravity and Radiation Increases Chromosome Aberrations

During space travel, humans are continuously exposed to two major environmental stresses, microgravity (μG) and space radiation. One of the fundamental questions is whether the two stressors are interactive. For over half a century, many studies were carried out in space, as well as using devices that simulated μG on the ground to investigate gravity effects on cells and organisms, and we have gained insights into how living organisms respond to μG. However, our knowledge on how to assess and manage human health risks in long-term mission to the Moon or Mars is drastically limited. For example, little information is available on how cells respond to simultaneous exposure to space radiation and μG. In this study, we analyzed the frequencies of chromosome aberrations (CA) in cultured human lymphoblastic TK6 cells exposed to X-ray or carbon ion under the simulated μG conditions. A higher frequency of both simple and complex types of CA were observed in cells exposed to radiation and μG simultaneously compared to CA frequency in cells exposed to radiation only. Our study shows that the dose response data on space radiation obtained at the 1G condition could lead to the underestimation of astronauts' potential risk for health deterioration, including cancer. This study also emphasizes the importance of obtaining data on the molecular and cellular responses to irradiation under μG conditions.

Why Do the Cosmic Rays Induce Aging?

The increasing duration of space missions involves a progressively higher exposure of astronauts to cosmic rays, whose most hazardous component is made up of High-Atomic number and High-Energy (HZE) ions. HZE ions interact along their tracks with biological molecules inducing changes on living material qualitatively different from that observed after irradiation for therapeutic purposes or following nuclear accidents. HZE ions trigger in cells different responses initialized by DNA damage and mitochondria dysregulation, which cause a prolonged state of sterile inflammation in the tissues. These cellular phenomena may explain why spending time in space was found to cause the onset of a series of diseases normally related to aging. These changes that mimic aging but take place more quickly make space flights also an opportunity to study the mechanisms underlying aging. In this short review, we describe the biological mechanisms underlying cell senescence and aging; the peculiar characteristics of HZE ions, their interaction with living matter and the effects on the organism; the key role of mitochondria in HZE ion-induced health effects and aging-related phenomena.

Effects of single-dose protons or oxygen ions on function and structure of the cardiovascular system in male Long Evans rats

PURPOSE

Studies are required to determine whether exposures to radiation encountered during manned missions in deep space may have adverse effects on the cardiovascular system. Most of the prior studies on effects of simulated space radiation on the heart and vasculature have been performed in mouse models. To provide data from a second animal species, two studies were performed to assess effects of high-energy charged particle radiation on the heart and abdominal aorta in a rat model.

MATERIALS AND METHODS

In study A, male Long Evans rats were exposed to whole-body protons (250 MeV, 0.5 Gy) or oxygen ions (¹⁶O, 600 MeV/n, 0.5 Gy), and ultrasonography was used to measure in vivo cardiac function and blood flow parameters at 3, 5, 9 and 12 months after radiation, followed by tissue collection at 12 months. In study B, male Long Evans rats were exposed to ¹⁶O (1 GeV/n, 0.01-0.25 Gy), and hearts collected at 6 to 7 and 12 months for histology and western-blots.

RESULTS

Both protons (250 MeV) and ¹⁶O (600 MeV/n) caused a decrease in left ventricular posterior wall thickness at 3-5 months, but did not change echocardiographic measures of cardiac function. In Pulsed-wave Doppler assessment of the abdominal aorta, an increase was seen in mean velocity, peak velocity, and velocity time integral at 12 months after ¹⁶O (600 MeV/n), suggesting a change in vascular function. There were no significant changes in histopathology or histological quantification of total collagens in heart or aorta. On the other hand, an increase was seen in a 75 kDa peptide of collagen type III in the left ventricle of rats exposed to protons (250 MeV) and ¹⁶O (600 MeV/n and 1 GeV/n), suggesting that radiation caused remodeling of existing collagens in the heart. ¹⁶O (600 MeV/n and 1 GeV/n) caused increases in left ventricular protein levels of immune cell markers CD2, CD4, CD8, and CD68.

CONCLUSION

A single low dose of whole body protons or ¹⁶O in male Long Evans rats did not change cardiac function or induce gross pathological changes in the heart or aorta, but induced mild changes in vascular function and remodeling of existing collagens in the heart. Altogether, studies in prior mouse models and the current work in rats indicate minor changes in cardiac function and structure after a low dose of single-ion radiation.

The effects of microgravity and space radiation on cardiovascular health: From low-Earth orbit and beyond

The unique conditions of space harbor considerable challenges for astronauts to overcome. Namely, the ionizing content of space radiation and the effects of microgravity have been implicated in the pathogenesis of cardiovascular disease. Post-flight carotid arterial stiffness was demonstrated in astronaut studies while early arteriosclerosis has been linked with microgravity-induced oxidative stress in cellular studies. Similarly, radiation has been shown to disrupt molecular pathways, enhance reactive oxygen species and increase risk of cardiovascular disease in exposed populations. These results may bear even more significance in space owing to the propensity for microgravity and space radiation to yield synergistic and/or additive interactions. Potential countermeasures such as α-tocopherol and captopril target these oxidative pathways and may help to protect against the effects of microgravity and radiation-induced cardiac damage. However, more research needs to be conducted in this area to facilitate a safe passage for humans to the Moon, Mars and beyond.

PolyRad - Protection Against Free Radical Damage

The effects of elevated levels of radiation contribute to the instability of pharmaceutical formulations in space compared to those on earth. Existing technologies are ineffective at maintaining the therapeutic efficacies of drugs in space. Thus, there is an urgent need to develop novel space-hardy formulations for preserving the stability and efficacy of drug formulations. This work aims to develop a novel approach for the protection of space pharmaceutical drug molecules from the radiation-induced damage to help extend or at least preserve their structural integrity and potency. To achieve this, free radical scavenging antioxidant, Trolox was conjugated on the surface of poly-lactic-co-glycolic acid (PLGA) nanoparticles for the protection of a candidate drug, melatonin that is used as a sleep aid medication in International Space Station (ISS). Melatonin-PLGA-PLL-Trolox nanoparticle as named as PolyRad was synthesized employing single oil in water (o/w) emulsion solvent evaporation method. PolyRad is spherical in shape and has an average diameter of ~600 nm with a low polydispersity index of 0.2. PolyRad and free melatonin (control) were irradiated by UV light after being exposed to a strong oxidant, hydrogen peroxide (H₂O₂). Bare melatonin lost ~80% of the active structure of the drug following irradiation with UV light or treatment with H₂O₂. In contrast, PolyRad protected >80% of the active structure of melatonin. The ability of PolyRad to protect melatonin structure was also carried out using 0, 1, 5 and 10 Gy gamma radiation. Gamma irradiation showed >98% active structures of melatonin encapsulated in PolyRads. Drug release and effectiveness of melatonin using PolyRad were evaluated on human umbilical vein endothelial cells (HUVEC) in vitro. Non-irradiated PolyRad demonstrated maximum drug release of ~70% after 72 h, while UV-irradiated and H₂O₂-treated PolyRad showed a maximum drug release of ~85%. Cytotoxicity of melatonin was carried out using both live/dead and MTT assays. Melatonin, non-radiated PolyRad and irradiated PolyRad inhibited the viability of HUVEC in a dose-dependent manner. Cell viability of melatonin, PolyRad alone without melatonin (PolyRad carrier control), non-radiated PolyRad, and irradiated PolyRad were ~98, 87, 75 and 70%, respectively at a concentration \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \sim $$\end{document}~ 0.01 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${mg}/{ml}$$\end{document}mg/ml (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$10\mu g/{ml}$$\end{document}10μg/ml). Taken together, PolyRad nanoparticle provides an attractive formulation platform for preventing damage to pharmaceutical drugs in potential space mission applications.

Changes in one-carbon metabolism and DNA methylation in the hearts of mice exposed to space environment-relevant doses of oxygen ions (16O)

Cardiovascular disease constitutes an important threat to humans after space missions beyond the Earth's magnetosphere. Epigenetic alterations have an important role in the etiology and pathogenesis of cardiovascular disease. Previous research in animal models has shown that protons and ⁵⁶Fe ions cause long-term changes in DNA methylation and expression of repetitive elements in the heart. However, astronauts will be exposed to a variety of ions, including the smaller fragmented products of heavy ions after they interact with the walls of the space craft. Here, we investigated the effects of ¹⁶O on the cardiac methylome and one-carbon metabolism in male C57BL/6 J mice. Left ventricles were examined 14 and 90 days after exposure to space-relevant doses of 0.1, 0.25, or 1 Gy of ¹⁶O (600 MeV/n). At 14 days, the two higher radiation doses elicited global DNA hypomethylation in the 5'-UTR of Long Interspersed Nuclear Elements 1 (LINE-1) compared to unirradiated, sham-treated mice, whereas specific LINE-1 elements exhibited hypermethylation at day 90. The pericentromeric major satellites were affected both at the DNA methylation and expression levels at the lowest radiation dose. DNA methylation was elevated, particularly after 90 days, while expression showed first a decrease followed by an increase in transcript abundance. Metabolomics analysis revealed that metabolites involved in homocysteine remethylation, central to DNA methylation, were unaffected by radiation, but the transsulfuration pathway was impacted after 90 days, with a large increase in cystathione levels at the lowest dose. In summary, we observed dynamic changes in the cardiac epigenome and metabolome three months after exposure to a single low dose of oxygen ions.

GeneLab Database Analyses Suggest Long-Term Impact of Space Radiation on the Cardiovascular System by the Activation of FYN Through Reactive Oxygen Species

Space radiation has recently been considered a risk factor for astronauts’ cardiac health. As an example, for the case of how to query and identify datasets within NASA’s GeneLab database and demonstrate the database utility, we used an unbiased systems biology method for identifying key genes/drivers for the contribution of space radiation on the cardiovascular system. This knowledge can contribute to designing appropriate experiments targeting these specific pathways. Microarray data from cardiomyocytes of male C57BL/6 mice followed-up for 28 days after exposure to 900 mGy of 1 GeV proton or 150 mGy of 1 GeV/n ⁵⁶Fe were compared to human endothelial cells (HUVECs) cultured for 7 days on the International Space Station (ISS). We observed common molecular pathways between simulated space radiation and HUVECs flown on the ISS. The analysis suggests FYN is the central driver/hub for the cardiovascular response to space radiation: the known oxidative stress induced immediately following radiation would only be transient and would upregulate FYN, which in turn would reduce reactive oxygen species (ROS) levels, protecting the cardiovascular system. The transcriptomic signature of exposure to protons was also much closer to the spaceflight signature than ⁵⁶Fe’s signature. To our knowledge, this is the first time GeneLab datasets were utilized to provide potential biological indications that the majority of ions on the ISS are protons, clearly illustrating the power of omics analysis. More generally, this work also demonstrates how to combine animal radiation studies done on the ground and spaceflight studies to evaluate human risk in space.

Effects of low-dose oxygen ions and protons on cardiac function and structure in male C57BL/6J mice

PURPOSE

Astronauts traveling beyond low-Earth orbit will be exposed to high linear-energy transfer charged particles. Because there is concern about the adverse effects of space radiation on the cardiovascular system, this study assessed cardiac function and structure and immune cell infiltration in a mouse model of charged-particle irradiation.

MATERIALS AND METHODS

Male C57BL/6 J mice were exposed to oxygen ions (¹⁶O, 600 MeV/n at 0.25-0.26 Gy/min to a total dose of 0, 0.05, 0.1, 0.25, or 1 Gy), protons (150 MeV, 0.35-0.55 Gy/min to 0, 0.5, or 1 Gy), or protons (150 MeV, 0.5 Gy) followed by ¹⁶O (600 MeV/n, 0.1 Gy). Separate groups of mice received ¹³⁷Cs γ-rays (1 Gy/min to 0, 0.5, 1, or 3 Gy) as a reference. Cardiac function and blood velocity were measured with ultrasonography at 3, 5, 7, and 9 months after irradiation. At 2 weeks, 3 months, and 9 months, cardiac tissue was collected to assess apoptosis, tissue remodeling, and markers of immune cells.

RESULTS

Ejection fraction and fractional shortening decreased at 3 and 7 months after ¹⁶O. These parameters did not change in mice exposed to γ-rays, protons, or protons followed by ¹⁶O. Each of the radiation exposures caused only small increases in cleaved caspase-3 and numbers of apoptotic nuclei. Changes in the levels of α-smooth muscle cell actin and a 75-kDa peptide of collagen type III in the left ventricle suggested tissue remodeling, but there was no significant change in total collagen deposition at 2 weeks, 3 months, and 9 months. Increases in protein amounts of cluster of differentiation (CD)2, CD68, and CD45 as measured with immunoblots at 2 weeks, 3 months, and 9 months after exposure to protons or ¹⁶O alone suggested immune cell infiltration. For type III collagen, CD2 and CD68, the efficacy in inducing protein abundance of CD2, CD68, and CD45 was ¹⁶O > protons > γ-rays > protons followed by ¹⁶O.

CONCLUSIONS

Low-dose, high-energy charged-particle irradiation caused mild changes in cardiac function and tissue remodeling in the mouse.

LGM2605 Reduces Space Radiation-Induced NLRP3 Inflammasome Activation and Damage in In Vitro Lung Vascular Networks

Updated measurements of charged particle fluxes during the transit from Earth to Mars as well as on site measurements by Curiosity of Martian surface radiation fluxes identified potential health hazards associated with radiation exposure for human space missions. Designing mitigation strategies of radiation risks to astronauts is critical. We investigated radiation-induced endothelial cell damage and its mitigation by LGM2605, a radioprotector with antioxidant and free radical scavenging properties. We used an in vitro model of lung vascular networks (flow-adapted endothelial cells; FAECs), exposed to gamma rays, low/higher linear energy transfer (LET) protons (3⁻4 or 8⁻10 keV/µm, respectively), and mixed field radiation sources (gamma and protons), given at mission-relevant doses (0.25 gray (Gy)⁻1 Gy). We evaluated endothelial inflammatory phenotype, NLRP3 inflammasome activation, and oxidative cell injury. LGM2605 (100 µM) was added 30 min post radiation exposure and gene expression changes evaluated 24 h later. Radiation induced a robust increase in mRNA levels of antioxidant enzymes post 0.25 Gy and 0.5 Gy gamma radiation, which was significantly decreased by LGM2605. Intercellular cell adhesion molecule-1 (ICAM-1) and NOD-like receptor protein 3 (NLRP3) induction by individual or mixed-field exposures were also significantly blunted by LGM2605. We conclude that LGM2605 is a likely candidate to reduce tissue damage from space-relevant radiation exposure.

Oxidative Stress as Cause, Consequence, or Biomarker of Altered Female Reproduction and Development in the Space Environment

Oxidative stress has been implicated in the pathophysiology of numerous terrestrial disease processes and associated with morbidity following spaceflight. Furthermore, oxidative stress has long been considered a causative agent in adverse reproductive outcomes. The purpose of this review is to summarize the pathogenesis of oxidative stress caused by cosmic radiation and microgravity, review the relationship between oxidative stress and reproductive outcomes in females, and explore what role spaceflight-induced oxidative damage may have on female reproductive and developmental outcomes.

Effects of Iron Overload and Oxidative Damage on the Musculoskeletal System in the Space Environment: Data from Spaceflights and Ground-Based Simulation Models

The space environment chiefly includes microgravity and radiation, which seriously threatens the health of astronauts. Bone loss and muscle atrophy are the two most significant changes in mammals after long-term residency in space. In this review, we summarized current understanding of the effects of microgravity and radiation on the musculoskeletal system and discussed the corresponding mechanisms that are related to iron overload and oxidative damage. Furthermore, we enumerated some countermeasures that have a therapeutic potential for bone loss and muscle atrophy through using iron chelators and antioxidants. Future studies for better understanding the mechanism of iron and redox homeostasis imbalance induced by the space environment and developing the countermeasures against iron overload and oxidative damage consequently may facilitate human to travel more safely in space.

High LET radiation shows no major cellular and functional effects on primary cardiomyocytes in vitro

It is well known that ionizing radiation causes adverse effects on various mammalian tissues. However, there is little information on the biological effects of heavy ion radiation on the heart. In order to fill this gap, we systematically examined DNA-damage induction and repair, as well as proliferation and apoptosis in avian cardiomyocyte cultures irradiated with heavy ions such as titanium and iron, relevant for manned space-flight, and carbon ions, as used for radiotherapy. Further, and to our knowledge for the first time, we analyzed the effect of heavy ion radiation on the electrophysiology of primary cardiomyocytes derived from chicken embryos using the non-invasive microelectrode array (MEA) technology. As electrophysiological endpoints beat rate and field action potential duration were analyzed. The cultures clearly exhibited the capacity to repair induced DNA damage almost completely within 24 h, even at doses of 7 Gy, and almost completely recovered from radiation-induced changes in proliferative behavior. Interestingly, no significant effects on apoptosis could be detected. Especially the functionality of primary cardiac cells exhibited a surprisingly high robustness against heavy ion radiation, even at doses of up to 7 Gy. In contrast to our previous study with X-rays the beat rate remained more or less unaffected after heavy ion radiation, independently of beam quality. The only change we could observe was an increase of the field action potential duration of up to 30% after titanium irradiation, diminishing within the following three days. This potentially pathological observation may be an indication that heavy ion irradiation at high doses could bear a long-term risk for cardiovascular disease induction.

Radiation-Induced Cardiovascular Disease: Mechanisms and Importance of Linear Energy Transfer

Radiation therapy (RT) in the form of photons and protons is a well-established treatment for cancer. More recently, heavy charged particles have been used to treat radioresistant and high-risk cancers. Radiation treatment is known to cause cardiovascular disease (CVD) which can occur acutely during treatment or years afterward in the form of accelerated atherosclerosis. Radiation-induced cardiovascular disease (RICVD) can be a limiting factor in treatment as well as a cause of morbidity and mortality in successfully treated patients. Inflammation plays a key role in both acute and chronic RICVD, but the underling pathophysiology is complex, involving DNA damage, reactive oxygen species, and chronic inflammation. While understanding of the molecular mechanisms of RICVD has increased, the growing number of patients receiving RT warrants further research to identify individuals at risk, plans for prevention, and targets for the treatment of RICVD. Research on RICVD is also relevant to the National Aeronautics and Space Administration (NASA) due to the prevalent space radiation environment encountered by astronauts. NASA's current research on RICVD can both contribute to and benefit from concurrent work with cell and animal studies informing radiotoxicities resulting from cancer therapy. This review summarizes the types of radiation currently in clinical use, models of RICVD, current knowledge of the mechanisms by which they cause CVD, and how this knowledge might apply to those exposed to various types of radiation.

Proceedings of the National Cancer Institute Workshop on Charged Particle Radiobiology

In April 2016, the National Cancer Institute hosted a multidisciplinary workshop to discuss the current knowledge of the radiobiological aspects of charged particles used in cancer therapy to identify gaps in that knowledge that might hinder the effective clinical use of charged particles and to propose research that could help fill those gaps. The workshop was organized into 10 topics ranging from biophysical models to clinical trials and included treatment optimization, relative biological effectiveness of tumors and normal tissues, hypofractionation with particles, combination with immunotherapy, "omics," hypoxia, and particle-induced second malignancies. Given that the most commonly used charged particle in the clinic currently is protons, much of the discussion revolved around evaluating the state of knowledge and current practice of using a relative biological effectiveness of 1.1 for protons. Discussion also included the potential advantages of heavier ions, notably carbon ions, because of their increased biological effectiveness, especially for tumors frequently considered to be radiation resistant, increased effectiveness in hypoxic cells, and potential for differentially altering immune responses. The participants identified a large number of research areas in which information is needed to inform the most effective use of charged particles in the future in clinical radiation therapy. This unique form of radiation therapy holds great promise for improving cancer treatment.

Heart in space: effect of the extraterrestrial environment on the cardiovascular system

National space agencies and private corporations aim at an extended presence of humans in space in the medium to long term. Together with currently suboptimal technology, microgravity and cosmic rays raise health concerns about deep-space exploration missions. Both of these physical factors affect the cardiovascular system, whose gravity-dependence is pronounced. Heart and vascular function are, therefore, susceptible to substantial changes in weightlessness. The altered cardiovascular function in space causes physiological problems in the postflight period. A compromised cardiovascular system can be excessively vulnerable to space radiation, synergistically resulting in increased damage. The space radiation dose is significantly lower than in patients undergoing radiotherapy, in whom cardiac damage is well-documented following cancer therapy in the thoracic region. Nevertheless, epidemiological findings suggest an increased risk of late cardiovascular disease even with low doses of radiation. Moreover, the peculiar biological effectiveness of heavy ions in cosmic rays might increase this risk substantially. However, whether radiation-induced cardiovascular effects have a threshold at low doses is still unclear. The main countermeasures to mitigate the effect of the space environment on cardiac function are physical exercise, antioxidants, nutraceuticals, and radiation shielding.

Redox Signaling and Its Impact on Skeletal and Vascular Responses to Spaceflight

Spaceflight entails exposure to numerous environmental challenges with the potential to contribute to both musculoskeletal and vascular dysfunction. The purpose of this review is to describe current understanding of microgravity and radiation impacts on the mammalian skeleton and associated vasculature at the level of the whole organism. Recent experiments from spaceflight and ground-based models have provided fresh insights into how these environmental stresses influence mechanisms that are related to redox signaling, oxidative stress, and tissue dysfunction. Emerging mechanistic knowledge on cellular defenses to radiation and other environmental stressors, including microgravity, are useful for both screening and developing interventions against spaceflight-induced deficits in bone and vascular function.

Different Sequences of Fractionated Low-Dose Proton and Single Iron-Radiation-Induced Divergent Biological Responses in the Heart

Deep-space travel presents risks of exposure to ionizing radiation composed of a spectrum of low-fluence protons (¹H) and high-charge and energy (HZE) iron nuclei (e.g., ⁵⁶Fe). When exposed to galactic cosmic rays, each cell in the body may be traversed by ¹H every 3-4 days and HZE nuclei every 3-4 months. The effects of low-dose sequential fractionated ¹H or HZE on the heart are unknown. In this animal model of simulated ionizing radiation, middle-aged (8-9 months old) male C57BL/6NT mice were exposed to radiation as follows: group 1, nonirradiated controls; group 2, three fractionated doses of 17 cGy ¹H every other day (¹H × 3); group 3, three fractionated doses of 17 cGy ¹H every other day followed by a single low dose of 15 cGy ⁵⁶Fe two days after the final ¹H dose (¹H × 3 + ⁵⁶Fe); and group 4, a single low dose of 15 cGy ⁵⁶Fe followed (after 2 days) by three fractionated doses of 17 cGy ¹H every other day (⁵⁶Fe + ¹H × 3). A subgroup of mice from each group underwent myocardial infarction (MI) surgery at 28 days postirradiation. Cardiac structure and function were assessed in all animals at days 7, 14 and 28 after MI surgery was performed. Compared to the control animals, the treatments that groups 2 and 3 received did not induce negative effects on cardiac function or structure. However, compared to all other groups, the animals in group 4, showed depressed left ventricular (LV) functions at 1 month with concomitant enhancement in cardiac fibrosis and induction of cardiac hypertrophy signaling at 3 months. In the irradiated and MI surgery groups compared to the control group, the treatments received by groups 2 and 4 did not induce negative effects at 1 month postirradiation and MI surgery. However, in group 3 after MI surgery, there was a 24% increase in mortality, significant decreases in LV function and a 35% increase in post-infarction size. These changes were associated with significant decreases in the angiogenic and cell survival signaling pathways. These data suggest that fractionated doses of radiation induces cellular and molecular changes that result in depressed heart functions both under basal conditions and particularly after myocardial infarction.

High Energy Particle Radiation-associated Oncogenic Transformation in Normal Mice: Insight into the Connection between Activation of Oncotargets and Oncogene Addiction

Concerns on high-energy particle radiation-induced tumorigenic transformation of normal tissue in astronauts, and in cancer patients undergoing radiotherapy, emphasizes the significance of elucidating the mechanisms involved in radiogenic transformation processes. Mostly used genetically modified or tumor-prone models are less reliable in determining human health risk in space or protracted post-treatment normal tissue toxicity. Here, in wild type C57BL/6 mice, we related the deregulation of distinctive set of tissue-specific oncotargets in major organs upon ⁵⁶Fe (600 MeV/amu; 0.5 Gy/min; 0.8 Gy) particle radiation and compared the response with low LET γ-radiation (¹³⁷Cs; 0.5 Gy/min; 2 Gy). One of the novel findings is the ‘tissue-independent’ activation of TAL2 upon high-energy radiation, and thus qualifies TAL2 as a potential biomarker for particle and other qualities of radiation. Heightened expression of TAL2 gene transcript, which sustained over four weeks post-irradiation foster the concept of oncogene addiction signaling in radiogenic transformation. The positive/negative expression of other selected oncotargets that expresses tissue-dependent manner indicated their role as a secondary driving force that addresses the diversity of tissue-dependent characteristics of tumorigenesis. This study, while reporting novel findings on radiogenic transformation of normal tissue when exposed to particle radiation, it also provides a platform for further investigation into different radiation quality, LET and dose/dose rate effect in healthy organs.

Apollo Lunar Astronauts Show Higher Cardiovascular Disease Mortality: Possible Deep Space Radiation Effects on the Vascular Endothelium

As multiple spacefaring nations contemplate extended manned missions to Mars and the Moon, health risks could be elevated as travel goes beyond the Earth's protective magnetosphere into the more intense deep space radiation environment. The primary purpose of this study was to determine whether mortality rates due to cardiovascular disease (CVD), cancer, accidents and all other causes of death differ in (1) astronauts who never flew orbital missions in space, (2) astronauts who flew only in low Earth orbit (LEO), and (3) Apollo lunar astronauts, the only humans to have traveled beyond Earth's magnetosphere. Results show there were no differences in CVD mortality rate between non-flight (9%) and LEO (11%) astronauts. However, the CVD mortality rate among Apollo lunar astronauts (43%) was 4-5 times higher than in non-flight and LEO astronauts. To test a possible mechanistic basis for these findings, a secondary purpose was to determine the long-term effects of simulated weightlessness and space-relevant total-body irradiation on vascular responsiveness in mice. The results demonstrate that space-relevant irradiation induces a sustained vascular endothelial cell dysfunction. Such impairment is known to lead to occlusive artery disease, and may be an important risk factor for CVD among astronauts exposed to deep space radiation.

Effects of ionizing radiation on the heart

This article provides an overview of studies addressing effects of ionizing radiation on the heart. Clinical studies have identified early and late manifestations of radiation-induced heart disease, a side effect of radiation therapy to tumors in the chest when all or part of the heart is situated in the radiation field. Studies in preclinical animal models have contributed to our understanding of the mechanisms by which radiation may injure the heart. More recent observations in human subjects suggest that ionizing radiation may have cardiovascular effects at lower doses than was previously thought. This has led to examinations of low-dose photons and low-dose charged particle irradiation in animal models. Lastly, studies have started to identify non-invasive methods for detection of cardiac radiation injury and interventions that may prevent or mitigate these adverse effects. Altogether, this ongoing research should increase our knowledge of biological mechanisms of cardiovascular radiation injury, identify non-invasive biomarkers for early detection, and potential interventions that may prevent or mitigate these adverse effects.

Radiation-induced changes in DNA methylation of repetitive elements in the mouse heart

DNA methylation is a key epigenetic mechanism, needed for proper control over the expression of genetic information and silencing of repetitive elements. Exposure to ionizing radiation, aside from its strong genotoxic potential, may also affect the methylation of DNA, within the repetitive elements, in particular. In this study, we exposed C57BL/6J male mice to low absorbed mean doses of two types of space radiation-proton (0.1 Gy, 150 MeV, dose rate 0.53 ± 0.08 Gy/min), and heavy iron ions ((56)Fe) (0.5 Gy, 600 MeV/n, dose rate 0.38 ± 0.06 Gy/min). Radiation-induced changes in cardiac DNA methylation associated with repetitive elements were detected. Specifically, modest hypomethylation of retrotransposon LINE-1 was observed at day 7 after irradiation with either protons or (56)Fe. This was followed by LINE-1, and other retrotransposons, ERV2 and SINE B1, as well as major satellite DNA hypermethylation at day 90 after irradiation with (56)Fe. These changes in DNA methylation were accompanied by alterations in the expression of DNA methylation machinery and affected the one-carbon metabolism pathway. Furthermore, loss of transposable elements expression was detected in the cardiac tissue at the 90-day time-point, paralleled by substantial accumulation of mRNA transcripts, associated with major satellites. Given that the one-carbon metabolism pathway can be modulated by dietary modifications, these findings suggest a potential strategy for the mitigation and, possibly, prevention of the negative effects exerted by ionizing radiation on the cardiovascular system. Additionally, we show that the methylation status and expression of repetitive elements may serve as early biomarkers of exposure to space radiation.

Ionizing radiation and heart risks

Cardiovascular disease and cancer are the two leading causes of morbidity and mortality worldwide. As advancements in radiation therapy (RT) have significantly increased the number of cancer survivors, the risk of radiation-induced cardiovascular disease (RICD) in this group is a growing concern. Recent epidemiological data suggest that accidental or occupational exposure to low dose radiation, in addition to therapeutic ionizing radiation, can result in cardiovascular complications. The progression of radiation-induced cardiotoxicity often takes years to manifest but is also multifaceted, as the heart may be affected by a variety of pathologies. The risk of cardiovascular disease development in RT cancer survivors has been known for 40 years and several risk factors have been identified in the last two decades. However, most of the early work focused on clinical symptoms and manifestations, rather than understanding cellular processes regulating homeostatic processes of the cardiovascular system in response to radiation. Recent studies have suggested that a different approach may be needed to refute the risk of cardiovascular disease following radiation exposure. In this review, we will focus on how different radiation types and doses may induce cardiovascular complications, highlighting clinical manifestations and the mechanisms involved in the pathophysiology of radiation-induced cardiotoxicity. We will finally discuss how current and future research on heart development and homeostasis can help reduce the incidence of RICD.

A priming dose of protons alters the early cardiac cellular and molecular response to (56)Fe irradiation

PURPOSE

Recent evidence suggests that the heart may be injured by ionizing radiation at lower doses than was previously thought. This raises concerns about the cardiovascular risks from exposure to radiation during space travel. Since space travel is associated with exposure to both protons from solar particle events and heavy ions from galactic cosmic rays, we here examined the effects of a "priming" dose of protons on the cardiac cellular and molecular response to a "challenge" dose of (56)Fe in a mouse model.

METHODS

Male C57BL/6 mice at 10 weeks of age were exposed to sham-irradiation, 0.1 Gy of protons (150 MeV), 0.5 Gy of (56)Fe (600 MeV/n), or 0.1 Gy of protons 24 hours prior to 0.5 Gy of (56)Fe. Hearts were obtained at 7 days post-irradiation and western-blots were used to determine protein markers of cardiac remodeling, inflammatory infiltration, and cell death.

RESULTS

Exposure to (56)Fe caused an increase in expression of α-smooth muscle cell actin, collagen type III, the inflammatory cell markers mast cell tryptase, CD2 and CD68, the endothelial glycoprotein thrombomodulin, and cleaved caspase 3. Of all proteins investigated, protons at a dose of 0.1 Gy induced a small increase only in cleaved caspase 3 levels. On the other hand, exposure to protons 24 hours before (56)Fe prevented all of the responses to (56)Fe.

CONCLUSIONS

This study shows that a low dose of protons may prime the heart to respond differently to a subsequent challenge dose of heavy ions. Further investigation is required to identify responses at additional time points, consequences for cardiac function, threshold dose levels, and mechanisms by which a proton priming dose may alter the response to heavy ions.

Space radiation and cardiovascular disease risk

Future long-distance space missions will be associated with significant exposures to ionizing radiation, and the health risks of these radiation exposures during manned missions need to be assessed. Recent Earth-based epidemiological studies in survivors of atomic bombs and after occupational and medical low dose radiation exposures have indicated that the cardiovascular system may be more sensitive to ionizing radiation than was previously thought. This has raised the concern of a cardiovascular disease risk from exposure to space radiation during long-distance space travel. Ground-based studies with animal and cell culture models play an important role in estimating health risks from space radiation exposure. Charged particle space radiation has dense ionization characteristics and may induce unique biological responses, appropriate simulation of the space radiation environment and careful consideration of the choice of the experimental model are critical. Recent studies have addressed cardiovascular effects of space radiation using such models and provided first results that aid in estimating cardiovascular disease risk, and several other studies are ongoing. Moreover, astronauts could potentially be administered pharmacological countermeasures against adverse effects of space radiation, and research is focused on the development of such compounds. Because the cardiovascular response to space radiation has not yet been clearly defined, the identification of potential pharmacological countermeasures against cardiovascular effects is still in its infancy.