"Modeled microgravity" affects cell response to ionizing radiation and increases genomic damage

Synergistic interplay between radiation and microgravity in spaceflight-related immunological health risks

Spaceflight poses a myriad of environmental stressors to astronauts´ physiology including microgravity and radiation. The individual impacts of microgravity and radiation on the immune system have been extensively investigated, though a comprehensive review on their combined effects on immune system outcomes is missing. Therefore, this review aims at understanding the synergistic, additive, and antagonistic interactions between microgravity and radiation and their impact on immune function as observed during spaceflight-analog studies such as rodent hindlimb unloading and cell culture rotating wall vessel models. These mimic some, but not all, of the physiological changes observed in astronauts during spaceflight and provide valuable information that should be considered when planning future missions. We provide guidelines for the design of further spaceflight-analog studies, incorporating influential factors such as age and sex for rodent models and standardizing the longitudinal evaluation of specific immunological alterations for both rodent and cellular models of spaceflight exposure.

Omics Studies of Tumor Cells under Microgravity Conditions

Cancer is defined as a group of diseases characterized by abnormal cell growth, expansion, and progression with metastasis. Various signaling pathways are involved in its development. Malignant tumors exhibit a high morbidity and mortality. Cancer research increased our knowledge about some of the underlying mechanisms, but to this day, our understanding of this disease is unclear. High throughput omics technology and bioinformatics were successful in detecting some of the unknown cancer mechanisms. However, novel groundbreaking research and ideas are necessary. A stay in orbit causes biochemical and molecular biological changes in human cancer cells which are first, and above all, due to microgravity (µg). The µg-environment provides conditions that are not reachable on Earth, which allow researchers to focus on signaling pathways controlling cell growth and metastasis. Cancer research in space already demonstrated how cancer cell-exposure to µg influenced several biological processes being involved in cancer. This novel approach has the potential to fight cancer and to develop future cancer strategies. Space research has been shown to impact biological processes in cancer cells like proliferation, apoptosis, cell survival, adhesion, migration, the cytoskeleton, the extracellular matrix, focal adhesion, and growth factors, among others. This concise review focuses on publications related to genetic, transcriptional, epigenetic, proteomic, and metabolomic studies on tumor cells exposed to real space conditions or to simulated µg using simulation devices. We discuss all omics studies investigating different tumor cell types from the brain and hematological system, sarcomas, as well as thyroid, prostate, breast, gynecologic, gastrointestinal, and lung cancers, in order to gain new and innovative ideas for understanding the basic biology of cancer.

Lost in Space? Unmasking the T Cell Reaction to Simulated Space Stressors

The space environment will expose astronauts to stressors like ionizing radiation, altered gravity fields and elevated cortisol levels, which pose a health risk. Understanding how the interplay between these stressors changes T cells' response is important to better characterize space-related immune dysfunction. We have exposed stimulated Jurkat cells to simulated space stressors (1 Gy, carbon ions/1 Gy photons, 1 µM hydrocortisone (HC), Mars, moon, and microgravity) in a single or combined manner. Pro-inflammatory cytokine IL-2 was measured in the supernatant of Jurkat cells and at the mRNA level. Results show that alone, HC, Mars gravity and microgravity significantly decrease IL-2 presence in the supernatant. 1 Gy carbon ion irradiation showed a smaller impact on IL-2 levels than photon irradiation. Combining exposure to different simulated space stressors seems to have less immunosuppressive effects. Gene expression was less impacted at the time-point collected. These findings showcase a complex T cell response to different conditions and suggest the importance of elevated cortisol levels in the context of space flight, also highlighting the need to use simulated partial gravity technologies to better understand the immune system's response to the space environment.

Simulating Space Conditions Evokes Different DNA Damage Responses in Immature and Mature Cells of the Human Hematopoietic System

The impact of space radiation and microgravity on DNA damage responses has been discussed controversially, largely due to the variety of model systems engaged. Here, we performed side-by-side analyses of human hematopoietic stem/progenitor cells (HSPC) and peripheral blood lymphocytes (PBL) cultivated in a 2D clinostat to simulate microgravity before, during and after photon and particle irradiation. We demonstrate that simulated microgravity (SMG) accelerates the early phase of non-homologous end joining (NHEJ)-mediated repair of simple, X-ray-induced DNA double-strand breaks (DSBs) in PBL, while repair kinetics in HSPC remained unaltered. Repair acceleration was lost with increasing LET of ion exposures, which increases the complexity of DSBs, precluding NHEJ and requiring end resection for successful repair. Such cell-type specific effect of SMG on DSB repair was dependent on the NF-кB pathway pre-activated in PBL but not HSPC. Already under unperturbed growth conditions HSPC and PBL suffered from SMG-induced replication stress associated with accumulation of single-stranded DNA and DSBs, respectively. We conclude that in PBL, SMG-induced DSBs promote repair of radiation-induced damage in an adaptive-like response. HSPC feature SMG-induced single-stranded DNA and FANCD2 foci, i.e., markers of persistent replication stress and senescence that may contribute to a premature decline of the immune system in space.

Analysis of Radiation Toxicity in Mammalian Cells Stably Transduced with Mitochondrial Stat3

A coordinated action between nuclear and mitochondrial activities is essential for a proper cellular response to genotoxic stress. Several nuclear transcription factors, including STAT3, translocate to mitochondria to exert mitochondrial function regulation; however, the role of mitochondrial STAT3 (mitoSTAT3) under stressed conditions is still poorly understood. In this study, we examined whether the stable expression of mitoSTAT3 wild-type or mutated at the conserved serine residue (Ser727), which is involved in the mitochondrial function of STAT3, can affect the DNA damage response to UVC radiation. To address this issue, we generated mammalian cells (NIH-3T3 and HCT-116 cells) stably transduced to express the mitochondrial-targeted Stat3 gene in its wild-type or Ser727 mutated forms. Our results show that cell proliferation is enhanced in mitoStat3-transduced cells under both non-stressed and stressed conditions. Once irradiated with UVC, cells expressing wild-type mitoSTAT3 showed the highest cell survival, which was associated with a significant decrease in cell death. Low levels of oxidative stress were detected in UVC-irradiated NIH-3T3 cells expressing mitoSTAT3 wild-type or serine-related dominant active form (Ser727D), confirming a role of mitochondrial STAT3 in minimizing oxidant cellular stress that provides an advantage for cell survival.

Skin microbiome considerations for long haul space flights

Dysbiosis of the human skin microbiome has long been associated with changes to the pH of the skin, dermal immune function and chronic skin conditions. Dermatological issues have been noted as the most prevalent medical presentation in the microgravity environment of space. The change in gravitational forces has been implicated in human immuno-suppression, also impacted by changes in the gastrointestinal-skin axis and its impact on Vitamin D metabolism, altered microbial gene expression in resident flora (leading changes in biofilm formation) and increased virulence factors in potential pathogens. There are also other stressors to the skin microbiome unique to space travel, including increased exposure to radiation, prolonged periods of dry washing technique, air quality and changes in microbe replication and growth parameters. Optimal microbiome health leads to enhanced skin barrier manufacture and maintenance, along with improved skin immune function and healing. In a microgravity environment expected to be experienced during long space flights, disruptions to the skin microbiome, coupled with increased virulence of pathological viruses and bacteria has implications for holistic skin health, astronaut cognitive function and mental health, and is coupled with slowed rates of wound healing. Scenario management for holistic skin health and restoration of microbiome homeostasis on long space flights require consideration.

A Current Overview of the Biological Effects of Combined Space Environmental Factors in Mammals

Distinct from Earth’s environment, space environmental factors mainly include space radiation, microgravity, hypomagnetic field, and disrupted light/dark cycles that cause physiological changes in astronauts. Numerous studies have demonstrated that space environmental factors can lead to muscle atrophy, bone loss, carcinogenesis, immune disorders, vascular function and cognitive impairment. Most current ground-based studies focused on single environmental factor biological effects. To promote manned space exploration, a better understanding of the biological effects of the spaceflight environment is necessary. This paper summarizes the latest research progress of the combined biological effects of double or multiple space environmental factors on mammalian cells, and discusses their possible molecular mechanisms, with the hope of providing a scientific theoretical basis to develop appropriate countermeasures for astronauts.

Cancer Studies under Space Conditions: Finding Answers Abroad

In this review article, we discuss the current state of knowledge in cancer research under real and simulated microgravity conditions and point out further research directions in this field. Outer space is an extremely hostile environment for human life, with radiation, microgravity, and vacuum posing significant hazards. Although the risk for cancer in astronauts is not clear, microgravity plays a thought-provoking role in the carcinogenesis of normal and cancer cells, causing such effects as multicellular spheroid formation, cytoskeleton rearrangement, alteration of gene expression and protein synthesis, and apoptosis. Furthermore, deleterious effects of radiation on cells seem to be accentuated under microgravity. Ground-based facilities have been used to study microgravity effects in addition to laborious experiments during parabolic flights or on space stations. Some potential 'gravisensors' have already been detected, and further identification of these mechanisms of mechanosensitivity could open up ways for therapeutic influence on cancer growth and apoptosis. These novel findings may help to find new effective cancer treatments and to provide health protection for humans on future long-term spaceflights and exploration of outer space.

NASA's Ground-Based Microgravity Simulation Facility

Since opportunities to conduct experiments in space are scarce, various microgravity simulators and analogs have been widely used in space biology ground studies. Even though microgravity simulators do not produce all of the biological effects observed in the true microgravity environment, they provide alternative test platforms that are effective, affordable, and readily available to facilitate microgravity research. The Microgravity Simulation Support Facility (MSSF) at the National Aeronautics and Space Administration (NASA) John F. Kennedy Space Center (KSC) has been established for conducting short duration experiments, typically less than 1 month, utilizing a variety of microgravity simulation devices for research at different gravity levels. The simulators include, but are not limited to, 2D Clinostats, 3D Clinostats, Random Positioning Machines, and Rotating Wall Vessels. In this chapter, we will discuss current MSSF capabilities, development concepts, and the physical characteristics of these microgravity simulators.

Genomic Changes Driven by Radiation-Induced DNA Damage and Microgravity in Human Cells

The space environment consists of a complex mixture of different types of ionizing radiation and altered gravity that represents a threat to humans during space missions. In particular, individual radiation sensitivity is strictly related to the risk of space radiation carcinogenesis. Therefore, in view of future missions to the Moon and Mars, there is an urgent need to estimate as accurately as possible the individual risk from space exposure to improve the safety of space exploration. In this review, we survey the combined effects from the two main physical components of the space environment, ionizing radiation and microgravity, to alter the genetics and epigenetics of human cells, considering both real and simulated space conditions. Data collected from studies on human cells are discussed for their potential use to estimate individual radiation carcinogenesis risk from space exposure.

Combined Environment Simulator for Low-Dose-Rate Radiation and Partial Gravity of Moon and Mars

Deep space exploration by humans has become more realistic, with planned returns to the Moon, travel to Mars, and beyond. Space radiation with a low dose rate would be a constant risk for space travelers. The combined effects of space radiation and partial gravity such as on the Moon and Mars are unknown. The difficulty for such research is that there are no good simulating systems on the ground to investigate these combined effects. To address this knowledge gap, we developed the Simulator of the environments on the Moon and Mars with Neutron irradiation and Gravity change (SwiNG) for in vitro experiments using disposable closed cell culture chambers. The device simulates partial gravity using a centrifuge in a three-dimensional clinostat. Six samples are exposed at once to neutrons at a low dose rate (1 mGy/day) using Californium-252 in the center of the centrifuge. The system is compact including two SwiNG devices in the incubator, one with and one without radiation source, with a cooling function. This simulator is highly convenient for ground-based biological experiments because of limited access to spaceflight experiments. SwiNG can contribute significantly to research on the combined effects of space radiation and partial gravity.

Biologic Impact of Different Ultra-Low-Fluence Irradiations in Human Fibroblasts

In this study, we aimed to evaluate the cellular response of healthy human fibroblasts induced by different types of ultra-low-fluence radiations, including gamma rays, neutrons and high linear energy transfer (LET) heavy ions. NB1RGB cells were pretreated with ultra-low-fluence radiations (~0.1 cGy/7-8 h) of ¹³⁷Cs gamma rays, ²⁴¹Am-Be neutrons, helium, carbon and iron ions before being exposed to an X-ray-challenging dose (1.5 Gy). Helium (LET = 2.3 keV/µm), carbon (LET = 13.3 keV/µm) and iron (LET = 200 keV/µm) ions were generated with the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. No differences in cell death-measured by colony-forming assay-were observed regardless of the radiation type applied. In contrast, mutation frequency, which was detected through cell transformation into 6-thioguanine resistant clones, was 1.9 and 4.0 times higher in cells pretreated with helium and carbon ions, respectively, compared to cells exposed to X-ray-challenging dose alone. Moreover, cells pretreated with iron ions or gamma-rays showed a mutation frequency similar to cells exposed to X-ray-challenging dose alone, while cells pretreated with neutrons had 0.15 times less mutations. These results show that cellular responses triggered by ultra-low-fluence irradiations are radiation-quality dependent. Altogether, this study shows that ultra-low-fluence irradiations with the same level as those reported in the International Space Station are capable of inducing different cellular responses, including radio-adaptive responses triggered by neutrons and genomic instability mediated by high-LET heavy ions, while electromagnetic radiations (gamma rays) seem to have no biologic impact.

Space Radiation Biology for "Living in Space"

Space travel has advanced significantly over the last six decades with astronauts spending up to 6 months at the International Space Station. Nonetheless, the living environment while in outer space is extremely challenging to astronauts. In particular, exposure to space radiation represents a serious potential long-term threat to the health of astronauts because the amount of radiation exposure accumulates during their time in space. Therefore, health risks associated with exposure to space radiation are an important topic in space travel, and characterizing space radiation in detail is essential for improving the safety of space missions. In the first part of this review, we provide an overview of the space radiation environment and briefly present current and future endeavors that monitor different space radiation environments. We then present research evaluating adverse biological effects caused by exposure to various space radiation environments and how these can be reduced. We especially consider the deleterious effects on cellular DNA and how cells activate DNA repair mechanisms. The latest technologies being developed, e.g., a fluorescent ubiquitination-based cell cycle indicator, to measure real-time cell cycle progression and DNA damage caused by exposure to ultraviolet radiation are presented. Progress in examining the combined effects of microgravity and radiation to animals and plants are summarized, and our current understanding of the relationship between psychological stress and radiation is presented. Finally, we provide details about protective agents and the study of organisms that are highly resistant to radiation and how their biological mechanisms may aid developing novel technologies that alleviate biological damage caused by radiation. Future research that furthers our understanding of the effects of space radiation on human health will facilitate risk-mitigating strategies to enable long-term space and planetary exploration.

Effect of simulated microgravity and ionizing radiation on expression profiles of miRNA, lncRNA, and mRNA in human lymphoblastoid cells

In space, multiple unique environmental factors, particularly microgravity and space radiation, pose a constant threat to astronaut health. MicroRNAs (miRNAs) and long noncoding RNAs (lncRNAs) are functional RNAs that play critical roles in regulating multiple cellular processes. To gain insight into the role of non-coding RNAs in response to radiation and microgravity, we analyzed RNA expression profiles in human lymphoblastoid TK6 cells incubated for 24 h under static or rotating conditions to stimulate microgravity in space, after 2-Gy γ-ray irradiation. The expression of 14 lncRNAs and 17 mRNAs (differentially-expressed genes, DEGs) was found to be significantly downregulated under simulated microgravity conditions. In contrast, irradiation upregulated 55 lncRNAs and 56 DEGs, whereas only one lncRNA, but no DEGs, was downregulated. Furthermore, two miRNAs, 70 lncRNAs, and 87 DEGs showed significantly altered expression in response to simulated microgravity after irradiation, and these changes were independently induced by irradiation and simulated microgravity. GO enrichment and KEGG pathway analyses indicated that the associated target genes showed similar patterns to the noncoding RNAs and were suggested to be involved in the immune/inflammatory response including LPS/TLR, TNF, and NF-κB signaling pathways. However, synergistic effects on RNA expression and cellular responses were also observed with a combination of simulated microgravity and irradiation based on microarray and RT-PCR analysis. Together, our results indicate that simulated microgravity and irradiation additively alter expression patterns but synergistically modulate the expression levels of RNAs and their target genes in human lymphoblastoid cells.

Expression Profile of Cell Cycle-Related Genes in Human Fibroblasts Exposed Simultaneously to Radiation and Simulated Microgravity

Multiple unique environmental factors such as space radiation and microgravity (μG) pose a serious threat to human gene stability during space travel. Recently, we reported that simultaneous exposure of human fibroblasts to simulated μG and radiation results in more chromosomal aberrations than in cells exposed to radiation alone. However, the mechanisms behind this remain unknown. The purpose of this study was thus to obtain comprehensive data on gene expression using a three-dimensional clinostat synchronized to a carbon (C)-ion or X-ray irradiation system. Human fibroblasts (1BR-hTERT) were maintained under standing or rotating conditions for 3 or 24 h after synchronized C-ion or X-ray irradiation at 1 Gy as part of a total culture time of 2 days. Among 57,773 genes analyzed with RNA sequencing, we focused particularly on the expression of 82 cell cycle-related genes after exposure to the radiation and simulated μG. The expression of cell cycle-suppressing genes (ABL1 and CDKN1A) decreased and that of cell cycle-promoting genes (CCNB1, CCND1, KPNA2, MCM4, MKI67, and STMN1) increased after C-ion irradiation under μG. The cell may pass through the G₁/S and G₂ checkpoints with DNA damage due to the combined effects of C-ions and μG, suggesting that increased genomic instability might occur in space.

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.

Increased Chromosome Aberrations in Cells Exposed Simultaneously to Simulated Microgravity and Radiation

Space radiation and microgravity (μG) are two major environmental stressors for humans in space travel. One of the fundamental questions in space biology research is whether the combined effects of μG and exposure to cosmic radiation are interactive. While studies addressing this question have been carried out for half a century in space or using simulated μG on the ground, the reported results are ambiguous. For the assessment and management of human health risks in future Moon and Mars missions, it is necessary to obtain more basic data on the molecular and cellular responses to the combined effects of radiation and µG. Recently we incorporated a μG⁻irradiation system consisting of a 3D clinostat synchronized to a carbon-ion or X-ray irradiation system. Our new experimental setup allows us to avoid stopping clinostat rotation during irradiation, which was required in all other previous experiments. Using this system, human fibroblasts were exposed to X-rays or carbon ions under the simulated μG condition, and chromosomes were collected with the premature chromosome condensation method in the first mitosis. Chromosome aberrations (CA) were quantified by the 3-color fluorescent in situ hybridization (FISH) method. Cells exposed to irradiation under the simulated μG condition showed a higher frequency of both simple and complex types of CA compared to cells irradiated under the static condition by either X-rays or carbon ions.

Synergistic Effects of Weightlessness, Isoproterenol, and Radiation on DNA Damage Response and Cytokine Production in Immune Cells

The implementation of rotating-wall vessels (RWVs) for studying the effect of lack of gravity has attracted attention, especially in the fields of stem cells, tissue regeneration, and cancer research. Immune cells incubated in RWVs exhibit several features of immunosuppression including impaired leukocyte proliferation, cytokine responses, and antibody production. Interestingly, stress hormones influence cellular immune pathways affected by microgravity, such as cell proliferation, apoptosis, DNA repair, and T cell activation. These pathways are crucial defense mechanisms that protect the cell from toxins, pathogens, and radiation. Despite the importance of the adrenergic receptor in regulating the immune system, the effect of microgravity on the adrenergic system has been poorly studied. Thus, we elected to investigate the synergistic effects of isoproterenol (a sympathomimetic drug), radiation, and microgravity in nonstimulated immune cells. Peripheral blood mononuclear cells were treated with the sympathomimetic drug isoproterenol, exposed to 0.8 or 2 Gy γ-radiation, and incubated in RWVs. Mixed model regression analyses showed significant synergistic effects on the expression of the β2-adrenergic receptor gene (ADRB2). Radiation alone increased ADRB2 expression, and cells incubated in microgravity had more DNA strand breaks than cells incubated in normal gravity. We observed radiation-induced cytokine production only in microgravity. Prior treatment with isoproterenol clearly prevents most of the microgravity-mediated effects. RWVs may be a useful tool to provide insight into novel regulatory pathways, providing benefit not only to astronauts but also to patients suffering from immune disorders or undergoing radiotherapy.

Functional validation of miRNAs targeting genes of DNA double-strand break repair to radiosensitize non-small lung cancer cells

DNA-Double strand breaks (DSBs) generated by radiation therapy represent the most efficient lesions to kill tumor cells, however, the inherent DSB repair efficiency of tumor cells can cause cellular radioresistance and impact on therapeutic outcome. Genes of DSB repair represent a target for cancer therapy since their down-regulation can impair the repair process making the cells more sensitive to radiation. In this study, we analyzed the combination of ionizing radiation (IR) along with microRNA-mediated targeting of genes involved in DSB repair to sensitize human non-small cell lung cancer (NSCLC) cells. MicroRNAs are natural occurring modulators of gene expression and therefore represent an attractive strategy to affect the expression of DSB repair genes. As possible IR-sensitizing targets genes we selected genes of homologous recombination (HR) and non-homologous end joining (NHEJ) pathway (i.e. RAD51, BRCA2, PRKDC, XRCC5, LIG1). We examined these genes to determine whether they may be real targets of selected miRNAs by functional and biological validation. The in vivo effectiveness of miRNA treatments has been examined in cells over-expressing miRNAs and treated with IR. Taken together, our results show that hsa-miR-96-5p and hsa-miR-874-3p can directly regulate the expression of target genes. When these miRNAs are combined with IR can decrease the survival of NSCLC cells to a higher extent than that exerted by radiation alone, and similarly to radiation combined with specific chemical inhibitors of HR and NHEJ repair pathway.

Interplay of space radiation and microgravity in DNA damage and DNA damage response

In space, multiple unique environmental factors, particularly microgravity and space radiation, pose constant threat to the DNA integrity of living organisms. Specifically, space radiation can cause damage to DNA directly, through the interaction of charged particles with the DNA molecules themselves, or indirectly through the production of free radicals. Although organisms have evolved strategies on Earth to confront such damage, space environmental conditions, especially microgravity, can impact DNA repair resulting in accumulation of severe DNA lesions. Ultimately these lesions, namely double strand breaks, chromosome aberrations, micronucleus formation, or mutations, can increase the risk for adverse health effects, such as cancer. How spaceflight factors affect DNA damage and the DNA damage response has been investigated since the early days of the human space program. Over the years, these experiments have been conducted either in space or using ground-based analogs. This review summarizes the evidence for DNA damage induction by space radiation and/or microgravity as well as spaceflight-related impacts on the DNA damage response. The review also discusses the conflicting results from studies aimed at addressing the question of potential synergies between microgravity and radiation with regard to DNA damage and cellular repair processes. We conclude that further experiments need to be performed in the true space environment in order to address this critical question.

Analysis of DNA-damage response to ionizing radiation in serum-shock synchronized human fibroblasts

Many aspects of cellular physiology, including cellular response to genotoxic stress, are related to the circadian rhythmicity induced by the molecular clock. The current study investigated if the cellular response to DNA damage is in relation to endogenous expression levels of the PER2 protein, a key component of the molecular regulatory system that confers rhythmicity in mammalian cells. Human normal fibroblasts (CCD-34Lu) were subjected to serum shock to induce circadian oscillations of the PER2 protein and then irradiated with γ- rays at times corresponding to the trough and peak expression of the PER2 protein. To better examine cellular response to DNA damage, the experiments performed in this study were carried out in non-proliferating CCD-34Lu fibroblasts in order to maintain the cell and circadian cycles separated while they were being exposed to genotoxic stress. Study results demonstrated that clonogenic cell survival, double-strand break repair kinetics, and TP53 protein levels were affected in the cells irradiated at the trough than in those irradiated at peak expression of the PER2 protein.

Cellular responses and gene expression profile changes due to bleomycin-induced DNA damage in human fibroblasts in space

Living organisms in space are constantly exposed to radiation, toxic chemicals or reactive oxygen species generated due to increased levels of environmental and psychological stresses. Understanding the impact of spaceflight factors, microgravity in particular, on cellular responses to DNA damage is essential for assessing the radiation risk for astronauts and the mutation rate in microorganisms. In a study conducted on the International Space Station, confluent human fibroblasts in culture were treated with bleomycin for three hours in the true microgravity environment. The degree of DNA damage was quantified by immunofluorescence staining for γ-H2AX, which is manifested in three types of staining patterns. Although similar percentages of these types of patterns were found between flight and ground cells, there was a slight shift in the distribution of foci counts in the flown cells with countable numbers of γ-H2AX foci. Comparison of the cells in confluent and in exponential growth conditions indicated that the proliferation rate between flight and the ground may be responsible for such a shift. We also performed a microarray analysis of gene expressions in response to bleomycin treatment. A qualitative comparison of the responsive pathways between the flown and ground cells showed similar responses with the p53 network being the top upstream regulator. The microarray data was confirmed with a PCR array analysis containing a set of genes involved in DNA damage signaling; with BBC3, CDKN1A, PCNA and PPM1D being significantly upregulated in both flight and ground cells after bleomycin treatment. Our results suggest that whether microgravity affects DNA damage response in space can be dependent on the cell type and cell growth condition.

The DNA damage response of C. elegans affected by gravity sensing and radiosensitivity during the Shenzhou-8 spaceflight

Space radiation and microgravity are recognized as primary and inevitable risk factors for humans traveling in space, but the reports regarding their synergistic effects remain inconclusive and vary across studies due to differences in the environmental conditions and intrinsic biological sensitivity. Thus, we studied the synergistic effects on transcriptional changes in the global genome and DNA damage response (DDR) by using dys-1 mutant and ced-1 mutant of C. elegans, which respectively presented microgravity-insensitivity and radiosensitivity when exposure to spaceflight condition (SF) and space radiation (SR). The dys-1 mutation induced similar transcriptional changes under both conditions, including the transcriptional distribution and function of altered genes. The majority of alterations were related to metabolic shift under both conditions, including transmembrane transport, lipid metabolic processes and proteolysis. Under SF and SR conditions, 12/14 and 10/13 altered pathways, respectively, were both grouped in the metabolism category. Out of the 778 genes involved in DDR, except eya-1 and ceh-34, 28 altered genes in dys-1 mutant showed no predicted protein interactions, or anti-correlated miRNAs during spaceflight. The ced-1 mutation induced similar changes under SF and SR; however, these effects were stronger than those of the dys-1 mutant. The additional genes identified were related to phosphorous/phosphate metabolic processes and growth rather than, metabolism, especially for environmental information processing under SR. Although the DDR profiles were significantly changed under both conditions, the ced-1 mutation favored DNA repair under SF and apoptosis under SR. Notably, 37 miRNAs were predicted to be involved in the DDR. Our study indicates that, the dys-1 mutation reduced the transcriptional response to SF, and the ced-1 mutation increased the response to SR, when compared with the wild type C. elegans. Although some effects were due to radiosensitivity, microgravity, depending on the dystrophin, exerts predominant effects on transcription in C. elegans during short-duration spaceflight.

Space experiment "Cellular Responses to Radiation in Space (CellRad)": Hardware and biological system tests

One factor contributing to the high uncertainty in radiation risk assessment for long-term space missions is the insufficient knowledge about possible interactions of radiation with other spaceflight environmental factors. Such factors, e.g. microgravity, have to be considered as possibly additive or even synergistic factors in cancerogenesis. Regarding the effects of microgravity on signal transduction, it cannot be excluded that microgravity alters the cellular response to cosmic radiation, which comprises a complex network of signaling pathways. The purpose of the experiment "Cellular Responses to Radiation in Space" (CellRad, formerly CERASP) is to study the effects of combined exposure to microgravity, radiation and general space flight conditions on mammalian cells, in particular Human Embryonic Kidney (HEK) cells that are stably transfected with different plasmids allowing monitoring of proliferation and the Nuclear Factor κB (NF-κB) pathway by means of fluorescent proteins. The cells will be seeded on ground in multiwell plate units (MPUs), transported to the ISS, and irradiated by an artificial radiation source after an adaptation period at 0 × g and 1 × g. After different incubation periods, the cells will be fixed by pumping a formaldehyde solution into the MPUs. Ground control samples will be treated in the same way. For implementation of CellRad in the Biolab on the International Space Station (ISS), tests of the hardware and the biological systems were performed. The sequence of different steps in MPU fabrication (cutting, drilling, cleaning, growth surface coating, and sterilization) was optimized in order to reach full biocompatibility. Different coatings of the foil used as growth surface revealed that coating with 0.1 mg/ml poly-D-lysine supports cell attachment better than collagen type I. The tests of prototype hardware (Science Model) proved its full functionality for automated medium change, irradiation and fixation of cells. Exposure of HEK cells to the β-rays emitted by the radiation source dose-dependently decreased cell growth and increased NF-κB activation. The signal of the fluorescent proteins after formaldehyde fixation was stable for at least six months after fixation, allowing storage of the MPUs after fixation for several months before the transport back to Earth and evaluation of the fluorescence intensity. In conclusion, these tests show the feasibility of CellRad on the ISS with the currently available transport mechanisms.

Changes in apoptotic microRNA and mRNA expression profiling in Caenorhabditis elegans during the Shenzhou-8 mission

Radiation and microgravity exposure have been proven to induce abnormal apoptosis in microRNA (miRNA) and mRNA expression, but whether space conditions, including radiation and microgravity, activate miRNAs to regulate the apoptosis is undetermined. For that purpose, we investigated miRNome and mRNA expression in the ced-1 Caenorhabditis elegans mutant vs the wild-type, both of which underwent spaceflight, spaceflight 1g-centrifuge control and ground control conditions during the Shenzhou-8 mission. Results showed that no morphological changes in the worms were detected, but differential miRNA expression increased from 43 (ground control condition) to 57 and 91 in spaceflight and spaceflight control conditions, respectively. Microgravity altered miRNA expression profiling by decreasing the number and significance of differentially expressed miRNA compared with 1 g incubation during spaceflight. Alterations in the miRNAs were involved in alterations in apoptosis, neurogenesis larval development, ATP metabolism and GTPase-mediated signal transduction. Among these, 17 altered miRNAs potentially involved in apoptosis were screened and showed obviously different expression signatures between space conditions. By integrated analysis of miRNA and mRNA, miR-797 and miR-81 may be involved in apoptosis by targeting the genes ced-10 and both drp-1 and hsp-1, respectively. Compared with ground condition, space conditions regulated apoptosis though a different manner on transcription, by altering expression of seven core apoptotic genes in spaceflight condition, and eight in spaceflight control condition. Results indicate that, miRNA of Caenorhabditis elegans probably regulates apoptotic gene expression in response to space environmental stress, and shows different behavior under microgravity condition compared with 1 g condition in the presence of space radiation.

Integration analysis of microRNA and mRNA expression profiles in human peripheral blood lymphocytes cultured in modeled microgravity

We analyzed miRNA and mRNA expression profiles in human peripheral blood lymphocytes (PBLs) incubated in microgravity condition, simulated by a ground-based rotating wall vessel (RWV) bioreactor. Our results show that 42 miRNAs were differentially expressed in MMG-incubated PBLs compared with 1 g incubated ones. Among these, miR-9-5p, miR-9-3p, miR-155-5p, miR-150-3p, and miR-378-3p were the most dysregulated. To improve the detection of functional miRNA-mRNA pairs, we performed gene expression profiles on the same samples assayed for miRNA profiling and we integrated miRNA and mRNA expression data. The functional classification of miRNA-correlated genes evidenced significant enrichment in the biological processes of immune/inflammatory response, signal transduction, regulation of response to stress, regulation of programmed cell death, and regulation of cell proliferation. We identified the correlation of miR-9-3p, miR-155-5p, miR-150-3p, and miR-378-3p expression with that of genes involved in immune/inflammatory response (e.g., IFNG and IL17F), apoptosis (e.g., PDCD4 and PTEN), and cell proliferation (e.g., NKX3-1 and GADD45A). Experimental assays of cell viability and apoptosis induction validated the results obtained by bioinformatics analyses demonstrating that in human PBLs the exposure to reduced gravitational force increases the frequency of apoptosis and decreases cell proliferation.

Changes in miRNA expression profile of space-flown Caenorhabditis elegans during Shenzhou-8 mission

Recent advances in the field of molecular biology have demonstrated that small non-coding microRNAs (miRNAs) have a broad effect on gene expression networks and play a key role in biological responses to environmental stressors. However, little is known about how space radiation exposure and altered gravity affect miRNA expression. The "International Space Biological Experiments" project was carried out in November 2011 by an international collaboration between China and Germany during the Shenzhou-8 (SZ-8) mission. To study the effects of spaceflight on Caenorhabditis elegans (C. elegans), we explored the expression profile miRNA changes in space-flown C. elegans. Dauer C. elegans larvae were taken by SZ-8 spacecraft and experienced the 16.5-day shuttle spaceflight. We performed miRNA microarray analysis, and the results showed that 23 miRNAs were altered in a complex space environment and different expression patterns were observed in the space synthetic and radiation environments. Most putative target genes of the altered miRNAs in the space synthetic environment were predicted to be involved in developmental processes instead of in the regulation of transcription, and the enrichment of these genes was due to space radiation. Furthermore, integration analysis of the miRNA and mRNA expression profiles confirmed that twelve genes were differently regulated by seven miRNAs. These genes may be involved in embryonic development, reproduction, transcription factor activity, oviposition in a space synthetic environment, positive regulation of growth and body morphogenesis in a space radiation environment. Specifically, we found that cel-miR-52, -55, and -56 of the miR-51 family were sensitive to space environmental stressors and could regulate biological behavioural responses and neprilysin activity through the different isoforms of T01C4.1 and F18A12.8. These findings suggest that C. elegans responded to spaceflight by altering the expression of miRNAs and some target genes that function in diverse regulatory pathways.

Simulated microgravity increases heavy ion radiation-induced apoptosis in human B lymphoblasts

AIMS

Microgravity and radiation, common in space, are the main factors influencing astronauts' health in space flight, but their combined effects on immune cells are extremely limited. Therefore, the effect of simulated microgravity on heavy ion radiation-induced apoptosis, and reactive oxygen species (ROS)-sensitive apoptosis signaling were investigated in human B lymphoblast HMy2.CIR cells.

MAIN METHODS

Simulated microgravity was achieved using a Rotating Wall Vessel Bioreactor at 37°C for 30 min. Heavy carbon-ion irradiation was carried out at 300 MeV/u, with a linear energy transfer (LET) value of 30 keV/μm and a dose rate of 1Gy/min. Cell survival was evaluated using the Trypan blue exclusion assay. Apoptosis was indicated by Annexin V/propidium iodide staining. ROS production was assessed by cytometry with a fluorescent probe dichlorofluorescein. Malondialdehyde was detected using a kit. Extracellular signal-regulated kinase (ERK), mitogen-activated protein kinase phosphatase-1 (MKP-1) and caspase-3 activation were measured by immunoblotting.

KEY FINDINGS

Simulated microgravity decreased heavy ion radiation-induced cell survival and increased apoptosis in HMy2.CIR cells. It also amplified heavy ion radiation-elicited intracellular ROS generation, which induced ROS-sensitive ERK/MKP-1/caspase-3 activation in HMy2.CIR cells. The above phenomena could be reversed by the antioxidants N-acetyl cysteine (NAC) and quercetin.

SIGNIFICANCE

These results illustrated that simulated microgravity increased heavy ion radiation-induced cell apoptosis, mediated by a ROS-sensitive signal pathway in human B lymphoblasts. Further, the antioxidants NAC and quercetin, especially NAC, might be good candidate drugs for protecting astronauts' and space travelers' health and safety.

Leukocyte activity is altered in a ground based murine model of microgravity and proton radiation exposure

Immune system adaptation during spaceflight is a concern in space medicine. Decreased circulating leukocytes observed during and after space flight infer suppressed immune responses and susceptibility to infection. The microgravity aspect of the space environment has been simulated on Earth to study adverse biological effects in astronauts. In this report, the hindlimb unloading (HU) model was employed to investigate the combined effects of solar particle event-like proton radiation and simulated microgravity on immune cell parameters including lymphocyte subtype populations and activity. Lymphocytes are a type of white blood cell critical for adaptive immune responses and T lymphocytes are regulators of cell-mediated immunity, controlling the entire immune response. Mice were suspended prior to and after proton radiation exposure (2 Gy dose) and total leukocyte numbers and splenic lymphocyte functionality were evaluated on days 4 or 21 after combined HU and radiation exposure. Total white blood cell (WBC), lymphocyte, neutrophil, and monocyte counts are reduced by approximately 65%, 70%, 55%, and 70%, respectively, compared to the non-treated control group at 4 days after combined exposure. Splenic lymphocyte subpopulations are altered at both time points investigated. At 21 days post-exposure to combined HU and proton radiation, T cell activation and proliferation were assessed in isolated lymphocytes. Cell surface expression of the Early Activation Marker, CD69, is decreased by 30% in the combined treatment group, compared to the non-treated control group and cell proliferation was suppressed by approximately 50%, compared to the non-treated control group. These findings reveal that the combined stressors (HU and proton radiation exposure) result in decreased leukocyte numbers and function, which could contribute to immune system dysfunction in crew members. This investigation is one of the first to report on combined proton radiation and simulated microgravity effects on hematopoietic, specifically immune cells.

The efficiency of homologous recombination and non-homologous end joining systems in repairing double-strand breaks during cell cycle progression

This study investigated the efficiency of Non-Homologous End Joining (NHEJ) and Homologous Recombination (HR) repair systems in rejoining DNA double-strand breaks (DSB) induced in CCD-34Lu cells by different γ-ray doses. The kinetics of DNA repair was assessed by analyzing the fluorescence decrease of γ-H2AX foci measured by SOID (Sum Of Integrated Density) parameter and counting foci number in the time-interval 0.5–24 hours after irradiation. Comparison of the two methods showed that the SOID parameter was useful in determining the amount and the persistence of DNA damage signal after exposure to high or low doses of ionizing radiation. The efficiency of DSB rejoining during the cell cycle was assessed by distinguishing G1, S, and G2 phase cells on the basis of nuclear fluorescence of the CENP-F protein. Six hours after irradiation, γ-H2AX foci resolution was higher in G2 compared to G1 cells in which both NHEJ and HR can cooperate. The rejoining of γ-H2AX foci in G2 phase cells was, moreover, decreased by RI-1, the chemical inhibitor of HR, demonstrating that homologous recombination is at work early after irradiation. The relevance of HR in DSB repair was assessed in DNA-PK-deficient M059J cells and in CCD-34Lu treated with the DNA-PKcs inhibitor, NU7026. In both conditions, the kinetics of γ-H2AX demonstrated that DSBs repair was markedly affected when NHEJ was absent or impaired, even in G2 phase cells in which HR should be at work. The recruitment of RAD51 at DSB sites was, moreover, delayed in M059J and in NU7026 treated-CCD-34Lu, with respect to DNA-PKcs proficient cells and continued for 24 hours despite the decrease in DNA repair. The impairment of NHEJ affected the efficiency of the HR system and significantly decreased cell survival after ionizing radiation, confirming that DSB rejoining is strictly dependent on the integrity of the NHEJ repair system.

Characterization of Epstein-Barr virus reactivation in a modeled spaceflight system

Epstein-Barr virus (EBV) is the causative agent of mononucleosis and is also associated with several malignancies, including Burkitt's lymphoma, Hodgkin's lymphoma, and nasopharyngeal carcinoma, among others. EBV reactivates during spaceflight, with EBV shedding in saliva increasing to levels ten times those observed pre-and post-flight. Although stress has been shown to increase reactivation of EBV, other factors such as radiation and microgravity have been hypothesized to contribute to reactivation in space. We used a modeled spaceflight environment to evaluate the influence of radiation and microgravity on EBV reactivation. BJAB (EBV-negative) and Raji (EBV-positive) cell lines were assessed for viability/apoptosis, viral antigen and reactive oxygen species expression, and DNA damage and repair. EBV-infected cells did not experience decreased viability and increased apoptosis due to modeled spaceflight, whereas an EBV-negative cell line did, suggesting that EBV infection provided protection against apoptosis and cell death. Radiation was the major contributor to EBV ZEBRA upregulation. Combining modeled microgravity and radiation increased DNA damage and reactive oxygen species while modeled microgravity alone decreased DNA repair in Raji cells. Additionally, EBV-infected cells had increased DNA damage compared to EBV-negative cells. Since EBV-infected cells do not undergo apoptosis as readily as uninfected cells, it is possible that virus-infected cells in EBV seropositive individuals may have an increased risk to accumulate DNA damage during spaceflight. More studies are warranted to investigate this possibility.

Simulated microgravity decreases apoptosis in fetal fibroblasts

Space travel is a major challenge for human beings. Especially, the mechanisms through which space conditions might alter animal development have been questioned for a long time. The two major physical stress factors that are of relevance in this context are space radiation and weightlessness. While it has been extensively shown that high doses of ionizing radiation induce deleterious effects on embryonic development, so far, little is known about the potential harmful effects of radiation in combination with microgravity on the developing organism. In the present study, we investigated the effects of simulated microgravity on irradiated STO mouse fetal fibroblast cells using a random positioning machine (RPM). Radiation-induced cell cycle changes were not affected when cells were subjected to simulated microgravity for 24 h. Moreover, no morphological differences were observed in irradiated samples exposed to simulated microgravity compared to cells that were exclusively irradiated. However, microgravity simulation significantly decreased the level of apoptosis at all doses as measured by caspase-3 activity and it prevented cells from undergoing radiation-induced size increase up to 1 Gy.

Analysis of miRNA and mRNA expression profiles highlights alterations in ionizing radiation response of human lymphocytes under modeled microgravity

BACKGROUND

Ionizing radiation (IR) can be extremely harmful for human cells since an improper DNA-damage response (DDR) to IR can contribute to carcinogenesis initiation. Perturbations in DDR pathway can originate from alteration in the functionality of the microRNA-mediated gene regulation, being microRNAs (miRNAs) small noncoding RNA that act as post-transcriptional regulators of gene expression. In this study we gained insight into the role of miRNAs in the regulation of DDR to IR under microgravity, a condition of weightlessness experienced by astronauts during space missions, which could have a synergistic action on cells, increasing the risk of radiation exposure.

METHODOLOGY/PRINCIPAL FINDINGS

We analyzed miRNA expression profile of human peripheral blood lymphocytes (PBL) incubated for 4 and 24 h in normal gravity (1 g) and in modeled microgravity (MMG) during the repair time after irradiation with 0.2 and 2Gy of γ-rays. Our results show that MMG alters miRNA expression signature of irradiated PBL by decreasing the number of radio-responsive miRNAs. Moreover, let-7i*, miR-7, miR-7-1*, miR-27a, miR-144, miR-200a, miR-598, miR-650 are deregulated by the combined action of radiation and MMG. Integrated analyses of miRNA and mRNA expression profiles, carried out on PBL of the same donors, identified significant miRNA-mRNA anti-correlations of DDR pathway. Gene Ontology analysis reports that the biological category of "Response to DNA damage" is enriched when PBL are incubated in 1 g but not in MMG. Moreover, some anti-correlated genes of p53-pathway show a different expression level between 1 g and MMG. Functional validation assays using luciferase reporter constructs confirmed miRNA-mRNA interactions derived from target prediction analyses.

CONCLUSIONS/SIGNIFICANCE

On the whole, by integrating the transcriptome and microRNome, we provide evidence that modeled microgravity can affects the DNA-damage response to IR in human PBL.

Effects of simulated microgravity on expression profile of microRNA in human lymphoblastoid cells

This study explores the changes in expression of microRNA (miRNA) and related genes under simulated microgravity conditions. In comparison with static 1 × g, microgravity has been shown to alter global gene expression patterns and protein levels in cultured cells or animals. miRNA has recently emerged as an important regulator of gene expression, possibly regulating as many as one-third of all human genes. However, very little is known about the effect of altered gravity on miRNA expression. To test the hypothesis that the miRNA expression profile would be altered in zero gravity resulting in altered regulation of gene expression leading to metabolic or functional changes in cells, we cultured TK6 human lymphoblastoid cells in a high aspect ratio vessel (bioreactor) for 72 h either in the rotating condition to model microgravity in space or in the static condition as a control. Expression of several miRNAs was changed significantly in the simulated microgravity condition including miR-150, miR-34a, miR-423-5p, miR-22, miR-141, miR-618, and miR-222. To confirm whether this altered miRNA expression correlates with gene expression and functional changes of the cells, we performed DNA microarray and validated the related genes using quantitative RT-PCR. Expression of several transcription factors including EGR2, ETS1, and c-REL was altered in simulated microgravity conditions. Taken together, the results reported here indicate that simulated microgravity alters the expression of miRNAs and genes in TK6 cells. To our knowledge, this study is the first to report the effects of simulated microgravity on the expression of miRNA and related genes.

Radiation-quality dependent cellular response in mutation induction in normal human cells

We studied cellular responses in normal human fibroblasts induced with low-dose (rate) or low-fluence irradiations of different radiation types, such as gamma rays, neutrons and high linear energy transfer (LET) heavy ions. The cells were pretreated with low-dose (rate) or low-fluence irradiations (approximately 1 mGy/7-8 h) of 137Cs gamma rays, 241Am-Be neutrons, helium, carbon and iron ions before irradiations with an X-ray challenging dose (1.5 Gy). Helium (LET = 2.3 keV/microm), carbon (LET = 13.3 keV/microm) and iron (LET = 200 keV/microm) ions were produced by the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. No difference in cell-killing effect, measured by a colony forming assay, was observed among the pretreatment with different radiation types. In mutation induction, which was detected in the hypoxanthine-guanine phosphoribosyltransferase (hprt) locus to measure 6-thioguanine resistant clones, there was no difference in mutation frequency induced by the X-ray challenging dose between unpretreated and gamma-ray pretreated cells. In the case of the pretreatment of heavy ions, X-ray-induced mutation was around 1.8 times higher in helium-ion pretreated and 4.0 times higher in carbon-ion pretreated cells than in unpretreated cells (X-ray challenging dose alone). However, the mutation frequency in cells pretreated with iron ions was the same level as either unpretreated or gamma-ray pretreated cells. In contrast, it was reduced at 0.15 times in cells pretreated with neutrons when compared to unpretreated cells. The results show that cellular responses caused by the influence of hprt mutation induced in cells pretreated with low-dose-rate or low-fluence irradiations of different radiation types were radiation-quality dependent manner.

DNA repair in modeled microgravity: double strand break rejoining activity in human lymphocytes irradiated with gamma-rays

Cell response to ionising radiation depends, besides on genetic and physiological features of the biological systems, on environmental conditions occurring during DNA repair. Many data showed that microgravity, experienced by astronauts during space flights or modeled on Earth, causes apoptosis, cytoskeletal alteration, cell growth inhibition, increased frequency of mutations and chromosome aberrations. In this study, we analysed the progression of the rejoining of double strand breaks (DSBs) in human peripheral blood lymphocytes (PBLs) irradiated with gamma-rays and incubated in static condition (1 g) or in modeled microgravity (MMG). gamma-H2AX foci formation and disappearance, monitored during the repair incubation, showed that the kinetics of DSBs rejoining was different in the two gravity conditions. The fraction of foci-positive cells decreased slower in MMG than in 1 g at 6 and 24 h after irradiation (P<0.01) and the mean number of gamma-H2AX foci per nucleus was significantly higher in MMG than in 1g at the same time-points (P<0.001). In the same samples we determined apoptotic level and the rate of DSB rejoining during post-irradiation incubation. A significant induction of apoptosis was observed in MMG at 24 h after irradiation (P<0.001), whereas at shorter times the level of apoptosis was slightly higher in MMG respect to 1 g. In accordance with the kinetics of gamma-H2AX foci, the slower rejoining of radiation-induced DSBs in MMG was observed by DNA fragmentation analyses during the repair incubation; the data of pulsed-field gel electrophoresis assay showed that the fraction of DNA released in the gel was significantly higher in PBL incubated in MMG after irradiation with respect to cells maintained in 1 g. Our results provide evidences that MMG incubation during DNA repair delayed the rate of radiation-induced DSB rejoining, and increased, as a consequence, the genotoxic effects of ionising radiation.

Bystander response in human lymphoblastoid TK6 cells

The mechanisms of the medium-mediated bystander response induced by gamma-rays in non-irradiated TK6 cells were investigated. Cell cultures were irradiated and the culture medium discarded immediately after irradiation and replaced with a fresh one. In cells incubated with conditioned medium from irradiated cells (CM), a significant decrease in cell viability and cloning efficiency was observed, together with a significant increase in apoptosis, also in directly irradiated cells. To examine whether bystander apoptosis involved the extrinsic pathway, an inhibitor of caspase-8 was added to CM cultures, which significantly decreased apoptosis to control levels. The addition to CM of ROS scavengers, Cu-Zn superoxide dismutase and N-acetylcysteine did not affect the induction of apoptosis. To assess whether CM treatment activates a DNA damage response, also the formation of gamma-H2AX foci, as markers of double-strand breaks and their colocalisation with 53-binding protein 1 (53BP1) and the protein mutated in the Nijmegen breakage syndrome 1 (NBS1) was analysed. In cultures treated for 2h with CM, 9-11% of cells showed gamma-H2AX foci, which partially or totally lacked colocalisation with 53BP1 and NBS1 foci. About 85% of irradiated cells were positive for gamma-H2AX foci, which colocalised with 53BP1 and NBS1 proteins. At 24h from irradiation, very few irradiated cells retained foci, fitting DNA repair kinetics. The number of foci-positive bystander cells also decreased to background values 24h after CM incubation. Our results suggest that irradiated TK6 cells release into the medium some soluble factors, not ROS, which are responsible for the cytotoxic effects induced in bystander cells. In our experimental system, the role of ROS appeared to be of minor importance in inducing cell mortality, but probably critical in activating the DNA damage response in the responsive fraction of bystander cells.

Does reduced gravity alter cellular response to ionizing radiation?

This review addresses the purported interplay between actual or simulated weightlessness and cellular response to ionizing radiation. Although weightlessness is known to alter several cellular functions and to affect signaling pathways implicated in cell proliferation, differentiation and death, its influence on cellular radiosensitivity has so far proven elusive. Renewed controversy as to whether reduced gravity enhances long-term radiation risk is fueled by recently published data that claim either overall enhancement of genomic damage or no increase of radiation-induced clastogenicity by modeled microgravity in irradiated human cells. In elucidating this crucial aspect of space radiation protection, ground-based experiments, such as those based on rotating-wall bioreactors, will increasingly be used and represent a more reproducible alternative to in-flight experiments. These low-shear vessels also make three-dimensional cellular co-cultures possible and thus allow to study the gravisensitivity of radioresponse in a context that better mimics cell-to-cell communication and hence in vivo cellular behavior.