Due to its germinative zone in the lens epithelium, the eye lens is a radiation-sensitive organ. These cells are actively proliferating during lifetime and finally differentiate into transparent lens fibers. In case cells are damaged, they cannot be eliminated from the lens which is covered by a capsule and not vascularized. Exposure of the eye lens to ionization radiation is thought to result in sub-capsular cortical lens opacification via various steps, starting with genetic damage of lens epithelial cells via changes in cell cycle control, apoptosis, differentiation, or other pathways controlling lens fiber cells’ differentiation, and cellular disorganization.


Due to higher local dose and different patterns of cellular energy deposition from high-LET components of GCR, higher efficiency in the induction of lens-damaging effects is assumed than for low-LET radiation. Therefore, animal experiments were mostly performed to determine the RBE of HZE particles to induce lens opacification and to detect possible dose rate effects. In rats, the RBE reached 50–100 for HZE particles within LET above 80 keV/μm [288] and fractionation of exposure did not reduce the cataractogenic effect [289]. Neutrons as secondary particles occurring in spacecraft and on planetary or moon surfaces had also a high RBE for cataract-induction in rats [290].


To determine the role of genetic predispositions, mice that are heterozygous for Ataxia telangiectasia mutated protein (ATM) were exposed to HZE particles and cataract formation was followed. ATM plays a central role in the DNA damage response (DDR). Heterozygosity for the ATM gene predisposes carriers for early onset time and progression of cataracts even without exposure to ionizing radiation [291]. Also after gamma ray and 1 GeV/n iron ion exposure, cataracts appear earlier in ATM heterozygous animals compared to wild-type mice and the RBE for HZE particle induced cataract formation ranged from 4 to 200, whereby the highest values were found for the lowest dose (10 mGy) and RBE decreases with increasing dose [292, 293]. In conclusion, HZE particles present in GCR and neutrons as part of the secondary radiation field are highly cataractogenic and the mechanisms such as long-term changes in gene expression, complex DNA damage, and chromosomal aberrations in eye lens epithelial cells (LECs) are still under investigation.


Exposure to space hazards, including microgravity and heavy ion exposure can cause harmful effects on the cardiovascular system during spaceflight. Upon entering microgravity, cephalad fluid shifts cause increased stroke volume and cardiac output. Furthermore, the cephalad fluid shift is also hypothesized to cause visual impairments due to increased cranial pressure [294]. During flight, mean arterial pressure is decreased, together with central venous pressure. Furthermore, decreased systemic vascular resistance, results from increased cardiac output, systemic arterial vasodilation, and decreased arterial pressure [295]. Other effects of microgravity exposure include hypovolemia, cardiac arrhythmia, cardiac atrophy, and orthostatic intolerance. Believed to be caused by fluid shifts and movement of interstitial water from the legs to the head, the fluid reduction and eventually hypovolemia results in a reduced number of red blood cells [296]. Moreover, cardiac atrophy occurs as a result of decreased metabolic demand and oxygen uptake during microgravity conditions. Together, cardiac deconditioning, i.e., hypovolemia, cardiac atrophy, and decreased cardiac output, causes a decreased exercise capacity and orthostatic intolerance post-flight [297].


While effects related to microgravity exposure during spaceflight are fairly well-known (albeit underexplored), impacts of the cardiovascular system from space radiation and heavy ion exposure during spaceflight are less known. Furthermore, studies from space analogs focusing on radiation effects have shown several effects on the cardiovascular system. Mice exposed to heavy ions show myocardial remodeling, resulting in hypertrophy and cardiac fibrosis [186]. Additionally, accelerated development of atherosclerosis has been found in mice after heavy ion exposure. Leading to a greater prevalence of myocardial infarction [298]. Both in vivo and in vitro models during space flight as well as using space analogs have been used to investigate underlying mechanisms of space-induced CVD. Important mechanisms include endothelial dysfunction, cellular apoptosis, cellular senescence, inflammation, and reactive oxygen species production [297].


Exposure to heavy ion, especially during long-term space mission, can also affect the central nervous system (CNS). The CNS is part of the nervous system and is composed of the brain and the spinal cord. It is responsible for perceiving any exterior information, transmitting, and subsequently processing it. Responsible for signal transmission are neurons, whereas glial cells (oligodendrocytes, microglia, or astrocytes) have diverse function such as the trophic support of neurons. As neurons are terminally differentiated and have a very restricted regeneration potential, damaged cells will usually not be replaced and thus damage might accumulate over months or years.


Acute CNS effects of ionizing radiation exposure are only observed after exposure to very high doses and can be expected in spaceflight only during very large Solar Particle Events (SPE) in case of insufficient shielding. Thus, for more than 20 years, possible effects of chronic low-dose exposure of the CNS to galactic cosmic rays (GCR) are discussed and a decrease in CNS performance of astronauts is suspected, which was also further evidenced in animal studies [299, 300]. Normally rodent animal experiments are performed at heavy ion accelerators simulating space radiation at doses below 1 Gy in a relatively short time, revealing impairment in cognitive performance, reduction of dendrites, reduced neurogenesis, and increased neuroinflammation [301–303]. As these effects can be seen even months after irradiation, late effects are possible even after exposure to lower doses [304].


Whereby it has to be considered that in these heavy ion accelerator experiments, the dose can only be applied in a short time, and prolonged exposure over several weeks or months mimicking the real situation in spaceflight is not often possible. With the use of new, low dose rate neutron irradiation facilities, it is now possible to expose rodents to a chronic low-dose as expected during space flights [305]. Also, mice that were irradiated with this chronic neutron irradiation (for 6 months) resulted in diminished hippocampal neuron excitability, a region which is essential for memory and learning, and disrupted hippocampal and cortical long-term potentiation. In addition, mice showed severe impairments in memory and learning tasks as well as distress behaviors [305].


One limit of experiments at the accelerator is that only radiation exposure of a few single radiation types can be studied, while in space, radiation exposure consists of a complex radiation mixture. It is still unclear if humans’ brains are affected to the same extent, but chronic low-dose radiation may cause problems for astronauts regarding decision-making processes or performance [306] (Box 10.8).


Since their discovery by Antonie van Leeuwenhoek in 1702, bdelloid rotifers and tardigrades have remained intriguing organisms. Their tolerance to desiccation at any stage of their life and their ability to survive a variety of stresses (e.g., low and high temperatures, absence of oxygen, vacuum, high level of ionizing radiation, etc.), makes them good candidates to study extreme resistance mechanisms in the context of space research. Tardigrades have a long history of space astrobiology experiments being among the first animals exposed to space vacuum and radiation. Recent experiments performed onboard of the ISS used bdelloid rotifers and tardigrades to study the adaptation to microgravity and cosmic radiation during spaceflight.


Bdelloid rotifers (Fig. 10.17) and tardigrades (Fig. 10.18) are among the smallest animals on Earth: most species are less than 1mm in size and contain ~1000 cells. Despite their small size, these animals have complete nervous, muscular, digestive, excretory, and reproductive systems. Mainly living in semi-terrestrial environments, such as lichens and mosses, most (but not all) bdelloid and tardigrade species are able to enter and survive complete desiccation (see Box 10.9 for definition) at any stage of their life cycle.

Two photographs of Adineta vaga depict the difference in the size and shape of its hydrated and desiccated forms. Egg production occurs in the hydrated form.

Fig. 10.17
Overview of the bdelloid rotifer Adineta vaga life cycle. Bdelloid rotifers live in limno-terrestrial habitats like mosses and lichens. Adapted to these environments, they can be desiccated at any stage of their life cycles including egg stage. When they are exposed to desiccation, adults adopt a “tun” shape allowing optimal desiccation resistance. Adineta vaga is about 200–250 μm long. (Credits B. Hespeels)

A set of 5 micrographs depicts the structural features of Echiniscus testudo, Paramacrobiotus areolatus, and the eggs of Macrobiotus kamilae and Paramacrobiotus areolatus.

Fig. 10.18
Morphology of adult tardigrades and eggs. (a, b) Lateral and dorsal views of Echiniscus testudo. (c) Dorsal view of Paramacrobiotus areolatus. (d, e) Global morphology of eggs laid by Macrobiotus kamilae and P. areolatus. Pictures were captured using scanning electron microscopy. (Illustration kindly provided by Daniel Stec and reprinted with his permission)


Two photographs of Adineta vaga depict the difference in the size and shape of its hydrated and desiccated forms. Egg production occurs in the hydrated form.


A set of 5 micrographs depicts the structural features of Echiniscus testudo, Paramacrobiotus areolatus, and the eggs of Macrobiotus kamilae and Paramacrobiotus areolatus.


When water starts to evaporate, these animals begin to contract their muscles and their body to adopt a “tun” shape allowing an optimal desiccation resistance [307–310]. This proper contraction of the body, followed by a specific organization of internal structures is a key step in enabling a successful recovery of desiccated animals after rehydration [308, 309]. The desiccation resistance and recovery rate vary between species [311–314]. The survival rate depends on the length of the desiccation period, the relative humidity, temperature, and animal age. Tardigrades desiccated over 10–20 years, within dry mosses stored at room temperature, were successfully rehydrated confirming their desiccation resistance for periods [315] [316]. While being frozen, these animals were shown to survive over 30 years of desiccation [317]. Bdelloid rotifers have also been shown to survive long periods of desiccation, up to 9 years [318]. As for tardigrades, cold temperatures seem to extend the capacity of desiccated bdelloids to cope with the long duration of metabolic arrest. In a recent publication by Shmakova et al. bdelloid rotifer specimens were recovered from frozen permafrost soil 24,000 years old [319]. If no data are still available for tardigrades, studying old permafrost samples may reveal other records of small animals’ life preservation. For example, some nematodes were described to successfully recover after melting from 30 to 40,000 years old samples [320].


Desiccation tolerance must be differentiated from drought tolerance. Many organisms are able to tolerate drought as a reduction in water availability in the environment for longer or shorter times. However, a reduced number is able to survive a loss of 90% or more of their body water content. Complete desiccation is reached when the water content decreases below 10% of the dried mass, not enough to form a monolayer around macromolecules, preventing enzymatic reactions and therefore metabolism.


Mostly found in habitats where physical parameters can change unpredictably, tardigrades and bdelloid rotifers were described to be able to cope with a wide range of physical extremes besides desiccation and freezing, such as UV radiation, high temperatures (exceeding 100 °C for a few minutes), high pressure or deep space vacuum [317, 321–327].


Among others, bdelloid rotifers and tardigrades were described to be highly resistant to low- and high-LET [328] radiation. In 2008, it was demonstrated for the first time that two bdelloid rotifer species, A. vaga and Philodina roseola, were resistant to ionizing radiation while being hydrated, surviving up to 1200 Gy of gamma radiation with fecundity (i.e., the total number of daughters produced by irradiated animals) and fertility (i.e., the capacity to produce at least one daughter) showing a dose response [329]. Later, it was demonstrated that desiccated bdelloid rotifers survive doses >5000 Gy of X-ray and proton radiation. These levels of radiation exposure were contrasting the Lethal Dose 50 (LD50) (i.e., dose required to kill 50% of the irradiated population) of mammalian cells which range from 2 to 6 Gy after X-ray irradiation. Similarly, desiccation-resistant tardigrades were described to survive high dose of X-ray and gamma ray (LD50 ranging between 3000 and 6000 Gy) (reviewed in [322]). Unexpectedly, radio-resistance of hydrated and desiccated tardigrades appeared to be more tolerant to high-LET radiation. For example [330], LD50 of the eutardigrade Richtersius coronifer was approx. 10,000 Gy. A major difference in comparison with bdelloid rotifers was that, despite a high survival after irradiation, most tardigrades were unable to produce fertile eggs for doses >100 Gy [322]. As an example, the tardigrade Hypsibius dujardini treated with gamma radiation had an estimated LD50/48 h for survival of ∼4200 Gy, and doses above 100 Gy dramatically impaired the production and hatching of laid eggs [331].


As an alternative to other animal models, the use of rotifers and tardigrades was proposed for space research. Indeed, these animals may contribute to better understanding damage and consequences induced by exposure to radiation and/or microgravity. How these organisms may respond and adapt to these stresses pave the road to the discovery of new molecules or candidate genes. Ultimately research outputs may be used to improve health span and protect astronauts or individuals subjected to radiation during space flights or medical treatments.


The use of rotifers and tardigrades as space research models was proposed because of the following aspects. (1) Complexity: they are Metazoans (multicellular animals), containing tissues and organs, having a complete gut and a complex muscular structure, yet being very simple animals. Rotifers and tardigrades are however made up of about 1000 cells, while a human is made up of several millions of cells. This simplification allows to disentangle complex problems through easier approaches. (2) Miniaturization: rotifers and tardigrades are small; experiments performed with numerous individuals require small vessels. (3) Distribution: rotifers and tardigrades are readily found in nature and are easily cultivated under controlled conditions. (4) Life span: rotifers and tardigrades have short life cycles that can be studied in a reasonable time period. (5) Reproductive mode: all bdelloid rotifers and some tardigrade species reproduce parthenogenetically. This reproduction system offers two key advantages: a rapid expansion of the population, and a high degree of reliability, as the genome is fully transmitted to the offspring. Therefore, the use of clonal lines reduces the biological variability noise in biological experiments. (6) Extremotolerance: both bdelloid rotifers and tardigrades were described to be able to deal with a high number of DNA DSBs and various stressors encountered by astronauts during space flight. Small extremotolerant animals can provide new perspectives in the adaptation of life to the space environment and ultimately lead to enhancing radio-resistance. For both clades, radiation resistance and radiation-sensitive species can be used in comparative experiments. (7) Storage: as most tardigrades and bdelloids survive desiccation and freezing, they can be stored easily before and after scientific experiments with limited impact on their biology and the scientific output. (8) Desiccation resistance: the desiccated state of tardigrades and bdelloid rotifers correlates with increased resistance to stresses, including deep space vacuum and extreme temperatures. These multiple properties and advantages for space experiments make bdelloid rotifers and tardigrades good candidates to test the limits of life during space exposure. An overview of space experiments involving tardigrades and rotifers is presented in the next two sub-sections.


In September 2007, tardigrades were exposed to LEO within the Biopan-6 experimental platform provided by the European Space Agency (ESA) (“Tardigrades in Space,” TARDIS. FOTON-M3 mission). During 10 days at LEO (258–281 km above sea level) samples of desiccated adult eutardigrades of the species Richtersius coronifer, Milnesium tardigradum, Echiniscus testudo, and Ramazzottius oberhaeuseri were exposed to space vacuum, cosmic radiations, and two different UV-radiation spectral ranges [323]. It was demonstrated that tardigrades were able to survive space vacuum and cosmic radiation with a survival rate ranging between 70% and 80%. Any impact on the reproductive capacities of exposed animals was reported. However, samples exposed to full solar radiation experienced high mortality. A small fraction of survivors died a few days post-rehydration without the production of any viable offspring. By filtering UV and restricting the exposure of desiccated tardigrades only to UVA and UVB, a significant part of desiccated tardigrades was able to be reactivated and was able to reproduce. Since the fertility of descendant generations of M. tardigradum was not impacted, it was suggested that survivors were able to repair a priori the damages induced by the spaceflight and did not transfer them to future generations [332].


In parallel to the TARDIS experiment (Fig. 10.19), two other experiments were launched onboard of the FOTON-M3: the (1) RoTaRad mission (Rotifers, Tardigrades, and Radiation) and (2) Tarse project (Tardigrade Resistance to Space Effects). RoTaRad experiment confirmed that desiccated tardigrades stored under controlled atmosphere were able to survive while being exposed to a combination of cosmic radiations and microgravity. However, the survival rates were reduced during this experiment likely due to the applied desiccation protocol [333]. With the Tarse project focusing on the eutardigrade Macrobiotus richtersi species, hydrated and desiccated individuals were exposed to the space environment for 12 days. In both states, microgravity and radiation had no effect on the survival rate, reproductive capacity, and DNA integrity of exposed animals. Despite the absence of visible morphological changes, it was nevertheless reported that the activity of key antioxidant proteins (including catalase and superoxide dismutase) was decreased during spaceflight. The amount of Heat Shock Proteins 70 and 90, known to be involved in stress resistances of tardigrades, did not differ after this short-term exposure to spaceflight.

Two photographs depict Bio pan facility with open lid on the outside of Foton capsule and the chambers in which tardigrades were placed, some are with sun filters.

Fig. 10.19
View of TARDIS experiment. (a) View of the exobiology Biopan platform containing TARDIS experiment. For 12 days in September 2007, approximately 3000 water bears were launched in space during the Foton-M3 mission. Reprinted with permission from ESA. (b) Details of the sample holder containing the tardigrades Richtersius coronifer. Tardigrades on the top level were exposed to the Sun and were optionally protected with filters. (Image kindly provided by K. Ingemar Jönsson and reprinted with his permission)


Two photographs depict Bio pan facility with open lid on the outside of Foton capsule and the chambers in which tardigrades were placed, some are with sun filters.


A few years later, the TARDIKISS experiments (Tardigrades In Space) were launched with the last Space Shuttle mission (STS-134 2011) [334]. During this 16-days mission, the enzyme activity of key antioxidants was investigated in desiccated tardigrades from the two species Paramacrobiotus richtersi and Ramazzottius oberhaeuseri [334]. Supporting the idea that desiccated animals were weakly affected by microgravity and cosmic radiation, comparative data analysis between flight and ground samples showed no significant differences in the enzymatic activity of antioxidants.


In June 2021, a fifth experiment was launched onboard of the ISS to investigate the short-term and multigenerational survival of tardigrades. The aim of the Cell Science-04 experiment (CS-04) was to evaluate the transcriptomic response of hydrated tardigrades cultured on the ISS using a dedicated cell culture system (Bioculture System, developed at NASA Ames Research Center). For this experiment, the tardigrade Hypsibius exemplaris was used as model species. Scientists are currently evaluating the ability of these animals to survive onboard of ISS for short and long periods of time (up to four generations). In parallel, the transcriptomic responses of these animals are being investigated to follow the evolution of the expression profiles of tardigrades in a microgravity environment. A progressive adaptation of tardigrades onboard of ISS may lead to a better understanding of the molecular responses involved in gravity sensing and will help expand research to secure astronaut’s health for future space missions. Among others, tardigrades were described to express several antioxidant proteins to face desiccation and radiation stresses [322, 335]. In particular, tardigrades were described to express specific proteins binding to DNA and protecting their genome from ROS induced by desiccation and ionizing radiation [336, 337].


How microgravity and cosmic radiation may affect desiccated bdelloid rotifers was tested for the first time in 1997. For their first exposure to space, dry samples of Macrotrachela quadricornifera were transported onboard of the space shuttle STS-81 for a total of 10 days. The data revealed a similar survival rate and reproductive fitness for ground controls and flight samples [312]. Since desiccated rotifers appeared to be protected from the impact of microgravity and cosmic radiation, at least in this short-term exposure experiment, researchers started to investigate the consequences of space flight on hydrated bdelloids. In absence of gravity, it has been hypothesized that the distribution of cytoskeletal elements or yolk granules in the egg cytoplasm is impacted. This abnormal organization of the cytoskeleton could impact the rotifer reproduction. Therefore, researchers first investigate the capacity of bdelloid rotifers to complete their embryological development under microgravity was initially investigated. Pre-flight experiments were performed under hyper-gravity environment (up to 20 g) and under simulated microgravity (as low as 0.0001 g) using a 3D random positioning machine (3DRPM). Results showed that the rotifer development remained constant regardless of the treatment experienced, except for some minor modifications in early embryos experiencing 20 g with no subsequent impact on the development. This first investigation suggests that bdelloid rotifers continue embryological development despite changes in g-force. Unfortunately, no data from flight experiment development associated with embryological development of bdelloid rotifers exposed to space environment was released post-flight.


Twenty years later, the bdelloid rotifer A. vaga was sent onboard of the ISS for two independent experiments. In December 2019, two autonomous hardware, each containing five culture bags loaded with 10,000 individuals, were transported onboard of ISS. Hydrated animals were exposed to launch conditions and exposed to 12 days of microgravity. At the same time, a ground reference experiment was implemented on Earth to compare the biological responses of rotifers to space conditions on ISS. The aim of this first experiment (RoB1, Fig. 10.20) was to compare the transcriptomic responses of hydrated A. vaga samples exposed to space environment with the ground control samples. Preliminary results confirmed the successful maintenance of hydrated bdelloid individuals on ISS, without additional food or oxygen supply and without astronaut intervention. All the replicates (ten) of the autonomous A.vaga cultures survived and reproduced on ISS with no visible impact on the morphology in space-exposed samples.

Three photographs depict Rob 1 hardware, P L T bags filled with rotifer cultures, and an astronaut keeping the Rob 1 hardware in K U B I K incubator going to the I S S.

Fig. 10.20
View of Rob1 hardware used to culture hydrated A. vaga individuals onboard of ISS (December 2019). Top left: Rob1 hardware after its assembly at the launch site at Kennedy Space Center. Rob1 hardware is a passive hardware containing five culture bags containing hydrated specimens of A. vaga. Hardware enables gas exchanges between rotifer cultures and the outside through a permeable membrane. Top right: View of the culture bags assembled inside Rob1 hardware. Culture bags, loaded with 10,000 A. vaga individuals each, are made of Teflon and ensure an optimal gas exchange between the culture medium and the outside. Bags are waterproof and avoid any leakage of the medium (composed of mineral water and sterile lettuce juice) or rotifers. Reprinted with permission of Marc Guillaume. Bottom left: View of ESA astronaut Luca Parmitano loading two Rob1 hardware on KUBIK. KUBIK is a small incubator, temperature-controlled, with removable inserts designed for self-contained microgravity experiments. (Reprinted with permission of NASA)


Three photographs depict Rob 1 hardware, P L T bags filled with rotifer cultures, and an astronaut keeping the Rob 1 hardware in K U B I K incubator going to the I S S.


While it is well documented that astronauts experience DNA damage when exposed to cosmic radiation, accumulating DNA mutations and/or genomic rearrangements [163], the combined effect of cosmic radiation and microgravity on the living organism is still debated. It is suspected that microgravity reduces the efficiency of DNA repair and increases cancer risk [207, 215, 338]. Several studies using simulated microgravity highlighted a decrease in DNA repair efficiency. However, no effects of spaceflight on the cellular capacity to repair artificially induced DNA was observed (see Moreno-Villanueva et al. [209] for review). In order to obtain more insights, bdelloid rotifers have been used as a model system to evaluate their DNA repair efficiency of induced DNA breaks in space environment as compared to Earth samples. By the end of 2020, desiccated and irradiated A. vaga individuals were sent onboard of ISS. Before launch, desiccated animals were irradiated with 500 Gy of X-ray or proton radiation. Onboard, bdelloids were rehydrated and cultivated for different time periods to (1) follow the putative DNA repair process occurring post-rehydration and (2) investigate whether these irradiated rotifers still produce offspring under microgravity. In addition, half of the samples were exposed to simulated gravity using a centrifuge on ISS. Finally, a ground experiment was conducted in parallel at the launch site at Kennedy Space Center for comparison. Data generated by this second space experiment (entitled RoB2, see Fig. 10.21) will enable: first, to compare the DNA repair kinetic of rehydrated bdelloids post irradiation in 1G, μG, and simulated 1G; second, to compare the radiation responses of rehydrated rotifers after exposure to low LET or high LET; and third, to compare the DNA repair efficiency in space and on Earth by isolating eggs or juveniles from the exposed samples and use whole genome sequencing to compare the genomic structure of these animals pre- and post-exposure. This space experiment with bdelloid rotifers will contribute to our understanding of DNA repair process activity in space, in the presence or absence of microgravity. Moreover, studying the molecular processes involved in the DDR process of A. vaga will be of huge interest for future space travel.

Two photographs depict a pair of gloved hands holding a Rob 2 hardware and a spaceman conducting an experiment with the samples of Adineta vaga in I S S.

Fig. 10.21
View of one Rob2 hardware used onboard of the ISS (left) and Astronauts checking the correct rehydration of A. vaga individuals. Sixteen pieces of hardware were sent to ISS, each containing 40,000 dry rotifers. Once onboard, rotifers were automatically rehydrated and cultivated 11 days before their fixation and download to Earth. (Reprinted with permission of Boris Hespeels and NASA)


Two photographs depict a pair of gloved hands holding a Rob 2 hardware and a spaceman conducting an experiment with the samples of Adineta vaga in I S S.


In general, the ongoing rotifer space experiments will contribute to a better understanding of the mechanisms involved in the protection and repair of damages induced by radiation. They pave the road to the discovery of new molecules or candidate genes that could ultimately be used to improve health span and protect astronauts or individuals subjected to radiation during space flights or medical treatments. This research is also of fundamental importance for the understanding of extreme biology and the questions raised on the origin of life and its ability to spread through outer space. A third experiment, supported by ESA, is under preparation to evaluate whether rotifers can survive full space exposure, outside ISS, as was previously reported for tardigrades.


Long space exploration missions, settlement on orbital stations, or future planetary settlement (e.g., on Mars) will require further development of Life Support Systems (LSS). The LSS are able to regenerate a great amount of essential resources for survival and represent an ideal solution since it is not technically and economically feasible in long space missions to transport a large amount of consumables from the Earth [339–341]. Bioregenerative Life Support Systems (BLSS) are an artificial closed ecosystem characterized by the same structure as a terrestrial ecosystem: producers (plants), consumers (humans/animals), and decomposers (microorganisms).


Among biological components within BLSS, higher plants would have the same role on Earth as producers. Through photosynthesis, plants would utilize carbon dioxide produced by space crew and provide oxygen and fresh food. Moreover, they would use nutrients derived from human wastes and guarantee water purification by transpiration. Furthermore, plant cultivation in space also would provide psychological support against isolation [342, 343].


Each organism in Space is subjected to several factors which are potential constraints for biological life. Among the environmental factors (e.g., altered gravity, interaction between microgravity and fluid-dynamics, modified conditions of pressure, temperature, confinement, etc.) limiting plant growth in space, ionizing radiation influences severely the development of organisms at molecular, morpho-structural, and physiological levels [163, 182, 344]. Indeed, ionizing radiation is considered one of the main constraints for the long permanence of humans in Space.


All organisms in extraterrestrial environments are subject to higher levels of ionizing radiation than on Earth and, notwithstanding the large number of studies aimed at understanding the effect of ionizing radiation on animals, the knowledge on plant reaction is limited. Available information is limited to horticultural model crops which are candidate for fresh food production in BLSS. Moreover, most experiments are based on the irradiation of dry seeds and data from irradiation tests using other biological models (e.g., seedlings, adult plants, actively growing tissues) are scanty.


In Fig. 10.22, a comparison of the responses of plants and mammals to ionizing radiation exposure is shown. Generally, plants are more resistant than mammals. Ionizing radiation is known to have differential effects on plant growth, development, and reproduction, ranging from detrimental outcomes at high doses, harmful consequences at intermediate levels, and stimulatory effects at very low doses. This phenomenon is called “hormesis.” Particularly, low doses of ionizing radiation have been reported to stimulate seed germination and root growth [345, 346].

An illustration depicts radiation hormesis, radiosensitivity, D N A repair mechanisms, high versus low L E T ionizing radiation, features conferring radio resistance in irradiated plants and mammals.

Fig. 10.22
A comparison among different responses of Plants (P) and Mammals (M) to ionizing radiation. (Reprinted with permission from Arena et al. [346])


An illustration depicts radiation hormesis, radiosensitivity, D N A repair mechanisms, high versus low L E T ionizing radiation, features conferring radio resistance in irradiated plants and mammals.


However, ionizing radiation can also induce dwarf growth that is a desirable trait under conditions of limited volume availability in missions on orbital stations or during exploration traveling. The increased radioresistance of plants is still a debated issue since it can be associated with a genetic basis, but it can also reflect biochemical and biomolecular mechanisms of shelter from genotoxic damage.


The severity of the effects of ionizing radiation on plants is dependent upon several factors including radiation-related parameters (e.g., dose, LET) and organism-related traits (e.g., species, cultivar, physiological status, and structural properties, as well as plant genome organization including the polyploidy) [345, 347].


In adult plants, in the case of organs at complete development, resistance to stressors can be often ascribed to integrated mechanisms of adaptation operating at morpho-structural and eco-physiological levels since the limits of major metabolic and physiological processes are dictated by the plant’s structure [348, 349]. Growth, reproduction, and, ultimately, survival of plants in Space depend on photosynthesis which is strongly responsive to ionizing radiation acting on the various components of the photosynthetic apparatus, such as pigment–protein complexes responsible for light absorption, electron transport carriers, and enzymes of carbon reduction cycle [345]. Ionizing radiation leads to several detrimental effects in photosynthetic apparatus, such as loss of functionality of photosystem II (PSII) and generation of free radicals causing photosynthetic membranes’ oxidation [350–352]. Changes in the total antioxidant pool and in the distribution of phenolic compounds in leaf tissues were observed in plants exposed to very high doses of X-rays, namely 50 and 100 Gy [353].


However, chronic exposure to low doses of ionizing radiation seems to enhance the activity of some antioxidant enzymes, providing plants with a radio-resistance [354, 355]. Moreover, the degree of plasticity of leaf cytological and anatomical traits in response to environmental changes can be responsible for enhancing or constraining processes such as light interception and gas exchanges, definitely affecting photosynthesis. Similarly, the correct functioning of the whole water transport system throughout the plant is responsible for water supply up to the leaves, necessary for efficient photosynthesis. The ability of xylem to transport water efficiently depends on the morphological features of its conduits and on the ultra-structural properties of conduit cell walls, whose main components can be differently affected by ionizing radiation.


Apart from a few findings mainly related to specific ultra-structural modifications occurring on irradiated seeds, the effect of cosmic radiation on organ/tissue organization, especially in relationship with eco-physiological traits, is still poorly explored. Moreover, most of the studies regard experiments with low-LET ionizing radiation [346, 355], and only a few data are available on the effects of chronic radiation exposure on plants in general, mainly deriving from nuclear accidents as Chernobyl in Ukraine (1986) and Fukushima in Japan (2011).


Regarding the complexity of their cells, all living organisms can be classified into two groups-prokaryotes and eukaryotes. Compared to prokaryotes, eukaryotic cells are highly organized and contain a cell nucleus. Prokaryotes are bacteria and archaea, while protists, plants (see Sect. 10.5.8), animals (see Sect. 10.5.7), and fungi (see Sect. 10.5.9) are eukaryotes.


In the following the effect of space radiation on in vitro models (conducted in a cell culture dish) and ex vivo models (experiments outside a living body) will be described.


In vitro models used in science, are very important, as they provide insight into cells. With this, the function of primary cells and cell lines of various origin (vertebrates including human, insects, and mussels) can be studied.


Ex vivo models or tissue explants allow studying complex functions and interactions of different cells within an organ. For these experiments, the living tissues are directly removed from a living organism or can be generated by means of pluripotent stem cells and cultivated under controlled conditions.


In comparison to cells in monolayer cultures (2D), cells in 3D cultures react completely differently. The biggest disadvantages of 2D cultures are the unnatural contact with a plastic or glass surface, the flat morphology of the cells on the growth surface that restricts intercellular contacts and the lack of an extracellular matrix which surrounds cells in vivo. These conditions modify the metabolism and functioning of cells and often result in the loss of the specific differentiation of a cell. The structure, function, and composition of organs and tissues can thus be better studied in 3D cell culture systems. They enable cell–cell and cell–extracellular matrix interactions in a three-dimensional space. 3D cultures are a very helpful tool before performing whole-animal studies. They can further be used to study the understanding of how processes in tissues are affected by spaceflight conditions, including space radiation and microgravity, which otherwise cannot be investigated in animal or human subject studies.


There are many different models of 3D cell cultures, including organoids, ex vivo tissue, or slice cultures, which are explained in the following. Furthermore, it is possible to create these models with 3D bioprinting, which have then a structure which closely resembles the organization of tissue or organs. In fact, the European Space Agency (ESA) recently summarized the capability science requirements for 3D bioprinting on the ISS to support medical treatment on long-term space missions.


In all given examples two or more cell types can be co-cultured, closely simulating the situation in organs or tissues, e.g., investigation of cellular differentiation processes in tissues, nerve-muscle function, tissue regeneration and repair, vascular tissue function, brain tissue homeostasis and aging, immune system processes or cardiac muscle function.


Human organoids, derived from stem cells or progenitor cells, are tiny self-organized organ-specific 3D cultures, recreating the physiological and cytoarchitecture of human organs. With this, the model reflects the in vivo situation much better than single cell cultures. For research purposes, it is feasible to create organoids that resemble the brain, kidney, lung, intestine, stomach, and liver.


Organoids will help to study the effect of space radiation on the overall response of organs, including cellular heterogeneity, cell-matrix interactions, cell-cell interactions, morphology, and functional changes [356, 357], which cannot be studied in in vitro systems. One major disadvantage compared to in vivo systems is the lack of microenvironment.


The effects of microgravity on human brain organoids were tested on the ISS during the Space Tango-human Brain investigation in 2019 (NASA). Of special interest was the effect on the brain cells including survival, migration, metabolism, and the formation of neuronal networks (Muotri, unpublished).


Spheroids are also 3D cell cultures, but in comparison to organoids, they form simple clusters into sphere-like formation, but they cannot self-assemble or regenerate. Whereby the cellular functions inside spheroids are closely correlated to the size, uniformity is especially important for reproducible results. To guarantee this, several methods for culturing are available such as hanging drops, scaffolds, liquid overlay technique, and hydrogels [358]. Nowadays spheroids are highly used to study the microenvironments of tumors or their response to radiotherapy.


Already in 2016, the SPHEROIDS project was launched on the ISS. Here, endothelial cells, which under simulated microgravity form small, rudimentary blood vessels, were exposed to real microgravity for 12 days on the ISS. The formation of spheroids under space conditions and under simulated microgravity on Earth were similar [359], underlining the important role of microgravity in spheroids formation.


Differences between the three types of cultures are summarized in Fig. 10.23.

A table depicts the cell types, derived from, morphology, represent, and examples of monolayer, spheroid, and organoid, with corresponding diagrams.

Fig. 10.23
Difference between the different cultures


A table depicts the cell types, derived from, morphology, represent, and examples of monolayer, spheroid, and organoid, with corresponding diagrams.


Organotypic slice cultures are tissue samples that are cut in thinly, about 300 μm, thick slice and are then cultivated on semipermeable insert. Most common are organotypic slice cultures that originate from different parts of the brain (e.g., hippocampus, cerebellum, or cortex) and can be kept in cultures for long term, while slices originating from liver tumors can only be kept in culture for a short time [360, 361]. Also, this 3D culture has the advantage that the composition and architecture of the extracellular matrix as well as the tissue are preserved. During analysis of the slices, it has to be considered that every slice, even from the same organ, has a partly different composition, cell counts, and viability, limiting the reproducibility of results produced by this method.


Organ cultures were developed from tissue and slice cultures. By using organ cultures, it is possible to study the functions of an organ in various conditions and states in an in vitro organ. Hereby, the entire organs or only a part of the organ are excised from the body and cultured. Also, with this method, the 3D structure of the tissue of choice is preserved.


For space exploration, the eye lens is of special interest, because it is amongst the most radiosensitive tissues in the human body. Ionizing radiation can cause a posterior sub-capsular cataract [287, 362, 363]. Whole lenses and lens epithelial cells in culture enable the study of early mechanisms of space radiation-induced cataractogenesis and of the relative biological efficiency of different space radiation components to induce early changes. With regard to the human lens in anatomy and size, the porcine eye is very similar. Thus, it is used to study the radiation response in the whole organ. Translation to the human eye lens can be enabled by using human-transformed epithelial cells or lens epithelial cells from donor patients. As the viability of eye lenses in cultures is limited to a few weeks, studies on radiation-induced full-blown cataract formation usually require animal experiments over their lifespan (Sect. 10.5).


In addition to that, the microgravity environment on the ISS suits perfectly to 3D print tissue cultures and later maybe entire organs. Compared to conditions on Earth where scaffolds or matrices are needed to form organoids, in space cells can easily self-organize into their precise structure. On the one hand, the bioprinted tissue could be used in the future to treat injured astronauts [364] and on the other hand the technique can be transferred to Earth and then be applied to the field of regenerative medicine for organ transplantations [365]. In July 2019, the 3D BioFabrication Facility (BFF—see Fig. 10.24) has arrived onboard of the ISS, with this it is now possible to study 3D bioprinting of different human tissues in space. Also, here real microgravity has the benefit that printed structures will not collapse, enabling also the printing of soft human tissue (NASA) (Box 10.10).

A photograph of the three-dimensional bio fabrication facility, the first bioprinter capable of manufacturing human tissue in the microgravity of space.

Fig. 10.24
NASA’s 3D BioFabrication Facility BFF. (Image JSC2019E037579, Credits NASA)


A photograph of the three-dimensional bio fabrication facility, the first bioprinter capable of manufacturing human tissue in the microgravity of space.


Several cell cultures system can be studied under space conditions

The microgravity environment on the ISS suits perfectly to print 3D tissue cultures


The microgravity environment on the ISS suits perfectly to print 3D tissue cultures


Understanding the effects of the space environment on microorganisms has witnessed recently considerable progress (whereas the main factors are microgravity, radiation, and vacuum). However, explicit knowledge of molecular mechanisms responsible for survival and adaptation in space is still missing. Space environment affects a variety of physiological features of microorganisms. The above features include metabolism, motility and proliferation rate, division of cells, and also virulence and biofilm production (Fig. 10.25) [366]. Molecular-level understanding of the above effects in space-exposed microorganisms is still lacking. It is believed that omics-based approach, together with classical phenotyping and physiological measurements, will be a useful toolbox for understanding mechanisms of microbial survivability in the harsh conditions of outer space. “Omics” stands for genomics, transcriptomics, proteomics, metabolomics, and more.

An illustration describes the stress response, membrane-associated readjustments, metabolic rearrangements, and genetic alterations for the survival of microbial entity in space environment.

Fig. 10.25
Molecular response experienced by microorganisms in the outer space environment revealed with the help of global and integrative –omics approaches of systems biology that have been recently used to study microorganisms exposed to real and simulated space conditions. (Reprinted with permission from Milojevic et al. [366])


An illustration describes the stress response, membrane-associated readjustments, metabolic rearrangements, and genetic alterations for the survival of microbial entity in space environment.


Systems biology is an interdisciplinary approach in biomedical research aiming at understanding the biological system at the organism, tissue, and cell level. Systems biology incorporates the results of –omics techniques, genome-scale metabolic and regulatory biomathematical models to understand molecular interactions, evolution, functional and phenotypical diversity, and molecular adaptation. The omics-based approach integrates various pieces of biological information from genomes, mRNA, and proteins to metabolites [367].


The –omics-based approach has recently opened a window for a deep insight into molecular machinery implicated in the survivability of space-exposed microorganisms by revealing expression, metabolic functioning, and regulation of the genes and proteins encoded by the genomes of “space travelers.” The diverse biological activities of microorganisms in space are affected by metabolic alterations caused in turn by genetic regulations (Fig. 10.26). It has been demonstrated by means of –omics-based approaches that exposed microbes switch to “energy saving mode.” Research identified some global regulatory molecules that drive molecular response of a few space-exposed microorganisms [366–369]. Various kinds of stress responses (e.g., general, osmotic, and oxidative) experienced by microorganisms in conditions of real and simulated outer space have been deciphered via –omics-assisted analyses [366]. Various genes with altered expression after microbs’ exposure to real and simulated outer space environment (Fig. 10.25) have been identified [366].

An illustration lists the genes and proteins expressed in different microbes during general stress response, osmotic stress response, tellurium resistance, and oxidative stress response in space.

Fig. 10.26
Stress responses experienced by microorganisms in outer space real and simulated conditions, revealed with –omics-assisted investigations. Proteins and genes of stress responses with altered abundance and expression after exposure of microorganisms to the outer space real and simulated environment [366]


An illustration lists the genes and proteins expressed in different microbes during general stress response, osmotic stress response, tellurium resistance, and oxidative stress response in space.


State-of-the-art –omics technologies have been successfully used to understand molecular mechanisms responsible for alterations of microbial virulence in space conditions (Fig. 10.27) [366].

An illustration lists factors of pathogenicity, virulence, and biofilm formation in space by K. pneumonia, S. aureus, D. radiodurans, B. cereus, P. aeruginosa, S. cerevisiae, C. albicans, Salmonella.

Fig. 10.27
Molecular alterations underlying microbial pathogenicity, virulence, and biofilm formation in the outer space environment, resolved with –omics-assisted investigations [366]


An illustration lists factors of pathogenicity, virulence, and biofilm formation in space by K. pneumonia, S. aureus, D. radiodurans, B. cereus, P. aeruginosa, S. cerevisiae, C. albicans, Salmonella.


Space exposure imposes stresses that affect microbial survival rates and may lead to certain discrepancies in –omics-assisted analysis of returned/exposed microorganisms. The composition of the cultivation medium influences the microbial space response [369], e.g., by providing specific antioxidants presented in rich medium, which may protect microbial cells against ionizing radiation. The majority of space experiments have been performed on satellites, where microorganisms are cultivated in environment protected from all factor but microgravity [366]. Direct exposure to real space environment outside the ISS followed by investigation with –omics techniques was performed on a few microbial species only [367, 370, 371]. Therefore, in order to broaden our knowledge of molecular mechanisms of microbial survivability in outer space, there is an urgent need for further experiments with direct exposure. Often, a multi-omics post-flight analysis has the problem of a limited number of microbiological samples exposed to the space environment. Therefore, the researchers should critically assess the design of outerspace experiments to provide a sufficient number of independent biological samples in order to enable statistically significant results in processing the –omics data. It is also extremely important to avoid artifacts: due to very high sensitivity of the –omics techniques of occasional occurrence of uncontrolled conditions, stress-related artifacts cannot be ruled out. In this context, it is highly desired to develop novel approaches for the efficient extraction of DNA, RNA, proteins, and metabolites simultaneously from the minimal amount of microbial cells [367, 372]. Furthermore, the absence of detailed reports regarding the environmental conditions during space exposure and corresponding ground control experiments is, unfortunately, a frequent reality that requires a critical reassessment of research planning. Providing a full record of controlled parameters (like temperature, humidity, and pressure profiles) during flight, simulated, and control experiments is highly desired to achieve a comprehensive and artifacts-free analysis of the effects of the space environment on the physiology and molecular machinery of microorganisms.


It has been proposed that in future space experiments, detailed metabolomic analysis of exposed microorganisms should be performed in addition to the proteotranscriptomic profiling. This novel approach has provided already plenty of new findings on fine molecular networks regulating the space response [367]. Recent works (e.g., the NASA twins’ study) used a multi-omics, systems biology analytical approach to analyze biomedical profiles of astronauts [120]. Results of performed targeted and untargeted metabolomics combined with proteomics effectively revealed the biomedical responses of a human body during a year-long spaceflight indicating mitochondrial stress as a consistent phenotype of spaceflight [120, 373]. Finally, the combination of molecular data with a genome-scale metabolic reconstruction of the respective species should be implemented, delivering the space-induced microbiome signatures [366].


Humans have all evolved in an environment containing a persistent low level of constant exposure to different endogenous and exogenous mutagenic agents, and consequently have developed many cellular mechanisms for either DNA protection or repair (see Chap. 2). However, when humans travel into space, these naturally evolved cellular mechanisms might not be enough as many major health threats from space radiation has been identified, e.g., central nervous system injury, cardiovascular diseases, immune dysfunction, cancer development, and premature aging. To reduce the risk of humans in space, there are some possible interventions which can limit the effects of space radiation. A dedicated review can be found elsewhere [374].


One way of reducing the health risk from space radiation exposure in humans is selecting more radioresistant humans during the selection campaigns of space agencies. The most used way is to perform in vitro adaptive response studies, in which cells collected from the candidates are used to measure their response to a fixed dose of ionizing radiation. While the results of these studies are not necessarily used during candidate selection, they hold great value in selecting the right people that will be more protected against space radiation. Another strategy would be to pharmacologically hamper the processes underlying the molecular (side) effects of space radiation exposure. Examples are the application of radioprotectors and geroprotectors, as well as supplementation with antioxidants or antioxidative capacity increasing compounds (see Chap. 11). While these pharmaceuticals hold great promise, many of them are still under investigation and not allowed to be used on humans.


An alternative method to elevate humans’ natural radiation protection capacity is inducing a hibernating or hypostasis state. It is well-known that natural hibernators become more radioresistant during their inactive state. The reason for this has not yet been fully elucidated. It is probably due to several factors related to slower cell metabolism and increased tissue hypoxia.


In recent years, a technique has been developed that allows hibernation to be reproduced even in those animals that would not usually be able to hibernate, such as rats. This technique is nowadays known as synthetic hibernation or synthetic torpor [375]. Although this research has a big potential to limit radiation-associated risks in space, it is quite far from practical use yet. Another futuristic method is the use of deuterium, the stable isotope of hydrogen. As carbon-deuterium bonds need more energy to break than normal carbon-hydrogen bonds, the necessary energy to break the hydrogen bonds between DNA bases would be higher, making deuterated DNA less sensitive than normal DNA to DNA damage following ionizing radiation exposure. However, a lot of issues have to be solved before deuterium could be applied in humans: lack of evolutionary adaption to catabolize organic compounds containing deuterium, consequent slower rate of vital metabolic reactions, and their potential toxic effects. Nevertheless, it has been shown that deuterated food or water intake helps to increase life or health spans from numerous model organisms.


Gene therapy stands for the use of genetic modifying techniques in order to achieve a therapeutic effect. In the context of radiation and radiation protection, this has been studied for several radioresistance mechanisms making these techniques interesting for deep space missions, where radiation protection concerns arise [374]


One of the strategies for gene therapy in radioresistance is the overexpression of endogenous antioxidants, for example, magnesium superoxide dismutase (MnSOD) that acts as a scavenger for reactive oxygen species produced after the interaction of radiation with the cell [376].


Another angle in which gene therapy can be useful for improving radioresistance is by enhancing the DNA damage repair such as the overexpression of certain repair proteins that are normally active in repairing the damage in the DNA strands after radiation exposure [377].


A promising approach takes its inspiration from extremophiles and their impressive radioresistance capabilities, in concrete, the tardigrades, a microscopic animal that is capable of surviving in extreme conditions. A protein identified in these organisms, termed damaged suppressor (Dsup), has been made to be expressed in human cell lines, reducing the number of DNA strand breaks and preserving cellular proliferative abilities after high doses of radiation [337].


Extreme conditions in a natural environment are only extreme from a human point of view. Extremophiles can only live under these conditions and depend on them. Often organisms living under these conditions are called “extremophiles” or “polyextremophiles” since most of them are coping with different extremes in their natural environment [378].


Plenty of different (poly) extremophiles in natural and human-made extreme environments exist in natural and human-made harsh environments. Examples include anaerobes, (hyper-) thermophiles, psychrophiles, halophiles, acidophiles, xerophiles, and piezophiles. For all the named organismic groups, cellular adaptation mechanisms are known that protect the cells themselves or enable them to live under extreme conditions in their natural habitat. In addition to the intracellular protection mechanisms, general protection mechanisms like spore formation are well-known. For example, the spore of the Bacterium Bacillus subtilis is characterized by a thick layer of peptidoglycan, a low water content inside the cell, a DNA conformation changed from B to A, and the presence of α/β-type small acid-soluble spore proteins which accumulate within the spore. In general, spores are more tolerant to inactivating physical stresses, like radiation as vegetative cells [379, 380]. Spore formation is known to be an answer to changing conditions in the environment that is used by microorganisms and fungi; special forms like the anhydrobiotic state are also observed in other eukaryotic cells like tardigrades, nematodes, and rotifers [381]. Spore formation and the anhydrobiotic state, as well as intracellular adaptation mechanisms, are relevant for possible survival after exposure to ionizing (space) radiation [323, 382]. In addition to spore formation, biofilm growth by the production of extracellular polymeric substances (EPS) also leads to a higher ionizing radiation tolerance [383].


Besides the named cellular adaptation mechanisms, there are also different intracellular adaptation mechanisms possible to cope with extreme environmental stresses. As described before, ionizing radiation exposure does not only lead to direct effects and intracellular damage, such as DNA strand breaks, it also leads to indirect effects, like ROS production. Hyperthermophilic Archaea, like Pyrococcus furiosus, are partly tolerant to the indirect effects of ionizing radiation, due to mechanisms protecting the DNA from the influence of ROS [384]. In these Archaea, DNA binding proteins play a major role as they bind and protect the DNA thereby limiting the accessibility of the DNA to ROS. In addition, increased expression of different enzymes like superoxide dismutase and the glutathione peroxidase can also reduce the level of intracellular ROS.


For the Bacterium Deinococcus radiodurans as well as for the Archaeon Halobacterium salinarum special intracellular Mn/Fe ratios are described: they demonstrate an intracellular accumulation of high amounts of manganese along with low iron levels, which contribute to their high radiation tolerance [385, 386]. This special Mn/Fe ratio was not found in radiation-tolerant anaerobic microorganisms. It is proposed that the low levels of IR-generated ROS under anaerobic conditions combined with highly constitutively expressed detoxification systems in these anaerobes are key to their radiation resistance and circumvent the need for the accumulation of Mn-antioxidant complexes in the cell [387].


Furthermore, polyploidy or the presence of several DNA copies within one single cell has been discussed to contribute to tolerance to desiccation and therefore also to ionizing radiation [388].


Halophilic organisms have different strategies to cope with a high salt concentration in their natural habitat. One option is the intracellular accumulation of salt or other compatible solutes [389]. It is also known that compatible solutes can contribute to the tolerance to ionizing radiation in halophilic microorganisms [389]. Additionally, protective mechanisms such as membrane pigments, including carotenoids, melanin, scytonemin, and bacterioruberin were found to be important in ionizing radiation protection in different organisms through the scavenging of hydroxyl radicals [390, 391].


In general, there is no direct adaptation of microorganisms to space conditions or space radiation known as all organisms evolved on Earth. Nevertheless, there have been and still are space experiments ongoing where the adaptability of different organisms is investigated during exposure to space conditions. In this context, we speak about the side effects of other tolerances or resistances which enable the organisms to endure space stressors. In general, organisms which are tolerant to desiccation developed mechanisms to repair the DNA which is damaged during the desiccation process. The same repair mechanisms can also be used to repair DNA damage caused by other stressors, such as ionizing radiation. One prominent example is the desiccation and radiation tolerance of the microorganism Deinococcus radiodurans. This organism uses the same cellular adaptation and repair strategies after exposure to drought and ionizing radiation exposure. However, not all desiccation-tolerant organisms are tolerant to (ionizing) radiation exposure [392]. The same is true for other repair machineries, where no direct correlation between hyper-/thermophilic organisms or the ability to produce compatible solutes and radiation tolerance could be identified [393, 394]. In addition, some microorganisms (e.g., Ignicoccus hospitalis) demonstrate a high survival rate after ionizing radiation exposure but possess a repair mechanism which is not known up to now [395].


As mentioned at the beginning of this chapter, protons account for nearly 87% of the total flux of the galactic cosmic radiation (GCR), helium ions—for approximately 12%, and the remaining heavy ions, or high-Z elements (HZE),—for less than 1%. However, the relative distribution of the effective dose is quite different. Multiplying the abundance by Z2 provides an estimate of the contribution to the dose. One should further consider the quality factor of the biological effectiveness of the corresponding radiation. As a result, HZE particles contribute approximately 89% of the total dose equivalent (mSv) in free space. Among the HZE particles in GCR, iron is the largest contributor (26%) to the effective dose [396].


Low dose rate irradiation is usually provided in a laboratory either by X-ray machines or radioactive sources, and neither mimics GCR well. The X-rays are low-energy radiation and do not mimic the penetrating capability of GCR. Usual radioactive sources emit α-, β-, and γ-radiation. While α-particles are helium nuclei abundant in GCR, the energy of typical α-particles is 4–9 MeV as compared with above 1000 MeV in GCR. As a result, α-particles cannot penetrate the thinnest screen (even the skin) and cannot be used for GCR simulation.


γ-Radiation has a much stronger penetration capability, and γ-emitters are used [397]. However, the biological effects of γ-radiation and ions are different. As for the β-radiation, in the sense of GCR simulation it combines the drawbacks of α and γ: having low penetrating capability, its biological effects are similar to γ and far from high-energy ions.


A partial solution has been found by using a unique artificial isotope Californium-252, which exhibits exceptionally high neutron emission. 252Cf is used, e.g., at the ESA test station and the new facility in Japan [398].


Although low-energy charged particles (up to about 20 MeV) do not reproduce the characteristics found in the GCR spectrum, low-energy facilities are widely available and are useful to help in the screening and the design of experiments that will be further carried out on higher energy accelerators (see next subsection).


Several accelerator types can be used to produce such low-energy beams, but electrostatic tandem accelerators are probably the most widely used. A schematic representation of such accelerator is given in Fig. 10.28. The first part consists of an ion source, which can produce any negative ion (with one extra electron) from hydrogen to uranium. The produced negative ion beam is then extracted from the source and guided to the main tube. The acceleration is carried out in two stages hence the name “tandem accelerator.” First, the negative ions are attracted to the positive high-voltage “terminal” located in the center of the tube. Then, negative ions can be stripped of part of their electrons (usually 2–3) in the stripper channel, turning to positive ions. These positive ions are repelled by the positive terminal voltage to the end of the tube, which is at ground potential. High-energy ions are focused by (usually superconducting) magnets and deflected into one of the beamlines, according to the particle energy, mass, and charge.

A diagram depicts particles from ion source passing through accelerator, energy selection magnet, aperture, beam switch and scanning with control, focusing unit, and sample. Detector gives the signal.

Fig. 10.28
Schematic view of the SNAKE (Superconducting nanoprobe for (kern) particle physics experiments) setup, including linear particle accelerator (orange), focusing unit (superconducting magnetic lens) and detection system with the particle detector and ultrafast high-voltage switch


A diagram depicts particles from ion source passing through accelerator, energy selection magnet, aperture, beam switch and scanning with control, focusing unit, and sample. Detector gives the signal.


Regarding the beam size, two configurations are used: microbeam and broad beam.


The initial accelerated ion beam is always a microbeam with a diameter often below 5 μm. Microbeams are a useful tool in studying the bystander effect (described in detail in Chap. 2). Indeed, such a beam permits to irradiate selectively one or more cells inside a population. This offers the possibility to either target the cell nucleus, the conventional target in radiobiology, the cytoplasm, or organelles. It also provides the advantage of knowing precisely the dose delivered to the cells and the number of particle shoots being determined in advance. In the context of space radiation, where the flux of high-mass particles is very low and the occurrence of a single shoot-in through a cell is very high, the bystander effect is a topic of crucial importance. Indeed, it is observed through a variety of endpoints: reduction in cell survival, double strand break induction, micronuclei, mutations, and expression of apoptosis, inflammation, and cell cycle-related genes.


Broad beams can be produced either by using scattering foils, by scanning microbeam, or by defocusing them. Beam homogeneity is controlled by plastic scintillators or silicon-based detectors.


The importance of accelerator-based studies was acknowledged by NASA decades ago. After preliminary research at the existing accelerators, it was decided to build a dedicated beamline. In 2003, the NASA Space Radiation Laboratory (NSRL) was commissioned at the Brookhaven National Laboratory (BNL). The NSRL layout is presented in Fig. 10.29. The facility is capable of supplying particles from protons (p) to gold (Au). Available beam energies range from 50 to 2500 MeV for protons and 50 to 1500 MeV per nucleon for ions between helium (He-2) and iron (Fe-56; Z = 26). Heavier ions with atomic numbers up to Z = 79 (Au) are limited to approximately 350–500 MeV per nucleon (https://www.bnl.gov/nsrl/).

An airborne photograph depicts the buildings of N S R L, target room, Linac, and E B I S enveloped by W fifth avenue and Michelson street.

Fig. 10.29
Aerial view and general layout of the NASA Space Radiation Laboratory (NSRL) facility in Upton, NY, USA. EBIS electron beam ion source. (Satellite view courtesy Google Earth)


An airborne photograph depicts the buildings of N S R L, target room, Linac, and E B I S enveloped by W fifth avenue and Michelson street.


The choice of Fe-56 ions is justified by a sharp decline in abundance for ions heavier than iron [396] while the chosen energy is around the peak of the galactic cosmic radiation spectrum. Moreover, the linear energy transfer is about 140 keV/μm, around the peak of effectiveness for late radiation effects [399]. The three key areas developed together to ultimately provide the GCR simulator at NSRL are illustrated in Fig. 10.30. Several important results have been obtained at NSRL. We can mention, for example, the observation that, despite being high-LET particles, heavy ions are not more effective than γ-radiation in the induction of leukemia in mice [400]. Another example is the discovery of specific types of brain damage caused by heavy ions [401], types that had not been known from X-ray studies.

An illustration lists the varied exploration missions, N S R L facility parameters, and animal and cell models in the simulation of G C R by NASA.

Fig. 10.30
Three key areas developed to provide the GCR simulator at NSRL. (Source: Simonsen et al. [396], reproduced with permission)


An illustration lists the varied exploration missions, N S R L facility parameters, and animal and cell models in the simulation of G C R by NASA.


The basic idea of a high-energy accelerator is illustrated schematically in Fig. 10.31. Each accelerating section itself consists of a sequence of resonant cavities in which the RF (radio frequency) electromagnetic field is oscillating. Ions traverse RF cavities subsequently; the timing of the passage of each cavity is synchronized with the direction and phase of the electric field—therefore, each ion is accelerated from cavity to cavity. In case of a linear accelerator (LINAC), the accelerating sections are positioned adjacently along a straight line. In case of a synchrotron (like EBIS in Fig. 10.31), the accelerating sections are positioned along a circumference, while the charged particle beam is bent between the sections by a magnetic field.

The diagram depicts an injector made of an ion gun and buncher and an accelerating section with radio frequency. The ion gun has a source and a grid gate. Charged particles’ bunches enter the buncher at a radio frequency.

Fig. 10.31
General layout of a linear high-energy particle accelerator. RF radio frequency


The diagram depicts an injector made of an ion gun and buncher and an accelerating section with radio frequency. The ion gun has a source and a grid gate. Charged particles’ bunches enter the buncher at a radio frequency.


The resonant frequency is usually either about 1 GHz (L-band of the RF spectrum) or about 3 GHz (S-band). The electromagnetic power for feeding the RF cavities is generated usually by a high-power klystron.


The accelerating cavities can be made either of normal-conducting metal (“warm” cavities usually made of copper) or of superconductor (usually, niobium). In the last case, cryogenic cooling to liquid-helium temperature is needed. Accelerating gradients are usually in the range of 10–30 MeV/m, e.g., the 3-km long SLAC accelerator commissioned back in the 1960s, accelerating electrons to the energy of 50 GeV, has an average accelerating gradient of about 17 MeV/m (with 3-GHz copper cavities).


Output of RF-driven particle accelerator necessarily consists of single bunches. Such bunches are called micropulse, and their duration is just a fraction of the oscillating field period. For example, for the S-band with period of about 300 ps = (3 GHz)–1, the typical micropulse duration is below 20 ps. The train of micropulse is called “macropulse.” While there is no theoretical limit for macropulse length, practically it is limited by the driving klystron pulse length: For S-band normal-conducting linacs the typical value is 5–20 μs, for L-band superconducting linacs—much longer (1 ms and more).


Particle beams are collimated and bent “magnetic lenses”—magnetic fields created by electromagnets or permanent magnets. These magnetic devices, governing the charged particle beam propagation, are called electron-optic (or ion-optic) devices. Such devices have some similarities to classical light optics in terms of mathematics, but in general comprise a separate field of knowledge. The reader interested in learning the field of particle beam optics is referred to the classical textbook of Reiser [402].


Although ionizing radiations were identified as the main showstopper to exploration mission, there are additional stresses in the space environment. While most of the factors below are not relevant to astronauts, they are important in studying extremophiles.


1.
Low pressure: The pressure varies from 10–1 Pa near Earth atmosphere to 10–14 Pa in deep space. Due to the degassing, pressure around the ISS is higher than in deep space ranging from 10−7 Pa in the Ram direction (e.g., front of the ISS relative to flight direction) to 10−4 Pa in the Wake direction (e.g., rear of the ISS relative to flight direction) [403].
 2.
Cold temperature: The low-pressure environment previously described drastically increases the molecular mean free path in space resulting in low heat transfer. Consequently, the temperature in deep space ranges from 3 to 4 K (–270 to –260°C) [404].
 3.
Solar radiation: Highly energetic phenomena are occurring in the Sun leading to the emission of high-intensity electromagnetic radiations. By moving away from the Sun, the emitted solar radiations are spread out over a large surface area reducing the solar irradiance with increased Sun-object distance. ISS, located at one astronomical unit distance from the Sun, is exposed to an approximate 1400 W/m2 heat flux. The associated electromagnetic spectrum extends from X-rays to radio waves with a higher proportion in the visible (47%), infrared (45%), and ultraviolet (7%) ranges [405].
 4.
Day and night cycles: As ISS orbits around the Earth and the latter around the Sun, the stress exposure has a cyclic temporal behavior with two main periods. The “day and night” cycle caused by the rotation around the Earth has a period of 91 min resulting in fluctuation in total irradiance and temperature of more than 1000 W/m2 and 5–10 °C. The second cycle (period of approximately 1 month) is due to the position change of ISS orbital plane relative to the Sun. In this case, greater variations of temperature (up to 60 °C) with a maximal temperature of about 50 °C were reported [406].


Low pressure: The pressure varies from 10–1 Pa near Earth atmosphere to 10–14 Pa in deep space. Due to the degassing, pressure around the ISS is higher than in deep space ranging from 10−7 Pa in the Ram direction (e.g., front of the ISS relative to flight direction) to 10−4 Pa in the Wake direction (e.g., rear of the ISS relative to flight direction) [403].


Cold temperature: The low-pressure environment previously described drastically increases the molecular mean free path in space resulting in low heat transfer. Consequently, the temperature in deep space ranges from 3 to 4 K (–270 to –260°C) [404].


Solar radiation: Highly energetic phenomena are occurring in the Sun leading to the emission of high-intensity electromagnetic radiations. By moving away from the Sun, the emitted solar radiations are spread out over a large surface area reducing the solar irradiance with increased Sun-object distance. ISS, located at one astronomical unit distance from the Sun, is exposed to an approximate 1400 W/m2 heat flux. The associated electromagnetic spectrum extends from X-rays to radio waves with a higher proportion in the visible (47%), infrared (45%), and ultraviolet (7%) ranges [405].


Day and night cycles: As ISS orbits around the Earth and the latter around the Sun, the stress exposure has a cyclic temporal behavior with two main periods. The “day and night” cycle caused by the rotation around the Earth has a period of 91 min resulting in fluctuation in total irradiance and temperature of more than 1000 W/m2 and 5–10 °C. The second cycle (period of approximately 1 month) is due to the position change of ISS orbital plane relative to the Sun. In this case, greater variations of temperature (up to 60 °C) with a maximal temperature of about 50 °C were reported [406].


To study the biological impact of the space environment, modules outside the ISS are an ideal environment to expose biological samples to LEO, where the conditions strongly differ from the ones encountered on Earth. However, the poor availability of these facilities stimulated the creation of exposure chambers on Earth capable of reproducing this LEO environment. The Laboratory for Analysis by Nuclear Reactions (LARN, University of Namur, Belgium, https://www.unamur.be/en/sci/physics/ur-en/larn-en) has developed an exposure module to simulate the aforementioned conditions on the ground for extended periods of time (several months). To this end, biological samples are placed into a vacuum chamber and exposed to various constraints. A cooling system located underneath the sample tray and an electromagnetic source reproducing the solar spectrum are controlled by a monitoring system capable to simulate the slow and fast cycles described above. In addition, a variety of neutral density filters and cut-off waveband filters enables to create multiple irradiance conditions within the same experiment, in order to investigate what part of the UV-visible spectrum is the most deleterious.


A similar facility also exists at DLR Cologne and has been recurrently used for pre-flight test programs and mission ground reference experiments for several astrobiological long-term space missions [405].


Q1.
Which types of radiation exist in space?
 Q2.
Are astronauts fully protected from radiation by spacecraft walls?
 Q3.
Can you describe the difference between the radiation environment on Earth (on ground) and the one at the surface of Mars?
 Q4.
What do we know now about specific health problems of astronauts?
 Q5.
What is the role of plants in Bioregenerative Life Support Systems (BLSS)?
 Q6.
What is the 3R Principle?
 Q7.
What kind of chronic effects on the CNS (central nervous system) were observed?
 Q8.
What is the difference between organoids and spheroids?
 Q9.
From which cells organoids can be cultured?
 Q10.
What is the main reason why bioprinting and other 3D cultures can be better cultured in Space?
 Q11.
What parameters should be considered if we would like to simulate conditions on Moon/Mars surface or in deep space (including stress unrelated to radiation)?
 Q12.
What is the interest to study the biological effects of low energy charged particles in the context of space radiation exposure?


Can you describe the difference between the radiation environment on Earth (on ground) and the one at the surface of Mars?


What do we know now about specific health problems of astronauts?


What is the role of plants in Bioregenerative Life Support Systems (BLSS)?


What kind of chronic effects on the CNS (central nervous system) were observed?


What is the main reason why bioprinting and other 3D cultures can be better cultured in Space?


What parameters should be considered if we would like to simulate conditions on Moon/Mars surface or in deep space (including stress unrelated to radiation)?


What is the interest to study the biological effects of low energy charged particles in the context of space radiation exposure?


SQ1.
Galactic Cosmic Rays, Solar Energetic Particles, Trapped radiation, and the solar wind
 SQ2.
No, the high penetrating character of GCRs and the cascades of secondary particles generated by the passage of GRCs ions through the spacecraft walls create an intravehicular field which is of high concern for the health risk of astronauts
 SQ3.
On Earth we are protected by both the atmosphere and the magnetic field: GCRs hitting the top of the atmosphere create particle showers but only a few of such secondaries (and very few of direct GCR ions) reach the ground. SEP are mostly shielded by the atmosphere and are of concern only for extreme events and mostly for high latitude/high-altitude flights for eventual biological risks, or on ground for infrastructures. On Mars, the very thin atmosphere offers very little shielding, and the exposure to GRCs, their secondary particles, and SEP is a concern.
 SQ4.
Though there are many concerns, the present evidence does not provide a conclusive answer. Astronauts as a cohort are not less healthy than US Air Force pilots, e.g., and much healthier than the general public (due to selection). The LNT-estimated cancer death risk for prolonged missions is considerable, but applicability of LNT for low dose rates is questionable.
 SQ5.
Plants in LSS remove carbon dioxide and provide oxygen, help water purification, can recycle wastes of the astronauts, and provide fresh food for the crew.
 SQ6.
Reduction (first R) of animal numbers, Refining (second) the test methods to lower the harm to the animal to a minimum, and Replace (third) animal experiments with alternative methods, when possible
 SQ7.
Reduction of dendrites, reduced neurogenesis and increased neuroinflammation, diminished hippocampal neuron excitability
 SQ8.
Spheroids are also a 3D cell cultures, but in comparison to organoids, they form simple clusters into sphere-like formation, but they cannot self-assemble or regenerate.
 SQ9.
Pluripotent stem cells
 SQ10.
Microgravity
 SQ11.
The particular profile of radiation spectrum can vary according to the location of interest. The ISS environment benefits from some protection granted by the Earth magnetic field. This is not the case for deep space or Mars, where the full spectrum of galactic cosmic rays should be considered. Solar proton events should be included, can vary in magnitude and their extent also depends on the distance to the sun. Beside radiation, day and night cycle can impact temperature conditions, which lead to biological effects on some model organisms. Pressure value and the presence or not of atmosphere should also be considered.
 SQ12.
Although the vast majority of particles in the GCR spectrum have very high energy, low energy charged particles are still produced in shielding materials and arise from fragmentation of heavier energetic nuclei. These low-energy particles often traverse shielding and remain of concern.


Galactic Cosmic Rays, Solar Energetic Particles, Trapped radiation, and the solar wind


No, the high penetrating character of GCRs and the cascades of secondary particles generated by the passage of GRCs ions through the spacecraft walls create an intravehicular field which is of high concern for the health risk of astronauts


On Earth we are protected by both the atmosphere and the magnetic field: GCRs hitting the top of the atmosphere create particle showers but only a few of such secondaries (and very few of direct GCR ions) reach the ground. SEP are mostly shielded by the atmosphere and are of concern only for extreme events and mostly for high latitude/high-altitude flights for eventual biological risks, or on ground for infrastructures. On Mars, the very thin atmosphere offers very little shielding, and the exposure to GRCs, their secondary particles, and SEP is a concern.


Though there are many concerns, the present evidence does not provide a conclusive answer. Astronauts as a cohort are not less healthy than US Air Force pilots, e.g., and much healthier than the general public (due to selection). The LNT-estimated cancer death risk for prolonged missions is considerable, but applicability of LNT for low dose rates is questionable.


Plants in LSS remove carbon dioxide and provide oxygen, help water purification, can recycle wastes of the astronauts, and provide fresh food for the crew.


Reduction (first R) of animal numbers, Refining (second) the test methods to lower the harm to the animal to a minimum, and Replace (third) animal experiments with alternative methods, when possible


Reduction of dendrites, reduced neurogenesis and increased neuroinflammation, diminished hippocampal neuron excitability


Spheroids are also a 3D cell cultures, but in comparison to organoids, they form simple clusters into sphere-like formation, but they cannot self-assemble or regenerate.


The particular profile of radiation spectrum can vary according to the location of interest. The ISS environment benefits from some protection granted by the Earth magnetic field. This is not the case for deep space or Mars, where the full spectrum of galactic cosmic rays should be considered. Solar proton events should be included, can vary in magnitude and their extent also depends on the distance to the sun. Beside radiation, day and night cycle can impact temperature conditions, which lead to biological effects on some model organisms. Pressure value and the presence or not of atmosphere should also be considered.


Although the vast majority of particles in the GCR spectrum have very high energy, low energy charged particles are still produced in shielding materials and arise from fragmentation of heavier energetic nuclei. These low-energy particles often traverse shielding and remain of concern.


Radiation protection aims to reduce unnecessary radiation exposure with the intention to minimize the harmful effects of radiation on human health. With increasing use of radiation technologies and radioisotopes in medicine and industry, the risk of radiological and nuclear accidents escalates, affecting human health. Nuclear power plants and industrial accidents pose a serious threat to public health. Emergency preparedness in an event of nuclear terrorism and nuclear warfare requires the use of existing radiomodifiers and public health measures such as sheltering in place and the use of personal protective equipment (PPE). New approaches are urgently needed for protecting the persons working in a radiation field, first responders, and general population in the form of safe, effective, and easily accessible radioprotective agents.


Cellular exposure to IR induces genomic instability or mutations predisposing to carcinogenesis and/or cell death. Upon exposure, radiation induces DNA damage, lipid peroxidation, oxidation of thiol groups located in the plasma membrane and membranes of the cellular organelles, DNA strand breaks, and base alterations in cells, tissues, and organs. These changes may trigger a series of cellular responses, including activation of DNA damage repair pathways, signal transduction responses, gene transcription, and immune and proinflammatory responses. Triggering these pathways helps to recover damaged cells or eliminate the dysfunctional cells. However, they may also result in the development of tissue toxicities. The radiation research program of the National Cancer Institute (NCI) has proposed the following pharmacological classification of agents with IR response modification properties according to the timing of administration (Fig. 11.1):

An illustration of the biological properties of radioprotectors, radiomitigators, and radiosensitizers.

Fig. 11.1
Classification of radiomodifiers with their biological properties


An illustration of the biological properties of radioprotectors, radiomitigators, and radiosensitizers.


A radioprotective agent/drug prevents harmful effects of radiation exposure while a radiosensitizing agent makes tumor cells more susceptible to radiation, in order to maximize the effect of radiotherapy while having less effect on normal tissues. Radiomitigators can attenuate IR damages even when they are delivered at the same time or after radiation exposition. The use of radiation-effect modulators (radioprotectors, radiomitigators, and/or radiosensitizers) can mitigate side effects and increase the efficacy of RT in cancer patients (Fig. 11.2).

An illustration of the use of radio modulators and their classification. Radio modulators increase the resistance of the body to irradiation by adaptively shifting the effectiveness of antioxidative protection. The three radio modulators are radioprotectors, radiomitigators, and radiosensitizers.

Fig. 11.2
The use of radioprotectors, radiomitigators, and radiosensitizers before, during, or after irradiation


An illustration of the use of radio modulators and their classification. Radio modulators increase the resistance of the body to irradiation by adaptively shifting the effectiveness of antioxidative protection. The three radio modulators are radioprotectors, radiomitigators, and radiosensitizers.


The extent of radiation damage to living cells and organisms depends on the type of radiation (alpha (α) particles, beta (β) particles, positrons, X-rays, gamma rays (γ-rays), UV, etc.). Attempts to protect against the damaging effects of radiation were made as early as 1949. Efforts are actively being continued to search for radioprotectors suitable to be used in specific scenarios of radiation exposure. Possible applications of radioprotectors are outlined in Fig. 11.3.

An illustration of the applications of radioprotectors. The two major applications are protecting personnel and providing safeguards. The different ways for protecting personnel and providing safeguards are listed.

Fig. 11.3
Various applications of radioprotectors


An illustration of the applications of radioprotectors. The two major applications are protecting personnel and providing safeguards. The different ways for protecting personnel and providing safeguards are listed.


Over the last few decades, many natural and synthetic compounds have been investigated for their potential as radioprotectors. Natural or synthetic radioprotectors are able to (i) reduce direct or indirect radiation damage, (ii) repair direct and indirect damage once they have occurred, and (iii) facilitate the repair of damaged cells or recover depleted cell populations [1].


It should be stressed that the majority of the compounds discussed below are currently not used in routine clinical practice and are still under preclinical or clinical evaluation.


Early development of synthetic radioprotectors focused on thiol compounds (e.g., amifostine) and their derivatives, which have been used in cancer patients, to prevent complications of RT. In addition, they have been thought to be useful in accidental radiation exposure scenarios [2]. However, the practical applicability of the majority of these synthetic compounds remained limited owing to their limited administration routes, narrow administration window for efficacy, high toxicity at high doses or at recurrent usage, and cost factors as well. Besides thiol compounds, various compounds with different chemical structures are being investigated to develop an ideal radioprotector; there is still an urgent need to identify and develop novel, nontoxic, effective, and biocompatible compounds which can adequately protect normal tissues with no sparing of the tumor cells.


An interest has been emerging in developing potential new candidate drugs from natural plants and phytochemicals. Plant products could bridge the gaps in the search for an ideal radioprotector due to its abundance, typically low toxicity, and relatively low cost.


An ideal radioprotective agent should (a) be efficient in providing multifaceted protection, (b) prevent direct and indirect acute or chronic effects on normal tissue, (c) be easily and comfortably administered without toxicity, (d) cause no or minimal adverse effects on the test organism, (e) have a sufficiently long time window of effectiveness after administration and also have a sufficiently long shelf life, (f) have an acceptable stability profile (both of bulk active product and formulated compound), (g) be compatible with a wide range of other drugs, (h) not protect tumors from IR, and (i) be easily accessible and economical and should not require special handling and transportation temperatures (Box 11.1).


Radioprotectors (synthetic compounds, natural plant extracts, and phytochemical derivatives) are designed to lessen the effects of radiation-induced damage in healthy tissues.

Radioprotective drugs are effective when administered prior to or during radiation exposure to reduce the radiation-induced injuries/toxicities.

Safe, novel, nontoxic, and easily accessible radioprotective agents are needed to be developed for human health.


Radioprotectors (synthetic compounds, natural plant extracts, and phytochemical derivatives) are designed to lessen the effects of radiation-induced damage in healthy tissues.


Radioprotective drugs are effective when administered prior to or during radiation exposure to reduce the radiation-induced injuries/toxicities.


Safe, novel, nontoxic, and easily accessible radioprotective agents are needed to be developed for human health.


Radioprotectors are diverse and elicit their action by various mechanisms (Fig. 11.4) such as:
Scavenging free radicals (either by suppressing the formation or by detoxifying radiation-induced free radical species).

Inducing hypoxia in cells in order to avoid synthesis of reactive oxygen species (ROS).

Increasing levels of antioxidant defenses such as GSH (reduced glutathione) and/or antioxidant enzymes (superoxide dismutase (SOD), glutathione peroxidase (GPx), thioreductase, catalase (CAT), etc.).

Triggering one or more cellular DNA damage repair pathways.

Impeding cell division or inhibiting apoptotic cell death.

Modulating redox-sensitive genes.

Modulating growth factors and cytokine production.

Controlling inflammatory response.

Chelating or decorporating radionuclides.

Promoting tissue regeneration (intestinal or hematopoietic and immunostimulant compounds), gene therapy, and/or stem cell therapy. In most cases, these molecules are administered after exposure to radiation, which is why they should be also considered radiomitigators.

An illustration of actions of radioprotectors. They are: scavenge free radicals, induce cellular radioprotectors and hypoxia in tissues, synchronize cells, modulate redox sensitive genes, growth factors, and cytokines, enhance D N A repair and stem cells, delay cell division, and inhibit apoptosis.

Fig. 11.4
Potential mechanism of action of radioprotectors against cell damage due to IR


Scavenging free radicals (either by suppressing the formation or by detoxifying radiation-induced free radical species).


Inducing hypoxia in cells in order to avoid synthesis of reactive oxygen species (ROS).


Increasing levels of antioxidant defenses such as GSH (reduced glutathione) and/or antioxidant enzymes (superoxide dismutase (SOD), glutathione peroxidase (GPx), thioreductase, catalase (CAT), etc.).


Promoting tissue regeneration (intestinal or hematopoietic and immunostimulant compounds), gene therapy, and/or stem cell therapy. In most cases, these molecules are administered after exposure to radiation, which is why they should be also considered radiomitigators.


An illustration of actions of radioprotectors. They are: scavenge free radicals, induce cellular radioprotectors and hypoxia in tissues, synchronize cells, modulate redox sensitive genes, growth factors, and cytokines, enhance D N A repair and stem cells, delay cell division, and inhibit apoptosis.


The most common mechanisms of radioprotection are the scavenging of free radicals, repair of DNA damages, inhibition of apoptosis or inflammation, increase antioxidant defenses, and modulation of growth factors, cytokines, and redox genes. Thus, the management of radiation exposure may require a holistic multimechanistic approach to achieve optimal radiation protection during RT of cancer patients and in cases of nuclear accidents or emergencies [3] (Box 11.2).


Radioprotectors can be screened for their effective emerging strategies, such as modulation of growth factors, cytokines, redox genes, and tissue renewal.

The radioprotective agents are often antioxidants, which may suppress or scavenge the radiation-induced free radicals from the cell.

These compounds are cofactors or can induce/stimulate antioxidants enzymes (like SOD, GPx, and) activity, which would likely lead to both prevent DNA damage and decrease in lipid peroxidation.

They may have the ability to enhance DNA repair, reduce the postradiation inflammatory response, or even delay cellular division allowing more time for cells to repair the DNA damage or undergo cell death.


Radioprotectors can be screened for their effective emerging strategies, such as modulation of growth factors, cytokines, redox genes, and tissue renewal.


The radioprotective agents are often antioxidants, which may suppress or scavenge the radiation-induced free radicals from the cell.


These compounds are cofactors or can induce/stimulate antioxidants enzymes (like SOD, GPx, and) activity, which would likely lead to both prevent DNA damage and decrease in lipid peroxidation.


They may have the ability to enhance DNA repair, reduce the postradiation inflammatory response, or even delay cellular division allowing more time for cells to repair the DNA damage or undergo cell death.