Molecular aspects of tritiated water and natural water in radiation biology

Insights into the modes of action of tritium on the early-life stages of zebrafish, Danio rerio, using transcriptomic and proteomic analyses

In the environment, populations are exposed to different kinds of ionizing radiation. Little is known about their modes of action on non-human species, and whether or not they are similar for alpha, beta and gamma radiations, considered as the reference. In this context, tritium effects (beta emitter) under the form of tritiated water (HTO) were investigated in zebrafish, a common model in toxicology and ecotoxicology with a fully sequenced genome. Experiments were conducted on early life stages, considered to be highly sensitive to pollutants, by exposing eggs to 0.4 mGy/h of HTO until 10 days post fertilization. Tritium internalization was quantified, and effects were investigated using a combined approach of transcriptomic and proteomic analyses. Results highlighted similarities in the biological pathways affected by HTO by both techniques, such as defence response, muscle integrity and contraction, and potential visual alterations. These results correlated well with previous data obtained on earlier developmental stages (1 and 4 dpf). Interestingly, HTO effects were partly overlapping those obtained after gamma irradiation, underlying potential common modes of action. This study, therefore, brought a body of evidence on the effects of HTO observed at the molecular level on zebrafish larvae. Further studies could investigate if the effects persist in adult organisms.

Prolonged effect associated with inflammatory response observed after exposure to low dose of tritium β-rays

Background: The value of relative biological effectiveness of tritium increases at low dose domain, which results in the suspicion of weighting factor of 1 for tritium after low dose exposure. Thus, present study was carried out to analyze the differences in the cellular responses at early and late period between low dose of tritium β-rays and γ-rays radiation.Methods: MCF-10A cells were exposed to low dose of tritium β-rays or γ-rays, then cellular behaviors, such as DNA double strand breaks (DSBs), apoptosis, reactive oxygen species (ROS) level and inflammatory relevant gene expression were analyzed at early and late period post-irradiation.Results: At early period the elimination of DSB foci produced by HTO is longer than γ-rays. High ROS level and a continual change of cell cycle distribution are observed in HTO radiation group. Based on the results of RNA sequencing, Ingenuity Pathway Analysis (IPA) indicates TNFR1 signaling and production of nitric oxide and ROS are activated as an acute response at 24 h post radiation. Moreover, it also shows a disturbance in cholesterol biosynthesis. The results of 30 days point that there is a lasting active inflammatory response, accompanying with a persistent high expression of relevant cytokines, such as TNF and IL1R.Conclusion: Compared to an acute response induced by γ-rays, a persistent inflammatory response exists in HTO-irradiated cells when cultured for 30 days, which might be related to accumulation of tritium in the form of organically bound tritium (OBT) in cellular DNA or lipids.

Tritiated Water Exposure in Zebrafish (Danio rerio): Effects on the Early-Life Stages

Tritium, a radioactive isotope of hydrogen of natural and anthropogenic origin, is ubiquitously present in the environment. Effluents of nuclear centers of production are significant anthropogenic sources. With the upcoming project of thermonuclear fusion, tritium releases in the environment may increase. It is therefore important to characterize the ecological risk linked to tritium. The effects of tritiated water (HTO) were therefore studied in zebrafish larvae exposed for 10 d to different dose rates, 1.1 × 10² , 4.1 × 10² , and 3.8 × 10³  µGy/h for larvae corresponding, respectively, to a water contamination of 10⁴ , 10⁵ , and 10⁶  Bq/mL of HTO. Those dose rates were higher than 10 μGy/h, which is the threshold recommended to start monitoring ecosystems where radiological contaminants are present. Mortality, embryo-larval development, immune toxicity, genotoxicity, neurotoxicity, and alterations of tissues were investigated. The results showed that HTO exposure induced DNA damage and reactive oxygen species production and modulated the expression of genes involved in detoxification processes. Moreover, modifications of the muscular tissues (degradation of myofibrils at 4 d post fertilization and disorganization of mitochondria at later stages) were observed. The results differed with HTO dose rates and with developmental stages. These results will drive future research for the development of new HTO-sensitive biomarkers and will allow us to progress in the characterization of the modes of action of tritium in fish. Environ Toxicol Chem 2020;39:648-658. © 2019 SETAC.

Effects of tritiated water on locomotion of zebrafish larvae: a new insight in tritium toxic effects on a vertebrate model species

Tritium (³H), a radioactive isotope of hydrogen, is ubiquitously present in the environment. In a previous study, we highlighted a mis-regulation of genes involved in muscle contraction, eye transparency and response to DNA damages after exposure of zebrafish embryo-larvae from 3 hpf to 96 hpf at 0.4 and 4 mGy/h of tritiated water (HTO). The present study aimed to link this gene mis-regulation to responses observed at higher biological levels. Analyses on spontaneous tail movement, locomotor activity and heart rate were performed. Histological sections of eyes were made to evaluate the impact of HTO on eye transparency and whole embryo immunostainings were realized to assess DNA double strand breaks repair using gamma-H2AX foci. We found a decrease of basal velocity as well as a decrease of response in 96 hpf larvae exposed at 0.4 mGy/h after a tactile stimulus as compared to controls. Histological sections of larvae eyes performed after the exposure to 4 mGy/h did not show obvious differences in lens transparency or retinal development between contaminated and control organisms. Gamma-H2AX foci detection revealed no differences in the number of foci between contaminated organisms and controls, for both dose rates. Overall, results highlighted more detrimental effects of HTO exposure on locomotor behavior in 96 hpf larvae exposed at the lowest dose rate. Those results could be linked to mis-regulation of genes involved in muscle contraction found in a previous study at the same dose rate. It appears that not all effects found at the molecular scale were confirmed using higher biological scales. These results could be due to a delay between gene expression modulation and the onset of physiological disruption or homeostatic mechanisms to deal with tritium effects. However, crossing data from different scales highlighted new pathways to explore, i.e. neurotoxic pathways, for better understanding HTO effects on organisms.

Tritiated water exposure disrupts myofibril structure and induces mis-regulation of eye opacity and DNA repair genes in zebrafish early life stages

Tritium (³H) is a radioactive isotope of hydrogen. In the environment, the most common form of tritium is tritiated water (HTO). The present study aimed to identify early biomarkers of HTO contamination through the use of an aquatic model, the zebrafish (Danio rerio). We used the zebrafish embryo-larvae model to investigate the modes of action of HTO exposure at dose rates of 0.4 and 4 mGy/h, dose rates expected to induce deleterious effects on fish. Zebrafish were exposed to HTO from 3 hpf (hours post fertilization) to 96 hpf. The transcriptomic effects were investigated 24 h and 96 h after the beginning of the contamination, using mRNAseq. Results suggested an impact of HTO contamination, regardless of the dose rate, on genes involved in muscle contraction (tnnt2d, tnni2a.4, slc6a1a or atp2a1l) and eye opacity (crygm2d9, crygmxl1, mipb or lim2.3) after 24 h of contamination. Interestingly, an opposite differential expression was highlighted in genes playing a role in muscle contraction and eye opacity in 24 hpf embryos when comparing dose rates, suggesting an onset of DNA protective mechanisms. The expression of h2afx and ddb2 involved in DNA repair was enhanced in response to HTO exposure. The entrainment of circadian clock and the response to H₂O₂ signalling pathways were enriched at 96 hpf at 0.4 mGy/h and in both stages after 4 mGy/h. Genes involved in ROS scavenging were differentially expressed only after 24 h of exposure for the lowest dose rate, suggesting the onset of early protective mechanisms against oxidative stress. Effects highlighted on muscle at the molecular scale were confirmed at a higher biological scale, as electron microscopy observations revealed sarcomere impairments in 96 hpf larvae for both dose rates. Together with other studies, the present work provides useful data to better understand modes of action of tritium on zebrafish embryos-larvae.

Radiobiological effects of tritiated water short-term exposure on V79 clonogenic cell survival

PURPOSE

We set out to improve the accuracy of absorbed dose calculations for in vitro measurements of the relative biological effectiveness (RBE) of tritiated water (HTO) for the clonogenic cell survival assay, also considering the influence of the end-of-track linear energy transfer (LET) of low-energy electrons.

MATERIALS AND METHODS

The COmputation Of Local Electron Release (COOLER) program was adopted to investigate the cell geometry and the tritium full beta-decay spectrum impact on the S-values and subsequently on the RBE of HTO for clonogenic cell survival at similar high dose rates (HDR).

RESULTS

S-values for cells growing in suspension are usually comparable to those for adherent cells. RBEs calculated at the 10% survival fraction through the use of the average energy are almost similar to those obtained with the beta-spectrum. For adherent cells, an RBE of 1.6 was found when HTO cell survival curves were compared to acute γ-ray exposures. Irrespective of the geometrical configuration, the RBE was 2.0 when the comparison was made with similar dose rates.

CONCLUSIONS

These results underline the importance of irradiating at equal dose rates and cell culture conditions when measuring in vitro RBE-values.

Transfer of tritium released into the marine environment by French nuclear facilities bordering the English Channel

Controlled amounts of liquid tritium are discharged as tritiated water (HTO) by the nuclear industry into the English Channel. Because the isotopic discrimination between 3H and H is small, organically bound tritium (OBT) and HTO should show the same T/H ratio under steady-state conditions. We report data collected from the environment in the English Channel. Tritium concentrations measured in seawater HTO, as well as in biota HTO and OBT, confirm that tritium transfers from HTO to OBT result in conservation of the T/H ratio (ca. 1 × 10(-16)). The kinetics of the turnover of tritium between seawater HTO, biota HTO, and OBT was investigated. HTO in two algae and a mollusk is shown to exchange rapidly with seawater HTO. However, the overall tritium turnover between HTO and the whole-organism OBT is a slow process with a tritium biological half-life on the order of months. Nonsteady-state conditions exist where there are sharp changes in seawater HTO. As a consequence, for kinetic reasons, the T/H ratio in OBT may deviate transiently from that observed in HTO of samples from the marine ecosystem. Dynamic modeling is thus more realistic for predicting tritium transfers to biota OBT under nonsteady-state conditions.

Distribution of tritium in estuarine waters: the role of organic matter

Tritium is an important environmental radionuclide whose reactivity with ligands and solids in aquatic systems is assumed to be limited. We studied the fractionation and sorption of tritium (added as tritiated water) in river water and seawater, and found that its distribution appears to be influenced by its affinity for organic matter. Tritium rapidly equilibrates with dissolved organic ligands that are retained by a reverse-phase C18 column, and with suspended sediment particles. Significantly, a measurable fraction of sorbed tritium associates with proteinaceous material that is potentially available to sediment-feeding organisms. These characteristics have not been reported previously and cannot be accounted for solely by isotopic exchange with hydrogen. Nevertheless, they are in qualitative agreement with available measurements of tritium in estuarine and coastal waters where its principal discharge is as tritiated water. Further research into the estuarine biogeochemical behaviour of tritium is required and radiological distribution coefficients and concentration factors that are assumed for this radionuclide may require reconsideration.

Impact of low doses of tritium on the marine mussel, Mytilus edulis: Genotoxic effects and tissue-specific bioconcentration

Despite growing scientific, public and regulatory concern over the discharge of radioactive substances, no serious attempts have been made to develop a rationale to evaluate the impact of environmentally relevant radionuclides in the aquatic environment. In this study, we have evaluated the genotoxic effects and tissue-specific concentration of tritium (added as tritiated water, HTO) in the adult life stage of the edible mussel, Mytilus edulis. The genotoxic effects were quantified in terms of the induction of: (a) micronuclei (MN), and (b) DNA single-strand breaks/alkali-labile sites using alkaline single-cell gel electrophoresis (Comet assay) in the haemocytes of exposed animals. The assays were optimised and validated using a range of concentrations (18-56 mgl(-1)) of ethylmethane sulfonate (EMS), a direct-acting reference genotoxic agent, over different exposure periods. Mussels were exposed to a series of concentrations of HTO equivalent to a dose range from 12 to 485 muGyh(-1) for 96 h, and different tissues and organs were then extracted and analysed. The study revealed a dose-dependent increase in the response for both the MN test and the Comet assay and for both EMS and HTO. In addition, HTO delivering dose rates below 500 muGyh(-1) was shown to be capable of inducing genetic damage in the haemocytes of these bivalves. The study also showed that inorganic tritium accumulated differentially in mussel tissues in a dose-dependent manner, with the gut accumulating the highest amount of radioactivity, followed by the gill, mantle, muscle, foot and byssus thread. The faeces and pseudo-faeces accumulated least radioactivity over the exposure period. Differential accumulation of radionuclides has significant implications for biomonitoring programmes, for this and other aquatic organisms. The study also suggests that the generic dose limits recommended by the International Atomic Energy Agency for the protection of aquatic biota might not be applicable to all aquatic organisms.

Prediction of total body lipid from total body water in rats. Part 2. In vivo estimation of total body lipid by tritium water dilution

Body composition data estimated for normal young, normal adult, and adult obese Wistar rats by a tritiated water (THO) dilution procedure (b. wt. range 119-237 g, n = 31; 305-597 g, n = 43; 274-381 g, n = 6) were compared with the results of whole body analysis. As for the predicted total body lipid (TBL), the content was calculated from the virtual THO-dilution space estimated by a simple micro-method and a regression equation between TBL [%] and DM [%] (dry matter) content (Part 1. Relations between directly measured major body components. Nahrung 35 (1991) 581. Due to different degrees of overestimations in the animal groups examined, the THO-space can be corrected via subtraction of 3.8%, 2.2% and 1.1% in the sequence young, obese, and adult groups. The derived TBL [%] data show a non-significant deviation of less than 1 g/100 g b. wt. in comparison to the three group means after direct lipid determinations. In the best case, i. e. after the additional elimination of extremely overestimated values for total body water (outliers) the combined tritium-derived lipid values (y) are correlated with the lipid data obtained analytically (x), both as proportion of the body weight, by the regression equation y = 0.848 + 0.963 x, r = 0.985, Sy.x = 1.77, n = 58. For a group of young rats (n = 14) after a pre-experimental supplementation with a sucrose solution, TBL values, not significantly different from analytically determined ones, could be calculated without any corrections.

HTO oral administration in mice. I: Threshold dose rate for haematopoietic death

Tritiated water in various concentrations was orally administered continuously to (C57BL/6N and C3H/He)F1 female mice in a closed animal chamber. Tritium radioactivity in various organ tissues was measured periodically after initiating tritiated water intake using an automatic sample combustion system and a liquid scintillation counter. After 7 days the specific radioactivity reached a plateau. These data allowed absorbed organ doses to be estimated. Within a range of 1.48 x 10(11) to 5.92 x 10(11) Bq/dm3 as the concentration of tritiated water in drinking water, the time of death after initiating the administration was about 2 weeks, a typical time for haematopoietic death. A linear relationship of times of death with tritiated water concentrations in drinking water was observed, on a log-log scale, between 1.85 x 10(10) Bq/dm3 and 1.48 x 10(11) Bq/dm3. At concentrations lower than 9.25 x 10(9) Bq/dm3, mice no longer died from haematopoietic failure. We conclude, therefore, that there should be a threshold dose rate for haematopoietic death.

The effects of tritium on embryo development: the embryotoxic effects of [3H]tryptophan

The effects of tritiated amino acids on the development of the early mouse embryo in vitro are described. [3H]tryptophan was extremely toxic, compared with [3H]glutamic acid, and inhibited growth of the two-cell embryo to the blastocyst stage. The LD50 values for the two amino acids were, respectively, 1 and 296 kBq/ml of medium. The two-cell embryo was more sensitive to the irradiation from incorporated [3H]tryptophan is selectively incorporated into two chromosomal proteins in the two-cell embryo, and that those proteins were stage-specific markers of development.

Radiation quality of tritium beta-rays: microdosimetric distributions for nanometer-size targets in a medium containing tritiated water

To elucidate the characteristics of the action of tritium beta-rays, the following parameters are derived: electron slowing down spectra of primary electrons (beta-rays) and delta-rays in a medium containing tritiated water; and frequency distributions for the microdosimetric quantity j (number of effective primary events per track per target), fj, for nanometer-size targets exposed to tritiated water. Features of the radiation quality of tritium beta-rays are discussed by comparing the present results with those for 60Co gamma-rays and 7 MeV electrons. It is concluded that, although tritium beta-rays, 60Co gamma-rays, and 7 MeV electrons are classified as the same low l.e.t. radiation, the radiation quality of tritium beta-rays is considerably different from those of 60Co gamma-rays and 7 MeV electrons, and has specific features such as a high average l.e.t., a small total electron fluence per unit absorbed dose, and a different microdosimetric distribution, fj, for nanometer-size targets.

1H- and 2H-n.m.r. studies of water in gamma-irradiated phosphatidylcholine multilamellar liposomes

1H- and 2H-n.m.r. studies of gamma-irradiation-induced variations in the dynamic structure and proportional amounts of free, trapped and bound water species in multilamellar liposomes are reported and discussed. Bound water is shown to increase with dose and to be present in two different structural states. A dose-dependent decrease in the 1H-n.m.r. relaxation times of bound water following gamma-irradiation is reported. Variations are suggested as being due to large scale changes at the bilayer surface.