Under UMB3A, we are studying the effect of chronic low to medium dose rate gamma radiation on organisms, with the help of NMBU’s Co-60 irradiation facility. Our main hypothesis has been that an organism’s capacity to mitigate oxidative stress -and thus maintain essential enzyme functions- determines its ability to repair the damage inflicted on essential macromolecules such as DNA. Indirect effects of radiation, particularly the formation of free radicals, can in turn damage cell components and cause disruption of signalling systems and metabolism.
The research has focused on three main topics, interspecies radiosensitivity comparison, life stage-dependent radiosensitivity and vulnerable cell types and cellular processes.
Figure 1. The Biological Effects Toolbox for interspecies comparison of radiosensitivity.
Species radiosensitivity. We have constructed a standardized framework (SOP) for exposure, dosimetry and effects assessment. A total of 12 different model species have been studied using a standard set of comparable dose rates, and biological endpoints to determine their sensitivity to gamma radiation (Fig. 1). For each species, the toxicity has been characterized using the adverse outcome pathways (AOP) framework. An important output from this has been to implement structured mechanistic insights of molecular toxicogenomics and quantitative toxic effects data into AOPs. For Daphnia magna, Song and co-authors (ref 4) developed an AOP for radiation-induced cellular Reactive Oxygen Species (ROS) as a molecular initiating event (MIE), connected to key events (KEs), such as increased oocyte apoptosis that leads to reduced fecundity as the adverse outcome (AO), which reduced reproduction at the population level (Fig. 2).
Figure 2. AOP network on radiation-induced excessive ROS leading to reduced reproduction.
This approach has generated a unique data set that enabled us to quantify differences in radiosensitivity between five different plant species ranging from radiation-tolerant algae and Arabidopsis to the more sensitive duckweed and pine trees (ref 1, 5, 7). All plant species showed similar DNA damage levels, but different effects with respect to development, growth and survival. This demonstrates that plants have very different capacities to tolerate and mitigate radiation-induced damage. We are currently investigating the mechanisms conferring the differences in tolerance between Arabidopsis and Norway spruce.
Life stage-dependent radiosensitivity and vulnerable cell types and cellular processes. Another important objective has been to determine which life stages are more sensitive to the harmful effects of ionizing radiation. We have conducted targeted irradiation of embryogenesis, juvenile and adult stages including gonadal development, in invertebrate, fish and a mammal model species.
Embryogenesis is a delicate process that involves a multitude of epigenetic and transcriptional programs required for cell differentiation, tissue establishment and organogenesis. We employed zebrafish to target the effects of radiation on the early embryogenesis and compared it with effects onto adult fish. The results showed a clear effect of low doses >1.5 mGy of gamma radiation on gene expression in embryos during the early gastrula stage, while prolonged exposure of 48 and 96 h showed a low but significant increase of adverse phenotypic effects at total dose > 4 mGy (ref 6).
A further hypothesis we have tested is that stem cell functions are susceptible to damage by radiation, and that ‘late effects’ such as developmental malformations, or reproductive defects, originate from damage to stem cell populations. We specifically targeted the gonadal development in young adult zebrafish and monitored the effects on reproduction and the resulting offspring. The adult fish tolerated a high total dose, but the radiation had a severe effect on the number and viability of their offspring. The heritable and epigenetic effects mechanisms were further studied as part of Umbrella 3C.
In the nematode C. elegans we have investigated the contribution of radiation-induced cellular ROS in vivo, and the interplay between defence mechanisms and cellular adverse effects (ref 2, 3). While somatic cells, including embryos, show a remarkable tolerance, reproduction is highly vulnerable to radiation (ref 3). We have shown that the reprotoxic effect of radiation is life stage-dependent, and that irradiation during the early larval stage caused the most damage. This stage is crucial to gonadal development, and we have identified spermatogenesis as the most radiosensitive process (ref 3). Molecular evidence indicates that radiation-induced DNA-damage during sperm meiosis leads to faulty spindle formation, chromosomal condensation and chromosomal segregation, which leads to a reduced number of spermatids (Fig. 3). The reduced number of spermatids accounts for >90% of the negative effect on reproduction. It is very interesting that radiation did not influence the function of the gonadal stem cells, even though the sperm formation was impaired and germ cell apoptosis was elevated. This is a clear demonstration that not all types of stem cells are vulnerable to radiation.
Figure 3. Proposed model for gamma radiation induced defective sperm meiosis in the C. elegans hermaphrodite. Repair of complex DNA damage such as DSB is initiated via histone demethylation by spr-5, which concomitantly represses set-17 regulated genes including msp. DNA damage onto the gametes causes defective spindle formation and chromosomal segregation. The concerted effect leads to reduced number of mature sperms with the downstream feedback inhibition of oocyte maturation and sheet cell contraction signalling.
Key publications:
- Xie L, Solhaug KA, Song Y, Brede DA, Lind OC, Salbu B, Tollefsen KE (2019). Modes of action and adverse effects of gamma radiation in an aquatic macrophyte Lemna minor. Sci Total Environ. 680: 23-34.
- Maremonti E, Eide DM, Rossbach LM, Lind OC, Salbu B, Brede DA (2020). In vivo assessment of reactive oxygen species production and oxidative stress effects induced by chronic exposure to gamma radiation in Caenorhabditis elegans. Free Radic. Biol. Med. 152: 583-596.
- Maremonti E, Eide DM, Oughton DH, Salbu B, Grammes F, Kassaye YA, Guédon R, Lecomte-Pradines C, Brede DA (2019). Gamma radiation induces life stage-dependent reprotoxicity in Caenorhabditis elegans via impairment of spermatogenesis. Sci Total Environ. 695: 133835.
- Song Y, Xie L, Lee Y, Brede DA, Lyne F, Kassaye Y, Thaulow J, Caldwell G, Salbu B, Tollefsen KE. (2020). Integrative assessment of low-dose gamma radiation effects on Daphnia magna reproduction: Toxicity pathway assembly and AOP development. Sci Total Environ. 705: 135912.
- Blagojevic D, Lee Y, Brede DA, Lind OC, Yakovlev I, Solhaug KA, Fossdal CG, Salbu B, Olsen JE (2019). Comparative sensitivity to gamma radiation at the organismal, cell and DNA level in young plants of Norway spruce, Scots pine and Arabidopsis thaliana. Planta 250(5): 1567-1590.
- Hurem S, Martín LM, Brede DA, Skjerve E, Nourizadeh-Lillabadi R, Lind OC, Christensen T, Berg V, Teien HC, Salbu B, Oughton DH, Aleström P, Lyche JL (2017). Dose-dependent effects of gamma radiation on the early zebrafish development and gene expression. PLoS One 12(6): e0179259.
- Gomes T, Xie L, Brede D, Lind OC, Solhaug KA, Salbu B, Tollefsen KE (2017). Sensitivity of the green algae Chlamydomonas reinhardtii to gamma radiation: Photosynthetic performance and ROS formation. Aquat Toxicol. 183: 1-10.
