Cellular responses to ionizing radiation change quickly over time during early development in zebrafish

Species-specific differences in DNA damage sensitivity at early developmental stage: A comparative study of sterlet (Acipenser ruthenus) and common carp (Cyprinus carpio)

DNA damage in embryos shapes the development of an organism. Understanding life stage-specific differences between fish species is essential for ecological risk assessment measures. We explored DNA damage sensitivity in two nonmodel fish species, sterlet (Acipenser ruthenus) and common carp (Cyprinus carpio). Embryos of these species were exposed to a model genotoxicant, camptothecin (CPT), during cleavage (2-cell) stage and gastrulation. Results revealed a species-specific DNA damage sensitivity only at cleavage stage. 3 nM CPT caused lethality in sterlet embryos while carp embryos hatched normally. Multiple nuclear abnormalities were observed in sterlet embryos by early gastrula stage. However, carp embryos exhibited nuclear abnormalities and DNA fragmentation at neurula stage only when exposed to 7 nM CPT. Moreover, increased expression of tp53 in carp embryos at gastrula stage suggests activation of apoptosis mechanism. These findings suggest that carp embryos activate DNA damage response more efficiently than sterlet embryos at same developmental stage.

Changes in repair pathways of radiation-induced DNA double-strand breaks at the midblastula transition in Xenopus embryo

Ionizing radiation (IR) causes DNA damage, particularly DNA double-strand breaks (DSBs), which have significant implications for genome stability. The major pathways of repairing DSBs are homologous recombination (HR) and nonhomologous end joining (NHEJ). However, the repair mechanism of IR-induced DSBs in embryos is not well understood, despite extensive research in somatic cells. The externally developing aquatic organism, Xenopus tropicalis, serves as a valuable model for studying embryo development. A significant increase in zygotic transcription occurs at the midblastula transition (MBT), resulting in a longer cell cycle and asynchronous cell divisions. This study examines the impact of X-ray irradiation on Xenopus embryos before and after the MBT. The findings reveal a heightened X-ray sensitivity in embryos prior to the MBT, indicating a distinct shift in the DNA repair pathway during embryo development. Importantly, we show a transition in the dominant DSB repair pathway from NHEJ to HR before and after the MBT. These results suggest that the MBT plays a crucial role in altering DSB repair mechanisms, thereby influencing the IR sensitivity of developing embryos.

A p21-GFP zebrafish model of senescence for rapid testing of senolytics in vivo

Senescence drives the onset and severity of multiple ageing-associated diseases and frailty. As a result, there has been an increased interest in mechanistic studies and in the search for compounds targeting senescent cells, known as senolytics. Mammalian models are commonly used to test senolytics and generate functional and toxicity data at the level of organs and systems, yet this is expensive and time consuming. Zebrafish share high homology in genes associated with human ageing and disease. They can be genetically modified relatively easily. In larvae, most organs develop within 5 days of fertilisation and are transparent, which allows tracking of fluorescent cells in vivo in real time, testing drug off-target toxicity and assessment of cellular and phenotypic changes. Here, we have generated a transgenic zebrafish line that expresses green fluorescent protein (GFP) under the promoter of a key senescence marker, p21. We show an increase in p21:GFP+ cells in larvae following exposure to ionising radiation and with natural ageing. p21:GFP+ cells display other markers of senescence, including senescence-associated β-galactosidase and IL6. The observed increase in senescent cells following irradiation is associated with a reduction in the thickness of muscle fibres and mobility, two important ageing phenotypes. We also show that quercetin and dasatinib, two senolytics currently in clinical trials, reduce the number of p21:GFP+ cells, in a rapid 5-day assay. This model provides an important tool to study senescence in a living organism, allowing the rapid selection of senolytics before moving to more expensive and time-consuming mammalian systems.

DNA repair genes play a variety of roles in the development of fish embryos

Embryogenesis is one of the most important life stages because it determines an organism's healthy growth. However, embryos of externally fertilizing species, such as most fish, are directly exposed to the environment during development and may be threatened by DNA damaging factors (pollutants, UV, reactive oxygen species). To counteract the negative effects of DNA fragmentation, fish embryos evolved complex damage response pathways. DNA repair pathways have been extensively studied in some fish species, such as zebrafish (Danio rerio). Our literature review, on the other hand, revealed a paucity of knowledge about DNA damage response and repair in non-model aquaculture fish species. Further, several pieces of evidence underlie the additional role of DNA repair genes and proteins in organogenesis, spatiotemporal localization in different tissue, and its indispensability for normal embryo development. In this review, we will summarize features of different DNA repair pathways in course of fish embryo development. We describe how the expression of DNA repair genes and proteins is regulated during development, their organogenetic roles, and how the expression of DNA repair genes changes in response to genotoxic stress. This will aid in addressing the link between genotoxic stress and embryo phenotype. Furthermore, available data indicate that embryos can repair damaged DNA, but the effects of early-life stress may manifest later in life as behavioral changes, neoplasia, or neurodegeneration. Overall, we conclude that more research on DNA repair in fish embryos is needed.

A systems biology analysis of reproductive toxicity effects induced by multigenerational exposure to ionizing radiation in C. elegans

Understanding the effects of chronic exposure to pollutants over generations is of primary importance for the protection of humans and the environment; however, to date, knowledge on the molecular mechanisms underlying multigenerational adverse effects is scarce. We employed a systems biology approach to analyze effects of chronic exposure to gamma radiation at molecular, tissue and individual levels in the nematode Caenorhabditis elegans. Our data show a decrease of 23% in the number of offspring on the first generation F0 and more than 40% in subsequent generations F1, F2 and F3. To unveil the impact on the germline, an in-depth analysis of reproductive processes involved in gametes formation was performed for all four generations. We measured a decrease in the number of mitotic germ cells accompanied by increased cell-cycle arrest in the distal part of the gonad. Further impact on the germline was manifested by decreased sperm quantity and quality. In order to obtain insight in the molecular mechanisms leading to decreased fecundity, gene expression was investigated via whole genome RNA sequencing. The transcriptomic analysis revealed modulation of transcription factors, as well as genes involved in stress response, unfolded protein response, lipid metabolism and reproduction. Furthermore, a drastic increase in the number of differentially expressed genes involved in defense response was measured in the last two generations, suggesting a cumulative stress effect of ionizing radiation exposure. Transcription factor binding site enrichment analysis and the use of transgenic strain identified daf-16/FOXO as a master regulator of genes differentially expressed in response to radiation. The presented data provide new knowledge with respect to the molecular mechanisms involved in reproductive toxic effects and accumulated stress resulting from multigenerational exposure to ionizing radiation.

Effects of gamma ionizing radiation exposure on Danio rerio embryo-larval stages - comparison with tritium exposure

The objective was to investigate the effects of ionizing radiation induced in zebrafish early life stages by coupling responses obtained at the molecular (genotoxicity, ROS production, gene expression) and phenotypic (tissue alteration, embryo-larval development) levels. Here we present results obtained after exposure of 3 hpf larvae to 10 days of gamma irradiation at 3.3 × 10¹, 1.3 × 10² and 1.2 × 10³ µGy/h, close to and higher than the benchmark for protection of ecosystems towards ionizing radiations of 10¹ µGy/h. Dose rates used in these studies were chosen to be in the 'derived consideration reference level' (DCRL) for gamma irradiation where deleterious effects can appear in freshwater fish. Also, these dose rates were similar to the ones already tested on tritium (beta ionizing radiation) in our previous work, in order to compare both types of ionizing radiation. Results showed that gamma irradiation did not induce any effect on survival and hatching. No effect was observed on DNA damages, but ROS production was increased. Muscle damages were observed for all tested dose rates, similarly to previous results obtained with tritium (beta ionizing radiation) at similar dose rates. Some molecular responses therefore appeared to be relevant for the study of gamma ionizing radiation effects in zebrafish.

Collimated Microbeam Reveals that the Proportion of Non-Damaged Cells in Irradiated Blastoderm Determines the Success of Development in Medaka (Oryzias latipes) Embryos

It has been widely accepted that prenatal exposure to ionizing radiation (IR) can affect embryonic and fetal development in mammals, depending on dose and gestational age of the exposure, however, the precise machinery underlying the IR-induced disturbance of embryonic development is still remained elusive. In this study, we examined the effects of gamma-ray irradiation on blastula embryos of medaka and found transient delay of brain development even when they hatched normally with low dose irradiation (2 and 5 Gy). In contrast, irradiation of higher dose of gamma-rays (10 Gy) killed the embryos with malformations before hatching. We then conducted targeted irradiation of blastoderm with a collimated carbon-ion microbeam. When a part (about 4, 10 and 25%) of blastoderm cells were injured by lethal dose (50 Gy) of carbon-ion microbeam irradiation, loss of about 10% or less of blastoderm cells induced only the transient delay of brain development and the embryos hatched normally, whereas embryos with about 25% of their blastoderm cells were irradiated stopped development at neurula stage and died. These findings strongly suggest that the developmental disturbance in the IR irradiated embryos is determined by the proportion of severely injured cells in the blastoderm.

Metformin fights against radiation-induced early developmental toxicity

Nuclear pollution intertwined accidental irradiation not only triggers acute and chronic radiation syndromes, but also endangers embryonic development in sight of uncontrollable gene mutation. Metformin (MET), a classic hypoglycemic drug, has been identified to possess multiple properties. In this study, we explored the radioprotective effects of MET on the developmental abnormalities and deformities induced by irradiation among three "star drugs". Specifically, zebrafish (Danio rerio) embryos exposed to 5.2 Gy gamma irradiation at 4 h post fertilization (hpf) showed overt developmental toxicity, including hatching delay, hatching rate decrease, developmental indexes reduction, morphological abnormalities occurrence and motor ability decline. However, MET treatment erased the radiation-induced phenotypes. In addition, MET degraded inflammatory reaction, hinders apoptosis response, and reprograms the development-related genes expression, such as sox2, sox3, sox19a and p53, in zebrafish embryos following radiation challenge. Together, our findings provide novel insights into metformin, and underpin that metformin might be employed as a promising radioprotector for radiation-induced early developmental toxicity in pre-clinical settings.

Ionising Radiation Induces Promoter DNA Hypomethylation and Perturbs Transcriptional Activity of Genes Involved in Morphogenesis during Gastrulation in Zebrafish

Embryonic development is particularly vulnerable to stress and DNA damage, as mutations can accumulate through cell proliferation in a wide number of cells and organs. However, the biological effects of chronic exposure to ionising radiation (IR) at low and moderate dose rates (< 6 mGy/h) remain largely controversial, raising concerns for environmental protection. The present study focuses on the molecular effects of IR (0.005 to 50 mGy/h) on zebrafish embryos at the gastrula stage (6 hpf), at both the transcriptomics and epigenetics levels. Our results show that exposure to IR modifies the expression of genes involved in mitochondrial activity from 0.5 to 50 mGy/h. In addition, important developmental pathways, namely, the Notch, retinoic acid, BMP and Wnt signalling pathways, were altered at 5 and 50 mGy/h. Transcriptional changes of genes involved in the morphogenesis of the ectoderm and mesoderm were detected at all dose rates, but were prominent from 0.5 to 50 mGy/h. At the epigenetic level, exposure to IR induced a hypomethylation of DNA in the promoter of genes that colocalised with both H3K27me3 and H3Kme4 histone marks and correlated with changes in transcriptional activity. Finally, pathway enrichment analysis demonstrated that the DNA methylation changes occurred in the promoter of important developmental genes, including morphogenesis of the ectoderm and mesoderm. Together, these results show that the transcriptional program regulating morphogenesis in gastrulating embryos was modified at dose rates greater than or equal to 0.5 mGy/h, which might predict potential neurogenesis and somitogenesis defects observed at similar dose rates later in development.

Stage-Specific Effects of Ionizing Radiation during Early Development

Early embryonic cells are sensitive to genotoxic stressors such as ionizing radiation. However, sensitivity to these stressors varies depending on the embryonic stage. Recently, the sensitivity and response to ionizing radiation were found to differ during the preimplantation period. The cellular and molecular mechanisms underlying the change during this period are beginning to be elucidated. In this review, we focus on the changes in radio-sensitivity and responses to ionizing radiation during the early developmental stages of the preimplantation (before gastrulation) period in mammals, Xenopus, and fish. Furthermore, we discuss the underlying cellular and molecular mechanisms and the similarities and differences between species.