Identification of radiation-sensitive mutants in the Medaka, Oryzias latipes

Biological effects of ionizing radiation on aquatic biota - A critical review

Ionizing radiation from radionuclides impacts marine aquatic biota and the scope of investigation must be wider than just invertebrates. We intend to detail and illustrate numerous biological effects that occur in both aquatic vertebrates and invertebrates, at various dose rates from all three kinds of ionizing radiation. The characteristics of radiation sources and dosages that would most effectively generate the intended effects in the irradiated organism were assessed once the biological differentiation between vertebrates and invertebrates was determined through multiple lines of evidence. We contend that invertebrates are still more radiosensitive than vertebrates, due to their small genome size, rapid reproduction rates and lifestyle, which help them to compensate for the effects of radiation induced declines in fecundity, life span and individual health. We also identified various research gaps in this field and suggest future directions to be investigated to remedy the lack of data available in this area.

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.

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.

A protective study of curcumin associated with Cr6+ induced oxidative stress, genetic damage, transcription of genes related to apoptosis and histopathology of fish, Channa punctatus (Bloch, 1793)

Ameliorative potential of curcumin against Cr⁶⁺-induced eco-toxicological manifestations was assessed in liver of exposed Channa punctatus (Actinopterygii) in six groups for 45 d; Group I as control. Group II with 3 mg/L of curcumin; group III with 7.89 mg/L of Cr⁶⁺. Groups IV, V and VI were simultaneously co-exposed with 7.89 mg/L of Cr⁶⁺ and three different curcumin concentrations, 1, 2, and 3 mg/L, respectively. In group III, SOD-CAT, GR significantly (p < 0.05) increased; decreased GSH level; elevated MN and AC frequencies; and a significant (p < 0.05) up-regulation of cat (2.72-fold), p53 (1.73-fold), bax (1.33-fold) and apaf-1 (2.13-fold) together with a significant (p < 0.05) down-regulation of bcl-2 (0.51-fold). Co-exposure significantly (p < 0.05) brought down activities of SOD-CAT, GR, raised GSH, decreased micronuclei and apoptotic frequencies along with recovery of histopathological anomalies in liver. This study establishes the protective role of curcumin against Cr⁶⁺-induced hepatotoxicity in fish.

Effect of gamma radiation on the growth, survival, hematology and histological parameters of rainbow trout (Oncorhynchus mykiss) larvae

Effects of low (1, 2.5 and 5Gy) and high doses (10, 20 and 40Gy) of gamma radiation were examined on the growth, survival, blood parameters and morphological changes of the intestines of rainbow trout (Oncorhynchus mykiss) larvae (103±20mg) after 12 weeks of exposure. Negative effects of gamma radiation on growth and survival were observed as radiation level and time increased. Changes were well documented at 10 and 20Gy. All the fish were dead at the dose of 40Gy. In all the treatments, levels of red blood cells (RBC), hematocrit (HCT) and hemoglobin (HB) were significantly (P<0.05) declined as the irradiation levels increased, whereas the amount of mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) did not change. No significant differences (P>0.05) were found in the levels of white blood cells (WBC), lymphocytes and monocytes. Destruction of the intestinal epithelium cells was indicated as the irradiation levels increased to 1Gy and above. The highest levels of growth, survival, specific growth rate (SGR), condition factor (CF) and protein efficiency rate (PER) were obtained in the control treatment. The results showed that gamma rays can be a potential means for damaging rainbow trout cells.

Co-expression of antioxidant enzymes with expression of p53, DNA repair, and heat shock protein genes in the gamma ray-irradiated hermaphroditic fish Kryptolebias marmoratus larvae

To investigate effects of gamma ray irradiation in the hermaphroditic fish, Kryptolebias marmoratus larvae, we checked expression of p53, DNA repair, and heat shock protein genes with several antioxidant enzyme activities by quantitative real-time RT-PCR and biochemical methods in response to different doses of gamma radiation. As a result, the level of gamma radiation-induced DNA damage was initiated after 4Gy of radiation, and biochemical and molecular damage became substantial from 8Gy. In particular, several DNA repair mechanism-related genes were significantly modulated in the 6Gy gamma radiation-exposed fish larvae, suggesting that upregulation of such DNA repair genes was closely associated with cell survival after gamma irradiation. The mRNA expression of p53 and most hsps was also significantly upregulated at high doses of gamma radiation related to cellular damage. This finding indicates that gamma radiation can induce oxidative stress with associated antioxidant enzyme activities, and linked to modulation of the expression of DNA repair-related genes as one of the defense mechanisms against radiation damage. This study provides a better understanding of the molecular mode of action of defense mechanisms upon gamma radiation in fish larvae.

DNA repair activity in fish and interest in ecotoxicology: a review

The knowledge of DNA repair in a target species is of first importance as it is the primary line of defense against genotoxicants, and a better knowledge of DNA repair capacity in fish could help to interpret genotoxicity data and/or assist in the choice of target species, developmental stage and tissues to focus on, both for environmental biomonitoring studies and DNA repair testing. This review focuses in a first part on what is presently known on a mechanistic basis, about the various DNA repair systems in fish, in vivo and in established cell lines. Data on base excision repair (BER), direct reversal with O⁶-alkylguanine transferase and double strand breaks repair, although rather scarce, are being reviewed, as well as nucleotide excision repair (NER) and photoreactivation repair (PER), which are by far the most studied repair mechanisms in fish. Most of these repair mechanisms seem to be strongly species and tissue dependent; they also depend on the developmental stage of the organisms. BER is efficient in vivo, although no data has been found on in vitro models. NER activity is quite low or even inexistent depending on the studies; however this lack is partly compensated by a strong PER activity, especially in early developmental stage. In a second part, a survey of the ecotoxicological studies integrating DNA repair as a parameter responding to single or mixture of contaminant is realized. Three main approaches are being used: the measurement of DNA repair gene expression after exposure, although it has not yet been clearly established whether gene expression is indicative of repair capacity; the monitoring of DNA damage removal by following DNA repair kinetics; and the modulation of DNA repair activity following exposure in situ, in order to assess the impact of exposure history on DNA repair capacity. Since all DNA repair processes are possible targets for environmental pollutants, we can also wonder at which extent such a modulation of repair capacities in fish could be the base for the development of new biomarkers of genotoxicity. Knowing the importance of the germ cell DNA integrity in the reproductive success of aquatic organisms, the DNA repair capacity of such cells deserve to be more studied, as well as DNA repair capacities of established fish cell lines. The limited amount of available data, which shows low/slow DNA repair capacities of fish cell lines compared with mammalian cell lines, concerned mainly the NER system; thus this point merits to be explored more deeply. Additionally, since some of the DNA repair systems appear more efficient in embryo larval stages, it would be of interest to consider embryonic cell lineages more closely.

DNA-PK inhibition causes a low level of H2AX phosphorylation and homologous recombination repair in Medaka (Oryzias latipes) cells

Nonhomologous end joining (NHEJ) and homologous recombination (HR) are known as DNA double-strand break (DSB) repair pathways. It has been reported that DNA-PK, a member of PI3 kinase family, promotes NHEJ and aberrant DNA-PK causes NHEJ deficiency. However, in this study, we demonstrate that a wild-type cell line treated with DNA-PK inhibitor and a mutant cell line with dysfunctional DNA-PK showed decreased HR efficiency in fish cells (Medaka, Oryzias latipes). Previously, we reported that the radiation-sensitive mutant RIC1 strain has a defect in the Histone H2AX phosphorylation after γ-irradiation. Here, we showed that a DNA-PK inhibitor, NU7026, treatment resulted in significant reduction in the number of γH2AX foci after γ-irradiation in wild-type cells, but had no significant effect in RIC1 cells. In addition, RIC1 cells showed significantly lower levels of DNA-PK kinase activity compared with wild-type cells. We investigated NHEJ and HR efficiency after induction of DSBs. Wild-type cells treated with NU7026 and RIC1 cells showed decreased HR efficiency. These results indicated that aberrant DNA-PK causes the reduction in the number of γH2AX foci and HR efficiency in RIC1 cells. We performed phosphorylated DNA-PKcs (Thr2609) and 53BP1 focus assay after γ-irradiation. RIC1 cells showed significant reduction in the number of phosphorylated DNA-PKcs foci and no deference in the number of 53BP1 foci compared with wild-type cells. These results suggest that low level of DNA-PK activity causes aberrant DNA-PKcs autophosphorylation in RIC1 cells. It is known that 53BP1 is involved in both DNA-PK dependent and independent NHEJ. Therefore we suggest that DNA-PK independent NHEJ repair DSBs under the condition of decreased DNA-PK activity, which causes reduction of HR efficiency.

Gamma irradiation-induced oxidative stress and developmental impairment in the hermaphroditic fish, Kryptolebias marmoratus embryo

This study investigated the effects of gamma radiation on the early developmental stages in hermaphroditic fish embryos of Kryptolebias marmoratus. The authors measured reactive oxygen species (ROS) level and antioxidant enzyme activities with the endpoint hatching rate after gamma irradiation of different embryonic stages. Then, the transcriptional changes of antioxidant enzyme-coding genes were evaluated by quantitative real-time reverse transcription polymerase chain reaction in response to gamma radiation on embryonic stages. Gamma radiation inhibited hatching rate and caused developmental impairment in a dose-dependent manner. Embryos showed tolerances in a developmental stage-dependent manner, indicating that early embryonic stages were more sensitive to the negative effects of gamma radiation than were later stages. After 5 Gy rate of radiation, the ROS level increased significantly at embryonic stages 2, 3, and 4 with a significant induction of all antioxidant enzyme activities. The expressions of glutathione S-transferase isoforms, catalase, superoxide dismutase (Mn-SOD, Cu/Zn-SOD), glutathione reductase, and glutathione peroxidase mRNA were upregulated in a dose-and-developmental stage-dependent manner. This finding indicates that gamma radiation can induce oxidative stress and subsequently modulates the expression of antioxidant enzyme-coding genes as one of the defense mechanisms. Interestingly, embryonic stage 1 exposed to gamma radiation showed a decreased expression in most antioxidant enzyme-coding genes, suggesting that this is also related to a lower hatching rate and developmental impairment. The results of this study provide a better understanding of the molecular mode of action of gamma radiation in aquatic organisms.

Investigating the embryo/larval toxic and genotoxic effects of γ irradiation on zebrafish eggs

Eggs/larval of freshwater fish (Danio rerio) were exposed to low dose rates of external gamma radiation (from 1 to 1000 mGy d(-1)) over a 20-day period, with the objective of testing the appropriateness of the 10 mGy d(-1) guideline suggested by the IAEA. The present study examines different endpoints, mortality and hatching time and success of embryos as well as the genotoxicity of γ-irradiations (after 48 h). The 20-day embryo-larval bioassay showed an enhanced larval resistance to starvation after chronic exposure to γ irradiation (from low 1 mGy d(-1) to high dose rate 1000 mGy d(-1)) and an acceleration in hatching time. Gamma irradiation led to increased genotoxic damage Ito zebrafish egg (40-50% DNA in tail in Comet assay) from the lowest dose rate (1 mGy d(-1)). Possible mechanisms of γ radiotoxicity and implications for radioprotection are discussed.

Cell lines derived from a medaka radiation-sensitive mutant have defects in DNA double-strand break responses

It was reported that the radiation-sensitive Medaka mutant "ric1" has a defect in the repair of DNA double-strand breaks (DSBs) induced by gamma-rays during early embryogenesis. To study the cellular response of a ric1 mutant to ionizing radiation (IR), we established the mutant embryonic cell lines RIC1-e9, RIC1-e42, RIC1-e43. Following exposure to gamma-irradiation, the DSBs in wild-type cells were repaired within 1 h, while those in RIC1 cells were not rejoined even after 2 h. Cell death was induced in the wild-type cells with cell fragmentation, but only a small proportion of the RIC1 cells underwent cell death, and without cell fragmentation. Although both wild-type and RIC1 cells showed mitotic inhibition immediately after gamma-irradiation, cell division was much slower to resume in the wild-type cells (20 h versus 12 h). In both wild-type and RIC1 cells, Ser139 phosphorylated H2AX (gammaH2AX) foci were formed after gamma-irradiation, however, the gammaH2AX foci disappeared more quickly in the RIC1 cell lines. These results suggest that the instability of gammaH2AX foci in RIC1 cells cause an aberration of the DNA damage response. As RIC1 cultured cells showed similar defective DNA repair as ric1 embryos and RIC1 cells revealed defective cell death and cell cycle checkpoint, they are useful for investigating DNA damage responses in vitro.

Communication of radiation-induced signals in vivo between DNA repair deficient and proficient medaka (Oryzias latipes)

Radiation-induced bystander effects are established consequences of exposure to ionizing radiation. The operation of this mechanism has been seen in vitro and also between fish, mammals, and plants in vive where stress signals from treated organisms induce responses in neighbors. In vitro research shows that DNA repair deficient cells produce more toxic bystander responses. To test this in vivo two strains of Japanese medaka were tested. One is a mutant, repair deficient strain (ric2) and the other, the wildtype repair proficient strain (CAB). Irradiated fish swam with unirradiated partners in a strain mix and match protocol. The data suggest that medaka produce signals, when exposed to radiation, that induce unirradiated fish ofthe same strain swimming with them to produce an altered response to that seen in bystanders to sham irradiated fish. More apoptosis was seen in bystanders to repair deficient fish. When the strains are mixed, the bystanders of either strain respond like the donor strain. Measurements of Bcl-2 and cmyc proteins in the explants confirmed these observations. A possible role for p53 was also identified in that the use of reporters with mutant p53 demonstrated that CAB signals killed all the reporter cells by apoptosis. Use of a similar but p53 wildtype cell line had no such effect. The data add to the body of knowledge showing that bystander signals operate at hierarchical levels of organization greater than the individual and may therefore have relevance in radioecology and (eco)systems biology.

Responses of embryonic germ cells of the radiation-sensitive Medaka mutant to gamma-irradiation

The radiation-sensitive mutant "ric1" has a defect in the repair mechanism of DNA double strand breaks induced by gamma-rays in early embryogenesis. In this study, the new radiation-sensitive Medaka (Oryzias latipes) strain, ric1olvas-GFP was established to monitor the development of germ cells in vivo. The development of germ cells was normal in ric1olvas-GFP, but embryonic germ cells at Stage 7 (32-cell stage) and Stage 33 (extensive proliferating stage of PGCs) showed higher radiosensitivity. There was no sex difference in germ cell radiosensitivity at Stage 7, but female embryos showed higher radiosensitivity than male at Stage 33. In embryos obtained by crossing ric1 female with olvas-GFP male, germ cells showed similar radiosensitivity to ric1olvas-GFP and increased sensitivity compared to embryos obtained from crossing wild-type female with olvas-GFP male at Stage 7. These results suggest that germ cells have the ric1 dependent DNA repair system during embryogenesis and the maternal ric1gene factor may play a critical role in radiosensitivity at an early developmental stage.

Radiation-induced untargeted germline mutations in Japanese medaka

Radiation has been shown to increase mutation frequencies at tandem repeat loci by indirect interactions of radiation with DNA. We studied germline mutations in chronically exposed Japanese medaka (Oryzias latipes) using microsatellite loci. After screening 26 randomly selected loci among unirradiated parents and their 200 offspring, we selected seven highly mutable loci (0.5-1.0 x 10(-2) mutants per locus per gamete) and two bonus loci for further study. To determine if radiation exposure increases mutation frequencies in these loci, medaka were chronically irradiated from subadults through maturation at relatively low dose rates of 68 mGy/d. Total doses for males and females were 10.4 and 3 Gy, respectively. The mean number of mutations for the offspring of exposed families (0.149+/-0.044) was significantly higher (P=0.018) than for control families (0.080+/-0.028), indicating induction of germline mutations from chronic irradiation. This increase in the microsatellite mutation rate is greater than expected from direct interaction of radiation with DNA, suggesting indirect, untargeted mechanism(s) for mutations. This study identified microsatellite loci with a high mutational background in medaka, variation among loci and families as important variables, and demonstrated the usefulness of this fish model for studying radiation-induced germline mutations.

Radiation-induced brain cell death can be observed in living medaka embryos

Medaka (Oryzias latipes) embryos at 25-26 and 28-30 stages were irradiated with a single acute dose of 10 Gy of X-ray, which is lower than the LD(50 )of the embryos. The effects on developing brains were examined under a stereomicroscope in living embryos until hatching. All the irradiated embryos survived; however, from 6 to 35 h after X-ray irradiation, massive clusters of optically opaque and round cells were observed either in the entire brain region (when irradiated at 25-26 stages) or mainly in the optic tectum (when irradiated at 28-30 stages). Histological examination and TUNEL showed that these cells are clusters of dead cells. These dead cell clusters disappeared thereafter, and the irradiated embryos continued to develop apparently normally. The grown irradiated embryos, however, had smaller brains and eyes than the nonirradiated control embryos. At hatching, the irradiated embryos exhibited histological abnormalities in the brain, particularly in the torus longitudinalis, and in the retina, although most of them hatched normally and survived. The results indicate that brain cell death and a reduced brain size can be observed in living irradiated embryos, and suggest that the medaka embryo is useful for screening the developmental neurotoxicity effects of various hazardous factors.

A systematic genome-wide screen for mutations affecting organogenesis in Medaka, Oryzias latipes

A large-scale mutagenesis screen was performed in Medaka to identify genes acting in diverse developmental processes. Mutations were identified in homozygous F3 progeny derived from ENU-treated founder males. In addition to the morphological inspection of live embryos, other approaches were used to detect abnormalities in organogenesis and in specific cellular processes, including germ cell migration, nerve tract formation, sensory organ differentiation and DNA repair. Among 2031 embryonic lethal mutations identified, 312 causing defects in organogenesis were selected for further analyses. From these, 126 mutations were characterized genetically and assigned to 105 genes. The similarity of the development of Medaka and zebrafish facilitated the comparison of mutant phenotypes, which indicated that many mutations in Medaka cause unique phenotypes so far unrecorded in zebrafish. Even when mutations of the two fish species cause a similar phenotype such as one-eyed-pinhead or parachute, more genes were found in Medaka than in zebrafish that produced the same phenotype when mutated. These observations suggest that many Medaka mutants represent new genes and, therefore, are important complements to the collection of zebrafish mutants that have proven so valuable for exploring genomic function in development.