Embryonic Medaka Model of Microglia in the Developing CNS Allowing In Vivo Analysis of Their Spatiotemporal Recruitment in Response to Irradiation

Retinal regeneration after injury induced by gamma-ray irradiation during early embryogenesis in medaka, Oryzias latipes

PURPOSE

Zebrafish, a small fish model, exhibits a multipotent ability for retinal regeneration after damage throughout its lifetime. Compared with zebrafish, birds and mammals exhibit such a regenerative capacity only during the embryonic period, and this capacity decreases with age. In medaka, another small fish model that has also been used extensively in biological research, the retina's inner nuclear layer (INL) failed to regenerate after injury in the hatchling at eight days postfertilization (dpf). We characterized the regenerative process of the embryonic retina when the retinal injury occurred during the early embryonic period in medaka.

METHODS

We employed a 10 Gy dose of gamma-ray irradiation to initiate retinal injury in medaka embryos at 3 dpf and performed histopathological analyses up to 21 dpf.

RESULTS

One day after irradiation, numerous apoptotic neurons were observed in the INL; however, these neurons were rarely observed in the ciliary marginal zone and the photoreceptor layer. Numerous pyknotic cells were clustered in the irradiated retina until two days after irradiation. These disappeared four days after irradiation, but the abnormal bridging structures between the INL and ganglion cell layer (GCL) were present until 11 days after irradiation, and the neural layers were completely regenerated 18 days after irradiation. After gamma-ray irradiation, the spindle-like Müller glial cells in the INL became rounder but did not lose their ability to express SOX2.

CONCLUSIONS

Irradiated retina at 3 dpf of medaka embryos could be completely regenerated at 18 days after irradiation (21 dpf), although the abnormal layer structures bridging the INL and GCL were transiently formed in the retinas of all the irradiated embryos. Four days after irradiation, embryonic medaka Müller glia were reduced in number but maintained SOX2 expression as in nonirradiated embryos. This finding contrasts with previous reports that 8 dpf medaka larvae could not fully regenerate damaged retinas because of loss of SOX2 expression.

3D reconstructed brain images reveal the possibility of the ogg1 gene to suppress the irradiation-induced apoptosis in embryonic brain in medaka (Oryzias latipes)

The accumulation of oxidative DNA lesions in neurons is associated with neurodegenerative disorders and diseases. Ogg1 (8-oxoG DNA glycosylase-1) is a primary repair enzyme to excise 7,8-dihydro-8-oxoguanine (8-oxoG), the most frequent mutagenic base lesion produced by oxidative DNA damage. We have developed ogg1-deficient medaka by screening with a high resolution melting (HRM) assay in Targeting-Induced Local Lesions In Genomes (TILLING) library. In this study, we identified that ogg1-deficient embryos have smaller brains than wild-type during the period of embryogenesis and larvae under normal conditions. To reveal the function of ogg1 when brain injury occurs during embryogenesis, we examined the induction of apoptosis in brains after exposure to gamma-rays with 10 Gy (137Cs, 7.3 Gy/min.) at 24 h post-irradiation both in wild-type and ogg1-deficient embryos. By acridine orange (AO) assay, clustered apoptosis in irradiated ogg1-deficient embryonic brains were distributed in a similar manner to those of irradiated wild-type embryos. To evaluate possible differences of gamma-ray induced apoptosis in both types of embryonic brains, we constructed 3D images of the whole brain based on serial histological sections. This analysis identified that the clustered apoptotic volume was about 3 times higher in brain of irradiated ogg1-deficient embryos (n = 3) compared to wild-type embryos (n = 3) (P = 0.04), suggesting that irradiation-induced apoptosis in medaka embryonic brain can be suppressed in the presence of functional ogg1. Collectively, reconstruction of 3D images can be a powerful approach to reveal slight differences in apoptosis induction post-irradiation.

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.

Protective effect of p53 knockout on 4-nonylphenol-induced nephrotoxicity in medaka (Oryzias latipes)

In the past few decades, environmental pollutants have become common because of misused nonionic surfactants and detergents. Nonylphenol ethoxylates (NPs) are one of the most important contaminants of water. Therefore, the present study aimed to investigate the protective blocking effect of apoptosis (deficient P53 gene) on 4-nonylphenol (4-NP)-induced nephrotoxicity of medaka (Oryzias latipes). We divided 36 fish into six groups: two different control groups of wild type (Wt; Hd-rR) control and p53 (-/-) control, and four different treated with 4-nonylphenol (50 μg/L and 100 μg/L) for 15 days. Histology, immunochemistry, and TUNEL assays confirmed that 4-NP causes nephrotoxicity. Our results showed that 4-NP administration significantly disturbed the kidney structure and function and 4-NP-treated fish showed dilated glomerular vessels, had less glomerular cellular content, decreased expression of glomerular proteins, and an increased level of apoptosis compared with a Wt control group (P < 0.05). As p53 is an apoptotic inducer, some protection in p53-deficient medaka was found as nephrotoxic effects of 4-NP were minimized significantly. Our study demonstrated for the first time to our knowledge that 4-NP induces apoptosis, causing nephrotoxicity in medaka. We found that blocking apoptosis blocking was able to protect the kidney from the toxic effects of 4-NP.

Radical change of apoptotic strategy following irradiation during later period of embryogenesis in medaka (Oryzias latipes)

Induction of apoptosis in response to various genotoxic stresses could block transmission of teratogenic mutations to progeny cells. The severity of biological effects following irradiation depends on the stage at which embryos are irradiated during embryogenesis. We reported previously that irradiation of medaka embryos 3 days post fertilization (dpf) with 10 Gy of gamma rays induced high incidence of apoptotic cells in the mid-brain, however, the embryos hatched normally and developed without apparent malformations. To determine the severity of biological effects following irradiation during a later period of embryogenesis, embryos of various developmental stages were irradiated with 15 Gy of gamma rays and examined for apoptotic induction at 24 h after irradiation in the brain, eyes and pharyngeal epithelium tissues, which are actively proliferating and sensitive to irradiation. Embryos irradiated at 3 dpf exhibited many apoptotic cells in these tissues, and all of them died due to severe malformations. In contrast, embryos irradiated at 5 dpf showed no apoptotic cells and subsequently hatched without apparent malformations. Embryos irradiated at 4 dpf had relatively low numbers of apoptotic cells compared to those irradiated at 3 dpf, thereafter most of them died within 1 week of hatching. In adult medaka, apoptotic cells were not found in these tissues following irradiation, suggesting that apoptosis occurs during a restricted time period of medaka embryogenesis throughout the life. No apoptotic cells were found in irradiated intestinal tissue, which is known to be susceptible to radiation damage in mammals, whereas many apoptotic cells were found in proliferating spermatogonial cells in the mature testis following irradiation. Taken together, with the exception of testicular tissue, the results suggest a limited period during medaka embryogenesis in which irradiation-induced apoptosis can occur.

Oocyte atresia in WT (HdrR) and P53 (-/-) medaka (Oryzias latipes) exposed to UVA

The negative effects of ambient ultraviolet (UVA) on the water environment have been recently highlighted; UVA can create deleterious effects by stimulating stress on pelagic organisms. Little is known about UVA effects on oocyte characteristics of female fish. In the present study we explored the effects of exposure to ecologically relevant levels of simulated UVA radiation on ovaries of two major strains WT (HdrR) and P53 (-/-) of medaka (Oryzias latipes) mature female. Fish were assigned to control and three UVA-exposed groups as (15 min, 30 min, and 60 min/day) for three days and sample selection was 24 h and 14 days after exposure. Histological alterations and oocyte atresia percentage were analyzed in the UVA-exposed fish compared to control. Alteration comprised hyperthrophied follicular cells with increased thickness, breakdown of egg chorion (zona radiata), damage of cortical alveoli, and distorted nucleus and cytoplasm. The atresia percentages significantly increased with higher UVA exposure dose and time for both the wild type and the p53 deficient fish. The wild type displayed significantly higher oocyte atresia percentage than the p53 mutant. These results suggested that UVA exposure provoked histological alterations in both p53 and WT medaka oocytes leading to follicular atresia, which reduce female reproductive ability and larval production. UVA oocyte response showed p53 dependent and independent histological alteration, however, the p53 mutant was less sensitive to UVA than the wild type in medaka fish.

Abscopal Activation of Microglia in Embryonic Fish Brain Following Targeted Irradiation with Heavy-Ion Microbeam

Microglia remove apoptotic cells by phagocytosis when the central nervous system is injured in vertebrates. Ionizing irradiation (IR) induces apoptosis and microglial activation in embryonic midbrain of medaka (Oryzias latipes), where apolipoprotein E (ApoE) is upregulated in the later phase of activation of microglia In this study, we found that another microglial marker, l-plastin (lymphocyte cytosolic protein 1), was upregulated at the initial phase of the IR-induced phagocytosis when activated microglia changed their morphology and increased motility to migrate. We further conducted targeted irradiation to the embryonic midbrain using a collimated microbeam of carbon ions (250 μm diameter) and found that the l-plastin upregulation was induced only in the microglia located in the irradiated area. Then, the activated microglia might migrate outside of the irradiated area and spread through over the embryonic brain, expressing ApoE and with activated morphology, for longer than 3 days after the irradiation. These findings suggest that l-plastin and ApoE can be the biomarkers of the activated microglia in the initial and later phase, respectively, in the medaka embryonic brain and that the abscopal and persisted activation of microglia by IR irradiation could be a cause of the abscopal and/or adverse effects following irradiation.

The notochord curvature in medaka (Oryzias latipes) embryos as a response to ultraviolet A irradiation

In the present work, the destructive effects of ultraviolet A (UVA; 366nm) irradiation on the developmental stages of Japanese medaka (Oryzias latipes) are revealed in terms of hatching success, mortality rate, and morphological malformations (yolk sac edema, body curvature, fin blistering, and dwarfism). Fertilized eggs in stage 4 were exposed to 15, 30, and 60min/day UVA for 3days in replicates. Fish were staged and aged following the stages established by Iwamatsu [1]. We observed and recorded the hatching time and deformed and dead embryos continuously. The hatching time was prolonged and the deformed and dead embryos numbers were increased by UVA dose increase. At stage 40, samples from each group were fixed to investigate their morphology and histopathology. Some morphological malformations were recorded after UVA exposure in both strains. Histopathological changes were represented as different shapes of curvature in notochord with collapse. The degree of collapsation was depended on the dose and time of UVA exposure. Our findings show that exposure to UVA irradiation caused less vertebral column curvature in medaka fry. Moreover, p53-deficient embryos were more tolerant than those of wild-type (Hd-rR) Japanese medaka. This study indicated the dangerous effects of the UVA on medaka.

Apoptotic cell death in erythrocytes of p53-deficient medaka (Oryzias latipes) after γ-irradiation

PURPOSE

Previous studies have examined the effects of γ-irradiation (γ-IR) on wild-type and p53 mutant Medaka (Oryzias latipes) 24 hours after irradiation and in the present work, apoptosis and alterations in erythrocytes of 4, 8 and 24 h and 14 days after gamma-ray irradiation were reported as genotoxic biomarkers of γ-irradiation.

MATERIALS AND METHODS

Sexually mature wild-type, WT (Hd-rR) and p53(-/-) adult female medaka (O. latipes) were exposed to 4 Gy dose of γ-IR and sampling were collected after 4, 8 and 24 h and 14 days.

RESULTS

Apoptosis and morphological alterations were observed from 4 h after irradiation and remarkably increased 8 h after irradiation in the wild-type. Apoptotic cell death has been observed 8 h after irradiation most prominently but subtle in p53 mutant medaka. All these phenotypes were recovered 14 days after irradiation in both strains. Although no micronuclei were seen in any group, nuclear abnormalities were observed in red blood cells. Both apoptosis and morphological alterations in erythrocytes were decreased after 24 and 14 days after γ-irradiation.

CONCLUSIONS

We conclude that apoptosis and malformations caused by 4 Gy γ-irradiation in the erythrocytes of medaka fish occurs from 4-24 h and the initial response until 8 h was p53-dependent.

Apoptosis and morphological alterations after UVA irradiation in red blood cells of p53 deficient Japanese medaka (Oryzias latipes)

Morphological alterations in red blood cells were described as hematological bioindicators of UVA exposure to investigate the sensitivity to UVA in wild type Japanese medaka (Oryzias latipes) and a p53 deficient mutant. The fewer abnormal red blood cells were observed in the p53 mutant fish under the control conditions. After exposure to different doses of UVA radiation (15min, 30min and 60min/day for 3days), cellular and nuclear alterations in red blood cells were analyzed in the UVA exposed fish compared with non-exposed controls and those alterations included acanthocytes, cell membrane lysis, swollen cells, teardrop-like cell, hemolyzed cells and sickle cells. Those alterations were increased after the UVA exposure both in wild type and the p53 deficient fish. Moreover, apoptosis analyzed by acridine orange assay showed increased number of apoptosis in red blood cells at the higher UVA exposure dose. No micronuclei but nuclear abnormalities as eccentric nucleus, nuclear budding, deformed nucleus, and bilobed nucleus were observed in each group. These results suggested that UVA exposure induced both p53 dependent and independent apoptosis and morphological alterations in red blood cells but less sensitive to UVA than Wild type in medaka fish.

Irradiation-injured brain tissues can self-renew in the absence of the pivotal tumor suppressor p53 in the medaka (Oryzias latipes) embryo

The tumor suppressor protein, p53, plays pivotal roles in regulating apoptosis and proliferation in the embryonic and adult central nervous system (CNS) following neuronal injuries such as those induced by ionizing radiation. There is increasing evidence that p53 negatively regulates the self-renewal of neural stem cells in the adult murine brain; however, it is still unknown whether p53 is essential for self-renewal in the injured developing CNS. Previously, we demonstrated that the numbers of apoptotic cells in medaka (Oryzias latipes) embryos decreased in the absence of p53 at 12-24 h after irradiation with 10-Gy gamma rays. Here, we used histology to examine the later morphological development of the irradiated medaka brain. In p53-deficient larvae, the embryonic brain possessed similar vacuoles in the brain and retina, although the vacuoles were much smaller and fewer than those found in wild-type embryos. At the time of hatching (6 days after irradiation), no brain abnormality was observed. In contrast, severe disorganized neuronal arrangements were still present in the brain of irradiated wild-type embryos. Our present results demonstrated that self-renewal of the brain tissue completed faster in the absence of p53 than wild type at the time of hatching because p53 reduces the acute severe neural apoptosis induced by irradiation, suggesting that p53 is not essential for tissue self-renewal in developing brain.