Toward a molecular equivalent dose: use of the medaka model in comparative risk assessment

Sequential histological changes in the liver of medaka exposed to methylazoxymethaol acetate

We performed a medaka bioassay for the carcinogenicity of methylazoxymethaol acetate (MAM-Ac) to examine the sequential histological changes in the liver from 3 days after exposure until tumor development. The medaka were exposed to MAM-Ac at a concentration of 2 ppm for 24 hours, and were necropsied at 3, 7, 10, 14, 21, 28, 35, 42, 49, 60, and 91 days after exposure. MAM-Ac induced four cases of hepatocellular adenoma and one case of hepatocellular carcinoma in 8 fish after 60 or 91 days of exposure. Histological changes in the liver until tumor development were divided into three phases. In the cytotoxic phase (1–10 days), MAM-Ac-exposed hepatocytes showed vacuolar degeneration and underwent necrosis and apoptosis, resulting in multiple foci of hepatocyte loss. In the repopulation phase (14–35 days), the areas of hepatocyte loss were filled with hepatic cysts and the remaining hepatocytes were surrounded by hepatic stellate-like cells (or spindle cells) and gradually disappeared. In the proliferation phase (42–91 days), the original hepatic parenchyma was regenerated and progressively replaced by regenerative hyperplastic nodules and/or liver neoplasms. The medaka retained a strong hepatocyte regenerative ability in response to liver injury. It is considered that this ability promotes the proliferation of initiated hepatocytes in multistep carcinogenesis and influences the development of liver tumor over a short period in medaka.

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.

Transgenic rat models for mutagenesis and carcinogenesis

Rats are a standard experimental animal for cancer bioassay and toxicological research for chemicals. Although the genetic analyses were behind mice, rats have been more frequently used for toxicological research than mice. This is partly because they live longer than mice and induce a wider variety of tumors, which are morphologically similar to those in humans. The body mass is larger than mice, which enables to take samples from organs for studies on pharmacokinetics or toxicokinetics. In addition, there are a number of chemicals that exhibit marked species differences in the carcinogenicity. These compounds are carcinogenic in rats but not in mice. Such examples are aflatoxin B1 and tamoxifen, both are carcinogenic to humans. Therefore, negative mutagenic/carcinogenic responses in mice do not guarantee that the chemical is not mutagenic/carcinogenic to rats or perhaps to humans. To facilitate research on in vivo mutagenesis and carcinogenesis, several transgenic rat models have been established. In general, the transgenic rats for mutagenesis are treated with chemicals longer than transgenic mice for more exact examination of the relationship between mutagenesis and carcinogenesis. Transgenic rat models for carcinogenesis are engineered mostly to understand mechanisms underlying chemical carcinogenesis. Here, we review papers dealing with the transgenic rat models for mutagenesis and carcinogenesis, and discuss the future perspective.

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.

Neoplastic and nonneoplastic liver lesions induced by dimethylnitrosamine in Japanese medaka fish

Small fish models have been used for decades in carcinogenicity testing. Demonstration of common morphological changes associated with specific mechanisms is a clear avenue by which data can be compared across divergent phyletic levels. Dimethylnitrosamine, used in rats to model human alcoholic cirrhosis and hepatic neoplasia, is also a potent hepatotoxin and carcinogen in fish. We recently reported some striking differences in the mutagenicity of DMN in lambda cII transgenic medaka fish vs. Big Blue(®) rats, but the pre-neoplastic and neoplastic commonalities between the two models are largely unknown. Here, we focus on these commonalities, with special emphasis on the TGF-β pathway and its corresponding role in DMN-induced hepatic neoplasia. Similar to mammals, hepatocellular necrosis, regeneration, and dysplasia; hepatic stellate cell and "spindle cell" proliferation; hepatocellular and biliary carcinomas; and TGF-β1 expression by dysplastic hepatocytes all occurred in DMN-exposed medaka. Positive TGF-β1 staining increased with increasing DMN exposure in bile preductular epithelial cells, intermediate cells, immature hepatocytes and fewer mature hepatocytes. Muscle specific actin identified hepatic stellate cells in DMN-exposed fish. Additional mechanistic comparisons between animal models at different phyletic levels will continue to facilitate the interspecies extrapolations that are so critical to toxicological risk assessments.

Quantifiable biomarkers of normal aging in the Japanese medaka fish (Oryzias latipes)

Background

Small laboratory fish share many anatomical and histological characteristics with other vertebrates, yet can be maintained in large numbers at low cost for lifetime studies. Here we characterize biomarkers associated with normal aging in the Japanese medaka (Oryzias latipes), a species that has been widely used in toxicology studies and has potential utility as a model organism for experimental aging research.

Principal Findings

The median lifespan of medaka was approximately 22 months under laboratory conditions. We performed quantitative histological analysis of tissues from age-grouped individuals representing young adults (6 months old), mature adults (16 months old), and adults that had survived beyond the median lifespan (24 months). Livers of 24-month old individuals showed extensive morphologic changes, including spongiosis hepatis, steatosis, ballooning degeneration, inflammation, and nuclear pyknosis. There were also phagolysosomes, vacuoles, and residual bodies in parenchymal cells and congestion of sinusoidal vessels. Livers of aged individuals were characterized by increases in lipofuscin deposits and in the number of TUNEL-positive apoptotic cells. Some of these degenerative characteristics were seen, to a lesser extent, in the livers of 16-month old individuals, but not in 6-month old individuals. The basal layer of the dermis showed an age-dependent decline in the number of dividing cells and an increase in senescence-associated β-galactosidase. The hearts of aged individuals were characterized by fibrosis and lipofuscin deposition. There was also a loss of pigmented cells from the retinal epithelium. By contrast, age-associated changes were not apparent in skeletal muscle, the ocular lens, or the brain.

Significance

The results provide a set of markers that can be used to trace the process of normal tissue aging in medaka and to evaluate the effect of environmental stressors.

A novel Escherichia coli-derived mutation detected with the Big Blue cII mutant selectable assay

Transgenic mouse mutation detection systems allow investigation of the origins and mechanisms of mutation associated with exogenous and endogenous mutagen exposures in individual tissues and cell types. In the past, selection assays for transgenic mutants have been contaminated with nonmurine-derived mutations and assay validation is critical to ensure murine in vivo origins of mutations. This is critical in studies of spontaneous mutations and extrapolation to endogenous mammalian genes. Herein, we provide one measure of the contribution of Escherichia coli (E. coli)-derived mutations to the Big Blue(R) cII transgene mutant selection assay. We report the first direct evidence of an E. coli-derived cII mutation identified among mutations recovered in the cII selective assay. An E. coli transposable 5 (Tn5) element IS50R inverted repeat (1,534 bp) was identified at base pair 414 in the cII transgene and the insertion generated a 9 bp target site duplication typical of this type of transposition. The bacterial transposition occurred only once in the assay of 25 x 10(6) plaque forming units and sequencing of 1,177 cII mutants. The observed frequency of this type of mutation is 4 x 10(-8) in retrieved lambda phage and 8.5 x 10(-4) in harvested cII mutants and thus a very rare occurrence in typical analyses of spontaneous in vivo mutations. Given that the frequency of transposition is equal to, or an order of magnitude higher, than the frequency of point mutations in E. coli, this article provides excellent validation for the murine origins of mutations detected using the cII mutant selection assay.