Recent Progress in the Generation of Transgenic Medaka (Oryzias latipes)

Cholesterol accumulation in ovarian follicles causes ovulation defects in Abca1a -/- Japanese medaka (Oryzias latipes)

ATP-binding cassette A1 (ABCA1) is a membrane protein, which exports excess cellular cholesterol to generate HDL to reduce the risk of the onset of cardiovascular diseases (CVD). In addition, ABCA1 exerts pleiotropic effects on such as inflammation, tissue repair, and cell proliferation and migration. In this study, we explored the novel physiological roles of ABCA1 using Japanese medaka (Oryzias latipes), a small teleost fish. Three Abca1 genes were found in the medaka genome. ABCA1A and ABCA1C exported cholesterol to generate nascent HDL as human ABCA1 when expressed in HEK293 cells. To investigate their physiological roles, each Abca1-deficient fish was generated using the CRISPR-Cas9 system. Abca1a ^-/- female medaka was found to be infertile, while Abca1b ^-/- and Abca1c ^-/- female medaka were fertile. In vitro ovarian follicle culture suggested that Abca1a deficiency causes ovulation defects. In the ovary, ABCA1A was expressed in theca cells, an outermost layer of the ovarian follicle. Total cholesterol content of Abca1a ^-/- ovary was significantly higher than that of the wild-type, while estrogen and progestin contents were compatible with those of the wild-type. Furthermore, cholesterol loading to the wild-type follicles caused ovulation defects. These results suggest that ABCA1A in theca cells regulates cholesterol content in the ovarian follicles and its deficiency inhibits successful ovulation through cholesterol accumulation in the ovarian follicle.

The genomic and genetic toolbox of the teleost medaka (Oryzias latipes)

The Japanese medaka, Oryzias latipes, is a vertebrate teleost model with a long history of genetic research. A number of unique features and established resources distinguish medaka from other vertebrate model systems. A large number of laboratory strains from different locations are available. Due to a high tolerance to inbreeding, many highly inbred strains have been established, thus providing a rich resource for genetic studies. Furthermore, closely related species native to different habitats in Southeast Asia permit comparative evolutionary studies. The transparency of embryos, larvae, and juveniles allows a detailed in vivo analysis of development. New tools to study diverse aspects of medaka biology are constantly being generated. Thus, medaka has become an important vertebrate model organism to study development, behavior, and physiology. In this review, we provide a comprehensive overview of established genetic and molecular-genetic tools that render medaka fish a full-fledged vertebrate system.

Asian medaka fishes offer new models for studying mechanisms of seawater adaptation

Japanese medaka (Oryzias latipes) is a freshwater (FW) teleost that is popular throughout the world for laboratory use. In this paper, we discuss the utility of Japanese medaka and related species for studying mechanisms of seawater (SW) adaptation. In addition to general advantages as an experimental animal such as their daily spawning activity, transparency of embryos, short generation time and established transgenic techniques, Japanese medaka have some adaptability to SW unlike the strictly stenohaline zebrafish (Danio rerio). Since other species in the genus Oryzias exhibit different degrees of adaptability to SW, comparative studies between Japanese medaka, where molecular-biological and genetic information is abundant, and other Oryzias species are expected to present varying approaches to solving the problems of SW adaptation. We introduce some examples of interspecies comparison for SW adaptabilities both in adult fish and in embryos. Oryzias species are good models for evolutionary, ecological and zoogeographical studies and a relationship between SW adaptability and geographic distribution has been suggested. Medaka fishes may thus deliver new insights into our understanding of how fish have expanded their distribution to a wide variety of osmotic environments.

Development and gene expression of nuclear transplants generated by transplantation of cultured cell nuclei into non-enucleated eggs in the medaka Oryzias latipes

To develop nuclear transplantation techniques for the medaka Oryzias latipes, nuclei of cultured cells from transgenic fish were transplanted into unfertilized eggs of the orange-red variety of O. latipes, without enucleation, in two experimental series. In the first experimental series, fibroblast cells cultured from the adult caudal fin were used as donors, which carried the green fluorescent protein (GFP) gene driven by the promoter of the medaka elongation factor 1alpha-A gene. Wild-type body color was another donor genetic marker used in this experimental series. In the second experimental series, cells cultured from 6-day-old embryos were used as donors, which carried the GFP genetic marker driven by the promoter of the medaka beta-actin gene. From more than 1000 eggs transplanted in each experiment, a considerable number of nuclear transplants developed to various embryonic stages showing stage- and tissue-specific expression of the donor genetic markers, although the expression was mosaic in many cases. Three and six of the transplanted eggs in the first and second experimental series (0.3 and 0.5%, respectively) hatched, and the hatchlings expressing the genetic markers survived for up to 3 weeks. The chromosome number varied among cells in a single transplant embryo. The results obtained in these experiments may help future cloning efforts in fish.

Estrogen inhibits development of yolk veins and causes blood clotting in transgenic medaka fish overexpressing estrogen receptor

We established three transgenic medaka fish lines overexpressing the medaka estrogen receptor under the constitutive medaka beta-actin promoter. The transgenic embryos became hypersensitive to estrogens (17 beta-estradiol and 17alpha-ethinylestradiol), and failed to develop yolk veins while blood clots formed in the blood island within 3 days after exposure to the estrogens. The embryos developed normally if exposed to estrogen after an early neurula stage, suggesting that the sensitive stage is before neurulation. The developmental defects were recovered by incubation with an anti-estrogen, tamoxifen. These results indicate that activation of estrogen receptor caused the estrogen-induced developmental defects. Our results show that the transgenic embryos can be used to assay the blood clotting activity of estrogenic compounds in vivo.

I-SceI meganuclease mediates highly efficient transgenesis in fish

The widespread use of fish as model systems is still limited by the mosaic distribution of cells transiently expressing transgenes leading to a low frequency of transgenic fish. Here we present a strategy that overcomes this problem. Transgenes of interest were flanked by two I-SceI meganuclease recognition sites, and co-injected together with the I-SceI meganuclease enzyme into medaka embryos (Oryzias latipes) at the one-cell stage. First, the promoter dependent expression was strongly enhanced. Already in F0, 76% of the embryos exhibited uniform promoter dependent expression compared to 26% when injections were performed without meganuclease. Second, the transgenesis frequency was raised to 30.5%. Even more striking was the increase in the germline transmission rate. Whereas in standard protocols it does not exceed a few percent, the number of transgenic F1 offspring of an identified founder fish reached the optimum of 50% in most lines resulting from meganuclease co-injection. Southern blot analysis showed that the individual integration loci contain only one or few copies of the transgene in tandem. At a lower rate this method also leads to enhancer trapping effects, novel patterns that are likely due to the integration of the transgene in the vicinity of enhancer elements. Meganuclease co-injection thus provides a simple and highly efficient tool to improve transgenesis by microinjection.