Hirohiko Kagawa

Bio: Hirohiko Kagawa is an academic researcher from National Institute for Basic Biology, Japan. The author has contributed to research in topics: Ovulation & Gonadotropin. The author has an hindex of 7, co-authored 8 publications receiving 653 citations.

Relationship between seasonal plasma estradiol-17 beta and testosterone levels and in vitro production by ovarian follicles of amago salmon (Oncorhynchus rhodurus).

Abstract: Plasma estradiol-17 beta and testosterone levels were assessed by radioimmunoassay during the sexual maturation of female amago salmon (Oncorhynchus rhodurus). Estradiol-17 beta levels gradually increased during vitellogenesis (June to September), reached a peak in September (about 16 ng/ml) and rapidly decreased in mature and ovulated fish (about 3-4 ng/ml) in October. The seasonal pattern of plasma testosterone levels lagged behind and followed that of estradiol-17 beta during vitellogenesis, but levels remained high in mature and ovulated fish (90-110 ng/ml). Estradiol-17 beta levels and the gonadosomatic index (GSI) values correlated well during vitellogenesis: GSI values showed a linear increase, and reached a peak (29.9 +/- 1.4) in October. Values were extremely low in ovulated fish (1.2 +/- 0.2). In vitro production of estradiol-17 beta and testosterone by ovarian follicles in response to partially purified chinook salmon gonadotropin (SG-G100) was examined monthly using 18-h incubations. Throughout the vitellogenic period SG-G100 stimulated both estradiol-17 beta and testosterone production: the steroidogenic response of follicles increased from June (about 2 ng/ml estradiol-17 beta; 0.1 ng/ml testosterone) to September (about 10 and 14 ng/ml, respectively). In October full-grown immature follicles which could be induced to mature in vitro by hormone treatment produced large amounts of testosterone (about 130 ng/ml) but not estradiol-17 beta. Postovulatory follicles also produced testosterone but the values were low (10 ng/ml) compared with full-grown immature follicles. Very low levels of estradiol-17 beta were produced by postovulatory follicles.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a decrease in aromatase activity in the ovarian granulosa cells of amago salmon (Oncorhynchus rhodurus) associated with final oocyte maturation.

Abstract: Aromatase activity in the isolated granulosa layers of amago salmon (Oncorhynchus rhodurus) ovarian follicles was examined during the course of vitellogenesis and final oocyte maturation and ovulation. Estradiol-17 beta production by isolated granulosa layers incubated with exogenous testosterone increased during the period of vitellogenesis to reach a peak in late vitellogenesis, and then declined rapidly, in association with the ability of the oocyte to mature in response to partially purified salmon gonadotropin (SG-G100). Extremely low levels of estradiol-17 beta were produced by granulosa layers from follicles which had undergone final oocyte maturation in vivo and by post-ovulatory follicles. SG-G100 had no discernible effect on estradiol-17 beta production. These results are discussed in relation to other studies on the endocrine control of steroidogenesis in this species.

Oocyte maturation in the amago salmon (Oncorhynchus rhodurus): In vitro effects of salmon gonadotropin, steroids, and cyanoketone (an inhibitor of 3β-hydroxy-Δ5-steroid dehydrogenase)

Abstract: The effect of partially purified chinook salmon gonadotropin (SG-G100) and a number of steroids on the induction of germinal vesicle breakdown (GVBD) in amago salmon (Oncorhynchus rhodurus) oocytes (with intact follicle layers) was investigated in vitro. SG-G100 was effective only at the highest concentration tested (1 microgram/ml). 17 alpha,20 beta-Dihydroxy-4-pregnen-3-one (17 alpha,20 beta-diOHprog) was the most potent maturation-inducing steroid tested, followed by 17 alpha-hydroxyprogesterone. Testosterone or deoxycorticosterone (DOC) enhanced the rate of GVBD in response to SG-G100. DOC also enhanced the response to 17 alpha,20 beta-diOHprog but testosterone was without effect, suggesting that DOC has a direct action on the oocyte while testosterone probably acts at the level of the follicle. Estradiol-17 beta had no effect on GVBD in response to SG-G100 or 17 alpha,20 beta-diOHprog. The action of SG-G100 was shown to be dependent on the synthesis of a second delta 4 steroidal mediator of maturation since cyanoketone, a specific inhibitor of 3 beta-hydroxy-delta 5-steroid dehydrogenase, completely abolished the maturational effects of the gonadotropin and pregnenolone but not delta 4 steroids. Radioimmunoassay of media in which oocytes were induced to mature in vitro with SG-G100 revealed significantly elevated levels of progesterone and 17 alpha,20 beta-diOHprog. Estradiol-17 beta levels, high in control media, were only elevated twofold by SG-G100. Levels of the two progestogens were extremely low or nondetectable in media in which oocytes were incubated with cyanoketone, while estradiol-17 beta levels remained high. These results are discussed in relation to other evidence indicating that 17 alpha,20 beta-diOHprog is the naturally occurring maturation-inducing steroid of amago salmon. The role of other steroid hormones, particularly the possible involvement of corticosteroids, in the control of final oocyte maturation in teleosts is explored.

Endocrine regulation of gametogenesis in fish.

Abstract: The pituitary-gonadal axis plays an important role in regulating gametogenesis in vertebrates. In most cases, gonadotropins act through the biosynthesis of gonadal steroid hormones which in turn mediate various stages of gametogenesis. A series of studies in our laboratory using several species of teleost fishes as experimental animals has provided new information about the endocrine regulation of gametogenesis, including oocyte growth, oocyte maturation, spermatogenesis and sperm maturation. This article briefly reviews our findings on the identification of steroidal mediators involved in each process of gametogenesis, and the sites and mechanisms of action of the mediators. These observations collectively demonstrate the appropriateness of using teleost fishes as valid models for examining hormonal influences on gametogenesis. Such models could also have applications and validity for vertebrates in general.

Regulation of oocyte maturation in fish.

Abstract: A period of oocyte growth is followed by a process called oocyte maturation (the resumption of meiosis) which occurs prior to ovulation and is a prerequisite for successful fertilization. Our studies using fish models have revealed that oocyte maturation is a three-step induction process involving gonadotropin (LH), maturation-inducing hormone (MIH), and maturation-promoting factor (MPF). LH acts on the ovarian follicle layer to produce MIH (17α, 20β-dihydroxy-4-pregnen-3-one, 17α, 20β-DP, in most fishes). The interaction of ovarian thecal and granulosa cell layers (two-cell type model), is required for the synthesis of 17α,20β-DP. The dramatic increase in the capacity of postvitellogenic follicles to produce 17α,20β-DP in response to LH is correlated with decreases in P450c17 (P450c17-I) and P450 aromatase (oP450arom) mRNA and increases in the novel form of P450c17 (P450c17-II) and 20β-hydroxysteroid dehydrogenase (20β-HSD) mRNA. Transcription factors such as Ad4BP/SF-1, Foxl2, and CREB may be involved in the regulation of expression of these steroidogenic enzymes. A distinct family of G-protein-coupled membrane-bound MIH receptors has been shown to mediate non-genomic actions of 17α, 20β-DP. The MIH signal induces the de novo synthesis of cyclin B from the stored mRNA, which activates a preexisting 35 kDa cdc2 kinase via phosphorylation of its threonine 161 by cyclin-dependent kinase activating kinase, thus producing the 34 kDa active cdc2 (active MPF). Upon egg activation, MPF is inactivated by degradation of cyclin B. This process is initiated by the 26S proteasome through the first cut in its NH2 terminus at lysine 57.

6 The Functional Morphology of Teleost Gonads

Abstract: Publisher Summary This chapter discusses the functional morphology of teleost gonads. Information presented in this chapter indicates that there are several processes of germ cell development, which are closely associated with changes in cellular activities of somatic cell elements. Recent studies on biochemical aspects of vitellogenesis in teleosts have shown that the hepatic and ovarian yolk proteins are similar to those of amphibian species. An in vitro method involving the separation of the follicular components has facilitated investigations of the detailed mechanism of the production of two major follicular steroid hormones. As a result of the usage of this technique, a two cell-type model involving thecal and granulosa cell layers has been proposed for the production of these two steroids for the first time in lower vertebrates. With further refinements this well characterized incubation procedure should provide an excellent system for studying the molecular basis of the mechanism of gonadotropin action on follicular steroidogenesis.