We previously reported that the prostaglandin E2 receptor subtype Ptger4b plays a role in ovulation in a teleost species, medaka and that ptger4b mRNA is drastically induced in preovulatory follicles prior to ovulation. The present study focuses on the hormonal regulation of ptger4b mRNA expression using this nonmammalian vertebrate model. Preovulatory follicles that had not been exposed to luteinizing hormone (Lh) in vivo were incubated in vitro with medaka recombinant Lh (rLh), which induced the ptger4b mRNA expression. The addition of trilostane, an inhibitor of 3beta-hydroxysteroid dehydrogenase, strongly inhibited rLh-induced ptger4b expression, and trilostane-suppressed ptger4b expression was restored to the level observed in rLh-treated follicles when 17alpha, 20beta-dihydroxy-4-pregnen-3-one was included in the culture. We determined that the expression of the progestin-activated transcription factor nuclear progestin receptor (Pgr) was also induced by medaka rLh in the follicle and that its expression preceded ptger4b expression. Forskolin treatment induced both pgr and ptger4b mRNA expression in the follicle. Follicular ptger4b mRNA expression was drastically suppressed by RU486, which was demonstrated to compete with 17alpha, 20beta-dihydroxy-4-pregnen-3-one for medaka Pgr in vitro, suggesting a role for Pgr in the expression of ptger4b mRNA. A chromatin immunoprecipitation assay with preovulatory follicles isolated from spawning medaka ovaries demonstrated direct binding of Pgr to the ptger4b promoter. These results indicate that ptger4b expression is regulated by a genomic mechanism involving Pgr.

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Ovulation is the process of liberating oocytes from the preovulatory follicles, and is observed in the ovaries of virtually all female vertebrate animals. Compared with mammalian species, there have been far fewer studies that address the ovulatory mechanisms of non-mammalian species. We have examined the molecular mechanism of follicle rupture during ovulation using the teleost model, medaka, or Oryzias latipes. Follicle rupture in medaka ovulation involves the cooperation of the tissue inhibitor of metalloproteinase-2b protein with at least three matrix metalloproteinases (MMP): membrane type-1 MMP (MT1-MMP), MT2-MMP, and gelatinase A. Our studies also indicate that the serine protease, i.e., plasmin, participates in the rupture for only a few hours prior to the activation of MMP-mediated hydrolysis at ovulation. The involvement of prostaglandin E2 (PGE2) in medaka ovulation was also demonstrated. Cyclooxygenase-2 and PGE2 receptor subtype EP4b were respectively shown to be an enzyme responsible for PGE2 synthesis and a receptor for the generated ligand in the preovulatory follicles. Based on the results obtained from our studies of fish, we discuss the similarities and differences in vertebrate ovulation compared with mammalian species.

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Ovulation, which is induced by the ovulatory luteinizing hormone (LH) surge, is a dynamic process that results in a discharge of one or more fertilizable oocytes from the ovarian follicle into the ovarian cavity or into the abdominal cavity. Follicle rupture is a core event of the ovulatory process and has been the subject of intensive investigation. Many studies have been performed in various vertebrate animals that focused on proteolysis during ovulation. Despite much effort, the proteases responsible for follicle rupture in ovulation have not yet been identified for mammalian species. However, studies conducted using the teleost medaka have recently provided valuable information about the follicle rupture process. Follicle rupture during medaka ovulation is accomplished by a two-step extracellular matrix (ECM) hydrolysis mechanism involving two distinct protease systems, the urokinase-type plasminogen activator-1 /plasmin and the matrix metalloproteinase system. In the 24-h spawning cycle of the fish, the former protease system is activated first, and the latter subsequently becomes active. Proteolytic activities of these systems are regulated by their intrinsic inhibitors. The endocrine regulation of the rupture was examined by investigating the expression of matrix metalloproteinase 15 (Mmp15), which is the only LH-inducible protease among those involved in the rupture process. At least two transcription factors, classical nuclear progestin receptor and CCAAT/enhancer-binding protein β, play critical roles in the expression of the protease transcript. This review also summarizes studies addressing follicle rupture during ovulation conducted using other teleost models to understand the current status of teleost ovulation studies.