Inhibition of medaka ovulation by gap junction blockers due to its disrupting effect on the transcriptional process of LH-induced Mmp15 expression.

Using medaka, we found that in vitro follicle ovulation, but not germinal vesicle breakdown, was inhibited by three gap junction blockers, carbenoxolone, mefloquine, and flufenamic acid. The blockers specifically inhibited follicular expression of matrix metalloproteinase-15 mRNA and the protein (mmp15/Mmp15), a protease indispensable for medaka ovulation, indicating that gap junctional communication may be required for successful ovulation and mmp15/Mmp15 expression. Further experiments using carbenoxolone as the representative of the gap junction blockers showed that expression of nuclear progestin receptor (Pgr), a transcription factor required for mmp15 expression, was not affected by carbenoxolone treatment, but the formation of phosphorylated Pgr was considerably suppressed. Carbenoxolone treatment caused a decrease in the Pgr binding to the promoter region of mmp15. mRNA expression of cyclin-dependent protein kinase-9 (cdk9) and cyclin I (ccni), whose translation products are demonstrated to be involved in Pgr phosphorylation in the medaka ovulating follicles, was suppressed by carbenoxolone treatment. Transcripts of connexin 34.5 (cx34.5) and connexin 35.4 (cx35.4) were dominantly expressed in the follicle cells of ovulating follicles. The results indicate that gap junctional communication plays an important role in medaka ovulation.

Follicle rupture during ovulation with an emphasis on recent progress in fish models.

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 ß, 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.

Prostaglandins in teleost ovulation: A review of the roles with a view to comparison with prostaglandins in mammalian ovulation.

Prostaglandins are well known to be central regulators of vertebrate ovulation. Studies addressing the role of prostaglandins in mammalian ovulation have established that they are involved in the processes of oocyte maturation and cumulus oocyte complex expansion. In contrast, despite the first indication of the role of prostaglandins in teleost ovulation appearing 40 years ago, the mechanistic background of their role has long been unknown. However, studies conducted on medaka over the past decade have provided valuable information. Emerging evidence indicates an indispensable role of prostaglandin E2 and its receptor subtype Ptger4b in the process of follicle rupture. In this review, we summarize studies addressing the role of prostaglandins in teleost ovulation and describe recent advances. To help understand differences from and similarities to ovulation in mammalian species, the findings on the roles of prostaglandins in mammalian ovulation are discussed in parallel.

Cell division and cadherin-mediated adhesion regulate lens epithelial cell movement in zebrafish.

In vertebrates, lens epithelial cells cover the anterior half of the lens fiber core. During development, lens epithelial cells proliferate, move posteriorly and differentiate into lens fiber cells after passing through the equator. To elucidate the mechanisms underlying lens epithelial cell movement, we conducted time-lapse imaging of zebrafish lens epithelium. Lens epithelial cells do not intermingle but maintain their relative positions during development. Cell division induces epithelial rearrangement, which subsequently promotes cell movement towards the equator. These data suggest that cell division is the major driving force for cell movement. In zebrafish, E-cadherin is expressed in lens epithelium, whereas N-cadherin is required for lens fiber growth. E-cadherin reduced lens epithelial cell movement, whereas N-cadherin enhanced it. Laser ablation experiments revealed that lens epithelium is governed by pulling tension, which is modulated by these cadherins. Thus, cell division and cadherin-mediated adhesion regulate lens epithelial cell movement via modulation of epithelial tension.

Expression of Membrane Progestin Receptors (mPRs) in Granulosa Cells of Medaka Preovulatory Follicles.

Membrane progestin receptor (mPR) α on the cell membrane of the oocyte is involved in the meiotic maturation of vertebrates, including teleosts, but little is known about the role of this membrane-bound follicular receptor. We investigated the ovarian expression of membrane progestin receptor (mPR) mRNA in medaka. In follicles that were destined to ovulate, transcripts of mPRα and mPRγ were expressed in the oocytes as well as the granulosa cells. Transcripts of mPRα and mPRγ were expressed at relatively constant levels in the whole ovary and in the preovulatory follicles throughout the 24-h spawning cycle. In vitro incubation of the preovulatory follicles with recombinant medaka luteinizing hormone caused no significant changes in the expression of mPRα and mPRγ mRNA, suggesting LH-independent follicular expression of these mPR genes. Using HEK293T cells expressing medaka mPRs, forskolin-elevated intracellular cAMP levels were found to be reduced on treatment of the cells with ligand 17α, 20ß-dihydroxy-4-pregnen-3-one (DHP), but only in the cells expressing mPRα. These results indicate that activation of mPRα and mPRγ with DHP may cause differential effects on the granulosa cells. Information obtained from the present study may help to elucidate the role of mPRα and mPRγ in the granulosa cells of the follicles.

The role of urokinase plasminogen activator and plasminogen activator inhibitor-1 in follicle rupture during ovulation in the teleost medaka.

We previously reported that the serine protease plasmin plays a role in follicle rupture during ovulation in the teleost medaka. In this study, we showed that urokinase-type plasminogen activator 1 (Plau1) is a physiological activator of plasminogen. Morphological analyses revealed that in the preovulatory follicle, plau1 mRNA was detected in association with follicle cells, while Plau1 protein was localized in the oocyte egg membrane. Both an inactive precursor and an active form of Plau1 were present at constant levels in the membrane fraction via the latter half of the 24-h spawning cycle. Plasminogen activator inhibitor-1 (Pai1) was detected in the follicle layer of the preovulatory follicle, but the protein level was low at approximately 7 h prior to ovulation. We showed that plasmin hydrolyzed laminin, which is a major component of the basement membrane and is situated between the granulosa and theca cells of the follicle. In vitro ovulation of large follicles was significantly inhibited by anti-Plau1 antibodies and active recombinant Pai1. Levels of Pai1 expression were increased in vivo at approximately 7 h prior to ovulation. Expression of Pai1 was also induced in vitro in the follicle with recombinant medaka luteinizing hormone (Lh). Lh-induced expression of pail mRNA was significantly suppressed by the presence of MDL (an adenylyl cyclase inhibitor), trilostane (a 3beta-hydroxysteroid dehydrogenase inhibitor), and RU486 (a nuclear progestin receptor antagonist). These results support our recent proposal of a sequential two-step ECM protein hydrolysis model for follicle rupture for medaka ovulation.

Luteinizing hormone-induced expression of Ptger4b, a prostaglandin E2 receptor indispensable for ovulation of the medaka Oryzias latipes, is regulated by a genomic mechanism involving nuclear progestin receptor.

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.

Recent advances in the understanding of teleost medaka ovulation: the roles of proteases and prostaglandins.

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.

New evidence for the involvement of prostaglandin receptor EP4b in ovulation of the medaka, Oryzias latipes.

A cDNA for a prostaglandin E(2) (PGE(2)) receptor subtype 4, EP4b (Ptger4b), was cloned from the medaka ovary. The effect of PGE(2) was examined using COS-7 cells expressing the recombinant Ptger4b protein. An increase in intracellular cAMP levels was observed when the cells were incubated with PGE(2), but the increase in cAMP levels was nullified by the addition of the EP4 antagonist GW627368X. The expression of ptger4b mRNA was drastically induced by the addition of pregnant mare serum gonadotropin to the in vitro culture of large preovulatory follicles. In in vitro ovulation studies of the effect of GW627368X addition on follicle ovulation, the critical timing of the PGE(2)/Ptger4b interaction was suggested to be between -1 and 0 h of ovulation. These results further substantiate that PGE(2)/Ptger4b signaling is involved in follicle rupture during ovulation in the medaka ovary.

Expression of cyclooxygenase-2 and prostaglandin receptor EP4b mRNA in the ovary of the medaka fish, Oryzias latipes: possible involvement in ovulation.

In vitro ovulation of mature medaka ovarian follicles was inhibited by inhibitors of cyclooxygenase (COX) or by an antagonist of the prostaglandin E(2) receptor (EP). Of the three medaka COX genes, ptgs2 was most dominantly expressed in the fish ovary. The ptgs2 transcript was detected in all sizes of growing follicles. In a 24-h spawning cycle, large-sized follicles contained ptgs2 mRNA at a fairly constant level. The levels of COX enzyme activity and prostaglandin E(2) were also constant in the large-sized follicles during the spawning cycle. The expression of prostaglandin E(2) receptor EP4b (ptger4b) mRNA was drastically upregulated in the large-sized follicles as the ovulation time approached. The current results indicate that prostaglandin E(2), which might be produced by COX-2, is involved in the ovulation of medaka, and that EP4b is likely the receptor responsible for exerting the action of prostaglandin E(2) in the process.