Role of gap junctions and protein kinase A during the development of oocyte maturational competence in Ayu (Plecoglossus altivelis)

Changes in Ovulation-Related Gene Expression during Induced Ovulation in the Amur Sturgeon (Acipenser schrenckii) Ovarian Follicles

The luteinizing hormone (LH) and maturation-inducing steroids (MIS), such as 17α,20β-dihydroxy-4-pregnen-3-one, regulate the final oocyte maturation in teleosts. Oocyte maturational competence (OMC) and ovulatory competence measure the sensitivity to MIS for oocyte maturation and ovulation, respectively. However, the molecular mechanisms underlying the acquisition of ovulatory competence remain unknown. Sturgeons are an excellent research model for investigating these mechanisms. We examined the seasonal profiles of OMC and ovulatory competence in vitro and the expression of 17 ovulation-related gene candidates using quantitative PCR in Amur sturgeon ovarian follicles. The ovulatory competence was induced by the LH-releasing hormone analog (LHRHa) priming injection after acquiring the OMC, which was spontaneously induced in spring or autumn. Seven genes, including the tissue-type plasminogen activator (plat), were enhanced following the LHRHa priming injection in ovarian follicles sampled from anovulated and ovulated fish. The activin receptor type 1 (acvr1) and prostaglandin G/H synthase 2 (ptgs2) were only upregulated in ovulated fish. Our results suggest that plat/plasmin and prostaglandin (PG)/PG receptor systems are essential for sturgeon ovulation, similar to other vertebrates. Notably, successful ovulation depends on a sufficient PG synthesis, and mediators activating the PG/PG receptor system are essential for acquiring the ovulatory competence. We provide the first report of ovulation-related gene alterations in the ovarian follicles of Amur sturgeons.

Dynamic transcriptome analysis of ovarian follicles in artificial maturing Japanese eel (Anguilla japonica)

Inducing maturation of the ovaries to enable the production of good-quality eggs is critical for the successful artificial breeding of Anguilla japonica. During the spawning season, however, the ovaries of A. japonica have been found to develop into asynchronous clutches, impeding the success of artificial breeding on a commercial scale. The dynamic molecular regulation of follicular development in the same individual was assessed by transcriptome analysis of the five stages of follicles, the pre-vitellogenic, early vitellogenic, midvitellogenic, late vitellogenic, and migratory nucleus stages in artificial maturing A. japonica. Comparisons across these developmental stages identified a total of 19,298 differentially expressed transcripts (DETs). Short time-series expression miner analysis across these DETs revealed four significant expression profiles. Gene Ontology function and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses found that some of the significantly enriched biological processes and metabolic pathways included those related to steroid hormone biosynthesis (cyp11a1, cyp17a1, cyp17a2, hsd17b1, and hsd17b12), cargo receptor activity (vtgr and vldlr), meiosis and ovulation (pgrs and mPRγ), hydration (cts and aqp1), and egg coat formation (zp). These genes and pathways were associated with serum 17β-estradiol concentrations and morphological changes. The levels of hsd17b12 and mPRγ mRNAs were much higher during the migratory nucleus stage, suggesting their respective involvement in the biosynthesis and functional pathway of the maturation-inducing steroid 17α,20β-dihydroxy-4-pregnen-3-one. The gene subtypes aqp1b and ctsd may regulate water influx into oocytes and yolk protein proteolysis, respectively. To our knowledge, the present study is the first to describe combined transcriptome profiling of asynchronously developing follicles in the same individual. The findings suggest that steroid hormone synthesis and nutrient absorption in follicular somatic cells play important roles during follicular development and maturation, despite the same external physiological surroundings.

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.

Endocrine and paracrine regulation of meiotic cell cycle progression in teleost oocytes: cAMP at the centre of complex intra-oocyte signalling events

Participation of major endocrine and/or local autocrine/paracrine factors and potential interplay between apparently disparate intra-oocyte signalling events during maintenance and withdrawal of meiotic prophase arrest has been an area of active research in recent years. Studies on oocyte maturation have contributed substantially in the discovery of some of the most important biochemical and cellular events like functional significance of novel membrane-associated steroid receptors, elucidation of maturation promoting factor (MPF), cytostatic factor (CSF) and other signalling cascades that entrain the cell cycle clock to hormonal stimuli. While follicular estrogen has largely been implicated in maintenance of prophase arrest, involvement of maturational steroid and membrane progestin receptor in resumption of meiotic G2-M1 transition in piscine oocytes has been shown earlier. Moreover, detection of ovarian IGF system, maturational gonadotropin stimulation of IGF ligands and potential synergism between maturational steroid and IGF1 in zebrafish oocytes are most recent advancements. Though endocrine/paracrine regulation of cyclic nucleotide-mediated signalling events in meiotic cell cycle progression is well established, involvement of PI3K/Akt signalling cascade has also been reported in fish, amphibian and mammalian oocytes. The major objective of this overview is to describe how fish oocytes maintain high cAMP/PKA activity and how steroid- and/or growth factor-mediated signalling cascade regulate this pathway for the withdrawal of meiotic arrest. Moreover, special emphasis is placed on some recent findings on interaction of PKA with some of the MPF-regulating components (e.g., synthesis of cyclin B or MEK/MAPK signalling cascade) for the maintenance of prophase arrest.

Molecular cloning of two gonadotropin receptors and their distinct mRNA expression profiles in daily oogenesis of the wrasse Pseudolabrus sieboldi

In fish, asynchronous development of ovarian follicles, the simultaneous advance of vitellogenesis and oocyte maturation in one ovary, is a rational reproductive strategy to spawn consecutively in one spawning season. In this study, to clarify the mode of action of follicle stimulating hormone (FSH) and luteinizing hormone (LH) in asynchronous ovarian follicle development in daily egg production, we cloned cDNAs of the follicle-stimulating hormone receptor (FSHR) and luteinizing hormone receptor (LHR) in the bambooleaf wrasse (Pseudolabrus sieboldi), which exhibits clear diurnal spawning rhythms over 1 month. In addition, different developmental stages of ovarian follicles were isolated from whole ovaries at various daily time points on 1 day in the spawning season, and mRNA expression levels of FSHR and LHR were analyzed. Sequence analysis showed distinct differences in the number of putative leucine-rich repeats at the extracellular domain between FSHR and LHR, suggesting a difference in ligand-specificity. Real-time PCR analyses revealed that FSHR mRNA was highly expressed in early yolk-stage follicles but decreased at the end of vitellogenesis. In contrast, the expression of LHR mRNA was maintained at low levels in vitellogenic stage follicles but markedly elevated at the end of the vitellogenic and early migratory nucleus stages, thereafter markedly dropping in the late migratory nucleus stage. The present results suggest that co-regulation of vitellogenesis and oocyte maturation in one ovary is controlled by the stage-distinctive expression levels of FSHR and LHR mRNA in ovarian follicles, and daily switching of sensitivity from FSH to LH is required for daily egg production.

The spatiotemporal expression of multiple coho salmon ovarian connexin genes and their hormonal regulation in vitro during oogenesis

BACKGROUND

Throughout oogenesis, cell-cell communication via gap junctions (GJs) between oocytes and surrounding follicle cells (theca and granulosa cells), and/or amongst follicle cells is required for successful follicular development. To gain a fundamental understanding of ovarian GJs in teleosts, gene transcripts encoding GJ proteins, connexins (cx), were identified in the coho salmon, Oncorhynchus kisutch, ovary. The spatiotemporal expression of four ovarian cx transcripts was assessed, as well as their potential regulation by follicle-stimulating hormone (FSH), luteinizing hormone (LH) and insulin-like growth factor 1 (IGF1).

METHODS

Salmonid ovarian transcriptomes were mined for cx genes. Four gene transcripts designated cx30.9, cx34.3, cx43.2, and cx44.9 were identified. Changes in gene expression across major stages of oogenesis were determined with real-time, quantitative RT-PCR (qPCR) and cx transcripts were localized to specific ovary cell-types by in situ hybridization. Further, salmon ovarian follicles were cultured with various concentrations of FSH, LH and IGF1 and effects of each hormone on cx gene expression were determined by qPCR.

RESULTS

Transcripts for cx30.9 and cx44.9 were highly expressed at the perinucleolus (PN)-stage and decreased thereafter. In contrast, transcripts for cx34.3 and cx43.2 were low at the PN-stage and increased during later stages of oogenesis, peaking at the mid vitellogenic (VIT)-stage and maturing (MAT)-stage, respectively. In situ hybridization revealed that transcripts for cx34.3 were only detected in granulosa cells, but other cx transcripts were detected in both oocytes and follicle cells. Transcripts for cx30.9 and cx44.9 were down-regulated by FSH and IGF1 at the lipid droplet (LD)-stage, whereas transcripts for cx34.3 were up-regulated by FSH and IGF1 at the LD-stage, and LH and IGF1 at the late VIT-stage. Transcripts for cx43.2 were down-regulated by IGF1 at the late VIT-stage and showed no response to gonadotropins.

CONCLUSION

Our findings demonstrate the presence and hormonal regulation of four different cx transcripts in the salmon ovary. Differences in the spatiotemporal expression profile and hormonal regulation of these cx transcripts likely relate to their different roles during ovarian follicle differentiation and development.

Regulation of membrane progestin receptors in the zebrafish ovary by gonadotropin, activin, TGF-beta and BMP-15

Progestin hormones are vital for inducing oocyte maturation in fish by binding to membrane progestin receptors (mPRs). The aim of this study was to examine the expression and regulation of mPRalpha and mPRbeta in zebrafish follicles. First, defolliculated fully grown oocytes were subjected to immunofluorescent staining using anti-mPRalpha and mPRbeta antibodies, and their expression on the oocyte membrane was confirmed. Second, total protein was collected from zebrafish follicles and Western blotting revealed that the level of mPRalpha and mPRbeta increased with follicle development. We have previously shown that several members of the transforming growth factor-beta (TGF-beta) superfamily, including TGF-beta1, activin-A, and bone morphogenetic protein (BMP)-15, regulate oocyte maturation in zebrafish. Therefore, the third major focus of this study was to test if these growth factors, as well as gonadotropins, regulate the expression of mPRs. Overexpression of BMP-15 significantly reduced, while knockdown of BMP-15 increased, mPRbeta levels. However, mPRalpha expression level remained unchanged with BMP-15 overexpression or knockdown. Treatment of follicles with human chorionic gonadotropin (hCG) resulted in an increased in mPRbeta, but not mPRalpha, expression levels. Activin-A induced the expression of mPRalpha and mPRbeta in a dose- and time-dependent manner. On the other hand, TGF-beta1 treatment suppressed the expression of mPRbeta, but not mPRalpha. Taken together, these findings further support the role of mPRs in oocyte maturation and suggest that gonadotropins, BMP-15, activin-A, and TGF-beta1 exert their regulatory effects on oocyte maturation in part by regulating mPR expression.

Oocyte maturation: converting the zebrafish oocyte to the fertilizable egg

The process of oogenesis culminates in steroid-induced oocyte maturation to produce the fertilizable egg. A quintessential biological entity, the egg is central to the production of new individuals. The result of egg fertilization by a sperm cell is the production of the mother of all stem cells (i.e. the zygote). Furthermore, the egg cytoplasm is the only one known to support reprogramming a transplanted nucleus to give rise to an individual (i.e. animal cloning). Zebrafish oocyte maturation is a complex event encompassing a number of cellular changes including germinal vesicle migration (GVM) and dissolution or breakdown (GVD), ooplasmic clearing (OC) with correlated yolk protein changes (YP), development of osmoregulation (OR) in fresh water, the formation of the future embryonic pole, the blastodisc (BF) and activatibility (AC) or cortical maturation. In zebrafish, and many other teleosts, 17alpha, 20beta-dihydroxy-4-pregnen-3-one (17alpha, 20beta-DP) has been shown to be the normal inducer of oocyte maturation. A 17alpha, 20beta-DP membrane-resident receptor mediates oocyte maturation via non-genomic mechanisms that are beginning to be understood. This paper will highlight some of the cellular markers resulting from the signaling initiated by 17alpha, 20beta-DP. By describing these markers, it is hoped that workers in the field will have additional tools to help further elucidate the signaling events of oocyte maturation.

Ovarian function of the trout preovulatory ovary: new insights from recent gene expression studies

During the preovulatory period the follicle-enclosed oocyte progressively acquires maturational and developmental competence. In addition, the follicle is also preparing for the release of the oocyte from the follicle at ovulation. Using real-time PCR and cDNA microarrays we have investigated the molecular mechanisms of oocyte competence acquisition and ovulation in rainbow trout (Oncorhynchus mykiss) by monitoring gene expression in the preovulatory ovary. These studies have demonstrated that many molecular events related to maturational competence and developmental competence acquisition, and ovulation occur concomitantly in the preovulatory ovarian follicle. Oocyte maturational competence acquisition is associated with a decrease of estrogen synthesis and signaling capacities. We also observed a differential expression of genes encoding for igfs and related binding protein, members of the TGF beta superfamily, proteins involved in ion and water transport, bone morphogenetic proteins, and cathepsins. In addition, our observation of a strong up-regulation, prior to ovulation, of genes encoding for proteins putatively involved in proteolysis, inflammation, coagulation, vasodilatation, and angiogenesis further supports the hypothesis comparing ovulation with an inflammatory-like reaction. Together, our results suggest that a finely tuned cross-talk exists between oocyte and follicular layers and between the ovulatory process and the oocyte maturational and developmental competence acquisition processes.

Heterologous gap junctions between granulosa cells and oocytes in ayu (Plecoglossus altivelis): formation and role during luteinizing hormone-dependent acquisition of oocyte maturational competence

Gap junctions (GJs) are intercellular channels. Molecules with a molecular weight of 1 kDa or less can pass back and forth between adjacent cells through GJs. Communication between oocytes and the somatic cells that surround them via GJs is known to play key roles to initiate oocyte maturation in many vertebrates. However, little is known of the detailed functions of ovarian GJs during oocyte maturation in fish. The oocyte maturation of fish is induced by a maturation-inducing steroid (MIS). The sensitivity of oocytes to the MIS is known as oocyte maturational competence (OMC) and is induced by luteinizing hormone (LH). However, LH receptors are found on the surface of granulosa cells rather than oocytes. We therefore proposed that the LH signals received by granulosa cells were passed to oocytes via GJs. This review describes current knowledge of the role of GJs between granulosa cells and oocytes during the LH-induced acquisition of OMC in fish.