Involvement of phosphatidylinositol 3 kinase in the progesterone-induced oocyte maturation in Rana dybowskii

In vitro induction of catfish, Clarias batrachus, oocyte maturation by conspecific vitellogenin 1 (CFVg1)

Present study demonstrates that conspecific vitellogenin1 (CFVg1) induces oocyte maturation in the catfish, Clarias batrachus. CFVg1 is able to develop fertilizable eggs in the Clarias batrachus. Therefore, different in vitro oocyte culture experiments were designed to see whether CFVg1 has efficacy of oocyte maturation and its pathway. In in vitro oocyte culture experiment, CFVg1 showed a dose- and time-dependent response and 64% maturation was obtained at the dose level of 10 µg/ml or more. CFVg1 induction of oocyte maturation was confirmed by co-incubating CFVg1 with CFVg1-antiserum (a-CFVg1), which inhibited the CFVg1-induced oocyte maturation. To answer issues lead to the understanding of the mechanism of vitellogenin (Vg) on oocyte maturation, trypsin digested CFVg1 and Indian major carp Cirhinus mrigala Vg HAI (Hydroxy appetite peak I) also showed significant level of maturation. Actinomycin-D and cycloheximide blocked the effect of CFVg1, indicating that CFVg1 acts through transcription and translation. Theophylline, the phosphodiesterase inhibitor, and cAMP also inhibited the stimulatory effect of CFVg1 on oocyte maturation, indicating indirectly that CFVg1-induced oocyte maturation by decreasing the intracellular cAMP possibly by activating the phosphodiesterase enzyme. Trilostane, the 3β-HSD-blocker, did not inhibit the CFVg1-induced oocyte maturation but wortmannin and Ly294002 two mechanistically different specific inhibitors of PI3 kinase blocked the oocyte maturation. The results thus indicate that oocyte maturation in catfish by Vg may be regulated by two pathways: (1) through decreasing the intraoocyte cAMP level by activating the cAMP-PKA pathway and (2) by cAMP-dependent PI3K/Akt pathway. Therefore, there might be role of vitellogenin itself in initiation of oocyte maturation.

Regulation of recombinant human insulin-induced maturational events in Clarias batrachus (L.) oocytes in vitro

Regulation of insulin-mediated resumption of meiotic maturation in catfish oocytes was investigated. Insulin stimulation of post-vitellogenic oocytes promotes the synthesis of cyclin B, histone H1 kinase activation and a germinal vesicle breakdown (GVBD) response in a dose-dependent and duration-dependent manner. The PI3K inhibitor wortmannin abrogates recombinant human (rh)-insulin action on histone H1 kinase activation and meiotic G2–M1 transition in denuded and follicle-enclosed oocytes in vitro. While the translational inhibitor cycloheximide attenuates rh-insulin action, priming with transcriptional blocker actinomycin D prevents insulin-stimulated maturational response appreciably, albeit in low amounts. Compared with rh-insulin, human chorionic gonadotrophin (hCG) stimulation of follicle-enclosed oocytes in vitro triggers a sharp increase in 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DHP) secreted in the incubation medium at 12 h. Interestingly, the insulin, but not the hCG-induced, maturational response shows less susceptibility to steroidogenesis inhibitors, trilostane or dl-aminoglutethimide. In addition, priming with phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX) or cell-permeable dbcAMP or adenylyl cyclase activator forskolin reverses the action of insulin on meiotic G2–M1 transition. Conversely, the adenylyl cyclase inhibitor, SQ 22536, or PKA inhibitor H89 promotes the resumption of meiosis alone and further potentiates the GVBD response in the presence of rh-insulin. Furthermore, insulin-mediated meiotic maturation involves the down-regulation of endogenous protein kinase A (PKA) activity in a manner sensitive to PI3K activation, suggesting potential involvement of a cross-talk between cAMP/PKA and insulin-mediated signalling cascade in catfish oocytes in vitro. Taken together, these results suggest that rh-insulin regulation of the maturational response in C. batrachus oocytes involves down-regulation of PKA, synthesis of cyclin B, and histone H1 kinase activation and demonstrates reduced sensitivity to steroidogenesis and transcriptional inhibition.

High cAMP attenuation of insulin-stimulated meiotic G2-M1 transition in zebrafish oocytes: interaction between the cAMP-dependent protein kinase (PKA) and the MAPK3/1 pathways

High intra-cellular cyclic nucleotide (cAMP) ensures prophase-I arrest and prevent steroid-induced meiotic G2-M1 transition in full-grown oocytes; however, relatively less information is available for cAMP regulation of growth factor-stimulated signalling events in the oocyte model. Here using zebrafish oocytes, we show that priming with dibutyryl cAMP (dbcAMP) or cAMP modulators, e.g. adenylate cyclase activator, forskolin or phosphodiesterase inhibitors (IBMX/cilostamide) block insulin action on germinal vesicle breakdown (GVBD) and histone H1 kinase activation. Though high cAMP priming attenuates insulin-induced MAPK3/1 (ERK1/2) phosphorylation (activation), following 2h of insulin stimulation it fails to block MAPK activation and GVBD. Further, insulin stimulation promotes down regulation of phospho-PKAc (inactivation) and PKA inhibition by H89/PKI-(6-22)-amide overcomes negative regulation by cAMP and induces GVBD and MAPK activation. Moreover, MEK1/2 inhibitor U0126 has no influence on H89-induced GVBD; however, it delays GVBD response in insulin-stimulated oocytes. MAPK activation by okadaic acid (OA) promotes GVBD; however, high dbcAMP abrogates OA action suggesting cross-talk between cAMP/PKA and MAPK-mediated signalling pathways may contribute significantly in maturing zebrafish oocyte.

Steroid-induced oocyte maturation in Indian shad Tenualosa ilisha (Hamilton, 1822) is dependent on phosphatidylinositol 3 kinase but not MAP kinase activation

Fully grown fish and amphibian oocytes exposed to a maturation-inducing steroid (MIS) activates multiple signal transduction pathways, leading to formation and activation of maturation-promoting factor (MPF) and induction of germinal vesicle breakdown (GVBD). The present study was to investigate if phosphatidylinositol 3 kinase (PI3 kinase) and mitogen-activated protein kinase (MAP kinase) activation are required for naturally occurring MIS, 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P)-induced cdc2 activation and oocyte maturation (OM) in Tenualosa ilisha. We observed that 17,20β-P-induced OM was significantly inhibited by PI3 kinase inhibitors Wortmannin and LY29400. 17,20 β-P was shown to activate PI3 kinase maximally at 90 min and cdc2 kinase at 16 h of treatment. Relative involvement of PI3 kinase, MAP kinase and cdc2 kinase in 17,20β-P-induced OM was examined. MAP kinase was rapidly phosphorylated and activated (60-120 min) after MIS treatment and this response preceded the activation of cdc2 kinase by several hours. A selective inhibitor of MAP kinase (MEK), PD98059, sufficiently blocked the phosphorylation and activation of MAP kinase. Inhibition of MAP kinase activity using PD98059 however, had no effect on MIS-induced cdc2 kinase activation and GVBD. These results demonstrate that activation of the PI3 kinase is required for 17,20β-P-induced cdc2 kinase activation and OM in T. ilisha. MAP kinase although was activated in response to 17,20β-P and PI3 kinase activation, it is not necessary for cdc2 activation and OM in this species.

Participation of PI3-kinase/Akt signalling in insulin stimulation of p34cdc2 activation in zebrafish oocyte: phosphodiesterase 3 as a potential downstream target

Exposure of fully grown oocytes to growth factors (insulin/IGFs) initiates various signalling cascades that culminate to final stages of oocyte maturation. Regulation of signalling pathways during growth factor-induced meiosis resumption in fish is not well characterized. Here we studied the participation of PI3K/Akt signalling pathway during recombinant human insulin (rh-insulin)-induced meiotic maturation in zebrafish (Danio rerio) oocytes. Priming of defolliculated oocytes in vitro with rh-insulin promotes germinal vesicle breakdown (GVBD) in a dose- and time-dependent manner, an effect sensitive to translation but not transcription inhibition. More than 80% of the oocytes underwent GVBD due to 0.8IU/ml rh-insulin within 10h of incubation and the kinetics of p34cdc2 kinase activation corresponded well with GVBD data. PI3K inhibitors, wortmannin and LY294002 blocked insulin, but not 17α, 20β-DHP-induced GVBD. Immunoblot analyses of oocyte extract revealed that phospho-PI3K (p85α) was up regulated within 30-60 min of insulin stimulation followed by phospho-Akt (Ser473) at 60-120 min. Though PI3K/Akt phosphorylation was largely unaffected, pre-incubation with phosphodiesterase (PDE) inhibitors, IBMX and cilostamide, but not rolipram completely blocked rh-insulin-induced p34cdc2 activation and GVBD. These results suggest that PDE3 may be one potential downstream target to PI3K/Akt signalling necessary for rh-insulin-induced GVBD in zebrafish.

Epidermal growth factor differentially regulates activin subunits in the zebrafish ovarian follicle cells via diverse signaling pathways

Epidermal growth factor (EGF) promotes oocyte maturation in the zebrafish and its effect is mediated via the activin system. However, the mechanisms by which EGF regulates activin subunits in the follicle cells remain unknown. The present study demonstrated that EGF controlled expression of three activin subunits (inhbaa, inhbab and inhbb) in the follicle cells via diverse signaling pathways. The expression of inhbaa and inhbb was often co-regulated via similar pathways. Suppression of MAPK3/1, p38 MAPK, PKC and PKA each blocked or partially reduced the stimulatory effects of EGF on the expression of inhbaa and inhbb while up-regulated that of inhbab. Conversely, inhibition of PI3K did not have any effect on the expression of inhbaa and inhbb but significantly suppressed the stimulatory effect of EGF on inhbab. In summary, EGF action in the zebrafish ovary involves activin system and its regulation of activin subunits is mediated by diverse signaling pathways downstream of EGFR.

Prediction of functional phosphorylation sites by incorporating evolutionary information

Protein phosphorylation is a ubiquitous protein post-translational modification, which plays an important role in cellular signaling systems underlying various physiological and pathological processes. Current in silico methods mainly focused on the prediction of phosphorylation sites, but rare methods considered whether a phosphorylation site is functional or not. Since functional phosphorylation sites are more valuable for further experimental research and a proportion of phosphorylation sites have no direct functional effects, the prediction of functional phosphorylation sites is quite necessary for this research area. Previous studies have shown that functional phosphorylation sites are more conserved than non-functional phosphorylation sites in evolution. Thus, in our method, we developed a web server by integrating existing phosphorylation site prediction methods, as well as both absolute and relative evolutionary conservation scores to predict the most likely functional phosphorylation sites. Using our method, we predicted the most likely functional sites of the human, rat and mouse proteomes and built a database for the predicted sites. By the analysis of overall prediction results, we demonstrated that protein phosphorylation plays an important role in all the enriched KEGG pathways. By the analysis of protein-specific prediction results, we demonstrated the usefulness of our method for individual protein studies. Our method would help to characterize the most likely functional phosphorylation sites for further studies in this research area.

Dual role of IGF-II in oocyte maturation in southern flounder Paralichthys lethostigma: up-regulation of mPRα and resumption of meiosis

Increasing evidence suggests a regulatory role for the IGF system in teleost oocyte maturation (OM). Our objectives were to determine if IGF-I and IGF-II regulate different stages of OM in southern flounder (Paralichthys lethostigma) and to identify the likely maturation-inducing steroid (MIS) in this species. The most abundant final product of ovarian steroidogenesis assays eluted at the position of 17,20β,21-trihydroxy-4-pregnen-3-one (20β-S). 20β-S was also more potent in inducing germinal vesicle breakdown (GVBD) of maturationally-competent oocytes than other teleost MISs. IGF-II (100 nM) induced maturational competence (OMC), as greater GVBD was induced after incubation with IGF-II+20β-S compared to that of the 20β-S+20β-S or IGF-II+no treatment group. Incubation with IGF-II (100 nM) for 4-8 h significantly increased ovarian membrane progestin receptor alpha (mPRα or Paqr7b) mRNA levels 12-15% and mPRα protein levels 75-101%. Further, the IGF-II-induced increase in mPRα protein concentrations was partially blocked by pretreatment with Wortmannin, a Pik3 inhibitor, and PD 098,059, a Mapk inhibitor. Both IGF-I and -II (100 nM) induced GVBD of maturationally-competent oocytes was blocked by incubation with cycloheximide. Incubation with D,L-Aminoglutethimide decreased IGF-II-induced GVBD but had no effect on IGF-I-induced GVBD. IGF-I and -II were also able to induce GVBD of maturationally-incompetent oocytes, and elicited 75% and 135% greater GVBD, respectively, than hCG+20β-S at 100 nM. In conclusion, we show that 20β-S is the likely MIS in this species and that IGF-I and -II are also able to induce GVBD. Further, IGF-II not only induces OMC but also up-regulates ovarian mPRα mRNA and protein through Pik3- and Mapk-dependent pathways. This is the first demonstration of mPRα regulation by an IGF in any vertebrate species.

Possible involvement of phosphatidylinositol 3-kinase, but not protein kinase B or glycogen synthase kinase 3beta, in progesterone-induced oocyte maturation in the Japanese brown frog, Rana japonica

It is known that amphibian oocytes undergo maturation through the formation and activation of maturation-promoting factor (MPF) in response to stimulation by the maturation-inducing hormone progesterone; however, the signal transduction pathway that links the hormonal stimulation on the oocyte surface to the activation of MPF in the oocyte cytoplasm remains a mystery. The aim of this study was to investigate whether the signal transduction mediated by phosphatidylinositol 3-kinase (PI3K), protein kinase B (PKB), and glycogen synthase kinase 3beta (GSK3beta) is involved in progesterone-induced oocyte maturation in the Japanese brown frog, Rana japonica. Inhibitors of PI3K, wortmannin and LY294002, inhibited progesterone-stimulated germinal vesicle breakdown (GVBD) only when the oocytes were treated at the initial phase of maturation, suggesting that PI3K is involved in the progesterone-induced maturation of Rana oocytes. However, we also obtained results suggesting that PKB and GSK3beta are not involved in Rana oocyte maturation. A constitutively active PKB expressed in the oocytes failed to induce GVBD in the absence of progesterone despite its high level of kinase activity. A Myc-tagged PKB expressed in the oocytes (used to monitor endogenous PKB activity) was not activated in the process of progesterone-induced oocyte maturation. Overexpression of GSK3beta, which is reported to retard the progress of Xenopus oocyte maturation, had no effect on Rana oocyte maturation. On the basis of these results, we propose that PI3K is involved in the initiation of Rana oocyte maturation, but that neither PKB nor GSK3beta is a component of the PI3K signal transduction pathway.

Steroid-induced oocyte maturation in Atlantic croaker (Micropogonias undulatus) is dependent on activation of the phosphatidylinositol 3-kinase/Akt signal transduction pathway

Exposure of fully grown fish and amphibian oocytes to a maturation-inducing steroid (MIS) activates numerous signal transduction pathways to initiate the final stage of oocyte maturation. These events culminate in the activation of maturation-promoting factor and germinal vesicle breakdown (GVBD). In most species, exposure to MIS causes a transient decrease in oocyte cAMP levels. Whether this reduction in oocyte cAMP concentration is sufficient to induce GVBD is unclear. The current study tested the hypothesis that activation of cAMP-independent signal transduction pathways by the naturally occurring MIS, 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S), is necessary for GVBD in Atlantic croaker (Micropogonias undulatus) oocytes. Results indicate that although 20beta-S treatment of oocyte membranes significantly reduced cAMP production, incubation of follicles with the cell-permeable cAMP-dependent protein kinase (Prka) inhibitors Rp-cAMP or KT5720 did not promote GVBD in the absence of 20beta-S. Additionally, treatment of follicles with the phosphodiesterase (Pde) inhibitors Cilostamide (Pde3) or Rolipram (Pde4) significantly reduced GVBD, but they were not able to completely block it. In contrast, pharmacologic inhibition of the cAMP-independent phosphatidylinositol 3-kinase (Pik3)/Akt signal transduction pathway using the Pik3 inhibitors Wortmannin or LY294002, or the Akt inhibitor ML-9, blocked 20beta-S-induced GVBD. Finally, mitogen-activated protein kinase (Mapk1/3) activity increased after treatment with 20beta-S; however, inhibition of Mapk1/3 activity using PD98059 or U0126 had no effect on GVBD. These results demonstrate that activation of cAMP-independent signaling pathways, especially the Pik3/Akt pathway, is necessary for 20beta-S-induced GVBD in Atlantic croaker oocytes.

Insulin-like growth factor-I induces oocyte maturational competence but not meiotic resumption in white bass (Morone chrysops) follicles in vitro: evidence for rapid evolution of insulin-like growth factor action

A combination of recombinant human (rh) insulin-like growth factor-I (IGF-I) (25 nM) and the maturation-inducing hormone (MIH), 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S; 72.5 nM), induced germinal vesicle breakdown (GVBD) in ovarian follicles of white bass incubated in vitro, whereas a four times greater concentration of each hormone was ineffective alone. These results indicate that IGF-I induces oocyte maturational competence (OMC) but not meiotic resumption in white bass. Culture medium concentrations of 20beta-S remained below detection limits for ovarian fragments incubated with rhIGF-I. Actinomycin D blocked GVBD in response to hCG but not to rhIGF-I plus 20beta-S, suggesting that IGF-I requires de novo translation but not transcription to induce OMC. Gap junction uncouplers, 1-octanol and 1-heptanol, and the phosphatidylinositiol 3-kinase (PI 3-K) inhibitors, wortmannin and LY 294002, attenuated hCG-, 20beta-S-, and rhIGF-I plus 20beta-S-induced GVBD. Although these inhibitors reduced hCG-induced progestin release, PI 3-K inhibitors did not alter MIH synthesis in some incubations and addition of 20beta-S to the incubations did not fully overcome the effects of either class of inhibitors, suggesting that decreasing MIH production is not their only inhibitory effect on gonadotropin (GtH) action. Our data suggest that gap junctions and PI 3-K activity are necessary for GtH and IGF-I to induce and maintain OMC in white bass. The induction of OMC but not meiotic resumption by IGF-I in white bass, compared with the induction of meiotic resumption but not OMC by IGF-I discovered in the congeneric striped bass suggests rapid evolution of the reproductive actions of IGF-I among temperate basses (genus Morone).