Effects of PKC activation on the meiotic maturation, fertilization and early embryonic development of mouse oocytes

Gene analysis of major signaling pathways regulated by gonadotropins in human ovarian granulosa tumor cells (KGN)†

The female reproductive function largely depends on timing and coordination between follicle-stimulating hormone (FSH) and luteinizing hormone. Even though it was suggested that these hormones act on granulosa cells via shared signaling pathways, mainly protein kinases A, B, and C (PKA, PKB, and PKC), there is still very little information available on how these signaling pathways are regulated by each hormone to provide such differences in gene expression throughout folliculogenesis. To obtain a global picture of the principal upstream factors involved in PKA, PKB, and PKC signaling in granulosa cells, human granulosa-like tumor cells (KGN) were treated with FSH or specific activators (forskolin, SC79, and phorbol 12-myristate 13-acetate) for each pathway to analyze gene expression with RNA-seq technology. Normalization and cutoffs (FC 1.5, P ≤ 0.05) revealed 3864 differentially expressed genes between treatments. Analysis of major upstream regulators showed that PKA is a master kinase of early cell differentiation as its activation resulted in the gene expression profile that accompanies granulosa cell differentiation. Our data also revealed that the activation of PKC in granulosa cells is also a strong differentiation signal that could control "advanced" differentiation in granulosa cells and the inflammatory cascade that occurs in the dominant follicle. According to our results, PKB activation provides support for PKA-stimulated gene expression and is also involved in granulosa cell survival throughout follicular development. Taken together, our results provide new information on PKA, PKB, and PKC signaling pathways and their roles in stimulating a follicle at the crossroad between maturation/ovulation and atresia.

Recent advances in mammalian reproductive biology

Reproductive biology is a uniquely important topic since it is about germ cells, which are central for transmitting genetic information from generation to generation. In this review, we discuss recent advances in mammalian germ cell development, including preimplantation development, fetal germ cell development and postnatal development of oocytes and sperm. We also discuss the etiologies of female and male infertility and describe the emerging technologies for studying reproductive biology such as gene editing and single-cell technologies.

PKCβ1 regulates meiotic cell cycle in mouse oocyte

PKCβI, a member of the classical protein kinase C family, plays key roles in regulating cell cycle transition. Here, we report the expression, localization and functions of PKCβI in mouse oocyte meiotic maturation. PKCβI and p-PKCβI (phosphor-PKCβI) were expressed from germinal vesicle (GV) stage to metaphase II (MII) stage. Confocal microscopy revealed that PKCβI was localized in the GV and evenly distributed in the cytoplasm after GV breakdown (GVBD), and it was concentrated at the midbody at telophase in meiotic oocytes. While, p-PKCβI was concentrated at the spindle poles at the metaphase stages and associated with midbody at telophase. Depletion of PKCβI by specific siRNA injection resulted in defective spindles, accompanied with spindle assembly checkpoint activation, metaphase I arrest and failure of first polar body (PB1) extrusion. Live cell imaging analysis also revealed that knockdown of PKCβI resulted in abnormal spindles, misaligned chromosomes, and meiotic arrest of oocytes arrest at the Pro-MI/MI stage. PKCβI depletion did not affect the G2/M transition, but its overexpression delayed the G2/M transition through regulating Cyclin B1 level and Cdc2 activity. Our findings reveal that PKCβI is a critical regulator of meiotic cell cycle progression in oocytes. Abbreviations: PKC, protein kinase C; COC, cumulus-oocyte complexes; GV, germinal vesicle; GVBD, germinal vesicle breakdown; Pro-MI, first pro-metaphase; MI, first metaphase; Tel I, telophase I; MII, second metaphase; PB1, first polar body; SAC, spindle assembly checkpoint.

A fast and reliable protocol for activation of porcine oocytes

Oocyte activation is physiologically triggered by the sperm during fertilization, however, production of porcine embryos by somatic cell nuclear transfer (SCNT), intracytoplasmic sperm injection (ICSI) or parthenogenetic activation (PA) requires artificial oocyte activation. Although effective protocols for artificial oocyte activation have been developed, current protocols require long exposures to non-specific inhibitors, which do not mimic the physiological process and may have detrimental consequences for embryo development. This study attempted to mimic the physiological activation events induced by fertilization, through the manipulation of Ca²⁺ and Zn²⁺ levels, and protein kinase C (PKC) as well as cyclin dependent kinase 1 (CDK1) activities, with the aim of developing an improved protocol for activation of porcine oocytes. In the first experiment, matured oocytes were exposed to ionomycin (Ion) for 5 min, and then treated with a specific CDK1 inhibitor (RO-3306) and/or PKC activator (OAG) for different time intervals. The highest rate of pronuclear (PN) formation (58.8%) was obtained when oocytes were treated with PKCa + CDK1i for 4 h. Second, PN formation and embryo development were evaluated in oocytes exposed for different times to a Zn²⁺ chelator (TPEN) following Ion treatment. This revealed that 15 min was the minimal exposure time to TPEN required to maximise oocyte activation and embryo development. Next, we observed that treatment with PKCa + CDK1i for 4 h after TPEN for 15 min decreased embryo development compared to TPEN alone. Lastly, we compared the efficiency of the Ion (5 min) plus TPEN (15 min) protocol (IT-20) with a control protocol used in our laboratory (CT-245) for production of PA, SCNT and ICSI embryos. In PA embryos, IT-20 resulted in higher cleavage (72% vs 49.2%) and blastocyst from cleaved embryos (65.5% vs 46.2%) compared to CT-245. In ICSI embryos, higher PN rates were obtained with the IT-20 protocol compared with CT-245 and the non-activated (N-A) group. Moreover, the two protocols were equally efficient for activation of SCNT embryos. Based on these findings, we propose that IT-20 is a fast and effective protocol for activation of porcine oocytes.

Oocyte maturation and fertilization in marine nemertean worms: using similar sorts of signaling pathways as in mammals, but often with differing results

In marine worms belonging to the phylum Nemertea, oocyte maturation and fertilization are regulated by the same general kinds of signals that control such processes in mammals. However, unlike mammalian oocytes that develop within follicles, nemertean oocytes characteristically lack a surrounding sheath of follicle cells and often respond differently to maturation-related cues than do mammalian oocytes. For example, elevators of cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP) levels promote the resumption of meiotic maturation (=germinal vesicle breakdown, GVBD) in nemertean oocytes, whereas increasing intraoocytic cAMP and cGMP typically blocks GVBD in mammals. Similarly, AMP-activated kinase (AMPK) signaling keeps nemertean oocytes from maturing, but in mouse oocytes, AMPK activation triggers GVBD. In addition, protein kinase C (PKC) activity is required for seawater-induced GVBD in nemerteans, whereas some PKCs have been shown to inhibit GVBD in mammals. Furthermore, although fertilization causes both types of oocytes to reorganize their endoplasmic reticulum and generate calcium oscillations that can involve soluble sperm factor activity and inositol 1,4,5-trisphosphate signaling, some discrepancies in the spatiotemporal patterns and underlying mechanisms of fertilization are also evident in nemerteans versus mammals. Thus, to characterize differences and similarities in gamete biology more fully, aspects of oocyte maturation and fertilization in marine nemertean worms are reviewed and briefly compared with related findings that have been published for mammalian oocytes. In addition, possible causes of the alternative responses displayed by oocytes in these two animal groups are addressed.

Specific protein kinase C isoforms α and βI are involved in follicle-stimulating hormone-induced mouse follicle-enclosed oocytes meiotic resumption

Protein kinase C (PKC) is involved in gonadotrophin-induced oocyte maturation. In the present study, we investigated the role of specific PKC isoforms in the process of follicle-stimulating hormone (FSH)-induced oocyte meiotic resumption. Small antral follicles (200–300 µm in diameter) were isolated from immature mice and cultured in vitro. FSH significantly induced follicle-enclosed oocytes (FEOs) meiotic resumption after 8 hr culture. However, the induced effect of FSH was dose-dependently inhibited by the specific PKC α and βI inhibitor Gö6976, and 100 nM Gö6976 completely blocked FSH function in oocyte meiotic resumption. Furthermore, FSH dramatically induced the expression of transcripts encoding epidermal growth factor (EGF)-like growth factors Areg, Btc, and Ereg mRNA levels, and up-regulated tyrosine phosphorylation level of EGF receptor (EGFR) in granulosa cells. Blocking the function of EGFR by AG1478 eliminated the effect of FSH-induced FEOs meiotic resumption, suggesting that FSH induced oocyte maturation through the activation of EGFR. FSH-induced phosphorylation of EGFR could also be inhibited by Gö6976. Next, we examined the effect of FSH on the expression and phosphorylation PKC α and βI. FSH induced the expression of PKC α at mRNA and protein level, and also up-regulated its phosphorylation level in granulosa cells after 8 hr culture. However, FSH had no effect on the expression of PKC βI but down-regulated its phosphorylation level. In conclusion, FSH-induced activation of PKC α alone, or together with the inactivation of PKC βI in granulosa cells, participates in mouse oocyte meiotic resumption, possibly by the activation of EGFR signaling pathway.

Comparative biology of cAMP-induced germinal vesicle breakdown in marine invertebrate oocytes

During maturation, oocytes must undergo a process of nuclear disassembly, or "germinal vesicle breakdown" (GVBD), that is regulated by signaling pathways involving cyclic AMP (cAMP). In vertebrate and starfish oocytes, cAMP elevation typically prevents GVBD. Alternatively, increased concentrations of intra-oocytic cAMP trigger, rather than inhibit, GVBD in several groups of marine invertebrates. To integrate what is known about the stimulation of GVBD by intra-oocytic cAMP, this article reviews published data for ascidian, bivalve, brittle star, jellyfish, and nemertean oocytes. The bulk of the review concentrates on the three most intensively analyzed groups known to display cAMP-induced GVBD-nemerteans, ascidians, and jellyfish. In addition, this synopsis also presents some previously unpublished findings regarding the stimulatory effects of intra-oocytic cAMP on GVBD in jellyfish and the annelid worm Pseudopotamilla occelata. Finally, factors that may account for the currently known distribution of cAMP-induced GVBD across animal groups are discussed.

Effect of age, GV transfer and modified nucleocytoplasmic ratio on PKCα in mouse oocytes and early embryos

Protein kinase C (PKC) is a family of Ser/Thr protein kinases that can be activated by Ca2+, phospholipid and diacylglycerol. There is evidence that PKC plays key roles in the meiotic maturation and activation of mammalian oocytes. The present study aimed to monitor the effect of age, germinal vesicle (GV) transfer and modified nucleoplasmic ratio on the subcellular distribution profile of PKCα, an important isozyme of PKC, in mouse oocytes undergoing meiotic maturation and following egg activation. Germinal vesicle oocytes were collected from 6–8-week-old and 12-month-old mice. Germinal vesicle-reconstructed oocytes and GV oocytes with one-half or one-third of the original oocyte volume were created using micromanipulation and electrofusion. The subcellular localization of PKCα was detected by immunocytochemistry and laser confocal microscopy. Our study showed that PKCα had a similar location pattern in oocytes and early embryos from young and old mice. PKCα was localized evenly in ooplasm, with weak staining in GV at the GV stage, and present in the entire meiosis II (MII) spindle at the MII stage. In pronuclear and 2-cell embryos, PKCα was concentrated in the nucleus except for the nucleolus. After the GV oocytes were reconstructed, the resultant MII oocytes and embryos showed a similar distribution of PKCα between reconstructed and unreconstructed controls. After one-half or two-thirds of the cytoplasm was removed from the GV oocytes, PKCα still had a similar location pattern in MII oocytes and early embryos from the GV oocytes with modified nucleoplasmic ratio. Our study showed that age, GV transfer and modified nucleocytoplasmic ratio does not affect distribution of PKCα during mouse oocyte maturation, activation, and early embryonic mitosis.

Pharmacological analyses of protein kinases regulating egg maturation in marine nemertean worms: a review and comparison with Mammalian eggs

For development to proceed normally, animal eggs must undergo a maturation process that ultimately depends on phosphorylations of key regulatory proteins. To analyze the kinases that mediate these phosphorylations, eggs of marine nemertean worms have been treated with pharmacological modulators of intracellular signaling pathways and subsequently probed with immunoblots employing phospho-specific antibodies. This article both reviews such analyses and compares them with those conducted on mammals, while focusing on how egg maturation in nemerteans is affected by signaling pathways involving cAMP, mitogen-activated protein kinases, Src-family kinases, protein kinase C isotypes, AMP-activated kinase, and the Cdc2 kinase of maturation-promoting factor.

Myristoylated alanine-rich C kinase substrate, but not Ca2+/calmodulin-dependent protein kinase II, is the mediator in cortical granules exocytosis

Sperm–egg fusion induces cortical granules exocytosis (CGE), a process that ensures the block to polyspermy. CGE can be induced independently by either a rise in intracellular calcium concentration or protein kinase C (PKC) activation. We have previously shown that myristoylated alanine-rich C kinase substrate (MARCKS) cross-links filamentous actin (F-actin) and regulates its reorganization. This activity is reduced either by PKC-induced MARCKS phosphorylation (PKC pathway) or by its direct binding to calmodulin (CaM; CaM pathway), both inducing MARCKS translocation, F-actin reorganization, and CGE. Currently, we examine the involvement of Ca2+/CaM-dependent protein kinase II (CaMKII) and MARCKS in promoting CGE and show that PKC pathway can compensate for lack of Ca2+/CaM pathway. Microinjecting eggs with either overexpressed protein or complementary RNA of constitutively active αCaMKII triggered resumption of second meiotic division, but induced CGE of an insignificant magnitude compared with CGE induced by wt αCaMKII. Microinjecting eggs with mutant-unphosphorylatable MARCKS reduced the intensity of 12-O-tetradecanoylphorbol 13-acetate or ionomycin-induced CGE by 50%, indicating that phosphorylation of MARCKS by novel and/or conventional PKCs (n/cPKCs) is a pivotal event associated with CGE. Moreover, we were able to demonstrate cPKCs involvement in ionomycin-induced MARCKS translocation and CGE. These results led us to propose that MARCKS, rather than CaMKII, as a key mediator of CGE.

Protein kinase C activity in mouse eggs regulates gamete membrane interaction

Gamete membrane interaction is critical to initiate the development of a new organism. The signaling pathways governing this event, however, are poorly understood. In this report, we provide the first evidence that protein kinase C activity in mouse eggs plays a crucial role in the regulation of this process. Stimulating PKC activity in mouse eggs by phorbol 12-myristate 13-acetate (PMA) drastically inhibited the egg's membrane ability to bind and fuse with sperm. Surprisingly, this significant reduction of gamete membrane interaction was also observed in eggs treated with the PKC inhibitors staurosporine and calphostin c. In further analysis, we found that while no change of egg actin cytoskeleton was detected after either PMA or calphostin c treatment, the structural morphology of egg surface microvilli was severely altered in the PMA-treated eggs, but not in the calphostin c-treated eggs. Moreover, sperm, which bound but did not fuse with the eggs treated with the anti-CD9 antibody KMC8, were liberated from the egg membrane after PMA, but not calphostin c, treatment. Taken together, these results suggest that egg PKC may be precisely balanced to regulate gamete membrane interaction in a biphasic mode, and this biphasic regulation is executed through two different mechanisms.

Acetylcholine increases Ca2+ influx by activation of CaMKII in mouse oocytes

IP3-induced Ca2+ release is the primary mechanism that is responsible for acetylcholine (ACh)-induced Ca2+ oscillation. However, other mechanisms remain to explain intracellular Ca2+ elevation. We here report that ACh induces Ca2+ influx via T-type Ca2+ channel by activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII), and the ACh-induced Ca2+ influx facilitates the generation of Ca2+ oscillation in the mouse ovulated oocytes (oocytes(MII)). ACh increased Ca2+ current by 50+/-21%, and produced Ca2+ oscillation. However, the currents and Ca2+ peaks were reduced in Ca2+ -free extracellular medium. ACh failed to activate Ca2+ current and to produce Ca2+ oscillation in oocytes pretreated with KN-93, a CaMKII inhibitor. KN-92, an inactive analogue of KN93, and PKC modulators could not prevent the effect of ACh. These results show that ACh increases T-type Ca2+ current by activation of CaMKII, independent of the PKC pathway, in the mouse oocytes.

In vivo and in vitro effects of FSH on oocyte maturation and developmental competence

There is increasing evidence demonstrating that oocyte quality depends on the events that occur before germinal vesicle breakdown (GVBD), suggesting that the oocyte must accumulate the appropriate information for meiotic resumption fertilization and early embryonic development before chromosome condensation. This situation seems to prevail in large mammals and particularly in the bovine where we have more information than in other species. Signaling events at two different levels controls the changes that must take place for follicular growth and attainment of oocyte developmental competence. The first signaling event comes from the proper differentiation of the follicle as it normally occurs in the dominant follicle in preparation for ovulation. The second signaling event occurs as the process of follicle differentiation signals directly to the oocyte, possibly through the cumulus cells, that conditions are suitable for further embryo development. The first signal, follicular differentiation, becomes possible though a rise and fall of FSH in the circulation, while the second signal might be mimicked partially by the same hormone acting on the cumulus cells. Although FSH is likely involved in these two signaling events, the processes involved are quite different and analysis of gene expression in granulosa, cumulus and oocyte is starting to reveal the complexity of this system. The next challenge is to combine these two pathways into a functional signaling cascade. To be successful and obtain meaningful information, these genomic analyses must be developed and performed in precisely defined conditions of follicular growth and differentiation or culture conditions. Functional genomics already started with the study of function of several genes and genes families in the regulation of follicular growth and follicle-oocyte co-differentiation (i.e. IGF and BMP genes families, EGF).

The PKC pathway and in particular its β1 isoform is clearly involved in meiotic arrest maintenance but poorly in FSH-induced meiosis resumption of the mouse cumulus cell enclosed oocyte

PKC modulators were used to investigate the role of the PKC pathway either on the maintenance of meiotic arrest or on FSH-induced maturation of mouse cumulus cell enclosed oocytes (CEOs). (1) Whereas PKC activation (PMA 8 microM) overcomed clearly the HX-maintained meiotic arrest (83.7 +/- 3.6% vs. 16.1 +/- 10.6% GVBD oocytes), PKC inhibition (Calphostin C 100 nM) did not. On the contrary, it better maintained the meiotic arrest than HX alone. (2) No significant effect of PKC activation or inhibition was observed. (3) HX alone maintained PKCbeta1 in the cytoplasm, whereas FSH and PKC activation induced partly its translocation into the nucleus. The results show that whereas the PKC pathway is clearly involved in maintenance of the meiotic arrest through PKCbeta1, it is not involved in FSH-induced meiosis of CEOs.

Expression profile of protein kinase C isozymes in preimplantation mouse development

In the preimplantation mouse embryo, the protein kinase C (PKC) family has been implicated in regulation of egg activation, progression of meiotic and mitotic cell cycles, embryo compaction, and blastulation, but the involvement of the individual isozymes is largely unknown. Here, using semiquantitative immunocytochemistry and confocal microscopy we analyze the relative amount and subcellular distribution of ten isozymes of PKC (alpha, betaI, betaII, gamma, delta, epsilon, eta, theta, zeta, iota/lambda) and a PKC-anchoring protein, receptor for activated C-kinase 1 (RACK1). Our results show that all of these isoforms of PKC are present between the two-cell and blastocyst stages of mouse preimplantation development, and that each has a distinct, dynamic pattern and level of expression. The data suggest that different complements of the isozymes are involved in various steps of preimplantation development, and will serve as a framework for further functional studies of the individual isozymes. In particular, there was a transient increase in the nuclear concentration of several isozymes at the early four-cell stage, suggesting that some of the PKC isozymes might be involved in regulation of nuclear organization and function in the early mouse embryo.

Subcellular localization of protein kinase C delta and epsilon affects transcriptional and post-transcriptional processes in four-cell mouse embryos

During mouse preimplantation development, two isozymes of protein kinase C (PKC), delta and epsilon, transiently localize to nuclei at the early four-cell stage. In order to study their functions at this stage, we altered the subcellular localization of these isozymes (ratio of nuclear to cytoplasmic concentrations) with peptides that specifically activate or inhibit translocation of each isozyme. The effects of altering nuclear concentration of each isozyme on transcription (5-bromouridine 5'-triphosphate (BrUTP) incorporation), amount and distribution of small nuclear ribonucleoproteins (snRNPs), nucleolar dynamics (immunocytochemistry for Smith antigen (Sm) protein) and the activity of embryonic alkaline phosphatase (EAP; histochemistry) were examined. We found that nuclear concentration of PKC epsilon correlated with total mRNA transcription. Higher nuclear concentrations of both PKC delta and epsilon decreased storage of snRNPs in Cajal bodies and decreased the number of nucleoli, but did not affect the nucleoplasmic concentration of snRNPs. Inhibiting translocation of PKC delta out of the nucleus at the early four-cell stage decreased cytoplasmic EAP activity, whereas inhibiting translocation of PKC epsilon increased EAP activity slightly. These results indicate that translocation of PKC delta and epsilon in and out of nuclei at the early four-cell stage in mice can affect transcription or message processing, and that sequestration of these PKC in nuclei can also affect the activity of a cytoplasmic protein (EAP).

Translocation of phospho-protein kinase Cs implies their roles in meiotic-spindle organization, polar-body emission and nuclear activity in mouse eggs

The protein kinase Cs (PKCs) are a family of Ser/Thr protein kinases categorized into three subfamilies: classical, novel, and atypical. The phosphorylation of PKC in germ cells is not well defined. In this study, we described the subcellular localization of phopho-PKC in the process of mouse oocyte maturation, fertilization, and early embryonic mitosis. Confocal microscopy revealed that phospho-PKC (pan) was distributed abundantly in the nucleus at the germinal vesicle stage. After germinal vesicle breakdown, phospho-PKC was localized in the vicinity of the condensed chromosomes, distributed in the whole meiotic spindle, and concentrated at the spindle poles. After metaphase I, phospho-PKC was translocated gradually to the spindle mid-zone during emission of the first polar body. After sperm penetration and electrical activation, the distribution of phospho-PKC was moved from the spindle poles to the spindle mid-zone. After the extrusion of the second polar body (PB2) phospho-PKC was localized in the area between the oocyte and the PB2. In fertilized eggs, phospho-PKC was concentrated in the pronuclei except for the nucleolus. Phospho-PKC was dispersed after pronuclear envelope breakdown, but distributed on the entire spindle at mitotic metaphase. The results suggest that PKC activation may play important roles in regulating spindle organization and stabilization, polar-body extrusion, and nuclear activity during mouse oocyte meiosis, fertilization, and early embryonic mitosis.