Effects of protein kinase C stimulation and free Ca2+ rise in mammalian egg activation

Chemical activation of mammalian oocytes and its application in camelid reproductive biotechnologies: A review

Mammalian oocyte activation is a critical process occurring post-gamete fusion, marked by a sequence of cellular events initiated by an upsurge in intracellular Ca2+. This surge in calcium orchestrates the activation/deactivation of specific kinases, leading to the subsequent inactivation of MPF and MAPK activities, alongside PKC activation. Despite various attempts to induce artificial activation using distinct chemical compounds as Ca2+ inducers and/or Ca2+-independent agents, the outcomes have proven suboptimal. Notably, incomplete suppression of MPF and MAPK activities persists, necessitating a combination of different agents for enhanced efficiency. Moreover, the inherent specificity of activation methods for each species precludes straightforward extrapolation between them. Consequently, optimization of protocols for each species and for each technique, such as PA, ICSI, and SCNT, is required. Despite recent strides in camelid biotechnologies, the field has seen little advancement in chemical activation methods. Only a limited number of chemical agents have been explored, and the effects of many remain unknown. In ICSI, despite obtaining blastocysts with different chemical compounds that induce Ca2+ and calcium-independent increases, viable offspring have not been obtained. However, SCNT has exhibited varying outcomes, successfully yielding viable offspring with a reduced number of chemical activators. This article comprehensively reviews the current understanding of the physiological activation of oocytes and the molecular mechanisms underlying chemical activation in mammals. The aim is to transfer and apply this knowledge to camelid reproductive biotechnologies, with emphasis on chemical activation in PA, ICSI, and SCNT.

Fertility: Store-Operated Ca2+ Entry in Germ Cells: Role in Egg Activation

At the time of fertilization, the sperm activates the egg and induces embryonic development by triggering an elevation in the egg's intracellular free Ca²⁺ concentration. In mammals the initial Ca²⁺ rise is followed by a series of repetitive Ca²⁺ transients (known as oscillations) that last for several hours. Although the source of Ca²⁺ during the signaling process is primarily the egg's smooth endoplasmic reticulum, the oscillations stop in the absence of extracellular Ca²⁺ indicating that a Ca²⁺ influx across the plasma membrane is essential to sustain them. Depletion of the intracellular stores using specific inhibitors generates a Ca²⁺ entry across the plasma membrane of eggs of various species, and a continuous influx of Ca²⁺ has been linked to the sperm-induced Ca²⁺ oscillations in the mouse; these data indicate that store-operated Ca²⁺ entry (SOCE) operates in eggs and may be the mechanism that maintains the long-lasting Ca²⁺ signal at fertilization. Recent findings suggest that the signaling proteins STIM1 and Orai1 are present in eggs; they are responsible for mediating SOCE, and their functions are essential for proper Ca²⁺ signaling at fertilization to support normal embryo development.

Oocyte activation and latent HIV-1 reactivation: AMPK as a common mechanism of action linking the beginnings of life and the potential eradication of HIV-1

In all mammalian species studied to date, the initiation of oocyte activation is orchestrated through alterations in intracellular calcium (Ca(2+)) signaling. Upon sperm binding to the oocyte plasma membrane, a sperm-associated phospholipase C (PLC) isoform, PLC zeta (PLCζ), is released into the oocyte cytoplasm. PLCζ hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to produce diacylglycerol (DAG), which activates protein kinase C (PKC), and inositol 1,4,5-trisphosphate (IP3), which induces the release of Ca(2+) from endoplasmic reticulum (ER) Ca(2+) stores. Subsequent Ca(2+) oscillations are generated that drive oocyte activation to completion. Ca(2+) ionophores such as ionomycin have been successfully used to induce artificial human oocyte activation, facilitating fertilization during intra-cytoplasmic sperm injection (ICSI) procedures. Early studies have also demonstrated that the PKC activator phorbol 12-myristate 13-acetate (PMA) acts synergistically with Ca(2+) ionophores to induce parthenogenetic activation of mouse oocytes. Interestingly, the Ca(2+)-induced signaling cascade characterizing sperm or chemically-induced oocyte activation, i.e. the "shock and live" approach, bears a striking resemblance to the reactivation of latently infected HIV-1 viral reservoirs via the so called "shock and kill" approach, a method currently being pursued to eradicate HIV-1 from infected individuals. PMA and ionomycin combined, used as positive controls in HIV-1 latency reversal studies, have been shown to be extremely efficient in reactivating latent HIV-1 in CD4(+) memory T cells by inducing T cell activation. Similar to oocyte activation, T cell activation by PMA and ionomycin induces an increase in intracellular Ca(2+) concentrations and activation of DAG, PKC, and downstream Ca(2+)-dependent signaling pathways necessary for proviral transcription. Interestingly, AMPK, a master regulator of cell metabolism that is activated thorough the induction of cellular stress (e.g. increase in Ca(2+) concentration, reactive oxygen species generation, increase in AMP/ATP ratio) is essential for oocyte maturation, T cell activation, and mitochondrial function. In addition to the AMPK kinase LKB1, CaMKK2, a Ca(2+)/calmodulin-dependent kinase that also activates AMPK, is present in and activated on T cell activation and is also present in mouse oocytes and persists until the zygote and two-cell stages. It is our hypothesis that AMPK activation represents a central node linking T cell activation-induced latent HIV-1 reactivation and both physiological and artificial oocyte activation. We further propose the novel observation that various compounds that have been shown to reactivate latent HIV-1 (e.g. PMA, ionomycin, metformin, bryostatin, resveratrol, etc.) or activate oocytes (PMA, ionomycin, ethanol, puromycin, etc.) either alone or in combination likely do so via stress-induced activation of AMPK.

Signal transduction in mammalian oocytes during fertilization

Mammalian embryo development begins when the fertilizing sperm triggers a series of elevations in the oocyte's intracellular free Ca(2+) concentration. The elevations are the result of repeated release and re-uptake of Ca(2+) stored in the smooth endoplasmic reticulum. Ca(2+) release is primarily mediated by the phosphoinositide signaling system of the oocyte. The system is stimulated when the sperm causes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG); IP3 then binds its receptor on the surface of the endoplasmic reticulum that induces Ca(2+) release. The manner in which the sperm generates IP3, the Ca(2+) mobilizing second messenger, has been the subject of extensive research for a long time. The sperm factor hypothesis has eventually gained general acceptance, according to which it is a molecule from the sperm that diffuses into the ooplasm and stimulates the phosphoinositide cascade. Much evidence now indicates that the sperm-derived factor is phospholipase C-zeta (PLCζ) that cleaves PIP2 and generates IP3, eventually leading to oocyte activation. A recent addition to the candidate sperm factor list is the post-acrosomal sheath WW domain-binding protein (PAWP), whose role at fertilization is currently under debate. Ca(2+) influx across the plasma membrane is also important as, in the absence of extracellular Ca(2+), the oscillations run down prematurely. In pig oocytes, the influx that sustains the oscillations seems to be regulated by the filling status of the stores, whereas in the mouse other mechanisms might be involved. This work summarizes the current understanding of Ca(2+) signaling in mammalian oocytes.

Optimized Protocols for In Vitro Maturation of Rat Oocytes Dramatically Improve Their Developmental Competence to a Level Similar to That of Ovulated Oocytes

The developmental capacity of in vitro-matured (IVM) oocytes is markedly lower than that of their in vivo-matured (IVO) counterparts, suggesting the need for optimization of IVM protocols in different species. There are few studies on IVM of rat oocytes, and there are even fewer attempts to improve ooplasmic maturation compared to those reported in other species. Furthermore, rat oocytes are well known to undergo spontaneous activation (SA) after leaving the oviduct; however, whether IVM rat oocytes have lower SA rates than IVO oocytes and can potentially be used for nuclear transfer is unknown. In this study, we investigated the effects of maturation protocols on cytoplasmic maturation of IVM rat oocytes and observed the possibility to reduce SA by using IVM rat oocytes. Ooplasmic maturation was assessed using multiple markers, including pre- and postimplantation development, meiotic progression, CG redistribution, redox state, and the expression of developmental potential- and apoptosis-related genes. The results showed that the best protocol consisting of modified Tissue Culture Medium-199 (TCM-199) supplemented with cysteamine/cystine and the cumulus cell monolayer dramatically improved the developmental competence of rat oocytes and supported both pre- and postimplantation development and other ooplasmic maturation makers to levels similar to that observed in ovulated oocytes. Rates of SA were significantly lower in IVM oocytes than in IVO oocytes when observed at the same intervals after nuclear maturation. In conclusion, we have optimized protocols for IVM of rat oocytes that sustain ooplasmic maturation to a level similar to ovulated oocytes. The results suggest that IVM rat oocytes might be used to reduce SA for rat cloning.

Lipid characterization of individual porcine oocytes by dual mode DESI-MS and data fusion

The development of sensitive measurements to analyze individual cells is of relevance to elucidate specialized roles or metabolic functions of each cell under physiological and pathological conditions. Lipids play multiple and critical roles in cellular functions and the application of analytical methods in the lipidomics area is of increasing interest. In this work, in vitro maturation of porcine oocytes was studied. Two independent sources of chemical information (represented by mass spectra in the positive and negative ion modes) from single oocytes (immature oocytes, 24-h and 44-h in vitro matured oocytes) were acquired by using desorption electrospray ionization-mass spectrometry (DESI-MS). Low and mid-level data fusion strategies are presented with the aim of better exploring the large amount of chemical information contained in the two mass spectrometric lipid profiles. Data were explored by principal component analysis (PCA) within the two multi-block approaches to include information on free fatty acids, phospholipids, cholesterol-related molecules, di- and triacylglycerols. After data fusion, clearer differences among immature and in vitro matured porcine oocytes were observed, which provide novel information regarding lipid metabolism throughout oocyte maturation. In particular, changes in TAG composition, as well as increase in fatty acid metabolism and membrane complexity were evidenced during the in vitro maturation process. This information can assist the improvement of in vitro embryo production for porcine species.

Role of Na+/Ca2+ exchanger (NCX) in modulating postovulatory aging of mouse and rat oocytes

We studied the role of the Na+/Ca2+ exchanger (NCX) in modulating oocyte postovulatory aging by observing changes in NCX contents and activities in aging mouse and rat oocytes. Whereas the NCX activity was measured by observing oocyte activation following culture with NCX inhibitor or activator, the NCX contents were determined by immunohistochemical quantification. Although NCX was active in freshly-ovulated rat oocytes recovered 13 h post hCG injection and in aged oocytes recovered 19 h post hCG in both species, it was not active in freshly-ovulated mouse oocytes. However, NCX became active when the freshly-ovulated mouse oocytes were activated with ethanol before culture. Measurement of cytoplasmic Ca2+ revealed Ca2+ increases always before NCX activation. Whereas levels of the reactive oxygen species (ROS) and the activation susceptibility increased, the density of NCX member 1 (NCX1) decreased significantly with oocyte aging in both species. While culture with H2O2 decreased the density of NCX1 significantly, culture with NaCl supplementation sustained the NCX1 density in mouse oocytes. It was concluded that (a) the NCX activity was involved in the modulation of oocyte aging and spontaneous activation; (b) ROS and Na+ regulated the NCX activity in aging oocytes by altering its density as well as functioning; and (c) cytoplasmic Ca2+ elevation was essential for NCX activation in the oocyte.

Calcium influx in mammalian eggs

Calcium (Ca2+) signals are involved in the regulation of oocyte maturation and play a critical role during fertilization. In the egg, Ca2+is stored in the lumen of the endoplasmic reticulum and a signal is generated when the stored Ca2+is released through specialized channels in the membrane of the endoplasmic reticulum to elevate the free Ca2+concentration in the cytoplasm. Extracellular Ca2+is also important, indicated by the fact that the mobilization of luminal Ca2+is typically followed by Ca2+entry across the plasma membrane. The transmembrane Ca2+flux replenishes the endoplasmic reticulum, and thus, it is essential to sustain prolonged Ca2+signals. It also seems to be responsible for the stimulation of important signaling cascades required for complete egg activation. Characterization of the pathway that mediates Ca2+entry implies that its major components include STIM1, a protein that senses the filling status of the stores, and ORAI1, a channel protein located in the plasma membrane. Defining the mechanism and functions of Ca2+entry will not only lead to a better understanding of egg physiology but may also help improving the efficiency of a number of assisted reproductive technologies.

Regulation of fusion of the nucleolar precursor bodies following activation of mouse oocytes: roles of the maturation-promoting factors and mitogen-activated protein kinases

Fusion of nucleoli or nucleolus precursor bodies (NPBs) has been observed during somatic cell interphase and pronuclear development of human zygotes; however, the underlying mechanism is unknown. NPB fusion and its regulation by mitogen-activated protein kinase (MAPK) and maturation-promoting factor (MPF) were studied in activated mouse oocytes. Small NPBs appeared about 4 h after ethanol activation, and took about 1.5 h to fuse into a large NPB, which persisted for about 10 h before disappearance. Analysis of the temporal windows for kinase action indicated that a high MAPK activity during the first 2 h and a low MPF activity during the first 3-4 h after activation were essential for subsequent NPB fusion. A preactivation decline in MAPK activity was associated with decreased NPB fusion following activation of aged oocytes. While MAPK inactivation by regulator U0126 prevented NPB fusion in oocytes activated by ethanol or 5 min Sr2+ treatments, it had no effect on oocytes fertilized or activated by 6 h Sr2+ treatment. In most cases, while rates of pronuclear formation did not differ, rates of NPB fusion differed significantly between different treatments. Our results suggest that: (i) the MAPK and MPF activities at the initial stage of activation regulate NPB fusion after pronuclear formation; (ii) pronuclear assembly and NPB fusion are two separable events that might be controlled by different mechanisms; and (iii) high MAPK activity and low MPF activity at the initial stage of activation is essential for NPB fusion when only one calcium rise is induced by ethanol, while inhibition of MAPK activity does not affect NPB fusion when the repetitive intracellular Ca2+ rises are induced after fertilization.

Alterations in the protein kinase C signaling activated by a parthenogenetic agent in oocytes from reproductively old mice

To investigate the effect of female age on oocyte developmental competence, we focused on protein kinase C (PKC), a major component of the signalling pathway involved in oocyte activation, and put forward the hypothesis that, as it occurs in many organs and tissues, aging affects PKC function in mouse oocytes. Biochemical activity of PKC along with the expression and subcellular distribution of some PKC isoforms were monitored in young and old mouse oocytes parthenogenetically activated by SrCl(2). We found that PKC activity increased reaching a level that was lower in old compared to young oocytes in association with an incomplete translocation of PKCbetaI to the plasma membrane. Moreover, old oocytes exhibited a reduced expression of PKCbeta1 and PKCalpha at the protein level, without significant effects on the expression of the Ca(2+)-independent PKCdelta. Detectable amounts of PKCbeta1 mRNA were observed in young and old oocytes at GV stage with no difference between the two groups of age. When meiotic progression to anaphase II up to first cleavage were analyzed, a delay in meiosis resumption and significantly lower rates of pronuclei formation and first cleavage were observed in old compared to young oocytes. Moreover, we found that, in contrast to SrCl(2), PMA (12-O-tetradecanoyl phorbol-13-acetate), a PKC agonist, was ineffective in activating old oocytes. Present findings provide evidence that aging affects the correct storage and activation of some PKCs, functional components of the machinery involved in oocyte activation, and suggest that these changes may negatively influence the activation competence of old oocytes.

Early zygotes are suitable recipients for bovine somatic nuclear transfer and result in cloned offspring

Cloning by somatic cell nuclear transfer (SCNT) subverts sperm-mediated fertilization that normally leads to physiological activation of the oocyte. Therefore, artificial activation is required and it is presently unclear what developmental consequences this has. In this study, we aimed to improve cattle cloning efficiency by utilizing a more physiological method of activating SCNT reconstructs. We carried outin vitrofertilization (IVF) of zona-intact bovine oocytes before SCNT. We removed the zona pellucida 4 h after insemination, stained the fertilized eggs with Hoechst 33342 and mechanically removed both male and female chromatin. The enucleated pre-activated cytoplasts were fused with male adult ear skin fibroblasts (‘IVF-NT’ group). Chemically activated SCNT embryos, produced according to our standard operating procedure for zona-free SCNT, served as controls. After 7 days,in vitrodevelopment to blastocysts of morphological grade 1–3 or grade 1–2 was very similar in both groups (39 vs 40% and 20 vs 21% respectively). However, post-implantation development was improved after sperm-mediated activation. Across four replicate runs, pregnancy establishment at day 35 was significantly higher for IVF-NT than for control SCNT embryos (30/49 = 61 vs 17/41 = 42% respectively;P< 0.05). Development into calves at term or weaning was also higher in the IVF-NT group compared with control SCNT (9/49 = 18 vs 3/41 = 7% and 6/49 = 12 vs 3/41 = 7%;P= 0.11 and 0.34 respectively).

Parthenogenetic activation of mouse oocytes by strontium chloride: a search for the best conditions

Strontium has been successfully used to induce activation of mouse oocytes in nuclear transfer and other experiments, but the optimum treatment conditions have not been studied systematically. When cumulus-free oocytes were treated with 10mM SrCl(2) for 0.5-5h, activation rates (88.4+/-4.1 to 91.2+/-2.7%) did not differ (mean+/-S.E.; P>0.2), but rate of blastulation (57.3+/-3.5%) and cell number per blastocyst (45.0+/-2.4) were the highest after treatment for 2.5h. When treated with 1-20mM SrCl(2) for 2.5h, the activation rate and cell number per blastocyst were higher (P<0.02) after 10mM SrCl(2) treatment than other treatments. The best activation and development were obtained with Ca(2+)-free Sr(2+) medium, but the activation rate was low (37.7+/-1.6%) in Ca(2+)-containing medium. Activation rates were the same, regardless of the presence or absence of cytochalasin B (CB) in the activating medium, but the blastulation rate was higher (P<0.001) in the presence of CB. Only 70% of the cumulus-enclosed oocytes were activated and 10% blastulated after a 10 min exposure to 1.6mM SrCl(2), and many lysed, with increased intensity of Sr(2+) treatment. The presence of CB in SrCl(2) medium markedly reduced lysis of cumulus-enclosed oocytes. Media M16 and CZB did not differ when used as activating media. Only 10.5% of the oocytes collected 13 h post hCG were activated by Sr(2+) treatment alone, with 34% blastulating, but rates of activation and blastulation increased (P<0.001) to 94 and 60%, respectively, when they were further treated with 6-dimethylaminopurine (6-DMAP). The total and ICM cell numbers were less (P<0.001) in parthenotes than in the in vivo fertilized embryos. In conclusion, the concentration and duration of SrCl(2) treatment and the presence or absence of CB in activating medium and cumulus cells had marked effects on mouse oocyte activation and development. To obtain the best activation and development, cumulus-free oocytes collected 18 h post hCG should be treated for 2.5h with 10mM SrCl(2) in Ca(2+)-free medium supplemented with 5 microg/mL of CB.

Relationship between low-molecular-weight insulin-like growth factor-binding proteins, caspase-3 activity, and oocyte quality

Bovine follicular atresia is associated with the apoptosis of granulosa cells and the subsequent loss of oocyte competence through the reduction of cellular contact (e.g., gap junctions). Several components of the insulin-like growth factor (IGF) system are thought to affect follicular atresia. Whereas the IGF-binding proteins (IGFBPs) are present in varying quantities throughout follicular development, IGFBP-5 appears to be present only during atresia, in parallel with its regulation in other tissue remodeling systems. However, to our knowledge, no connection has yet been made between atresia, low-molecular-weight IGFBP content, and oocyte quality in the bovine ovary. Caspases are actively involved in ovarian follicular atresia, and apoptosis in antral follicles is caspase-3-dependent. Hence, the aim of the present study was to investigate the use of these factors in the assessment of oocyte quality and developmental potential. Oocytes were aspirated, morphologically classified, and individually matured in vitro. The follicular fluid and granulosa cells of these follicles were analyzed for IGFBP profile and caspase-3 activity, respectively. A significant correlation was found between the presence of low-molecular-weight IGFBPs in bovine follicular fluid and caspase-3 activity of granulosa cells isolated from individual follicles. The highest percentage of development to the blastocyst stage was observed in oocytes from slightly atretic follicles. This group of oocytes contained an equal proportion of oocytes at grades 1-3. These data demonstrate that low-molecular-weight IGFBP profile is a more reliable method than the traditional morphological assessment of oocytes and can be used as an effective marker of developmentally competent oocytes. Importantly, these results have implications for the use of noninvasive follicular fluid markers in the selection of competent oocytes to improve outcomes of in vitro fertilization.

Activation of protein kinase C induces mitogen-activated protein kinase dephosphorylation and pronucleus formation in rat oocytes

Mammalian oocytes are arrested at metaphase of the second meiotic division (MII) before fertilization. When oocytes are stimulated by spermatozoa, they exit MII stage and complete meiosis. It has been suggested that an immediate increase in intracellular free calcium concentration and inactivation of maturation promoting factor (MPF) are required for oocyte activation. However, the underlying mechanism is still unclear. In the present study, we investigated the role of protein kinase C (PKC) and mitogen-activated protein (MAP) kinase, and their interplay in rat oocyte activation. We found that MAP kinase became dephosphorylated in correlation with pronucleus formation after fertilization. Protein kinase C activators, phorbol 12-myriatate 13-acetate (PMA) and 1,2-dioctanoyl-rac-glycerol (diC8), triggered dephosphorylation of MAP kinase and pronucleus formation in a dose-dependent and time-dependent manner. Dephosphorylation of MAP kinase was also correlated with pronucleus formation when oocytes were treated with PKC activators. Effects of PKC activators were abolished by the PKC inhibitors, calphostin C and staurosporine, as well as a protein phosphatase blocker, okadaic acid (OA). These results suggest that PKC activation may cause rat oocyte pronucleus formation via MAP kinase dephosphorylation, which is probably mediated by OA-sensitive protein phosphatases. We also provide evidence supporting the involvement of such a process in fertilization.

A role for protein kinase C during rat egg activation

Upon sperm-egg interaction, an increase in intracellular calcium concentration ([Ca(2+)](i)) is observed. Several studies reported that cortical reaction (CR) can be triggered not only by a [Ca(2+)](i) rise but also by protein kinase C (PKC) activation. Because the CR is regarded as a Ca(2+)-dependent exocytotic process and because the calcium-dependent conventional PKCs (cPKC) alpha and beta II are considered as exocytosis mediators in various cell systems, we chose to study activation of the cPKC in the rat egg during in vivo fertilization and parthenogenetic activation. By using immunohistochemistry and confocal microscopy techniques, we demonstrated, for the first time, the activation of the cPKC alpha, beta I, and beta II during in vivo fertilization. All three isozymes examined presented translocation to the egg's plasma membrane as early as the sperm-binding stage. However, the kinetics of their translocation was not identical. Activation of cPKC alpha was obtained by the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) or by 1-oleoyl-2-acetylglycerol (OAG) but not by the calcium ionophore ionomycin. PKC alpha translocation was first detected 5-10 min after exposure to TPA and reached a maximum at 20 min, whereas in eggs activated by OAG, translocation of PKC alpha was observed almost immediately and reached a maximum within 5 min. These results suggest that, although [Ca(2+)](i) elevation on its own does not activate PKC alpha, it may accelerate OAG-induced PKC alpha activation. We also demonstrate a successful inhibition of the CR by a myristoylated PKC pseudosubstrate (myrPKCPsi), a specific PKC inhibitor. Our study suggests that exocytosis can be triggered independently either by a [Ca(2+)](i) rise or by PKC.

Molecular and biochemical regulation of early mammalian development

Fertilization initiates a rapid series of changes that restructures the egg into the zygote and initiates the program of early development. These changes in the cell occur while the genetic complement of the egg and sperm are in a highly condensed state and unable to participate in transcription. The egg cytoplasm, formed by the maternal genome, contains the necessary components that mediate the early restructuring of egg into zygote. These changes are mediated by a series of cytoplasmic signal transduction events initiated by the rise in [Ca2+]i caused when the sperm penetrates the egg. The structural changes that the egg undergoes are rapid and result in the extensive remodeling of this specialized cell. Protein kinase C (PKC) and calcium/calmodulin-dependent protein kinase II (CaM KII) are two pivotal signaling agents that mediate several of these rapid modifications in cell structure. Studies indicate the meiotic spindle serves as an architectural element in the egg that acts to colocalize elements from several of the key signaling pathways and may provide a means for these pathways to interact. In mammals, transcription begins earlier than in zygotes from other classes of organisms, starting several hours after fertilization in the male and female pronuclei and continuing in the embryonic nuclei. Studies indicate that nuclei undergo an initial state that is permissive for transcription, and then in Gap 2 of the two-cell embryo, enter a transcriptionally repressive state. These changes have been linked to the times during the cell cycle when the DNA is replicated, and also have been proposed as a requirement for proper initiation of the program of early development.

Cytoplasmic Ca2+ oscillation coordinates the formation of actin filaments in the sea urchin eggs activated with phorbol ester

Changes in the intracellular Ca2+ concentration ([Ca2+]i) and the formation of actin filaments were investigated in unfertilized eggs of the sea urchin Hemicentrotus pulcherrimus after activation with a phorbol ester, 12-O-tetradecanoyl phorbol13-acetate (TPA). Intracellular Ca2+ oscillation was observed using a fluorescent Ca2+ indicator dye, calcium green dextran. From about 20 to 80 min after the addition of TPA to 100 microM, there was a rise in [Ca2+]i, which was followed by Ca2+ oscillation. A change in [Ca2+]i in response to TPA was not observed in eggs that had been injected with heparin, an inositol 1,4,5-triphosphate (IP3) receptor antagonist. Therefore, long-term exposure to a high concentration of TPA seems to induce Ca2+ release via the IP3 pathway, as well as causing the release of diacylglycerol from membrane lipids. Moreover, the elongation of actin filaments occurred in the cytoplasm during the rise in [Ca2+]i. Actin filaments also formed when TPA-induced cytoplasmic alkalization was inhibited by exposure to Na(+)-free sea water. These results suggest that the observed cytoplasmic formation of actin filaments may be related to change in the cytoplasmic [Ca2(+)]i, and not intracellular pH, induced by TPA. These phenomena may be similar to the changes in actin construction that occur during cell cycle events.

Fertilization signalling and protein-tyrosine kinases

Fertilization is initiated by species-specific gamete cell recognition, i.e. sperm-egg interaction, followed by a rapid and sustained activation of multiple cellular and biochemical events, collectively called 'egg activation', which is indispensable for successful formation of zygotic nucleus and later embryogenesis. It is well known that sperm-induced egg activation is mediated by a transient release of calcium ions that originates from the sperm entry point and propagates through the entire egg cytoplasm. It is unclear, however, what kind of upstream events prelude to the calcium transient after sperm-egg interaction. Recently, much attention has been paid to the role of protein-tyrosine phosphorylation in egg activation process by a number of studies on some well-established model organisms. These includes marine invertebrates, frogs, and mammals. In this review, we will summarize the recent findings that begin to uncover a 'missing link' between sperm-egg interaction and egg activation with emphasis on the role of egg protein-tyrosine kinases (PTKs) in Xenopus egg fertilization.

Comparison of Oocyte-Activating Agents for Mouse Cloning

Since somatic cell components are unable to activate oocytes following injection or fusion, enucleated oocytes receiving adult somatic cells during the cloning process must be activated artificially for their development. We compared the efficiency of four types of oocyte-activating agents: strontium, ethanol, single electric pulse, and spermatozoa. Although strontium was the best in supporting preimplantation development of reconstructed mouse oocytes, there was no significant difference among the four agents with respect to the rate of postimplantation embryo development and the birth of live offspring.

MAP kinase activity is downregulated by phorbol ester during mouse oocyte maturation and egg activation in vitro

The effects of protein kinase C (PKC) stimulator, phorbol 12-myriatate 13-acetate (PMA), on meiotic cell cycle regulation and mitogen-activated protein (MAP) kinase changes have been studied in mouse oocytes and eggs. The results showed that MAP kinase activation itself was not necessary for germinal vesicle breakdown (GVBD), but the ability of the ooplasm to phosphorylate MAP kinase was a prerequisite for this event. At concentrations of 1.6 nM, PMA effectively inhibited GVBD and MAP kinase activation, suggesting that PMA inhibits GVBD by inhibiting molecule(s) upstream to MAP kinase. At concentrations of 16.2 nM, PMA induced metaphase-interphase transition more effectively in eggs collected 19 hr after human chorionic gonadotropin (hCG) administration than in those collected 15 hr after hCG administration. The degree of MAP kinase activity decrease was well correlated with the time course and proportion of pronuclear formation. On the other hand, when the effect of PMA on cell cycle progression was abolished by protein phosphatase inhibitor, okadaic acid, MAP kinase was superactivated. The biologically inactive 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD) had no evident effects on either GVBD and interphase transition or on MAP kinase activity. Furthermore, the effects of PMA on oocyte GVBD, egg activation, and MAP kinase activity could be overcome by the specific PKC inhibitor, calphostin C, suggesting the possible involvement of this enzyme in the regulation of MAP kinase activity. The results suggest that activation of PKC by PMA entrains a cascade of events that ultimately inhibits MAP kinase activation and GVBD in mouse oocytes and induces MAP kinase inactivation and metaphase-interphase transition in mouse eggs.

Mechanisms leading to cortical reaction in the mammalian egg

Activation of the mammalian egg results in cortical reaction (CR), which is correlated with an increase in intracellular Ca2+ concentration and PKC activation. The CR is a gradual rather then an "all or none" response, and can be regulated by different concentrations of parthenogenetic activators. To evaluate the biological significance of parthenogenetic induced CR, rat eggs were fertilized or activated by different concentrations of ionomycin and TPA. Cortical granules (CG) were monitored by electron microscopy, while the CG exudate was visualized by Lens culinaris lectin and Texas Red, using light and confocal microscopy. The ability of the CR to trigger a full block to polyspermy was examined in an IVF system. Our study demonstrates the existence of light and dark CG, which differ by number, distribution in the egg cortex, and sensitivity to parthenogenetic activators. Sperm penetration or high concentration of activators, trigger depletion of both light and dark CG, leading to a full CR. Low concentration of activators altered the CG density, the ratio of dark/light CG, and induced partial CR that was sufficient to cause a block to polyspermy. The results imply that Ca2+ rise or PKC activation have different effects on light and dark CG. In recently fertilized or parthenogenetically activated eggs, CG exudate appeared as evenly distributed spots, whereas in more advanced stages of fertilization the exudate was scattered as patchy aggregates. This observation suggests a difference in the dispersion of CG exudate after fertilization as compared to parthenogenetic activation.

Increase of intracellular Ca2+ and relocation of E-cadherin during experimental decompaction of mouse embryos

To determine the role of intracellular Ca2+ in compaction, the first morphogenetic event in embryogenesis, we analyzed preimplantation mouse embryos under several decompacting conditions, including depletion of extracellular Ca2+, blocking of Ca2+ channels, and inhibition of microfilaments, calmodulin, and intracellular Ca2+ release. Those treatments induced decompaction of mouse morulae and simultaneously induced changes in cytosolic free Ca2+ concentration and deregionalization of E-cadherin and fodrin. When morulae were allowed to recompact, the location of both proteins recovered. In contrast, actin did not change its cortical location with compaction nor with decompaction-recompaction. Calmodulin localized in areas opposite to cell-cell contacts in eight-cell stage embryos before and after compaction. Inhibition of calmodulin with trifluoperazine induced its delocalization while morulae decompacted. A nonspecific rise of intracellular free Ca2+ provoked by ionomycin did not affect the compacted shape. Moreover, the same decompacting treatments when applied to uncompacted embryos did not produce any change in intracellular Ca2+. Our results demonstrate that in preimplantation mouse embryos experimentally induced stage-specific changes of cell shape are accompanied by changes of intracellular free Ca2+ and redistribution of the cytoskeleton-related proteins E-cadherin, fodrin, and calmodulin. We conclude that intracellular Ca2+ specifically is involved in compaction and probably regulates the function and localization of cytoskeleton elements.

Profile of protein kinase C isozymes and their possible role in mammalian egg activation

Western blot analysis was used to investigate protein kinase C (PKC) profile of rat eggs. The presence of eight PKC isozymes was demonstrated: conventional PKC alpha, beta and gamma; novel PKC delta, epsilon and mu; atypical PKC zeta and lambda. PKC alpha was detected by RT-PCR as well. PKC translocation from the cytosol to the plasma membrane served as a marker for enzyme activation. Immunofluorescence confocal microscopy demonstrated a relatively uniform distribution of PKC alpha, betaI, and betaII throughout the cytosol of metaphase II arrested eggs. PKC accumulation at the plasma membrane was detected 5 min after exposure to 12-O-tetradecanoyl phorbol-13-acetate and increased with time, thus demonstrating activation of these PKCs.

Protein kinase C is required for the disappearance of MPF upon artificial activation in mouse eggs

The aim of the present study was to investigate the implication of protein kinase C (PKC) in the mouse egg activation process. We used OAG (1-oleoyl-2-acetyl-sn-glycerol) as a PKC activator, calphostin C as a specific PKC inhibitor, and the calcium ionophore A23187 as a standard parthenogenetic agent. The exposure of zona-free eggs to 150 microM or 50 microM OAG for 10 min resulted in meiosis II completion in approximately 80% of instances. By contrast, at a lower concentration (25 microM), the PKC stimulator was ineffective as parthenogenetic agent. Shortly after the application of 150 microM OAG, the cytosolic Ca2+ concentration ([Ca2+]i) increased transiently in all the eggs examined, whereas after the addition of 50 microM OAG, [Ca2+]i remained unchanged for at least 20 min. During this period, the activity of M-phase promoting factor (MPF) dramatically decreased and most of the eggs entered anaphase except when the PKC was inhibited by calphostin C. Similarly, MPF inactivation and meiosis resumption were prevented in calphostin C-loaded eggs following treatment with A23187, even though the ionophore-induced Ca2+ signalling was not affected. Taken together, our results indicate that stimulation of PKC is a sufficient and necessary event to induce meiosis resumption in mouse eggs and strongly suggest that, in this species, the mechanism by which a transient calcium burst triggers MPF inactivation involves a PKC-dependent pathway.

Activation of protein kinase C induces cortical granule exocytosis in a Ca(2+)-independent manner, but not the resumption of cell cycle in porcine eggs

The effects of protein kinase C (PKC) activation on meiotic resumption and cortical granule (CG) exocytosis as well as its dependence on Ca2+ in porcine eggs matured in vitro were studied. Cortical granule release was judged by both confocal laser microscopy after the eggs were labeled with fluorescein isothiocyanate-peanut agglutinin (FITC-PNA) and electron microscopy. Meiotic resumption and pronuclear formation were observed after eggs were stained with acetic orcein. When eggs were treated with PKC activators, 1-oleyl-2-acetyl-glycerol (OAG) or phorbol 12-myristate 13-acetate (PMA), the pronuclear formation percentage was significantly lower than that of Ca2+ ionophore A23187-treated group, but not statistically different from that in negative control group (P > 0.05), and most of the eggs were still arrested at metaphase II stage, suggesting that PKC activation does not induce the resumption of meiosis and pronuclear formation. In contrast, PKC activation induced 89.1% to 100% of the eggs completely or partially released their CG in different groups, not statistically different from A23187-treated group, and this effect could be overcome by PKC inhibition. When the intracellular free Ca2+ was chelated with acetoxymethal ester form of 1,2-bis(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM), and then treated with PMA or OAG in Ca(2+)-free medium, the proportions of eggs with CG release were 90.9% and 78.1%, respectively, not statistically different from the above-treated groups, suggesting that CG exocytosis induced by PKC activation is independent of Ca2+ rise. The results indicate that different events of porcine egg activation may be uncoupled from one another.

Unique protein kinase C profile in mouse oocytes: lack of calcium-dependent conventional isoforms suggested by rtPCR and Western blotting

rtPCR and Western blotting were used to determine which members of the PKC family are present in both immature and mature mouse oocytes. Using isoform-specific PCR primers and antibodies PKC-delta and -lambda were detected while such techniques failed to observe the conventional isoforms of PKC-alpha, -beta, -gamma. This isoform profile was confirmed using an alternative PCR strategy, which allowed discrimination of PCR products derived from conventional and novel PKC isoforms. In addition PKC-epsilon, -eta, -theta and -zeta were not detected by rtPCR. These results suggest that the predominant isoforms in oocytes are PKC-delta and -lambda.

Activation of protein kinase C after fertilization is required for remodeling the mouse egg into the zygote

Fertilization of the mammalian egg initiates numerous biochemical and structural changes which remodel the egg into a single-celled zygote. To date, the most extensively studied phenomenon of fertilization in virtually all species has been the relationship between sperm penetration and the induction of the initial rise in intracellular-free calcium ([Ca2+]i) concentration within the egg. In contrast, relatively few studies have focused on the biochemical events following this rise in calcium, and even fewer studies have directly linked the biochemical events to the structural changes which must ensue for proper development of the embryo. In this study, we exploited recently developed technologies to investigate the action of protein kinase C (PKC), a presumed downstream transducer of the initial rise in [Ca2+]i, during fertilization and artificial activation with calcium ionophore or phorbol 12-myristate 13-acetate (PMA). The newly developed myristoylated PKC pseudosubstrate (myrPKC psi) was used to specifically inhibit PKC, thereby averting the trauma of injecting the egg with nonmyristoylated PKC psi. Following fertilization, eggs which were pretreated with myr-PKC psi were not capable of forming a second polar body and pronuclear formation was significantly inhibited. Spatial and temporal localization of PKC using confocal microscopy to visualize the PKC reporter dye, Rim-1, demonstrated localization of PKC to the lateral aspects of the forming second polar body after fertilization, or after artificial activation with calcium ionophore or PMA. In vivo biochemical analysis of eggs which were fertilized or artificially activated demonstrated that PKC activity rose at the same time (40 min) as the second polar body formed and then subsided over the next 5 hr post activation. From these data, we conclude that PKC plays an integral role in directing the transformation from egg to embryo.

Study of protein kinase C antagonists on cortical granule exocytosis and cell-cycle resumption in fertilized mouse eggs

Although pharmacological agonists of protein kinase C (PKC) stimulate some events of mammalian egg activation, including cortical granule (CG) exocytosis, it is not known if these events are dependent on PKC activation during the normal process of fertilization. In order to examine the potential role of PKC in CG exocytosis, this study investigated whether PKC agonists faithfully mimic CG release and whether PKC antagonists block fertilization-induced CG release in mature mouse eggs. Phorbol ester (TPA, 2.5 ng/ml) treatment resulted in an atypical pattern of CG release in which there was a greater net loss of CGs in the equatorial region of the egg than in the region opposite the spindle. This pattern also was in contrast to that during fertilization, in which CG release occurred randomly throughout the cortex. Fertilization experiments utilized two different PKC inhibitors, bisindolylmaleimide (5 microM) and chelerytherine (0.8 microM), targeted to both the "conserved" substrate and ATP binding domains of PKC. Simultaneous use of both inhibitors at maximal concentrations (compatible with fertilization and above their IC50S) resulted in no detectable inhibition of CG release in treated fertilized eggs compared to controls. In addition no inhibition of anaphase onset was observed in treated fertilized eggs. Activity of the inhibitors was verified by demonstrating that they blocked the induction of CG loss by TPA. Moreover, 1 microM staurosporine, a potent but less specific antagonist of PKC, also did not block CG loss whereas the metaphase-anaphase transition was temporarily inhibited. The results indicate that TPA does not faithfully mimic CG release in fertilized eggs, that a role for PKC in CG release at fertilization remains to be established, and that other calcium-dependent effectors may be involved in CG exocytosis.

Calcium- and meiotic-spindle-independent activation of pig oocytes by the inhibition of staurosporine-sensitive protein kinases

The dependence of pig oocyte activation (both nuclear activation and cortical granule exocytosis) induced by staurosporine on intracellular Ca2+ rise and spindle assembly was studied. Nuclear activation was evaluated by pronuclear (PN) formation, cleavage and their developmental ability, and cortical granule (CG) exocytosis was assessed by electron microscopy and laser confocal microscopy of oocytes labelled with fluorescein isothiocyanate-peanut agglutinin. Exposure of pig oocytes of 0.3 and 3 microM protein kinase inhibitor staurosporine for 30 min resulted in the nuclear activation in 71.8% and 85.7% of the oocytes, respectively. The pronuclei in activated oocytes contained several compact nucleoli. When the cleaved 2-cell oocytes were further cultured in vitro, 93.5% developed beyond the 4-cell stage, and 12.9% developed to the morula stage after 4 days of culture. Of the oocytes treated with 3 microM staurosporine, 62.5% and 9.4% released their CGs partially and completely, respectively. The nuclear activation induced by staurosporine was overcome by the prior treatment of oocytes with okadaic acid, resulting in only 33.3% of the oocytes undergoing nuclear activation. However, when oocytes were exposed first to 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethanal ester), a cell permeate calcium chelator, or Colcemid, a meiotic spindle disrupter, and then to staurosporine, nuclear activation was observed in 74.2% and 82.3% of the oocytes, respectively. These data were the same as those in oocytes treated only with staurosporine (85.7%). The present study indicates that pig oocytes can be activated by the inhibition of staurosporine-sensitive protein kinase(s), and that this activation is dependent upon mitogen-activated protein kinase but independent of the intracellular Ca2+ rise and spindle integrity.

Transient release of calcium from inositol 1,4,5-trisphosphate-specific stores regulates mouse preimplantation development

Inositol 1,4,5-trisphosphate can regulate growth and differentiation by modulating the release of intracellular Ca2+ in a variety of cellular systems, and it is involved in oocyte activation. Recent studies suggest that mammalian preimplantation development may also be regulated by the release of Ca2+ from intracellular stores. The rate of cavitation and cell division was accelerated after a transient elevation of intracellular Ca2+ levels was induced in morulae by exposure to ethanol or ionomycin. Embryos exposed to BAPTA-AM, a chelator of intracellular Ca2+, exhibited a brief dose-dependent reduction in basal Ca2+ levels, a temporal inhibition of ionophore-induced Ca2+ signalling and a subsequent delay in blastocoel formation. BAPTA-AM at 0.5 microM did not significantly alter the basal intracellular calcium level, but chelated Ca2+ that was released after ethanol exposure and thereby attenuated the ethanol-induced acceleration of cavitation. BAPTA-AM also inhibited cell division to the 16-cell stage in a dose-dependent manner, which correlated with the inhibition of cavitation. Thimerosal and inositol 1,4,5-trisphosphate significantly elevated the intracellular Ca2+ concentration in mouse morula-stage embryos, providing evidence for the existence of inositol 1,4,5-trisphosphate-sensitive Ca2+ stores. Although caffeine failed to release intracellular Ca2+, ryanodine induced a small biphasic release of Ca2+, suggesting that ryanodine-sensitive Ca2+ stores may also exist in mouse embryos. Morulae exposed to the calmodulin inhibitor W-7 exhibited a dose-dependent delay in blastocoel formation. A 4 hour exposure to 10 microM W-7 did not significantly alter cavitation, but attenuated the ionophore-induced stimulation of blastocoel formation. This finding suggests that the developmental effects produced through Ca2+ signalling are mediated by calmodulin. Our results demonstrate that Ca2+ release in mouse morulae occurs predominantly through the inositol 1,4,5-trisphosphate receptor, and that alteration of intracellular Ca2+ levels can accelerate or delay embryonic growth and differentiation, providing a mechanistic link between the regulation of oocyte and embryonic development.

Effects of alcohols on murine preimplantation development: relationship to relative membrane disordering potency

During in vitro culture of murine preimplantation embryos, we have observed that exposure to 0.1% ethanol induces an immediate increase in intracellular calcium levels and subsequently accelerates embryogenesis. If the observed effects of ethanol on developing embryos is mediated by its membrane disordering potency, we hypothesized that the relative membrane disordering potencies of related alcohols would correspondingly effect embryonic intracellular calcium levels and developmental rates. Two-cell embryos were exposed to 0.1% ethanol or 0.05 to 1.0% (w/v) n-butanol, n-propanol, isopropanol, 1,2-propanediol, glycerol, or methanol for 24 hr at 37 degrees C, and development to the blastocyst stage was monitored after 5 days. n-Butanol, n-propanol, isopropanol, and methanol treatment caused a dose-dependent inhibition (p < 0.01) of development to the blastocyst stage, whereas 1,2-propanediol or glycerol neither accelerated nor inhibited development. In a second experiment, 8-cell morulae were treated with 1,2-propanediol or glycerol, and cavitation rates were examined. There was no significant difference from control embryos in the onset of cavitation or the blastocoel expansion rate of 1,2-propanediol- or glycerol-exposed embryos, whereas exposure to 0.1% ethanol accelerate cavitation (p > 0.05). In a third experiment, morulae were exposed to 0.1% or 1.0% of each alcohol and were monitored for changes in intracellular calcium levels using the fluorescent indicator, fluo-3-acetoxymethyl ester. There was an immediate increase in intracellular calcium levels when morulae were treated with 1.0% ethanol or n-butanol, but only ethanol induced an increase (p < 0.05) in the level of intracellular calcium at 0.1%. These data suggest that ethanol is unique in its ability to accelerate embryogenesis and that the membrane disordering potency of ethanol does not directly underlie its effects on intracellular calcium release and the acceleration of preimplantation development.

Calcium oscillations and protein synthesis inhibition synergistically activate mouse oocytes

We have examined the ability of the two parthenogenetic agents, strontium (Sr2+) and cycloheximide, to activate mouse oocytes. We demonstrate that Sr2+ and cycloheximide act synergistically to promote parthenogenetic activation up to the pronuclear stage in oocytes collected immediately after ovulation. These two agents appeared to act independently, since incubation in Sr2+ media triggered a series of intracellular Ca2+ rises without affecting protein synthesis and cycloheximide inhibited protein synthesis without causing any intracellular Ca2+ changes. In addition, cycloheximide did not alter the pattern of Ca2+ oscillations induced by Sr2+. In contrast, we show that another commonly used parthenogenetic activation treatment, the Ca2+ ionophore A23187, has dual effects. Exposure of oocytes to the Ca2+ ionophore, A 23187, in Ca(2+)- and Mg(2+)-free medium leads to the activation of young oocytes. However, as well as generating a Ca2+ increase, the treatment of mouse oocytes with A23187 and Ca(2+)- and Mg(2+)-free media led to a marked inhibition of protein synthesis. Our data show that parthenogenetic agents may have two important loci for activating mammalian oocytes and that the combined effect on Ca2+ release and protein synthesis is most effective.

Highly effective method of human oocyte activation

Fresh and aged unfertilised human oocytes were activated by electroporation and by exposure to isotonic solution of mannitol supplemented with low concentrations of calcium, magnesium and chloride. Over 95% of the fresh oocytes were activated, all showing formation of one pronucleus and extrusion of the second polar body. Oocytes activated 1 and 2 days post-collection showed activation rates of 66.6% and 64.1%, respectively; however, the proportion of one-pronucleate oocytes in these groups was significantly lower (61.6% and 23.5%, respectively). There was no difference in the activation efficiency between the two activation modes. Twelve activated oocytes from the freshly collected group cleaved when left in culture. It is concluded that, in the human, a brief exposure to isotonic solution of mannitol with low concentrations of calcium, magnesium and chloride is a very effective activation stimulus.

The exit of mouse oocytes from meiotic M-phase requires an intact spindle during intracellular calcium release

To study the role of the metaphase spindle during the period of oocyte activation, mouse oocytes were fertilised or activated parthenogenetically in the presence or absence of the microtubule inhibitor nocodazole. In both cases, nocodazole caused the disappearance of the spindle and prevented the passage of the oocytes into interphase. However, the calcium spiking responses of the oocytes were not affected by nocodazole, being repetitive after fertilisation and a single spike after activation. If, after their activation or fertilisation in nocodazole, oocytes were later removed from the drug, only those that had been fertilised progressed into interphase. This progress was associated with continuing calcium spiking. Moreover, both the spiking and the progress to interphase could be blocked or reduced in incidence by removal of external calcium or addition of 5,5′-dimethyl BAPTA-AM. Oocytes that had been activated by ethanol in the presence of nocodazole and then removed from it, to allow re-formation of the spindle, only progressed into interphase if given a second exposure to ethanol, thereby eliciting a second calcium transient. These results show that exit from meiotic M-phase requires the simultaneous presence of a fully intact spindle during the release of calcium and that those factors leading to the degradation of cyclin B are only activated transiently. Since cyclin is being degraded continuously in the metaphase-II-arrested mouse oocyte and since this degradation is microtubule-dependent, these data suggest that the superimposition of a high concentration of intracellular calcium is required to tilt the equilibrium further in favour of cyclin degradation if exit from M-phase is to occur.

Oocyte reaction to penetrating sperm

Before fertilisation the egg is metabolically quiescent and its nucleus is arrested at metaphase of the second meiosis. After sperm-egg fusion, the arrested nucleus resumes meiosis, and then changes into the female pronucleus. Such a sequence of morphological and biochemical events is called “activation”. The inital responses of the egg to activation by the sperm include cortical granule exocytosis and resumption of meiosis.

Diacylglycerol-enhanced electrical activation of porcine oocytes matured in vitro

This study was undertaken to examine the effect of second messengers added to the electroporation medium on electric pulse-induced artificial activation of meiotic Metaphase II porcine oocytes. Six separate experiments evaluated second messengers added to electroporation medium. When added to electroporation medium, neither phospholipase C (PLC: 0 to 2.5 Units/ml), D-myo-inositol triphosphate (IP(3): 0 to 10,000 muM), nor guanosine 5'-O-(3-thiotriphosphate; GTP-gamma-S: 0 to 100 muM) had any effect (P> 0.05) on activation rates. However, addition of 1,2-dioctanoyl-sn-glycerol (DiC(8)) increased activation rates in a dose-dependent response. At a level of 1,000 muM, DiC(8) resulted in a higher activation rate (P< 0,05) than 0.0, 0.1, 1 or 10 muM of DiC(8) with a pulse, and the 1,000 and 10,000 muM of DiC(8) no-pulse control groups. Effects of DiC(8) (1,000 muM) and IP(3) (100 muM) in combination or individually were investigated. At 1,000 muM, DiC(8) caused a higher rate of activation (P< 0.05) than 100 muM IP(3), but the result was not different from DiC(8) + IP(3). In another experiment, no difference (P> 0.05) was observed between DiC(8), GTP-gamma-S and IP(3), but DiC(8) + GTP-gamma-S + IP(3) + PLC yielded a higher (P< 0.05) activation rate than PLC or the rate of the controls. No significant development (blastocyst) was observed after 5 days of culture in any of the experiments. Protein profiles of activated oocytes, determined by 1D SDS-PAGE, were characteristic of pronuclear-stage embryos. These data indicate that the addition of DiC(8) to the electroporation medium synergistically enhances the rate of activation of electrically stimulated in vitro-matured porcine oocytes.

Protein kinase C-dependent and independent events in mouse egg activation

The involvement of calcium- or protein kinase C (PKC)-dependent pathways in cortical granule exocytosis (CGE) and pronucleus formation was examined in mouse eggs using the specific PKC stimulator OAG (1-oleyl-2-acetyl-sn-glycerol) at different external calcium concentrations ([Ca2+]e) ranging from 1.7 mM to 0.1 microM. A 10 min exposure of eggs to 150 microM OAG in the presence of 1.7 mM [Ca2+]e caused a large calcium influx, cortical granule release and 82% activation. The increased permeability of the egg membrane to Ca2+ ions after OAG treatment lasted 20 min. At [Ca2+]e lower than 1.7 mM, both OAG-induced calcium influx and CGE decreased, reaching a non-detectable level at 0.1 microM and 100 microM [Ca2+]e, respectively. Resumption of meiosis was not affected by [Ca2+]e above 200 microM but it was reduced at any lower [Ca2+]e, with a minimum activation frequency of 46% at 0.1 microM [Ca2+]e. Loading of eggs with > or = 3 microM of the calcium chelator BAPTA AM (1,2-bis(o-aminophenoxy)ethane-N',N',N',N'-tetraacetic acid-acetoxymethyl ester) prior to OAG treatment caused a reduction in meiosis resumption with 50% of eggs forming pronuclei. Potent inhibitors of PKC, such as acridine orange and sphingosine, did not interfere with OAG-induced CGE. Conversely, these compounds prevented OAG-induced pronucleus formation in a dose-dependent manner with an IC50 (inhibiting concentration, 50%) of 5 microM and 30 microM for acridine orange and sphingosine, respectively. Microinjection of inositol 1,4,5-trisphosphate into eggs at 0.1 microM elicited Ca2+ release from intracellular stores and the cortical reaction, but failed to stimulate pronucleus formation. These results indicate that, in mouse eggs, CGE is a PKC-independent event, and that the transition from M-phase to interphase may require PKC activity for stimulation.

Electroporation of inositol 1,4,5-triphosphate induces repetitive calcium oscillations in murine oocytes

The purpose of these experiments was to determine the effect of electroporation of IP3 into the cytosol of murine secondary oocytes and evaluate any alterations in [Ca2+]i resulting from Ca2+ release from intracellular stores. In addition, we evaluated the effect of ethanol (ETOH) on the release of Ca2+ from intracellular stores. Oocytes were loaded with the Ca2+ indicator fluo-3 by incubation in 100 microliters drops of medium containing 2 microM fluo-3/AM for 60 min at 37 degrees C. Changes in fluorescence were monitored by use of an inverted microscope which had been connected to a spectrofluorometer. Fluorescent intensity measurements were acquired for a minimum of 416 sec time span or up to 1,248 sec, with integration readings of 1 sec duration obtained every 2 sec throughout the measurement period. The experimental design consisted of comparing the rise in [Ca2+]i of fluo-3 loaded secondary oocytes subjected to electroporation in PBS and Ca(2+)-free PBS, each containing 25 microM IP3, to that elicited by PBS and Ca(2+)-free PBS containing a final concentration of 7% ETOH. Non-pulsed control secondary oocytes were placed in PBS + 25 microM IP3 during monitoring of [Ca2+]i fluorescence. Pulsed control secondary oocytes were placed in Ca(2+)-free PBS, subjected to electroporation pulse, and monitored for [Ca2+]i fluorescence. Electroporation of IP3 was accomplished by placing fluo-3 loaded secondary oocytes between the electrodes of a glass slide fusion chamber which had been overlaid with 300 microliters of PBS + 25 microM IP3 or Ca(2+)-free PBS + 25 microM IP3.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase C modulators influence meiosis kinetics but not fertilizability of mouse oocytes

The role of protein kinase C (PKC) in the successive steps of mouse oocyte meiotic process was investigated. We have used either OAG, an analog of diacylglycerol, or mezerein, a nonphorbol ester diterpene, less tumor promoting than phorbol esters, as PKC activators, and staurosporine as PKC inhibitor. Cumulus-free oocytes were cultured in minimum essential medium with each of these PKC modulators and maturation stages were screened every two hours until the end of the process. Both PKC activators prevented GVBD at each tested dose for 4 hr (OAG) and 8 hr (mezerein), and decreased the frequencies of PB oocytes. The inhibitory effects of both activators were dose dependent and reversible. The addition of OAG to the culture medium after GVBD occurrence (i.e., after 4 hrs) did not affect PB extrusion whereas similar addition of mezerein significantly decreased the frequency of PB oocytes. Inhibition of PKC by staurosporine accelerated GVBD and increased the frequency of PB extrusion. When staurosporine was added after GVBD, PB extrusion occurred earlier but PB oocyte frequency was not increased. Fertilizability was not affected when oocyte maturation occurred in the presence of any of these substances despite the delay in maturation process. These results clearly indicate that the PKC pathway is involved in mouse oocyte meiotic process: activation of the enzyme would arrest meiotic process whereas its inhibition would participate in meiosis induction.

Parthenogenetic activation pattern and microtubular organization of the mouse oocyte after exposure to 1,2-propanediol

We have studied the effect of 1,2-propanediol (PROH) on cumulus-oocyte complexes from the mouse. We determined the morphological survival rate, the pattern of parthenogenetic activation, and the microtubular and chromosomal organization. Cumulus-oocyte complexes were collected at 16 h post hCG from superovulated female hybrid mice. These cumulus-intact oocytes were exposed to 1.5 or 3 M PROH for 6, 12, or 18 min at 0, 22, or 37 degrees C. The cryoprotectant was diluted out in a 1 M sucrose solution at 22 degrees C. After 5-6 h at 37 degrees C, oocytes were denuded and examined under Nomarski optics. The results show that PROH can induce degeneration and parthenogenetic activation in the mouse oocyte in a concentration, temperature, and time-dependent way. As the activation stimulus was strengthened, an increasing proportion of oocytes shifted from parthenogenetic activation with polar body extrusion to parthenogenetic activation with polar body retention and even to immediate cleavage. Nontoxic and nonactivating conditions involved mainly exposure to 1.5 M PROH at 0 degrees C. Spindle integrity and chromosomal organization were analyzed for exposure to 1.5 and 3 M PROH for 12 min at 0 degrees C. The separate effect of cooling and exposure to 1 M sucrose were also evaluated. Microtubules were visualized by monoclonal anti-alpha-tubulin labeling followed by immunogold-silver staining. Cooling and exposure to 1 M sucrose or to 1.5 M PROH did not induce major abnormalities in the microtubular or chromosomal organization. On the other hand, a significant percentage of deformities such as spindle size reduction and loss of bipolarity were observed after exposure to 3 M PROH. The results of the present study demonstrate that the use of PROH as a single cryoprotectant for the freezing of mature unfertilized oocytes cannot be recommended in procedures involving ambient temperature or concentrations exceeding 1.5 M PROH. On the other hand, the potential beneficial effect of low temperatures may outweigh the effect of concentration at subzero temperatures and could be explored further in the tailoring of conditions for slow controlled freezing.

Meiosis, egg activation, and nuclear envelope breakdown are differentially reliant on Ca2+, whereas germinal vesicle breakdown is Ca2+ independent in the mouse oocyte

During early development, intracellular Ca2+ mobilization is not only essential for fertilization, but has also been implicated during other meiotic and mitotic events, such as germinal vesicle breakdown (GVBD) and nuclear envelope breakdown (NEBD). In this study, the roles of intracellular and extracellular Ca2+ were examined during meiotic maturation and reinitiation at parthenogenetic activation and during first mitosis in a single species using the same methodologies. Cumulus-free metaphase II mouse oocytes immediately resumed anaphase upon the induction of a large, transient Ca2+ elevation. This resumption of meiosis and associated events, such as cortical granule discharge, were not sensitive to extracellular Ca2+ removal, but were blocked by intracellular Ca2+ chelators. In contrast, meiosis I was dependent on external Ca2+; in its absence, the formation and function of the first meiotic spindle was delayed, the first polar body did not form and an interphase-like state was induced. GVBD was not dependent on external Ca2+ and showed no associated Ca2+ changes. NEBD at first mitosis in fertilized eggs, on the other hand, was frequently, but not always associated with a brief Ca2+ transient and was dependent on Ca2+ mobilization. We conclude that GVBD is Ca2+ independent, but that the dependence of NEBD on Ca2+ suggests regulation by more than one pathway. As cells develop from Ca(2+)-independent germinal vesicle oocytes to internal Ca(2+)-dependent pronuclear eggs, internal Ca2+ pools increase by approximately fourfold.

Repetitive calcium transients and the role of calcium in exocytosis and cell cycle activation in the mouse egg

The role of calcium in cortical granule exocytosis and activation of the cell cycle at fertilization was examined in the mouse egg using the calcium chelator BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) and the fluorescent calcium indicator fluo-3. BAPTA and fluo-3 were introduced into zona-free mouse eggs by a 30-min incubation with 0.01-50 microM BAPTA acetoxymethyl ester (AM) and/or 1-20 microM fluo-3 AM prior to in vitro fertilization. Incubation of eggs in greater than or equal to 5.0 microM BAPTA AM inhibited cortical granule exocytosis in all cases. Introduction of the calcium chelator into the egg blocked second polar body formation at greater than or equal to 1.0 microM BAPTA AM. Sperm entry occurred in all eggs regardless of the BAPTA AM concentration. Sperm induce a large transient increase in calcium lasting 2.3 +/- 0.6 min, followed by repetitive transients lasting 0.5 +/- 0.1 min and occurring at 3.4 +/- 1.4-min intervals. Incubation with greater than or equal to 5.0 microM BAPTA AM inhibited all calcium transients. Introduction of BAPTA also inhibited calcium transients, exocytosis, and the resumption of meiosis following application of the calcium ionophore A23187 or SrCl2, which activate eggs. These results demonstrate that the calcium increase at fertilization is required for cortical granule exocytosis and resumption of the cell cycle in a mammalian egg.

Chromatin behaviour under influence of puromycin and 6-DMAP at different stages of mouse oocyte maturation

Preovulatory mouse oocytes were cultured in vitro up to each subsequent stages of maturation: germinal vesicle (GV), germinal vesicle breakdown (GVBD), groups of not yet individualized bivalents, circular bivalents, late prometaphase I, metaphase I, anaphase I and telophase I. The stages were identified in living oocytes by fluorescence microscopy using Hoechst 33342 as a specific vital dye. Oocytes from each stage of development developed in vitro and ovulated metaphase II oocytes were subsequently cultured in the presence of puromycin or 6-dimethylaminopurine (6-DMAP), an inhibitor of protein phosphorylation. The effects on chromatin of these drugs were studied during and at the end of culture by fluorescence and electron microscopy. We found that puromycin and 6-DMAP stop meiosis when applied at all stages of oocyte maturation, except for metaphase II. Oocytes at this stage are activated by puromycin. Reaction of the oocytes to the two drugs is different at GV and at metaphase II. All of the other stages react to the drugs by chromatin compaction, which can be followed by chromatin decondensation to form a nucleus. Our results suggest that late prophase chromatin condensation, bivalent individualization and retention of their individuality, as well as individualization of monovalents from telophase and retention of their individuality at metaphase II, are dependent on protein phosphorylation. The events occurring between metaphase I and telophase I are independent of protein synthesis and phosphorylation. The events occurring between metaphase II and formation of the nucleus are independent of protein synthesis.

The four dimensions of calcium signalling in Xenopus oocytes

This review focuses on the inositol phosphate/Ca2+ signalling pathway in Xenopus oocytes. The known characteristics of the individual elements of this cascade--from the membrane receptors to the intracellular Ca2+ stores--will be covered. Based on this knowledge, a simple model will then try to account for the behaviour of the newly recognized oscillations of free intracellular Ca2+ and propagated Ca2+ waves. Finally, some of the potential physiological functions of the inositol phosphate pathway will be summarized. Although there is no systematic attempt to contrast the findings in the oocyte to those in other cells, the readers of this journal will not fail to notice a high degree of similarity. Although this may seem unexciting at first, it suggests that the inositol phosphate signalling pathway may be strikingly conserved across species.