Link between fertilization-induced Ca2+ oscillations and relief from metaphase II arrest in mammalian eggs: a model based on calmodulin-dependent kinase II 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.

Inhibition of CDK4/6 kinases causes production of aneuploid oocytes by inactivating the spindle assembly checkpoint and accelerating first meiotic progression

Cyclin D-CDK4/6 complex mediates the transition from the G1 to S phase in mammalian somatic cells. Meiotic oocytes pass through the G2/M transition and complete the first meiosis to reach maturation at the metaphase of meiosis II without intervening S phase, while Cyclin D-CDK4/6 complex is found to express during meiotic progression. Whether Cyclin D-CDK4/6 complex regulates meiotic cell cycle progression is not known. Here, we found its different role in oocyte meiosis: Cyclin D-CDK4/6 complex served as a regulator of spindle assembly checkpoint (SAC) to prevent aneuploidy in meiosis I. Inhibition of CDK4/6 kinases disrupted spindle assembly, chromosome alignment and kinetochore-microtubule attachments, but unexpectedly accelerated meiotic progression by inactivating SAC, consequently resulting in production of aneuploid oocytes. Further studies showed that the MPF activity decrease before first polar body extrusion was accelerated probably by inactivation of the SAC to promote ubiquitin-mediated cyclin B1 degradation. Taken together, these data reveal a novel role of Cyclin D-CDK4/6 complex in mediating control of the SAC in female meiosis I.

Diagnosis and Treatment of Male Infertility-Related Fertilization Failure

Infertility affects approximately 15% of reproductive-aged couples worldwide, of which up to 30% of the cases are caused by male factors alone. The origin of male infertility is mostly attributed to sperm abnormalities, of which many are caused by genetic defects. The development of intracytoplasmic sperm injection (ICSI) has helped to circumvent most male infertility conditions. However, there is still a challenging group of infertile males whose sperm, although having normal sperm parameters, are unable to activate the oocyte, even after ICSI treatment. While ICSI generally allows fertilization rates of 70 to 80%, total fertilization failure (FF) still occurs in 1 to 3% of ICSI cycles. Phospholipase C zeta (PLCζ) has been demonstrated to be a critical sperm oocyte activating factor (SOAF) and the absence, reduced, or altered forms of PLCζ have been shown to cause male infertility-related FF. The purpose of this review is to (i) summarize the current knowledge on PLCζ as the critical sperm factor for successful fertilization, as well as to discuss the existence of alternative sperm-induced oocyte activation mechanisms, (ii) describe the diagnostic tests available to determine the cause of FF, and (iii) summarize the beneficial effect of assisted oocyte activation (AOA) to overcome FF.

Post-translational regulation of the maternal-to-zygotic transition

The maternal-to-zygotic transition (MZT) is essential for the developmental control handed from maternal products to newly synthesized zygotic genome in the earliest stages of embryogenesis, including maternal component (mRNAs and proteins) degradation and zygotic genome activation (ZGA). Various protein post-translational modifications have been identified during the MZT, such as phosphorylation, methylation and ubiquitination. Precise post-translational regulation mechanisms are essential for the timely transition of early embryonic development. In this review, we summarize recent progress regarding the molecular mechanisms underlying post-translational regulation of maternal component degradation and ZGA during the MZT and discuss some important issues in the field.

A lack of coordination between sister-chromatids segregation and cytokinesis in the oocytes of B6.YTIR (XY) sex-reversed female mice

The B6.Y^TIR (XY) mouse develops bilateral ovaries despite the expression of the testis-determining gene Sry during gonadal differentiation. We reported that the oocytes of the XY female are defective in their cytoplasm, resulting in a failure in the second meiotic division after activation or fertilization in vitro. However, the mechanism of meiotic failure or the cause of infertility remained to be clarified. In the present study, we obtained mature oocytes from XY females by superovulation and confirmed that these oocytes also fail in zygotic development. By using confocal microscopy 3D-analysis, we demonstrated that meiotic spindles were properly positioned and oriented in the MII-oocytes from XY females. After parthenogenic activation, fewer oocytes from XY females extruded the second polar body, and in those oocytes, sister-chromatids were often separated but neither set entered the second polar body. ARP2, F-actin, and ORC4, known to play roles in asymmetric meiotic division, were initially localized along the ooplasmic membrane and concentrated over the MII-spindle but lost their cortical polarity after activation while the sister-chromatids moved away from the oolemma in the oocytes from XY females. Our results indicate that the second polar body extrusion is uncoupled from the sister-chromatids separation in the oocytes from XY female mouse.

Calcineurin role in porcine oocyte activation

Calcineurin is required for oocyte exit from meiotic block in metaphase II (MII) stage in invertebrates and also in lower vertebrates. However, the role of calcineurin in mammalian oocyte activation is still unclear. The aim of this study was to determine whether calcineurin is involved in the processes regulating porcine oocyte activation. Indirect immunofluorescence demonstrated localization of both calcineurin subunits, CnA and CnB, especially in the cortex area of MII oocytes, in vitro fertilized and also parthenogenetically activated oocytes. After activation, the fluorescence intensity of the protein in the cortex area of oocytes remains unchanged; the protein calcineurin in the cytoplasm was recorded mainly around the pronuclei. Treatment of matured oocytes with calcineurin inhibitors, cyclosporin A (CsA) and hymenistatin I (HS-I), followed by activation with calcium ionophore A23187, significantly decreased the rate of activated oocytes compared to oocytes that were treated only with calcium ionophore (Ca-Io), (CsA+Ca-Io 25.0% v. Ca-Io 83.3%; HS-I+Ca-Io 32.5% v. Ca-Io 85.0%). Compared to the control, CsA treatment of matured oocytes followed by activation with Ca-Io did not affect the activity level of metaphase-promoting factor (MPF) and mitogen-activated protein kinase (MAPK) in activated oocytes evaluated by kinase activity assay. Simultaneous staining of calcineurin and cortical granule content in matured oocytes showed that calcineurin distributed in the cortical area of the oocyte has not been colocalized with cortical granules content. On the other hand, the calcineurin inhibition before parthenogenetic activation leads to a reduction of the cortical reaction level compared to oocytes that were not treated with CsA (complete exocytosis: CsA+Ca-Io 2.6% v. Ca-Io 83.9%; sum of cortical granule brightness: CsA + Ca-Io 0.69 v. Ca-Io 0.15). Our results showed that calcineurin is involved in the process of pig oocyte activation and cortical granule exocytosis; however this regulation seems to be MPF and MAPK independent.

Modulation of cell cycle control during oocyte-to-embryo transitions

Ex ovo omnia--all animals come from eggs--this statement made in 1651 by the English physician William Harvey marks a seminal break with the doctrine that all essential characteristics of offspring are contributed by their fathers, while mothers contribute only a material substrate. More than 360 years later, we now have a comprehensive understanding of how haploid gametes are generated during meiosis to allow the formation of diploid offspring when sperm and egg cells fuse. In most species, immature oocytes are arrested in prophase I and this arrest is maintained for few days (fruit flies) or for decades (humans). After completion of the first meiotic division, most vertebrate eggs arrest again at metaphase of meiosis II. Upon fertilization, this second meiotic arrest point is released and embryos enter highly specialized early embryonic divisions. In this review, we discuss how the standard somatic cell cycle is modulated to meet the specific requirements of different developmental stages. Specifically, we focus on cell cycle regulation in mature vertebrate eggs arrested at metaphase II (MII-arrest), the first mitotic cell cycle, and early embryonic divisions.

Parthenogenetic activation of bovine oocytes using single and combined strontium, ionomycin and 6-dimethylaminopurine treatments

In vitro-matured (IVM) bovine oocytes were activated with single and combined treatments of strontium (S), ionomycin (I) and 6-DMAP (D). Using oocytes IVM for 26 h, we observed that activation altered cell cycle kinetics (faster progression, MIII arrest, or direct transition from MII to pronuclear stage) when compared toin vitrofertilization. The effect of oocyte age on early parthenogenesis was assessed in oocytes IVM for 22, 26 and 30 h. Better results in pronuclear development were obtained in treatments ISD (81.7%) at 22 h; D (66.7%), IS (63.3%), ID (73.3%) and ISD (76.7%) at 26 h; and D (86.7%), IS (85.0%) and ID (78.3%) at 30 h. Higher cleavage occurred on ISD (80.0%) at 22 h; ID (83.3%) and ISD (91.7%) at 26 h; and I (86.7%), IS (90.0%), ID (85.0%) and ISD (95.0%) at 30 h. More blastocysts were achieved in ID (25.0%) and ISD (18.3%) at 22 h; and in ID at 26 h (45.0%) and 30 h (50.0%). We also observed that IS allowed higher haploid (77.4%) embryonic development, whilst ID was better for diploid (89.1%) development. It was concluded that association of S and D without I was not effective for blastocyst development; treatments using S were less influenced by oocyte age, but when S was associated with D there was a detrimental effect on aged oocytes; treatment ISD promoted higher activation and cleavage rates in young oocytes and ID protocol was the best for producing blastocysts.

Differential expression of oxygen-regulated genes in bovine blastocysts

Low oxygen conditions (2%) during post-compaction culture of bovine blastocysts improve embryo quality, which is associated with a small yet significant increase in the expression of glucose transporter 1 (GLUT-1), suggesting a role of oxygen in embryo development mediated through oxygen-sensitive gene expression. However, bovine embryos to at least the blastocyst stage lack a key regulator of oxygen-sensitive gene expression, hypoxia-inducible factor 1alpha (HIF1alpha). A second, less well-characterized protein (HIF2alpha) is, however, detectable from the 8-cell stage of development. Here we use differential display to determine additional gene targets in bovine embryos in response to low oxygen conditions. While development to the blastocyst stage was unaffected by the oxygen concentration used during post-compaction culture, differential display identified oxygen-regulation of myotrophin and anaphase promoting complex 1 expression, with significantly lower levels observed following culture under 20% oxygen than 2% oxygen. These results further support the hypothesis that the level of gene expression of specific transcripts by bovine embryos alters in response to changes in the oxygen environment post-compaction. Specifically, we have identified two oxygen-sensitive genes that are potentially regulated by HIF2 in the bovine blastocyst.

Morphology of Dromedary Camel Oocytes and their Ability to Spontaneous and Chemical Parthenogenetic Activation

The present work was conducted to examine (1) the morphology of dromedary cumulus-oocytes complexes (COCs), (2) to study the incidence of spontaneous development of oocytes in vivo and (3) to assess the ability of in vitro matured dromedary oocytes to chemical parthenogenetic activation compared with in vitro fertilized (IVF) oocytes. COCs were recovered from dromedary ovaries classified according to their morphology into six categories. Oocyte diameter was measured using eye piece micrometer. For chemical activation, COCs with at least three layers of cumulus-cells were in vitro matured (IVM) in TCM 199 + 10 microg/ml FSH + 10 IU hCG/ml + 10% FCS + 50 microg/ml gentamycin. COCs were incubated for 40 h at 38.5 degrees C under 5% CO2 in humidified air. After IVM, matured oocytes with first polar body (first Pb) were divided into two groups. Group 1: activated in 7% ethanol (E) for 5 min followed by culture in 2 mM 6-dimethylaminopurin (6-DMAP, E D, subgroup 1) or 10 microg/ml cycloheximide (CHX, E CHX, subgroup 2) for 3.5 h at 38.5 degrees C under 5% CO2. In group 2, oocytes were activated using 50 microM Ca A23187 (Ca A) for 5 min followed by culture in 2 mM 6-DMAP (Ca D, subgroup 3) or 10 microg/ml CHX(Ca CHX, subgroup 4) for 3.5 h at 38.5 degrees C under 5% CO2. For control group, IVM oocytes were fertilized using frozen-thawed camel spermatozoa separated by swim-up method then suspended in Fert-TALP medium supplemented with 6 mg/ml BSA (FAF) + 10 microg/ml heparin. In all groups, oocytes were in vitro cultured in SOFaa medium + 5% FCS and 5 microg/ml insulin + 50 microg/ml gentamycin. Cleavage rate and embryo development were checked on Days 2, 5 and 8. An average of 11.3 +/- 0.3 COCs were recovered/dromedary ovary. Categories 1 and 2 represented 33.1% and 34.8%, respectively, and were significantly higher (p < 0.01) than the other categories (19.1, 9.2 and 2.6% for categories 3-5, respectively). Category 6 (embryo-like structures) represented 1.2% of the recovered oocytes, staining of these embryo-like structures with orcien dye indicated the presence of divided cells with condensed nuclei. Dromedary oocytes averaged 166.2 +/- 2.6 microm in diameter with black cytoplasm. Chemical activation of IVM dromedary oocyte with first Pb in 7% ethanol or 50 microM Ca A followed by culture in 2 mM 6-DMAP showed significantly higher (p < 0.01) cleavage and developmental rates to the morula stage than oocytes activated using 7% ethanol or 50 microM Ca A followed by 10 microg/ml CHX or in vitro fertilized control group. Higher (p < 0.01) proportion of oocytes sequentially cultured in 10 microg/ml CHX or that in vitro fertilized were arrested at the 2-4-cell stage compared with that cultured in 6-DMAP.

Activation of fertilized and nuclear transfer eggs

In all animal species, initiation of embryonic development occurs shortly after the joining together of the gametes from each of the sexes. The first of these steps, referred to as "egg activation", is a series of molecular events that results in the syngamy of the two haploid genomes and the beginning of cellular divisions for the new diploid embryo. For many years it has been known that the incoming sperm drives this process, as an unfertilized egg will remain dormant until it can no longer sustain normal metabolic processes. Until recently, it was also believed that the sperm was the only cell capable of creating a viable embryo and offspring. Recent advances in cell biology have allowed researchers to not only understand the molecular mechanisms of egg activation, but to exploit the use of pharmacological agents to bypass sperm-induced egg activation for the creation of animals by somatic cell nuclear transfer. This chapter will focus on the molecular events of egg activation in mammals as they take place during fertilization, and will discuss how these mechanisms are successfully bypassed in processes such as somatic cell nuclear transfer.

Calmodulin-dependent protein kinase II triggers mouse egg activation and embryo development in the absence of Ca2+ oscillations

Fertilization in mammalian eggs is accompanied by oscillatory changes in intracellular Ca(2+) concentration, which are critical for initiating and completing egg activation events and the developmental program. Ca(2+)/Camodulin-dependent protein kinase II (CaMKII) is a multifunctional enzyme that is postulated to be the downstream transducer of the Ca(2+) signal in many cell types. We tested the hypothesis that CaMKII is the major integrator of Ca(2+)-induced egg activation events and embryo development by microinjecting a cRNA that encodes a constitutively active (Ca(2+)-independent) mutant form of CaMKII (CA-CaMKII) into mouse eggs. Expression of this cRNA, which does not increase intracellular Ca(2+), induced a sustained rise in CaMKII activity and triggered egg activation events, including cell cycle resumption, and degradation and recruitment of maternal mRNAs; cortical granule exocytosis, however, did not occur normally. Furthermore, when mouse eggs were injected with sperm devoid of Ca(2+)-releasing activity and activated with either CA-CaMKII cRNA or by SrCl(2), similar rates and incidence of development to the blastocyst stage were observed. These results strongly suggest that CaMKII is a major integrator of the Ca(2+) changes that occur following fertilization.

When a sperm meets an egg: block to polyspermy

Embryonic development is initiated after the fertilizing spermatozoon enters the egg and triggers a series of events known as egg activation. Activation results in an increase in intracellular calcium concentration, cortical granule exocytosis (CGE), cell cycle resumption and recruitment of maternal mRNA. CGE is an evolutionary developed mechanism that causes modification of the zona pellucida to prevent penetration of additional spermatozoa, ensuring successful egg activation and embryo development. The egg CGE is a unique and convenient mammalian model for studying the different proteins participating at the membrane fusion cascade, which, unlike other secretory cells, occurs only once in the egg's lifespan. This article highlights a number of proteins, ascribed to participate in CGE and thus the block to polyspermy. CGE can be triggered either by a calcium dependent pathway, or via protein kinase C (PKC) activation that requires a very low calcium concentration. In a recent study, we suggested that the filamentous actin (F-actin) at the egg's cortex is a dynamic network. It can be maneuvered towards allowing CGE by activated actin associated proteins and/or by activated PKC and its down stream proteins, such as myristoylated alanine-rich C kinase substrate (MARCKS). MARCKS, a protein known to cross-link F-actin in other cell types, was found to be expressed and colocalized with actin in non-activated MII eggs. We further demonstrated MARCKS dissociation from actin after activation by ionomycin, a process that can lead to the breakdown of the actin network, thus allowing CGE. The more we know of the intricate process of CGE and of the proteins participating in it, the more the assisted reproductive procedures might benefit from that knowledge.

Calcium oscillations and mammalian egg activation

Fertilization in all species studied to date induces an increase in the intracellular concentration of free calcium ions ([Ca2+]i) within the egg. In mammals, this [Ca2+]i signal is delivered in the form of long-lasting [Ca2+]i oscillations that begin shortly after fusion of the gametes and persist beyond the time of completion of meiosis. While not fully elucidated, recent evidence supports the notion that the sperm delivers into the ooplasm a trigger of oscillations, the so-called sperm factor (SF). The recent discovery that mammalian sperm harbor a specific phospholipase C (PLC), PLCzeta has consolidated this view. The fertilizing sperm, and presumably PLCzeta promote Ca2+ release in eggs via the production of inositol 1,4,5-trisphosphate (IP3), which binds and gates its receptor, the type-1 IP3 receptor, located on the endoplasmic reticulum, the Ca2+ store of the cell. Repetitive Ca2+ release in this manner results in a positive cumulative effect on downstream signaling molecules that are responsible for the completion of all the events comprising egg activation. This review will discuss recent advances in our understanding of how [Ca2+]i oscillations are initiated and regulated in mammals, highlight areas of discrepancies, and emphasize the need to better characterize the downstream molecular cascades that are dependent on [Ca2+]i oscillations and that may impact embryo development.

Telomere biology in mammalian germ cells and during development

The development of an organism is a strictly regulated program in which controlled gene expression guarantees the establishment of a specific phenotype. The chromosome termini or so-called telomeres preserve the integrity of the genome within developing cells. In the germline, during early development, and in highly proliferative organs, human telomeres are balanced between shortening processes with each cell division and elongation by telomerase, but once terminally differentiated or mature the equilibrium is shifted to gradual shortening by repression of the telomerase enzyme. Telomere length is to a large extent genetically determined and the neonatal telomere length equilibrium is, in fact, a matter of evolution. Gradual telomere shortening in normal human somatic cells during consecutive rounds of replication eventually leads to critically short telomeres that induce replicative senescence in vitro and probably in vivo. Hence, a molecular clock is set during development, which determines the replicative potential of cells during extrauterine life. Telomeres might be directly or indirectly implicated in longevity determination in vivo, and information on telomere length setting in utero and beyond should help elucidate presumed causal connections between early growth and aging disorders later in life. Only limited information exists concerning the mechanisms underlying overall telomere length regulation in the germline and during early development, especially in humans. The intent of this review is to focus on recent advances in our understanding of telomere biology in germline cells as well as during development (pre- and postimplantation periods) in an attempt to summarize our knowledge about telomere length determination and its importance for normal development in utero and the occurrence of the aging and abnormal phenotype later on.

Simulation of calcium waves in ascidian eggs: insights into the origin of the pacemaker sites and the possible nature of the sperm factor

Fertilization triggers repetitive waves of cytosolic Ca2+ in the egg of many species. The mechanism involved in the generation of Ca2+ waves has been studied in much detail in mature ascidian eggs, by raising artificially the level of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] or of its poorly metabolizable analogue, glycero-myo-phosphatidylinositol 4,5-bisphosphate [gPtdIns(4,5)P2]. Here, we use this strategy and the experimental results it provides to develop a realistic theoretical model for repetitive Ca2+ wave generation and propagation in mature eggs. The model takes into account the heterogeneous spatial distribution of the endoplasmic reticulum. Our results corroborate the hypothesis that Ca2+ wave pacemakers are associated with cortical accumulations of endoplasmic reticulum. The model is first tested and validated by the adequate match between its theoretical predictions and the observed effects of localized injections of massive amounts of Ins(1,4,5)P3 analogues. In a second step, we use the model to make some propositions about the possible characteristics of the sperm factor. We find that to account for the spatial characteristics of the first series of Ca2+ waves seen at fertilization in ascidian eggs, it has to be assumed that, if the sperm factor is a phospholipase C, it is Ca2+-sensitive and highly diffusible. Although the actual state of knowledge does not allow us to explain the observed relocalization of the Ca2+ wave pacemaker site, the model corroborates the assumption that PtdIns(4,5)P2, the substrate for phospholipase C is distributed over the entire egg. We also predict that the dose of sperm factor injected into the egg should modulate the temporal characteristics of the first, long-lasting fertilization wave.

Fertilization stimulates long-lasting oscillations of CaMKII activity in mouse eggs

Elucidation of the biochemical mechanisms by which specific proteins transduce the all important intracellular calcium (Ca2+) signal at fertilization into events of egg activation will increase our understanding of the regulation of the onset of development and the extent to which these signals can be experimentally modified. Previously, we reported data supporting the hypothesis that mouse eggs have the capability to generate oscillations of the activity of Ca2+ and calmodulin-dependent kinase II (CaMKII), regulating the cell cycle and secretion. This study directly demonstrates transient increases of enzyme activity in relatively close synchrony with Ca2+ oscillations for the first hour of fertilization in single mouse eggs monitored for both Ca2+ and CaMKII activity. The extent of the enzyme activity increase was correlated with the level of intracellular Ca2+. After a rise in activity, the decrease in activity did not appear to be due to negative feedback from elevated Ca2+ or CaMKII activity over time, since enzyme activity persisted after 8 min of elevated Ca2+ from 7% ethanol activation. The contribution of CaMKII from a single sperm to the rise in CaMKII activity at fertilization appeared to be negligible. Also, long-term cell cycle inhibition was observed in fertilized eggs with the CaMKII antagonist myrAIP (50 microM), which did not inhibit the first large Ca2+ transient or subsequent early oscillations but did reduce the percentage of eggs fertilized. Thus, mammalian eggs appear to drive many activation events over time to completion with repeated short bursts of Ca2+ oscillation-dependent CaMKII activity, rather than by a steady-state, continuously elevated level of CaMKII activity that is maintained by periodic Ca2+ oscillations.

Activation and early parthenogenesis of bovine oocytes treated with ethanol and strontium

Efficient artificial activation is indispensable for the success of cloning programs. Strontium has been shown to effectively activate mouse oocytes for nuclear transfer procedures, however, there is limited information on its use for bovine oocytes. The present study had as objectives: (1). to assess the ability of strontium to induce activation and parthenogenetic development in bovine oocytes of different maturational ages in comparison with ethanol; and (2). to verify whether the combination of both treatments improves activation and parthenogenetic development rates. Bovine oocytes were in vitro matured for 24, 26, 28, and 30 h, and treated with ethanol (E, 7% for 5 min) or strontium chloride (S, 10mM SrCl(2) for 5h) alone or in combination: ethanol+strontium (ES) and strontium+ethanol (SE). Activated oocytes were cultured in vitro in synthetic oviductal fluid (SOF) medium and assessed for pronuclear formation (15-16 h), cleavage (46-48 h) and development to the blastocyst stage (D7). Treatment with ethanol and strontium promoted similar results regarding pronuclear formation (E, 20-66.7%; S, 26.7-53.3%; P>0.05) and cleavage (E, 12.8-40.6%; S, 16.1-41.9%; P>0.05), regardless of oocyte age. The actions of both strontium and ethanol were influenced by oocyte age: ethanol induced greater activation rates after 28 and 30 h of maturation (48.4 and 66.7% versus 20.0 and 23.3% for 24 and 26 h, respectively; P<0.05) and strontium after 30 h (53.3%) was superior to 24 and 26 h (26.7% for both). Blastocyst development rates were minimal in all treatments (0.0-6.3%; P>0.05), however, when the mean (+/-S.D.) cell number in blastocysts at the same maturational period was compared, strontium treatment was superior to ethanol for activation rates (82+/-5.7 and 89.5+/-7.8 versus 54 and 61, at 28 and 30 h, respectively). Improved results were obtained by combined treatments. The combination of ethanol and strontium resulted in similar pronuclear formation (ES, 36.7-83.9%; SE, 53.1-90.3%) and cleavage rates (ES, 31.3-81.3%; SE, 65.6-80.7%). Regarding embryo development, there was no difference (P>0.05) between treatments, and blastocysts were only obtained in treatment SE at 24 and 26 h (6.5% for both). It is concluded that, SrCl(2) induces activation and parthenogenetic development in bovine oocytes.

Cellular and molecular mechanisms leading to cortical reaction and polyspermy block in mammalian eggs

Following fusion of sperm and egg, the contents of cortical granules (CG), a kind of special organelle in the egg, release into the perivitelline space (cortical reaction), causing the zona pellucida to become refractory to sperm binding and penetration (zona reaction). Accumulating evidence demonstrates that mammalian cortical reaction is probably mediated by activation of the inositol phosphate (PIP(2)) cascade. The sperm-egg fusion, mediated by GTP-binding protein (G-protein), may elicit the generation of two second messengers, inositol 1,4,5 triphosphate (IP(3)) and diacylglycerol (DAG). The former induces Ca(2+) release from intracellular stores and the latter activates protein kinase C (PKC), leading to CG exocytosis. Calmodulin-dependent kinase II (CaMKII) may act as a switch in the transduction of the calcium signal. The CG exudates cause zona sperm receptor modification and zona hardening, and thus block polyspermic penetration. Oolemma modification after sperm-egg fusion and formation of CG envelope following cortical reaction also contribute to polyspermy block.

Oscillatory CaMKII activity in mouse egg activation

Fertilization-induced intracellular calcium (Ca(2+)) oscillations stimulate the onset of mammalian development, and little is known about the biochemical mechanism by which these Ca(2+) signals are transduced into the events of egg activation. This study addresses the hypothesis that transient increases in Ca(2+) similar to those at fertilization stimulate oscillatory Ca(2+)/calmodulin-dependent kinase II (CaMKII) enzyme activity, incrementally driving the events of egg activation. Since groups of fertilized eggs normally oscillate asynchronously, synchronous oscillatory Ca(2+) signaling with a frequency similar to fertilization was experimentally induced in unfertilized mouse eggs by using ionomycin and manipulating extracellular calcium. Coanalysis of intracellular Ca(2+) levels and CaMKII activity in the same population of eggs demonstrated a rapid and transient enzyme response to each increase in Ca(2+). Enzyme activity increased 370% during the first Ca(2+) rise, representing about 60% of maximal activity, and had decreased to basal levels within 5 min from the time Ca(2+) reached its peak value. Single fertilized eggs monitored for Ca(2+) had a mean increase in CaMKII activity of 185%. One and two ionomycin-induced Ca(2+) transients resulted in 39 and 49% mean cortical granule (CG) loss, respectively, while CG exocytosis and resumption of meiosis were inhibited by a CaMKII antagonist. These studies demonstrate that changes in the level of Ca(2+) and in CaMKII activity can be studied in the same cell and that CaMKII activity is exquisitely sensitive to experimentally induced oscillations of Ca(2+) in vivo. The data support the hypothesis that CaMKII activity oscillates for a period of time after normal fertilization and temporally regulates many events of egg activation.

Signalling in mammalian egg activation: role of protein kinases

Embryonic development is initiated after the fertilizing spermatozoon enters the egg and triggers a process known as 'egg activation'. Activation results in an increase in intracellular calcium concentration, cortical granule exocytosis (CGE), cell cycle resumption and recruitment of maternal mRNA. Various treatments can induce parthenogenetic activation characterized by the same manifestations. Signal transduction pathways similar to those known for somatic cells mediate the mammalian egg activation. This review focuses on the signal transduction pathways that occur during mammalian fertilization and during parthenogenetic egg activation. We discuss the possibility that members of the protein tyrosine kinase (PTKs) families, the Src family PTKs in particular, operate during egg activation and that protein kinase C can induce CGE.

Attributes and dynamics of the endoplasmic reticulum in mammalian eggs

The endoplasmic reticulum is a multifunctional continuous network of membrane-enclosed sacs and tubules that extends throughout the cell. The endoplasmic reticulum is the site of protein synthesis and assembly, as well as lipid and membrane synthesis. Additionally, the endoplasmic reticulum contains calcium pumps, intraluminal calcium storage proteins, and specific calcium-releasing channels. Thus, this membrane system plays a central role in intracellular signaling through the storage and release of calcium. At fertilization, the sperm triggers a large and dramatic release of calcium from the endoplasmic reticulum, which activates the egg to begin development. The ability of the egg to fully elevate calcium depends on biochemical and structural changes during oocyte maturation. The sensitivity of the calcium-releasing system increases and the endoplasmic reticulum is reorganized during maturation of the oocyte; together, these dynamic changes place a substantial calcium storage compartment just beneath the membrane, near the site of sperm-egg fusion. Localization of the calcium store may also contribute to the long-lasting calcium oscillations that are characteristic of mammalian fertilization. Examination of the endoplasmic reticulum in living eggs is leading to a better understanding of calcium release at fertilization.

Telomerase activity in bovine embryos during early development

The telomere is the end structure of the DNA molecule. Telomerase is the ribonuclear enzyme that helps the cell's telomere to elongate; otherwise, the telomere will shorten with each cell division through conventional DNA replication. In most mammalian species, telomerase activity is present in germ cells but not in somatic cells. Recent research shows that telomerase activity is also present in early embryos, but to our knowledge, the dynamics of this enzyme during early embryo development have not been studied. In the present work, we conducted telomerase activity assays on bovine embryos fertilized in vitro and harvested at different stages from zygote to blastocyst. A polymerase chain reaction-based assay (Telomeric Repeat Amplification Protocol) was used to detect the telomerase activity in these embryos. We demonstrated that the telomerase activity is present in the early embryos, but that its level varies with the different developmental stages. The activity was relatively low in mature oocytes. It increased after in vitro fertilization and then decreased gradually until the embryo reached the eight-cell stage. After the eight-cell stage, the telomerase activity increased again and reached its highest level in the blastocyst stage. This study provides insight regarding how telomerase activity and, possibly, the length of the telomere are reprogrammed during early embryo development.

A specific inhibitor of p34(cdc2)/cyclin B suppresses fertilization-induced calcium oscillations in mouse eggs

Fertilization-induced Ca(2+) oscillations in mouse eggs cease at the time of pronuclear formation when maturation-promoting factor (MPF) is inactivated, but the Ca(2+) oscillations are ceaseless if eggs are arrested at metaphase by colcemid, which maintains the activity of MPF. To determine the possible role of MPF in regulation of cytoplasmic Ca(2+) excitability, roscovitine, a specific inhibitor of p34(cdc2)/cyclin B kinase, was used to inactivate MPF, and its effect on fertilization-induced Ca(2+) oscillations was investigated. Our results showed that roscovitine at >/= 50 microM suppressed fertilization-induced Ca(2+) oscillations in normal and colcemid-treated metaphase II (MII) eggs after the first 1-2 Ca(2+) spikes. Roscovitine inhibition of fertilization-induced Ca(2+) oscillations could be reversed by extensive washing of the eggs. Histone H1 kinase activity in colcemid-treated MII eggs was similarly inhibited by roscovitine, which suggested that the cessation of fertilization-induced Ca(2+) oscillations is due to the inactivation of MPF. Thimerosal-induced Ca(2+) oscillations in Ca(2+)-, Mg(2+)-free medium was also suppressed by roscovitine, suggesting a general inhibitory effect of roscovitine on Ca(2+) oscillations. The inhibition may be achieved by disruption of Ca(2+) release and refilling of the calcium store. Thapsigargin, an inhibitor of the endoplasmic reticulum Ca-ATPase, induced significantly less Ca(2+) release in roscovitine-treated eggs than in the non-drug-treated eggs. Taken together, our results suggest that MPF plays an important role in regulation of the cytoplasmic Ca(2+) excitability in mouse eggs.

The cortical endoplasmic reticulum (ER) of the mouse egg: localization of ER clusters in relation to the generation of repetitive calcium waves

The endoplasmic reticulum (ER) of the mature mouse egg consists of a fine tubular network and pronounced accumulations in the cortex. The ER was visualized both in intact eggs and with in vitro preparations of the cortex using the fluorescent lipophilic dye, DiI. Immunofluorescent labeling of the ER in isolated cortical preparations demonstrated that the ER clusters contain inositol 1,4, 5-trisphosphate (IP(3)) receptors, indicating an important involvement in sperm-induced Ca(2+) transients, which are triggered by IP(3). We imaged the ER during fertilization and the subsequent Ca(2+) transients and found that the clusters remained intact throughout this period. Recovery of fluorescence after photobleaching established that the ER clusters are continuous with the reticular ER network and that these structures remain stable and continuous throughout the time of fertilization-induced Ca(2+) transients; continuity also remained during IP(3) injection. These results indicate that, in contrast to echinoderm eggs, the ER of mouse eggs does not become disrupted when it releases Ca(2+)at fertilization. The localization and apparent stability of the cortical ER clusters may be important in generating Ca(2+) oscillations, which are characteristic of fertilized mammalian eggs. Imaging of intracellular Ca(2+) revealed that Ca(2+) transients originate in the hemisphere of the egg that contains abundant ER clusters, thus the mouse contains a stable cortical pacemaker responsible for generating Ca(2+) waves.

Downregulation of delta CaM kinase II in human tumor cells

Over two dozen alternative splice variants of CaMK-II, the type II Ca(2+)/CaM-dependent protein kinase, are encoded from four genes (alpha, beta, gamma and delta) in mammalian cells. Isozymes of alpha and beta CaMK-II are well characterized in brain; however, an understanding of the relative endogenous levels of CaMK-II isozymes in a wide variety of non-neuronal cells has not yet been described. In this study, we have demonstrated that CaMK-II consists primarily of the 54 kDa delta CaMK-II (delta(2) or delta(C)) isozyme in rodent fibroblasts. beta and gamma CaMK-II isozymes are minor and alpha CaMK-II was not expressed. The primary delta CaMK-II in human fibroblasts and the MCF10A mammary epithelial cell line was the 52 kDa delta(4) CaMK-II, an isozyme identical to delta(2) except for a missing 21-amino-acid C-terminal tail. delta CaMK-II levels were diminished in both human and rodent fibroblasts after SV40 transformation and in the mammary adenocarcinoma MCF7 cell line when compared to MCF10A cells. In fact, most tumor cells exhibited CaMK-II specific activities which were two- to tenfold lower than in untransformed fibroblasts. We conducted complementary CaMK-II studies on the NGF-induced differentiation of rat PC-12 cells. Although no new synthesis of CaMK-II occurs, neurite outgrowth in these cells is accompanied by a preferential activation of delta CaMK-II. Endogenous delta CaMK-II has a perinuclear distribution in fibroblasts and extends along neurites in PC-12 cells. These findings point to a role for delta CaMK-II isozymes in cellular differentiation.

Comparative biology of calcium signaling during fertilization and egg activation in animals

During animal fertilizations, each oocyte or egg must produce a proper intracellular calcium signal for development to proceed normally. As a supplement to recent synopses of fertilization-induced calcium responses in mammals, this paper reviews the spatiotemporal properties of calcium signaling during fertilization and egg activation in marine invertebrates and compares these patterns with what has been reported for other animals. Based on the current database, fertilization causes most oocytes or eggs to generate multiple wavelike calcium oscillations that arise at least in part from the release of internal calcium stores sensitive to inositol 1,4,5-trisphosphate (IP3). Such calcium waves are modulated by upstream pathways involving oolemmal receptors and/or soluble sperm factors and in turn regulate calcium-sensitive targets required for subsequent development. Both "protostome" animals (e.g., mollusks, annelids, and arthropods) and "deuterostomes" (e.g., echinoderms and chordates) display fertilization-induced calcium waves, IP3-mediated calcium signaling, and the ability to use a combination of external calcium influx and internal calcium release. Such findings fail to support the dichotomy in calcium signaling modes that had previously been proposed for protostomes vs deuterostomes and instead suggest that various features of fertilization-induced calcium signals are widely shared throughout the animal kingdom.

CaM kinase II as frequency decoder of Ca2+ oscillations

In many cell types, Ca2+ signals are organized in the form of repetitive spikes. The frequency of these intracellular Ca2+ oscillations increases with the level of stimulation, suggesting the existence of a frequency encoding phenomenon. The question arises as to how the frequency of Ca2+ oscillations can be decoded inside the cell. Ca2+/calmodulin kinase II has long been proposed as an attractive candidate, as it is a key target of Ca2+ signals. By immobilizing the Ca2+/calmodulin kinase II and subjecting it to pulses of Ca2+ of variable amplitude, duration, and frequency, De Koninck and Schulman have shown for the first time that the autonomous activity of Ca2+/calmodulin kinase II is highly sensitive to the temporal pattern of Ca2+ oscillations.