Incompetence of preovulatory mouse oocytes to undergo cortical granule exocytosis following induced calcium oscillations

Improved low-invasive mRNA electroporation method into immature mouse oocytes visualizes protein dynamics during development†

One of the major causes of oocyte quality deterioration, chromosome segregation abnormalities manifest mainly during meiosis I, which occurs before and during ovulation. However, currently, there is a technical limitation in the introduction of mRNA into premature oocytes without impairing embryonic developmental ability. In this study, we established a low-invasive electroporation (EP) method to introduce mRNA into pre-ovulatory, germinal vesicle (GV) mouse oocytes in an easier manner than the traditional microinjection method. The EP method with an optimized impedance value resulted in the efficient introduction of mRNAs encoding enhanced green fluorescent protein (EGFP) into the GV oocytes surrounded by cumulus cells at a survival rate of 95.0%. Furthermore, the introduction of histone H2B-EGFP mRNA into the GV oocytes labeled most of the oocytes without affecting the blastocyst development rate, indicating the feasibility of the visualization of oocyte chromosomal dynamics that enable us to assay chromosomal integrity in oocyte maturation and cell count in embryonic development. The establishment of this EP method offers extensive assays to select pre-implantation embryos and enables the surveying of essential factors for mammalian oocyte quality determination.

Calcium signaling in oocyte quality and functionality and its application

Calcium (Ca2+) is a second messenger for many signal pathways, and changes in intracellular Ca2+ concentration ([Ca2+]i) are an important signaling mechanism in the oocyte maturation, activation, fertilization, function regulation of granulosa and cumulus cells and offspring development. Ca2+ oscillations occur during oocyte maturation and fertilization, which are maintained by Ca2+ stores and extracellular Ca2+ ([Ca2+]e). Abnormalities in Ca2+ signaling can affect the release of the first polar body, the first meiotic division, and chromosome and spindle morphology. Well-studied aspects of Ca2+ signaling in the oocyte are oocyte activation and fertilization. Oocyte activation, driven by sperm-specific phospholipase PLCζ, is initiated by concerted intracellular patterns of Ca2+ release, termed Ca2+ oscillations. Ca2+ oscillations persist for a long time during fertilization and are coordinately engaged by a variety of Ca2+ channels, pumps, regulatory proteins and their partners. Calcium signaling also regulates granulosa and cumulus cells' function, which further affects oocyte maturation and fertilization outcome. Clinically, there are several physical and chemical options for treating fertilization failure through oocyte activation. Additionally, various exogenous compounds or drugs can cause ovarian dysfunction and female infertility by inducing abnormal Ca2+ signaling or Ca2+ dyshomeostasis in oocytes and granulosa cells. Therefore, the reproductive health risks caused by adverse stresses should arouse our attention. This review will systematically summarize the latest research progress on the aforementioned aspects and propose further research directions on calcium signaling in female reproduction.

SNAP23 is required for constitutive and regulated exocytosis in mouse oocytes†

Mammalian oocytes are stored in the ovary for prolonged periods, and arrested in meiotic prophase. During this period, their plasma membranes are constantly being recycled by endocytosis and exocytosis. However, the function of this membrane turnover is unknown. Here, we investigated the requirement for exocytosis in the maintenance of meiotic arrest. Using Trim-away, a newly developed method for rapidly and specifically depleting proteins in oocytes, we have identified the SNARE protein, SNAP23, to be required for meiotic arrest. Degradation of SNAP23 causes premature meiotic resumption in follicle-enclosed oocytes. The reduction in SNAP23 is associated with loss of gap junction communication between the oocyte and surrounding follicle cells. Reduction of SNAP23 protein also inhibits regulated exocytosis in response to a Ca2+ stimulus (cortical granule exocytosis), as measured by lectin staining and cleavage of ZP2. Our results show an essential role for SNAP23 in two key processes that occur in mouse oocytes and eggs.

Experimental strategies towards increasing intracellular mitochondrial activity in oocytes: A systematic review

PURPOSE

The mitochondrial complement is critical in sustaining the earliest stages of life. To improve the Assisted Reproductive Technology (ART), current methods of interest were evaluated for increasing the activity and copy number of mitochondria in the oocyte cell.

METHODS

This covered the researches from 1966 to September 2015.

RESULTS

The results provided ten methods that can be studied individually or simultaneously.

CONCLUSION

Though the use of these techniques generated great concern about heteroplasmy observation in humans, it seems that with study on these suggested methods there is real hope for effective treatments of old oocyte or oocytes containing mitochondrial problems in the near future.

The zinc spark is an inorganic signature of human egg activation

Egg activation refers to events required for transition of a gamete into an embryo, including establishment of the polyspermy block, completion of meiosis, entry into mitosis, selective recruitment and degradation of maternal mRNA, and pronuclear development. Here we show that zinc fluxes accompany human egg activation. We monitored calcium and zinc dynamics in individual human eggs using selective fluorophores following activation with calcium-ionomycin, ionomycin, or hPLCζ cRNA microinjection. These egg activation methods, as expected, induced rises in intracellular calcium levels and also triggered the coordinated release of zinc into the extracellular space in a prominent “zinc spark.” The ability of the gamete to mount a zinc spark response was meiotic-stage dependent. Moreover, chelation of intracellular zinc alone was sufficient to induce cell cycle resumption and transition of a meiotic cell into a mitotic one. Together, these results demonstrate critical functions for zinc dynamics and establish the zinc spark as an extracellular marker of early human development.

RyR channel-mediated increase of cytosolic free calcium level signals cyclin B1 degradation during abortive spontaneous egg activation in rat

In few mammalian species including rat, post-ovulatory aging induces abortive spontaneous egg activation (SEA), which is morphologically characterized by exit from metaphase-II (M-II) arrest. A possibility exists that the RyR channel-mediated insufficient increase of cytosolic free Ca(2+) level could be one of the causes for post-ovulatory aging-induced abortive SEA. To test this possibility, eggs collected after 17 h post-hCG surge were cultured with or without various concentrations of nifedipine (NF), ruthenium red (RR), and KN-93 for 3 h in vitro. Morphological changes characteristic of abortive SEA, cytosolic free Ca(2+) level, cyclin B1 level, and meiotic status were analyzed. Data of the present study indicate that NF and RR inhibited post-ovulatory aging-induced abortive SEA in a concentration-dependent manner. Further, RR protected against RyR channel as well as caffeine-mediated increase of cytosolic free Ca(2+) level. In addition, KN-93 inhibited post-ovulatory aging-induced abortive SEA in a concentration-dependent manner. An increase of cytosolic free Ca(2+) level was associated with a reduction of cyclin B1 level during post-ovulatory aging-induced abortive SEA. These data indirectly suggest the involvement of RyR channels in the increase of cytosolic free Ca(2+) level. The increased cytosolic free Ca(2+) level triggers cyclin B1 degradation possibly through CaMK-II activity during post-ovulatory aging-induced abortive SEA in rat eggs cultured in vitro.

Prophase I mouse oocytes are deficient in the ability to respond to fertilization by decreasing membrane receptivity to sperm and establishing a membrane block to polyspermy

Changes occurring as the prophase I oocyte matures to metaphase II are critical for the acquisition of competence for normal egg activation and early embryogenesis. A prophase I oocyte cannot respond to a fertilizing sperm as a metaphase II egg does, including the ability to prevent polyspermic fertilization. Studies here demonstrate that the competence for the membrane block to polyspermy is deficient in prophase I mouse oocytes. In vitro fertilization experiments using identical insemination conditions result in monospermy in 87% of zona pellucida (ZP)-free metaphase II eggs, while 92% of ZP-free prophase I oocytes have four or more fused sperm. The membrane block is associated with a postfertilization reduction in the capacity to support sperm binding, but this reduction in sperm-binding capacity is both less robust and slower to develop in fertilized prophase I oocytes. Fertilization of oocytes is dependent on the tetraspanin CD9, but little to no release of CD9 from the oocyte membrane is detected, suggesting that release of CD9-containing vesicles is not essential for fertilization. The deficiency in membrane block establishment in prophase I oocytes correlates with abnormalities in two postfertilization cytoskeletal changes: sperm-induced cortical remodeling that results in fertilization cone formation and a postfertilization increase in effective cortical tension. These data indicate that cortical maturation is a component of cytoplasmic maturation during the oocyte-to-egg transition and that the egg cortex has to be appropriately primed and tuned to be responsive to a fertilizing sperm.

Expression and functional activity of PACAP and its receptors on cumulus cells: effects on oocyte maturation

Pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptor PAC1-R (PACAP type 1 receptor) are transiently expressed in granulosa cells (GCs) of mouse preovulatory follicles and affect several parameters associated with the ovulatory process. We investigated the expression of PACAP and its receptors in cumulus cells (CCs) after the LH surge and their role on cumulus expansion/apoptosis and oocyte maturation. PACAP and PAC1-R expression increased in CCs isolated at different times after treatment with human chorionic gonadotropin (hCG). Moreover, PACAP was able to reverse the inhibition of oocyte meiotic maturation caused by hypoxantine in cumulus cell-oocyte complexes (COCs) and efficiently promoted male pronuclear formation after fertilisation. PACAP was also able to induce cumulus expansion and prevent CC apoptosis. Our results demonstrated the induction of PACAP and its receptors in CCs by LH and EGF, suggesting that PACAP may play a significant role in the complex interactions of gonadotropin and growth factors during ovulation and fertilisation.

Anaphase-promoting complex control in female mouse meiosis

Entry into, and passage through, the two meiotic divisions of the oocyte has to be highly coordinated to ensure proper segregation of chromosomes. This coordination ensures that the hallmark stops and starts of the meiotic process occur at the right time to prevent aneuploidy. The Anaphase-Promoting Complex is an activity mostly studied in the mitotic cell cycle division, where it has essential functions during mitosis. As detailed here the Anaphase-Promoting Complex also plays vital roles in controlling at least three meiotic events: maintenance of prophase I arrest, timely and faithful segregation of homologous chromosomes in meiosis I, and the meiotic arrest following ovulation.

Regulation of peripheral spindle movement and spindle rotation during mouse oocyte meiosis: new perspectives

Spindle movement, including spindle migration during first meiosis and spindle rotation during second meiosis, is essential for asymmetric divisions in mouse oocytes. Previous studies by others and us have shown that microfilaments are required for both spindle migration and rotation. In the present study, we aimed to further investigate the mechanism controlling spindle movement during mouse oocyte meiosis. By employing drug treatment and immunofluorescence microscopy, we showed that dynamic microtubule assembly was involved in both spindle migration and rotation. Furthermore, we found that the calcium/CaM/CaMKII pathway was important for regulating spindle rotation.

Meiosis in oocytes: predisposition to aneuploidy and its increased incidence with age

Mammalian oocytes begin meiosis in the fetal ovary, but only complete it when fertilized in the adult reproductive tract. This review examines the cell biology of this protracted process: from entry of primordial germ cells into meiosis to conception. The defining feature of meiosis is two consecutive cell divisions (meiosis I and II) and two cell cycle arrests: at the germinal vesicle (GV), dictyate stage of prophase I and at metaphase II. These arrests are spanned by three key events, the focus of this review: (i) passage from mitosis to GV arrest during fetal life, regulated by retinoic acid; (ii) passage through meiosis I and (iii) completion of meiosis II following fertilization, both meiotic divisions being regulated by cyclin-dependent kinase (CDK1) activity. Meiosis I in human oocytes is associated with an age-related high rate of chromosomal mis-segregation, such as trisomy 21 (Down's syndrome), resulting in aneuploid conceptuses. Although aneuploidy is likely to be multifactorial, oocytes from older women may be predisposed to be becoming aneuploid as a consequence of an age-long decline in the cohesive ties holding chromosomes together. Such loss goes undetected by the oocyte during meiosis I either because its ability to respond and block division also deteriorates with age, or as a consequence of being inherently unable to respond to the types of segregation defects induced by cohesion loss.

Establishment of the mammalian membrane block to polyspermy: evidence for calcium-dependent and -independent regulation

One crucial result of egg activation is the establishment of blocks on the zona pellucida and the egg plasma membrane to prevent fertilization by additional sperm. The mechanism(s) by which a mammalian egg regulates the establishment of the membrane block to polyspermy is largely unknown. Since Ca(2+) signaling regulates several egg activation events, this study investigates how sperm-induced Ca(2+) transients affect the membrane block to polyspermy, building on our previous work (Biology of Reproduction 67:1342). We demonstrate that mouse eggs that experience only one sperm-induced Ca(2+) transient establish a membrane block that is less effective, than in eggs that experience normal sperm-induced Ca(2+) transients but that is more effective than in eggs with completely suppressed [Ca(2+)](cyt) increases. Sperm-induced increases in [Ca(2+)](cyt) regulate the timing of membrane block establishment, as this block is established more slowly in eggs that experience one or no sperm-induced Ca(2+) transients. Finally, our studies produce the intriguing discovery that there is also a Ca(2+)-independent event that is associated with fertilization in the pathway leading to membrane block establishment. Taken together, these data indicate that Ca(2+) plays a role in facilitating membrane block establishment by regulating the timing with which this change in egg membrane function occurs, and also that the membrane block differs from other post-fertilization egg activation responses as Ca(2+) is not the only stimulus. The membrane block to polyspermy in mammalian eggs is likely to be the culmination of multiple post-fertilization events that together modify the egg membrane's receptivity to sperm.

The role of Src family kinases in egg activation

The Src family kinases (SFKs) are believed to mediate some of the early events of egg activation at fertilization--intracellular Ca2+ increase and resumption of the second meiotic division (RMII). SFKs are both necessary and sufficient for triggering intracellular Ca2+ increase in eggs of sea urchin, sea star, Xenopus etc, but their role in mammalian eggs is not entirely determined. In this study we examined the involvement of SFKs in the events leading to Ca2+ increase in rat eggs and demonstrated their involvement in RMII. Microinjecting mRNAs of active forms of Fyn or c-Yes but not of c-Src, into ovulated eggs, triggered RMII without evoking Ca2+ increase. A specific SFKs inhibitor (SU6656) or dominant-negative (DN) forms of Fyn or c-Yes were unable to block Ca2+ oscillations rather, modulated them, in fertilized eggs or in parthenogenetically activated eggs. Moreover, inhibiting SFKs activity blocked RMII and decreased the level of cyclin B1 degradation. Our results imply participation of SFKs in the signal transduction pathway leading to egg activation, but not in the one leading to Ca2+ increase. We propose that SFKs act downstream to Ca2+ increase at the level of M-phase promoting factor (MPF).

The MEK inhibitor, U0126, alters fertilization-induced [Ca2+]i oscillation parameters and secretion: differential effects associated with in vivo and in vitro meiotic maturation

Although mitogen-activated protein kinase (MAPK) is a well-known cell cycle regulator, emerging studies have also implicated its activity in the regulation of intracellular calcium concentration ([Ca2+](i)) and secretion. Those studies raise the hypothesis that MAPK activity during oocyte maturation and early fertilization is required for normal egg Ca2+ oscillations and cortical granule (CG) secretion. We extend the findings of [Lee, B., Vermassen, E., Yoon, S.-Y., Vanderheyden, V., Ito, J., Alfandari, D., De Smedt, H., Parys, J.B., Fissore, R.A., 2006. Phosphorylation of IP(3)R1 and the regulation of [Ca2+](i) responses at fertilization: a role for the MAP kinase pathway. Development 133, 4355-4365] by demonstrating acute effects on Ca2+ oscillation frequency, amplitude, and duration in fertilized mouse eggs matured in vitro with the MAPK inhibitor, U0126. Frequency was increased, whereas amplitude and duration were greatly decreased. These effects were significantly reduced in eggs matured in vivo and fertilized in the presence of the inhibitor. Ionomycin studies indicated that intracellular Ca2+ stores were differentially affected in eggs matured in vitro with U0126. Consistent with these effects on [Ca2+](i) elevation, fertilization-induced CG exocytosis and metaphase II exit were also reduced in in vitro-matured eggs with U0126, but not in those similarly treated after in vivo maturation. These results indicate that MAPK targets Ca2+ regulatory proteins during both maturation and fertilization, as well as provide a new hypothesis for MAPK function, which is to indirectly regulate events of early development by controlling Ca2+ oscillation parameters.

Comparison of Ca2+ and CaMKII responses in IVF and ICSI in the mouse

Novel methods of egg activation in human assisted reproductive technologies and animal somatic cell nuclear transfer are likely to alter the signalling process that occurs during normal fertilization. Intracytoplasmic sperm injection (ICSI) bypasses the normal processes of the acrosome reaction, sperm-egg fusion, and processing of the sperm plasma membrane, as well as alters some parameters of intracellular calcium ([Ca(2+)](i)) dynamics (reported previously by Kurokawa and Fissore (2003)). Herein, we extend these studies to determine if ICSI alters the activity of the Ca(2+)-dependent protein, Ca(2+)/calmodulin-dependent kinase II (CaMKII), which is responsible for the completion of meiosis in vertebrate eggs. After ICSI or in vitro fertilization (IVF), individual mouse eggs were monitored for their relative changes in both [Ca(2+)](i) and CaMKII activity during the first [Ca(2+)](i) rise and a subsequent rise associated with second polar body extrusion. The duration of the first [Ca(2+)](i) rise was greater in ICSI than in IVF, but the amplitude of the rise was transiently higher for IVF than ICSI. However, a similar mean CaMKII activity was observed in both procedures. During polar body extrusion, the amplitude and duration of the Ca(2+) rises were increased by a small amount in ICSI compared with IVF, whereas the CaMKII activities were similar. Thus, compared with IVF, ICSI is not associated with decreased or delayed CaMKII activity in response to these Ca(2+) signals in the mouse.

Secretory mechanisms and Ca2+ signaling in gametes: similarities to regulated neuroendocrine secretion in somatic cells and involvement in emerging pathologies

Recent studies demonstrate that regulated secretion in probably all mammalian cells, from gonadotropes to gametes, utilizes similar signaling systems, intracellular Ca(2+) regulation, Ca(2+)-dependent proteins, cytoskeletal participation, and SNARE-mediated fusion. Thus, highly specialized cells, like sperm and eggs, should no longer be considered to have evolved a cell-type specific secretory mechanism. In gametes, Ca(2+)-dependent proteins and enzymes transduce elevations of intracellular Ca(2+) into secretory events, i.e., exocytosis of the acrosome in sperm and cortical granules in the egg. Just as secretory deficiencies have clinical consequences in endocrine and exocrine cells, failure of secretion of cortical granules or the acrosome can result in failure of normal fertilization or fertilization followed by abnormal development. With the advent of human in vitro fertilization, such gamete pathologies have been recently identified and have led to new clinical procedures to achieve normal fertilization and pregnancies. A better understanding of the common Ca(2+)-dependent secretory pathways in both gametes and somatic cells should be beneficial to investigating mis-regulation in either cell type.

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.

Characterization of calcium oscillation patterns in caprine oocytes induced by IVF or an activation technique used in nuclear transfer

Routine activation of nuclear transfer (NT) eggs involves the application of a single intracellular calcium [Ca2+]i rise, stimulated by an electrical pulse, as opposed to [Ca2+]i oscillations, which is the natural mode of sperm-induced activation at fertilization in all mammalian species tested to date. It has yet to be shown that caprine oocytes exhibit an increase in calcium at fertilization in a manner similar to other mammals. The objective of the present study was to evaluate and characterize the ([Ca2+]i) oscillation patterns of caprine metaphase II (MII) oocytes during IVF and during an activation techniques used in nuclear transfer. Additionally, the effect of cytochalasin B (cyto B) in the NT process was evaluated for its impact on [Ca2+]i oscillations and subsequent embryo development. Mature in vitro and in vivo derived caprine oocytes were activated by 5 microM ionomycin, an electrical pulse(s), or IVF. The intracellular Ca2+ response was determined using the [Ca2+]i indicator Fura-2 dextran (Fura-2D). Ova treated with ionomycin or stimulated by an electrical pulse exhibited a single [Ca2+]i rise, whereas IVF-derived oocytes showed oscillations. IVF [Ca2+]i showed some variation, with 62% of in vitro matured oocytes exhibiting oscillations, whereas 8% of in vivo matured oocytes exhibited oscillations demonstrating a correlation between [Ca2+]i responses and maturation technique. Knowing the [Ca2+]i profile of activated eggs, one may be able to optimize the activation methodology used in a production nuclear transfer setting which could potentially improve development to term for NT embryos.

Strontium-induced rat egg activation

Parthenogenetic agents that evoke cytosolic calcium concentration ([Ca2+]i) oscillations similar to those evoked by sperm, mimic fertilization more faithfully than agents that trigger a single [Ca2+]i transient. Strontium chloride (SrCl2) binds to and activates the Ca2+-binding site on the inositol 1,4,5-trisphosphate receptor and evokes [Ca2+]i oscillations. Although SrCl2 has been reported to activate mouse eggs, little is known regarding the pattern of the [Ca2+]i oscillations it evokes in rat eggs and their effect on the early events of egg activation: cortical granule exocytosis (CGE) and completion of meiosis (CM). In the current study we investigated the effect of various concentrations of SrCl2 (2, 4 or 6 mM) on [Ca2+]i, by monitoring [Ca2+]i oscillations in fura-2-loaded rat eggs. Treatment with 2 mM SrCl2 was optimal for inducing the first [Ca2+]i transient, which was similar in duration to that triggered by sperm. However, the frequency and duration of the subsequent [Ca2+]i oscillations were lower and longer in SrCl2-activated than in sperm-activated eggs. The degree of CGE was identical in eggs activated by either sperm or SrCl2, as assessed by semi-quantitative immunohistochemistry combined with confocal microscopy. Evoking 1, 2 or 10 [Ca2+]i oscillations (8, 15 or 60 min in SrCl2 respectively) had no effect on the intensity of fluorescent CGE reporter dyes, while 60-min exposure to SrCl2 caused a delay in CM. Our results demonstrate that SrCl2 is an effective parthenogenetic agent that mimics rat egg activation by sperm, as judged by the generation of [Ca2+]i oscillations, CGE and CM.

Antagonists of myosin light chain kinase and of myosin II inhibit specific events of egg activation in fertilized mouse eggs

Although recent studies have demonstrated the importance of calcium/calmodulin (Ca(2+)/CAM) signaling in mammalian fertilization, many targets of Ca(2+)/CAM have not been investigated and represent potentially important regulatory pathways to transduce the Ca2+ signal that is responsible for most events of egg activation. A well-established Ca(2+)/CAM-dependent enzyme is myosin light chain kinase (MYLK2), the downstream target of which is myosin II, an isoform of myosin known to be important in cytokinesis. In fertilized mouse eggs, established inhibitors of MYLK2 and myosin II were investigated for their effects on events of egg activation. The MYLK2 antagonist, ML-7, did not decrease the activity of Ca(2+)/CAM protein kinase II or the elevation of intracellular Ca2+, and it did not delay the onset of Ca2+ oscillations. In contrast, ML-7 inhibited second polar body (PB) formation in a dose-dependent manner and reduced cortical granule (CG) exocytosis by a mean of approximately 50%. The myosin II isoform-specific inhibitor, blebbistatin, had similar inhibitory effects. Although both antagonists had no effect on anaphase onset, they inhibited second PB formation by preventing spindle rotation before telophase II and normal contractile ring constriction. To our knowledge, this is the first report that MYLK2 and myosin II are involved in regulating the position of the meiotic spindle, formation of the second PB, and CG exocytosis. The present results suggest that MYLK2 is one of a family of CAM-dependent proteins that act as multifunctional regulators and transduce the Ca2+ signal at fertilization.

Cortical granules behave differently in mouse oocytes matured under different conditions

BACKGROUND

To better understand the differences between in vivo (IVO) and in vitro (IVM) matured oocytes, we studied the chronological changes in cortical granule (CG) distribution and nuclear progression during maturation, and the competence of CG release and embryo development of mouse oocytes matured under different conditions.

METHODS

Oocytes matured in vivo or in different culture media were used and CG distribution and release were assessed by fluorescein isothiocyanate-labelled Lens culinaris agglutinin and laser confocal microscopy.

RESULTS

Tempos of nuclear maturation and CG redistribution were slower, and competence for CG exocytosis, cleavage and blastulation were lower in the IVM oocytes than in the IVO oocytes. These parameters also differed among oocytes matured in different culture media. Hypoxanthine (HX, 4 mM) blocked germinal vesicle breakdown (GVBD), postponed CG migration and prevented CG-free domain (CGFD) formation. Cycloheximide (CHX) facilitated both GVBD and CG migration, but inhibited CGFD formation. The presence of serum in maturation media enhanced CG release after aging or activation of oocytes. Maintenance of germinal vesicle intact for some time by a trace amount (0.18 mM) of HX was beneficial to oocyte cytoplasmic maturation.

CONCLUSION

CGs behaved differently in mouse oocytes matured under different conditions, and cytoplasmic maturity was not fully achieved in the IVM oocytes.

Egg activation events are regulated by the duration of a sustained [Ca2+]cyt signal in the mouse

Although the dynamics of oscillations of cytosolic Ca2+ concentration ([Ca2+]cyt) play important roles in early mammalian development, the impact of the duration when [Ca2+]cyt is elevated is not known. To determine the sensitivity of fertilization-associated responses [i.e., cortical granule exocytosis, resumption of the cell cycle, Ca2+/calmodulin-dependent protein kinase II (CaMKII) activity, recruitment of maternal mRNAs] and developmental competence of the parthenotes to the duration of a [Ca2+]cyt transient, unfertilized mouse eggs were subjected to a prolonged [Ca2+]cyt change for 15, 25, or 50 min by means of repetitive Ca2+ electropermeabilization at 2-min intervals. The initiation and completion of fertilization-associated responses are correlated with the duration of time in which the [Ca2+]cyt is elevated, with the exception that autonomous CaMKII activity is down-regulated with prolonged elevated [Ca2+]cyt. Activated eggs from 25- or 50-min treatments readily develop to the blastocyst stage with no sign of apoptosis or necrosis and some implant. Ca2+ influx into unfertilized eggs causes neither Ca2+ release from intracellular stores nor rapid removal of cytosolic Ca2+. Thus, the total Ca2+ signal input appears to be an important regulatory parameter that ensures completion of fertilization-associated events and oocytes have a surprising degree of tolerance for a prolonged change in [Ca2+]cyt.

Impaired fertility in T-stock female mice after superovulation

Superovulation of female mice with exogenous gonadotrophins is routinely used for increasing the number of eggs ovulated by each female in reproductive and developmental studies. We report an unusual effect of superovulation on fertilization in mice.In vivomatings of superovulated T-stock females with B6C3F1 males resulted in a two-fold reduction (P< 0.001) in the frequencies of fertilized eggs compared with control B6C3F1 matings. In addition, approximately 22 h after mating, only 15% of fertilized eggs recovered in T-stock females had reached the metaphase stage of the first cleavage division versus 87% in B6C3F1 females (P< 0.0001). Matings with T-stock males did not improve the reproductive performance of T-stock females. To investigate the possible cause(s) for the impaired fertilization and zygotic development, the experiments were repeated usingin vitrofertilization. Under these conditions, the frequencies of fertilized eggs were not different in superovulated T-stock and B6C3F1 females (51.7 ± 6.0 and 64.5 ± 3.8%,P= 0.10). There was a seven-fold increase in the frequencies of fertilized eggs that completed the first cell cycle of development afterin vitroversusin vivofertilization in T-stock females. These results rule out an intrinsic deficiency of the T-stock oocyte as the main reason for the impaired fertility afterin vivomatings, and suggest that superovulation of T-stock females may induce a hostile oviductal and uterine environment with dramatic effects on fertilization and zygotic development.

Membrane and cortical abnormalities in post-ovulatory aged eggs: analysis of fertilizability and establishment of the membrane block to polyspermy

Fertilization at increased times after ovulation is associated with poor reproductive outcomes. This study examines the effects of post-ovulatory ageing on egg membrane function through analyses of mouse eggs collected at 13 and 22 h post-HCG ('young' and 'aged' eggs, respectively). Experiments in which fertilized zona pellucida-free young and aged eggs are challenged with additional sperm reveal that aged eggs are less able to establish a membrane block to prevent polyspermy, since sperm penetrate 24% of fertilized aged eggs but are unable to penetrate fertilized young eggs. This is not due to a failure of aged eggs to respond to fertilization, as the extent of sperm-induced cortical granule exocytosis is similar in aged and young eggs. Post-ovulatory ageing also affects egg membrane receptivity to sperm as a subset of zona pellucida-free aged eggs are slow to fertilize or resistant to fertilization. Sperm binding to young and aged eggs is similar, but aged eggs develop cytoskeletal abnormalities that may affect membrane/cortical function, such as the ability of the egg membrane to support sperm-egg fusion. These data demonstrate that the poor reproductive outcomes associated with post-ovulatory ageing could be a result of reduced fertilization, due to reduced egg membrane receptivity to sperm, or a result of increased incidence of polyspermy, due to the reduced ability to establish a membrane block to polyspermy. This analysis of egg membrane function deficiencies provides insights into post-ovulatory ageing and has implications for assisted reproductive technologies.

Sperm penetration of immature and maturing oocytes does not affect phosphorylation of mitogen-activated protein kinase in pigs

Pig oocytes cultured in vitro for 0, 25, 33 and 44 h were inseminated by frozen-thawed ejaculated sperm. At specified times after insemination, sperm penetration, cell cycle progression and mitogen-activated protein kinase (MAPK) phosphorylation were evaluated. It was shown that: (1) oocytes at various maturational stages could be penetrated by sperm; (2) sperm penetration did not affect meiotic cell cycle progression; (3) sperm penetration of germinal vesicle (GV) oocytes and maturing oocytes did not alter MAPK phosphorylation; and (4) when premetaphase I (pre-MI) and metaphase I (MI) oocytes, in which MAPK was activated, were fertilised, no evident MAPK dephosphorylation was detected as in metaphase II oocytes. The data suggest that sperm penetration before oocyte maturation does not affect MAPK phosphorylation and that the machinery inactivating MAPK upon fertilisation is not developed in maturing (pre-MI to MI) oocytes.

A C-type lectin associated and translocated with cortical granules during oocyte maturation and egg fertilization in fish

Oocyte maturation and egg fertilization in both vertebrates and invertebrates are marked by orchestrated cytoplasmic translocation of secretory vesicles known as cortical granules. It is thought that such redistribution of cellular content is critical for asymmetrical cell division during early development, but the mechanism and regulation of the process is poorly understood. Here we report the identification, purification and cDNA cloning of a C-type lectin from oocytes of a freshwater fish species gibel carp (Carassius auratus gibelio). The purified protein has been demonstrated to have lectin activity and to be a Ca(2+)-dependent C-type lectin by hemagglutination activity assay. Immunocytochemistry revealed that the lectin is associated with cortical granules, gradually translocated to the cell surface during oocyte maturation, and discharged to the egg envelope upon fertilization. Interestingly, the lectin becomes phosphorylated on threonine residues upon induction of exocytosis by fertilization and returns to its original state after morula stage of embryonic development, suggesting that this posttranslational modification may represent a critical molecular switch for early embryonic development.

The thiol reagent, thimerosal, irreversibly inhibits meiosis reinitiation in mouse oocyte when applied during a very early and narrow temporal window: a pharmacological analysis

The effect of the sulfhydryl reagent, thimerosal (TMS) on meiosis resumption in germinal vesicle (GV)-stage denuded mouse oocytes was studied. It irreversibly inhibits both GV breakdown (GVBD) and the first polar body (pb1) extrusion in concentration- and time-dependent manners, the most striking result being the very early and narrow temporal window during which denuded primary oocytes released from their follicle are susceptible to a pulse of the drug. This inhibition is bypassed by dithiothreitol (DTT) with an efficiency declining with time, while thiosalicylic acid (TA), an analog of TMS devoid of the mercury atom, has no effect on meiosis reinitiation. These results strongly suggest that the inhibitory effect of TMS is a consequence of its sulfhydryl group oxidising activity. The molecular target(s) of this inhibitory oxidation should however be identified. In contrast to DTT, okadaic acid (OA), known to bypass the inhibitory effect of drugs interfering with protein kinase activities, only induces chromatin condensation and GVBD in TMS-pulsed oocytes with a delay of about 8 hr as compared to the control situation. This confirms that a very early thiol oxidation induced by TMS exerts a much more dramatic effect on resumption on meiosis than any pharmacological manipulation of protein kinase activities leading to activation of MPF.

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.

Maturation-associated increase in IP3 receptor type 1: role in conferring increased IP3 sensitivity and Ca2+ oscillatory behavior in mouse eggs

Maturation of mouse oocytes is accompanied by an increase in sensitivity to inositol 1,4,5-trisphosphate (IP(3))-mediated release of intracellular calcium. To test the hypothesis that the maturation-associated 1.5- to 2.0-fold increase in the mass of the type 1 IP(3) receptor (IP(3)R-1) confers this increase in IP(3) sensitivity, we employed RNA interference to prevent this change in IP(3)R-1 protein level. Microinjection into germinal vesicle (GV)-intact oocytes of dsRNA corresponding to the IP(3)R-1 sequence resulted in a >90% reduction in the amount of maternal IP(3)R-1 mRNA and prevented the maturation-associated increase in the mass of the IP(3)R-1 protein. These injected oocytes matured to metaphase II, and there was no effect on the maturation-associated increases in p34(cdc2)/cyclin B kinase and MAP kinase activities or the global pattern of protein synthesis. IP(3)-induced cortical granule exocytosis was significantly decreased in these eggs when compared with controls previously injected with enhanced green fluorescent protein (EGFP) dsRNA. Following insemination, the IP(3)R-1 dsRNA-injected eggs displayed significantly fewer Ca(2+) transients than controls, and the duration of the first Ca(2+) transient was about half that of controls. These results support the hypothesis that the maturation-associated increase in the mass of IP(3)R-1 confers the increase in IP(3)-sensitivity that is observed following oocyte maturation and is necessary for the proper Ca(2+) oscillatory pattern following insemination.

The origin of monozygotic twinning

The incidence of monozygotic twinning appears to be increasing within the field of assisted human reproduction. Many theories have been put forward as to how and when this occurs. Whatever the cause, the normal events of embryo development, which necessarily involve axis formation, patterning and polarization, need to be adhered to in order to obtain a viable offspring. This paper describes the course of development in terms of axis formation and polarity and offers suggestions as to how either a disruption of this or duplication events in the course of the formation of these parameters could prevent or contribute to a twinning event. The likelihood of twinning occurring at any point is discussed in terms of the establishment of polarity and axes.

Fertilization of mouse oocytes from in vitro-matured preantral follicles using classical in vitro fertilization or intracytoplasmic sperm injection

Early preantral mouse follicles with a diameter of 110-160 microm were cultured in vitro for 10 or 12 days. Mature oocytes were retrieved following hCG, and fertilization was attempted either by in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI). Two-cell and blastocyst formation rates and blastocyst cell numbers were compared between 10-day and 12-day in vitro-matured oocytes versus in vivo-matured oocytes. Uncleaved IVF oocytes were subjected to chromosome analysis. The 2-cell formation rate was significantly improved by ICSI compared with IVF both in 10-day (72.1% versus 56.1%; P = 0.03) and 12-day cultures (74.1% versus 54.5%; P = 0.028). Cytogenetic analysis of uncleaved MII oocytes following IVF showed that about 30% of MII oocytes showed no sign of sperm penetration. The blastocyst formation rate was significantly lower in 12-day versus 10-day cultures, whether fertilization was by IVF (40.7% versus 62.4%, P = 0.016) or by ICSI (32.5% versus 57.1%, P = 0.035). Blastocyst cell numbers from IVF and ICSI 10-day groups were similar and both significantly higher (P < 0.001) than from IVF 12-day cultures. All above expressed values were significantly higher for in vivo-matured oocytes. In conclusion, fertilization of oocytes from in vitro-matured mouse preantral follicles can be optimized with ICSI, giving significantly higher 2-cell formation rates than IVF. Blastocyst formation rate was not influenced by the technique of fertilization but rather by the extent of the in vitro culture period. Best results on preimplantation development of oocytes for in vitro-matured preantral follicles were obtained with ICSI on oocytes from 10-day in vitro cultures.

Translocation of the classic protein kinase C isoforms in porcine oocytes: implications of protein kinase C involvement in the regulation of nuclear activity and cortical granule exocytosis

Protein kinase C (PKC) is a family of Ser/Thr protein kinases categorized into three subfamilies: classical, novel, and atypical. The subcellular localization of classical PKCalpha, -betaI, and -gamma in the process of porcine oocyte maturation, fertilization, and parthenogenetic activation and their involvement in cortical granule (CG) exocytosis were investigated. The results of Western blot showed that PKCalpha, -betaI, and -gamma were expressed in the oocytes at the germinal vesicle (GV) and metaphase II (MII) stages. Confocal microscopy revealed that the three PKC isoforms were concentrated in the GV but evenly distributed in the cytoplasm of MII eggs. PKCalpha and -gamma were translocated to the plasma membrane soon after sperm penetration. cPKCs migrated into the pronucleus in fertilized eggs. Following treatment with a PKC activator, phorbol 12-myristate 13-acetate (PMA), CGs were released and PKCalpha and -gamma were translocated to the membrane. The CG exocytosis and PKC redistribution induced by PMA could be blocked by the PKC inhibitor staurosporine. Parthenogenetic stimulation with ionophore A23187 or electrical pulse also induced cPKC translocation and CG exocytosis. Eggs injected with PKCalpha isoform-specific antibody failed to undergo CG exocytosis after PMA treatment or fertilization. The results suggest that cPKCs, especially the alpha-isotype, regulate nuclear function and CG exocytosis in porcine eggs.

The dynamics of plasma membrane PtdIns(4,5)P2 at fertilization of mouse eggs

A series of intracellular Ca2+ oscillations are responsible for triggering egg activation and cortical granule exocytosis at fertilization in mammals. These Ca2+ oscillations are generated by an increase in inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], which results from the hydrolysis of phosphatidylinositol 4,5-bisphosphate[PtdIns(4,5)P2]. Using confocal imaging to simultaneously monitor Ca2+ and plasma membrane PtdIns(4,5)P2in single living mouse eggs we have sought to establish the relationship between the kinetics of PtdIns(4,5)P2 metabolism and the Ca2+ oscillations at fertilization. We report that there is no detectable net loss of plasma membrane PtdIns(4,5)P2either during the latent period or during the subsequent Ca2+oscillations. When phosphatidylinositol 4-kinase is inhibited with micromolar wortmannin a limited decrease in plasma membrane PtdIns(4,5)P2 is detected in half the eggs studied. Although we were unable to detect a widespread loss of PtdIns(4,5)P2, we found that fertilization triggers a net increase in plasma membrane PtdIns(4,5)P2 that is localized to the vegetal cortex. The fertilization-induced increase in PtdIns(4,5)P2 follows the increase in Ca2+, is blocked by Ca2+ buffers and can be mimicked, albeit with slower kinetics, by photoreleasing Ins(1,4,5)P3. Inhibition of Ca2+-dependent exocytosis of cortical granules, without interfering with Ca2+ transients, inhibits the PtdIns(4,5)P2 increase. The increase appears to be due to de novo synthesis since it is inhibited by micromolar wortmannin. Finally,there is no increase in PtdIns(4,5)P2 in immature oocytes that are not competent to extrude cortical granules. These studies suggest that fertilization does not deplete plasma membrane PtdIns(4,5)P2 and that one of the pathways for increasing PtdIns(4,5)P2 at fertilization is invoked by exocytosis of cortical granules.

The biology of cortical granules

An egg-that took weeks to months to make in the adult-can be extraordinarily transformed within minutes during its fertilization. This review will focus on the molecular biology of the specialized secretory vesicles of fertilization, the cortical granules. We will discuss their role in the fertilization process, their contents, how they are made, and the molecular mechanisms that regulate their secretion at fertilization. This population of secretory vesicles has inherent interest for our understanding of the fertilization process. In addition, they have import because they enhance our understanding of the basic processes of secretory vesicle construction and regulation, since oocytes across species utilize this vesicle type. Here, we examine diverse animals in a comparative approach to help us understand how these vesicles function throughout phylogeny and to establish conserved themes of function.

Identification of a translocation deficiency in cortical granule secretion in preovulatory mouse oocytes

Preovulatory, germinal vesicle (GV)-stage mouse oocytes are unable to undergo normal cortical granule (CG) secretion. Full secretory competence is observed by metaphase II (MII) of meiosis and involves the development of calcium response mechanisms. To identify the deficient or inhibited step in CG secretion, preovulatory GV-stage oocytes were stimulated and tested for their ability to undergo translocation, docking, and/or fusion. The mean CG distance to the plasma membrane was not reduced in fertilized or sperm fraction-injected, GV-stage oocytes relative to that in control GV-stage oocytes. In addition, analysis of individual CG distances to the plasma membrane indicated no subpopulation of CGs competent to translocate. Further analysis demonstrated that secretory incompetence likely is not due to a lack of proximity of CGs to the egg's primary calcium store, the endoplasmic reticulum. Calcium/calmodulin-dependent protein kinase II (CaMKII), which is reportedly involved in secretory granule translocation and secretion in many cells, including eggs, was investigated. A 60-kDa CaMKII isoform detected by Western blot analysis increased 150% during oocyte maturation. The CaMKII activity assays indicated that MII-stage eggs correspondingly have 110% more maximal activity than GV-stage oocytes. These data demonstrate that the primary secretory deficiency is due to a failure of CG translocation, and that a maturation-associated increase in CaMKII correlates with the acquisition of secretory competence and the ability of the egg to undergo normal activation.

Activation of rabbit oocytes: the impact of the Ca2+ signal regime on development

Postfertilization manipulation of mammalian embryos results in various developmental alterations. To determine whether the manipulation of the Ca2+ regime causing oocyte activation is a valuable experimental means in helping understand the biological process by which embryos integrate signals from outside and later regulate gene expression, we linked Ca2+ signal parameters i.e. amplitude, number and frequency, with the efficiency and quality of postimplantation development. Freshly ovulated rabbit oocytes were subjected to repetitive and modulated Ca2+ influx. The results provide three major pieces of information. Firstly, the Ca2+ stimulus is the most efficient signal activating mammalian eggs when it is applied in a repetitive manner, the amplitude being the crucial factor. Secondly, the dynamics of early cleavage does not appear to be determined by either the frequency or the amplitude of modulation of the Ca2+ signal that activates the oocyte. Thirdly, amplitude and temporal modulation of the Ca2+ signal in the early minutes influences the developmental performance and the morphology of the rabbit parthenogenetic conceptus at day 11.5 of pregnancy. The results demonstrate the importance of epigenetic events during postfertilization as well as the possible uses of Ca2+ modulation in studying long term developmental effects.

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.

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.