A novel trans-complementation assay suggests full mammalian oocyte activation is coordinately initiated by multiple, submembrane sperm components

To initiate normal embryonic development, an egg must receive a signal to become activated at fertilization. We here report that the ability of demembranated sperm heads to activate is abolished after incubation over the range 20-44 degreesC and is sensitive to reducing agents. On the basis of this observation, we have developed a microinjection-based, trans-complementation assay in order to dissect the heat-inactivated sperm-borne oocyte-activating factor(s) (SOAF). We demonstrate that the failure of heat-inactivated sperm heads to activate an egg is rescued by coinjection with dithiothreitol-solubilized SOAF from demembranated sperm heads. The solubilized SOAF (SOAFs) is trypsin sensitive and is liberated from demembranated heads in a temperature-dependent manner that inversely correlates with the ability of sperm heads to activate. This argues that SOAFs is a proteinaceous molecular species required to initiate activation. Injection of oocytes with mouse or hamster sperm cytosolic factors, but not SOAFs alone, induced resumption of meiosis, further suggesting that these cytosolic factors and SOAF are distinct. Collectively, these data strongly suggest that full mammalian oocyte activation is initiated by the coordinated action of one or more heat-sensitive protein constituents of the perinuclear matrix and at least one heat-stable submembrane component.     

Injection of sperm cytosolic factor into mouse metaphase II oocytes induces different developmental fates according to the frequency of [Ca(2+)](i) oscillations and oocyte age

Intracellular calcium ([Ca(2+)](i)) rises are a hallmark of mammalian fertilization and are associated with normal activation of embryonic development. Injection of mammalian sperm cytosolic factor (SCF) into oocytes has been shown to trigger [Ca(2+)](i) rises similar to those observed during fertilization, and to initiate normal embryonic development. However, Ca(2+) release has also been shown to be associated with cell death, but the mechanisms of the detrimental effects of Ca(2+) stimulation on development have not yet been investigated. Thus, studies were undertaken using SCF to test the effects of [Ca(2+)](i) oscillations on oocyte activation in freshly ovulated and aged oocytes. Injections of 1 mg/ml SCF into freshly ovulated mouse metaphase II oocytes, which evoked Ca(2+) responses with low frequency and short duration, induced normal activation and cleavage to the two-cell stage. Conversely, injection of 15 mg/ml SCF, which triggered high-frequency and persistent Ca(2+) responses, induced abnormal activation that was characterized by abnormal chromatin configurations, inhibition of DNA synthesis, and lack of first mitotic spindle assembly. More importantly, fertilization-like Ca(2+) responses induced by injection of 1 mg/ml SCF triggered cell death, rather than activation, in in vitro-aged oocytes. These oocytes exhibited extensive cytoplasmic and DNA fragmentation that was accompanied by activation of protein caspases, all of which are signs of apoptotic cell death. Fewer similarly aged oocytes that were either unstimulated or activated with 7% ethanol underwent fragmentation. Together, these results suggest that [Ca(2+)](i) oscillations are required to activate freshly ovulated oocytes, but if initiated at abnormally high frequency and duration or if induced in aged oocytes, the [Ca(2+)](i) oscillations may trigger premature termination of embryonic development.     

Fate of postacrosomal perinuclear theca recognized by monoclonal antibody MN13 after sperm head microinjection and its role in oocyte activation in mice

Monoclonal antibody (mAb) MN13 labels mouse sperm head postacrosomal perinuclear theca (PT), which is possibly involved in oocyte activation during fertilization. The antigenic site is expressed after mild sonication followed by treatment with dithiothreitol (DTT) or heat (45 degrees C), and is visible as a thick band in the postacrosomal region. The presence of protease inhibitors in the sonication medium suppresses the exposure of MN13 epitope (MN13p), suggesting the involvement of a proteolytic reaction in this process. Spermatozoa do not express MN13p after the induction of acrosome exocytosis by Ca(2+) ionophore, zona binding, or during zona penetration, a strategy that ensures safe delivery of postacrosomal PT proteins to oocytes after fusion. MN13 labeling was not detectable during fertilization by zona-free in vitro fertilization, suggesting that the antigenic site does not react with proteolytic enzymes during sperm-oocyte fusion and the antibody does not recognize the nascent epitope. Microinjection of sperm heads prepared by sonication and DTT treatment led to the activation of metaphase II oocytes. The oocyte activating function of such sperm heads was significantly diminished after labeling with MN13 prior to intracytoplasmic sperm injection (ICSI), but labeling with antiequatorin antibody MN9 activated oocytes with a frequency similar to that of unlabeled sperm heads. The sperm heads in inactive oocytes formed premature chromosome condensations (PCCs), which were invested by independent metaphase-like spindles. These observations indicate that the postacrosomal PT recognized by mAb MN13 is involved in oocyte activation. MN13p is dissociated from sperm heads during the early stages of decondensation after ICSI. In activated oocytes, MN13-labeled fine granules were redistributed in the midzone spindle region, whereas in inactive oocytes they formed a ring around the polar regions of the metaphase II and PCC spindles.     

STIM1 is required for Ca2+ signaling during mammalian fertilization

During fertilization in mammals, a series of oscillations in the oocyte's intracellular free Ca(2+) concentration is responsible for oocyte activation and stimulation of embryonic development. The oscillations are associated with influx of Ca(2+) across the plasma membrane that is probably triggered by the depletion of the intracellular stores, a mechanism known as store-operated Ca(2+) entry. Recently, STIM1 has been identified in oocytes as a key component of the machinery that generates the Ca(2+) influx after store depletion. In this study, the involvement of STIM1 in the sperm-induced Ca(2+) oscillations and its significance in supporting subsequent embryo development were investigated. Downregulation of STIM1 levels in pig oocytes by siRNA completely inhibited the repetitive Ca(2+) signal triggered by the fertilizing sperm. In addition, a significantly lower percentage of oocytes cleaved or formed blastocysts when STIM1 was downregulated prior to fertilization compared to the control groups. Restoring STIM1 levels after fertilization in such oocytes by means of mRNA injection could not rescue embryonic development that in most cases was arrested at the 2-cell stage. On the other hand, STIM1 overexpression prior to fertilization did not alter the pattern of sperm-induced Ca(2+) oscillations and development of these fertilized oocytes up to the blastocyst stage was also similar to that registered in the control group. Finally, downregulation of STIM1 had no effect on oocyte activation when activation was stimulated artificially by inducing a single large elevation in the oocyte's intracellular free Ca(2+) concentration. These findings suggest that STIM1 is essential for normal fertilization as it is involved in the maintenance of the long-lasting repetitive Ca(2+) signal.     

Mammalian phospholipase Czeta induces oocyte activation from the sperm perinuclear matrix

Mammalian sperm-borne oocyte activating factor (SOAF) induces oocyte activation from a compartment that engages the oocyte cytoplasm, but it is not known how. A SOAF-containing extract (SE) was solubilized from the submembrane perinuclear matrix, a domain that enters the egg. SE initiated activation sufficient for full development. Microinjection coupled to tandem mass spectrometry enabled functional correlation profiling of fractionated SE without a priori assumptions about its chemical nature. Phospholipase C-zeta (PLCzeta) correlated absolutely with activating ability. Immunoblotting confirmed this and showed that the perinuclear matrix is the major site of 72-kDa PLCzeta. Oocyte activation was efficiently induced by 1.25 fg of sperm PLCzeta, corresponding to a fraction of one sperm equivalent (approximately 0.03). Immunofluorescence microscopy localized sperm head PLCzeta to a post-acrosomal region that becomes rapidly exposed to the ooplasm following gamete fusion. This multifaceted approach suggests a mechanism by which PLCzeta originates from an oocyte-penetrating assembly--the sperm perinuclear matrix--to induce mammalian oocyte activation at fertilization.     

Ca(2+) oscillations induced by a cytosolic sperm protein factor are mediated by a maternal machinery that functions only once in mammalian eggs

At fertilization in mammals, the sperm activates the egg by inducing a series of oscillations in the intracellular free Ca(2+) concentration. There is evidence showing that this oscillatory event is triggered by a sperm-derived protein factor which diffuses into egg cytoplasm after gamete membrane fusion. At present the identity of this factor and its precise mechanism of action is unknown. Here, we studied the specificity of action of the sperm factor in triggering Ca(2+) oscillations in mammalian eggs. In doing so, we examined the patterns of Ca(2+) signaling in mouse eggs, zygotes, parthenogenetic eggs and maturing oocytes following the stimulation of bovine sperm extracts which contain the sperm factor. It is observed that the sperm factor could induce Ca(2+) oscillations in metaphase eggs, maturing oocytes and parthenogenetically activated eggs but not in the zygotes. We present evidence that Ca(2+) oscillations induced by the sperm factor require a maternal machinery. This machinery functions only once in mammalian oocytes and eggs, and is inactivated by sperm-derived components but not by parthenogenetic activation. In addition, it is found that neither InsP(3) receptor sensitivity to InsP(3) nor Ca(2+) pool size are the determinants that cause the fertilized egg to lose its ability to generate sperm-factor-induced Ca(2+) oscillations at metaphase. In conclusion, our study suggests that the orderly sequence of Ca(2+) oscillations in mammalian eggs at fertilization is critically dependent upon the presence of a functional maternal machinery that determines whether the sperm-factor-induced Ca(2+) oscillations can persist.     

Activation, pronuclear formation, and development in vitro of pig oocytes following intracytoplasmic injection of freeze-dried spermatozoa

The fertilization of pig oocytes following intracytoplasmic injection of freeze-dried spermatozoa was evaluated. Activation and male pronuclear (MPN) formation were better in oocytes injected with isolated freeze-dried sperm heads than whole freeze-dried spermatozoa, but cleaved embryos were generally difficult to develop to the morula or blastocyst stage. When spermatozoa were freeze-dried for 24 h, oocyte activation and MPN formation in activated oocytes after sperm head injection were inhibited. Embryo development to the blastocyst stage was only obtained after injecting sperm heads isolated from spermatozoa freeze-dried for 4 h and stored at 4 degrees C. The proportion of embryos that developed to the blastocyst stage was not increased by the treatment of injected oocytes with Ca ionophore (5-10 microM). Increasing the sperm storage time did not affect oocyte activation or MPN formation, but blastocyst development was observed only after 1 mo of storage. These results demonstrate that pig oocytes can be fertilized with appropriately freeze-dried spermatozoa and that the fertilized oocytes can develop to the blastocyst stage.     

Patterns of intracellular calcium oscillations in horse oocytes fertilized by intracytoplasmic sperm injection: possible explanations for the low success of this assisted reproduction technique in the horse

In all species studied, fertilization induces intracellular Ca2+ ([Ca2+]i) oscillations required for oocyte activation and embryonic development. This species-specific pattern has not been studied in the equine, partly due to the difficulties linked to in vitro fertilization in this species. Therefore, the objective of this study was to use intracytoplasmic sperm injection (ICSI) to investigate fertilization-induced [Ca2+]i signaling and, possibly, ascertain problems linked to the success of this technology in the horse. In vivo- and in vitro-matured mare oocytes were injected with a single motile stallion sperm. Few oocytes displayed [Ca2+]i responses regardless of oocyte source and we hypothesized that this may result from insufficient release of the sperm-borne active molecule (sperm factor) into the oocyte. However, permeabilization of sperm membranes with Triton-X or by sonication did not alleviate the deficient [Ca2+]i responses in mare oocytes. Thus, we hypothesized that a step downstream of release, possibly required for sperm factor function, is not appropriately accomplished in horse oocytes. To test this, ICSI-fertilized horse oocytes were fused to unfertilized mouse oocytes, which are known to respond with [Ca2+]i oscillations to injection of stallion sperm, and [Ca2+]i monitoring was performed. Such pairs consistently displayed [Ca2+]i responses demonstrating that the sperm factor is appropriately released into the ooplasm of horse oocytes, but that these are unable to activate and/or provide the appropriate substrate that is required for the sperm factor delivered by ICSI to initiate oscillations. These findings may have implications to improve the success of ICSI in the equine and other livestock species.     

Injections of porcine sperm extracts trigger fertilization-like calcium oscillations in oocytes of a marine worm

The precise mechanisms by which sperm trigger calcium transients in eggs or oocytes during fertilization remain unknown. Based on time-lapse confocal microscopy, we show that intracellular injections of porcine sperm extracts cause the oocytes of a marine nemertean worm to undergo repetitive calcium oscillations resembling those obtained during normal fertilizations. Such findings are consistent with the view that fertilization involves a soluble sperm factor (SF) which is capable of eliciting calcium transients without binding to externally situated receptors on the oocyte plasmalemma. This study also describes for the first time the wave-like propagation patterns of SF-induced calcium transients that are generated in a heterologous combination of gametes obtained from different phyla of animals. Such cross-reactivity between distantly related taxa suggests that the intracellular signaling pathways triggered by sperm factors can be well conserved.     

Impact of oxidative stress in aged mouse oocytes on calcium oscillations at fertilization

In vivo post-ovulatory aging of oocytes significantly affects the development of oocytes and embryos. Also, oocyte aging alters the regulation of the intracellular calcium concentration, thus affecting Ca(2+) oscillations in fertilized oocytes. Because reactive oxygen species (ROS) are known to significantly perturb Ca(2+) homeostasis mainly through direct effects on the machinery involved in intracellular Ca(2+) storage, we hypothesized that the poor development of aged oocytes that may have been exposed to oxidative stress for a prolonged time might arise from impaired Ca(2+)-oscillation-dependent signaling. The fertilization rates of aged oocytes and of fresh oocytes treated with 100 microM hydrogen peroxide (H(2)O(2)) for 10 min were significantly lower than that of fresh oocytes. Comparing within the fertilized oocytes, blastocyst formation was decreased while embryo fragmentation was increased similarly in the aged and H(2)O(2)-treated fresh oocytes. The frequency of Ca(2+) oscillations was significantly increased whereas the amplitude of individual Ca(2+) transients was lowered in the aged and H(2)O(2)-treated fresh oocytes. The rates of rise and decline in individual Ca(2+) transients were decreased in these oocytes, indicating impaired Ca(2+) handling. When lipid peroxidation was assessed using 4,4-difluoro-5-(4-phenyl-1,3-buttadienyl)-4-bora-3a, 4a-diaza-s-indacene-3-undecanoic acid (C11-BODIPY) in unfertilized oocytes placed in a 5% CO(2) in air atmosphere, the green fluorescence (indicating lipid peroxidation) increased faster in the aged oocytes than in the fresh oocytes. Furthermore, the green fluorescence in the aged oocytes was already approximately 20 times higher than that in the fresh oocytes at the beginning of the measurements. These findings support the idea that Ca(2+) oscillations play a key role in the development of fertilized aged oocytes.     

Animal-vegetal axis patterning mechanisms in the early sea urchin embryo

During mouse fertilization the spermatozoon induces a series of low-frequency long-lasting Ca(2+) oscillations. It is generally accepted that these oscillations are due to Ca(2+) release through the inositol 1,4,5-trisphosphate (InsP(3)) receptor. However, InsP(3) microinjection does not mimic sperm-induced Ca(2+) oscillations, leading to the suggestion that the spermatozoon causes Ca(2+) release by sensitizing the InsP(3) receptor to basal levels of InsP(3). This contradicts recent evidence that the spermatozoon triggers Ca(2+) oscillations by introducing a phospholipase C or else an activator of phospholipase C. Here we show for the first time that sperm-induced Ca(2+) oscillations may be mimicked by the photolysis of caged InsP(3) in both mouse metaphase II eggs and germinal vesicle stage oocytes. Eggs, and also oocytes that had displayed spontaneous Ca(2+) oscillations, gave long-lasting Ca(2+) oscillations when fertilized or when caged InsP(3) was photolyzed. In contrast, oocytes that had shown no spontaneous Ca(2+) oscillations did not generate many oscillations when fertilized or following photolysis of caged InsP(3). Fertilization in eggs was most closely mimicked when InsP(3) was uncaged at relatively low amounts for extended periods. Here we observed an initial Ca(2+) transient with superimposed spikes, followed by a series of single transients with a low frequency; all characteristics of the Ca(2+) changes at fertilization. We therefore show that InsP(3) can mimic the distinctive pattern of Ca(2+) release in mammalian eggs at fertilization. It is proposed that a sperm Ca(2+)-releasing factor operates by generating a continuous small amount of InsP(3) over an extended period of time, consistent with the evidence for the involvement of a phospholipase C.     

Parthenogenetic activation of bovine oocytes using bovine and murine phospholipase C zeta

Background  

During natural fertilization, sperm fusion with the oocyte induces long lasting intracellular calcium oscillations which in turn are responsible for oocyte activation. PLCZ1 has been identified as the factor that the sperm delivers into the egg to induce such a response. We tested the hypothesis that PLCZ1 cRNA injection can be used to activate bovine oocytes.     

A soluble extract from human spermatozoa activates ascidian oocytes

A soluble extract from human spermatozoa induced calcium oscillations and extrusion of the first polar body when injected into oocytes of the ascidian Ciona intestinalis . The properties of calcium oscillations and time of polar body extrusion precisely mimic oocyte activation induced by C. intestinalis sperm or sperm extracts. The data suggest that human sperm extracts can activate oocytes of different phyla by the same mechanism as homologous spermatozoa. Injection of inositol 1,4,5-trisphosphate (IP₃) into C. intestinalis oocytes mimicked to some extent the initial stages of oocyte activation, but the results demonstrate that ascidian oocyte activation by human sperm extract cannot be explained solely in terms of IP₃-induced calcium release. Injection of other calcium releasing second messengers, cyclic adenosine diphosphate ribose, or calcium ions, does not lead to oocyte activation or release intracellular calcium in ascidian oocytes. It was concluded that human spermatozoa contain one or more molecules that can trigger intracellular calcium release in oocytes from different phyla.     

Defective calcium release during in vitro fertilization of maturing oocytes of LT/Sv mice

Oocytes of LT/Sv mice have anomalous cytoplasmic and nuclear maturation. Here, we show that in contrast to the oocytes of wild-type mice, a significant fraction of LT/Sv oocytes remains arrested at the metaphase of the first meiotic division and is unable to undergo sperm-induced activation when fertilized 15 hours after the resumption of meiosis. We also show that LT/Sv oocytes experimentally induced to resume meiosis and to reach metaphase II are unable to undergo activation in response to sperm penetration. However, the ability for sperm-induced activation developed during prolonged in vitro culture. Both types of LT/Sv oocytes, i.e. metaphase I and those that were experimentally induced to reach metaphase II, underwent activation when they were fertilized 21 hours after germinal vesicle breakdown (GVBD). Thus, the ability of LT/Sv oocytes to become activated by sperm depends on cytoplasmic maturation rather than on nuclear maturation i.e. on the progression of meiotic division. We also show that sperm penetration induces fewer Ca(2+) transients in LT/Sv oocytes than in control wild-type oocytes. In addition, we found that the levels of mRNA encoding different isoforms of protein kinase C (alpha, delta and zeta), that are involved in meiotic maturation and signal transduction during fertilization, differed between metaphase I LT/Sv oocytes which cannot be activated by sperm, and those which are able to undergo activation after fertilization. However, no significant differences between these oocytes were found at the level of mRNA encoding IP(3) receptors which participate in calcium release during oocyte fertilization.     

Porcine oocyte activation induced by a cytosolic sperm factor

It is not known how the fertilizing sperm elicits the release of Ca(2+) from the oocyte's intracellular stores. We investigated whether a crude extract isolated from boar sperm could induce the Ca(2+) release and trigger subsequent early and late activation events upon injection into matured porcine oocytes. The sperm extract induced an immediate rise in the intracellular free Ca(2+) concentration in all oocytes tested, which was followed by repetitive Ca(2+) transients in 11 out of 14 oocytes. Heat or trypsin treatment of the extract totally abolished the Ca(2+) releasing activity of the sperm factor. The injected oocytes showed cortical granule exocytosis, they resumed meiosis and entered first interphase: pronuclei were formed in 89.2% (132/148) of the cases. Pronuclear formation was accompanied by the appearance of a new 22 kDa protein as normally seen at fertilization. Of the successfully injected oocytes 51.7% (105/203) cleaved and 2.0% (4/203) developed to the blastocyst stage after being cultured for 7 days in NCSU 23 medium. Injection of the carrier medium could not trigger these changes. The results indicate that the sperm might activate porcine oocytes by introducing a soluble factor into the oocyte's cytoplasm after gamete fusion.     

Increased sensitivity and clustering of elementary Ca2+ release events during oocyte maturation

The universal signal for egg activation at fertilization is a rise in cytoplasmic Ca(2+) with defined spatial and temporal kinetics. Mammalian and amphibian eggs acquire the ability to produce such Ca(2+) signals during a maturation period that precedes fertilization and encompasses resumption of meiosis and progression to metaphase II. In Xenopus, immature oocytes produce fast, saltatory Ca(2+) waves that can be oscillatory in nature in response to IP(3). In contrast, mature eggs produce a single continuous, sweeping Ca(2+) wave in response to IP(3) or sperm fusion. The mechanisms mediating the differentiation of Ca(2+) signaling during oocyte maturation are not well understood. Here, I characterized elementary Ca(2+) release events (Ca(2+) puffs) in oocytes and eggs and show that the sensitivity of IP(3)-dependent Ca(2+) release is greatly enhanced during oocyte maturation. Furthermore, Ca(2+) puffs in eggs have a larger spatial fingerprint, yet are short lived compared to oocyte puffs. Most interestingly, Ca(2+) puffs cluster during oocyte maturation resulting in a continuum of Ca(2+) release sites over space in eggs. These changes in the spatial distribution of elementary Ca(2+) release events during oocyte maturation explain the continuous nature and slower speed of the fertilization Ca(2+) wave.     

Initiation and organization of events during the first cell cycle in mammals: applications in cloning

The technology of cloning involves transplanting a diploid nucleus into a mature oocyte cytoplast. The cytoplast is then activated to initiate the first cell cycle of development as a nuclear transplant embryo. Initiation and regulation of events during the first cell cycle are, therefore, critical for proper reprogramming of the donor nucleus and development as a cloned embryo. Activation is normally induced by the sperm and is mediated by a series of intracellular free calcium ([Ca(2+)](i)) oscillations that last for several hours. Although it is not known precisely how the sperm induces activation, current evidence favors the delivery, by the sperm, of a soluble protein factor that causes the production of IP3. IP3 acts to open a Ca(2+) channel in the endoplasmic reticulum and release Ca(2+) into the cytosol. A variety of methods have been used to duplicate or replace the sperm-induced [Ca(2+)](i) increase to cause activation in nuclear transplant embryos. It has been found that treatments that cause a single transient [Ca(2+)](i) activate some oocytes with the level of activation increasing as the oocyte ages. Attempts have been made to extend the period of time over which [Ca(2+)](i) oscillations occur. This has been successful in increasing activation rates of less mature oocytes but the techniques are still cumbersome. An alternative method, that has been very successful, is the combination of a treatment that elevates [Ca(2+)](i) and a treatment that maintains low levels of maturation promoting factor for several hours after the initial [Ca(2+)](i) elevation. The sperm also contributes the centrosome that organizes microtubules during the first cell cycle. One current hypothesis for regulation of sperm centrosomal activity consists of a dephosphorylation of sperm connecting piece proteins following sperm entry into the oocyte and activation of the oocyte. Dephosphorylation of these proteins results in the disassembly of the connecting piece and assembly of a functional centrosome. In nuclear transfer, centrosomal components are contributed by the donor cell. If the cell is fused to the cytoplast before centriole replication then a single aster forms. If the cell is fused after centriole replication then two asters form. In either case and even in parthenogenetic oocytes, which do not have centrioles, the first cell cycle progresses to metaphase. However, progress is slow and some defects are observed in the assembly of chromosomes into a metaphase plate.     

Calcium and other ion dynamics during gamete maturation and fertilization

Ion currents and cytosolic free calcium ([Ca(2+)](i)) elevations are crucial events in triggering the complex machinery involved in both gamete maturation and fertilization. Oocyte maturation is triggered by hormone signaling which causes ion currents and [Ca(2+)](i) increase. Extracellular calcium seems to be required for meiosis progression since: (i) calcium depletion in the maturation medium severely affects oocyte developmental competence; (ii) the activity of plasma membrane L-type Ca(2+) currents decreases during maturation; (iii) the exposure to verapamil, a specific Ca(2+) channel blocker, decreases in vitro maturation efficiency. In spermatozoa, maturation initiates inside the epididymis and ends in the female genital tract. During their journey through the female reproductive tract, sperm undergo a dramatic selection and capacitation achieving fertilization competence. Adhesion to the tubal epithelium extends sperm life through depression of [Ca(2+)](i) until capacitation signals trigger an [Ca(2+)](i) elevation followed by sperm release. At fertilization, egg-sperm interaction evokes well-described transient and almost simultaneous events: i.e., fertilization current, a change in resting potential, and an increase in free [Ca(2+)](i) concentration. These events, termed oocyte activation, are the direct consequence of sperm interaction via either activation of a receptor or entry of a sperm factor. The latter hypothesis has been recently supported by the discovery of PCLzeta, a sperm-specific isozyme triggering a dramatic [Ca(2+)](i) increase via inositol 1,4,5-trisphosphate (IP(3)) production. The course of ion currents and [Ca(2+)](i) transients during maturation and fertilization plays a pivotal role in correct embryo development.     

Failure of male pronucleus formation is the major cause of lack of fertilization and embryo development in pig oocytes subjected to intracytoplasmic sperm injection

The objectives of this study were 1) to compare the efficiency of intracytoplasmic sperm injection (ICSI) with and without additional artificial stimulation using frozen-thawed sperm and in vitro-matured porcine oocytes and 2) to determine the nuclear anomalies of ICSI oocytes that failed to fertilize or develop. In experiments 1 and 2, we evaluated the effects of additional activation treatments, e.g., electrical stimulus, Ca ionophore (A23187), and/or cycloheximide, on fertilization and development of ICSI porcine oocytes. Significantly higher fertilization, cleavage, and blastocyst rates were obtained for oocytes treated with a combination of ICSI and electrical activation (EA) (P < 0.05) than for those treated with ICSI alone. However, different combinations of electrical and chemical activation treatments did not further improve the rates of fertilization, cleavage, and blastocyst development for ICSI embryos. To elucidate the association between sperm head decondensation and oocyte activation and to investigate the cause of embryonic development failure, in experiment 3 we evaluated the nuclear morphology of oocytes 16-20 h after ICSI. Nearly 100% of oocytes showed female pronucleus formation after ICSI regardless of activation treatment. However, failure of male pronucleus formation with intact or swelling sperm heads was observed in some ICSI embryos, suggesting that these embryos underwent cell division with the female pronucleus only. Artificial activation (EA and A23187) had a beneficial effect on embryonic development, sperm decondensation was independent of the resumption of meiosis, and the failure of formation of a male pronucleus was the major cause for fertilization failure in porcine ICSI embryos.     

Inositol 1,4,5-trisphosphate receptors are downregulated in mouse oocytes in response to sperm or adenophostin A but not to increases in intracellular Ca(2+) or egg activation

Fertilization in mammals stimulates a series of Ca(2+) oscillations that continue for 3-4 h. Cell-cycle-dependent changes in the ability to release Ca(2+) are one mechanism that leads to the inhibition of Ca(2+) transients after fertilization. The downregulation of InsP(3)Rs at fertilization may be an additional mechanism for inhibiting Ca(2+) transients. In the present study we examine the mechanism of this InsP(3)R downregulation. We find that neither egg activation nor Ca(2+) transients are necessary or sufficient for the stimulation of InsP(3)R downregulation. First, parthenogenetic activation fails to stimulate downregulation. Second, downregulation persists when fertilization-induced Ca(2+) transients and egg activation are inhibited using BAPTA. Third, downregulation can be induced in immature oocytes that do not undergo egg activation. Other than fertilization, the only stimulus that downregulated InsP(3)Rs was microinjection of the potent InsP(3)R agonist adenophostin A. InsP(3)R downregulation was inhibited by the cysteine protease inhibitor ALLN but MG132 and lactacystin were not effective. Finally, we have injected maturing oocytes with adenophostin A and produced MII eggs depleted of InsP(3)Rs. We show that sperm-induced Ca(2+) signaling is inhibited in such InsP(3)R-depleted eggs. These data show that InsP(3)R binding is sufficient for downregulation and that Ca(2+) signaling at fertilization is mediated via the InsP(3)R.