The appearance of glycoconjugates associated with cortical granule release during mouse fertilization

For the first time we have shown with appropriately labelled lectins that fucosyl- and sialyl-rich glycoconjugates are released into the perivitelline space of the mouse oocyte after activation by the fertilizing spermatozoon or artificial activation by the calcium ionophore A23187 or ethanol. The glycoconjugates show a punctate distribution over the oocyte surface except for the microvilli-free area overlying the second meiotic spindle from which they are absent. Their appearance in the perivitelline space is associated with the release of the cortical granule suggesting that they represent part of the cortical granule exudate. Soon after the glycoconjugates appear, they begin to aggregate. The process continues until the beginning of cytokinesis at first cleavage when a single large aggregate is found within the cleavage furrow. Most of the labelled glycoconjugates disappear by the late 2-cell stage and no evidence was found for their presence during the later preimplantation period. This technique is suitable for monitoring the kinetics of the cortical reaction in mammalian oocytes and investigating the importance of the glycoconjugates in early preimplantation period.     

Characterization of protein kinase C-delta in mouse oocytes throughout meiotic maturation and following egg activation

Changes in protein kinase C (PKC) activity influence the progression of meiosis; however, the specific function of the various PKC isoforms in female gametes is not known. In the current study, the protein expression and subcellular distribution profile of PKC-delta (PKC-delta), a novel isoform of the PKC family, was determined in mouse oocytes undergoing meiotic maturation and following egg activation. The full-length protein was observed as a doublet (76 and 78 kDa) on Western blot analysis. A smaller (47 kDa) carboxyl-terminal fragment, presumably the truncated catalytic domain of PKC-delta, was also strongly expressed. Both the full-length protein and the catalytic fragment became phosphorylated coincident with the resumption of meiosis and remained phosphorylated throughout metaphase II (MII) arrest. Immunofluorescence staining showed PKC-delta distributed diffusely throughout the cytoplasm of oocytes during maturation and associated with the spindle apparatus during the first meiotic division. Discrete foci of the protein also localized with the chromosomes in some mature eggs. Following the completion of meiosis, PKC-delta became dephosphorylated within 2 h of in vitro fertilization or parthenogenetic activation. The protein also accumulated in the nuclei of early embryos and was phosphorylated during M-phase of the initial mitotic cleavage division. By the two-cell stage, expression of the truncated catalytic fragment was minimal. These data demonstrate that the subcellular distribution and posttranslational modification of PKC-delta is cell cycle dependent, suggesting that its activity and/or function likely vary with the progression of meiosis and egg activation.     

Down-regulation of the inositol 1,4,5-trisphosphate receptor in mouse eggs following fertilization or parthenogenetic activation

Fertilization in mammalian eggs is characterized by the presence of intracellular calcium ([Ca(2+)]i) oscillations. In mouse eggs, these oscillations cease after a variable period of time and this is accompanied by a decrease in inositol 1,4,5-trisphosphate receptor (IP3R) responsiveness and down-regulation of the IP3R type 1 (IP3R-1). To investigate the signaling pathway responsible for inducing IP3R-1 down-regulation during fertilization, mouse eggs were exposed to or injected with several Ca(2+)-releasing agonists and the amounts of IP3R-1 immunoreactivity evaluated by Western blotting. Exposure to ethanol or ionomycin, which induce a single [Ca(2+)]i rise, failed to signal down-regulation of IP3R-1. However, [Ca(2+)]i oscillations induced by injection of boar sperm fractions (SF), which presumably stimulate production of IP3, or adenophostin A, an IP3R agonist, both induced down-regulation of IP3R-1 of a magnitude similar to or greater than that observed after fertilization. Exposure to thimerosal, an oxidizing agent that modifies the IP3R without stimulating production of IP3, also initiated down-regulation of IP3R-1, although oscillations initiated by SrCl(2) failed to evoke down-regulation of IP3R-1. The degradation of IP3R-1 in mouse eggs appears to be mediated by the proteasome pathway because it was inhibited by preincubation with lactacystin, a very specific proteasome inhibitor. We therefore suggest that persistent stimulation of the phosphoinositide pathway in mouse eggs by the sperm during fertilization or by injection of SF leads to down-regulation of the IP3R-1.     

Perivitelline space of mammalian oocytes: extracellular matrix of unfertilized oocytes and formation of a cortical granule envelope following fertilization.

Extracellular matrices (ECM) present around unfertilized and fertilized mammalian oocytes were studied ultrastructurally in samples prepared in the presence of ruthenium red to facilitate stabilization of extracellular materials. Unfertilized mouse, hamster, and human oocytes have an ECM comprising granules and filaments in their perivitelline spaces (PVS). This matrix is more abundant in the human than in hamsters and mice. The granule/filament matrix appears identical to the matrix seen between cumulus and corona radiata cells following ruthenium red processing and previously shown to comprise protein and hyaluronic acid. By including ruthenium red during fixation, it is possible to demonstrate the existence of cortical granule exudate in the PVS of fertilized oocytes from hamsters, mice, and humans. Much of the cortical granule exudate is trapped in the PVS and forms a new coat around the fertilized oocyte. This material is particulate when stained with ruthenium red and appears to be uniformly dispersed around the entire oocyte surface. We refer to this new coat as the cortical granule envelope. This envelope is observed in the PVS of all developmental stages up to and including blastocysts in all three species. Following hatching of mouse and hamster blastocysts, the cortical granule envelope is no longer present. Possible functions of this envelope are discussed.     

Characterization of a sperm factor for egg activation at fertilization of the newt Cynops pyrrhogaster

Eggs of the newt, Cynops pyrrhogaster, arrested at the second meiotic metaphase are activated by sperm at fertilization and then complete meiosis to initiate development. We highly purified a sperm factor for egg activation from a sperm extract with several chromatographies. The purified fraction containing only a 45 kDa protein induced egg activation accompanied by an intracellular Ca2+ increase when injected into unfertilized eggs. Although injection of mouse phospholipase C (PLC) zeta-mRNA caused a Ca2+ increase and egg activation, partial amino acid sequences of the 45 kDa protein were homologous to those of Xenopus citrate synthase, but not to PLCs. An anti-porcine citrate synthase antibody recognized the 45 kDa protein both in the purified fraction and in the sperm extract. Treatment with the anti-citrate synthase antibody reduced the egg-activation activity in the sperm extract. Injection of porcine citrate synthase or mRNA of Xenopus citrate synthase induced a Ca2+ increase and caused egg activation. A large amount of the 45 kDa protein was localized in two lines elongated from the neck to the middle piece of sperm. These results indicate that the 45 kDa protein is a major component of the sperm factor for egg activation at newt fertilization.     

High hydrostatic pressure and the dissection of fertilization responses : I. The relationship between cortical granule exocytosis and proton efflux during fertilization of the sea urchin egg

High hydrostatic pressure applied between sperm attachment and the onset of cortical granule exocytosis will inhibit this exocytotic event in sea urchin eggs. Such pressure-treated zygotes, nevertheless, are activated and capable of development. Thus, this technique can be used as a tool to study the relationship between cortical granule breakdown and other fertilization-related responses. We have studied whether the exocytosis of cortical granules is necessary for proton efflux (acid release) to occur. Our results indicate that although Ca²⁺ is released while the eggs are under pressure (a prerequisite for the following events to take place), cortical granule exocytosis and acid release are pressure-sensitive and completely inhibited at pressures above 400 atm (6000 psi) and 275 atm (4000 psi), respectively. However, upon decompression, acid release is initiated which amounts to 65–70% of that seen in the unpressurized controls, suggesting that the efflux mechanism does not require cortical granule exocytosis and must result from some modification of the original plasma membrane of the egg. The remaining 30–35% of the acid release is related to cortical granule exocytosis, since it can be obtained upon induction of the cortical granule fusion 30 min later under atmospheric pressure. The initiation of acid release after decompression indicates that the efflux mechanism is not transiently turned on at fertilization, but undergoing long-term modification; the recovery of the ability to induce cortical granule fusion after fertilization under pressure suggests a refilling of cytoplasmic Ca²⁺ stores within this time course.     

Monoclonal antibody to a common antigen of secretory granule membranes: intracellular localization and recycling of the antigen after secretion.

Monoclonal antibody (MAb) 170-5 was generated to the secretory granule membrane of rat parotid acinar cells. The MAb recognized integral membrane glycoproteins (SG 170 antigen) localized on the luminal side of the secretory granules with N-linked carbohydrates, molecular weights 92, 84, 76, 69, and 65 KD. Immunohistochemical studies indicated that the SG 170 antigen was found in the secretory granules of both exocrine and endocrine cells and in the lysosomes of various cells in the rat. Immunoelectron microscopy with immunogold revealed that the antigen was present on the membrane of the secretory granules, lysosomes, the Golgi vesicles, and condensing vacuoles in pancreatic and parotid acinar cells and in AR42J rat pancreatic tumor cells; the Golgi stacks exhibited no immunoreaction. The common localization of the antigen in the secretory granule membranes indicated that this antigen may play an essential role in regulated secretion. Employing HRP-labeled MAb 170-5, we followed the retrieval of the antigen after exocytosis in AR42J cells. The MAb was internalized specifically with antigen-mediated endocytosis. It was transported to endosomes, subsequently to the trans-Golgi network, and then packaged into secretory granules. However, the Golgi stacks revealed no uptake of the labeled antibody.     

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.     

The localization of PI and PIP kinase activities in the sea urchin egg and their modulation following fertilization

Phosphatidylinositol phosphate (PIP) kinase activity is localized to the cortical region of unfertilized sea urchin eggs, while phosphatidylinositol (PI) kinase activity is found in both cortical and noncortical membranes. Following fertilization PIP kinase activity decreases, while PI kinase activity remains unchanged. The selective loss of PIP kinase activity is related to cortical granule exocytosis since the drop in activity does not occur if exocytosis is prevented by high hydrostatic pressure. When isolated cortices are exposed to elevated concentrations of calcium, both the PI and PIP kinase activities increase, suggesting that activation of these enzymes might occur when calcium levels increase within the fertilized egg prior to cortical granule exocytosis. The polyamine spermine also stimulates the formation of phosphatidylinositol bisphosphate at physiological concentrations.     

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.     

Monoclonal antibodies to rabbit sperm autoantigens. I. Inhibition of in vitro fertilization and localization on the egg

Biochemical and antigenic similarities exist among members of what can be considered a family of low molecular weight rabbit sperm autoantigens. These autoantigens are intrinsic plasma membrane glycoproteins specific to spermatogenic cells and spermatozoa. The amino acid and carbohydrate compositions of rabbit sperm autoantigen-1 (RSA-1) and RSA-2 were compared and monoclonal antibodies (mAb) were raised in mice against rabbit sperm autoantigens. The epitopes recognized by the antibodies were present on RSA-1, 2 and 3. A monoclonal anti-RSA-1, 2 and 3 (designated A.F. 1) was used to localize the antigen on spermatozoa and testis cells and investigate the epitope's tissue specificity. This mAb inhibited in vitro fertilization but did not block the sperm from dispersing the cumulus cells surrounding the egg. The mAb also demonstrated the presence of RSA-1, 2 and 3 on the plasma membrane of the egg after fertilization. It is concluded that the RSA family plays a central role in zona penetration.     

Characterization, fate, and function of hamster cortical granule components

Little is known about the composition and function of mammalian cortical granules. In this study, lectins were used as tools to: (1) estimate the number and molecular weight of glycoconjugates in hamster cortical granules and show what sugars are associated with each glycoconjugate; (2) identify cortical granule components that remain associated with the oolemma, cortical granule envelope, and/or zona pellucida of fertilized oocytes and preimplantation embryos; and (3) examine the role of cortical granule glycoconjugates in preimplantation embryogenesis. Microscopic examination of unfertilized oocytes revealed that the lectins PNA, DBA, WGA, RCA(120), Con A, and LCA bound to hamster cortical granules. Moreover, LCA and Con A labeled the zona pellucida, cortical granule envelope, and plasma membrane of fertilized and artificially activated oocytes and two and eight cell embryos. Lectin blots of unfertilized oocytes had at least 12 glycoconjugates that were recognized by one or more lectins. Nine of these glycoconjugates are found in the cortical granule envelope and/or are associated with the zona pellucida and plasma membrane following fertilization. In vivo functional studies showed that the binding of Con A to one or more mannosylated cortical granule components inhibited blastomere cleavage in two-cell embryos. Our data show that hamster cortical granules contain approximately 12 glycoconjugates of which nine remain associated extracellularly with the fertilized oocyte after the cortical reaction and that one or more play a role in regulating cleavage divisions.     

Characterization of a developmentally regulated mouse embryonic antigen

Summary A mammalian embryonic cell surface glycoprotein (ESGp), whose expression and biochemical structure seem to be developmentally regulated, has been isolated and characterized. The molecule expressed in two cell through morula stage mouse embryos has a molecular weight, by electrophoretic analyses, of 90 kDa. At the blastocyst stage, however, the molecule migrates as a broad, heterogeneous band ranging from 90 to 110 kDa. Evidence obtained from studies of embryonal carcinoma (EC) cells indicates that this band is actually a composite of three distinct molecules (molecular weight 90, 95, and 105 to 110 kDa), each of which is synthesized uniquely by one of the different cell types of the blastocyst: the embryonic ectoderm and visceral and parietal endoderms, respectively. A survey of various mouse tissues and cell lines has revealed that undifferentiated cells express the low molecular weight form (90 kDa) characteristic of embryonic ectoderm, whereas differentiated cells and adult tissues express the high molecular weight form (110 kDa) characteristic of parietal endoderm. Only the EC visceral endoderm cell analogues have been shown to express the intermediate molecule (95 kDa). In embryos, the antigen is uniformly distributed over the cell surface during early cleavage stages (two to eight cell); just before compaction, however, it seems to redistribute and becomes polarized at the outside exposed edges of blastomeres. In cultured EC cells, ESGp is found only in areas of cell-to-cell contact; free-standing surfaces of cells are negative for expression. It is possible, therefore, that ESGp may be involved in the intercellular adhesion of both EC cells and compacting embryos. This work was supported by grant R01 HD23402 from the National Institutes of Health, Bethesda, MD.     

Premature cortical granule loss does not prevent sperm penetration of mouse eggs.

The role of cortical granules in the mouse egg's plasmalemma block to polyspermy was investigated by examining the effect of premature granule loss on egg fertility. Granule loss, quantitated by transmission electron microscopic examination, was induced in zona-free eggs by exposure to the divalent cation ionophore, A23187, or by mechanical removal of zonae. Egg exposure to ionophore led to the loss of approximately 50% of the egg's complement of granules in the absence of nuclear activation (parthenogenesis), while complete cortical granule loss accompanied the parthenogenetic activation seen in a limited population of mechanically stimulated eggs. Aged eggs underwent nuclear activation without a dramatic reduction in granule complements. The fertility of treated zona-free eggs was identical to that of controls, as measured by the percentage of eggs penetrated and by the mean number of sperm recovered per egg. Moreover, both ionophore-treated and aged eggs subsequently underwent a normal sperm-induced block response. Exposure of zona-intact eggs to ionophore was also without effect on egg fertility. These results indicate that cortical granules are not involved in the plasmalemma block to polyspermy in the mouse.     

The fertilization potential is not necessary for the block to polyspermy or the activation of development in the medaka egg

The egg of the medaka, Oryzias latipes, was impaled with two microelectrodes so that its membrane potential could be clamped at a constant level during fertilization. Fertilization occurred at all membrane potentials between ?80 and +48 mV. Therefore, there is apparently no electrical block to polyspermy in this egg. In 16 of these eggs the membrane potential was also clamped at a constant level during the 6- to 14-min period after fertilization and the eggs' subsequent development was studied. All of these eggs developed normally up to at least the beating heart stage. Therefore, the fertilization potential is not necessary for further development. When the egg is clamped at levels more negative than ?25 mV, the injected clamping current is usually biphasic just after fertilization with an inward current phase preceding a longer outward phase. The inward current phase corresponds well in time with the membrane depolarization normally triggered by fertilization. The outward current phase was observed in all eggs studied and the more positive the holding potential, the longer was the outward current duration.     

The sperm entry site during fertilization of the zebrafish egg: localization of actin.

The sperm entry site (SES) of zebrafish (Brachydanio rerio) eggs was studied before and during fertilization by fluorescence, scanning, and transmission electron microscopy. Rhodamine phalloidin (RhPh), used to detect polymerized filamentous actin, was localized to microvilli of the SES and to cytoplasm subjacent to the plasma membrane in the unfertilized egg. The distribution of RhPh staining at the SES correlated with the ultrastructural localization of a submembranous electrondense layer of cortical cytoplasm approximately 500 nm thick and containing 5- to 6-nm filaments. Actin, therefore, was organized at the SES as a tightly knit meshwork of filaments prior to fertilization. Contact between the fertilizing sperm and the filamentous actin network was observed by 15-20 sec postinsemination or just before the onset of fertilization cone formation. Growing fertilization cones of either artificially activated or inseminated eggs exhibited intense RhPh staining and substantial increase in thickness of the actin meshwork. Collectively, TEM and RhPh fluorescence images of inseminated eggs demonstrated that the submembranous actin became rearranged in fertilization cones to form a thickened meshwork around the sperm nucleus during incorporation. The results reported here suggest that activation of the egg triggers a dramatic polymerization of actin beneath the plasma membrane of the fertilization cone. Furthermore, the actin involved in sperm incorporation is sensitive to the action of cytochalasins.     

First cleavage of the mouse embryo responds to change in egg shape at fertilization

Although mouse development is regulative, the cleavage pattern of the embryo is not random. The first cleavage tends to relate to the site of the previous meiosis. Sperm entry might provide a second cue, but evidence for and against this is indirect and has been debated. To resolve whether sperm entry position relates to the first cleavage, we have followed development from fertilization by time-lapse imaging. This directly showed cytokinesis passes close to the site of the previous meiosis and to both the sperm entry site and trajectory of the male pronucleus in a significant majority of eggs. We detected asymmetric distribution of Par6 protein in relation to the site of meiosis, but not sperm entry. Unexpectedly, we found the egg becomes flattened upon fertilization in an actin-mediated process. The sperm entry position tends to lie at one end of the short axis along which cleavage will pass. When we manipulated eggs to change their shape, this repositioned the cleavage plane such that eggs divided along their experimentally imposed short axis. Such manipulated eggs were able to develop to term, emphasizing the regulative nature of their development.     

The localization of LAP2 beta during pronuclear formation in bovine oocytes after fertilization or activation

We have shown that the assembly of lamin-associated polypeptide (LAP) 2beta was detected surrounding the chromatin mass around the time of extrusion of the second polar body (PB) in some fertilized oocytes, but not in most activated oocytes, by using A23187 and cycloheximide (CaA + CH). Here, we immunohistologically analysed the correlation between LAP2beta assembly and chromatin condensation in fertilized and activated oocytes during the second meiosis. In bovine cumulus cells, the onset of LAP2beta assembly was observed around anaphase chromosomes with strongly phosphorylated histone H3. No LAP2beta assembled around the chromosomes in the first and second polar bodies and the alternative oocyte chromatin (oCh) if histone H3 was phosphorylated. Only histone H3 of oCh was completely dephosphorylated during the telophase II/G1 transition (Tel II/G1), and then LAP2beta assembled around only the oCh without phosphorylated histone H3. In the oocytes activated by CaA + CH, LAP2beta did not assemble around the condensed oCh during the Tel II/G1 transition, although their histone H3 dephosphorylation occurred rather rapidly compared with that of the fertilized oocytes. The patterns of histone H3 dephosphorylation and LAP2beta assembly in oocytes activated by CaA alone showed greater similarity to those in fertilized oocytes than to those in oocytes activated by CaA + CH. These results show that LAP2beta assembles around only oCh after complete dephosphorylation of histone H3 after fertilization and activation using CaA alone, and that the timing of histone H3 dephosphorylation and LAP2beta assembly in these oocytes is different from that of somatic cells. The results also indicate that CH treatment inhibits LAP2beta assembly around oCh but not histone H3 dephosphorylation.     

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.