Involvement of Calcium Signaling and the Actin Cytoskeleton in the Membrane Block to Polyspermy in Mouse Eggs1

RanGTP and the actin cytoskeleton keep paternal and maternal chromosomes apart during fertilization

Zygotes require two accurate sets of parental chromosomes, one each from the mother and the father, to undergo normal embryogenesis. However, upon egg-sperm fusion in vertebrates, the zygote has three sets of chromosomes, one from the sperm and two from the egg. The zygote therefore eliminates one set of maternal chromosomes (but not the paternal chromosomes) into the polar body through meiosis, but how the paternal chromosomes are protected from maternal meiosis has been unclear. Here we report that RanGTP and F-actin dynamics prevent egg-sperm fusion in proximity to maternal chromosomes. RanGTP prevents the localization of Juno and CD9, egg membrane proteins that mediate sperm fusion, at the cell surface in proximity to maternal chromosomes. Following egg-sperm fusion, F-actin keeps paternal chromosomes away from maternal chromosomes. Disruption of these mechanisms causes the elimination of paternal chromosomes during maternal meiosis. This study reveals a novel critical mechanism that prevents aneuploidy in zygotes.

Cellular and molecular aspects of oocyte maturation and fertilization: a perspective from the actin cytoskeleton

ABSTRACT

Much of the scientific knowledge on oocyte maturation, fertilization, and embryonic development has come from the experiments using gametes of marine organisms that reproduce by external fertilization. In particular, echinoderm eggs have enabled the study of structural and biochemical changes related to meiotic maturation and fertilization owing to the abundant availability of large and transparent oocytes and eggs. Thus, in vitro studies of oocyte maturation and sperm-induced egg activation in starfish are carried out under experimental conditions that resemble those occurring in nature. During the maturation process, immature oocytes of starfish are released from the prophase of the first meiotic division, and acquire the competence to be fertilized through a highly programmed sequence of morphological and physiological changes at the oocyte surface. In addition, the changes in the cortical and nuclear regions are essential for normal and monospermic fertilization. This review summarizes the current state of research on the cortical actin cytoskeleton in mediating structural and physiological changes during oocyte maturation and sperm and egg activation in starfish and sea urchin. The common denominator in these studies with echinoderms is that exquisite rearrangements of the egg cortical actin filaments play pivotal roles in gamete interactions, Ca2+ signaling, exocytosis of cortical granules, and control of monospermic fertilization. In this review, we also compare findings from studies using invertebrate eggs with what is known about the contributions made by the actin cytoskeleton in mammalian eggs. Since the cortical actin cytoskeleton affects microvillar morphology, movement, and positioning of organelles and vesicles, and the topography of the egg surface, these changes have impacts on the fertilization process, as has been suggested by recent morphological studies on starfish oocytes and eggs using scanning electron microscopy. Drawing the parallelism between vitelline layer of echinoderm eggs and the zona pellucida of mammalian eggs, we also discuss the importance of the egg surface in mediating monospermic fertilization.

GRAPHICAL ABSTRACT

Preventing polyspermy in mammalian eggs-Contributions of the membrane block and other mechanisms

The egg's blocks to polyspermy (fertilization of an egg by more than one sperm) were originally identified in marine and aquatic species with external fertilization, but polyspermy matters in mammalian reproduction too. Embryonic triploidy is a noteworthy event associated with pregnancy complications and loss. Polyspermy is a major cause of triploidy with up to 80% of triploid conceptuses being the result of dispermic fertilization. The mammalian female reproductive tract regulates the number of sperm that reach the site of fertilization, but mammals also utilize egg-based blocks to polyspermy. The egg-based blocks occur on the mammalian egg coat (the zona pellucida) and the egg plasma membrane, with apparent variation between different mammalian species regarding the extent to which one or both are used. The zona pellucida block to polyspermy has some similarities to the slow block in water-dwelling species, but the mammalian membrane block to polyspermy differs substantially from the fast electrical block that has been characterized in marine and aquatic species. This review discusses what is known about the incidence of polyspermy in mammals and about the mammalian membrane block to polyspermy, as well as notes some lesser-characterized potential mechanisms contributing to polyspermy prevention in mammals.

Sperm-borne phospholipase C zeta-1 ensures monospermic fertilization in mice

Sperm entry in mammalian oocytes triggers intracellular Ca²⁺ oscillations that initiate resumption of the meiotic cell cycle and subsequent activations. Here, we show that phospholipase C zeta 1 (PLCζ1) is the long-sought sperm-borne oocyte activation factor (SOAF). Plcz1 gene knockout (KO) mouse spermatozoa fail to induce Ca²⁺ changes in intracytoplasmic sperm injection (ICSI). In contrast to ICSI, Plcz1 KO spermatozoa induced atypical patterns of Ca²⁺ changes in normal fertilizations, and most of the fertilized oocytes ceased development at the 1–2-cell stage because of oocyte activation failure or polyspermy. We further discovered that both zona pellucida block to polyspermy (ZPBP) and plasma membrane block to polyspermy (PMBP) were delayed in oocytes fertilized with Plcz1 KO spermatozoa. With the observation that polyspermy is rare in astacin-like metalloendopeptidase (Astl) KO female oocytes that lack ZPBP, we conclude that PMPB plays more critical role than ZPBP in vivo. Finally, we obtained healthy pups from male mice carrying human infertile PLCZ1 mutation by single sperm ICSI supplemented with Plcz1 mRNA injection. These results suggest that mammalian spermatozoa have a primitive oocyte activation mechanism and that PLCζ1 is a SOAF that ensures oocyte activation steps for monospermic fertilization in mammals.

Viable offspring after imaging of Ca2+ oscillations and visualization of the cortical reaction in mouse eggs

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During mammalian fertilization, egg Ca 2+ oscillations are known to play pivotal roles in triggering downstream events such as resumption of the cell cycle and the establishment of blocks to polyspermy. However, viable offspring have not been obtained after monitoring Ca 2+ oscillations, and their spatiotemporal links to subsequent events are still to be examined. Therefore, the development of imaging methods to avoid phototoxic damage while labeling these events is required. Here, we examined the usefulness of genetically encoded Ca 2+ indicators for optical imaging (GECOs), in combination with spinning-disk confocal imaging. The Ca 2+ imaging of fertilized mouse eggs with GEM-, G-, or R-GECO recorded successful oscillations (8.19 ± 0.31, 7.56 ± 0.23, or 7.53 ± 0.27 spikes in the first 2 h, respectively), similar to those obtained with chemical indicators. Then, in vitro viability tests revealed that imaging with G- or R-GECO did not interfere with the rate of development to the blastocyst stage (61.8 or 70.0%, respectively, vs 75.0% in control). Furthermore, two-cell transfer to recipient female mice after imaging with G- or R-GECO resulted in a similar birthrate (53.3 or 52.0%, respectively) to that of controls (48.7%). Next, we assessed the quality of the cortical reaction (CR) in artificially activated or fertilized eggs using fluorescently labeled Lens culinaris agglutinin fluorescein isothiocyanate. Multicolor imaging demonstrated that the first few Ca 2+ spikes are sufficient for the completion of the CR and subsequent hardening of the zona pellucida in mouse eggs. These methods provide a framework for studying Ca 2+ dynamics in mammalian fertilization.

Sperm Meets Egg: The Genetics of Mammalian Fertilization

Fertilization is the culminating event of sexual reproduction, which involves the union of the sperm and egg to form a single, genetically distinct organism. Despite the fundamental role of fertilization, the basic mechanisms involved have remained poorly understood. However, these mechanisms must involve an ordered schedule of cellular recognition events between the sperm and egg to ensure successful fusion. In this article, we review recent progress in our molecular understanding of mammalian fertilization, highlighting the areas in which genetic approaches have been particularly informative and focusing especially on the roles of secreted and cell surface proteins, expressed in a sex-specific manner, that mediate sperm-egg interactions. We discuss how the sperm interacts with the female reproductive tract, zona pellucida, and the oolemma. Finally, we review recent progress made in elucidating the mechanisms that reduce polyspermy and ensure that eggs normally fuse with only a single sperm.

Cortical mechanics and myosin-II abnormalities associated with post-ovulatory aging: implications for functional defects in aged eggs

STUDY HYPOTHESIS

Cellular aging of the egg following ovulation, also known as post-ovulatory aging, is associated with aberrant cortical mechanics and actomyosin cytoskeleton functions.

STUDY FINDING

Post-ovulatory aging is associated with dysfunction of non-muscle myosin-II, and pharmacologically induced myosin-II dysfunction produces some of the same deficiencies observed in aged eggs.

WHAT IS KNOWN ALREADY

Reproductive success is reduced with delayed fertilization and when copulation or insemination occurs at increased times after ovulation. Post-ovulatory aged eggs have several abnormalities in the plasma membrane and cortex, including reduced egg membrane receptivity to sperm, aberrant sperm-induced cortical remodeling and formation of fertilization cones at the site of sperm entry, and reduced ability to establish a membrane block to prevent polyspermic fertilization.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Ovulated mouse eggs were collected at 21-22 h post-human chorionic gonadotrophin (hCG) (aged eggs) or at 13-14 h post-hCG (young eggs), or young eggs were treated with the myosin light chain kinase (MLCK) inhibitor ML-7, to test the hypothesis that disruption of myosin-II function could mimic some of the effects of post-ovulatory aging. Eggs were subjected to various analyses. Cytoskeletal proteins in eggs and parthenogenesis were assessed using fluorescence microscopy, with further analysis of cytoskeletal proteins in immunoblotting experiments. Cortical tension was measured through micropipette aspiration assays. Egg membrane receptivity to sperm was assessed in in vitro fertilization (IVF) assays. Membrane topography was examined by low-vacuum scanning electron microscopy (SEM).

MAIN RESULTS AND THE ROLE OF CHANCE

Aged eggs have decreased levels and abnormal localizations of phosphorylated myosin-II regulatory light chain (pMRLC; P = 0.0062). Cortical tension, which is mediated in part by myosin-II, is reduced in aged mouse eggs when compared with young eggs, by ∼40% in the cortical region where the metaphase II spindle is sequestered and by ∼50% in the domain to which sperm bind and fuse (P < 0.0001). Aging-associated parthenogenesis is partly rescued by treating eggs with a zinc ionophore (P = 0.003), as is parthenogenesis induced by inhibition of mitogen-activated kinase (MAPK) 3/1 [also known as extracellular signal-regulated kinase (ERK)1/2] or MLCK. Inhibition of MLCK with ML-7 also results in effects that mimic those of post-ovulatory aging: fertilized ML-7-treated eggs show both impaired fertilization and increased extents of polyspermy, and ML-7-treated young eggs have several membrane abnormalities that are shared by post-ovulatory aged eggs.

LIMITATIONS, REASONS FOR CAUTION

These studies were done with mouse oocytes, and it remains to be fully determined how these findings from mouse oocytes would compare with other species. For studies using methods not amenable to analysis of large sample sizes and data are limited to what images one can capture (e.g. SEM), data should be interpreted conservatively.

WIDER IMPLICATIONS OF THE FINDINGS

These data provide insights into causes of reproductive failures at later post-copulatory times.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTERESTS

This project was supported by R01 HD037696 and R01 HD045671 from the NIH to J.P.E. Cortical tension studies were supported by R01 GM66817 to D.N.R. The authors declare there are no financial conflicts of interest.

Size-specific follicle selection improves mouse oocyte reproductive outcomes

Encapsulated in vitro follicle growth (eIVFG) has great potential to provide an additional fertility preservation option for young women and girls with cancer or other reproductive health threatening diseases. Currently, follicles are cultured for a defined period of time and analyzed as a cohort. However, follicle growth is not synchronous, and culturing follicles for insufficient or excessive times can result in compromised gamete quality. Our objective is to determine whether the selection of follicles based on size, rather than absolute culture time, better predict follicle maturity and oocyte quality. Multilayer secondary mouse follicles were isolated and encapsulated in 0.25% alginate. Follicles were cultured individually either for defined time periods or up to specific follicle diameter ranges, at which point several reproductive endpoints were analyzed. The metaphase II (MII) percentage after oocyte maturation on day 6 was the highest (85%) when follicles were cultured for specific days. However, if follicles were cultured to a terminal diameter of 300-350 μm irrespective of absolute time in culture, 93% of the oocytes reached MII. More than 90% of MII oocytes matured from follicles with diameters of 300-350 μm showed normal spindle morphology and chromosome alignment, 85% of oocytes showed two pronuclei after IVF, 81% developed into the two-cell embryo stage and 38% developed to the blastocyst stage, all significantly higher than the percentages in the other follicle size groups. Our study demonstrates that size-specific follicle selection can be used as a non-invasive marker to identify high-quality oocytes and improve reproductive outcomes during eIVFG.

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.

Fertilization induces a transient exposure of phosphatidylserine in mouse eggs

Phosphatidylserine (PS) is normally localized to the inner leaflet of the plasma membrane and the requirement of PS translocation to the outer leaflet in cellular processes other than apoptosis has been demonstrated recently. In this work we investigated the occurrence of PS mobilization in mouse eggs, which express flippase Atp8a1 and scramblases Plscr1 and 3, as determined by RT-PCR; these enzyme are responsible for PS distribution in cell membranes. We find a dramatic increase in binding of flouresceinated-Annexin-V, which specifically binds to PS, following fertilization or parthenogenetic activation induced by SrCl₂ treatment. This increase was not observed when eggs were first treated with BAPTA-AM, indicating that an increase in intracellular Ca²⁺ concentration was required for PS exposure. Fluorescence was observed over the entire egg surface with the exception of the regions overlying the meiotic spindle and sperm entry site. PS exposure was also observed in activated eggs obtained from CaMKIIγ null females, which are unable to exit metaphase II arrest despite displaying Ca²⁺ spikes. In contrast, PS exposure was not observed in TPEN-activated eggs, which exit metaphase II arrest in the absence of Ca²⁺ release. PS exposure was also observed when eggs were activated with ethanol but not with a Ca²⁺ ionophore, suggesting that the Ca²⁺ source and concentration are relevant for PS exposure. Last, treatment with cytochalasin D, which disrupts microfilaments, or jasplakinolide, which stabilizes microfilaments, prior to egg activation showed that PS externalization is an actin-dependent process. Thus, the Ca²⁺ rise during egg activation results in a transient exposure of PS in fertilized eggs that is not associated with apoptosis.

Evaluation of zona pellucida function for sperm penetration during in vitro fertilization in pigs

In porcine oocytes, the function of the zona pellucida (ZP) with regard to sperm penetration or prevention of polyspermy is not well understood. In the present study, we investigated the effects of the ZP on sperm penetration during in vitro fertilization (IVF). We collected in vitro-matured oocytes with a first polar body (ZP+ oocytes). Some of them were freed from the ZP (ZP− oocytes) by two treatments (pronase and mechanical pipetting), and the effects of these treatments on sperm penetration parameters (sperm penetration rate and numbers of penetrated sperm per oocyte) were evaluated. There was no evident difference in the parameters between the two groups. Secondly, we compared the sperm penetration parameters of ZP+ and ZP− oocytes using frozen-thawed epididymal spermatozoa from four boars. Sperm penetration into ZP+ oocytes was found to be accelerated relative to ZP− oocytes. Thirdly, we evaluated the sperm penetration of ZP+ and ZP− oocytes at 1−10 h after IVF (3 h gamete co-incubation). The proportions of oocytes penetrated by sperm increased significantly with time in both groups; however, the number of penetrated sperm per oocyte did not increase in ZP− oocytes. Finally, we performed IVF using ZP− oocytes divided into control (3 h) and prolonged gamete co-incubation (5 h) groups. Greater numbers of sperm penetrated in the 5 h group than in the control group. These results suggest that the ZP and oolemma are not competent factors for prevention of polyspermy in our present porcine IVF system. However, it appears that ZP removal is one of the possibilities for reducing polyspermic penetration in vitro in pigs.

Degradation of actin nucleators affects cortical polarity of aged mouse oocytes

OBJECTIVE

To investigate the molecular mechanism of mouse oocyte polarity loss during aging.

DESIGN

Experimental study.

SETTING

Academic basic research laboratory.

ANIMAL(S)

Mice.

INTERVENTION(S)

Oocytes were collected 16 hours after injection of hCG and cultured in M16 medium for an additional 14 hours with or without caffeine.

MAIN OUTCOME MEASURE(S)

Expression and localizations of actin nucleators actin-related protein 2/3 complex, JMY, and WAVE2 were examined by immunofluorescence staining, and their messenger RNA levels were examined by real-time reverse transcription-polymerase chain reaction.

RESULT(S)

The protein and messenger RNA levels of actin-related protein 2/3 complex, JMY, and WAVE2 were decreased in aged oocytes, but the levels were normal in caffeine-treated aged oocytes.

CONCLUSION(S)

Our data indicated that the loss of oocyte polarity may be due to the degradation of actin nucleators in aged oocytes.

Effects of ubiquitin C-terminal hydrolase L1 deficiency on mouse ova

Maternal proteins are rapidly degraded by the ubiquitin-proteasome system during oocyte maturation in mice. Ubiquitin C-terminal hydrolase L1 (UCHL1) is highly and specifically expressed in mouse ova and is involved in the polyspermy block. However, the role of UCHL1 in the underlying mechanism of polyspermy block is poorly understood. To address this issue, we performed a comprehensive proteomic analysis to identify maternal proteins that were relevant to the role of UCHL1 in mouse ova using UCHL1-deficient gad. Furthermore, we assessed morphological features in gad mouse ova using transmission electron microscopy. NACHT, LRR, and PYD domain-containing (NALP) family proteins and endoplasmic reticulum (ER) chaperones were identified by proteomic analysis. We also found that the 'maternal antigen that embryos require' (NLRP5 (MATER)) protein level increased significantly in gad mouse ova compared with that in wild-type mice. In an ultrastructural study, gad mouse ova contained less ER in the cortex than in wild-type mice. These results provide new insights into the role of UCHL1 in the mechanism of polyspermy block in mouse ova.

Ca2+ signaling during mammalian fertilization: requirements, players, and adaptations

Changes in the intracellular concentration of calcium ([Ca(2+)](i)) represent a vital signaling mechanism enabling communication among cells and between cells and the environment. The initiation of embryo development depends on a [Ca(2+)](i) increase(s) in the egg, which is generally induced during fertilization. The [Ca(2+)](i) increase signals egg activation, which is the first stage in embryo development, and that consist of biochemical and structural changes that transform eggs into zygotes. The spatiotemporal patterns of [Ca(2+)](i) at fertilization show variability, most likely reflecting adaptations to fertilizing conditions and to the duration of embryonic cell cycles. In mammals, the focus of this review, the fertilization [Ca(2+)](i) signal displays unique properties in that it is initiated after gamete fusion by release of a sperm-derived factor and by periodic and extended [Ca(2+)](i) responses. Here, we will discuss the events of egg activation regulated by increases in [Ca(2+)](i), the possible downstream targets that effect these egg activation events, and the property and identity of molecules both in sperm and eggs that underpin the initiation and persistence of the [Ca(2+)](i) responses in these species.

Cortical mechanics and meiosis II completion in mammalian oocytes are mediated by myosin-II and Ezrin-Radixin-Moesin (ERM) proteins

Cell division is inherently mechanical, with cell mechanics being a critical determinant governing the cell shape changes that accompany progression through the cell cycle. The mechanical properties of symmetrically dividing mitotic cells have been well characterized, whereas the contribution of cellular mechanics to the strikingly asymmetric divisions of female meiosis is very poorly understood. Progression of the mammalian oocyte through meiosis involves remodeling of the cortex and proper orientation of the meiotic spindle, and thus we hypothesized that cortical tension and stiffness would change through meiotic maturation and fertilization to facilitate and/or direct cellular remodeling. This work shows that tension in mouse oocytes drops about sixfold during meiotic maturation from prophase I to metaphase II and then increases ∼1.6-fold upon fertilization. The metaphase II egg is polarized, with tension differing ∼2.5-fold between the cortex over the meiotic spindle and the opposite cortex, suggesting that meiotic maturation is accompanied by assembly of a cortical domain with stiffer mechanics as part of the process to achieve asymmetric cytokinesis. We further demonstrate that actin, myosin-II, and the ERM (Ezrin/Radixin/Moesin) family of proteins are enriched in complementary cortical domains and mediate cellular mechanics in mammalian eggs. Manipulation of actin, myosin-II, and ERM function alters tension levels and also is associated with dramatic spindle abnormalities with completion of meiosis II after fertilization. Thus, myosin-II and ERM proteins modulate mechanical properties in oocytes, contributing to cell polarity and to completion of meiosis.

Impact of marine drugs on cytoskeleton-mediated reproductive events

Marine organisms represent an important source of novel bioactive compounds, often showing unique modes of action. Such drugs may be useful tools to study complex processes such as reproduction; which is characterized by many crucial steps that start at gamete maturation and activation and virtually end at the first developmental stages. During these processes cytoskeletal elements such as microfilaments and microtubules play a key-role. In this review we describe: (i) the involvement of such structures in both cellular and in vitro processes; (ii) the toxins that target the cytoskeletal elements and dynamics; (iii) the main steps of reproduction and the marine drugs that interfere with these cytoskeleton-mediated processes. We show that marine drugs, acting on microfilaments and microtubules, exert a wide range of impacts on reproductive events including sperm maturation and motility, oocyte maturation, fertilization, and early embryo development.

Decrease in CD9 content and reorganization of microvilli may contribute to the oolemma block to sperm penetration during fertilization of mouse oocyte

Tetraspanin CD9 is the only protein of the oocyte membrane (oolemma) known to be required for the fusion of gametes during fertilization in the mouse. Using electron microscopy and immunostaining we examined the differences in localization of CD9 between ovulated oocytes, zygotes and parthenogenetically activated eggs (parthenogenotes). Changes in ultrastructure of oolemma, which take place in oocytes after fertilization or artificial activation, were also assessed. We demonstrated that after fertilization the level of CD9 present on microvilli of zygote was two times lower than its level on the oolemma of the oocyte. In addition, we showed that the distribution of microvilli is less uniform in the zygotes than in the unfertilized oocytes. We propose that the changes of microvilli distribution and their CD9 content are responsible for the development of the oocyte membrane block to sperm penetration.

Inositol 1,4,5-trisphosphate receptor 1 degradation in mouse eggs and impact on [Ca2+]i oscillations

The initiation of normal embryo development depends on the completion of all events of egg activation. In all species to date, egg activation requires an increase(s) in the intracellular concentration of calcium ([Ca(2+)](i)), which is almost entirely mediated by inositol 1,4,5-trisphosphate receptor 1 (IP(3)R1). In mammalian eggs, fertilization-induced [Ca(2+)](i) responses exhibit a periodic pattern that are called [Ca(2+)](i) oscillations. These [Ca(2+)](i) oscillations are robust at the beginning of fertilization, which occurs at the second metaphase of meiosis, but wane as zygotes approach the pronuclear stage, time after which in the mouse oscillations cease altogether. Underlying this change in frequency are cellular and biochemical changes associated with egg activation, including degradation of IP(3)R1, progression through the cell cycle, and reorganization of intracellular organelles. In this study, we investigated the system requirements for IP(3)R1 degradation and examined the impact of the IP(3)R1 levels on the pattern of [Ca(2+)](i) oscillations. Using microinjection of IP(3) and of its analogs and conditions that prevent the development of [Ca(2+)](i) oscillations, we show that IP(3)R1 degradation requires uniform and persistently elevated levels of IP(3). We also established that progressive degradation of the IP(3)R1 results in [Ca(2+)](i) oscillations with diminished periodicity while a near complete depletion of IP(3)R1s precludes the initiation of [Ca(2+)](i) oscillations. These results provide insights into the mechanism involved in the generation of [Ca(2+)](i) oscillations in mouse eggs.

Beta1 integrin is an adhesion protein for sperm binding to eggs

We investigated the role of beta(1) integrin in mammalian fertilization and the mode of inhibition of fertilinbeta-derived polymers. We determined that polymers displaying the Glu-Cys-Asp peptide from the fertilinbeta disintegrin domain mediate inhibition of mammalian fertilization through a beta(1) integrin receptor on the egg surface. Inhibition of fertilization is a consequence of competition with sperm binding to the cell surface, not activation of an egg-signaling pathway. The presence of the beta(1) integrin on the egg surface increases the rate of sperm attachment but does not alter the total number of sperm that can attach or fuse to the egg. We conclude that the presence of beta(1) integrin enhances the initial adhesion of sperm to the egg plasma membrane and that subsequent attachment and fusion are mediated by additional egg and sperm proteins present in the beta(1) integrin complex. Therefore, the mechanisms by which sperm fertilize wild-type and beta(1) knockout eggs are different.

Sperm–egg adhesion and fusion in mammals

Fertilisation is an orchestrated, stepwise process during which the participating male and female gametes undergo irreversible changes, losing some of their structural components while contributing others to the resultant zygote. Following sperm penetration through the egg coat, the sperm plasma membrane fuses with its oocyte counterpart, the oolemma. At least two plasma membrane proteins essential for sperm–oolemma fusion – IZUMO and CD9 on the male and female gametes, respectively – have been identified recently by classical cell biology approaches and confirmed by gene deletion. Oolemma-associated tetraspanin CD81, closely related to CD9, also appears to have an essential role in fusion. Additional proteins that may have nonessential yet still facilitating roles in sperm–oolemma adhesion and fusion include oolemma-anchored integrins and oocyte-expressed retroviral envelope proteins, sperm disintegrins, and sperm-borne proteins of epididymal origin such as CRISP1 and CRISP2. This review discusses these components of the gamete fusion mechanism within the framework of gamete structure, membrane biology, cell signalling and cytoskeletal dynamics, and revisits the topic of antipolyspermy defence at the oolemma level. Harnessing the mechanisms of sperm–egg fusion is of importance to animal biotechnology and to human assisted fertilisation, wherein male patients with reduced sperm fusibility have been identified.

Reduction of mouse egg surface integrin alpha9 subunit (ITGA9) reduces the egg's ability to support sperm-egg binding and fusion

The involvement of egg integrins in mammalian sperm-egg interactions has been controversial, with data from integrin inhibitor studies contrasting with evidence from knockouts showing that specific integrin subunits are not essential for fertility. An alpha(4)/alpha(9) (ITGA4/ITGA9) integrin subfamily member has been implicated in fertilization but not extensively examined, so we tested the following three hypotheses: 1) an ITGA4/ITGA9 integrin participates in sperm-egg interactions, 2) short-term acute knockdown by RNA interference of integrin subunits would result in a fertilization phenotype differing from that of chronic depletion via knockout, and 3) detection of a fertilization phenotype is sensitive to in vitro fertilization (IVF) assay conditions. We show that mouse and human eggs express the alpha(9) integrin subunit (ITGA9). RNA interference-mediated knockdown resulted in reduced levels of Itga9 mRNA and surface protein in mouse eggs. RNA interference attempts to knockdown ITGA9's likely beta partner, beta(1) (ITGB1), resulted in reduced Itgb1 mRNA but no reduction in ITGB1 surface protein. Therefore, studies using a function-blocking anti-ITGB1 antibody tested the hypothesis that ITGB1 participates in gamete interactions. Analyses of sperm-egg interactions with Itga9-knockdown eggs and anti-ITGB1 antibody-treated eggs in IVF assays using specific sperm:egg ratios revealed the following: 1) a reduction, but not complete loss, of sperm-egg binding and fusion was observed and 2) the reduction of sperm-egg binding and fusion was not detected in inseminations with high sperm:egg ratios. These data demonstrate that ITGA9 and ITGB1 participate in sperm-egg interactions but clearly are not the only molecules involved. This also shows that careful design of IVF parameters allows detection of deficiencies in gamete interactions.

Chloride Is essential for capacitation and for the capacitation-associated increase in tyrosine phosphorylation

After epididymal maturation, sperm capacitation, which encompasses a complex series of molecular events, endows the sperm with the ability to fertilize an egg. This process can be mimicked in vitro in defined media, the composition of which is based on the electrolyte concentration of the oviductal fluid. It is well established that capacitation requires Na(+), HCO(3)(-), Ca(2+), and a cholesterol acceptor; however, little is known about the function of Cl(-) during this important process. To determine whether Cl(-), in addition to maintaining osmolarity, actively participates in signaling pathways that regulate capacitation, Cl(-) was replaced by either methanesulfonate or gluconate two nonpermeable anions. The absence of Cl(-) did not affect sperm viability, but capacitation-associated processes such as the increase in tyrosine phosphorylation, the increase in cAMP levels, hyperactivation, the zona pellucidae-induced acrosome reaction, and most importantly, fertilization were abolished or significantly reduced. Interestingly, the addition of cyclic AMP agonists to sperm incubated in Cl(-)-free medium rescued the increase in tyrosine phosphorylation and hyperactivation suggesting that Cl(-) acts upstream of the cAMP/protein kinase A signaling pathway. To investigate Cl(-) transport, sperm incubated in complete capacitation medium were exposed to a battery of anion transport inhibitors. Among them, bumetanide and furosemide, two blockers of Na(+)/K(+)/Cl(-) cotransporters (NKCC), inhibited all capacitation-associated events, suggesting that these transporters may mediate Cl(-) movements in sperm. Consistent with these results, Western blots using anti-NKCC1 antibodies showed the presence of this cotransporter in mature sperm.

Alteration of the cortical actin cytoskeleton deregulates Ca2+ signaling, monospermic fertilization, and sperm entry

BACKGROUND

When preparing for fertilization, oocytes undergo meiotic maturation during which structural changes occur in the endoplasmic reticulum (ER) that lead to a more efficient calcium response. During meiotic maturation and subsequent fertilization, the actin cytoskeleton also undergoes dramatic restructuring. We have recently observed that rearrangements of the actin cytoskeleton induced by actin-depolymerizing agents, or by actin-binding proteins, strongly modulate intracellular calcium (Ca2+) signals during the maturation process. However, the significance of the dynamic changes in F-actin within the fertilized egg has been largely unclear.

METHODOLOGY/PRINCIPAL FINDINGS

We have measured changes in intracellular Ca2+ signals and F-actin structures during fertilization. We also report the unexpected observation that the conventional antagonist of the InsP(3) receptor, heparin, hyperpolymerizes the cortical actin cytoskeleton in postmeiotic eggs. Using heparin and other pharmacological agents that either hypo- or hyperpolymerize the cortical actin, we demonstrate that nearly all aspects of the fertilization process are profoundly affected by the dynamic restructuring of the egg cortical actin cytoskeleton.

CONCLUSIONS/SIGNIFICANCE

Our findings identify important roles for subplasmalemmal actin fibers in the process of sperm-egg interaction and in the subsequent events related to fertilization: the generation of Ca2+ signals, sperm penetration, cortical granule exocytosis, and the block to polyspermy.

Protein kinase C activity in mouse eggs regulates gamete membrane interaction

Gamete membrane interaction is critical to initiate the development of a new organism. The signaling pathways governing this event, however, are poorly understood. In this report, we provide the first evidence that protein kinase C activity in mouse eggs plays a crucial role in the regulation of this process. Stimulating PKC activity in mouse eggs by phorbol 12-myristate 13-acetate (PMA) drastically inhibited the egg's membrane ability to bind and fuse with sperm. Surprisingly, this significant reduction of gamete membrane interaction was also observed in eggs treated with the PKC inhibitors staurosporine and calphostin c. In further analysis, we found that while no change of egg actin cytoskeleton was detected after either PMA or calphostin c treatment, the structural morphology of egg surface microvilli was severely altered in the PMA-treated eggs, but not in the calphostin c-treated eggs. Moreover, sperm, which bound but did not fuse with the eggs treated with the anti-CD9 antibody KMC8, were liberated from the egg membrane after PMA, but not calphostin c, treatment. Taken together, these results suggest that egg PKC may be precisely balanced to regulate gamete membrane interaction in a biphasic mode, and this biphasic regulation is executed through two different mechanisms.

CaMKII can participate in but is not sufficient for the establishment of the membrane block to polyspermy in mouse eggs

Fertilization triggers initiation of development and establishment of blocks on the egg coat and plasma membrane to prevent fertilization by multiple sperm (polyspermy). The mechanism(s) by which mammalian eggs establish the membrane block to polyspermy is largely unknown. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) appears to be the key regulator of several egg activation events (completion of meiosis, progression to embryonic interphase, recruitment of maternal mRNAs). Since sperm-induced increases in cytosolic Ca(2+) play a role in establishment of the membrane block to polyspermy in mouse eggs, we hypothesized that CaMKII was a Ca(2+)-dependent effector leading to this change in egg membrane function. To test this hypothesis, we modulated CaMKII activity in two ways: activating eggs parthenogenetically by introducing constitutively active CaMKIIalpha (CA-CaMKII) into unfertilized eggs, and inhibiting endogenous CaMKII in fertilized eggs with myristoylated autocamtide 2-related inhibitory peptide (myrAIP). We find that eggs treated with myrAIP establish a less effective membrane block to polyspermy than do control eggs, but that CA-CaMKII is not sufficient for membrane block establishment, despite the fact that CA-CaMKII-activated eggs undergo other egg activation events. This suggests that: (1) CaMKII activity contributes to the membrane block, but this not faithfully mimicked by CA-CaMKII and furthermore, other pathways, in addition to those activated by Ca(2+) and CaMKII, also participate in membrane block establishment; (2) CA-CaMKII has a range of effects as a parthenogenetic trigger of egg activation (high levels of cell cycle resumption, modest levels of cortical granule exocytosis, and no membrane block establishment).

In vitro development of polyspermic porcine oocytes: Relationship between early fragmentation and excessive number of penetrating spermatozoa

Embryo development during in vitro culture of polyspermic porcine oocytes was investigated in the present study. After in vitro fertilization (IVF) of in vitro matured oocytes, putative zygotes were centrifuged to visualize pronuclei. Two pronuclear (2PN) and poly-pronuclear (PPN) zygotes were selected and cultured in vitro. Their development to the blastocyst stage and total cell numbers, dead cell rates and ploidy at the blastocyst stage and morphology of resultant embryos after first cleavage were compared. A cleavage rate of PPN embryos was lower than that of 2PN (61.3% and 82.2%, respectively), however, the ability of cleaved embryos to develop to the blastocyst stage did not differ between the PPN and the 2PN groups (22.4% and 32.9%, respectively). Also there was no difference in total cell numbers and rates of dead cells between PPN and 2PN blastocysts. The majority of blastocysts in 2PN group were found to be diploid. In contrast, blastocysts in PPN group showed heterogeneous status in their ploidy including polyploidy and mixoploidy, whereas a remarkable proportion (31.3%) of them was found to be diploid. After the first cleavage (at 36 h after IVF), there was no difference in the number of nuclei/embryo between the two groups, nevertheless embryos in PPN group had significantly higher numbers of blastomeres than that of embryos in 2PN group, mainly due to an increased frequency of anuclear blastomeres. The present results indicate that correction of embryo ploidy in polyspermic embryos can occur during IVC. Nevertheless the frequency of partial fragmentation in polyspermic embryos is increased.

Calcium and sperm components in the establishment of the membrane block to polyspermy: studies of ICSI and activation with sperm factor

One important result of egg activation is the establishment of blocks to prevent polyspermic fertilization; these blocks are established on the zona pellucida and the egg plasma membrane. This study examines what the sperm brings to the egg to induce the establishment of the membrane block to polyspermy, building on past evidence that membrane block establishment does not occur in response to parthenogenetic stimuli that induce a single transient increase in cytosolic Ca2+ or intracytoplasmic sperm injection (ICSI). We test the hypotheses that (i) sperm-associated Ca2+ release activity triggers membrane block establishment; (ii) introduction of sperm contents via variations on ICSI protocols (resulting in improved Ca2+ transients, egg activation and embryo development over traditional ICSI protocols) triggers membrane block establishment and (iii) sperm adhesion [binding of an extracellular sperm ligand(s) to an egg receptor(s)] combined with sperm-associated Ca2+ release activity triggers membrane block establishment. Interestingly, none of these stimuli induced establishment of the membrane block to polyspermy in mouse eggs. However, the sperm-associated remodeling of the egg cortical cytoskeleton differs between conventionally fertilized and ICSI-fertilized eggs; taken with our previous data implicating actin microfilaments in membrane block establishment, this raises the possibility that cortical reorganization may be a contributing factor. In sum, fertilization-like Ca2+ transients, either alone or combined with sperm-egg binding, are not sufficient for membrane block establishment, but that an event(s) associated with gamete interaction plays a role in this membrane function change.

Effect of maturation medium on in vitro cleavage of canine oocytes fertilized with fresh and cooled homologous semen

This study evaluated the effect of three maturation media on the development of in vitro-matured and in vitro-fertilized dog oocytes. In Experiment 1 (non-comparative experiment) canine cumulus–oocyte complexes (COCs) were matured in vitro in TCM199 supplemented with estrous cow serum (10%) + gonadotropins + steroid (treatment A), TCM199 + estrous cow serum (10%) (treatment B), or TCM199 + polyvinylpyrrolidone (PVP) (4%) (treatment C). All maturation media contained a final concentration of 1 μg/ml of human somatotropin (hST). Oocytes were fertilized with fresh ejaculated sperm and development was assessed by cleavage. The objective of Experiment 2 (comparative experiment) was to compare the rates of cleavage and developmental capacity of COCs matured in vitro in same medium as in Experiment 1, and fertilized either with fresh ejaculated or with cooled extended homologous spermatozoa. In Experiments 1 and 2, oocytes fertilized with fresh semen were in vitro-matured for 48 h, while in Experiment 2 COCs fertilized with cooled semen were matured in vitro for 72 h. The results of Experiments 1 and 2 demonstrated that cleavage was not influenced by the oocyte's maturation environment. The results of Experiment 1 showed that pronucleus formation + cleavage (day 7 after IVF) was similar among treatments A, B and C (p = 0.277). Also, in Experiment 2, pronucleus formation + cleavage (day 7 after IVF) was not different for oocytes fertilized in vitro either with fresh or cooled semen and maturated in media A (p = 0.190), B (p = 0.393) or C (p = 0.687). In both experiments, the numbers of embryos that developed to the 6–8-cell stage were higher for oocytes matured in medium A and fertilized with fresh semen, when compared with numbers of oocytes matured in media B and C. Embryo development to the 6–8-cell stage of oocytes fertilized either with fresh or cooled sperm was observed in treatments A and C in Experiment 2. Cumulus cell expansion was similar among treatments in Experiment 1. In Experiment 2, cumulus cell expansion among treatments A, B and C was similar after 48 h or 72 h of IVM. In both experiments, the greatest expansion category seen was for category 2 (outer cumulus cells slightly expanded). No correlation between cumulus expansion and cleavage were observed. Polyspermy rates in oocytes matured in medium A, and fertilized with fresh sperm were not significantly different from polyspermy rates observed using media B and C, in both experiments. Our findings indicate that treatments A, B and C are similarly effective for the cleavage of dog oocytes. Furthermore, it was demonstrated that canine oocytes matured in vitro could be fertilized by homologous cooled spermatozoa and progress to cleavage.

Activation of fertilized and nuclear transfer eggs

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

Oocyte CD9 is enriched on the microvillar membrane and required for normal microvillar shape and distribution

Microvilli are found on the surface of many cell types, including the mammalian oocyte, where they are thought to act in initial contact of sperm and oocyte plasma membranes. CD9 is currently the only oocyte protein known to be required for sperm-oocyte fusion. We found CD9 is localized to the oocyte microvillar membrane using transmission electron microscopy (TEM). Scanning electron microscopy (SEM) showed that CD9 null oocytes, which are unable to fuse with sperm, have an altered length, thickness and density of their microvilli. One aspect of this change in morphology was quantified using TEM by measuring the radius of curvature at the microvillar tips. A small radius of curvature is thought to promote fusibility and the radius of curvature of microvillar tips on CD9 wild-type oocytes was found to be half that of the CD9 null oocytes. We found that oocyte CD9 co-immunoprecipitates with two Ig superfamily cis partners, EWI-2 and EWI-F, which could have a role in linking CD9 to the oocyte microvillar actin core. We also examined latrunculin B-treated oocytes, which are known to have reduced fusion ability, and found altered microvillar morphology by SEM and TEM. Our data suggest that microvilli may participate in sperm-oocyte fusion. Microvilli could act as a platform to concentrate adhesion/fusion proteins and/or provide a membrane protrusion with a low radius of curvature. They may also have a dynamic interaction with the sperm that serves to capture the sperm cell and bring it into close contact with the oocyte plasma membrane.

The absence of a Ca(2+) signal during mouse egg activation can affect parthenogenetic preimplantation development, gene expression patterns, and blastocyst quality

A series of Ca(2+) oscillations during mammalian fertilization is necessary and sufficient to stimulate meiotic resumption and pronuclear formation. It is not known how effectively development continues in the absence of the initial Ca(2+) signal. We have triggered parthenogenetic egg activation with cycloheximide that causes no Ca(2+) increase, with ethanol that causes a single large Ca(2+) increase, or with Sr(2+) that causes Ca(2+) oscillations. Eggs were co-treated with cytochalasin D to make them diploid and they formed pronuclei and two-cell embryos at high rates with each activation treatment. However, far fewer of the embryos that were activated by cycloheximide reached the blastocyst stagecompared tothose activated by Sr(2+) orethanol. Any cycloheximide-activated embryos that reached the blastocyst stage had a smaller inner cell mass number and a greater rate of apoptosis than Sr(2+)-activated embryos. The poor development of cycloheximide-activated embryos was due to the lack of Ca(2+) increase because they developed to blastocyst stages at high rates when co-treated with Sr(2+) or ethanol. Embryos activated by either Sr(2+) or cycloheximide showed similar signs of initial embryonic genome activation (EGA) when measured using a reporter gene. However, microarray analysis of gene expression at the eight-cell stage showed that activation by Sr(2+) leads to a distinct pattern of gene expression from that seen with embryos activated by cycloheximide. These data suggest that activation of mouse eggs in the absence of a Ca(2+) signal does not affect initial parthenogenetic events, but can influence later gene expression and development.

Regulation of dynamic events by microfilaments during oocyte maturation and fertilization

Actin filaments (microfilaments) regulate various dynamic events during oocyte meiotic maturation and fertilization. In most species, microfilaments are not required for germinal vesicle breakdown and meiotic spindle formation, but they mediate peripheral nucleus (chromosome) migration, cortical spindle anchorage, homologous chromosome separation, cortex development/maintenance, polarity establishment, and first polar body emission during oocyte maturation. Peripheral cortical granule migration is controlled by microfilaments, while mitochondria movement is mediated by microtubules. During fertilization, microfilaments are involved in sperm incorporation, spindle rotation (mouse), cortical granule exocytosis, second polar body emission and cleavage ring formation, but are not required for pronuclear apposition (except for the mouse). Many of the events are driven by the dynamic interactions between myosin and actin filaments whose polymerization is regulated by RhoA, Cdc42, Arp2/3 and other signaling molecules. Studies have also shown that oocyte cortex organization and polarity formation mediated by actin filaments are regulated by mitogen-activated protein kinase, myosin light-chain kinase, protein kinase C and its substrate p-MARKS as well as PAR proteins. The completion of several dynamic events, including homologous chromosome separation, spindle anchorage, spindle rotation, vesicle organelle transport and pronuclear apposition (mouse), requires interactions between microfilaments and microtubules, but determination of how the two systems of the cytoskeleton precisely cross-link, and which proteins link microfilaments to microtubules to perform functions in eggs, requires further studies. Finally, the meaning of microfilament-mediated oocyte polarity versus embryo polarity and embryo development in different species (Drosophila, Xenopus and mouse) is discussed.

Cumulus cells affect distribution and function of the cytoskeleton and organelles in porcine oocytes

Mammalian oocytes grow and undergo meiosis within ovarian follicles. Oocytes are arrested at the first meiotic prophase, being controlled or influenced by follicular somatic cells. Under the influence of gonadotropins, immature oocytes resume meiosis. During meiotic progression, some cytoplasmic changes occur, so-called cytoplasmic maturation. However, porcine follicular oocytes vary greatly in developmental competence. The present review summarizes recent studies highlighting the importance of cumulus cells in maintaining the developmental ability and in reorganizing the cytoskeleton and organelles of porcine oocytes. Factors affecting wide variation of the nuclear and cytoplasmic maturation observed in the porcine oocytes are discussed. (Reprod Med Biol 2006; 5: 183-194).

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.

CD9 protein appears on growing mouse oocytes at the time when they develop the ability to fuse with spermatozoa

CD9 is a member of the tetraspanin superfamily proteins and is the only protein on the mouse oocyte which is known to be indispensable in sperm–egg fusion. Here, using indirect immunofluorescence we show that CD9 appears on the oolemma during the early stages of the growth of the oocyte, when it measures 13–22 μm in diameter. When the oocyte reaches a diameter of 17–22 μm, the density of CD9 in its oolemma is similar to the density of this protein in the cell membrane of the fully grown secondary oocyte. The appearance of CD9 in growing oocytes correlates with the previously reported time of the acquisition of fusibility between the spermatozoon and the egg. Accordingly we propose that during oogenesis the development of the ability of the oolemma to fuse with sperm may be regulated by synthesis of CD9 by the oocyte.

Calcium oscillations and mammalian egg activation

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

Targeted disruption of tyrosylprotein sulfotransferase-2, an enzyme that catalyzes post-translational protein tyrosine O-sulfation, causes male infertility

Tyrosine O-sulfation is a post-translational modification mediated by one of two Golgi tyrosylprotein sulfotransferases (TPST-1 and -2) expressed in all mammalian cells. Tyrosine sulfation plays an important role in the function of some known TPST substrates by enhancing protein-protein interactions. To explore the role of these enzymes in vivo and gain insight into other potential TPST substrates, TPST-2-deficient mice were generated by targeted disruption of the Tpst2 gene. Tpst2(+/-) mice appear normal and, when interbred, yield litters of normal size with a Mendelian distribution of the targeted mutation. Tpst2(-/-) mice have moderately delayed growth but appear healthy and attain normal body weight by 10 weeks of age. In contrast to Tpst1(-/-) males that have normal fertility, Tpst2(-/-) males are infertile. Tpst2(-/-) sperm are normal in number, morphology, and motility in normal media and appear to capacitate and undergo acrosomal exocytosis normally. However, they are severely defective in their motility in viscous media and in their ability to fertilize zona pellucida-intact eggs. Adhesion of Tpst2(-/-) sperm to the egg plasma membrane is reduced compared with wild type sperm, but sperm-egg fusion is similar or even increased. These data strongly suggest that tyrosine sulfation of unidentified substrate(s) play a crucial role in these processes and document for the first time the critical importance of post-translational tyrosine sulfation in male fertility.

Mammalian membrane block to polyspermy: new insights into how mammalian eggs prevent fertilisation by multiple sperm

To inhibit fertilisation by more than one sperm (a condition known as polyspermy), eggs have developed preventative mechanisms known as blocks to polyspermy. The block at the level of the egg extracellular coat (the zona pellucida in mammals, the vitelline envelope in non-mammals) has been well characterised in many different animal species and the block at the level of the egg plasma membrane is understood in some non-mammalian species. However, virtually nothing is known about the membrane block to polyspermy in mammalian eggs, despite data dating back 50–90 years that provide evidence for its existence. In the present review, we will discuss the background on blocks to polyspermy used by animal eggs and then focus on the membrane block to polyspermy in mammalian eggs. This will include a summary of classical studies that provide evidence for this block in mammalian eggs, assays used to study the mammalian membrane block and what has been elucidated from recent experimental studies about the cellular signalling events that lead to membrane block establishment and the mechanism of how the membrane block may prevent additional fertilisation.

Ultrastructural Dynamics of Human Reproduction, from Ovulation to Fertilization and Early Embryo Development1

This study describes the updated, fine structure of human gametes, the human fertilization process, and human embryos, mainly derived from assisted reproductive technology (ART). As clearly shown, the ultrastructure of human reproduction is a peculiar multistep process, which differs in part from that of other mammalian models, having some unique features. Particular attention has been devoted to the (1) sperm ultrastructure, likely "Tygerberg (Kruger) strict morphology criteria"; (2) mature oocyte, in which the MII spindle is barrel shaped, anastral, and lacking centrioles; (3) three-dimensional microarchitecture of the zona pellucida with its unique supramolecular filamentous organization; (4) sperm-egg interactions with the peculiarity of the sperm centrosome that activates the egg and organizes the sperm aster and mitotic spindles of the embryo; and (5) presence of viable cumulus cells whose metabolic activity is closely related to egg and embryo behavior in in vitro as well as in vivo conditions, in a sort of extraovarian "microfollicular unit." Even if the ultrastructural morphodynamic features of human fertilization are well understood, our knowledge about in vivo fertilization is still very limited and the complex sequence of in vivo biological steps involved in human reproduction is only partially reproduced in current ART procedures.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

Phosphorylated MARCKS: A novel centrosome component that also defines a peripheral subdomain of the cortical actin cap in mouse eggs

MARCKS (myristoylated alanine-rich C-kinase substrate) is a major substrate for protein kinase C (PKC), a kinase that has multiple functions during oocyte maturation and egg activation, for example, spindle function and cytoskeleton reorganization. We examined temporal and spatial changes in p-MARCKS localization during maturation of mouse oocytes and found that p-MARCKS is a novel centrosome component based its co-localization with pericentrin and gamma-tubulin within microtubule organizing centers (MTOCs). Like pericentrin, p-MARCKS staining at the MI spindle poles was asymmetric. Based on this asymmetry, we found that one end of the spindle was preferentially extruded with the first polar body. At MII, however, the spindle poles had symmetrical p-MARCKS staining. p-MARCKS also was enriched in the periphery of the actin cap overlying the MI or MII spindle to form a ring-shaped subdomain. Because phosphorylation of MARCKS modulates its actin crosslinking function, this localization suggests p-MARCKS functions as part of the contractile apparatus during polar body emission. Our finding that an activator of conventional and novel PKC isoforms did not increase the amount of p-MARCKS suggested that an atypical isoform was responsible for MARCKS phosphorylation. Consistent with this idea, immunostaining revealed that the staining patterns of p-MARCKS and the active form of the atypical PKC zeta/lambda isoform(s) were very similar. These results show that p-MARCKS is a novel centrosome component and also defines a previously unrecognized subdomain of the actin cap overlying the spindle.