Ca2+ signalling during fertilization of echinoderm eggs

De novo assembly of a transcriptome from the eggs and early embryos of Astropecten aranciacus

Starfish have been instrumental in many fields of biological and ecological research. Oocytes of Astropecten aranciacus, a common species native to the Mediterranean Sea and the East Atlantic, have long been used as an experimental model to study meiotic maturation, fertilization, intracellular Ca²⁺ signaling, and cell cycle controls. However, investigation of the underlying molecular mechanisms has often been hampered by the overall lack of DNA or protein sequences for the species. In this study, we have assembled a transcriptome for this species from the oocytes, eggs, zygotes, and early embryos, which are known to have the highest RNA sequence complexity. Annotation of the transcriptome identified over 32,000 transcripts including the ones that encode 13 distinct cyclins and as many cyclin-dependent kinases (CDK), as well as the expected components of intracellular Ca²⁺ signaling toolkit. Although the mRNAs of cyclin and CDK families did not undergo significant abundance changes through the stages from oocyte to early embryo, as judged by real-time PCR, the transcript encoding Mos, a negative regulator of mitotic cell cycle, was drastically reduced during the period of rapid cleavages. Molecular phylogenetic analysis using the homologous amino acid sequences of cytochrome oxidase subunit I from A. aranciacus and 30 other starfish species indicated that Paxillosida, to which A. aranciacus belongs, is not likely to be the most basal order in Asteroidea. Taken together, the first transcriptome we assembled in this species is expected to enable us to perform comparative studies and to design gene-specific molecular tools with which to tackle long-standing biological questions.

Gene expression profiling during the embryo-to-larva transition in the giant red sea urchin Mesocentrotus franciscanus

In echinoderms, major morphological transitions during early development are attributed to different genetic interactions and changes in global expression patterns that shape the regulatory program for the specification of embryonic territories. In order more thoroughly to understand these biological and molecular processes, we examined the transcriptome structure and expression profiles during the embryo‐to‐larva transition of a keystone species, the giant red sea urchin Mesocentrotus franciscanus. Using a de novo assembly approach, we obtained 176,885 transcripts from which 60,439 (34%) had significant alignments to known proteins. From these transcripts, ~80% were functionally annotated allowing the identification of ~2,600 functional, structural, and regulatory genes involved in developmental process. Analysis of expression profiles between gastrula and pluteus stages of M. franciscanus revealed 791 differentially expressed genes with 251 GO overrepresented terms. For gastrula, up‐regulated GO terms were mainly linked to cell differentiation and signal transduction involved in cell cycle checkpoints. In the pluteus stage, major GO terms were associated with phosphoprotein phosphatase activity, muscle contraction, and olfactory behavior, among others. Our evolutionary comparative analysis revealed that several of these genes and functional pathways are highly conserved among echinoids, holothuroids, and ophiuroids.

Development of Ca2+-release mechanisms during oocyte maturation of the starfish Asterina pectinifera

An important step for successful fertilization and further development is the increase in intracellular Ca2+ in the activated oocyte. It has been known that starfish oocytes become increasingly sensitive to inositol 1,4,5-trisphosphate (IP3) during meiotic maturation to exhibit highly efficient IP3-induced Ca2+ release (IICR) by the time of germinal vesicle breakdown (GVBD). However, we noted that the peak level of intracellular Ca2+ increase after insemination is already high in the maturing oocytes before GVBD. Using maturing oocytes before GVBD, we investigated Ca2+ release mechanisms other than IICR. We report here that Ca2+-release mechanisms dependent on nicotinic acid adenine dinucleotide phosphate (NAADP) and nicotinamide adenine dinucleotide (NADP), the precursor of NAADP, became functional prior to the development of IICR mechanisms. As with IP3, but unlike NAADP, the Ca2+ stores responsive to NADP are sensitized during the meiotic maturation induced by 1-methyladenine (1-MA). This suggests that the process may represent a physiological response to the maturation hormone. NADP-dependent Ca2+ release in immature oocytes, however, did not induce oocyte maturation by itself, but was enhanced by the conditions mimicking the increases of intracellular Ca2+ and pH that take place in the maturing oocytes of starfish.

Egg Activation at Fertilization by a Soluble Sperm Protein

The most fundamental unresolved issue of fertilization is to define how the sperm activates the egg to begin embryo development. Egg activation at fertilization in all species thus far examined is caused by some form of transient increase in the cytoplasmic free Ca2+ concentration. What has not been clear, however, is precisely how the sperm triggers the large changes in Ca2+ observed within the egg cytoplasm. Here, we review the studies indicating that the fertilizing sperm stimulates a cytosolic Ca2+ increase in the egg specifically by delivering a soluble factor that diffuses into the cytosolic space of the egg upon gamete membrane fusion. Evidence is primarily considered in species of eggs where the sperm has been shown to elicit a cytosolic Ca2+ increase by initiating Ca2+ release from intracellular Ca2+ stores. We suggest that our best understanding of these signaling events is in mammals, where the sperm triggers a prolonged series of intracellular Ca2+ oscillations. The strongest empirical studies to date suggest that mammalian sperm-triggered Ca2+ oscillations are caused by the introduction of a sperm-specific protein, called phospholipase C-zeta (PLCζ) that generates inositol trisphosphate within the egg. We will discuss the role and mechanism of action of PLCζ in detail at a molecular and cellular level. We will also consider some of the evidence that a soluble sperm protein might be involved in egg activation in nonmammalian species.

Identification of phospholipase activity in Rhinella arenarum sperm extract capable of inducing oocyte activation

Egg activation, which includes cortical granule exocytosis, resumption and completion of meiosis and pronuclear formation culminates in the first mitotic cleavage. However, the mechanism through which the fertilizing sperm induces this phenomenon is still controversial. We investigated the effect of the microinjection of homologous sperm soluble fractions obtained by fast protein liquid chromatography (FPLC) from reacted sperm (without acrosome) and non-reacted sperm on the activation of Rhinella arenarum oocytes matured in vitro. The FPLC-purified sperm fraction obtained from reacted or non-reacted sperm is able to induce oocyte activation when it is microinjected. This fraction has a 24 kDa protein and showed phospholipase C (PLC) activity in vitro, which was inhibited by D-609 but not by n-butanol or neomycin, suggesting that it is a PLC that is specific for phosphatidylcholine (PC-PLC). The assays conducted using inhibitors of inositol triphosphate (IP3) and ryanodine receptors (RyRs) indicate that the fraction with biological activity would act mainly through the cADPr (cyclic ADP ribose) pathway. Moreover, protein kinase C (PKC) inhibition blocks the activation produced by the same fraction. Immunocytochemical studies indicate that this PC-PLC can be found throughout the sperm head.

Cell surface changes in the egg at fertilization

An egg changes dramatically at fertilization. These changes include its developmental potential, its physiology, its gene expression profile, and its cell surface. This review highlights the changes in the cell surface of the egg that occur in response to sperm. These changes include modifications to the extracellular matrix, to the plasma membrane, and to the secretory vesicles whose contents direct many of these events. In some species, these changes occur within minutes of fertilization, and are sufficiently dramatic so that they can be seen by the light microscope. Many of these morphological changes were documented in remarkable detail early in the 1900 s by Ernest Everett Just. A recent conference in honor of his contributions stimulated this overview. We highlight the major cell surface changes that occur in echinoderms, one of Just's preferred research organisms.

Impact of marine drugs on animal reproductive processes

The discovery and description of bioactive substances from natural sources has been a research topic for the last 50 years. In this respect, marine animals have been used to extract many new compounds exerting different actions. Reproduction is a complex process whose main steps are the production and maturation of gametes, their activation, the fertilisation and the beginning of development. In the literature it has been shown that many substances extracted from marine organisms may have profound influence on the reproductive behaviour, function and reproductive strategies and survival of species. However, despite the central importance of reproduction and thus the maintenance of species, there are still few studies on how reproductive mechanisms are impacted by marine bioactive drugs. At present, studies in either marine and terrestrial animals have been particularly important in identifying what specific fine reproductive mechanisms are affected by marine-derived substances. In this review we describe the main steps of the biology of reproduction and the impact of substances from marine environment and organisms on the reproductive processes.

Characterization of phospholipases C beta and gamma and their possible roles in Chaetopterus egg activation

Intracellular calcium release from the endoplasmic reticulum is a hallmark at egg activation of both vertebrates and invertebrates. This fertilization-associated calcium release results from generation of the second messenger inositol 1,4,5-trisphosphate (IP(3)) by one or more phospholipases C (PLC). We characterized Chaetopterus PLCbeta and gamma by reverse transcription/degenerate oligonucleotide primed PCR and rapid amplification of cDNA end PCR. Phylogenetic analyses suggested that the deduced PLCbeta protein shared the greatest homology with mammalian PLCbeta4; the deduced PLCgamma protein shared the greatest homology with starfish PLCgamma and diverged from mammalian PLCgamma before mammalian the PLCgamma1 and gamma2 isoforms diverged. Western blot analyses with specific anti-PLCbeta and gamma antibodies, respectively, revealed that 135 and 150 kDa proteins were expressed in eggs. The general PLC antagonist U-73122 blocked fertilization-induced egg activation; however, the inactive analog, U-73343, had no effect on egg activation. We further tested whether egg activation was G protein-PLCbeta and/or protein tyrosine kinase-PLCgamma dependent. Cholera and pertussis toxins, well-known effectors of G proteins, had no effect on egg activation; while two antagonists of PTK, genistein and tyrphostin B42, inhibited both fertilization-induced and artificial egg activation. Taken together, our studies suggested that PLC activity from eggs contributes to Chaetopterus egg activation and PLCgamma might play an important role during this biological process.

Measuring Ca2+-signalling at fertilization in the sea urchin Psammechinus miliaris: alterations of this Ca2+-signal by copper and 2,4,6-tribromophenol

During fertilization, eggs undergo a temporary rise in the intracellular concentration of free Ca(2+) ions. Using the membrane permeable acetoxymethylester of the fluorescent calcium indicator dye Fura-2, Fura-2 AM, the Ca(2+)-signal at fertilization was not detectable in eggs of the sea urchin Psammechinus miliaris. However, after treatment of the eggs with Fura-2 AM in combination with MK571, an inhibitor for multidrug resistance associated proteins, clear Ca(2+)-signals at fertilization could be measured without microinjection of the dye. We used this methodology to detect possible alterations of Ca(2+)-signalling at fertilization by exposure of eggs to environmental pollutants. For this purpose, the heavy metal copper, the bromophenol 2,4,6-tribromophenol, the organic compound bisphenol A and the polycyclic aromatic hydrocarbon phenanthrene were tested for their potential to inhibit fertilization success of P. miliaris. Copper and 2,4,6-tribromophenol showed a dose-dependent effect on fertilization rates of P. miliaris and significantly inhibited fertilization at 6.3 microM Cu(2+) and 1 microM 2,4,6-tribromphenol. Bisphenol A significantly inhibited fertilization success at 438 microM while phenanthrene had no effect up to 56 microM. 6.3 microM copper and 100 microM 2,4,6-tribromophenol significantly increased the Ca(2+)-signal at fertilization. This alteration may contribute to the reduced fertilization rates of P. miliaris after exposure to copper and 2,4,6-tribromophenol.

Subcellular localization of calcium and Ca-ATPase activity during nuclear maturation in Bufo arenarum oocytes

The localization of calcium and Ca-ATPase activity in Bufo arenarum oocytes was investigated by ultracytochemical techniques during progesterone-induced nuclear maturation, under in vitro conditions. No Ca2+ deposits were detected in either control oocytes or progesterone-treated ones for 1-2 h. At the time when nuclear migration started, electron dense deposits of Ca2+ were visible in vesicles, endoplasmic reticulum cisternae and in the space between the annulate lamellae membranes. Furthermore, Ca-ATPase activity was also detected in these membrane structures. As maturation progressed, the cation deposits were observed in the cytomembrane structures, which underwent an important reorganization and redistribution. Thus, they moved from the subcortex and became located predominantly in the oocyte cortex area when nuclear maturation ended. Ca2+ stores were observed in vesicles surrounding or between the cortical granules, which are aligned close to the plasma membrane. The positive Ca-ATPase reaction in these membrane structures could indicate that the calcium deposit is an ATP-dependent process. Our results suggest that during oocyte maturation calcium would be stored in membrane structures where it remains available for release at the time of fertilization. Data obtained under our experimental conditions indicate that calcium from the extracellular medium would be important for the oocyte maturation process.

Expression of multiple Src family kinases in sea urchin eggs and their function in Ca2+ release at fertilization

Egg activation at fertilization in deuterostomes requires a rise in intracellular Ca(2+), which is released from the egg's endoplasmic reticulum. In sea urchins, a Src Family Kinase (SpSFK1) is necessary for the PLCgamma-mediated signaling event that initiates this Ca(2+) release (Giusti, A.F., O'Neill, F.J., Yamasu, K., Foltz, K.R. and Jaffe, L.A., 2003. Function of a sea urchin egg Src family kinase in initiating Ca2+ release at fertilization. Dev. Biol. 256, 367-378.). Annotation of the Strongylocentrotus purpuratus genome sequence led to the identification of additional, predicted SFKs (Bradham, C.A., Foltz, D.R., Beane, W.S., Amone, M.I., Rizzo, F., Coffman, J.A., Mushegian, A., Goel, M., Morales, J., Geneviere, A.M., Lapraz, F., Robertson, A.J., Kelkar, H., Loza-Coll, M., Townley, I.K., Raisch, M., Roux, M.M., Lepage, T., Gache, C., McClay, D.R., Manning, G., 2006. The sea urchin kinome: a first look. Dev. Biol. 300, 180-193.; Roux, M.M., Townley, I.K., Raisch, M., Reade, A., Bradham, C., Humphreys, G., Gunaratne, H.J., Killian, C.E., Moy, G., Su, Y.H., Ettensohn, C.A., Wilt, F., Vacquier, V.D., Burke, R.D., Wessel, G. and Foltz, K.R., 2006. A functional genomic and proteomic perspective of sea urchin calcium signaling and egg activation. Dev. Biol. 300, 416-433.). Here, we describe the cloning and characterization of these 4 additional SFKs and test their function during the initial Ca(2+) release at fertilization using the dominant-interfering microinjection method coupled with Ca(2+) recording. While two of the new SFKs (SpFrk and SpSFK3) are necessary for Ca(2+) release, SpSFK5 appears dispensable for early egg to embryo transition events. Interestingly, SpSFK7 may be involved in preventing precocious release of Ca(2+). Binding studies indicate that only SpSFK1 is capable of direct interaction with PLCgamma. Immunolocalization studies suggest that one or more SpSFK and PLCgamma are localized to the egg cortex and at the site of sperm-egg interaction. Collectively, these data indicate that more than one SFK is involved in the Ca(2+) release pathway at fertilization.

Activation of amphibian oocytes by sperm extracts

In the fertilization of most animals, egg activation is accompanied by an increase in cytoplasmatic Ca2+; however, the mechanism through which the fertilizing sperm induce this phenomenon is still controversial. An increase in intracellular free Ca2+is required to trigger egg activation events, a process that includes cortical granule exocytosis, resumption and completion of meiosis and DNA replication, and culminates in the first mitotic cleavage. In this work, we investigated the effect of microinjection and incubation of different fractions of homologous sperm extract on the activation ofBufo arenarumoocytes maturedin vitro. Two heat treatment-sensitive fractions obtained by chromatography were able to induce oocyte activation. The sperm fraction, which contained a 24 kDa protein, induced 90% activation when it was microinjected into the oocytes. Whilst the sperm fraction, which contained a 36 kDa protein, was able to induce about 70% activation only when it was applied on the oocyte surface.

Calcium signalling in early embryos

The onset of development in most species studied is triggered by one of the largest and longest calcium transients known to us. It is the most studied and best understood aspect of the calcium signals that accompany and control development. Its properties and mechanisms demonstrate what embryos are capable of and thus how the less-understood calcium signals later in development may be generated. The downstream targets of the fertilization calcium signal have also been identified, providing some pointers to the probable targets of calcium signals further on in the process of development. In one species or another, the fertilization calcium signal involves all the known calcium-releasing second messengers and many of the known calcium-signalling mechanisms. These calcium signals also usually take the form of a propagating calcium wave or waves. Fertilization causes the cell cycle to resume, and therefore fertilization signals are cell-cycle signals. In some early embryonic cell cycles, calcium signals also control the progress through each cell cycle, controlling mitosis. Studies of these early embryonic calcium-signalling mechanisms provide a background to the calcium-signalling events discussed in the articles in this issue.

Characterization of Xenopus egg membrane microdomains containing uroplakin Ib/III complex: roles of their molecular interactions for subcellular localization and signal transduction

A single-transmembrane protein uroplakin III (UPIII) and its tetraspanin binding-partner uroplakin Ib (UPIb) are members of the UP proteins that were originally identified in mammalian urothelium. In Xenopus laevis eggs, these proteins: xUPIII and xUPIb, are components of the cholesterol-enriched membrane microdomains or "rafts" and involved in the sperm-egg membrane interaction and subsequent egg activation signaling via Src tyrosine kinase at fertilization. Here, we investigate whether the xUPIII-xUPIb complex is in close proximity to CD9, a tetraspanin that has been implicated in the sperm-egg fusion in the mouse and GM1, a ganglioside typically enriched in egg rafts. Preparation of the egg membrane microdomains using different non-ionic detergents (Brij 98 and Triton X-100), chemical cross-linking, co-immunoprecipitation, in vitro kinase assay and in vitro fertilization experiments demonstrated that GM1, but not CD9, is in association with the xUPIII-xUPIb complex and contributes to the sperm-dependent egg activation. Transfection experiments using HEK293 cells demonstrated that xUPIII and xUPIb localized efficiently to the cholesterol-dependent membrane microdomains when they were co-expressed, whereas co-expression of xUPIII and CD9, instead of xUPIb, did not show this effect. Furthermore, xUPIII and xUPIb were shown to suppress kinase activity of the wild type, but not a constitutively active form of, Xenopus Src protein co-expressed in HEK293 cells. These results provide novel insight into the molecular architecture of the egg membrane microdomains containing xUPIII, xUPIb and Src, which may contribute to the understanding of sperm-egg interaction and signaling during Xenopus fertilization.

Intracellular Ca2+ increase induces post-fertilization events via MAP kinase dephosphorylation in eggs of the hydrozoan jellyfish Cladonema pacificum

Naturally spawned eggs of the hydrozoan jellyfish Cladonema pacificum are arrested at G1-like pronuclear stage until fertilization. Fertilized eggs of Cladonema undergo a series of post-fertilization events, including loss of sperm-attracting ability, expression of adhesive materials on the egg surface, and initiation of cell cycle leading to DNA synthesis and cleavage. Here, we investigate whether these events are regulated by changes in intracellular Ca2+ concentration and mitogen-activated protein kinase (MAP kinase) activity in Cladonema eggs. We found that MAP kinase is maintained in the phosphorylated form in unfertilized eggs. Initiation of sperm-induced Ca2+ increase, which is the first sign of fertilization, was immediately followed by MAP kinase dephosphorylation within a few minutes of fertilization. The fertilized eggs typically stopped sperm attraction by an additional 5 min and became sticky around this time. They further underwent cytokinesis yielding 2-cell embryos at approximately 1 h post-fertilization, which was preceded by DNA synthesis evidenced by BrdU incorporation into the nuclei. Injection of inositol 1,4,5-trisphosphate (IP3) into unfertilized eggs, which produced a Ca2+ increase similar to that seen at fertilization, triggered MAP kinase dephosphorylation and the above post-fertilization events without insemination. Conversely, injection of BAPTA/Ca2+ into fertilized eggs at approximately 10 s after the initiation of Ca2+ increase immediately lowered the elevating Ca2+ level and inhibited the subsequent post-fertilization events. Treatment with U0126, an inhibitor of MAP kinase kinase (MEK), triggered the post-fertilization events in unfertilized eggs, where MAP kinase dephosphorylation but not Ca2+ increase was generated. Conversely, preinjection of the glutathione S-transferase (GST) fusion protein of MAP kinase kinase kinase (Mos), which maintained the phosphorylated state of MAP kinase, blocked the post-fertilization events in fertilized eggs without preventing a Ca2+ increase. These results strongly suggest that all of the three post-fertilization events, cessation of sperm attraction, expression of surface adhesion, and progression of cell cycle, lie downstream of MAP kinase dephosphorylation that is triggered by a Ca2+ increase.

Calcium-responsive contractility during fertilization in sea urchin eggs

Fertilization triggers a reorganization of oocyte cytoskeleton, and in sea urchins, there is a dramatic increase in cortical F-actin. However, the role that myosin II plays during fertilization remains largely unexplored. Myosin II is localized to the cortical cytoskeleton both before and after fertilization and to examine myosin II contractility in living cells, Lytechinus pictus eggs were observed by time-lapse microscopy. Upon sperm binding, a cell surface deflection traversed the egg that was followed by and dependent on the calcium wave. The calcium-dependence of surface contractility could be reproduced in unfertilized eggs, where mobilization of intracellular calcium in unfertilized eggs under compression resulted in a marked contractile response. Lastly, inhibition of myosin II delayed absorption of the fertilization cone, suggesting that myosin II not only responds to the same signals that activate eggs but also participates in the remodeling of the cortical actomyosin cytoskeleton during the first zygotic cell cycle.

When microspores decide to become embryos — cellular and molecular changesThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

Cultured microspores can be induced to develop into fully functional haploid embryos instead of mature pollen. The ability of these cells to change their development in response to environmental stimuli is an exceptional example of totipotency in plants. Discovering the triggers of embryo development in microspore cultures could lead to a greater understanding of the early stages of embryogenesis in plants and might be used to increase the range of crop plants to which microspore culture can be applied for the production of double haploid homozygous lines. The information might also help to define the general characteristics of pluripotent cells in any organism. In this review, the changes that occur in cellular organization and gene expression in early-stage microspore cultures of several species are discussed. Responding cells in these cultures enlarge, their nuclei are repositioned to their cell centres, and their cytoplasms become filled with fragmented vacuoles. We used flow cytometry to track cellular changes in canola ( Brassica napus L.) microspore cultures as well as microarray analyses and real-time PCR to compare gene expression in embryogenic and nonembryogenic cells. A model for embryogenic cell activation in plants that involves alkalinization, Ca2+ signaling, and changes in GTPase activity that lead to significant changes in gene expression is discussed.

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.

Reactive oxygen species and Udx1 during early sea urchin development

Sea urchin fertilization is marked by a massive conversion of molecular oxygen to hydrogen peroxide by a sea urchin dual oxidase, Udx1. This enzyme is essential for completing the physical block to polyspermy. Yet, its expression is maintained during development, as indicated by the presence of both Udx1 mRNA and Udx1 protein enriched at the surface of all non-mesenchymal blastomeres. When hydrogen peroxide synthesis by Udx1 is inhibited, either pharmacologically or by specific antibody injection, cleavage is delayed. Application of exogenous hydrogen peroxide, however, partially rescues a fraction of these defective embryos. We also report an unequal distribution of reactive oxygen species between sister blastomeres during early cleavage stages, suggesting a functional role for Udx1 in intracellular signaling.

Distinct roles for multiple Src family kinases at fertilization

Egg activation at fertilization requires the release of Ca2+ from the endoplasmic reticulum of the egg. Recent evidence indicates that Src family kinases (SFKs) function in the signaling pathway that initiates this Ca2+ release in the eggs of many deuterostomes. We have identified three SFKs expressed in starfish (Asterina miniata) eggs, designated AmSFK1, AmSFK2 and AmSFK3. Antibodies made against the unique domains of each AmSFK protein revealed that all three are expressed in eggs and localized primarily to the membrane fraction. Both AmSFK1 and AmSFK3 (but not AmSFK2) are necessary for egg activation, as determined by injection of starfish oocytes with dominant-interfering Src homology 2 (SH2) domains, which specifically delay and reduce the initial release of Ca2+ at fertilization. AmSFK3 exhibits a very rapid and transient kinase activity in response to fertilization, peaking at 30 seconds post sperm addition. AmSFK1 kinase activity also increases transiently at fertilization, but peaks later, at 2 minutes. These results indicate that there are multiple SFKs present in starfish eggs with distinct, perhaps sequential, signaling roles.

Spatiotemporal characteristics and mechanisms of intracellular Ca(2+) increases at fertilization in eggs of jellyfish (Phylum Cnidaria, Class Hydrozoa)

We have clarified, for the first time, the spatiotemporal patterns of intracellular Ca(2+) increases at fertilization and the Ca(2+)-mobilizing mechanisms in eggs of hydrozoan jellyfish, which belong to the evolutionarily old diploblastic phylum, Cnidaria. An initial Ca(2+) increase just after fertilization took the form of a Ca(2+) wave starting from one cortical region of the egg and propagating to its antipode in all of four hydrozoan species tested: Cytaeis uchidae, Cladonema pacificum, Clytia sp., and Gonionema vertens. The initiation site of the Ca(2+) wave was restricted to the animal pole, which is known to be the only area of sperm-egg fusion in hydrozoan eggs, and the wave propagating velocity was estimated to be 4.2-5.9 mum/s. After a Ca(2+) peak had been attained by the initial Ca(2+) wave, the elevated Ca(2+) gradually declined and returned nearly to the resting value at 7-10 min following fertilization. Injection of inositol 1,4,5-trisphosphate (IP(3)), an agonist of IP(3) receptors (IP(3)R), was highly effective in inducing a Ca(2+) increase in unfertilized eggs; IP(3) at a final intracellular concentration of 12-60 nM produced a fully propagating Ca(2+) wave equivalent to that observed at fertilization. In contrast, a higher concentration of cyclic ADP-ribose (cADPR), an agonist of ryanodine receptors (RyR), only generated a localized Ca(2+) increase that did not propagate in the egg. In addition, caffeine, another stimulator of RyR, was completely without effect. Sperm-induced Ca(2+) increases in Gonionema eggs were severely affected by preinjection of heparin, an inhibitor of Ca(2+) release from IP(3)R. These results strongly suggest that there is a well-developed IP(3)R-, but not RyR-mediated Ca(2+) release mechanism in hydrozoan eggs and that the former system primarily functions at fertilization. Our present data also demonstrate that the spatial characteristics and mechanisms of Ca(2+) increases at fertilization in hydrozoan eggs resemble those reported in higher triploblastic animals.

SH2 domain-mediated activation of an SRC family kinase is not required to initiate Ca2+ release at fertilization in mouse eggs

SRC family kinases (SFKs) function in initiating Ca2+release at fertilization in several species in the vertebrate evolutionary line, but whether they play a similar role in mammalian fertilization has been uncertain. We investigated this question by first determining which SFK proteins are expressed in mouse eggs, and then measuring Ca2+release at fertilization in the presence of dominant negative inhibitors. FYN and YES proteins were found in mouse eggs, but other SFKs were not detected; based on this, we injected mouse eggs with a mixture of FYN and YES Src homology 2 (SH2) domains. These SH2 domains were effective inhibitors of Ca2+release at fertilization in starfish eggs, but did not inhibit Ca2+release at fertilization in mouse eggs. Thus the mechanism by which sperm initiate Ca2+release in mouse eggs does not depend on SH2 domain-mediated activation of an SFK. We also tested the small molecule SFK inhibitor SU6656, and found that it became compartmentalized in the egg cytoplasm, thus suggesting caution in the use of this inhibitor. Our findings indicate that although the initiation of Ca2+release at fertilization of mammalian eggs occurs by a pathway that has many similarities to that in evolutionarily earlier animal groups, the requirement for SH2 domain-mediated activation of an SFK is not conserved.

The oxidative burst at fertilization is dependent upon activation of the dual oxidase Udx1

The sea urchin egg is a quiescent cell...until fertilization, when the egg is activated. The classic respiratory burst at fertilization is the result of prodigious hydrogen peroxide production, but the mechanism for this synthesis is not known. Here we quantitate the kinetics of hydrogen peroxide synthesis at a single-cell level using an imaging photon detector, showing that 60 nM hydrogen peroxide accumulates within the perivitelline space of each zygote. We find that the NADPH oxidation activity is enriched at the cell surface and is sensitive to a pharmacological inhibitor of NADPH oxidase enzymes. Finally, we show that a sea urchin dual oxidase homolog, Udx1, is responsible for generating the hydrogen peroxide necessary for the physical block to polyspermy. Phylogenetic analysis of the enzymatic modules in Udx1 suggests a potentially conserved role for the dual oxidase family in hydrogen peroxide production and regulation during fertilization.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

Activation of multidrug efflux transporter activity at fertilization in sea urchin embryos (Strongylocentrotus purpuratus)

This study presents functional and molecular evidence for acquisition of multidrug transporter-mediated efflux activity as a consequence of fertilization in the sea urchin. Sea urchin eggs and embryos express low levels of efflux transporter genes with homology to the multidrug resistance associated protein (mrp) and permeability glycoprotein (p-gp) families of ABC transporters. The corresponding efflux activity is low in unfertilized eggs but is dramatically upregulated within 25 min of fertilization; the expression of this activity does not involve de novo gene expression and is insensitive to inhibitors of transcription and translation indicating activation of pre-existing transporter protein. Our study, using specific inhibitors of efflux transporters, indicates that the major activity is from one or more mrp-like transporters. The expression of activity at fertilization requires microfilaments, suggesting that the transporters are in vesicles and moved to the surface after fertilization. Pharmacological inhibition of mrp-mediated efflux activity with MK571 sensitizes embryos to the toxic compound vinblastine, confirming that one role for the efflux transport activity is embryo protection from xenobiotics. In addition, inhibition of mrp activity with MK571 alone retards mitosis indicating that mrp-like activity may also be required for early cell divisions.

[Establishment and expression of embryonic axes: comparisons between different model organisms]

In an accompanying article (C. Sardet et al. m/s 2004; 20 : 414-423) we reviewed determinants of polarity in early development and the mechanisms which regulate their localization and expression. Such determinants have for the moment been identified in only a few species: the insect Drosophila melanogaster, the worm Caenorhabditis elegans, the frog Xenopus laevis and the ascidians Ciona intestinalis and Holocynthia roretzi. Although oogenesis, fertilization, and cell divisions in these embryos differ considerably, with respect to early polarities certain common themes emerge, such as the importance of cortical mRNAs, the PAR polarity proteins, and reorganizations mediated by the cytoskeleton. Here we highlight similarities and differences in axis establishment between these species, describing them in a chronological order from oocyte to gastrula, and add two more classical model organisms, sea urchin and mouse, to complete the comparisons depicted in the form of a Poster which can be downloaded from the site http://biodev.obs-vlfr.fr/biomarcell.

The cytosolic sperm factor that triggers Ca2+ oscillations and egg activation in mammals is a novel phospholipase C: PLCzeta

When sperm activate eggs at fertilization the signal for activation involves increases in the intracellular free Ca2+ concentration. In mammals the Ca2+ changes at fertilization consist of intracellular Ca2+ oscillations that are driven by the generation of inositol 1,4,5-trisphosphate (InsP3). It is not established how sperm trigger the increases in InsP3 and Ca2+ at fertilization. One theory suggests that sperm initiate signals to activate the egg by introducing a specific factor into the egg cytoplasm after membrane fusion. This theory has been mainly based upon the observation that injecting a cytosolic sperm protein factor into eggs can trigger the same pattern of Ca2+ oscillations induced by the sperm. We have recently shown that this soluble sperm factor protein is a novel form of phospholipase C (PLC), and it is referred to as PLCzeta(zeta). We describe the evidence that led to the identification of PLCzeta and discuss the issues relating to its potential role in fertilization.

Are Src family kinases involved in cell cycle resumption in rat eggs?

The earliest visible indications for the transition to embryos in mammalian eggs, known as egg activation, are cortical granules exocytosis (CGE) and resumption of meiosis (RM); these events are triggered by the fertilizing spermatozoon through a series of Ca2+ transients. The pathways, within the egg, leading to the intracellular Ca2+ release and to the downstream cellular events, are currently under intensive investigation. The involvement of Src family kinases (SFKs) in Ca2+ release at fertilization is well supported in marine invertebrate eggs but not in mammalian eggs. In a previous study we have shown the expression and localization of Fyn, the first SFK member demonstrated in the mammalian egg. The purpose of the current study was to identify other common SFKs and resolve their function during activation of mammalian eggs. All three kinases examined: Fyn, c-Src and c-Yes are distributed throughout the egg cytoplasm. However, Fyn and c-Yes tend to concentrate at the egg cortex, though only Fyn is localized to the spindle as well. The different localizations of the various SFKs imply the possibility of their different functions within the egg. To examine whether SFKs participate in the signal transduction pathways during egg activation, we employed selective inhibitors of the SFKs activity (PP2 and SU6656). The results demonstrate that RM, which is triggered by Ca2+ elevation, is an SFK-dependent process, while CGE, triggered by either Ca2+ elevation or protein kinase C (PKC), is not. The possible involvement of SFKs in the signal transduction pathways that lead from the sperm-egg fusion site downstream of the Ca2+ release remains unclear.

Identification of a starfish egg PLC-gamma that regulates Ca2+ release at fertilization

At fertilization, eggs undergo a cytoplasmic free Ca2+ rise, which is necessary for stimulating embryogenesis. In starfish eggs, studies using inhibitors designed against vertebrate proteins have shown that this Ca2+ rise requires an egg Src family kinase (SFK) that directly or indirectly activates phospholipase C-gamma (PLC-gamma) to produce IP3, which triggers Ca2+ release from the egg's endoplasmic reticulum (ER) [reviewed in Semin. Cell Dev. Biol. 12 (2001) 45]. To examine in more detail the endogenous factors in starfish eggs that are required for Ca2+ release at fertilization, an oocyte cDNA encoding PLC-gamma was isolated from the starfish Asterina miniata. This cDNA, designated AmPLC-gamma, encodes a protein with 49% identity to mammalian PLC-gamma1. A 58-kDa Src family kinase interacted with recombinant AmPLC-gamma Src homology 2 (SH2) domains in a specific, fertilization-responsive manner. Immunoprecipitations of sea urchin egg PLC-gamma using an affinity-purified antibody directed against AmPLC-gamma revealed fertilization-dependent phosphorylation of PLC-gamma. Injecting starfish eggs with the tandem SH2 domains of AmPLC-gamma (which inhibits PLC-gamma activation) specifically inhibited Ca2+ release at fertilization. These results indicate that an endogenous starfish egg PLC-gamma interacts with an egg SFK and mediates Ca2+ release at fertilization via a PLC-gamma SH2 domain-mediated mechanism.

Molecular dissection of egg fertilization signaling with the aid of tyrosine kinase-specific inhibitor and activator strategies

Fertilization is triggered by sperm-egg interaction and fusion that initiate a transient rise(s) in the free intracellular calcium ([Ca(2+)](i)) that is responsible for a series of biochemical and cell biological events, so-called "egg activation". Calcium-dependent egg activation leads to the initiation of developmental program that culminates in the birth of individuals. A growing body of knowledge has uncovered the molecular mechanisms underlying sperm-induced transient [Ca(2+)](i) increase(s) to some extent; namely, in most animals so far studied, a second messenger inositol 1,4,5-trisphosphate (IP(3)) seems to play a pivotal role in inducing [Ca(2+)](i) transient(s) at fertilization. However, signaling mechanisms used by sperm to initiate IP(3)-[Ca(2+)](i) transient pathway have not been elucidated. To approach this problem, we have employed African clawed frog, Xenopus laevis, as a model animal and conducted experiments designed specifically to determine the role of the Src family protein-tyrosine kinases (SFKs or Src family PTKs) in the sperm-induced egg activation. This review compiles information about the use of PTK-specific inhibitors and activators for analyzing signal transduction events in egg fertilization. Specifically, we focus on molecular identification of Xenopus Src and the signaling mechanism of the Src-dependent egg activation that has been established recently. We also summarize recent advances in understanding the role of the Src family kinases in egg fertilization of other model organisms, and discuss future directions of the field.

The charged milieu: a major player in fertilization reactions

In previous studies, we have found that negatively charged, but not uncharged, amino acids and sugars block sea urchin fertilization. These studies were developed from modeling work in non-living systems using derivatized agarose beads that suggested that charge-charge bonding may control at least some adhesive interactions. In the present study, the effects of positively charged, negatively charged and uncharged molecules were examined in the sea urchin sperm-egg system in over 300 individual trials. The results indicate that depending on the specific molecules utilized, both sperm and egg are exquisitely sensitive to charged but not uncharged molecules and to pH changes in sea water caused by some of the charged molecules. It is shown that egg activation, as well as sperm motility and sperm-egg interactions, can be affected by charged molecules. One compound, fructose-1-phosphate blocked fertilization in S. purpuratus sea urchins but not in Lytechinus pictus sea urchins. These findings indicate that charge alone cannot explain all the results. In this case, the presence of a ketone instead of an aldehyde group indicates that species-specific components may control fertilization reactions. The present study is a comprehensive survey of the effects of charge, pH and molecular structure on the fertilization activation continuum in a model system of sea urchins.

MAP kinases regulate unfertilized egg apoptosis and fertilization suppresses death via Ca2+signaling

The default fate for eggs from many species is death by apoptosis and thus, successful fertilization depends upon suppression of the maternal death program. Little is known about the molecular triggers which activate this process or how the fertilization signal suppresses the default maternal apoptotic pathway. The MAP kinase (MAPK) family member, ERK, plays a universal and critical role in several stages of oocyte meiotic maturation, and fertilization results in ERK inactivation. In somatic cells, ERK and other MAPK family members, p38 and JNK, provide opposing signals to regulate apoptosis, however, it is not known whether MAPKs play a regulatory role in egg apoptosis, nor whether suppression of apoptosis by fertilization is mediated by MAPK activity. Here we demonstrate that MAPKs are involved in starfish egg apoptosis and we investigate the relationship between the fertilization induced signaling pathway and MAPK activation. ERK is active in post-meiotic eggs just until apoptosis onset and then p38, JNK and a third kinase are activated, and remain active through execution. Sequential activation of ERK and p38 is necessary for apoptosis, and newly synthesized proteins are required both upstream of ERK and downstream of p38 for activation of the full apoptotic program. Fertilization causes a dramatic rise in intracellular Ca2+, and we report that Ca2+ provides a necessary and sufficient pro-survival signal. The Ca2+ pathway following fertilization of both young and aged eggs causes ERK to be rapidly inactivated, but fertilization cannot rescue aged eggs from death, indicating that ERK inactivation is not sufficient to suppress apoptosis.

Echinoderm eggs and embryos: procurement and culture

The protocols outlined here hopefully will provide researchers with healthy, beautiful echinoderm oocytes, eggs, and embryos for experimental use. The large size of echinoderm oocytes and eggs, the ease with which they can be manipulated, and (in many species) their optical clarity, make them an ideal model system for studying not only the events specific to oocyte maturation and fertilization, but also for investigating more general questions regarding cell cycle regulation in an in vivo system. The quick rate at which development proceeds after fertilization to produce transparent embryos and larva makes the echinoderm an advantageous organism for studying deuterostome embryogenesis. Continued use of the echinoderms as model systems will undoubtedly uncover exciting answers to questions regarding fertilization, cell cycle regulation, morphogenesis, and how developmental events are controlled.

Phospholipase C-dependent Ca2+ release by worm and mammal sperm factors

Egg activation in all animals evidently requires the synthesis of inositol 1,4,5-trisphosphate (InsP(3)) from phosphatidylinositol 4,5-bisphosphate (PIP(2)) by phospholipase C (PLC). Depending on the organism, InsP(3) elicits either calcium oscillations or a single wave, which in turn initiates development. A soluble component in boar sperm that activates mammalian eggs has been suggested to be a PLC isoform. We tested this hypothesis in vitro using egg microsomes of Chaetopterus. Boar sperm factor elicited Ca(2+) release from the microsomes by an InsP(3)-dependent mechanism. The PLC inhibitor U-73122, but not its inactive analog U-73343, blocked the response to sperm factor but not to InsP(3). U-73122 also inhibited the activation of fertilized and parthenogenetic eggs. Chaetopterus sperm also contained a similar activity. These results strongly support the hypothesis that sperm PLCs are ubiquitous mediators of egg activation at fertilization.

Function of a sea urchin egg Src family kinase in initiating Ca2+ release at fertilization

Egg activation at fertilization requires the release of Ca(2+) from the egg's endoplasmic reticulum, and recent evidence has indicated that a Src family kinase (SFK) may function in initiating this signaling pathway in echinoderm eggs. Here, we identify and characterize a SFK from the sea urchin Strongylocentrotus purpuratus, SpSFK1. SpSFK1 RNA is present in eggs, and an antibody made against a SpSFK1 peptide recognizes an approximately 58-kDa egg membrane-associated protein in eggs of S. purpuratus as well as another sea urchin Lytechinus variegatus. Injection of both species of sea urchin eggs with dominant-interfering Src homology 2 domains of SpSFK1 delays and reduces the release of Ca(2+) at fertilization. Injection of an antibody against SpSFK1 into S. purpuratus eggs also causes a small increase in the delay between sperm-egg fusion and Ca(2+) release. In contrast, when injected into eggs of L. variegatus, this same antibody has a dramatic stimulatory effect: it causes PLCgamma-dependent Ca(2+) release like that occurring at fertilization. Correspondingly, in lysates of L. variegatus eggs, but not S. purpuratus eggs, the antibody stimulates SFK activity. Injection of L. variegatus eggs with another antibody that recognizes the L. variegatus egg SFK also causes PLCgamma-dependent Ca(2+) release like that at fertilization. These results indicate that activation of a Src family kinase present in sea urchin eggs is necessary to cause Ca(2+) release at fertilization and is capable of stimulating Ca(2+) release in the unfertilized egg via PLCgamma, as at fertilization.

Molecular mechanisms utilized by alternative c-kit gene products in the control of spermatogonial proliferation and sperm-mediated egg activation

The c-kit proto-oncogene plays a dual role in the control of male fertility in mice through two alternative gene products: (1). c-kit [the transmembrane tyrosine kinase receptor for stem cell factor (SCF)], which is expressed and functional in differentiating spermatogonia of the postnatal testis, in which c-kit is essential for pre-meiotic proliferation; and (2). tr-kit, an intracellular protein which is specifically accumulated during spermiogenesis through the use of an alternative intronic promoter, and which is able to trigger mouse egg activation when microinjected into the cytoplasm of metaphase II arrested oocytes. Here, we summarize the most recent findings about the molecular pathways through which c-kit regulates cell cycle progression in mitotic germ cells, and those through which sperm-derived tr-kit triggers parthenogenetic completion of meiosis II and pronuclear formation in microinjected mouse eggs.

Calcium wave pacemakers in eggs

During the past 25 years, the characterization of sperm-triggered calcium signals in eggs has progressed from the discovery of a single calcium increase at fertilization in the medaka fish to the observation of repetitive calcium waves initiated by multiple meiotic calcium wave pacemakers in the ascidian. In eggs of all animal species, sperm-triggered inositol (1,4,5)-trisphosphate [Ins(1,4,5)P(3)] production regulates the vast array of calcium wave patterns observed in the different species. The spatial organization of calcium waves is driven either by the intracellular distribution of the calcium release machinery or by the localized and dynamic production of calcium-releasing second messengers. In the highly polarized egg cell, cortical endoplasmic reticulum (ER)-rich clusters act as pacemaker sites dedicated to the initiation of global calcium waves. The extensive ER network made of interconnected ER-rich domains supports calcium wave propagation throughout the egg. Fertilization triggers two types of calcium wave pacemakers depending on the species: in mice, the pacemaker site in the vegetal cortex of the egg is probably a site that has enhanced sensitivity to Ins(1,4,5)P(3); in ascidians, the calcium wave pacemaker may rely on a local source of Ins(1,4,5)P(3) production apposed to a cluster of ER in the vegetal cortex.

Egg activation at fertilization: where it all begins

A centrally important factor in initiating egg activation at fertilization is a rise in free Ca(2+) in the egg cytosol. In echinoderm, ascidian, and vertebrate eggs, the Ca(2+) rise occurs as a result of inositol trisphosphate-mediated release of Ca(2+) from the endoplasmic reticulum. The release of Ca(2+) at fertilization in echinoderm and ascidian eggs requires SH2 domain-mediated activation of a Src family kinase (SFK) and phospholipase C (PLC)gamma. Though some evidence indicates that a SFK and PLC may also function at fertilization in vertebrate eggs, SH2 domain-mediated activation of PLC gamma appears not to be required. Much work has focused on identifying factors from sperm that initiate egg activation at fertilization, either as a result of sperm-egg contact or sperm-egg fusion. Current evidence from studies of ascidian and mammalian fertilization favors a fusion-mediated mechanism; this is supported by experiments indicating that injection of sperm extracts into eggs causes Ca(2+) release by the same pathway as fertilization.

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

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

Postmeiotic unfertilized starfish eggs die by apoptosis

Fertilization of starfish eggs during meiosis results in rapid progression to embryogenesis as soon as meiosis II is completed. Unfertilized eggs complete meiosis and arrest in postmeiotic interphase for an, until now, indeterminate time. If they remain unfertilized, the mature postmeiotic eggs ultimately die. The aim of this study is to characterize the mechanism of death in postmeiotic unfertilized starfish eggs. We report that, in two species of starfish, in the absence of fertilization, postmeiotic interphase arrest persists for 16-20 h, after which time the cells synchronously and rapidly die. Dying eggs extrude membrane blebs, undergo cytoplasmic contraction and darkening, and fragment into vesicles in a manner reminiscent of apoptotic cells. The DNA of dying eggs is condensed, fragmented, and labeled by the TUNEL assay. Taken together, these data suggest that the default fate of postmeiotic starfish eggs, like their mammalian counterparts, is death by apoptosis. We further report that the onset and execution of apoptosis in this system is dependent on ongoing protein synthesis and is inhibited by a rise in intracellular Ca(2+), an essential component of the fertilization signaling pathway. We propose starfish eggs as a useful model to study developmentally regulated apoptosis.

Evidence that phospholipase C from the sperm is not responsible for initiating Ca(2+) release at fertilization in mouse eggs

Release of Ca(2+) from intracellular stores at fertilization of mammalian eggs is mediated by inositol 1,4,5-trisphosphate (IP3), but the mechanism by which the sperm initiates IP3 production is not yet understood. We tested the hypothesis that phospholipase C (PLC) activity introduced into the mouse egg as a consequence of sperm-egg fusion is responsible for causing Ca(2+) release. We demonstrated that microinjecting purified, recombinant PLCgamma1 protein into mouse eggs caused Ca(2+) oscillations like those seen at fertilization. However, the PLC activity in the minimum amount of purified PLCgamma1 protein needed to elicit Ca(2+) release when injected into eggs was approximately 500-900 times the PLC activity contained in a single sperm. This indicates that a single mouse sperm does not contain enough PLC activity to be responsible for causing Ca(2+) release at fertilization. We also examined whether phosphatidylinositol 3-kinase (PI3K) could have a role in this process, and found that several inhibitors of PI3K-mediated signaling had no effect on Ca(2+) release at fertilization.

Ovulation triggers activation of Drosophila oocytes

Drosophila melanogaster mature oocytes in ovaries are arrested at metaphase I of meiosis. Eggs that have reached the uterus have released this arrest. It was not known where in the female reproductive tract egg activation occurs and what triggers it. We investigated when and where the egg is activated in Drosophila in vivo and at what meiotic stage the egg is fertilized. We found that changes in the egg's envelope's permeability, one feature of activation, initiate during ovulation, even while most of the egg is still within the ovary. The egg becomes impermeable as it proceeds down the oviducts; the process is complete by the time the egg is in the uterus. Cross-linking of vitelline membrane protein sV23 also increases progressively as the egg moves through the oviducts and the uterus. Activation also triggers meiosis to resume before the egg reaches the uterus, such that the earliest eggs that reach the uterus are in anaphase I. We discuss models for Drosophila egg activation in vivo.