Calcium release at fertilization in starfish eggs is mediated by phospholipase Cgamma

Calcium signaling in oocyte quality and functionality and its application

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

Starfish oocytes of A. pectinifera reveal marked differences in sperm-induced electrical and intracellular calcium changes during oocyte maturation and at fertilization

Although changes in membrane potential and intracellular Ca²⁺ (Ca_i ) during fertilization in starfish oocytes have been known for long time, little is known precisely about how and what kind of channels are involved during oocyte maturation and in fertilization, and how the mechanisms of changes in Ca_i in oocytes develop during oocyte maturation. Since in starfish, oocyte maturation-inducing hormone, 1-methyladenine (1MA) is well known, we took advantage of it to investigate the developmental process of channel-function and changes in Ca_i in three different developmental stages using 1MA. Sperm-induced membrane current at voltage clamp and changes in Ca_i in starfish oocytes, Asterina pectinifera, were examined in stages of immature, partly mature (a state in 15-20 min after sufficient concentration, 1 µM of 1MA addition, or 30-40 min exposure to subthreshold concentration of 1MA), and mature oocytes (MO). We found some immature and many partly MOs showed fluctuating responses in membrane current, membrane potential, and corresponding changes in Ca_i , which are distinct from those in MOs. The responses in immature and partly MOs indicate physiologically characteristic responses of insufficient changes in Ca_i and its corresponding electrical responses at the partial developmental stage during maturation. Our data should shed light on the mechanism of egg activation and oocyte maturation in terms of examining membrane current and corresponding changes in Ca_i .

Identification of SH2 Domain-Mediated Protein Interactions that Operate at Fertilization in the Sea Star Patiria miniata

The signaling mechanisms controlling internal calcium release at fertilization in animals are still largely unknown. Echinoderms, such as the sea star Patiria miniata, produce abundant and easily accessible sperm and eggs. In addition, eggs are naturally synchronized at the same cell cycle stage, collectively making these animals an attractive model to study the signaling proteins controlling fertilization. However, the lack of antibodies to identify proteins in this model system has slowed progress in identifying key signaling molecules. With the advances in mass spectrometry, we present a method for identifying tyrosine phosphorylated proteins binding to GST-tagged SH2 domains in sea star cell lysates for downstream mass spectrometry analysis.

Phospholipase Cζ (PLCζ) versus postacrosomal sheath WW domain-binding protein (PAWP): Which molecule will survive as a sperm factor?

During mammalian fertilization, sperm is fused with the oocyte's membrane, triggering the resumption of meiosis from the metaphase II arrest, the extrusion of the second polar body, and the exocytosis of cortical granules; these events are collectively called 'oocyte activation.' In all species studied to date, the transient rise in the cytosolic level of calcium (in particular, the repeated calcium increases called 'calcium oscillations' in mammals) is required for these events. Researchers have focused on identifying the factor(s) that can induce calcium oscillations during fertilization. Sperm‐specific phospholipase C, i.e., PLC zeta (PLCζ), is a strong candidate of the factor(s), and several research groups using different species obtained evidence that PLCζ is a sperm factor that can induce calcium oscillations during fertilization. However, postacrosomal sheath Tryptophan‐Tryptophan (WW)—domain‐binding protein (PAWP) was recently shown to have a pivotal role in inducing calcium oscillations in some species. In this review, we focus on PLCζ and PAWP as sperm factors, and we discuss this controversy: Which of these two molecules survives as a sperm factor?

Modulators of calcium signalling at fertilization

Calcium (Ca2+) signals initiate egg activation across the animal kingdom and in at least some plants. These signals are crucial for the success of development and, in the case of mammals, health of the offspring. The mechanisms associated with fertilization that trigger these signals and the molecules that regulate their characteristic patterns vary widely. With few exceptions, a major contributor to fertilization-induced elevation in cytoplasmic Ca2+ is release from endoplasmic reticulum stores through the IP3 receptor. In some cases, Ca2+ influx from the extracellular space and/or release from alternative intracellular stores contribute to the rise in cytoplasmic Ca2+. Following the Ca2+ rise, the reuptake of Ca2+ into intracellular stores or efflux of Ca2+ out of the egg drive the return of cytoplasmic Ca2+ back to baseline levels. The molecular mediators of these Ca2+ fluxes in different organisms include Ca2+ release channels, uptake channels, exchangers and pumps. The functions of these mediators are regulated by their particular activating mechanisms but also by alterations in their expression and spatial organization. We discuss here the molecular basis for modulation of Ca2+ signalling at fertilization, highlighting differences across several animal phyla, and we mention key areas where questions remain.

Insemination or phosphatidic acid induces an outwardly spiraling disk of elevated Ca2+ to produce the Ca2+ wave during Xenopus laevis fertilization

During Xenopus fertilization, the initial intracellular calcium ((Ca²⁺)_i) release at the sperm-egg binding site (hot spot) has not been described without the use of inhibitors, nor related to underlying ER structure. Without inhibitors, we now report that sperm induce an initial hot spot after sperm addition to Xenopus eggs that was ~25 µm. This area is consistent with the size of ER patches and clusters of IP3 receptors that have enhanced activity. Furthermore, we find a new mechanism for the fertilization (Ca²⁺)_i wave; instead of outward diffusion of inositol 1,4,5-trisphosphate (IP₃), we find that the wave was generated by an outward, clockwise rotation of a ~63 µm disk of elevated (Ca²⁺)_i moving very rapidly at ~65 µm/s. We also suggest a new mechanism for the acceleration of the fertilization (Ca²⁺)_i wave as the disk accelerated and was joined by other rotating disks (some rotating counterclockwise) at a time when the speed of the (Ca²⁺)_i wave increases. To examine the role of phosphatidic acid (PA) in the release of (Ca²⁺)_i during Xenopus fertilization, we find that two inhibitors of PA production delayed the appearance of fertilization hot spots by ~9-12 min but did not reduce the size of hot spots and actually accelerated the later (Ca²⁺)_i wave. Surprisingly, global addition of PA to Xenopus eggs induced localized hot spots at a time and size that was similar to those induced after sperm addition. In contrast, sperm induce a rapid (Ca²⁺)_i wave (~4 µm/s) within ~30 s after hot spot appearance, whereas hot spots induced by PA required an ~32 min to induce a very slow (~1 µm/s) (Ca²⁺)_i wave with a lower peak of (Ca²⁺)_i. Thus, PA may not be required for the initial release of (Ca²⁺)_i at the sperm-egg binding site, but mimics sperm by inducing a similarly sized localized (Ca²⁺)_i release. As compared with sperm, PA may induce a weak, slow (Ca²⁺)_i wave by slowly increasing IP3 receptor clustering. Addition of PA to Xenopus oocytes, or Ca²⁺ ionophore to either Xenopus oocytes or eggs, did not induce hot spots but a global (Ca²⁺)_i wave that rapidly moved at ~12 µm/s.

The eggstraordinary story of how life begins

More than 15 years have elapsed since the identification of phospholipase C ζ1 (PLCζ) from a genomic search for mouse testis/sperm-specific PLCs. This molecule was proposed to represent the sperm factor responsible for the initiation of calcium (Ca²⁺ ) oscillations required for egg activation and embryo development in mammals. Supporting evidence for this role emerged from studies documenting its expression in all mammals and other vertebrate species, the physiological Ca²⁺ rises induced by injection of its messenger RNA into mammalian and nonmammalian eggs, and the lack of expression in infertile males that fail intracytoplasmic sperm injection. In the last year, genetic animal models have added support to its role as the long sought-after sperm factor. In this review, we highlight the findings that demonstrated the role of Ca²⁺ as the universal signal of egg activation and the experimental buildup that culminated with the identification of PLCζ as the soluble sperm factor. We also discuss the structural-functional properties that make PLCζ especially suited to evoke oscillations in eggs. Lastly, we examine unresolved aspects of the function and regulation of PLCζ and whether or not it is the only sperm factor in mammalian sperm.

The role of Src family kinases in starfish egg fertilisation

A common early feature in the activation of all eggs during fertilisation is an increase in the level of intra-cellular free calcium (Ca2+) that, in most species, propagates as a wave across the egg (reviewed in Strieker, 1999). In echinoderms, this Ca2+ release is the result of a signal transduction cascade that requires phospholipase Cγ (PLCγ)-mediated production of inositol trisphosphate (IP3) (Carroll et al., 1997, 1999). PLCγ is most commonly regulated by tyrosine phosphorylation (Rhee & Bae, 1997), indicating that a tyrosine kinase is a likely upstream regulator of PLCγ enzymatic activity at fertilisation. In support of this hypothesis, an increase in tyrosine kinase activity and an increase in tyrosine-phosphorylated proteins at fertilisation has been observed in echinoderm eggs (Satoh & Garbers, 1985; Ciapa & Epel, 1991; Kinsey, 1997). Moreover, the tyrosine kinase inhibitors genistein (Shen et al., 1999) and PP1 (Abassi et al., 2000) have been used to show that in sea urchin eggs a tyrosine kinase activity is required for normal Ca2+ release in response to fertilisation.

In eggs of the starfish Asterina miniata, a Src-type tyrosine kinase has been identified as a potential regulator of PLCγ activity at fertilisation (Giusti et al., 1999a). This kinase exhibits a rapid fertilisation-dependent association specifically with the Src Homology 2 (SH2) domains of PLCγ. Moreover, the timing of this association correlates with an increase in the tyrosine kinase activity bound to the PLCγ SH2 domains, and neither the Src kinase nor the associated kinase activity was observed to associate with the PLCγ SH2 domains after treating eggs with the calcium ionophore A23187 (Giusti et al., 1999a). These data identify an egg Src family kinase as a potential upstream regulator of PLCγ during starfish egg fertilisation.

Starfish as a Model System for Analyzing Signal Transduction During Fertilization

The starfish oocyte and egg offer advantages for use as a model system for signal transduction research. Some of these have been recognized for over a century, including the ease of procuring gametes, in vitro fertilization, and culturing the embryos. New advances, particularly in genomics, have also opened up opportunities for the use of these animals. In this chapter, we give a few examples of the historical use of the starfish for research in cell biology and then describe some new areas in which we believe the starfish can contribute to our understanding of signal transduction-particularly in fertilization.

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.

Sperm-induced Ca2+ release during egg activation in mammals

This review discusses the role that the sperm-specific phospholipase C zeta (PLCζ) is proposed to play during the fertilization of mammalian eggs. At fertilization, the sperm initiates development by causing a series of oscillations in cytosolic concentrations of calcium [Ca(2)] within the egg. PLCζ mimics the sperm at fertilization, causing the same pattern of Ca(2+) release as seen at fertilization. Introducing PLCζ into mouse eggs also mimics a number of other features of the way in which the fertilizing sperm triggers Ca(2+) oscillations. We discuss the localization of PLCζ within the egg and present a hypothesis about the localization of PLCζ within the sperm before the initiation of fertilization.

Biotinylation of oocyte cell surface proteins of the starfish Patiria miniata

Understanding the signal transduction processes that occur during oocyte maturation and fertilization requires knowledge of the constituent proteins from the cell surface to relevant intracellular compartments. To identify starfish oocyte and egg cell surface proteins, a biotinylation method was adapted from prior protocols using B cells, leukocytes, mouse oocytes, and sea urchin eggs (Cole et al. Mol Immunol 24:699-705, 1987; Flaherty and Swann NJ. Mol Reprod Dev 35:285-292, 1993; Haley and Wessel. Dev Biol 272:191-202, 2004; Hurley et al. J Immunol Methods 85:195-202, 1985). This method utilizes the water-soluble Sulfo-NHS-Biotin, which does not cross the egg plasma membrane. The process of biotinylation does not appear to have any effect on the process of oocyte maturation or fertilization. Furthermore, it can be used with either vitelline-intact or vitelline-free oocytes and allows the proteins to be visualized successfully through immunoblotting, immunoprecipitation, or by scanning confocal microscopy.

Uncovering the genetic basis for early isogamete differentiation: a case study of Ectocarpus siliculosus

BACKGROUND

The phenomenon of sexual reproduction characterizes nearly all eukaryotes, with anisogamy being the most prevalent form of gamete discrimination. Since dimorphic gametes most likely descend from equal-sized specialized germ cells, identifying the genetic bases of the early functional diversification in isogametes can provide better understanding of the evolution of sexual dimorphism. However, despite the potential importance to the evolutionary biology field, no comprehensive survey of the transcriptome profiling in isomorphic gametes has been reported hitherto.

RESULTS

Gamete differentiation on the genomic level was investigated using Ectocarpus siliculosus, a model organism for brown algal lineage which displays an isogamous sexual reproduction cycle. Transcriptome libraries of male and female gametes were generated using Next Generation Sequencing technology (SOLiD) and analyzed to identify differentially regulated genes and pathways with potential roles in fertilization and gamete specialization. Gamete transcriptomes showed a high level of complexity with a large portion of gender specific gene expression. Our results indicate that over 4,000 of expressed genes are differentially regulated between male and female, including sequences related to cell movement, carbohydrate and lipid metabolism, signaling, transport and RNA processing.

CONCLUSIONS

This first comprehensive transcriptomic study of protist isogametes describes considerable adaptation to distinct sexual roles, suggesting that functional anisogamy precedes morphological differentiation. Several sex-biased genes and pathways with a putative role in reproduction were identified, providing the basis for more detailed investigations of the mechanisms underlying evolution of mating types and sexual dimorphism.

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.

Comparative biology of sperm factors and fertilization-induced calcium signals across the animal kingdom

Fertilization causes mature oocytes or eggs to increase their concentrations of intracellular calcium ions (Ca²⁺) in all animals that have been examined, and such Ca²⁺ elevations, in turn, provide key activating signals that are required for non-parthenogenetic development. Several lines of evidence indicate that the Ca²⁺ transients produced during fertilization in mammals and other taxa are triggered by soluble factors that sperm deliver into oocytes after gamete fusion. Thus, for a broad-based analysis of Ca²⁺ dynamics during fertilization in animals, this article begins by summarizing data on soluble sperm factors in non-mammalian species, and subsequently reviews various topics related to a sperm-specific phospholipase C, called PLCζ, which is believed to be the predominant activator of mammalian oocytes. After characterizing initiation processes that involve sperm factors or alternative triggering mechanisms, the spatiotemporal patterns of Ca²⁺ signals in fertilized oocytes or eggs are compared in a taxon-by-taxon manner, and broadly classified as either a single major transient or a series of repetitive oscillations. Both solitary and oscillatory types of fertilization-induced Ca²⁺ signals are typically propagated as global waves that depend on Ca²⁺ release from the endoplasmic reticulum in response to increased concentrations of inositol 1,4,5-trisphosphate (IP₃). Thus, for taxa where relevant data are available, upstream pathways that elevate intraoocytic IP3 levels during fertilization are described, while other less-common modes of producing Ca²⁺ transients are also examined. In addition, the importance of fertilization-induced Ca²⁺ signals for activating development is underscored by noting some major downstream effects of these signals in various animals.

Starting a new life: sperm PLC-zeta mobilizes the Ca2+ signal that induces egg activation and embryo development: an essential phospholipase C with implications for male infertility

We have discovered that a single sperm protein, phospholipase C-zeta (PLCζ), can stimulate intracellular Ca(2+) signalling in the unfertilized oocyte ('egg') culminating in the initiation of embryonic development. Upon fertilization by a spermatozoon, the earliest observed signalling event in the dormant egg is a large, transient increase in free Ca(2+) concentration. The fertilized egg responds to the intracellular Ca(2+) rise by completing meiosis. In mammalian eggs, the Ca(2+) signal is delivered as a train of long-lasting cytoplasmic Ca(2+) oscillations that begin soon after gamete fusion and persist beyond the completion of meiosis. Sperm PLCζ effects Ca(2+) release from egg intracellular stores by hydrolyzing the membrane lipid PIP(2) and consequent stimulation of the inositol 1,4,5-trisphosphate (InsP(3) ) receptor Ca(2+) -signalling pathway, leading to egg activation and early embryogenesis. Recent advances have refined our understanding of how PLCζ induces Ca(2+) oscillations in the egg and also suggest its potential dysfunction as a cause of male infertility.

Gamete membrane microdomains and their associated molecules in fertilization signaling

Fertilization is the fundamental system of biological reproduction in many organisms, including animals, plants, and algae. A growing body of knowledge has emerged to explain how fertilization and activation of development are accomplished. Studies on the molecular mechanisms of fertilization are in progress for a wide variety of multicellular organisms. In this review, we summarize recent findings and debates about the long-standing questions concerning fertilization: how egg and sperm become competent for their interaction with each other, how the binding and fusion of these gamete cells are made possible, and how the fertilized eggs initiate development to a newborn. We will focus on the structure and function of the membrane microdomains (MDs) of egg and sperm that may serve as a platform or signaling center for the aforementioned cellular functions. In particular, we provide evidence that MDs of eggs from the African clawed frog, Xenopus laevis, play a pivotal role in receiving extracellular signals from fertilizing sperm and then transmitting them to the egg cytoplasm, where the tyrosine kinase Src is present and responsible for the subsequent signaling events collectively called egg activation. The presence of a new signaling axis involving uroplakin III, an MD-associated transmembrane protein, and Src in this system will be highlighted and discussed.

Protein tyrosine kinase signaling during oocyte maturation and fertilization

The oocyte is a highly specialized cell capable of accumulating and storing energy supplies as well as maternal transcripts and pre-positioned signal transduction components needed for zygotic development, undergoing meiosis under control of paracrine signals from the follicle, fusing with a single sperm during fertilization, and zygotic development. The oocyte accomplishes this diverse series of events by establishing an array of signal transduction pathway components that include a select collection of protein tyrosine kinases (PTKs) that are expressed at levels significantly higher than most other cell types. This array of PTKs includes cytosolic kinases such as SRC-family PTKs (FYN and YES), and FAK kinases, as well as FER. These kinases typically exhibit distinct patterns of localization and in some cases are translocated from one subcellular compartment to another during meiosis. Significant differences exist in the extent to which PTK-mediated pathways are used by oocytes from species that fertilize externally versus internally. The PTK activation profiles as well as calcium signaling pattern seems to correlate with the extent to which a rapid block to polyspermy is required by the biology of each species. Suppression of each of the SRC-family PTKs as well as FER kinase results in failure of meiotic maturation or zygote development, indicating that these PTKs are important for oocyte quality and developmental potential. Future studies will hopefully reveal the extent to which these factors impact clinical assisted reproductive techniques in domestic animals and humans.

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.

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.

Central Role of Calcium-Dependent Tyrosine Kinase PYK2 in Endothelial Nitric Oxide Synthase–Mediated Angiogenic Response and Vascular Function

Background— The involvement of Ca 2+ -dependent tyrosine kinase PYK2 in the Akt/endothelial NO synthase pathway remains to be determined.

Methods and Results— Blood flow recovery and neovessel formation after hind-limb ischemia were impaired in PYK2 −/− mice with reduced mobilization of endothelial progenitors. Vascular endothelial growth factor (VEGF)–mediated cytoplasmic Ca 2+ mobilization and Ca 2+ -independent Akt activation were markedly decreased in the PYK2-deficient aortic endothelial cells, whereas the Ca 2+ -independent AMP-activated protein kinase/protein kinase-A pathway that phosphorylates endothelial NO synthase was not impaired. Acetylcholine-mediated aortic vasorelaxation and cGMP production were significantly decreased. Vascular endothelial growth factor–dependent migration, tube formation, and actin cytoskeletal reorganization associated with Rac1 activation were inhibited in PYK2-deficient endothelial cells. PI3-kinase is associated with vascular endothelial growth factor–induced PYK2/Src complex, and inhibition of Src blocked Akt activation. The vascular endothelial growth factor–mediated Src association with PLCγ1 and phosphorylation of 783 Tyr-PLCγ1 also were abolished by PYK2 deficiency.

Conclusion— These findings demonstrate that PYK2 is closely involved in receptor- or ischemia-activated signaling events via Src/PLCγ1 and Src/PI3-kinase/Akt pathways, leading to endothelial NO synthase phosphorylation, and thus modulates endothelial NO synthase–mediated vasoactive function and angiogenic response.

Ca2+ signaling differentiation during oocyte maturation

Oocyte maturation is an essential cellular differentiation pathway that prepares the egg for activation at fertilization leading to the initiation of embryogenesis. An integral attribute of oocyte maturation is the remodeling of Ca2+ signaling pathways endowing the egg with the capacity to produce a specialized Ca2+ transient at fertilization that is necessary and sufficient for egg activation. Consequently, mechanistic elucidation of Ca2+ signaling differentiation during oocyte maturation is fundamental to our understanding of egg activation, and offers a glimpse into Ca2+ signaling regulation during the cell cycle.

Flipping the switch: How a sperm activates the egg at fertilization

Sperm interaction with an egg in animals was first documented 160 years ago in sea urchins by Alphonse Derbès (1847) when he noted the formation of an "envelope" following the sperm's "approach" to the egg. The "envelope" in sea urchins is an obvious phenotype of fertilization in this animal and over the past 35 years has served to indicate a presence of calcium released from cytoplasmic stores essential to activate the egg. The mechanism of calcium release has been intensely studied because it is a universal regulator of cellular activity, and recently several intersecting pathways of calcium release have been defined. Here we examine these various mechanisms with special emphasis on recent work in eggs of both sea urchins and mice.

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.

Possible role of calcium on oocyte development after intracytoplasmic sperm injection in quail (Coturnix japonica)

Although a rise in intracellular calcium concentration of vertebrate oocytes plays a pivotal role for the initiation of fertilization or oocyte activation, no study on this subject has been reported in birds. This study was conducted to study the role of intracellular calcium in relation to fertilization in avian oocytes. First, immediately after a quail oocyte was injected with a sperm, it was treated with strontium chloride as an inducer for intracellular calcium rise at doses of 0, 2.5, 5, 7.5, 10 mM for 4 hr in the culture medium and was followed by 20-hr culture. Treatment with 5 mM of strontium chloride induced blastodermal development in 24.2% of injected eggs, although no oocytes developed without strontium treatment. Second, quail oocytes were injected with a sperm and 0.1 M calcium chloride or a sperm and saline solution, cultured without calcium for 4 hr and was followed by 20-hr culture without strontium. The calcium solution induced blastodermal development in 20.5% of the oocytes, although no oocytes developed without calcium treatment. Third, quail oocytes were injected with 1,2-bis (o-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA) as a calcium chelator, cultured with strontium (5 mM) for 4 hr followed by 20-hr culture without strontium. Only one oocyte developed after BAPTA and strontium treatment of 36 oocytes examined. Developmental stages of all the oocytes ranged from IV to VII. These results suggest that intracellular calcium rise may participate in quail oocyte activation and allow fertilization and blastodermal development.

A functional genomic and proteomic perspective of sea urchin calcium signaling and egg activation

The sea urchin egg has a rich history of contributions to our understanding of fundamental questions of egg activation at fertilization. Within seconds of sperm-egg interaction, calcium is released from the egg endoplasmic reticulum, launching the zygote into the mitotic cell cycle and the developmental program. The sequence of the Strongylocentrotus purpuratus genome offers unique opportunities to apply functional genomic and proteomic approaches to investigate the repertoire and regulation of Ca(2+) signaling and homeostasis modules present in the egg and zygote. The sea urchin "calcium toolkit" as predicted by the genome is described. Emphasis is on the Ca(2+) signaling modules operating during egg activation, but the Ca(2+) signaling repertoire has ramifications for later developmental events and adult physiology as well. Presented here are the mechanisms that control the initial release of Ca(2+) at fertilization and additional signaling components predicted by the genome and found to be expressed and operating in eggs at fertilization. The initial release of Ca(2+) serves to coordinate egg activation, which is largely a phenomenon of post-translational modifications, especially dynamic protein phosphorylation. Functional proteomics can now be used to identify the phosphoproteome in general and specific kinase targets in particular. This approach is described along with findings to date. Key outstanding questions regarding the activation of the developmental program are framed in the context of what has been learned from the genome and how this knowledge can be applied to functional studies.

When a sperm meets an egg: block to polyspermy

Embryonic development is initiated after the fertilizing spermatozoon enters the egg and triggers a series of events known as egg activation. Activation results in an increase in intracellular calcium concentration, cortical granule exocytosis (CGE), cell cycle resumption and recruitment of maternal mRNA. CGE is an evolutionary developed mechanism that causes modification of the zona pellucida to prevent penetration of additional spermatozoa, ensuring successful egg activation and embryo development. The egg CGE is a unique and convenient mammalian model for studying the different proteins participating at the membrane fusion cascade, which, unlike other secretory cells, occurs only once in the egg's lifespan. This article highlights a number of proteins, ascribed to participate in CGE and thus the block to polyspermy. CGE can be triggered either by a calcium dependent pathway, or via protein kinase C (PKC) activation that requires a very low calcium concentration. In a recent study, we suggested that the filamentous actin (F-actin) at the egg's cortex is a dynamic network. It can be maneuvered towards allowing CGE by activated actin associated proteins and/or by activated PKC and its down stream proteins, such as myristoylated alanine-rich C kinase substrate (MARCKS). MARCKS, a protein known to cross-link F-actin in other cell types, was found to be expressed and colocalized with actin in non-activated MII eggs. We further demonstrated MARCKS dissociation from actin after activation by ionomycin, a process that can lead to the breakdown of the actin network, thus allowing CGE. The more we know of the intricate process of CGE and of the proteins participating in it, the more the assisted reproductive procedures might benefit from that knowledge.

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.

Functional, biochemical, and chromatographic characterization of the complete [Ca2+]i oscillation-inducing activity of porcine sperm

A cytosolic sperm protein(s), referred to as sperm factor (SF), is delivered into eggs by the sperm during mammalian fertilization to induce repetitive increases in the intracellular concentration of free Ca2+ ([Ca2+]i) that are referred to as [Ca2+]i oscillations. [Ca2+]i oscillations are essential for egg activation and early embryonic development. Recent evidence shows that the novel sperm-specific phospholipase C (PLC), PLCzeta, may be the long sought after [Ca2+]i oscillation-inducing SF. Here, we demonstrate the complete extraction of SF from porcine sperm and show that regardless of the method of extraction a single molecule/complex appears to be responsible for the [Ca2+]i oscillation-inducing activity of these extracts. Consistent with this notion, all sperm fractions that induced [Ca2+]i oscillations, including FPLC-purified fractions, exhibited high in vitro PLC activity at basal Ca2+ levels (0.1-5 microM), a hallmark of PLCzeta. Notably, we detected immunoreactive 72-kDa PLCzeta in an inactive fraction, and several fractions capable of inducing oscillations were devoid of 72-kDa PLCzeta. Nonetheless, in the latter fractions, proteolytic fragments, presumably corresponding to cleaved forms of PLCzeta, were detected by immunoblotting. Therefore, our findings corroborate the hypothesis that a sperm-specific PLC is the main component of the [Ca2+]i oscillation-inducing activity of sperm but provide evidence that the presence of 72-kDa PLCzeta does not precisely correspond with the Ca2+ releasing activity of porcine sperm fractions.

NAADP and InsP3 play distinct roles at fertilization in starfish oocytes

NAADP participates in the response of starfish oocytes to sperm by triggering the fertilization potential (FP) through the activation of a Ca2+ current which depolarizes the membrane to the threshold of activation of the voltage-gated Ca2+ channels. The aim of this study was to investigate whether this Ca2+ influx is linked to the onset of the concomitant InsP3-mediated Ca2+ wave by simultaneously employing Ca2+ imaging and single-electrode intracellular recording techniques. In control oocytes, the sperm-induced membrane depolarization always preceded by a few seconds the onset of the Ca2+ wave. Strikingly, the self-desensitization of NAADP receptors either abolished the Ca2+ response or resulted in abnormal oocyte activation, i.e., the membrane depolarization followed the Ca2+ wave and the oocyte was polyspermic. The inhibition of InsP3 signaling only impaired the propagation of the Ca2+ wave and shortened the FP. The duration of FP was also reduced in low-Na+ sea water. Finally, uncaged InsP3 produced a Ca2+ increase, which depolarized the membrane upon the activation of a Ca2+-sensitive cation current. These results support the hypothesis that Ca2+ entry during the NAADP-triggered FP is required for the onset of the Ca2+ wave at fertilization. The InsP3-mediated Ca2+ wave, in turn, may interact with the NAADP-evoked depolarization by activating a Ca2+-dependent Na+ entry.

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.

Mechanisms of Ca2+ liberation at fertilization

The mechanisms underlying the Ca2+ release at fertilization of several animal organisms are reported. Four main classical theories are described, i.e., that of Ca2+ release following simple sperm contact and a G protein stimulation; that of simple sperm contact followed by a tyrosine kinase receptor activation; that of the necessity of introduction by sperm into the egg of molecules for Ca2+ release; and that the molecule introduced into the marine eggs for Ca2+ release is the same Ca2+. Two other mechanisms for Ca2+ release are also illustrated: that of ryanodine receptor stimulation and that of NAADP formation.

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.

Kinase activation through dimerization by human SH2-B

The isoforms of SH2-B, APS, and Lnk form a family of signaling proteins that have been described as activators, mediators, or inhibitors of cytokine and growth factor signaling. We now show that the three alternatively spliced isoforms of human SH2-B readily homodimerize in yeast two-hybrid and cellular transfections assays, and this is mediated specifically by a unique domain in its amino terminus. Consistent with previous reports, we further show that the SH2 domains of SH2-B and APS bind JAK2 at Tyr813. These findings suggested a model in which two molecules of SH2-B or APS homodimerize with their SH2 domains bound to two JAK2 molecules, creating heterotetrameric JAK2-(SH2-B)2-JAK2 or JAK2-(APS)2-JAK2 complexes. We further show that APS and SH2-B isoforms heterodimerize. At lower levels of SH2-B or APS expression, dimerization approximates two JAK2 molecules to induce transactivation. At higher relative concentrations of SH2-B or APS, kinase activation is blocked. SH2-B or APS homodimerization and SH2-B/APS heterodimerization thus provide direct mechanisms for activating and inhibiting JAK2 and other kinases from the inside of the cell and for potentiating or attenuating cytokine and growth factor receptor signaling when ligands are present.

NAADP triggers the fertilization potential in starfish oocytes

In invertebrates oocytes or eggs, the fertilization or activation potential establishes the fast electrical block to polyspermy and, in some species, provides the Ca2+ influx which contributes to the following intracellular Ca2+ wave. In echinoderms, the molecule triggering the activation potential is still unknown. The aim of this study was to assess whether nicotinic acid-adenine dinucleotide phosphate (NAADP) elicited the fertilization potential in starfish oocytes. The changes in membrane potential induced by the sperm were measured in oocytes held at a low resting potential, so that the Ca2+-action potential was inactivated and only the initial slower depolarization caused by the sperm could be studied. Decreasing extracellular Na+ concentration did not prevent the onset of the fertilization potential, while removal of external Ca2+ abolished it. The pre-incubation with SK&F 96365 and verapamil and the pre-injection of BAPTA inhibited the fertilization potential, while the injection of heparin only reduced its duration. The biophysical and pharmacological properties of the sperm-elicited depolarization were similar to those displayed by the NAADP-activated Ca2+-mediated current recently described in starfish oocytes. Indeed, the desensitization of NAADP-receptors prevented the onset of the fertilization potential. Taken together, these data suggest that NAADP could trigger the fertilization potential in starfish oocytes.

βγ subunits of heterotrimeric G-proteins contribute to Ca2+ release at fertilization in the sea urchin

A cytoplasmic Ca2+ transient is required for egg activation at fertilization in all animals. The pathway leading to release of Ca2+ from the endoplasmic reticulum in echinoderms includes activation of a SRC homolog, followed by phospholipase Cγ activation, and formation of inositol trisphosphate. However, the upstream activators or modulators of this signaling pathway are not known. We recently identified four Gα subunits of heterotrimeric G-proteins present in the sea urchin egg, and here we find that activation of G-proteins of the Gαs and Gαq type, but not Gαi or Gα12 type, is required for normal Ca2+ dynamics at fertilization. The effects of these G-proteins are mediated by the Gβγ subunits, occur upstream of the cytoplasmic Ca2+ release, and influence both the amplitude of Ca2+ release and the duration of the lag phase. We propose integration of the G-protein input into the framework of signaling at sea urchin fertilization.

A PKC wave follows the calcium wave after activation of Xenopus eggs

Calcium waves are well-known hallmarks of egg activation that trigger resumption of the cell cycle and development of the embryo. These waves rapidly and efficiently assure that activation signals are transmitted to all regions of the egg. Although the mechanism by which the calcium wave propagates across an egg as large as that of Xenopus is not known, two models prevail. One model is a wave of calcium-induced calcium release (CICR) and the other is propagation by inositol-induced calcium release (IICR). IICR requires a wave of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis, generating two second messengers, IP3, which then releases calcium and DAG, which activates protein kinase C (PKC). We show here that a wave of PKC-green fluorescent protein travels across the egg immediately following, and at the same velocity as, the calcium wave. This is the first example of a PKC wave in a vertebrate egg and supports the IICR model of wave propagation.

Evidence that activation of Src family kinase is not required for fertilization-associated [Ca2+]i oscillations in mouse eggs

Recent evidence in marine invertebrate, frog, and zebrafish eggs suggests the involvement of a Src family kinase (SFK) in fertilization-induced Ca2+ release. In the present study, we have investigated whether activation of an SFK is required for initiation of intracellular Ca2+ ([Ca2+]i) oscillations in mouse fertilization. We detected a Hck-like protein and tyrosine-phosphorylated proteins in soluble and insoluble sperm fractions, respectively. However, the presence of these proteins did not correspond to the active fractions of porcine sperm extracts (pSE). Moreover, [Ca2+]i oscillations induced by pSE in mouse eggs were unaltered by pre-incubation of pSE with specific SFK inhibitors such as 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazol[3,4-d]-pyrimidine (PP2) or lavendustin A, despite the fact that the inhibitors were shown to be active both in vivo and in vitro. Another SFK inhibitor, peptide A, blocked oscillations when incubated with pSE prior to injection into eggs, but this inhibition required more than ten times the concentration reportedly required to inhibit SFK activity. In addition, pre-injection or pre-incubation of eggs with these inhibitors did not affect the ability of pSE to trigger [Ca2+]i oscillations in mouse eggs. Microinjection of a recombinant c-Src protein or mRNAs encoding constitutively active Src proteins did not induce [Ca2+]i release. Finally, when sperm and eggs, both of which were pre-treated with PP2, were fertilized, [Ca2+]i oscillations occurred normally. We can therefore conclude that activation of an SFK is neither necessary nor sufficient for triggering fertilization-induced [Ca2+]i oscillations.

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.

Reconstitution of Src-dependent phospholipase Cgamma phosphorylation and transient calcium release by using membrane rafts and cell-free extracts from Xenopus eggs

We reported previously that egg membrane rafts serve as a subcellular microdomain for sperm-dependent tyrosine kinase signaling in Xenopus fertilization. Moreover, we demonstrated that raft-associated Src tyrosine kinase was activated by sperm in vitro. Here we show that egg rafts incubated with sperm or hydrogen peroxide (H2O2) can promote Src-dependent phosphorylation of phospholipase Cgamma (PLCgamma) and transient calcium release in the extracts of unfertilized Xenopus eggs. In vivo egg activation by sperm or H2O2 also promotes tyrosine phosphorylation and raft-translocalization of PLCgamma. Immunodepletion of PLCgamma from the egg extracts inhibits the raft-dependent calcium release. Rafts prepared from H2O2-activated eggs also promote Src-dependent dephosphorylation of p42 mitogen-activated protein kinase and cell cycle transition from metaphase II to interphase in egg extracts. PLCgamma phosphorylation and calcium release in egg extracts can be promoted by rafts prepared from COS-7 cells expressing the Xenopus Src gene. These results demonstrate that the signaling events elicited by fertilization in Xenopus eggs can be reconstituted in vitro. The development of such experimental platforms will allow us to dissect the molecular mechanism of sperm-dependent activation of raft-associated Src and subsequent up-regulation of PLCgamma and egg activation machinery in Xenopus eggs.