SEARCH

SEARCH BY CITATION

Abstract

  1. Top of page
  2. Abstract
  3. THE NATURE OF SPERM-INDUCED Ca2+ RELEASE IN EGGS
  4. WHAT IS THE SPERM FACTOR?
  5. PLCζ: THE TRIGGER OF [Ca2+]i OSCILLATIONS DURING MAMMALIAN FERTILIZATION
  6. REGULATION OF SPERM/PLCζ-INDUCED [Ca2+]i OSCILLATIONS
  7. [Ca2+]i OSCILLATIONS AND ACTIVATION OF DEVELOPMENT
  8. [Ca2+]i OSCILLATIONS AND NUCLEAR REPROGRAMMING
  9. CONCLUSIONS AND FUTURE DIRECTIONS
  10. LIRERATURE CITED

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), PLCζ has consolidated this view. The fertilizing sperm, and presumably PLCζ 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. © 2005 Wiley-Liss, Inc.

In mammals, ovulation occurs once fully grown oocytes have progressed to an arrested state in the metaphase stage of the second meiotic division (MII). The process leading up to this arrest, termed oocyte maturation, renders the oocyte fully competent to respond to the forthcoming signaling events that take place during fertilization. Part of this preparation period entails the recruitment and translation of stored maternal mRNAs that encode for effectors of the pathways involved in cell cycle progression and calcium (Ca2+) release, and a structural reorganization of key calcium-sensitive elements and organelles (Fujiwara et al., 1993; Mehlmann et al., 1995, 1996; Shirashi et al., 1995; Machaca, 2004). The culmination of these events results in a perfectly harmonized system that is poised to respond in the most sensitive and efficient manner to the incoming sperm.

Exit from the MII arrest and meiotic resumption, referred to as “egg activation,” is made possible by the fertilizing sperm, which evokes in the egg an increase in the concentration of intracellular free calcium ions ([Ca2+]i). This rise in [Ca2+]i is necessary and sufficient for the completion of all the events of egg activation (Schultz and Kopf, 1995), including exocytosis of the cortical granule material to block polyspermy (Cran et al., 1988), resumption of the meiotic cell cycle through Ca2+-dependent destruction of cyclin B (Hyslop et al., 2004), pronuclear formation, recruitment of maternal mRNAs, and initiation of mitotic divisions that unveil the complete developmental program (Schultz and Kopf, 1995; Ducibella et al., 2002).

Although the precise mechanism by which the sperm initiates the Ca2+ release that is responsible for triggering embryonic development has not yet been made fully clear, it has been established in all species studied to date that it involves the activation of the phosphoinositide (PI) pathway (Turner et al., 1984; Stith et al., 1994). Activation of the PI pathway in eggs (Fig. 1) results in the production of inositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DAG) via the hydrolysis of phosphatidyl 4,5-bisphosphate (PIP2) by a phosphoinositide-specific phospholipase C (PLC) isoform (Parrington et al., 1998; Rongish et al., 1999; Rice et al., 2000). Both products of the PI pathway are involved in shaping [Ca2+]i responses. Increase in the intracellular concentrations of IP3 is responsible for mediating Ca2+ release by binding and gating its receptor, the type I IP3 receptor (IP3R-1, Miyazaki et al., 1993), a tetrameric ligand-gated Ca2+ channel located on the endoplasmic reticulum (ER) membrane, the main Ca2+ store of the cell (Koch, 1990; Berridge, 2002). Production of DAG, either directly (Bazzi and Nelsestuen, 1989), or indirectly via activation of protein kinase C (PKC), may be involved in the regulation of Ca2+ influx (Halet et al., 2004). Notably, in spite of the universal requirement for a [Ca2+]i increase in egg activation, the Ca2+ release that accompanies fertilization varies widely among species. For instance, in some lower vertebrates and marine animals this Ca2+ release takes the form of a single, rather long (10 min) transient, whereas mammalian eggs show persistent and repetitive changes in [Ca2+]i (reviewed in Stricker, 1999) beginning shortly after fertilization (Lawrence et al., 1997) and lasting for several hours. Interestingly, in mouse eggs, sperm-initiated [Ca2+]i responses cease in concurrence with pronuclear formation (Jones et al., 1995; Marangos et al., 2003; Jellerette et al., 2004), while in other mammalian species oscillations persist throughout the first cell cycle (Fissore et al., 1992; Nakada et al., 1995). Therefore, the differences in [Ca2+]i responses among phyla and even within classes suggest, at the very least, evolutionary divergence of the mechanism(s) leading to activation of the PI pathway.

thumbnail image

Figure 1. Proposed mechanisms of [Ca2+]i oscillations during mammalian fertilization. Upon gamete fusion, the fertilizing sperm delivers into the ooplasm a soluble substance, presently thought to be PLCζ capable of hydrolyzing PIP2 into the two signaling molecules, IP3 and DAG. IP3 is involved in intracellular Ca2+ release by binding and gating its receptor, the IP3R-1, located in the ER, the Ca2+ store of the cell, while DAG may play an integral role in signaling events at the plasma membrane. DAG together with the elevations in [Ca2+]i may further target and activate PKC to the plasma membrane, where PKC may regulate Ca2+ influx to refill the intracellular stores making possible the persistence of oscillations. The channels and mechanisms responsible for the influx of Ca2+ are unknown, although the transient receptor potential (TRP) channels and the store-operated Ca2+ entry mechanism have been both demonstrated in mammalian eggs.

Download figure to PowerPoint

Consistent with the fundamental role of Ca2+ in egg activation and the protracted nature of this response in mammals, the egg's Ca2+ releasing machinery undergoes significant reorganization and modification(s) prior to fertilization. For instance, during oocyte maturation, the spatial distribution of the ER and of the IP3R-1 changes from the perinuclear region of the germinal vesicle (GV) to distinct focal clusters at the cortex of the MII egg (Mehlmann et al., 1996; FitzHarris et al., 2003). Additionally, at this time, the overall mass of IP3R-1 is increased due to the recruitment and translation of IP3R-1 mRNA (Miyazaki et al., 1993; Shiraishi et al., 1995; Mehlmann et al., 1996; Xu et al., 2003). Lastly, while not well understood, direct modification of IP3R-1 or of other proteins involved in Ca2+ homeostasis such as Ca2+ pumps and/or plasma membrane Ca2+ channels by the prevailing kinase environment in the maturing oocyte may also take place during this prelude to fertilization. Collectively, these adaptations are thought to lead to a maximal (Fujiwara et al., 1993; Mehlmann and Kline, 1994) and spatially organized series of oscillations (Kline et al., 1999; Deguchi et al., 2000), the occurrence of which is pivotal for the initiation of normal development (Ozil, 1990; Vitullo and Ozil, 1992; Miyazaki et al., 1993; Ozil and Huneau, 2001; Ducibella et al., 2002; Ozil et al., 2005). This review will discuss recent advances in understanding how the fertilizing sperm initiates [Ca2+]i oscillations in mammalian eggs, the cellular and molecular regulation of Ca2+ release in eggs, and the requirement for the Ca2+ regime in egg activation.

THE NATURE OF SPERM-INDUCED Ca2+ RELEASE IN EGGS

  1. Top of page
  2. Abstract
  3. THE NATURE OF SPERM-INDUCED Ca2+ RELEASE IN EGGS
  4. WHAT IS THE SPERM FACTOR?
  5. PLCζ: THE TRIGGER OF [Ca2+]i OSCILLATIONS DURING MAMMALIAN FERTILIZATION
  6. REGULATION OF SPERM/PLCζ-INDUCED [Ca2+]i OSCILLATIONS
  7. [Ca2+]i OSCILLATIONS AND ACTIVATION OF DEVELOPMENT
  8. [Ca2+]i OSCILLATIONS AND NUCLEAR REPROGRAMMING
  9. CONCLUSIONS AND FUTURE DIRECTIONS
  10. LIRERATURE CITED

There has been much debate and speculation as to the mechanism(s) that triggers activation of the PI pathway and [Ca2+]i oscillations during mammalian fertilization, and this has led to the proposal of several theories to explain this phenomenon. One early hypothesis, referred to as the “conduit hypothesis,” proposed that sperm fusion allows Ca2+ to passively enter the egg. However, the findings that the initial fertilization Ca2+ responses proceed unaltered in the absence of extracellular Ca2+ ([Ca2+]e) (Stricker, 1996), clearly demonstrated that [Ca2+]e is not necessary for the initiation of [Ca2+]i oscillations. The “receptor hypothesis” suggests that upon sperm-egg membrane contact, receptor-ligand interactions on the surface of the gametes relay intracellular signaling events that initiate Ca2+ release in the egg. One of the signaling cascades thought to be engaged by the interaction of gametes is that mediated by protein tyrosine kinases (PTKs). Specifically, the Src-family of PTKs (SFKs) may activate PLCγs (Carroll et al., 1997; Shearer et al., 1999; Giusti et al., 1999a,b, 2000; Sato et al., 2000, 2003; Tokmakov et al., 2002), thereby triggering Ca2+ release through the production of IP3. In accordance with this notion, PTK and PLCγ activity are upregulated shortly following fertilization in echinoderm and fish eggs (Ciapa and Epel, 1991; Rongish et al., 1999; O'Neill et al., 2004), and fertilization-triggered Ca2+ signaling in egg extracts was reconstituted by adding “activated” membrane raft fractions (Sato et al., 2003), which suggests the presence of a receptor-mediated SFK/PLCγ activation model in Xenopus fertilization. Furthermore, inhibition of PLCγ activation by a dominant negative approach using over expression of PLCγ SH2 domains prevented the sperm-induced [Ca2+]i rise in sea urchin (Carroll et al., 1997) and starfish eggs (Runft et al., 2004). Nonetheless, extensive pharmacological studies (Kurokawa et al., 2004) along with dominant negative approaches (Mehlmann et al., 1998; Mehlmann and Jaffe, 2005) and injection of recombinant PLCγ (Mehlmann et al., 2001) failed to show any involvement of this pathway in evoking [Ca2+]i responses in mammalian eggs. This is in spite the fact that PLCγ and SFK isoforms are expressed in mouse eggs (Dupont et al., 1996; Mehlmann and Jaffe, 2005), and that stimulation of its activity in these cells by exogenous expression of tr-kit, a sperm tyrosine kinase which activates Fyn, a SFK present in mouse and rat eggs (Talmor et al., 1998), was able to induce exit from meiosis and pronuclear formation (Sette et al., 2002). Therefore, while available evidence argues against a role for SFKs in the initiation of [Ca2+]i oscillations in mammalian fertilization, possible downstream effects of this signaling cascade on other events of eggs activation need further investigation (Talmor-Cohen et al., 2004). Lastly, extensive studies, which initially relied on the injection of activators and inhibitors of α subunits of Gq proteins, implicated the activation of PLCβ isoforms in the initiation of Ca2+ release in mammalian fertilization (Miyazaki, 1988; Fissore and Robl, 1994; Moore et al., 1994). However, the subsequent findings demonstrating that inhibition of Gαq subunits by injection of a function-blocking antibody was without effect on fertilization-induced [Ca2+]i oscillations (Williams et al., 1998), in conjunction with the apparent normal fertility in most strain of mice lacking one of the PLCβ isoforms (Choi et al., 2001), suggest that the contribution of this pathway to the initiation of oscillations in mammalian fertilization is negligible (Figs. 1 and 2).

thumbnail image

Figure 2. The effect of the Ca2+ regime on egg activation. A: Depiction of typical Ca2+ traces from fertilized mouse and cow eggs. Although the interval between [Ca2+]i oscillations differ significantly between these two mammalian species (15–20 min in the mouse vs. 30–40 min in the cow), the Ca2+ stimulus is transduced through a conserved mechanism to achieve egg activation and embryonic development. B: Within minutes of sperm-egg fusion, the egg displays a series of long-lasting oscillations in [Ca2+]i. In the mouse, [Ca2+]i oscillations are entrained with the cell cycle, and cease shortly before the formation of the pronuclei. A subsequent rise is observed at the time of PNBD and, on occasions, oscillations are observed during the first embryonic mitosis (mitosis I). Egg activation events exhibit different [Ca2+]i requirements, with the first several spikes being responsible for the exocytosis of cortical granules and block to polyspermy (I), whereas additional [Ca2+]i transients are needed to induce the resumption of meiosis including second meiotic polar body extrusion and to initiate the recruitment of maternal mRNAs (II). Further stimulation appears necessary to promote pronuclear formation and the initiation of embryonic mitosis (III). The relative Ca2+ requirement for the completion of each egg activation event is represented by horizontal bars (dark areas within the bar denotes ∼ the number of [Ca2+]i rises needed to initiate each event; shaded areas denote ∼ the number of [Ca2+]i rises required to complete each event; modified from Ducibella et al., 2002). Egg activation events in which a direct link to the Ca2+ activation stimulus needs further demonstration are marked with a question mark (?) inside the corresponding bar. Each [Ca2+]i rise (black trace) induces a concomitant increase in the activity of CaMKII (blue trace), which is thought to be one of the molecules that translate the Ca2+ stimulus into events of egg activation. The activity of CaMKII outside the first hour post-fertilization is currently unknown (denoted by a question mark).

Download figure to PowerPoint

Given the inability of the aforementioned experimental paradigms to recapitulate the initiation of [Ca2+]i oscillations in mammals, consensus began to coalesce on the need for a novel mechanism to explain the initiation of oscillations in these species. The hypothesis that emerged, “the fusion hypothesis,” proposed that upon gamete fusion the sperm delivers a factor, commonly referred to as the sperm factor (SF), into the ooplasm capable of activating the PI pathway and oscillations (Parrington et al., 1996). The initial, and sole, experimental support for this hypothesis was the demonstration that injection of sperm extracts into mammalian eggs was able to replicate the pattern of oscillations initiated by the sperm (Stice and Robl, 1990; Swann, 1990; Stricker, 1997; Wu et al., 1997). Curiously, the first demonstration of this mechanism was obtained in sea urchin eggs (Dale et al., 1985), a species in which, paradoxically, the hypothesis under discussion may not account for the mechanism of fertilization (see above). The advent of intracytoplasmic sperm injection (ICSI, Palermo et al., 1992), a technique by which an intact sperm delivered into the ooplasm is capable of initiating fertilization-like [Ca2+]i responses (Tesarik and Testart, 1994) and embryo development to term, consolidated the concept that a sperm product was responsible for initiating oscillations in mammalian eggs. While the success of ICSI unquestionably implicates the sperm as the harbor of the factor that triggers oscillations, close examination of the ICSI-induced [Ca2+]i responses underscores the notion that events that take place prior to or during interaction of the gamete membranes are pivotal for normal fertilization. For instance, in mouse eggs, ICSI-initiated oscillations occur less frequently after the first hour and show premature termination (Kurokawa and Fissore, 2003). More revealing still are the findings in large domestic species where fertilization by ICSI fails altogether to initiate [Ca2+]i oscillations (Bedford et al., 2004; and unpublished data [Malcuit, Kurokawa, and Fissore]).

WHAT IS THE SPERM FACTOR?

  1. Top of page
  2. Abstract
  3. THE NATURE OF SPERM-INDUCED Ca2+ RELEASE IN EGGS
  4. WHAT IS THE SPERM FACTOR?
  5. PLCζ: THE TRIGGER OF [Ca2+]i OSCILLATIONS DURING MAMMALIAN FERTILIZATION
  6. REGULATION OF SPERM/PLCζ-INDUCED [Ca2+]i OSCILLATIONS
  7. [Ca2+]i OSCILLATIONS AND ACTIVATION OF DEVELOPMENT
  8. [Ca2+]i OSCILLATIONS AND NUCLEAR REPROGRAMMING
  9. CONCLUSIONS AND FUTURE DIRECTIONS
  10. LIRERATURE CITED

Despite the fact that the identity of the molecule(s) responsible for the SF activity has yet to be fully elucidated, glimpses of the properties of the putative molecule(s) have emerged from fertilization and biochemical studies (Wu et al., 1997, 2001; Kimura et al., 1998; Jones et al., 2000; Perry et al., 2000; Rice et al., 2000). First, while early studies assumed that all SF activity was rapidly released into the ooplasm (Stice and Robl, 1990; Swann, 1990), subsequent studies revealed that the Ca2+-inducing activity was also present in detergent-resistant sperm domains, most likely the sperm perinuclear theca (Kimura et al., 1998; Perry et al., 1999, 2000; Knott et al., 2003; Sutovsky et al., 2003). Consistent with the concept of SF distribution to several sperm compartments was the demonstration that complete release of SF activity into the ooplasm required ∼2 h (Knott et al., 2003). Second, consistent with its perinuclear localization in the sperm, in vitro fertilization and ICSI studies showed that sperm's Ca2+-releasing activity could be recovered after fertilization, as it associated with the pronuclei of the developing zygotes (Kono et al., 1995, 1996; Knott et al., 2003). Third, in vitro PLC assays using sperm extracts showed that these extracts possessed high PLC activity, nearly twice as high as the activity present in other tissues known to express several PLCs isoforms (Jones et al., 2000; Rice et al., 2000). Importantly, the PLC activity of sperm extracts is prominent even in the presence of basal [Ca2+]e, which is very relevant given that this molecule is expected to initiate oscillations in mammalian eggs, which at the time of fertilization show [Ca2+]i basal levels of ∼0.1 µM. Therefore, since several PLC isoforms are expressed in mammalian sperm (Choi et al., 2001; Fukami et al., 2001; Parrington et al., 2002), these enzymes surfaced as logical candidates to be the SF. Importantly, injection of recombinant proteins representing most of the isoforms expressed in sperm failed to initiate oscillations, or it did so at non-physiological concentrations (Mehlmann et al., 2001). Furthermore, chromatographic fractionation of sperm extracts revealed that none of the known PLCs were present in the fractions with [Ca2+]i oscillation-inducing activity (Wu et al., 2001; Parrington et al., 2002). Hence, if a sperm PLC were to be the SF, it had to be a novel PLC. Towards this end, a novel sperm-specific PLC, PLCζ (Saunders et al., 2002), was screened out of mouse expressed sequence tags. Initial studies revealed that PLCζ exhibits [Ca2+]i oscillation-inducing activity ascribed, thus far, only to the sperm or SF. Hence, PLCζ may be the long sought-after SF.

PLCζ: THE TRIGGER OF [Ca2+]i OSCILLATIONS DURING MAMMALIAN FERTILIZATION

  1. Top of page
  2. Abstract
  3. THE NATURE OF SPERM-INDUCED Ca2+ RELEASE IN EGGS
  4. WHAT IS THE SPERM FACTOR?
  5. PLCζ: THE TRIGGER OF [Ca2+]i OSCILLATIONS DURING MAMMALIAN FERTILIZATION
  6. REGULATION OF SPERM/PLCζ-INDUCED [Ca2+]i OSCILLATIONS
  7. [Ca2+]i OSCILLATIONS AND ACTIVATION OF DEVELOPMENT
  8. [Ca2+]i OSCILLATIONS AND NUCLEAR REPROGRAMMING
  9. CONCLUSIONS AND FUTURE DIRECTIONS
  10. LIRERATURE CITED

Since its discovery from mouse testis (Saunders et al., 2002), PLCζ has also been identified and cloned in humans and primates (Cox et al., 2002), and highly homologous sequences have been reported for several other species including the bovine and porcine (Genbank accession numbers GI:55669158 and GI:32400660, respectively). PLCζ is to date, the most elementary of PLC isoforms identified. In concurrence with the modular organization of other PLCs (Rhee, 2001), PLCζ consists of four Ca2+-binding EF hands, X and Y catalytic domains, and the Ca2+-dependent phospholipid-binding C2 domain (Essen et al., 1997). Notably, PLCζ lacks the typical pleckstrin homology (PH) domain, which has been found in all previously identified PLC isoforms (Williams and Katan, 1996). In support of its purported role as the SF, injection of recombinant PLCζ (Fujimoto et al., 2004; Kouchi et al., 2004) or PLCζ cRNA has been shown to evoke sperm-like- oscillations in mouse (Cox et al., 2002; Saunders et al., 2002; Fujimoto et al., 2004; Kouchi et al., 2004; Larman et al., 2004), human (Rogers et al., 2004), and bovine eggs (Malcuit et al., 2005). In addition, in vitro PLC assays using recombinant PLCζ revealed high enzymatic activity at basal [Ca2+]e concentrations (Kouchi et al., 2004). Moreover, PLCζ cRNA-induced oscillations that ceased at ∼ the time of pronuclear formation, which is comparable to what is observed after natural fertilization (Larman et al., 2004; Yoda et al., 2004) and, like the sperm, zygotes activated by injection of PLCζ cRNA showed high rates of in vitro development to the blastocyst stage (Cox et al., 2002; Saunders et al., 2002). Lastly, a recent report in mouse sperm has localized PLCζ to the post-acrosomal region of mouse sperm (Fujimoto et al., 2004), which is the first area thought to come in contact with the ooplasm after the fusion of gametes (Sutovsky et al., 2003). Collectively, the evidence in support of PLCζ as the mammalian SF is compelling. Nevertheless, important questions remain to be addressed regarding the expression, localization, and storage of PLCζ in sperm, its mechanism of release into the ooplasm, and mechanism(s) of activation once in the egg. Additionally, it needs to be demonstrated whether or not PLCζ represents the sole inducer of [Ca2+]i oscillations during fertilization in all mammals. In regards to the latter, two recent reports are worth discussing. First, using transgenic-mediated RNA interference, male mice were created with decreased levels of PLCζ protein. In vitro fertilization (IVF) using sperm from these males resulted in attenuated [Ca2+]i responses, and one of the males produced greatly reduced litter sizes despite inducing normal rates of in vitro embryonic development (Knott et al., 2005). Second, a recent report showed that following fractionation of sperm extracts, the presence of immunoreactive 72-kDa PLCζ correlated with the ability of these fractions to induce egg activation (Fujimoto et al., 2004). Importantly, several of the active fractions were devoid of 72-kDa PLCζ, and others had greatly reduced amounts of immunoreactive? PLCζ. Nevertheless, no mechanism(s) was proposed to account for the egg activation capacity in the fractions devoid of 72-kDa PLCζ. Perhaps the answers to these questions will only be fully realized once a PLCζ knockout is created.

REGULATION OF SPERM/PLCζ-INDUCED [Ca2+]i OSCILLATIONS

  1. Top of page
  2. Abstract
  3. THE NATURE OF SPERM-INDUCED Ca2+ RELEASE IN EGGS
  4. WHAT IS THE SPERM FACTOR?
  5. PLCζ: THE TRIGGER OF [Ca2+]i OSCILLATIONS DURING MAMMALIAN FERTILIZATION
  6. REGULATION OF SPERM/PLCζ-INDUCED [Ca2+]i OSCILLATIONS
  7. [Ca2+]i OSCILLATIONS AND ACTIVATION OF DEVELOPMENT
  8. [Ca2+]i OSCILLATIONS AND NUCLEAR REPROGRAMMING
  9. CONCLUSIONS AND FUTURE DIRECTIONS
  10. LIRERATURE CITED

Having been initiated several minutes after sperm-egg fusion (Lawrence et al., 1997), oscillations in mammalian eggs persist for several hours at regular intervals of ∼15–30 min (Fig. 2A); in the mouse, oscillations cease at the time of pronuclear formation, revealing an association of Ca2+ release with the cell cycle (Jones et al., 1995; Nixon et al., 2000; Marangos et al., 2003). The mechanism(s) that underlies this correspondence have not yet been fully elucidated, although eggs seem to use several means to synchronize the cell cycle and [Ca2+]i oscillations. It is well established that the sperm's [Ca2+]i oscillation-inducing factor is targeted to the forming pronucleus of the developing zygote, and that, consistent with this, pronuclei of fertilized zygotes, but not of parthenogenetic zygotes, possess the ability to initiate [Ca2+]i oscillations when fused with MII eggs (Kono et al., 1995). Moreover, a [Ca2+]i transient ensues when the pronucleus of fertilized zygotes breaks down (PNBD) as the embryo enters the first mitosis (mitosis I) (Kono et al., 1996; Day et al., 2000). Therefore, the recent finding that PLCζ is targeted to the pronucleus by a stretch of basic amino acids in the linker domain of the protein implicated pronuclear sequestration of PLCζ as one of the mechanism entraining the cell cycle and [Ca2+]i oscillations in mouse zygotes (Larman et al., 2004). Demonstration of PLCζ localization was performed by fluorescent antibody labeling of a Myc-tagged version of PLCζ and this approach was also used to disclose the information that, upon PNBD, [Ca2+]i responses and Myc-PLCζ reappeared in the cytoplasm. In this study, mutation of a single amino acid of the nuclear localization signal abrogated pronuclear localization and extended oscillations. Interestingly, real time evaluation of PLCζ distribution using a yellow fluorescent protein “Venus”-fused PLCζ reported that [Ca2+]i oscillations ceased prior to the accumulation of PLCζ in the pronucleus (Yoda et al., 2004). Moreover, the findings in the bovine demonstrating that [Ca2+]i oscillations persist through interphase (Fissore et al., 1992; Nakada et al., 1995), despite the recently reported conservation of key basic amino acids in the linker region of bovine PLCζ is not consistent with the notion that PLCζ sequestration represents the sole mechanism coordinating the cell cycle with oscillations.

Besides the discrepancy of PLCζ distribution and cessation of oscillations reported by Yoda et al. (2004), other findings raised the possibility that other cellular mechanisms may assist in the cell cycle coordination of oscillations in the mouse. For instance, close examination of the pattern of sperm-initiated oscillations shows a gradual, but significant, increase in the intervals between rises well before formation of the pronuclei (Kurokawa and Fissore, 2003). What is more, interphase stage injections of either IP3 (Jones et al., 1995; FitzHarris et al., 2003) or adenophostin A, a non-hydrolyzable IP3 analogue, failed to initiate oscillations at this stage (Jellerette et al., 2004). Together, we interpreted the results to mean that the Ca2+ release machinery, and most likely the IP3R-1 is less responsive to IP3 and this is responsible, at least in part, for the termination of oscillations as zygotes transition into interphase (Jellerette et al., 2004).

Several mechanisms could account for decreased responsiveness of IP3R-1 in zygotes. Following fertilization, the concentration of IP3R-1 in zygotes steadily declines to stabilize at ∼50% of the initial IP3R-1 mass (Parrington et al., 1998; Brind et al., 2000; Jellerette et al., 2000). This IP3R-1 downregulation, which is driven by binding of IP3 to its receptor (Zhu and Wojcikiewicz, 2000; Khan and Joseph, 2003), might severely alter the ability to sustain oscillations as fertilization progresses. Consistent with this notion, it was recently demonstrated that elimination of ∼50% of IP3R-1 mass resulted in premature termination of oscillations in mouse eggs (Xu et al., 2003), although the impact that the gradual decline of receptor mass has on fertilization associated oscillations has not been thoroughly tested. The responsiveness of the IP3R-1 in zygotes may be also compromised by the declining activity of M-phase kinases (FitzHarris et al., 2003; Jellerette et al., 2004). For instance, several M-phase phosphorylation consensus sites have been identified in IP3R-1 and seem to undergo active modifications during the cell cycle in somatic cells (Malathi et al., 2003) and in mouse eggs/zygotes (Jellerette et al., 2004). M-phase kinases, and most specifically cdk1, may also affect IP3R-1 conductivity and presence of oscillations by regulating the organization of the ER. The presence of ER and IP3R-1 clusters, which is associated with active cdk1, correlates with a greater Ca2+-releasing ability of IP3R-1 and hence, a propensity for oscillations in a variety of eggs (Stricker et al., 1998; FitzHarris et al., 2003; Stricker and Smythe, 2003; Machaca, 2004). Nonetheless, whether this reflects a specific effect of receptor clustering or is due, at least in part, to other cell cycle-associated cellular changes remains to be determined. Lastly, the effect of M-phases kinases on Ca2+ influx pathways, as reportedly takes place in Xenopus oocytes (Machaca and Haun, 2002), cannot be discounted as playing a role in the presence and persistence of oscillations. Therefore, the cell-cycle association of [Ca2+]i oscillations in mammalian eggs may represent the coordinated modification of several cellular mechanisms; elucidating the precise contribution of each then, should be a task for the near future.

[Ca2+]i OSCILLATIONS AND ACTIVATION OF DEVELOPMENT

  1. Top of page
  2. Abstract
  3. THE NATURE OF SPERM-INDUCED Ca2+ RELEASE IN EGGS
  4. WHAT IS THE SPERM FACTOR?
  5. PLCζ: THE TRIGGER OF [Ca2+]i OSCILLATIONS DURING MAMMALIAN FERTILIZATION
  6. REGULATION OF SPERM/PLCζ-INDUCED [Ca2+]i OSCILLATIONS
  7. [Ca2+]i OSCILLATIONS AND ACTIVATION OF DEVELOPMENT
  8. [Ca2+]i OSCILLATIONS AND NUCLEAR REPROGRAMMING
  9. CONCLUSIONS AND FUTURE DIRECTIONS
  10. LIRERATURE CITED

In spite of the realization that the activation of embryonic development in mammals relies on the initiation of [Ca2+]i oscillations, there is remarkably little known as to what are the “molecular effectors” in the egg that translate the transient [Ca2+]i elevations into cellular events. It is well established that unlike echinoderm and amphibian fertilization, initiation of multiple rises is required to promote complete exit from meiosis in mammals (for review see Stricker, 1999). Subsequent studies, first in Xenopus eggs, have lent support to the notion that [Ca2+]i elevations mediate meiotic resumption by activating the type II Ca2+/calmodulin-dependent protein kinase (CaMKII), which by an as yet unknown mechanism, leads to activation of the anaphase promoting complex (APC) (Yamamoto et al., 2005) presumably through destruction of a member of the early mitotic inhibitor (Emi) proteins (Reimann et al., 2001; Reimann and Jackson, 2002; Tung et al., 2005). Interestingly, in mammals, each [Ca2+]i rise results in a concomitant activation of CaMKII (Markoulaki et al., 2003, 2004), and this is envisioned to be responsible for the steady increase in the rate of cyclin degradation through maintenance of APC activity (Dupont, 1998; Marangos and Carroll, 2004b). The APC, an E3 ubiquitin ligase, targets cell cycle proteins such as cyclin B for proteasomal degradation following their poly-ubiquitination (King et al., 1995; Yu et al., 1996; Nixon et al., 2002; Marangos and Carroll, 2004a,b). The loss of cylin B, the regulatory subunit of MPF (Gautier et al., 1990) and limiting factor for the maintenance of the metaphase state, results in the exit from M-phase and thus the resumption of meiosis. Interestingly, this Ca2+-mediated cell-cycle transition is thought to be unique to eggs (Marangos and Carroll, 2004b), and requires stable levels of cytostatic factor (CSF) (Hyslop et al., 2004]), which is itself, unique to eggs (Masui, 1974).

If the only function of Ca2+ oscillations were to promote exit of meiosis, then elevations of [Ca2+]i according to disparate, but not excessive protocols, should lead to equal rates of activation and development. Nonetheless, elegant studies by Ozil and co-workers have demonstrated this not to be the case. In their studies, activation protocols that adhered more closely to the sperm-like patterns of oscillations resulted in higher rates of implantation and development (Ozil, 1990; Ozil and Huneau, 2001; Ozil et al., 2005). In these studies the amplitude, frequency, and number of [Ca2+]i rises were modulated by electroporation of [Ca2+]e into the ooplasm of rabbit and mouse eggs. Each parameter tested had a marked effect on the rates of egg activation as well as post-implantation development. In line with the developmental benefits of Ca2+, more recent studies revealed that different events of egg activation have dissimilar requirements regarding their initiation and completion, with the completion of each event needing higher number of rises (Ducibella et al., 2002; Ozil et al., 2005). Especially relevant to embryo development, is the possible impact of the [Ca2+]i activating stimulus on the recruitment of specific maternal RNAs (Ducibella et al., 2002; Ozil et al., 2005), the recruitment and translations of which may be critical for the activation of the zygotic genome (Aoki et al., 2003). For instance, it was shown that “recruitment of maternal RNAs,” as evidenced by the detection of new protein synthesis, was initiated by the administration of eight [Ca2+]e pulses but only became fully apparent, or “fertilization-like,” in zygotes receiving a total of 24 [Ca2+]e pulses (Fig. 2B and Ducibella et al., 2002). While these results imply a correspondence between [Ca2+]i oscillations and protein synthesis, it is important to discriminate whether the observed changes in protein profiles were not simply due to progression into zygotic interphase, which seemed to exhibit similar requirements for [Ca2+]e pulses (Ducibella et al., 2002). Nonetheless, the approach used in this study, followed by identification of the proteins regulated by Ca2+ pulses, could prove invaluable in elucidating yet uncharacterized signaling pathways during egg activation.

How then does the egg transduce these compounding [Ca2+]i rises into an effector? It has recently been shown that just as concentrations of [Ca2+]i oscillate in response to IP3 production, so too does the activity of CaMKII (Tatone et al., 2002; Markoulaki et al., 2003, 2004). For instance, the activity of CaMKII not only increased in response to augmenting levels of [Ca2+]i, but oscillated in close synchrony with each [Ca2+]i transient after fertilization, as determined in single eggs by simultaneous monitoring of [Ca2+]i levels and kinase activity (Markoulaki et al., 2003, 2004). Hence, given that each [Ca2+]i transient during fertilization accounts for a single event during which CaMKII acts upon its targets, it can be speculated that the compounding effect of each [Ca2+]i rise is required for brief periods of CaMKII activity (De Koninck and Schulman, 1998), the culmination of which results in successful egg activation. Importantly, a recent report in hippocampal dendrites has demonstrated that CaMKII directly phosphorylates the cytoplasmic polyadenylation element binding protein (CPEB) that is directly responsible for the polyadenylation, recruitment, and translation of mRNAs (Atkins et al., 2004). A similar mechanism, therefore, could potentially function in eggs. This would provide a direct link between the results mentioned above in which repetitive Ca2+ pulses modulated protein expression patterns from stored maternal mRNAs (Ducibella et al., 2002; Ozil et al., 2005). It is worth noting that other kinases such as PKC (Halet et al., 2004), or proteins such as actin (McAvey et al., 2002) and calreticulin (Tutuncu et al., 2004), may also serve as effectors of [Ca2+]i rises during egg activation, although additional research is required to realize the extent of their participation.

[Ca2+]i OSCILLATIONS AND NUCLEAR REPROGRAMMING

  1. Top of page
  2. Abstract
  3. THE NATURE OF SPERM-INDUCED Ca2+ RELEASE IN EGGS
  4. WHAT IS THE SPERM FACTOR?
  5. PLCζ: THE TRIGGER OF [Ca2+]i OSCILLATIONS DURING MAMMALIAN FERTILIZATION
  6. REGULATION OF SPERM/PLCζ-INDUCED [Ca2+]i OSCILLATIONS
  7. [Ca2+]i OSCILLATIONS AND ACTIVATION OF DEVELOPMENT
  8. [Ca2+]i OSCILLATIONS AND NUCLEAR REPROGRAMMING
  9. CONCLUSIONS AND FUTURE DIRECTIONS
  10. LIRERATURE CITED

Besides the possible aforementioned impact that the Ca2+ activating stimulus may have on zygotic gene expression via recruitment and translation of specific maternal mRNAs, an equally intriguing possibility is that the fertilization-initiated [Ca2+]i responses are also involved in the nuclear remodeling of the parental genomes, which commences soon after fertilization (Dean et al., 2003). After fertilization, the gametes undergo extensive reconfiguration, which is required for activation of the zygotic genome (for review see Latham, 1999). Of the two gametes, the paternal genome undergoes the most rapid and dramatic change, as is subjected to the exchange of protamines for maternal histones and this occurs in concert with sperm nuclear decondensation (Adenot et al., 1997; Santos et al., 2002). Additionally, the paternal genome undergoes near complete DNA demethylation of CpG dinucleotides before reaching the pronuclear stage (Barton et al., 2001; Santos et al., 2002). While the precise nature of the molecular mechanisms that underlie the transformation of the sperm nucleus into a pronucleus is not fully elucidated, given that this reconfiguration coincides with presence of [Ca2+]i oscillations, a role for Ca2+-dependent processes in such events cannot be discounted.

Nuclear remodeling and epigenetic modification of the zygotic genome also entails covalent modifications of the nucleosomes' core histones (Stein et al., 1997; Strahl and Allis, 2000). These changes involve phosphorylation, acetylation, and methylation of histones as well as the removal by opposing enzymes of these modifications from the target amino acids in these proteins, which are the basic amino acids lysine and arginine (Fischle et al., 2003; Sarmento et al., 2004). While histone methylases have been identified in somatic cells and eggs, the enzymes that remove these modifications are not presently well known. Importantly, mouse oocytes and eggs reportedly abundantly express a peptidylarginine demethyliminase-like molecule, ePAD, which functionally demethylates histone by the conversion of methylarginine to citrulline, and the subsequent release of methylimine (Wright et al., 2003; Sarmento et al., 2004; Wang et al., 2004). Relevant to the possible role of Ca2+ on nuclear reprogramming, is the finding that PADs are Ca2+ sensitive enzymes (Vossenaar et al., 2004; Wang et al., 2004). Therefore, it will be important to address the issue of the Ca2+-dependence of ePAD for its functional role on nuclear remodeling and reprogramming of the fertilized zygote, and to determine if maximal ePAD activity requires [Ca2+]i oscillations similar to those seen during fertilization.

CONCLUSIONS AND FUTURE DIRECTIONS

  1. Top of page
  2. Abstract
  3. THE NATURE OF SPERM-INDUCED Ca2+ RELEASE IN EGGS
  4. WHAT IS THE SPERM FACTOR?
  5. PLCζ: THE TRIGGER OF [Ca2+]i OSCILLATIONS DURING MAMMALIAN FERTILIZATION
  6. REGULATION OF SPERM/PLCζ-INDUCED [Ca2+]i OSCILLATIONS
  7. [Ca2+]i OSCILLATIONS AND ACTIVATION OF DEVELOPMENT
  8. [Ca2+]i OSCILLATIONS AND NUCLEAR REPROGRAMMING
  9. CONCLUSIONS AND FUTURE DIRECTIONS
  10. LIRERATURE CITED

The mechanisms of fertilization-induced [Ca2+]i oscillations and egg activation are finally becoming unraveled. We are just now learning how [Ca2+]i oscillations are induced by the sperm and how these [Ca2+]i rises are transduced into egg activation events. We must continue to reveal the downstream pathways that translate [Ca2+]i rises into cellular and molecular events, and determine if a link exist between [Ca2+]i oscillations and nuclear reprogramming. Elucidation of these pathways and mechanisms will make it possible to determine whether altered [Ca2+]i oscillations, such as those that may occur in zygotes generated by assisted reproductive techniques, may affect the health of the offspring later in life. This is of the utmost importance in light of the popularity of ICSI as a means for assisted reproduction in fertility clinics around the globe. Not only will the study of these events help elucidate the precise mechanisms by which the sperm acts during fertilization, but will also facilitate the development of more efficient exogenous activation methods that may more closely unleash the molecular events induced by fertilization, such that we may ensure that all offspring produced by such means begin their development in the most physiological way possible.

LIRERATURE CITED

  1. Top of page
  2. Abstract
  3. THE NATURE OF SPERM-INDUCED Ca2+ RELEASE IN EGGS
  4. WHAT IS THE SPERM FACTOR?
  5. PLCζ: THE TRIGGER OF [Ca2+]i OSCILLATIONS DURING MAMMALIAN FERTILIZATION
  6. REGULATION OF SPERM/PLCζ-INDUCED [Ca2+]i OSCILLATIONS
  7. [Ca2+]i OSCILLATIONS AND ACTIVATION OF DEVELOPMENT
  8. [Ca2+]i OSCILLATIONS AND NUCLEAR REPROGRAMMING
  9. CONCLUSIONS AND FUTURE DIRECTIONS
  10. LIRERATURE CITED
  • Adenot PG, Mercier Y, Renard JP, Thompson EM. 1997. Differential H4 acetylation of paternal and maternal chromatin precedes DNA replication and differential transcriptional activity in pronuclei of 1-cell mouse embryos. Development 124(22): 46154625.
  • Aoki F, Hara KT, Schultz RM. 2003. Acquisition of transcriptional competence in the 1-cell mouse embryo: Requirement for recruitment of maternal mRNAs. Mol Reprod Dev 64(3): 270274.
  • Atkins CM, Nozaki N, Shigeri Y, Soderling TR. 2004. Cytoplasmic polyadenylation element binding protein-dependent protein synthesis is regulated by calcium/calmodulin-dependent protein kinase II. J Neurosci 24(22): 51935201.
  • Barton SC, Arney KL, Shi W, Niveleau A, Fundele R, Surani MA, Haaf T. 2001. Genome-wide methylation patterns in normal and uniparental early mouse embryos. Hum Mol Genet 10(26): 29832987.
  • Bazzi MD, Nelsestuen GL. 1989. Differences in the effects of phorbol esters and diacylglycerols on protein kinase C. Biochemistry 28(24): 93179323.
  • Bedford SJ, Kurokawa M, Hinrichs K, Fissore RA. 2004. Patterns of intracellular calcium oscillations in horse oocytes fertilized by intracytoplasmic sperm injection: Possible explanations for the low success of this assisted reproduction technique in the horse. Biol Reprod 70(4): 936944.
  • Berridge MJ. 2002. The endoplasmic reticulum: A multifunctional signaling organelle. Cell Calcium 32(5–6): 235249.
  • Brind S, Swann K, Carroll J. 2000. Inositol 1,4,5-trisphosphate receptors are downregulated in mouse oocytes in response to sperm or adenophostin A but not to increases in intracellular Ca2+ or egg activation. Dev Biol 223(2): 251265.
  • Carroll DJ, Ramarao CS, Mehlmann LM, Roche S, Terasaki M, Jaffe LA. 1997. Calcium release at fertilization in starfish eggs is mediated by phospholipase C gamma. J Cell Biol 138(6): 13031311.
  • Choi D, Lee E, Hwang S, Jun K, Kim D, Yoon BK, Shin HS, Lee JH. 2001. The biological significance of phospholipase C beta 1 gene mutation in mouse sperm in the acrosome reaction, fertilization, and embryo development. J Assist Reprod Genet 18(5): 305310.
  • Ciapa B, Epel D. 1991. A rapid change in phosphorylation on tyrosine accompanies fertilization of sea urchin eggs. FEBS 295(1–3): 167170.
  • Cox LJ, Larman MG, Saunders CM, Hashimoto K, Swann K, Lai FA. 2002. Sperm phospholipase C zeta from humans and cynomolgus monkeys triggers Ca2+ oscillations, activation and development of mouse oocytes. Reproduction 124(5): 611623.
  • Cran DG, Moor RM, Irvine RF. 1988. Initiation of the cortical reaction in hamster and sheep oocytes in response to inositol trisphosphate. J Cell Sci 91(Pt 1): 139144.
  • Dale B, DeFelice LJ, Ehrenstein G. 1985. Injection of a soluble sperm fraction into sea-urchin eggs triggers the cortical reaction. Experientia 41(8): 10681070.
  • Day ML, McGuinness OM, Berridge MJ, Johnson MH. 2000. Regulation of fertilization-induced Ca2+ spiking in the mouse zygote. Cell Calcium 28(1): 4754.
  • De Koninck P, Schulman H. 1998. Sensitivity of CaM kinase II to the frequency of Ca2+ oscillations. Science 279(5348): 227230.
  • Dean W, Santos F, Reik W. 2003. Epigenetic reprogramming in early mammalian development and following somatic nuclear transfer. Semin Cell Dev Biol 14(1): 93100.
  • Deguchi R, Shirakawa H, Oda S, Mohri T, Miyazaki S. 2000. Spatiotemporal analysis of Ca2+ waves in relation to the sperm entry site and animal-vegetal axis during Ca2+ oscillations in fertilized mouse eggs. Dev Biol 218(2): 299313.
  • Ducibella T, Huneau D, Angelichio E, Xu Z, Schultz RM, Kopf GS, Fissore R, Madoux S, Ozil JP. 2002. Egg-to-embryo transition is driven by differential responses to Ca2+ oscillation number. Dev Biol 250(2): 280291.
  • Dupont G. 1998. Link between fertilization-induced Ca2+ oscillations and relief from metaphase II arrest in mammalian eggs: A model based on calmodulin-dependent kinase II activation. Biophys Chem 72(1–2): 153167.
  • Dupont G, McGuinness OM, Johnson MH, Berridge MJ, Borgese F. 1996. Phospholipase C in mouse oocytes: Characterization of beta and gamma isoforms and their possible involvement in sperm-induced Ca2+ spiking. Biochem J 316(Pt 2): 583591.
  • Essen LO, Perisic O, Lynch DE, Katan M, Williams RL. 1997. A ternary metal binding site in the C2 domain of phosphoinositide-specific phospholipase C delta1. Biochemistry 36(10): 27532762.
  • Fischle W, Wang Y, Allis CD. 2003. Binary switches and modification cassettes in histone biology and beyond. Nature 425(6957): 475479.
  • Fissore RA, Robl JM. 1994. Mechanism of calcium oscillations in fertilized rabbit eggs. Dev Biol 166(2): 634642.
  • Fissore RA, Dobrinsky JR, Balise JJ, Duby RT, Robl JM. 1992. Patterns of intracellular Ca2+ concentrations in fertilized bovine eggs. Biol Reprod 47(6): 960969.
  • FitzHarris G, Marangos P, Carroll J. 2003. Cell cycle-dependent regulation of structure of endoplasmic reticulum and inositol 1,4,5-trisphosphate-induced Ca2+ release in mouse oocytes and embryos. Mol Biol Cell 14(1): 288301.
  • Fujimoto S, Yoshida N, Fukui T, Amanai M, Isobe T, Itagaki C, Izumi T, Perry AC. 2004. Mammalian phospholipase C zeta induces oocyte activation from the sperm perinuclear matrix. Dev Biol 274(2): 370383.
  • Fujiwara T, Nakada K, Shirakawa H, Miyazaki S. 1993. Development of inositol trisphosphate-induced calcium release mechanism during maturation of hamster oocytes. Dev Biol 156(1): 6979.
  • Fukami K, Nakao K, Inoue T, Kataoka Y, Kurokawa M, Fissore RA, Nakamura K, Katsuki M, Mikoshiba K, Yoshida N, Takenawa T. 2001. Requirement of phospholipase C delta4 for the zona pellucida-induced acrosome reaction. Science 292(5518): 920923.
  • Gautier J, Minshull J, Lohka M, Glotzer M, Hunt T, Maller JL. 1990. Cyclin is a component of maturation-promoting factor from Xenopus. Cell 60(3): 487494.
  • Giusti AF, Carroll DJ, Abassi YA, Foltz KR. 1999a. Evidence that a starfish egg Src family tyrosine kinase associates with PLC gamma1 SH2 domains at fertilization. Dev Biol 208(1): 189199.
  • Giusti AF, Carroll DJ, Abassi YA, Terasaki M, Foltz KR, Jaffe LA. 1999b. Requirement of a Src family kinase for initiating calcium release at fertilization in starfish eggs. J Biol Chem 274(41): 2931829322.
  • Giusti AF, Xu W, Hinkle B, Terasaki M, Jaffe LA. 2000. Evidence that fertilization activates starfish eggs by sequential activation of a Src-like kinase and phospholipase C gamma. J Biol Chem 275(22): 1678816794.
  • Halet G, Tunwell R, Parkinson SJ, Carroll J. 2004. Conventional PKCs regulate the temporal pattern of Ca2+ oscillations at fertilization in mouse eggs. J Cell Biol 164(7): 10331044.
  • Hyslop LA, Nixon VL, Levasseur M, Chapman F, Chiba K, McDougall A, Venables JP, Elliott DJ, Jones KT. 2004. Ca2+ promoted cyclin B1 degradation in mouse oocytes requires the establishment of a metaphase arrest. Dev Biol 269(1): 206219.
  • Jellerette T, He CL, Wu H, Parys JB, Fissore RA. 2000. Down-regulation of the inositol 1,4,5-trisphosphate receptor in mouse eggs following fertilization or parthenogenetic activation. Dev Biol 223(2): 238250.
  • Jellerette T, Kurokawa M, Lee B, Malcuit C, Yoon SY, Smyth J, Vermassen E, De Smedt H, Parys JB, Fissore RA. 2004. Cell cycle-coupled [Ca2+]i oscillations in mouse zygotes and function of the inositol 1,4,5-trisphosphate receptor-1. Dev Biol 274(1): 94109.
  • Jones KT, Carroll J, Merriman JA, Whittingham DG, Kono T. 1995. Repetitive sperm-induced Ca2+ transients in mouse oocytes are cell cycle dependent. Development 121(10): 32593266.
  • Jones KT, Matsuda M, Parrington J, Katan M, Swann K. 2000. Different Ca2+-releasing abilities of sperm extracts compared with tissue extracts and phospholipase C isoforms in sea urchin egg homogenate and mouse eggs. Biochem J 346(Pt 3): 743749.
  • Khan MT, Joseph SK. 2003. Proteolysis of type I inositol 1,4,5-trisphosphate receptor in WB rat liver cells. Biochem J 375(Pt 3): 603611.
  • Kimura Y, Yanagimachi R, Kuretake S, Bortkiewicz H, Perry AC, Yanagimachi H. 1998. Analysis of mouse oocyte activation suggests the involvement of sperm perinuclear material. Biol Reprod 58(6): 14071415.
  • King RW, Peters JM, Tugendreich S, Rolfe M, Hieter P, Kirschner MW. 1995. A 20S complex containing CDC27 and CDC16 catalyzes the mitosis-specific conjugation of ubiquitin to cyclin B. Cell 81(2): 279288.
  • Kline D, Mehlmann L, Fox C, Terasaki M. 1999. The cortical endoplasmic reticulum (ER) of the mouse egg: Localization of ER clusters in relation to the generation of repetitive calcium waves. Dev Biol 215(2): 431442.
  • Knott JG, Kurokawa M, Fissore RA. 2003. Release of the Ca2+ oscillation-inducing sperm factor during mouse fertilization. Dev Biol 260(2): 536547.
  • Knott JG, Kurokawa M, Fissore RA, Schultz RM, Williams CJ. 2005. Transgenic RNA interference reveals role for mouse sperm phospholipase C zeta in triggering Ca2+ oscillations during fertilization. Biol Reprod 72(4): 992996.
  • Koch GL. 1990. The endoplasmic reticulum and calcium storage. Bioessays 12(11): 527531.
  • Kono T, Carroll J, Swann K, Whittingham DG. 1995. Nuclei from fertilized mouse embryos have calcium-releasing activity. Development 121(4): 11231128.
  • Kono T, Jones KT, Bos-Mikich A, Whittingham DG, Carroll J. 1996. A cell cycle-associated change in Ca2+ releasing activity leads to the generation of Ca2+ transients in mouse embryos during the first mitotic division. J Cell Biol 132(5): 915923.
  • Kouchi Z, Fukami K, Shikano T, Oda S, Nakamura Y, Takenawa T, Miyazaki S. 2004. Recombinant phospholipase C zeta has high Ca2+ sensitivity and induces Ca2+ oscillations in mouse eggs. J Biol Chem 279(11): 1040810412.
  • Kurokawa M, Fissore RA. 2003. ICSI-generated mouse zygotes exhibit altered calcium oscillations, inositol 1,4,5-trisphosphate receptor-1 down-regulation, and embryo development. Mol Hum Reprod 9(9): 523533.
  • Kurokawa M, Sato K, Smyth J, Wu H, Fukami K, Takenawa T, Fissore RA. 2004. Evidence that activation of Src family kinase is not required for fertilization-associated [Ca2+]i oscillations in mouse eggs. Reproduction 127(4): 441454.
  • Larman MG, Saunders CM, Carroll J, Lai FA, Swann K. 2004. Cell cycle-dependent Ca2+ oscillations in mouse embryos are regulated by nuclear targeting of PLC zeta. J Cell Sci 117(Pt 12): 25132521.
  • Latham KE. 1999. Mechanisms and control of embryonic genome activation in mammalian embryos. Int Rev Cytol 193: 71124.
  • Lawrence Y, Whitaker M, Swann K. 1997. Sperm-egg fusion is the prelude to the initial Ca2+ increase at fertilization in the mouse. Development 124(1): 233241.
  • Machaca K. 2004. Increased sensitivity and clustering of elementary Ca2+ release events during oocyte maturation. Dev Biol 275(1): 170182.
  • Machaca K, Haun S. 2002. Induction of maturation-promoting factor during Xenopus oocyte maturation uncouples Ca2+ store depletion from store-operated Ca2+ entry. J Cell Biol 156(1): 7585.
  • Malathi K, Kohyama S, Ho M, Soghoian D, Li X, Silane M, Berenstein A, Jayaraman T. 2003. Inositol 1,4,5-trisphosphate receptor (type 1) phosphorylation and modulation by Cdc2. J Cell Biochem 90(6): 11861196.
  • Malcuit C, Knott JG, He C, Wainwright T, Parys JB, Robl JM, Fissore RA. 2005. Fertilization and Inositol 1,4,5-Trisphosphate (IP3)-induced calcium release in Type-1 Inositol 1,4,5-Trisphosphate receptor down-regulated bovine eggs. Biol Reprod 73: 213.
  • Marangos P, Carroll J. 2004a. The dynamics of cyclin B1 distribution during meiosis I in mouse oocytes. Reproduction 128(2): 153162.
  • Marangos P, Carroll J. 2004b. Fertilization and InsP3-induced Ca2+ release stimulate a persistent increase in the rate of degradation of cyclin B1 specifically in mature mouse oocytes. Dev Biol 272(1): 2638.
  • Marangos P, FitzHarris G, Carroll J. 2003. Ca2+ oscillations at fertilization in mammals are regulated by the formation of pronuclei. Development 130(7): 14611472.
  • Markoulaki S, Matson S, Abbott AL, Ducibella T. 2003. Oscillatory CaMKII activity in mouse egg activation. Dev Biol 258(2): 464474.
  • Markoulaki S, Matson S, Ducibella T. 2004. Fertilization stimulates long-lasting oscillations of CaMKII activity in mouse eggs. Dev Biol 272(1): 1525.
  • Masui Y. 1974. A cytostatic factor in amphibian oocytes: Its extraction and partial characterization. J Exp Zool 187(1): 141147.
  • McAvey BA, Wortzman GB, Williams CJ, Evans JP. 2002. Involvement of calcium signaling and the actin cytoskeleton in the membrane block to polyspermy in mouse eggs. Biol Reprod 67(4): 13421352.
  • Mehlmann LM, Jaffe LA. 2005. SH2 domain-mediated activation of an SRC family kinase is not required to initiate Ca2+ release at fertilization in mouse eggs. Reproduction 129(5): 557564.
  • Mehlmann LM, Kline D. 1994. Regulation of intracellular calcium in the mouse egg: Calcium release in response to sperm or inositol trisphosphate is enhanced after meiotic maturation. Biol Reprod 51(6): 10881098.
  • Mehlmann LM, Terasaki M, Jaffe LA, Kline D. 1995. Reorganization of the endoplasmic reticulum during meiotic maturation of the mouse oocyte. Dev Biol 170(2): 607615.
  • Mehlmann LM, Mikoshiba K, Kline D. 1996. Redistribution and increase in cortical inositol 1,4,5-trisphosphate receptors after meiotic maturation of the mouse oocyte. Dev Biol 180(2): 489498.
  • Mehlmann LM, Carpenter G, Rhee SG, Jaffe LA. 1998. SH2 domain-mediated activation of phospholipase C gamma is not required to initiate Ca2+ release at fertilization of mouse eggs. Dev Biol 203(1): 221232.
  • Mehlmann LM, Chattopadhyay A, Carpenter G, Jaffe LA. 2001. Evidence that phospholipase C from the sperm is not responsible for initiating Ca2+ release at fertilization in mouse eggs. Dev Biol 236(2): 492501.
  • Miyazaki S. 1988. Inositol 1,4,5-trisphosphate-induced calcium release and guanine nucleotide-binding protein-mediated periodic calcium rises in golden hamster eggs. J Cell Biol 106(2): 345353.
  • Miyazaki S, Shirakawa H, Nakada K, Honda Y. 1993. Essential role of the inositol 1,4,5-trisphosphate receptor/Ca2+ release channel in Ca2+ waves and Ca2+ oscillations at fertilization of mammalian eggs. Dev Biol 158(1): 6278.
  • Moore GD, Ayabe T, Visconti PE, Schultz RM, Kopf GS. 1994. Roles of heterotrimeric and monomeric G proteins in sperm-induced activation of mouse eggs. Development 120(11): 33133323.
  • Nakada K, Mizuno J, Shiraishi K, Endo K, Miyazaki S. 1995. Initiation, persistence, and cessation of the series of intracellular Ca2+ responses during fertilization of bovine eggs. J Reprod Dev 41: 7784.
  • Nixon VL, McDougall A, Jones KT. 2000. Ca2+ oscillations and the cell cycle at fertilisation of mammalian and ascidian eggs. Biol Cell 92(3–4): 187196.
  • Nixon VL, Levasseur M, McDougall A, Jones KT. 2002. Ca2+ oscillations promote APC/C-dependent cyclin B1 degradation during metaphase arrest and completion of meiosis in fertilizing mouse eggs. Curr Biol 12(9): 746750.
  • O'Neill FJ, Gillett J, Foltz KR. 2004. Distinct roles for multiple Src family kinases at fertilization. J Cell Sci 117(Pt 25): 62276238.
  • Ozil JP. 1990. The parthenogenetic development of rabbit oocytes after repetitive pulsatile electrical stimulation. Development 109(1): 117127.
  • Ozil JP, Huneau D. 2001. Activation of rabbit oocytes: The impact of the Ca2+ signal regime on development. Development 128(6): 917928.
  • Ozil JP, Markoulaki S, Toth S, Matson S, Banrezes B, Knott JG, Schultz RM, Huneau D, Ducibella T. 2005. Egg activation events are regulated by the duration of a sustained [Ca2+]cyt signal in the mouse. Dev Biol 282: 3954.
  • Palermo G, Joris H, Devroey P, Van Steirteghem AC. 1992. Pregnancies after intracytoplasmic injection of single spermatozoon into an oocyte. Lancet 340(8810): 1718.
  • Parrington J, Swann K, Shevchenko VI, Sesay AK, Lai FA. 1996. Calcium oscillations in mammalian eggs triggered by a soluble sperm protein. Nature 379(6563): 364368.
  • Parrington J, Brind S, De Smedt H, Gangeswaran R, Lai FA, Wojcikiewicz R, Carroll J. 1998. Expression of inositol 1,4,5-trisphosphate receptors in mouse oocytes and early embryos: The type I isoform is upregulated in oocytes and downregulated after fertilization. Dev Biol 203(2): 451461.
  • Parrington J, Jones ML, Tunwell R, Devader C, Katan M, Swann K. 2002. Phospholipase C isoforms in mammalian spermatozoa: Potential components of the sperm factor that causes Ca2+ release in eggs. Reproduction 123(1): 3139.
  • Perry AC, Wakayama T, Yanagimachi R. 1999. A novel trans-complementation assay suggests full mammalian oocyte activation is coordinately initiated by multiple, submembrane sperm components. Biol Reprod 60(3): 747755.
  • Perry AC, Wakayama T, Cooke IM, Yanagimachi R. 2000. Mammalian oocyte activation by the synergistic action of discrete sperm head components: Induction of calcium transients and involvement of proteolysis. Dev Biol 217(2): 386393.
  • Reimann JD, Jackson PK. 2002. Emi1 is required for cytostatic factor arrest in vertebrate eggs. Nature 416(6883): 850854.
  • Reimann JD, Gardner BE, Margottin-Goguet F, Jackson PK. 2001. Emi1 regulates the anaphase-promoting complex by a different mechanism than Mad2 proteins. Genes Dev 15(24): 32783285.
  • Rhee SG. 2001. Regulation of phosphoinositide-specific phospholipase C. Annu Rev Biochem 70: 281312.
  • Rice A, Parrington J, Jones KT, Swann K. 2000. Mammalian sperm contain a Ca2+-sensitive phospholipase C activity that can generate InsP3 from PIP2 associated with intracellular organelles. Dev Biol 228(1): 125135.
  • Rogers NT, Hobson E, Pickering S, Lai FA, Braude P, Swann K. 2004. Phospholipase Czeta causes Ca2+ oscillations and parthenogenetic activation of human oocytes. Reproduction 128(6): 697702.
  • Rongish BJ, Wu W, Kinsey WH. 1999. Fertilization-induced activation of phospholipase C in the sea urchin egg. Dev Biol 215(2): 147154.
  • Runft LL, Carroll DJ, Gillett J, Giusti AF, O'Neill FJ, Foltz KR. 2004. Identification of a starfish egg PLC gamma that regulates Ca2+ release at fertilization. Dev Biol 269(1): 220236.
  • Santos F, Hendrich B, Reik W, Dean W. 2002. Dynamic reprogramming of DNA methylation in the early mouse embryo. Dev Biol 241(1): 172182.
  • Sarmento OF, Digilio LC, Wang Y, Perlin J, Herr JC, Allis CD, Coonrod SA. 2004. Dynamic alterations of specific histone modifications during early murine development. J Cell Sci 117(Pt 19): 44494459.
  • Sato K, Tokmakov AA, Iwasaki T, Fukami Y. 2000. Tyrosine kinase-dependent activation of phospholipase Cgamma is required for calcium transient in Xenopus egg fertilization. Dev Biol 224(2): 453469.
  • Sato K, Tokmakov AA, He CL, Kurokawa M, Iwasaki T, Shirouzu M, Fissore RA, Yokoyama S, Fukami Y. 2003. Reconstitution of Src-dependent phospholipase Cgamma phosphorylation and transient calcium release by using membrane rafts and cell-free extracts from Xenopus eggs. J Biol Chem 278(40): 3841338420.
  • Saunders CM, Larman MG, Parrington J, Cox LJ, Royse J, Blayney LM, Swann K, Lai FA. 2002. PLC zeta: A sperm-specific trigger of Ca(2+) oscillations in eggs and embryo development. Development 129(15): 35333544.
  • Schultz RM, Kopf GS. 1995. Molecular basis of mammalian egg activation. Curr Top Dev Biol 30: 2162.
  • Sette C, Paronetto MP, Barchi M, Bevilacqua A, Geremia R, Rossi P. 2002. Tr-kit-induced resumption of the cell cycle in mouse eggs requires activation of a Src-like kinase. EMBO J 21(20): 53865395.
  • Shearer J, De Nadai C, Emily-Fenouil F, Gache C, Whitaker M, Ciapa B. 1999. Role of phospholipase Cgamma at fertilization and during mitosis in sea urchin eggs and embryos. Development 126(10): 22732284.
  • Shiraishi K, Okada A, Shirakawa H, Nakanishi S, Mikoshiba K, Miyazaki S. 1995. Developmental changes in the distribution of the endoplasmic reticulum and inositol 1,4,5-trisphosphate receptors and the spatial pattern of Ca2+ release during maturation of hamster oocytes. Dev Biol 170(2): 594606.
  • Stein P, Worrad DM, Belyaev ND, Turner BM, Schultz RM. 1997. Stage-dependent redistributions of acetylated histones in nuclei of the early preimplantation mouse embryo. Mol Reprod Dev 47(4): 421429.
  • Stice SL, Robl JM. 1990. Activation of mammalian oocytes by a factor obtained from rabbit sperm. Mol Reprod Dev 25(3): 272280.
  • Stith BJ, Espinoza R, Roberts D, Smart T. 1994. Sperm increase inositol 1,4,5-trisphosphate mass in Xenopuslaevis eggs preinjected with calcium buffers or heparin. Dev Biol 165(1): 206215.
  • Strahl BD, Allis CD. 2000. The language of covalent histone modifications. Nature 403(6765): 4145.
  • Stricker SA. 1996. Repetitive calcium waves induced by fertilization in the nemertean worm Cerebratulus lacteus. Dev Biol 176(2): 243263.
  • Stricker SA. 1997. Intracellular injections of a soluble sperm factor trigger calcium oscillations and meiotic maturation in unfertilized oocytes of a marine worm. Dev Biol 186(2): 185201.
  • Stricker SA. 1999. Comparative biology of calcium signaling during fertilization and egg activation in animals. Dev Biol 211(2): 157176.
  • Stricker SA, Smythe TL. 2003. Endoplasmic reticulum reorganizations and Ca2+ signaling in maturing and fertilized oocytes of marine protostome worms: the roles of MAPKs and MPF. Development 130(13): 28672879.
  • Stricker SA, Silva R, Smythe T. 1998. Calcium and endoplasmic reticulum dynamics during oocyte maturation and fertilization in the marine worm Cerebratulus lacteus. Dev Biol 203(2): 305322.
  • Sutovsky P, Manandhar G, Wu A, Oko R. 2003. Interactions of sperm perinuclear theca with the oocyte: Implications for oocyte activation, anti-polyspermy defense, and assisted reproduction. Microsc Res Tech 61(4): 362378.
  • Swann K. 1990. A cytosolic sperm factor stimulates repetitive calcium increases and mimics fertilization in hamster eggs. Development 110(4): 12951302.
  • Talmor A, Kinsey WH, Shalgi R. 1998. Expression and immunolocalization of p59c-fyn tyrosine kinase in rat eggs. Dev Biol 194(1): 3846.
  • Talmor-Cohen A, Tomashov-Matar R, Eliyahu E, Shapiro R, Shalgi R. 2004. Are Src family kinases involved in cell cycle resumption in rat eggs? Reproduction 127(4): 455463.
  • Tatone C, Delle Monache S, Iorio R, Caserta D, Di Cola M, Colonna R. 2002. Possible role for Ca2+ calmodulin-dependent protein kinase II as an effector of the fertilization Ca2+ signal in mouse oocyte activation. Mol Hum Reprod 8(8): 750757.
  • Tesarik J, Testart J. 1994. Treatment of sperm-injected human oocytes with Ca2+ ionophore supports the development of Ca2+ oscillations. Biol Reprod 51(3): 385391.
  • Tokmakov AA, Sato KI, Iwasaki T, Fukami Y. 2002. Src kinase induces calcium release in Xenopus egg extracts via PLC gamma and IP3-dependent mechanism. Cell Calcium 32(1): 1120.
  • Tung JJ, Hansen DV, Ban KH, Loktev AV, Summers MK, Adler JR 3rd, Jackson PK. 2005. A role for the anaphase-promoting complex inhibitor Emi2/XErp1, a homolog of early mitotic inhibitor 1, in cytostatic factor arrest of Xenopus eggs. Proc Natl Acad Sci 102(12): 43184323.
  • Turner PR, Sheetz MP, Jaffe LA. 1984. Fertilization increases the polyphosphoinositide content of sea urchin eggs. Nature 310(5976): 414415.
  • Tutuncu L, Stein P, Ord TS, Jorgez CJ, Williams CJ. 2004. Calreticulin on the mouse egg surface mediates transmembrane signaling linked to cell cycle resumption. Dev Biol 270(1): 246260.
  • Vitullo AD, Ozil JP. 1992. Repetitive calcium stimuli drive meiotic resumption and pronuclear development during mouse oocyte activation. Dev Biol 151(1): 128136.
  • Vossenaar ER, Radstake TR, van der Heijden A, van Mansum MA, Dieteren C, de Rooij DJ, Barrera P, Zendman AJ, van Venrooij WJ. 2004. Expression and activity of citrullinating peptidylarginine deiminase enzymes in monocytes and macrophages. Ann Rheum Dis 63(4): 373381.
  • Wang Y, Wysocka J, Sayegh J, Lee YH, Perlin JR, Leonelli L, Sonbuchner LS, McDonald CH, Cook RG, Dou Y, Roeder RG, Clarke S, Stallcup MR, Allis CD, Coonrod SA. 2004. Human PAD4 regulates histone arginine methylation levels via demethylimination. Science 306(5694): 279283.
  • Williams RL, Katan M. 1996. Structural views of phosphoinositide-specific phospholipase C: Signalling the way ahead. Structure 4(12): 13871394.
  • Williams CJ, Mehlmann LM, Jaffe LA, Kopf GS, Schultz RM. 1998. Evidence that Gq family G proteins do not function in mouse egg activation at fertilization. Dev Biol 198(1): 116127.
  • Wright PW, Bolling LC, Calvert ME, Sarmento OF, Berkeley EV, Shea MC, Hao Z, Jayes FC, Bush LA, Shetty J, Shore AN, Reddi PP, Tung KS, Samy E, Allietta MM, Sherman NE, Herr JC, Coonrod SA. 2003. ePAD, an oocyte and early embryo-abundant peptidylarginine deiminase-like protein that localizes to egg cytoplasmic sheets. Dev Biol 256(1): 7388.
  • Wu H, He CL, Fissore RA. 1997. Injection of a porcine sperm factor triggers calcium oscillations in mouse oocytes and bovine eggs. Mol Reprod Dev 46(2): 176189.
  • Wu H, Smyth J, Luzzi V, Fukami K, Takenawa T, Black SL, Allbritton NL, Fissore RA. 2001. Sperm factor induces intracellular free calcium oscillations by stimulating the phosphoinositide pathway. Biol Reprod 64(5): 13381349.
  • Xu Z, Williams CJ, Kopf GS, Schultz RM. 2003. Maturation-associated increase in IP3 receptor type 1: Role in conferring increased IP3 sensitivity and Ca2+ oscillatory behavior in mouse eggs. Dev Biol 254(2): 163171.
  • Yamamoto TM, Iwabuchi M, Ohsumi K, Kishimoto T. 2005. APC/C-Cdc20-mediated degradation of cyclin B participates in CSF arrest in unfertilized Xenopus eggs. Dev Biol 279(2): 345355.
  • Yoda A, Oda S, Shikano T, Kouchi Z, Awaji T, Shirakawa H, Kinoshita K, Miyazaki S. 2004. Ca2+ oscillation-inducing phospholipase C zeta expressed in mouse eggs is accumulated to the pronucleus during egg activation. Dev Biol 268(2): 245257.
  • Yu H, King RW, Peters JM, Kirschner MW. 1996. Identification of a novel ubiquitin-conjugating enzyme involved in mitotic cyclin degradation. Curr Biol 6(4): 455466.
  • Zhu CC, Wojcikiewicz RJ. 2000. Ligand binding directly stimulates ubiquitination of the inositol 1,4,5-trisphosphate receptor. Biochem J 348(Pt 3): 551556.