Cell cycle-coupled [Ca(2+)](i) oscillations in mouse zygotes and function of the inositol 1,4,5-trisphosphate receptor-1

Zn2+ is essential for Ca2+ oscillations in mouse eggs

Changes in the intracellular concentration of free calcium (Ca2+) underpin egg activation and initiation of development in animals and plants. In mammals, the Ca2+ release is periodical, known as Ca2+ oscillations, and mediated by the type 1 inositol 1,4,5-trisphosphate receptor (IP3R1). Another divalent cation, zinc (Zn2+), increases exponentially during oocyte maturation and is vital for meiotic transitions, arrests, and polyspermy prevention. It is unknown if these pivotal cations interplay during fertilization. Here, using mouse eggs, we showed that basal concentrations of labile Zn2+ are indispensable for sperm-initiated Ca2+ oscillations because Zn2+-deficient conditions induced by cell-permeable chelators abrogated Ca2+ responses evoked by fertilization and other physiological and pharmacological agonists. We also found that chemically or genetically generated eggs with lower levels of labile Zn2+ displayed reduced IP3R1 sensitivity and diminished ER Ca2+ leak despite the stable content of the stores and IP3R1 mass. Resupplying Zn2+ restarted Ca2+ oscillations, but excessive Zn2+ prevented and terminated them, hindering IP3R1 responsiveness. The findings suggest that a window of Zn2+ concentrations is required for Ca2+ responses and IP3R1 function in eggs, ensuring optimal response to fertilization and egg activation.

Zn2+ is Essential for Ca2+ Oscillations in Mouse Eggs

Changes in the intracellular concentration of free calcium (Ca2+) underpin egg activation and initiation of development in animals and plants. In mammals, the Ca2+ release is periodical, known as Ca2+ oscillations, and mediated by the type 1 inositol 1,4,5-trisphosphate receptor (IP3R1). Another divalent cation, zinc (Zn2+), increases exponentially during oocyte maturation and is vital for meiotic transitions, arrests, and polyspermy prevention. It is unknown if these pivotal cations interplay during fertilization. Here, using mouse eggs, we showed that basal concentrations of labile Zn2+ are indispensable for sperm-initiated Ca2+ oscillations because Zn2+-deficient conditions induced by cell-permeable chelators abrogated Ca2+ responses evoked by fertilization and other physiological and pharmacological agonists. We also found that chemically- or genetically generated eggs with lower levels of labile Zn2+ displayed reduced IP3R1 sensitivity and diminished ER Ca2+ leak despite the stable content of the stores and IP3R1 mass. Resupplying Zn2+ restarted Ca2+ oscillations, but excessive Zn2+ prevented and terminated them, hindering IP3R1 responsiveness. The findings suggest that a window of Zn2+ concentrations is required for Ca2+ responses and IP3R1 function in eggs, ensuring optimal response to fertilization and egg activation.

Voltage Dependent N Type Calcium Channel in Mouse Egg Fertilization

Repetitive changes in the intracellular calcium concentration ([Ca²⁺]i) triggers egg activation, including cortical granule exocytosis, resumption of second meiosis, block to polyspermy, and initiating embryonic development. [Ca²⁺]i oscillations that continue for several hours, are required for the early events of egg activation and possibly connected to further development to the blastocyst stage. The sources of Ca²⁺ ion elevation during [Ca²⁺]i oscillations are Ca²⁺ release from endoplasmic reticulum through inositol 1,4,5 tri-phosphate receptor and Ca²⁺ ion influx through Ca²⁺ channel on the plasma membrane. Ca²⁺ channels have been characterized into voltage-dependent Ca²⁺ channels (VDCCs), ligand-gated Ca²⁺ channel, and leak-channel. VDCCs expressed on muscle cell or neuron is specified into L, T, N, P, Q, and R type VDCs by their activation threshold or their sensitivity to peptide toxins isolated from cone snails and spiders. The present study was aimed to investigate the localization pattern of N and P/Q type voltage-dependent calcium channels in mouse eggs and the role in fertilization. [Ca²⁺]i oscillation was observed in a Ca²⁺ contained medium with sperm factor or adenophostin A injection but disappeared in Ca²⁺ free medium. Ca²⁺ influx was decreased by Lat A. N-VDCC specific inhibitor, ω-Conotoxin CVIIA induced abnormal [Ca²⁺]i oscillation profiles in SrCl₂ treatment. N or P/Q type VDC were distributed on the plasma membrane in cortical cluster form, not in the cytoplasm. Ca²⁺ influx is essential for [Ca²⁺]i oscillation during mammalian fertilization. This Ca²⁺ influx might be controlled through the N or P/Q type VDCCs. Abnormal VDCCs expression of eggs could be tested in fertilization failure or low fertilization eggs in subfertility women.

Ca2+ Signaling and Homeostasis in Mammalian Oocytes and Eggs

Changes in the intracellular concentration of calcium ([Ca2+]i) represent a vital signaling mechanism enabling communication between and among cells as well as with the environment. Cells have developed a sophisticated set of molecules, "the Ca2+ toolkit," to adapt [Ca2+]i changes to specific cellular functions. Mammalian oocytes and eggs, the subject of this review, are not an exception, and in fact the initiation of embryo devolvement in all species is entirely dependent on distinct [Ca2+]i responses. Here, we review the components of the Ca2+ toolkit present in mammalian oocytes and eggs, the regulatory mechanisms that allow these cells to accumulate Ca2+ in the endoplasmic reticulum, release it, and maintain basal and stable cytoplasmic concentrations. We also discuss electrophysiological and genetic studies that have uncovered Ca2+ influx channels in oocytes and eggs, and we analyze evidence supporting the role of a sperm-specific phospholipase C isoform as the trigger of Ca2+ oscillations during mammalian fertilization including its implication in fertility.

Postovulatory ageing modifies sperm-induced Ca2+ oscillations in mouse oocytes through a conditions-dependent, multi-pathway mechanism

Postovulatory ageing of mammalian oocytes occurs between their ovulation and fertilization and has been shown to decrease their developmental capabilities. Aged oocytes display numerous abnormalities, including altered Ca²⁺ signalling. Fertilization-induced Ca²⁺ oscillations are essential for activation of the embryonic development, therefore maintaining proper Ca²⁺ homeostasis is crucial for the oocyte quality. In the present paper, we show that the mechanism underlying age-dependent alterations in the pattern of sperm-triggered Ca²⁺ oscillations is more complex and multifaceted than previously believed. Using time-lapse imaging accompanied by immunostaining and molecular analyses, we found that postovulatory ageing affects the amount of Ca²⁺ stored in the cell, expression of Ca²⁺ pump SERCA2, amount of available ATP and distribution of endoplasmic reticulum and mitochondria in a manner often strongly depending on ageing conditions (in vitro vs. in vivo). Importantly, those changes do not have to be caused by oxidative stress, usually linked with the ageing process, as they occur even if the amount of reactive oxygen species remains low. Instead, our results suggest that aberrations in Ca²⁺ signalling may be a synergistic result of ageing-related alterations of the cell cycle, cytoskeleton, and mitochondrial functionality.

Role of Type 1 Inositol 1,4,5-triphosphate Receptors in Mammalian Oocytes

The ability of oocytes to undergo normal fertilization and embryo development is acquired during oocyte maturation which is transition from the germinal vesicle stage (GV), germinal vesicle breakdown (GVBD) to metaphase of meiosis II (MII). Part of this process includes redistribution of inositol 1,4, 5-triphosphate receptor (IP3R), a predominant Ca²⁺ channel on the endoplasmic reticulum membrane. Type 1 IP3R (IP3R1) is expressed in mouse oocytes dominantly. At GV stage, IP3R1 are arranged as a network throughout the cytoplasm with minute accumulation around the nucleus. At MII stage, IP3R1 diffuses to the entire cytoplasm in a more reticular manner, and obvious clusters of IP3R1 are observed at the cortex of the egg. This structural reorganization provides acquisition of [Ca²⁺]_i oscillatory activity during fertilization. In this review, general properties of IP3R1 in somatic cells and mammalian oocyte are introduced.

Transcriptional status of mouse oocytes corresponds with their ability to generate Ca2+ release

In fully grown ovarian follicles both transcriptionally active (NSN) and inactive (SN) oocytes are present. NSN oocytes have been shown to display lower developmental potential. It is possible that oocytes that have not completed transcription before meiosis resumption accumulate less RNA and proteins required for their further development, including those responsible for regulation of Ca2+ homeostasis. Oscillations of the cytoplasmic concentration of free Ca2+ ions ([Ca2+]i) are triggered in oocytes by a fertilizing spermatozoon and are crucial for inducing and regulating further embryonic development. We showed that NSN-derived oocytes express less inositol 1,4,5-triphosphate receptor type 1 (IP3R1), store less Ca2+ ions and generate weaker spontaneous [Ca2+]i oscillations during maturation than SN oocytes. Consequently, NSN oocytes display aberrant [Ca2+]i oscillations at fertilization. We speculate that this defective regulation of Ca2+ homeostasis might be one of the factors responsible for the lower developmental potential of NSN oocytes.

Calcium influx and sperm-evoked calcium responses during oocyte maturation and egg activation

Under the guidance and regulation of hormone signaling, large majority of mammalian oocytes go through twice cell cycle arrest-resumption prior to the fertilized egg splits: oocyte maturation and egg activation. Cytosolic free calcium elevations and endoplasmic reticulum calcium store alternations are actively involved in triggering the complex machineries and events during oogenesis. Among these, calcium influx had been implicated in the replenishment of endoplasmic reticulum store during oocyte maturation and calcium oscillation during egg activation. This process also drove successful fertilization and early embryo development. Store-operated Ca²⁺ entry, acts as the principal force of calcium influx, is composed of STIM1 and Orai1 on the plasma membrane. Besides, transient receptor potential channels also participate in the process of calcium inwards. In this review, we summarize the recent researches on the spatial-temporal distribution of store-operated calcium entry components and transient receptor potential channels. Questions about how these channels play function for calcium influx and what impacts these channels have on oocytes are discussed. At the time of sperm-egg fusion, sperm-specific factor(s) diffuse and enable eggs to mount intracellular calcium oscillations. In this review, we also focus on the basic knowledge and the modes of action of the potential sperm factor phospholipase C zeta, as well as the downstream receptor, type 1 inositol 1,4,5-trisphosphate receptor. From the achievement in the previous several decades, it is easy to find that there are too many doubtful points in the field that need researchers take into consideration and take action in the future.

The effect of M-phase stage-dependent kinase inhibitors on inositol 1,4,5-trisphosphate receptor 1 (IP3 R1) expression and localization in pig oocytes

At fertilization, inositol 1,4,5-trisphosphate receptor type 1 (IP3 R1) has a crucial role in Ca(2+) release in mammals. Expression levels, localization and phosphorylation of IP3 R1 are important for its function, but it still remains unclear which molecule(s) regulates IP3 R1 behavior in pig oocytes. We examined whether there was a difference in localization of IP3 R1 after in vitro or in vivo maturation of pig oocytes. In mouse oocytes, large clusters of IP3 R1 were formed in the cortex of the oocyte except in a ring-shaped band of cortex adjacent to the spindle. However, no such clusters of IP3 R1 were observed in pig oocytes and there was no difference in its localization between in vitro and in vivo matured oocytes. We next tried to clarify which factor(s) regulates IP3 R1 localization, phosphorylation and expression using M-phase stage-dependent kinase inhibitors. Our results show that treatments with roscovitine (p34(cdc2) kinase inhibitor) or U0126 (mitogen-activated protein kinase inhibitor) did not affect IP3 R1 expression or localization in pig oocytes, although the latter strongly inhibited phosphorylation. However, treatment with BI-2536, an inhibitor of polo-like kinase 1 (Plk1), dramatically decreased the expression level of IP3 R1 in pig oocytes in a dose-dependent manner. From these results, it is suggested that Plk1 is involved in the regulation of IP3 R1 expression in pig oocytes.

Role of caspase-3 cleaved IP3 R1 on Ca(2+) homeostasis and developmental competence of mouse oocytes and eggs

Apoptosis in most cell types is accompanied by altered Ca(2+) homeostasis. During apoptosis, caspase-3 mediated cleavage of the type 1 inositol 1,4,5-trisphosphate receptor (IP3 R1) generates a 95-kDa C-terminal fragment (C-IP3 R1), which represents the channel domain of the receptor. Aged mouse eggs display abnormal Ca(2+) homeostasis and express C-IP3 R1, although whether or not C-IP3 R1 expression contributes to Ca(2+) misregulation or a decrease in developmental competency is unknown. We sought to answer these questions by injecting in mouse oocytes and eggs cRNAs encoding C-IP3 R1. We found that: (1) expression of C-IP3 R1 in eggs lowered the Ca(2+) content of the endoplasmic reticulum (ER), although, as C-IP3 R1 is quickly degraded at this stage, its expression did not impair pre-implantation embryo development; (2) expression of C-IP3 R1 in eggs enhanced fragmentation associated with aging; (3) endogenous IP3 R1 is required for aging associated apoptosis, as its down-regulation prevented fragmentation, and expression of C-IP3 R1 in eggs with downregulated IP3 R1 partly restored fragmentation; (4) C-IP3 R1 expression in GV oocytes resulted in persistent levels of protein, which abolished the increase in the ER releasable Ca(2+) pool that occurs during maturation, undermined the Ca(2+) oscillatory ability of matured eggs and their activation potential. Collectively, this study supports a role for IP3 R1 and C-IP3 R1 in regulating Ca(2+) homeostasis and the ER Ca(2+) content during oocyte maturation. Nevertheless, the role of C-IP3 R1 on Ca(2+) homeostasis in aged eggs seems minor, as in MII eggs the majority of endogenous IP3 R1 remains intact and C-IP3 R1 undergoes rapid turnover.

How to make a good egg!: The need for remodeling of oocyte Ca(2+) signaling to mediate the egg-to-embryo transition

The egg-to-embryo transition marks the initiation of multicellular organismal development and is mediated by a specialized Ca(2+) transient at fertilization. This explosive Ca(2+) signal has captured the interest and imagination of scientists for many decades, given its cataclysmic nature and necessity for the egg-to-embryo transition. Learning how the egg acquires the competency to generate this Ca(2+) transient at fertilization is essential to our understanding of the mechanisms controlling egg and the transition to embryogenesis. In this review we discuss our current knowledge of how Ca(2+) signaling pathways remodel during oocyte maturation in preparation for fertilization with a special emphasis on the frog oocyte as additional reviews in this issue will touch on this in other species.

Caffeine alleviates the deterioration of Ca(2+) release mechanisms and fragmentation of in vitro-aged mouse eggs

The developmental competence of mammalian eggs is compromised by postovulatory aging. We and others have found that in these eggs, the intracellular calcium ([Ca(2+)](i)) responses required for egg activation and initiation of development are altered. Nevertheless, the mechanism(s) underlying this defective Ca(2+) release is not well known. Here, we investigated if the function of IP(3)R1, the major Ca(2+) release channel at fertilization, was undermined in in vitro-aged mouse eggs. We found that in aged eggs, IP(3)R1 displayed reduced function as many of the changes acquired during maturation that enhance IP(3)R1 Ca(2+) conductivity, such as phosphorylation, receptor reorganization and increased Ca(2+) store content ([Ca(2+)](ER)), were lost with increasing postovulatory time. IP(3)R1 fragmentation, possibly associated with the activation of caspase-3, was also observed in these eggs. Many of these changes were prevented when the postovulatory aging of eggs was carried out in the presence of caffeine, which minimized the decline in IP(3)R(1) function and maintained [Ca(2+)](ER) content. Caffeine also maintained mitochondrial membrane potential, as measured by JC-1 fluorescence. We therefore conclude that [Ca(2+)](i) responses in aged eggs are undermined by reduced IP(3)R1 sensitivity, decreased [Ca(2+)](ER) , and compromised mitochondrial function, and that addition of caffeine ameliorates most of these aging-associated changes. Understanding the molecular basis of the protective effects of caffeine will be useful in elucidating, and possibly reversing, the signaling pathway(s) compromised by in vitro culture of eggs.

Regulation of inositol 1,4,5-trisphosphate receptor function during mouse oocyte maturation

At the time of fertilization, an increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) underlies egg activation and initiation of development in all species studied to date. The inositol 1,4,5-trisphosphate receptor (IP(3)R1), which is mostly located in the endoplasmic reticulum (ER) mediates the majority of this Ca(2+) release. The sensitivity of IP(3)R1, that is, its Ca(2+) releasing capability, is increased during oocyte maturation so that the optimum [Ca(2+)](i) response concurs with fertilization, which in mammals occurs at metaphase of second meiosis. Multiple IP(3)R1 modifications affect its sensitivity, including phosphorylation, sub-cellular localization, and ER Ca(2+) concentration ([Ca(2+)](ER)). Here, we evaluated using mouse oocytes how each of these factors affected IP(3)R1 sensitivity. The capacity for IP(3)-induced Ca(2+) release markedly increased at the germinal vesicle breakdown stage, although oocytes only acquire the ability to initiate fertilization-like oscillations at later stages of maturation. The increase in IP(3)R1 sensitivity was underpinned by an increase in [Ca(2+)](ER) and receptor phosphorylation(s) but not by changes in IP(3)R1 cellular distribution, as inhibition of the former factors reduced Ca(2+) release, whereas inhibition of the latter had no impact. Therefore, the results suggest that the regulation of [Ca(2+)](ER) and IP(3)R1 phosphorylation during maturation enhance IP(3)R1 sensitivity rendering oocytes competent to initiate oscillations at the expected time of fertilization. The temporal discrepancy between the initiation of changes in IP(3)R1 sensitivity and acquisition of mature oscillatory capacity suggest that other mechanisms that regulate Ca(2+) homeostasis also shape the pattern of oscillations in mammalian eggs.

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

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

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

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

Regulation of inositol 1,4,5-trisphosphate receptor type 1 function during oocyte maturation by MPM-2 phosphorylation

Egg activation and further embryo development require a sperm-induced intracellular Ca(2+) signal at the time of fertilization. Prior to fertilization, the egg's Ca(2+) machinery is therefore optimized. To this end, during oocyte maturation, the sensitivity, i.e. the Ca(2+) releasing ability, of the inositol 1,4,5-trisphosphate receptor type 1 (IP(3)R1), which is responsible for most of this Ca(2+) release, markedly increases. In this study, the recently discovered specific Polo-like kinase (Plk) inhibitor BI2536 was used to investigate the role of Plk1 in this process. BI2536 inactivates Plk1 in oocytes at the early stages of maturation and significantly decreases IP(3)R1 phosphorylation at an MPM-2 epitope at this stage. Moreover, this decrease in Plk1-dependent MPM-2 phosphorylation significantly lowers IP(3)R1 sensitivity. Finally, using in vitro phosphorylation techniques we identified T(2656) as a major Plk1 site on IP(3)R1. We therefore propose that the initial increase in IP(3)R1 sensitivity during oocyte maturation is underpinned by IP(3)R1 phosphorylation at an MPM-2 epitope(s).

Acetylcholine rescues two-cell block through activation of IP3 receptors and Ca2+/calmodulin-dependent kinase II in an ICR mouse strain

Acetylcholine (ACh) causes early activation events in mouse oocytes, but little is known about its precise role in the early embryonic development of mice. We aimed to determine whether and how ACh is capable of rescuing two-cell block in an in vitro culture system. ACh evoked different transient Ca(2+) patterns showing a higher Ca(2+) peak in the two-cell stage embryos (two-cells) than observed in mature oocytes. In early two-cells subjected to an in vitro two-cell block, xestospongin C (Xes-C), an IP3 receptor antagonist, significantly decreased the level of the ACh-induced Ca(2+) increase. The reduction in the ACh-induced Ca(2+) increase by Xes-C in late two-cells was lower than that in early two-cells. Furthermore, KN62 and KN93, both CaMKII inhibitors, were found to reduce the magnitude of the ACh-induced Ca(2+) increase in early two-cells. The addition of ACh to the culture medium showed an ability to rescue in vitro two-cell block. However, the addition of ACh together with both Xes-C and CaMKII inhibitors or with either inhibitor separately had no effect on the rescue of two-cell block. Long-term exposure of late two-cells to ACh decreased morula and early blastocyst development and ACh had a differential effect on early and late two-cells. These results indicate that ACh likely rescues the in vitro two-cell block through activation of IP3R- and/or CaMKII-dependent signal transduction pathways.

Kinase-dependent regulation of inositol 1,4,5-trisphosphate-dependent Ca2+ release during oocyte maturation

Fertilization induces a species-specific Ca(2+) transient with specialized spatial and temporal dynamics, which are essential to temporally encode egg activation events such as the block to polyspermy and resumption of meiosis. Eggs acquire the competence to produce the fertilization-specific Ca(2+) transient during oocyte maturation, which encompasses dramatic potentiation of inositol 1,4,5-trisphosphate (IP(3))-dependent Ca(2+) release. Here we show that increased IP(3) receptor (IP(3)R) sensitivity is initiated at the germinal vesicle breakdown stage of maturation, which correlates with maturation promoting factor (MPF) activation. Extensive phosphopeptide mapping of the IP(3)R resulted in approximately 70% coverage and identified three residues, Thr-931, Thr-1136, and Ser-114, which are specifically phosphorylated during maturation. Phospho-specific antibody analyses show that Thr-1136 phosphorylation requires MPF activation. Activation of either MPF or the mitogen-activated protein kinase cascade independently, functionally sensitizes IP(3)-dependent Ca(2+) release. Collectively, these data argue that the kinase cascades driving meiotic maturation potentiates IP(3)-dependent Ca(2+) release, possibly trough direct phosphorylation of the IP(3)R.

Regulation of inositol 1,4,5-trisphosphate-induced Ca2+ release by reversible phosphorylation and dephosphorylation

The inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) is a universal intracellular Ca2+-release channel. It is activated after cell stimulation and plays a crucial role in the initiation and propagation of the complex spatio-temporal Ca2+ signals that control cellular processes as different as fertilization, cell division, cell migration, differentiation, metabolism, muscle contraction, secretion, neuronal processing, and ultimately cell death. To achieve these various functions, often in a single cell, exquisite control of the Ca2+ release is needed. This review aims to highlight how protein kinases and protein phosphatases can interact with the IP3R or with associated proteins and so provide a large potential for fine tuning the Ca2+-release activity and for creating efficient Ca2+ signals in subcellular microdomains.

Biological differences between in vitro produced bovine embryos and parthenotes

Parthenotes may represent an alternate ethical source of stem cells, once biological differences between parthenotes and embryos can be understood. In this study, we analyzed development, trophectoderm (TE) differentiation, apoptosis/necrosis, and ploidy in parthenotes and in vitro produced bovine embryos. Subsequently, using real-time PCR, we analyzed the expression of genes expected to underlie the observed differences at the blastocyst stage. In vitro matured oocytes were either fertilized or activated with ionomycin +6-DMAP and cultured in simple medium. Parthenotes showed enhanced blastocyst development and diploidy and reduced TE cell counts. Apoptotic and necrotic indexes did not vary, but parthenotes evidenced a higher relative proportion of apoptotic cells between inner cell mass and TE. The pluripotence-related POU5F1 and the methylation DNMT3A genes were downregulated in parthenotes. Among pregnancy recognition genes, TP-1 was upregulated in parthenotes, while PGRMC1 and PLAC8 did not change. Expression of p66(shc) and BAX/BCL2 ratio were higher, and p53 lower, in parthenotes. Among metabolism genes, SLC2A1 was downregulated, while AKR1B1, PTGS2, H6PD, and TXN were upregulated in parthenotes, and SLC2A5 did not differ. Among genes involved in compaction/blastulation, GJA1 was downregulated in parthenotes, but no differences were detected within ATP1A1 and CDH1. Within parthenotes, the expression levels of SLC2A1, TP-1, and H6PD, and possibly AKR1B1, resemble patterns described in female embryos. The pro-apoptotic profile is more pronounced in parthenotes than in embryos, which may differ in their way to channel apoptotic stimuli, through p66(shc) and p53 respectively, and in their mechanisms to control pluripotency and de novo methylation.

Inositol 1,4,5-trisphosphate receptor 1, a widespread Ca2+ channel, is a novel substrate of polo-like kinase 1 in eggs

To initiate embryo development, the sperm induces in the egg release of intracellular calcium ([Ca2+](i)). During oocyte maturation, the inositol 1,4,5-trisphosphate receptor (IP(3)R1), the channel implicated, undergoes modifications that enhance its function. We found that IP(3)R1 becomes phosphorylated during maturation at an MPM-2 epitope and that this persists until the fertilization-associated [Ca2+](i) responses cease. We also reported that maturation without ERK activity diminishes IP(3)R1 MPM-2 reactivity and [Ca2+](i) responses. Here, we show that IP(3)R1 is a novel target for Polo-like kinase1 (Plk1), a conserved M-phase kinase, which phosphorylates it at an MPM-2 epitope. Plk1 and IP(3)R1 interact in an M-phase preferential manner, and they exhibit close co-localization in the spindle/spindle poles area. This co-localization is reduced in the absence of ERK activity, as the ERK pathway regulates spindle organization and IP(3)R1 cortical re-distribution. We propose that IP(3)R1 phosphorylation by Plk1, and possibly by other M-phase kinases, underlies the delivery of spatially and temporally regulated [Ca2+](i) signals during meiosis/mitosis and cytokinesis.

Parthenogenetic activation of bovine oocytes using bovine and murine phospholipase C zeta

BACKGROUND

During natural fertilization, sperm fusion with the oocyte induces long lasting intracellular calcium oscillations which in turn are responsible for oocyte activation. PLCZ1 has been identified as the factor that the sperm delivers into the egg to induce such a response. We tested the hypothesis that PLCZ1 cRNA injection can be used to activate bovine oocytes.

RESULTS

Mouse and bovine PLCZ1 cRNAs were injected into matured bovine oocytes at different concentrations. Within the concentrations tested, mouse PLCZ1 injection activated bovine oocytes at a maximum rate when the pipette concentration of cRNA ranged from 0.25 to 1 mug/muL, while bovine PLCZ1 was optimal at 0.1 mug/muL. At their most effective concentrations, PLCZ1 induced parthenogenetic development at rates similar to those observed using other activation stimuli such as Ionomycin/CHX and Ionomycin/DMAP. Injection of mouse and bovine PLCZ1 cRNA induced dose-dependent sperm-like calcium oscillations whose frequency increased over time. Injection of bovine and mouse PLCZ1 cRNA also induced IP3R-1 degradation, although bovine PLCZ1 cRNA evoked greater receptor degradation than its mouse counterpart.

CONCLUSION

Injection of PLCZ1 cRNA efficiently activated bovine oocytes by inducing a sperm-like calcium oscillatory pattern. Importantly, the high rate of aneuploidy encountered in parthenogenetic embryos activated by certain chemical means was not observed in PLCZ1 activated embryos.

The roles of Ca2+, downstream protein kinases, and oscillatory signaling in regulating fertilization and the activation of development

Reviews in Developmental Biology have covered the pathways that generate the all-important intracellular calcium (Ca(2+)) signal at fertilization [Miyazaki, S., Shirakawa, H., Nakada, K., Honda, Y., 1993a. Essential role of the inositol 1,4,5-trisphosphate receptor/Ca(2+) release channel in Ca(2+) waves and Ca(2+) oscillations at fertilization of mammalian eggs. Dev. Biol. 158, 62-78; Runft, L., Jaffe, L., Mehlmann, L., 2002. Egg activation at fertilization: where it all begins. Dev. Biol. 245, 237-254] and the different temporal responses of Ca(2+) in many organisms [Stricker, S., 1999. Comparative biology of calcium signaling during fertilization and egg activation in animals. Dev. Biol. 211, 157-176]. Those reviews raise the importance of identifying how Ca(2+) causes the events of egg activation (EEA) and to what extent these temporal Ca(2+) responses encode developmental information. This review covers recent studies that have analyzed how these Ca(2+) signals are interpreted by specific proteins, and how these proteins regulate various EEA responsible for the onset of development. Many of these proteins are protein kinases (CaMKII, PKC, MPF, MAPK, MLCK) whose activity is directly or indirectly regulated by Ca(2+), and whose amount increases during late oocyte maturation. We cover biochemical progress in defining the signaling pathways between Ca(2+) and the EEA, as well as discuss how oscillatory or multiple Ca(2+) signals are likely to have specific advantages biochemically and/or developmentally. These emerging concepts are put into historical context, emphasizing that key contributions have come from many organisms. The intricate interdependence of Ca(2+), Ca(2+)-dependent proteins, and the EEA raise many new questions for future investigations that will provide insight into the extent to which fertilization-associated signaling has long-range implications for development. In addition, answers to these questions should be beneficial to establishing parameters of egg quality for human and animal IVF, as well as improving egg activation protocols for somatic cell nuclear transfer to generate stem cells and save endangered species.

Release of phospholipase C ζand [Ca2+]i oscillation-inducing activity during mammalian fertilization

During fertilization of mammalian eggs a factor from the sperm, the sperm factor (SF), is released into the ooplasm and induces persistent [Ca2+]ioscillations that are required for egg activation and embryo development. A sperm-specific phospholipase C (PLC), PLCz, is thought to be the SF. Here, we investigated whether the SF activity and PLCζare simultaneously and completely released into the ooplasm soon after sperm entry. To accomplish this, we enucleated sperm heads within 90 min of intracytoplasmic sperm injection (ICSI) and monitored the persistence of the [Ca2+]ioscillations in eggs in which the sperm had been withdrawn. We also stained the enucleatedsperm heads to ascertain the presence/absence of PLCζ. Our results show that by 90 min all the SF activity had been released from the sperm, as fertilized enucleated eggs oscillated as fertilized controls, even in cases in which oscillations were prolonged by arresting eggs at metaphase. In addition, we found that the released SF activity became associated with the pronucleus (PN), as induction of PN envelope breakdown evoked comparable [Ca2+]iresponses in enucleated and non-manipulated zygotes. Lastly, we found that PLCzlocalized to the equatorial area of bull sperm and to the post-acrosomal region of mouse sperm and that by 90 min after ICSI all the sperm’s PLCζimmunoreactivity was lost in both species. Altogether, our findings show that during fertilization the SF activity and PLCζimmunoreactivity are simultaneously released from the sperm, suggesting that PLCζmay be the only [Ca2+]ioscillation-inducing factor of mammalian sperm.

A novel mechanism controls the Ca2+ oscillations triggered by activation of ascidian eggs and has an absolute requirement for Cdk1 activity

Fertilisation in ascidians triggers a series of periodic rises in cytosolic Ca2+ that are essential for release from metaphase I arrest and progression through meiosis II. These sperm-triggered Ca2+ oscillations are switched off at exit from meiosis II. Ascidian zygotes provided the first demonstration of the positive feedback loop whereby elevated Cdk1 activity maintained these Ca2+ oscillations. Since then it has been reported that Cdk1 sensitises the type I inositol trisphosphate [Ins(1,4,5)P3] receptor in somatic cells, and that sperm-triggered Ca2+ oscillations in mouse zygotes stop because the forming pronuclei sequester phospholipase C zeta that was delivered to the egg by the fertilising sperm.

Here, using enucleation, we demonstrate in ascidian eggs that Ca2+ spiking stops at the correct time in the absence of pronuclei. Sequestration of sperm factor is therefore not involved in terminating Ca2+ spiking for these eggs. Instead we found that microinjection of the Cdk1 inhibitor p21 blocked Ca2+ spiking induced by ascidian sperm extract (ASE). However, such eggs were still capable of releasing Ca2+ in response to Ins(1,4,5)P3 receptor agonists, indicating that ASE-triggered Ca2+ oscillations can stop even though the response to Ins(1,4,5)P3 remained elevated. These data suggest that Cdk1 activity promotes Ins(1,4,5)P3 production in the presence of the sperm factor, rather than sensitising the Ca2+ releasing machinery to Ins(1,4,5)P3. These findings suggest a new link between this cell cycle kinase and the Ins(1,4,5)P3 pathway.

Changes in endoplasmic reticulum structure during mouse oocyte maturation are controlled by the cytoskeleton and cytoplasmic dynein

Oocyte maturation in mouse is associated with a dramatic reorganisation of the endoplasmic reticulum (ER) from a network of cytoplasmic accumulations in the germinal vesicle-stage oocyte (GV) to a network of distinctive cortical clusters in the metaphase II egg (MII). Multiple lines of evidence suggest that this redistribution of the ER is important to prepare the oocyte for the generation of repetitive Ca2+ transients which trigger egg activation at fertilisation. The aim of the current study was therefore to investigate the timecourse and mechanism of ER reorganisation during oocyte maturation. The ER is first restructured at the time of GV-breakdown (GVBD) into a dense network of membranes which envelop and invade the developing meiotic spindle. GVBD is essential for the initiation of ER reorganisation, since ER structure does not change in GV-arrested oocytes. ER reorganisation is also prevented by the microtubule inhibitor nocodazole and by the inhibition of cytoplasmic dynein, a microtubule-associated motor protein. ER redistribution at GVBD is therefore dynein-driven and cell cycle-dependent. Following GVBD the dense network of ER surrounds the spindle during its migration to the oocyte cortex. Cortical clusters of ER are formed close to the time of, but independently of the metaphase I-metaphase II transition. Formation of the characteristic ER clusters is prevented by the depolymerisation of microfilaments, but not of microtubules. These experiments reveal that ER reorganisation during oocyte maturation is a complex multi-step process involving distinct microtubule- and microfilament-dependent phases and indicate a role for dynein in the cytoplasmic changes which prepare the oocyte for fertilisation.

The MEK inhibitor, U0126, alters fertilization-induced [Ca2+]i oscillation parameters and secretion: differential effects associated with in vivo and in vitro meiotic maturation

Although mitogen-activated protein kinase (MAPK) is a well-known cell cycle regulator, emerging studies have also implicated its activity in the regulation of intracellular calcium concentration ([Ca2+](i)) and secretion. Those studies raise the hypothesis that MAPK activity during oocyte maturation and early fertilization is required for normal egg Ca2+ oscillations and cortical granule (CG) secretion. We extend the findings of [Lee, B., Vermassen, E., Yoon, S.-Y., Vanderheyden, V., Ito, J., Alfandari, D., De Smedt, H., Parys, J.B., Fissore, R.A., 2006. Phosphorylation of IP(3)R1 and the regulation of [Ca2+](i) responses at fertilization: a role for the MAP kinase pathway. Development 133, 4355-4365] by demonstrating acute effects on Ca2+ oscillation frequency, amplitude, and duration in fertilized mouse eggs matured in vitro with the MAPK inhibitor, U0126. Frequency was increased, whereas amplitude and duration were greatly decreased. These effects were significantly reduced in eggs matured in vivo and fertilized in the presence of the inhibitor. Ionomycin studies indicated that intracellular Ca2+ stores were differentially affected in eggs matured in vitro with U0126. Consistent with these effects on [Ca2+](i) elevation, fertilization-induced CG exocytosis and metaphase II exit were also reduced in in vitro-matured eggs with U0126, but not in those similarly treated after in vivo maturation. These results indicate that MAPK targets Ca2+ regulatory proteins during both maturation and fertilization, as well as provide a new hypothesis for MAPK function, which is to indirectly regulate events of early development by controlling Ca2+ oscillation parameters.

Comparison of Ca2+ and CaMKII responses in IVF and ICSI in the mouse

Novel methods of egg activation in human assisted reproductive technologies and animal somatic cell nuclear transfer are likely to alter the signalling process that occurs during normal fertilization. Intracytoplasmic sperm injection (ICSI) bypasses the normal processes of the acrosome reaction, sperm-egg fusion, and processing of the sperm plasma membrane, as well as alters some parameters of intracellular calcium ([Ca(2+)](i)) dynamics (reported previously by Kurokawa and Fissore (2003)). Herein, we extend these studies to determine if ICSI alters the activity of the Ca(2+)-dependent protein, Ca(2+)/calmodulin-dependent kinase II (CaMKII), which is responsible for the completion of meiosis in vertebrate eggs. After ICSI or in vitro fertilization (IVF), individual mouse eggs were monitored for their relative changes in both [Ca(2+)](i) and CaMKII activity during the first [Ca(2+)](i) rise and a subsequent rise associated with second polar body extrusion. The duration of the first [Ca(2+)](i) rise was greater in ICSI than in IVF, but the amplitude of the rise was transiently higher for IVF than ICSI. However, a similar mean CaMKII activity was observed in both procedures. During polar body extrusion, the amplitude and duration of the Ca(2+) rises were increased by a small amount in ICSI compared with IVF, whereas the CaMKII activities were similar. Thus, compared with IVF, ICSI is not associated with decreased or delayed CaMKII activity in response to these Ca(2+) signals in the mouse.

Phosphorylation of IP3R1 and the regulation of [Ca2+]i responses at fertilization: a role for the MAP kinase pathway

A sperm-induced intracellular Ca2+ signal ([Ca2+]i) underlies the initiation of embryo development in most species studied to date. The inositol 1,4,5 trisphosphate receptor type 1 (IP3R1) in mammals, or its homologue in other species, is thought to mediate the majority of this Ca2+ release. IP3R1-mediated Ca2+ release is regulated during oocyte maturation such that it reaches maximal effectiveness at the time of fertilization, which, in mammalian eggs, occurs at the metaphase stage of the second meiosis (MII). Consistent with this, the [Ca2+]i oscillations associated with fertilization in these species occur most prominently during the MII stage. In this study, we have examined the molecular underpinnings of IP3R1 function in eggs. Using mouse and Xenopus eggs, we show that IP3R1 is phosphorylated during both maturation and the first cell cycle at a MPM2-detectable epitope(s), which is known to be a target of kinases controlling the cell cycle. In vitro phosphorylation studies reveal that MAPK/ERK2, one of the M-phase kinases, phosphorylates IP3R1 at at least one highly conserved site, and that its mutation abrogates IP3R1 phosphorylation in this domain. Our studies also found that activation of the MAPK/ERK pathway is required for the IP3R1 MPM2 reactivity observed in mouse eggs, and that eggs deprived of the MAPK/ERK pathway during maturation fail to mount normal [Ca2+]i oscillations in response to agonists and show compromised IP3R1 function. These findings identify IP3R1 phosphorylation by M-phase kinases as a regulatory mechanism of IP3R1 function in eggs that serves to optimize [Ca2+]i release at fertilization.

Remodelling of calcium signalling during liver regeneration in the rat

BACKGROUND/AIMS

During liver regeneration, a network of cytokines and growth factors interact with hepatocytes, helping to restore the liver mass and functions after partial tissue loss. Agonists that trigger Ca2+ signals in the liver contribute to this process, although little is known about calcium signalling during liver regeneration.

RESULTS

We observed two phases in which the hepatocyte response to calcium-mobilising agonists was greatly reduced versus control cells at 24h and five days after partial hepatectomy. We found that both phases of hepatocyte desensitisation involved the down-regulation of cell surface receptors and the type II InsP3 receptor. Single cell studies with flash photolysis of caged InsP3 revealed that InsP3-mediated Ca2+ release was slower in regenerating hepatocytes at 24, 48 h and 5 days than in control cells. Also, the temporal pattern of vasopressin-elicited intracellular calcium oscillations studied on fura2-loaded cells was altered, with the duration of each Ca2+ peak being longer. Finally, we showed an association between hepatocyte desensitisation and progression through the cell cycle towards the S phase at 24 h after hepatectomy.

CONCLUSIONS

Our study supports the remodelling of hepatocyte calcium signalling during liver regeneration, and that this change is partly linked with cell cycle progression.

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.

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.

The presence of X- and Y-chromosomes in oocytes leads to impairment in the progression of the second meiotic division

The oocytes of B6.Y(TIR) sex-reversed female mice can be fertilized but the resultant embryos die at early cleavage stages. In the present study, we examined chromosome segregation at meiotic divisions in the oocytes of XY female mice, compared to those of XX littermates. The timing and frequency of oocyte maturation in culture were comparable between the oocytes from both types of females. At the first meiotic division, the X- and Y-chromosomes segregated independently and were retained in oocytes at equal frequencies. However, more oocytes retained the correct number of chromosomes than anticipated from random segregation. The oocytes that had reached MII-stage were activated by fertilization or incubation with SrCl(2). As expected, the majority of oocytes from XX females completed the second meiotic division and reached the 2-cell stage in 24 h. By contrast, more than half of oocytes from XY females initially remained at the MII-stage while the rest precociously entered interphase after SrCl(2) activation; very few oocytes were seen at the second anaphase or telophase and they often showed impairment of sister-chromatid separation. Eventually the majority of oocytes entered interphase and formed pronuclei, but very few reached the 2-cell stage. Similar results were obtained after fertilization. We conclude that the XY chromosomal composition in oocyte leads to impairment in the progression of the second meiotic division.

Differing mechanisms of cAMP- versus seawater-induced oocyte maturation in marine nemertean worms I. The roles of serine/threonine kinases and phosphatases

Unlike in most animals, oocytes of marine nemertean worms initiate maturation (=germinal vesicle breakdown, GVBD) following an increase, rather than a decrease, in intraoocytic cAMP. To analyze how serine/threonine (Ser/Thr) kinase cascades involving mitogen-activated protein kinase (MAPK), maturation-promoting factor (MPF), cAMP-dependent protein kinase (PKA), and phosphatidylinositol 3-kinase (PI3K) regulate nemertean GVBD, oocytes of Cerebratulus sp. were treated with pharmacological modulators and stimulated with cAMP-elevating drugs or seawater (SW) alone. Both cAMP elevators and SW triggered GVBD while activating MAPK, its target p90Rsk, and MPF. Similarly, neither cAMP- nor SW-induced GVBD was affected by several Ser/Thr phosphatase inhibitors, and both stimuli apparently accelerated GVBD via a MAPK-independent, PI3K-dependent mechanism. However, inhibitors of Raf-1, a kinase that activates MAPK kinase, blocked GVBD and MAPK activation during SW-, but not cAMP-induced maturation. In addition, MPF blockers more effectively reduced GVBD and MAPK activity in SW versus in cAMP-elevating treatments. Moreover, the two maturation-inducing stimuli yielded disparate patterns of PKA-related MAPK activations and phosphorylations of putative PKA substrates. Collectively, such findings suggest that in maturing oocytes of Cerebratulus sp., Ser/Thr kinase cascades differ during cAMP- versus SW-induced GVBD in several ways, including MAPK activation modes, MPF-feedback loops, and PKA-related signaling pathways. Additional differences in cAMP- versus SW-induced oocyte maturation are also described in the accompanying study that deals with the roles of tyrosine kinase signaling during GVBD.

ERK binds, phosphorylates InsP3 type 1 receptor and regulates intracellular calcium dynamics in DT40 cells

Modulation on the duration of intracellular Ca(2+) transients is essential for B-cell activation. We have previously shown that extracellular-signal-regulated kinase (ERK) can phosphorylate inositol 1,4,5-trisphosphate receptor type 1 (IP(3)R1) at serine 436 and regulate its calcium channel activity. Here we investigate the potential physiological interaction between ERK and IP(3)R1 using chicken DT40 B-cell line in which different mutants are expressed. The interaction between ERK and IP(3)R1 is confirmed by co-immunoprecipitation and fluorescence resonance energy transfer (FRET) assays. This constitutive interaction is independent of either ERK kinase activation or IP(3)R1 phosphorylation status. Back phosphorylation analysis further shows that type 1 IP(3)R (IP(3)R1) is phosphorylated by ERK in anti-IgM-activated DT40 cells. Finally, our data show that the phosphorylation of Ser 436 in the IP(3)-binding domain of IP(3)R1 leads to less Ca(2+) release from endoplasmic reticulum (ER) microsomes and accelerates the declining of calcium increase in DT40 cells in response to anti-IgM stimulation.

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.

Embryonic Stem Cells from Parthenotes

While human embryonic stem cells (hESCs) hold tremendous therapeutic potential, they also create societal and ethical dilemmas. Adult and placental stem cells represent two alternatives to the hESC, but may have technical limitations. An additional alternative is the stem cell derived from parthenogenesis. Parthenogenesis is a reproductive mechanism that is common in lower organisms and produces a live birth from an oocyte activated in the absence of sperm. However, parthenogenetic embryos will develop to the blastocyst stage and so can serve as a source of embryonic stem cells. Parthenogenetic ESCs (pESCs) have been shown to have the properties of self-renewal and the capacity to generate cell derivatives from the three germ layers, confirmed by contributions to chimeric animals and/or teratoma formation when injected into SCID mice. Therefore, this mechanism for generating stem cells has the ethical advantage of not involving the destruction of viable embryos. Moreover, the cells do not involve the union of male and female and so genetic material will be derived exclusively from the female oocyte donor (with the attendant potential immunological advantages). This chapter describes the biology underlying parthenogenesis, as well as provides detailed technical considerations for the production of pESCs.

Cdc2/Cyclin B1 Interacts with and Modulates Inositol 1,4,5-Trisphosphate Receptor (Type 1) Functions

The resistance of inositol 1,4,5-trisphosphate receptor (IP3R)-deficient cells to multiple forms of apoptosis demonstrates the importance of IP3-gated calcium (Ca2+) release to cellular apoptosis. However, the specific upstream biochemical events leading to IP3-gated Ca2+ release during apoptosis induction are not known. We have shown previously that the cyclin-dependent kinase 1/cyclin B (cdk1/CyB or cdc2/CyB) complex phosphorylates IP3R1 in vitro and in vivo at Ser421 and Thr799. In this study, we show that: 1) the cdc2/CyB complex directly interacts with IP3R1 through Arg391, Arg441, and Arg871; 2) IP3R1 phosphorylation at Thr799 by the cdc2/CyB complex increases IP3 binding; and 3) cdc2/CyB phosphorylation increases IP3-gated Ca2+ release. Taken together, these results demonstrate that cdc2/CyB phosphorylation positively regulates IP3-gated Ca2+ signaling. In addition, identification of a CyB docking site(s) on IP3R1 demonstrates, for the first time, a direct interaction between a cell cycle component and an intracellular calcium release channel. Blocking this phosphorylation event with a specific peptide inhibitor(s) may constitute a new therapy for the treatment of several human immune disorders.

An increase in [Ca2+]i is sufficient but not necessary for driving mitosis in early mouse embryos

An increase in intracellular Ca2+ concentration ([Ca2+]i) has been shown to drive sea-urchin embryos and some fibroblasts through nuclear-envelope breakdown (NEBD) and the metaphase-to-anaphase transition. Mitotic Ca2+ transients can be pan-cellular global events or localized to the perinuclear region. It is not known whether Ca2+ is a universal regulator of mitosis or whether its role is confined to specific cell types. To test the hypothesis that Ca2+ is a universal regulator of mitosis, we have investigated the role of Ca2+ in mitosis in one-cell mouse embryos. Fertilized embryos generate Ca2+ transients during the first mitotic division. Imposing a Ca2+ transient by photorelease of inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] resulted in acceleration of mitosis entry, suggesting that a [Ca2+]i increase is capable of triggering mitosis. Mitotic Ca2+ transients were inhibited using three independent approaches: injection of intracellular Ca2+ buffers; downregulation of Ins(1,4,5)P3 receptors; and removal of extracellular Ca2+. None of the interventions had any effects on the timing of NEBD or cytokinesis. The possibility that NEBD is driven by localized perinuclear Ca2+ transients was examined using two-photon microscopy but no Ca2+-dependent increases in fluorescence were found to precede NEBD. Finally, the second mitotic division took place in the absence of any detectable [Ca2+]i increase. Thus, although an induced [Ca2+]i increase can accelerate mitosis entry, neither cytosolic nor perinuclear [Ca2+] increases appear to be necessary for progression through mitosis in mouse embryos.

Strontium promotes calcium oscillations in mouse meiotic oocytes and early embryos through InsP3 receptors, and requires activation of phospholipase and the synergistic action of InsP3

BACKGROUND

Sr2+ is the most efficient agent for mouse oocyte activation and functions by inducing Ca2+ oscillations. However, its specific mechanism of action remains unknown. Here we investigated the specificity and possible mechanism of Sr2+-induced Ca2+ oscillations in mouse oocytes and early embryos.

METHODS

Ca2+ oscillations in oocytes and embryos were measured by ratiometric fluorescence imaging using fura-2AM. The role of phospholipase C (PLC) and inositol trisphosphate (InsP3) receptors in Sr2+-induced Ca2+ oscillations was examined by selective inhibitors.

RESULTS

Sr2+ can induce Ca2+ oscillations in both immature and mature oocytes, and in early embryos. A cell cycle stage-dependent phenomenon to Sr2+ stimulation was observed in 1-cell embryos. By using a low molecular weight heparin to antagonize the function of InsP3 receptors, we were able to show that InsP3 receptors are essential for Sr2+-induced Ca2+ oscillations. Treating metaphase II (MII) oocytes with the PLC inhibitor, U73122, abolished Sr2+-induced increases in Ca2+. This inhibitory effect of U73122 could be rescued by microinjection of InsP3, indicating that Sr2+-induced Ca2+ oscillations require the synergistic action of InsP3.

CONCLUSIONS

Sr2+-induced calcium oscillations in mouse oocytes and early embryos are mediated through InsP3 receptors, and require PLC activation and the synergistic action of InsP3.

Fertilization and Inositol 1,4,5-Trisphosphate (IP3)-Induced Calcium Release in Type-1 Inositol 1,4,5-Trisphosphate Receptor Down-Regulated Bovine Eggs1

It is widely believed that stimulation of the phosphoinositide pathway and production of 1,4,5-inositol trisphosphate (IP(3)) underlies the oscillatory changes in the concentration of intracellular free calcium ions ([Ca(2+)](i)) seen during mammalian fertilization. IP(3) promotes Ca(2+) release in eggs by binding to its receptor, the type-1 IP(3) receptor (IP(3)R-1, also known as ITPR1), a ligand-gated Ca(2+) channel located in the membrane of the endoplasmic reticulum, the main Ca(2+) store of the cell. While IP(3)R-1 has been shown to mediate all Ca(2+) release during mouse fertilization, whether or not it plays such an essential role in fertilization-induced Ca(2+) release in large domestic species such as bovine and porcine is presently not known. Accordingly, we have generated metaphase II bovine eggs with a approximately 70%-80% reduction in the number of intact IP(3)R-1 by inducing receptor down-regulation during oocyte maturation. We did so by injecting the nonhydrolyzable IP(3) analogue, adenophostin A. Functional Ca(2+) release analysis revealed that IP(3)R-1 is the predominant Ca(2+) release channel in bovine eggs, requiring as little as 20% of total intact receptor to mount persistent [Ca(2+)](i) oscillations in response to fertilization, expression of PLCzeta (also known as PLCZ1), and adenophostin A. However, lower concentrations of IP(3) and near-physiological concentrations of porcine sperm extract were unable to trigger [Ca(2+)](i) oscillations in this reduced IP(3)R-1 model. Furthermore, we present evidence that the sensitivity of bovine IP(3)R-1 is impaired at the first embryonic interphase. Together, these results demonstrate the essential role of IP(3)R-1-mediated Ca(2+) release during fertilization in bovine eggs, and identify cell cycle regulatory mechanisms of [Ca(2+)](i) oscillations at the level of IP(3)R-1.