Regulation of diacylglycerol production and protein kinase C stimulation during sperm- and PLCzeta-mediated mouse egg activation

The Therapeutic and Diagnostic Potential of Phospholipase C Zeta, Oocyte Activation, and Calcium in Treating Human Infertility

Oocyte activation, a fundamental event during mammalian fertilisation, is initiated by concerted intracellular patterns of calcium (Ca2+) release, termed Ca2+ oscillations, predominantly driven by testis-specific phospholipase C zeta (PLCζ). Ca2+ exerts a pivotal role in not just regulating oocyte activation and driving fertilisation, but also in influencing the quality of embryogenesis. In humans, a failure of Ca2+ release, or defects in related mechanisms, have been reported to result in infertility. Furthermore, mutations in the PLCζ gene and abnormalities in sperm PLCζ protein and RNA, have been strongly associated with forms of male infertility where oocyte activation is deficient. Concurrently, specific patterns and profiles of PLCζ in human sperm have been linked to parameters of semen quality, suggesting the potential for PLCζ as a powerful target for both therapeutics and diagnostics of human fertility. However, further to PLCζ and given the strong role played by Ca2+ in fertilisation, targets down- and up-stream of this process may also present a significantly similar level of promise. Herein, we systematically summarise recent advancements and controversies in the field to update expanding clinical associations between Ca2+-release, PLCζ, oocyte activation and human fertility. We discuss how such associations may potentially underlie defective embryogenesis and recurrent implantation failure following fertility treatments, alongside potential diagnostic and therapeutic avenues presented by oocyte activation for the diagnosis and treatment of human infertility.

Role of zinc in female reproduction

Zinc is a critical component in a number of conserved processes that regulate female germ cell growth, fertility, and pregnancy. During follicle development, a sufficient intracellular concentration of zinc in the oocyte maintains meiotic arrest at prophase I until the germ cell is ready to undergo maturation. An adequate supply of zinc is necessary for the oocyte to form a fertilization-competent egg as dietary zinc deficiency or chelation of zinc disrupts maturation and reduces the oocyte quality. Following sperm fusion to the egg to initiate the acrosomal reaction, a quick release of zinc, known as the zinc spark, induces egg activation in addition to facilitating zona pellucida hardening and reducing sperm motility to prevent polyspermy. Symmetric division, proliferation, and differentiation of the preimplantation embryo rely on zinc availability, both during the oocyte development and post-fertilization. Further, the fetal contribution to the placenta, fetal limb growth, and neural tube development are hindered in females challenged with zinc deficiency during pregnancy. In this review, we discuss the role of zinc in germ cell development, fertilization, and pregnancy with a focus on recent studies in mammalian females. We further detail the fundamental zinc-mediated reproductive processes that have only been explored in non-mammalian species and speculate on the role of zinc in similar mechanisms of female mammals. The evidence collected over the last decade highlights the necessity of zinc for normal fertility and healthy pregnancy outcomes, which suggests zinc supplementation should be considered for reproductive age women at risk of zinc deficiency.

Label-Free Quantitative Proteomic Profiling of LAD2 Mast Cell Releasates Reveals the Mechanism of Tween-80-Induced Anaphylactoid Reaction

PURPOSE

Tween-80 is one of the most important causes resulting in anaphylactoid reaction. However, its mechanism remains unclear. Proteomic characterizations of mast cells' excreta in response to Tween-80 are assayed to investigate the mechanism of anaphylactoid reaction.

EXPERIMENTAL DESIGN

A label-free LCMS/MS-based proteomics is used to analyze Tween-80-stimulated Laboratory of Allergic Diseases 2 (LAD2) mast cells releasates. The results of proteomic are analyzed by bioinformatics analysis. Western blotting is used to verify the expression of proteins.

RESULTS

Overall, endocytosis, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), and calcium signaling pathways play important roles in Tween-80-induced LAD2 cells activation by bioinformatics analysis. The expressions of relative proteins including actin-related protein 2/3 complexes, vacuolar protein sorting-associated protein, phosphorylation of transcription factor of P105 and P65, phosphorylation of inositol 1,4,5-trisphosphate receptor (IP₃ R), phosphoinositide phospholipase Cγ (PLCγ), and protein kinase C (PKC), are significantly increased in Tween-80 group compared to control. Tween-80 might be internalized via endocytosis, which induces degranulation by PLCγ/PKC pathways mediated calcium influx, and promotes the generation of inflammatory mediators via NF-κB pathway resulting in anaphylactoid reaction.

Oocyte Activation and Fertilisation: Crucial Contributors from the Sperm and Oocyte

This chapter intends to summarise the importance of sperm- and oocyte-derived factors in the processes of sperm-oocyte binding and oocyte activation. First, we describe the initial interaction between sperm and the zona pellucida, with particular regard to acrosome exocytosis. We then describe how sperm and oocyte membranes fuse, with special reference to the discovery of the sperm protein IZUMO1 and its interaction with the oocyte membrane receptor JUNO. We then focus specifically upon oocyte activation, the fundamental process by which the oocyte is alleviated from metaphase II arrest by a sperm-soluble factor. The identity of this sperm factor has been the source of much debate recently, although mounting evidence, from several different laboratories, provides strong support for phospholipase C ζ (PLCζ), a sperm-specific phospholipase. Herein, we discuss the evidence in support of PLCζ and evaluate the potential role of other candidate proteins, such as post-acrosomal WW-binding domain protein (PAWP/WBP2NL). Since the cascade of downstream events triggered by the sperm-borne oocyte activation factor heavily relies upon specialised cellular machinery within the oocyte, we also discuss the critical role of oocyte-borne factors, such as the inositol trisphosphate receptor (IP₃R), protein kinase C (PKC), store-operated calcium entry (SOCE) and calcium/calmodulin-dependent protein kinase II (CaMKII), during the process of oocyte activation. In order to place the implications of these various factors and processes into a clinical context, we proceed to describe their potential association with oocyte activation failure and discuss how clinical techniques such as the in vitro maturation of oocytes may affect oocyte activation ability. Finally, we contemplate the role of artificial oocyte activating agents in the clinical rescue of oocyte activation deficiency and discuss options for more endogenous alternatives.

Fertility: Store-Operated Ca2+ Entry in Germ Cells: Role in Egg Activation

At the time of fertilization, the sperm activates the egg and induces embryonic development by triggering an elevation in the egg's intracellular free Ca²⁺ concentration. In mammals the initial Ca²⁺ rise is followed by a series of repetitive Ca²⁺ transients (known as oscillations) that last for several hours. Although the source of Ca²⁺ during the signaling process is primarily the egg's smooth endoplasmic reticulum, the oscillations stop in the absence of extracellular Ca²⁺ indicating that a Ca²⁺ influx across the plasma membrane is essential to sustain them. Depletion of the intracellular stores using specific inhibitors generates a Ca²⁺ entry across the plasma membrane of eggs of various species, and a continuous influx of Ca²⁺ has been linked to the sperm-induced Ca²⁺ oscillations in the mouse; these data indicate that store-operated Ca²⁺ entry (SOCE) operates in eggs and may be the mechanism that maintains the long-lasting Ca²⁺ signal at fertilization. Recent findings suggest that the signaling proteins STIM1 and Orai1 are present in eggs; they are responsible for mediating SOCE, and their functions are essential for proper Ca²⁺ signaling at fertilization to support normal embryo development.

The sperm phospholipase C-ζ and Ca2+ signalling at fertilization in mammals

A series of intracellular oscillations in the free cytosolic Ca(2+) concentration is responsible for activating mammalian eggs at fertilization, thus initiating embryo development. It has been proposed that the sperm causes these Ca(2+) oscillations after membrane fusion by delivering a soluble protein into the egg cytoplasm. We previously identified sperm-specific phospholipase C (PLC)-ζ as a protein that can trigger the same pattern of Ca(2+) oscillations in eggs seen at fertilization. PLCζ appears to be the elusive sperm factor mediating egg activation in mammals. It has potential therapeutic use in infertility treatments to improve the rate of egg activation and early embryo development after intra-cytoplasmic sperm injection. A stable form of recombinant human PLCζ could be a prototype for use in such in vitro fertilization (IVF) treatments. We do not yet understand exactly how PLCζ causes inositol 1,4,5-trisphosphate (InsP3) production in eggs. Sperm PLCζ is distinct among mammalian PI-specific PLCs in that it is far more potent in triggering Ca(2+) oscillations in eggs than other PLCs, but it lacks a PH domain that would otherwise be considered essential for binding to the phosphatidylinositol 4,5-bisphosphate (PIP2) substrate. PLCζ is also unusual in that it does not appear to interact with or hydrolyse plasma membrane PIP2. We consider how other regions of PLCζ may mediate its binding to PIP2 in eggs and how interaction of PLCζ with egg-specific factors could enable the hydrolysis of internal sources of PIP2.

Molecular triggers of egg activation at fertilization in mammals

In mammals, the sperm activates the development of the egg by triggering a series of oscillations in the cytosolic-free Ca(2+) concentration (Ca(2+) i). The sperm triggers these cytosolic Ca(2+i) oscillations after sperm-egg membrane fusion, as well as after intracytoplasmic sperm injection (ICSI). These Ca(2+) i oscillations are triggered by a protein located inside the sperm. The identity of the sperm protein has been debated over many years, but all the repeatable data now suggest that it is phospholipase Czeta (PLCζ). The main downstream target of Ca(2+) i oscillations is calmodulin-dependent protein kinase II (CAMKII (CAMK2A)), which phosphorylates EMI2 and WEE1B to inactivate the M-phase promoting factor protein kinase activity (MPF) and this ultimately triggers meiotic resumption. A later decline in the activity of mitogen-activated protein kinase (MAPK) then leads to the completion of activation which is marked by the formation of pronuclei and entry into interphase of the first cell cycle. The early cytosolic Ca(2+) increases also trigger exocytosis via a mechanism that does not involve CAMKII. We discuss some recent developments in our understanding of these triggers for egg activation within the framework of cytosolic Ca(2+) signaling.

PLCζ sequence, protein levels, and distribution in human sperm do not correlate with semen characteristics and fertilization rates after ICSI

PURPOSE

Sperm-borne PLCζ protein induces Ca(2+) oscillations in the oocyte and is believed to play a major role during oocyte activation. However, its implication in fertilization failure following ICSI is still debated. We analyzed PLCζ gene sequence, protein expression level, and localization in both patients with previous failed fertilization by ICSI and sperm donors with proven fertility in order to assess the association of PLCζ with both sperm characteristics and ability to fertilize.

METHODS

Semen from 15 patients and 13 sperm donors with proven fertility was included in the study. Analysis of the PLCζ gene sequence, protein expression through Western blot, and protein localization by immunofluorescence were performed.

RESULTS

Two patients with total fertilization failure presented mutations in heterozygosis in the PLCζ gene. Comparison with donor sample sequences displayed comparable SNP allele frequency. Distribution pattern of PLCζ did not vary significantly between donor and patient samples. Levels of PLCζ protein in sperm cells showed an interindividual variability both in patient and donor samples. Several SNPs previously reported in infertile patients were also present in fertile men.

CONCLUSION

Failed fertilization occurs even when levels and distribution of PLCζ protein are within normal range. PLCζ seems to be a necessary but not sufficient factor in determining the molecular pathway involved in oocyte activation.

Signal transduction in mammalian oocytes during fertilization

Mammalian embryo development begins when the fertilizing sperm triggers a series of elevations in the oocyte's intracellular free Ca(2+) concentration. The elevations are the result of repeated release and re-uptake of Ca(2+) stored in the smooth endoplasmic reticulum. Ca(2+) release is primarily mediated by the phosphoinositide signaling system of the oocyte. The system is stimulated when the sperm causes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG); IP3 then binds its receptor on the surface of the endoplasmic reticulum that induces Ca(2+) release. The manner in which the sperm generates IP3, the Ca(2+) mobilizing second messenger, has been the subject of extensive research for a long time. The sperm factor hypothesis has eventually gained general acceptance, according to which it is a molecule from the sperm that diffuses into the ooplasm and stimulates the phosphoinositide cascade. Much evidence now indicates that the sperm-derived factor is phospholipase C-zeta (PLCζ) that cleaves PIP2 and generates IP3, eventually leading to oocyte activation. A recent addition to the candidate sperm factor list is the post-acrosomal sheath WW domain-binding protein (PAWP), whose role at fertilization is currently under debate. Ca(2+) influx across the plasma membrane is also important as, in the absence of extracellular Ca(2+), the oscillations run down prematurely. In pig oocytes, the influx that sustains the oscillations seems to be regulated by the filling status of the stores, whereas in the mouse other mechanisms might be involved. This work summarizes the current understanding of Ca(2+) signaling in mammalian oocytes.

Egg Activation at Fertilization by a Soluble Sperm Protein

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

Lack of protein kinase C-delta (PKCδ) disrupts fertilization and embryonic development

This study tested the function of protein kinase C delta (PKCδ) during fertilization and embryonic development using gene-knockout (Prkcd(-/-)) mice. Fertility analysis revealed that Prkcd(-/-) mating pairs produce significantly fewer pups per litter than wild-type pairs (P < 0.05), and exhibit a high incidence of embryonic loss post-implantation. Both Prkcd(-/-) male as well as Prkcd(-/-) female mice mated to Prkcd(+/+) controls also showed reduced litter sizes, with a selective loss of Prkcd-null pups. Further analysis of the females demonstrated comparable in vitro fertilization outcomes between control and Prkcd(-/-) oocytes fertilized with wild-type sperm. Pregnant Prkcd(-/-) females, however, exhibited a reduced number of total implantations, suggesting a possible disruption in early embryo quality and/or implantation. In turn, male gamete analysis revealed that Prkcd(-/-) sperm demonstrated a decreased capacity to penetrate the zona pellucida (P < 0.05), necessary for successful fertilization. Moreover, we identified phosphorylated PKCδ as a component of the sperm acrosome, indicating a potential role for this kinase in acrosome exocytosis. Therefore, loss of PKCδ disrupts key reproductive functions in both males and females that limit fertility.

Sperm-induced Ca2+ release during egg activation in mammals

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

Phospholipase C zeta (PLCζ): oocyte activation and clinical links to male factor infertility

Mounting scientific and clinical evidence supports the key role played by phospholipase C zeta (PLCζ), a sperm-specific protein, in the activation of oocytes following fertilisation. Lacking a pleckstrin homology domain, PLCζ remains the smallest known mammalian PLC and was first identified in 2002. Since then, PLCζ has been the target for a multitude of studies in both mammalian and non-mammalian species focused upon its fundamental biochemical activity and crucial role as the mediator of oocyte activation. The earliest event subsequent to gamete fusion is the onset of a series of intracellular calcium oscillations within the oocyte, which are known to modulate cortical granule exocytosis, release meiotic arrest, regulate gene expression, recruit maternal mRNA, and initiate embryogenesis. Collectively these processes are known as 'oocyte activation' and together, represent a fundamental mechanism for early embryonic development. Evidence suggests that these processes are initiated and controlled by calcium release from ooplasmic sources in response to PLCζ activity via the inositol-1,4,5-triphosphate (IP3) pathway. While the biochemical action of PLCζ has been extensively studied, especially in relation to the EF-hands, X-Y linker, and C2 domain, all of which play critical roles for in vivo activity, there are still key gaps in our knowledge, particularly in terms of regulation and interaction with other proteins within the oocyte. Moreover, increasing clinical evidence has revealed a strong correlation between certain types of male infertility and the aberrant expression, localisation, structure and function of PLCζ in human sperm, particularly in cases of recurrent intracytoplasmic sperm injection (ICSI) failure, globozoospermia, and oocyte activation deficiency (OAD). In addition, two heterozygous substitution mutations have been identified in the coding sequence of PLCζ in one particular patient causing disruption to the catalytic X and Y domains and resulting in infertility. Although, such cases can be treated via the use of artificial oocyte activators (AOAs) such as calcium ionophores, significant concern remains over the use of such chemical agents, largely due to the fact that calcium release manifests as a single transient, rather than a series of oscillations as observed during normal fertilisation. Current interest in PLCζ is thus to develop a series of prognostic, diagnostic and therapeutic approaches which could first identify male patients that are deficient in PLCζ and then rescue oocyte activation ability via assisted reproductive technology (ART) and a pure, functionally-active, recombinant human PLCζ protein. While significant progress has been made in such areas over recent years, there is a clear need to translate scientific findings to clinical settings in order to maximise successful outcome for patients.

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

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

Calcium influx in mammalian eggs

Calcium (Ca2+) signals are involved in the regulation of oocyte maturation and play a critical role during fertilization. In the egg, Ca2+is stored in the lumen of the endoplasmic reticulum and a signal is generated when the stored Ca2+is released through specialized channels in the membrane of the endoplasmic reticulum to elevate the free Ca2+concentration in the cytoplasm. Extracellular Ca2+is also important, indicated by the fact that the mobilization of luminal Ca2+is typically followed by Ca2+entry across the plasma membrane. The transmembrane Ca2+flux replenishes the endoplasmic reticulum, and thus, it is essential to sustain prolonged Ca2+signals. It also seems to be responsible for the stimulation of important signaling cascades required for complete egg activation. Characterization of the pathway that mediates Ca2+entry implies that its major components include STIM1, a protein that senses the filling status of the stores, and ORAI1, a channel protein located in the plasma membrane. Defining the mechanism and functions of Ca2+entry will not only lead to a better understanding of egg physiology but may also help improving the efficiency of a number of assisted reproductive technologies.

The dynamics of PKC-induced phosphorylation triggered by Ca2+ oscillations in mouse eggs

Fertilization of mammalian eggs is characterized by a series of Ca(2+) oscillations triggered by a phospholipase C activity. These Ca(2+) increases and the parallel generation of diacylglycerol (DAG) stimulate protein kinase C (PKC). However, the dynamics of PKC activity have not been directly measured in living eggs. Here, we have monitored the dynamics of PKC-induced phosphorylation in mouse eggs, alongside Ca(2+) oscillations, using fluorescent C-kinase activity reporter (CKAR) probes. Ca(2+) oscillations triggered either by sperm, phospholipase C zeta (PLCζ) or Sr(2+) all caused repetitive increases in PKC-induced phosphorylation, as detected by CKAR in the cytoplasm or plasma membrane. The CKAR responses lasted for several minutes in both the cytoplasm and plasma membrane then returned to baseline values before subsequent Ca(2+) transients. High frequency oscillations caused by PLCζ led to an integration of PKC-induced phosphorylation. The conventional PKC inhibitor, Gö6976, could inhibit CKAR increases in response to thapsigargin or ionomycin, but not the repetitive responses seen at fertilization. Repetitive increases in PKCδ activity were also detected during Ca(2+) oscillations using an isoform-specific δCKAR. However, PKCδ may already be mostly active in unfertilized eggs, since phorbol esters were effective at stimulating δCKAR only after fertilization, and the PKCδ-specific inhibitor, rottlerin, decreased the CKAR signals in unfertilized eggs. These data show that PKC-induced phosphorylation outlasts each Ca(2+) increase in mouse eggs but that signal integration only occurs at a non-physiological, high Ca(2+) oscillation frequency. The results also suggest that Ca(2+) -induced DAG formation on intracellular membranes may stimulate PKC activity oscillations at fertilization.

Calcium signaling in mammalian egg activation and embryo development: the influence of subcellular localization

Calcium (Ca(2+) ) signals drive the fundamental events surrounding fertilization and the activation of development in all species examined to date. Initial studies of Ca(2+) signaling at fertilization in marine animals were tightly linked to new discoveries of bioluminescent proteins and their use as fluorescent Ca(2+) sensors. Since that time, there has been rapid progress in our understanding of the key functions for Ca(2+) in many cell types and of the impact of cellular localization on Ca(2+) signaling pathways. In this review, which focuses on mammalian egg activation, we consider how Ca(2+) is regulated and stored at different stages of oocyte development and examine the functions of molecules that serve as both regulators of Ca(2+) release and effectors of Ca(2+) signals. We then summarize studies exploring how Ca(2+) directs downstream effectors mediating both egg activation and later signaling events required for successful preimplantation embryo development. Throughout this review, we focus attention on how localization of Ca(2+) signals influences downstream signaling events, and attempt to highlight gaps in our knowledge that are ripe for future research.

Calcium influx-mediated signaling is required for complete mouse egg activation

Mammalian fertilization is accompanied by oscillations in egg cytoplasmic calcium (Ca(2+)) concentrations that are critical for completion of egg activation. These oscillations are initiated by Ca(2+) release from inositol 1,4,5-trisphosphate (IP(3))-sensitive intracellular stores. We tested the hypothesis that Ca(2+) influx across the plasma membrane was a requisite component of egg activation signaling, and not simply a Ca(2+) source for store repletion. Using intracytoplasmic sperm injection (ICSI) and standard in vitro fertilization (IVF), we found that Ca(2+) influx was not required to initiate resumption of meiosis II. However, even if multiple oscillations in intracellular Ca(2+) occurred, in the absence of Ca(2+) influx, the fertilized eggs failed to emit the second polar body, resulting in formation of three pronuclei. Additional experiments using the Ca(2+) chelator, BAPTA/AM, demonstrated that Ca(2+) influx is sufficient to support polar body emission and pronucleus formation after only a single sperm-induced Ca(2+) transient, whereas BAPTA/AM-treated ICSI or fertilized eggs cultured in Ca(2+)-free medium remained arrested in metaphase II. Inhibition of store-operated Ca(2+) entry had no effect on ICSI-induced egg activation, so Ca(2+) influx through alternative channels must participate in egg activation signaling. Ca(2+) influx appears to be upstream of CaMKIIγ activity because eggs can be parthenogenetically activated with a constitutively active form of CaMKIIγ in the absence of extracellular Ca(2+). These results suggest that Ca(2+) influx at fertilization not only maintains Ca(2+) oscillations by replenishing Ca(2+) stores, but also activates critical signaling pathways upstream of CaMKIIγ that are required for second polar body emission.

Loss of activity mutations in phospholipase C zeta (PLCζ) abolishes calcium oscillatory ability of human recombinant protein in mouse oocytes

BACKGROUND

Mammalian oocyte activation occurs via a series of intracellular calcium (Ca(2+)) oscillations thought to be induced by a sperm-specific phospholipase C zeta (PLCζ). There is now strong evidence to indicate that certain types of human male infertility are caused by failure of the sperm to activate the oocyte in an appropriate manner. Molecular analysis of the PLCζ gene of a male patient with oocyte activation deficiency has previously identified a point mutation causing a histidine to proline substitution at PLCζ residue 398 (PLCζ(H398P)), leading to abnormal Ca(2+) release profiles and reduced oocyte activation efficiency.

METHODS AND RESULTS

In the present study, we used HEK293T cells to produce recombinant human wild-type PLCζ (PLCζ(WT)) protein which, upon microinjection into mouse oocytes, induced Ca(2+) oscillations characteristic of oocyte activation. Injection of recombinant PLCζ(H398P) was unable to elicit Ca(2+) oscillations in mouse oocytes. Loss of activity mutations, such as PLCζ(H398P) and an artificially induced frameshift mutation (PLCζ(ΔYC2)) did not affect Ca(2+) release when over-expressed in HEK293T cells, whereas PLCζ(WT) inhibited adenosine triphosphate-activated Ca(2+) release. Confocal imaging of fluorescently tagged PLCζ isoforms in HEK293T cells suggested a cytoplasmic pattern of localization, while quantitative analysis of fluorescence levels showed that PLCζ(WT) > PLCζ(H398P) > PLCζ(ΔYC2), indicating that loss of activity mutations may lead to protein instability. This was further indicated by the low proportion of sperm and the lower levels of total PLCζ immunofluorescence from the patient exhibiting PLCζ(H398P) compared with fertile controls.

CONCLUSIONS

We demonstrate, for the first time, the production of active recombinant human PLCζ protein which retained the ability to elicit characteristic Ca(2+) oscillations in mouse oocytes, an ability which was eliminated by an infertility-linked mutation. These findings advance our understanding of PLCζ, and provide a critical step forward in obtaining purified PLCζ protein as a potential therapeutic agent for oocyte activation deficiency.

Paternal exposure to cyclophosphamide affects the progression of sperm chromatin decondensation and activates a DNA damage response in the prepronuclear rat zygote

Spermatozoon decondensation in the zygote leads to the initiation of chromatin remodeling during which protamines are removed and replaced with maternal histones. We hypothesize that damage to male germ cells induced by paternal exposure to cyclophosphamide may alter the timing of spermatozoal decondensation and the pattern of chromatin remodeling in the prepronuclear rat zygote. A specific order of sperm decondensation was observed, starting at the posterior end, proceeding to the ventral sides, followed by the tip, and finally the midbody region of the sperm head nucleus; subgroups of partially decondensed type a sperm nuclei were defined as types a1, a2, a3, and a4. Based on their frequencies relative to controls, paternal exposure to cyclophosphamide accelerated the timing of spermatozoal decondensation. Two distinct patterns of chromatin remodeling were observed for totally decondensed (type b) and recondensing (type c) sperm nuclei: H4K12ac showed a homogenous staining, whereas H3S10ph displayed a ring-like staining around the sperm nucleus; the distribution of these posttranslationally modified histones was not affected by cyclophosphamide exposure. In contrast, paternal cyclophosphamide treatment increased the number of gammaH2AX foci found in decondensing sperm nuclei. Small foci were significantly increased in type a2 and a3 nuclei, whereas a significant increase in the numbers of large foci was found in type b and c nuclei. This increase in gammaH2AX foci in the decondensing male genome suggests that damage recognition and repair pathways are initiated in prepronuclear rat zygotes. Thus, exposure of male rats to chronic low doses of cyclophosphamide accelerates spermatozoal decondensation and leads to the activation of gammaH2AX recognition of DNA damage in the male genome of the prepronuclear zygote.