The absence of a Ca(2+) signal during mouse egg activation can affect parthenogenetic preimplantation development, gene expression patterns, and blastocyst quality

Significance of oxidative stress and antioxidant capacity tests as biomarkers of premature ovarian insufficiency: A case control study

BACKGROUND

Premature ovarian insufficiency (POI) is a condition that causes secondary amenorrhea owing to ovarian hypofunction at an early stage. Early follicular depletion results in intractable infertility, thereby considerably reducing the quality of life of females. Given the continuum in weakened ovarian function, progressing from incipient ovarian failure (IOF) to transitional ovarian failure and further to POI, it is necessary to develop biomarkers for predicting POI. The oxidative stress states in IOF and POI were comprehensively evaluated via oxidative stress [diacron-reactive oxygen metabolites (d-ROMs)] test and antioxidant capacity [biological antioxidant potential (BAP)].

AIM

To explore the possibilities of oxidative stress and antioxidant capacity as biomarkers for the early detection of POI.

METHODS

Females presenting with secondary amenorrhea over 4 mo and a follicle stimulating hormone level of > 40 mIU/mL were categorized into the POI group. Females presenting with a normal menstrual cycle and a follicle stimulating hormone level of > 10.2 mIU/mL were categorized into the IOF group. Healthy females without ovarian hypofunction were categorized into the control group. Among females aged < 40 years who visited our hospital from January 2021 to June 2022, we recruited 11 patients into both POI and IOF groups. For the potential antioxidant capacity, the relative oxidative stress index (BAP/d-ROMs × 100) was calculated, and the oxidative stress defense system was comprehensively evaluated.

RESULTS

d-ROMs were significantly higher in the POI and IOF groups than in the control group, (478.2 ± 58.7 U.CARR, 434.5 ± 60.6 U.CARR, and 341.1 ± 35.1 U.CARR, respectively) (U.CARR is equivalent to 0.08 mg/dL of hydrogen peroxide). However, no significant difference was found between the POI and IOF groups. Regarding BAP, no significant difference was found between the control, IOF, and POI groups (2078.5 ± 157.4 μmol/L, 2116.2 ± 240.2 μmol/L, and 2029.0 ± 186.4 μmol/L, respectively). The oxidative stress index was significantly higher in the POI and IOF groups than in the control group (23.7 ± 3.3, 20.7 ± 3.6, and 16.5 ± 2.1, respectively). However, no significant difference was found between the POI and IOF groups.

CONCLUSION

High levels of oxidative stress suggest that evaluating the oxidative stress state may be a useful indicator for the early detection of POI.

Embryo development and live birth resulted from artificial oocyte activation after microdissection testicular sperm extraction with ICSI in patients with non-obstructive azoospermia

INTRODUCTION

The application of microdissection testicular sperm extraction (micro-TESE) to retrieve the sperm of patients with non-obstructive azoospermia (NOA) has greatly increased. Patients with NOA often have poor quality sperm. Unfortunately, there are few studies on artificial oocyte activation (AOA) performed on patients who successfully retrieved motile and immotile sperm by micro-TESE after intracytoplasmic sperm injection (ICSI). Therefore, this study sought to obtain more comprehensive evidence-based data and embryo development outcomes to aid consultation of patients with NOA who opted to receive assisted reproductive techniques and to determine whether AOA needs to be performed in different motile sperm after ICSI.

METHODS

This retrospective study involved 235 patients with NOA who underwent micro-TESE to retrieve adequate sperm for ICSI between January 2018 and December 2020. A total of 331 ICSI cycles were performed in the 235 couples. Embryological, clinical, and neonatal outcomes were demonstrated comprehensively between motile sperm and immotile sperm using AOA and non-AOA treatment.

RESULTS

Motile sperm injection with AOA (group 1) showed significantly higher fertility rate (72.77% vs. 67.59%, p=0.005), 2 pronucleus (2PN) fertility rate (64.33% vs. 60.22%, p=0.036), and miscarriage rate (17.65% vs. 2.44%, p=0.018) compared with motile sperm injection with non-AOA (group 2). Group 1 had comparable available embryo rate (41.29% vs. 40.74%, p=0.817), good embryo rate (13.44% vs. 15.44%, p=0.265), and without an embryo for transfer rate (10.85% vs. 9.90%, p=0.815) compared with group 2. Immotile sperm injection with AOA (group 3) displayed significantly higher fertility rate (78.56% vs. 67.59%, p=0.000), 2PN fertility rate (67.36% vs. 60.22%, p=0.001), without an embryo for transfer rate (23.76% vs. 9.90%, p=0.008), and miscarriage rate (20.00% vs. 2.44%, p=0.014), but significantly lower available embryo rate (26.63% vs.40.74%, p=0.000) and good embryo rate (15.44% vs. 6.99%, p=0.000) compared with group 2. In groups 1, 2, and 3, the rates of implantation (34.87%, 31.85% and 28.00%, respectively; p=0.408), clinical pregnancy (43.87%, 41.00%, and 34.48%, respectively; p=0.360) and live birth (36.13%, 40.00%, and 27.59%, respectively; p=0.194) were similar.

DISCUSSION

For those patients with NOA from whom adequate sperm were retrieved for ICSI, AOA could improve fertilization rate, but not embryo quality and live birth outcomes. For patients with NOA and only immotile sperm, AOA can help achieve acceptable fertilization rate and live birth outcomes. AOA is recommended for patients with NOA only when immotile sperm are injected.

Calcium ionophore improves embryonic development and pregnancy outcomes in patients with previous developmental problems in ICSI cycles

BACKGROUND

Calcium (Ca²⁺) ionophores are now mainly considered as efficient treatments for fertilization failure. Recently, its application for rescuing poor embryo development was proposed but still non-routine. This study aimed to explore whether Ca²⁺ ionophore improves embryo development and pregnancy outcomes in patients with poor embryo development in previous intracytoplasmic sperm injection (ICSI) cycles.

METHODS

This study included 97 patients undergoing assisted oocyte activation (AOA) with Ca²⁺ ionophore (calcimycin, A23187) treatment. Preimplantation embryonic development and clinical outcomes were compared between ICSI-AOA cycles (AOA group) and previous ICSI cycles of the same patients in which poor embryo developmental potential was present (non-AOA group). Subgroups stratified by maternal age (< 35, 35-40, ≥ 40 years, respectively) were analyzed separately.

RESULTS

A total of 642 MII oocytes were collected in AOA group, and 689 in non-AOA group. Significantly higher day 3 good quality embryo rate (P = 0.034), good quality blastocyst formation rate (P <  0.001), and utilization rate (P <  0.001) were seen in AOA group. Similar results were seen in each subgroup. For pregnancy outcomes, there were significant differences in clinical pregnancy rate (P = 0.039) and live birth rate (P = 0.045) in total group. In subgroup aged < 35 years, biochemical (P = 0.038), clinical (P = 0.041), and ongoing pregnancy rate (P = 0.037) in AOA group were significantly higher than that in non-AOA group. No significant improvement for clinical outcomes for subgroups aged 35-40 and aged ≥40.

CONCLUSION

The study suggests that calcimycin could improve preimplantation development and pregnancy outcomes in patients aged < 35 years with embryo developmental problems in previous ICSI cycles.

Fertilization, Oocyte Activation, Calcium Release and Epigenetic Remodelling: Lessons From Cancer Models

Oocyte activation deficiency (OAD) is the basis of Total Fertilisation Failure (TFF) and is attributed to mutations in the PLCζ gene—termed male factor infertility. This derives abnormal Ca²⁺ oscillations and could be the main cause of primary disruptions in the gene expression of Ca²⁺-related proteins. Epigenetic mechanisms are universally accepted as key regulators of gene expression. However, epigenetic dysregulations have not been considered as potential mechanisms of oocyte-borne OAD. Herein, we discuss changes in the DNA methylome during oogenesis and embryogenesis. We further highlight key pathways comprising the oocyte Ca²⁺ toolkit, which could be targets of epigenetic alterations, especially aberrations in DNA methylation. Considering that the vast majority of epigenetic modifications examined during fertilization revolve around alterations in DNA methylation, we aim in this article to associate Ca²⁺-specific mechanisms with these alterations. To strengthen this perspective, we bring evidence from cancer research on the intricate link between DNA methylation and Ca²⁺ signaling as cancer research has examined such questions in a lot more detail. From a therapeutic standpoint, if our hypothesis is proven to be correct, this will explain the cause of TFF in idiopathic cases and will open doors for novel therapeutic targets.

Comparative study of preimplantation development following distinct assisted oocyte activation protocols in a PLC-zeta knockout mouse model

Mammalian fertilization encompasses a series of Ca2+ oscillations initiated by the sperm factor phospholipase C zeta (PLCζ). Some studies have shown that altering the Ca2+ oscillatory regime at fertilization affects preimplantation blastocyst development. However, assisted oocyte activation (AOA) protocols can induce oocyte activation in a manner that diverges profoundly from the physiological Ca2+ profiling. In our study, we used the newly developed PLCζ-null sperm to investigate the independent effect of AOA on mouse preimplantation embryogenesis. Based on previous findings, we hypothesized that AOA protocols with Ca2+ oscillatory responses might improve blastocyst formation rates and differing Ca2+ profiles might alter blastocyst transcriptomes. A total of 326 MII B6D2F1-oocytes were used to describe Ca2+ profiles and to compare embryonic development and individual blastocyst transcriptomes between four control conditions: C1 (in-vivo fertilization), C2 (ICSI control sperm), C3 (parthenogenesis) and C4 (ICSI-PLCζ-KO sperm) and four AOA groups: AOA1 (human recombinant PLCζ), AOA2 (Sr2+), AOA3 (ionomycin) and AOA4 (TPEN). All groups revealed remarkable variations in their Ca2+ profiles; however, oocyte activation rates were comparable between the controls (91.1% ± 13.8%) and AOA (86.9% ± 11.1%) groups. AOA methods which enable Ca2+ oscillatory responses (AOA1: 41% and AOA2: 75%) or single Ca2+ transients (AOA3: 50%) showed no significantly different blastocyst rates compared to ICSI control group (C2: 70%). In contrast, we observed a significant decrease in compaction (53% vs. 83%) and blastocyst rates (41% vs. 70%) in the absence of an initial Ca2+ trigger (AOA4) compared with the C2 group. Transcription profiles did not identify significant differences in gene expression levels between the ICSI control group (C2) and the four AOA groups.

Use of phospholipase C zeta analysis to identify candidates for artificial oocyte activation: a case series of clinical pregnancies and a proposed algorithm for patient management

OBJECTIVE

To investigate the applicability of phospholipase C zeta (PLCζ) analysis in assisting the clinical decision-making process when considering artificial oocyte activation (AOA) for infertile males in assisted reproductive technology.

DESIGN

Fifty-six males (43 infertile/13 fertile) were screened using our PLCζ assay.

SETTING

Fertility unit/university laboratory.

PATIENT(S)

Infertile males with abnormal sperm morphology or total fertilization failure, low fertilization rate (<50%), or repeated fertilization failure in assisted reproductive technology.

INTERVENTION(S)

We analyzed PLCζ levels in sperm from fertile and infertile males. Eligible patients subsequently underwent intracytoplasmic sperm injection (ICSI)/artificial oocyte activation (AOA) with calcimycin (GM508).

MAIN OUTCOME MEASURE(S)

PLCζ localization and level and the proportion of sperm expressing PLCζ. Thresholds of PLCζ deficiency, fertilization rates, pregnancy rates, and live birth rates of AOA and non-AOA cycles.

RESULT(S)

Compared with 13 fertile controls, 34 of the 43 infertile males had significantly lower levels of PLCζ and/or a significantly lower proportion of sperm exhibiting PLCζ. Of these 34 patients, 15 showed a significant PLCζ reduction in both parameters, which we termed "PLCζ deficiency." Five PLCζ-deficient patients opted for AOA; all five achieved fertilization, and four achieved clinical pregnancies and live births. The fertilization rate improved significantly from 18.6% (ICSI) to 56.8% (ICSI/AOA). The clinical pregnancy rate and live birth rate with AOA were both 40% per initiated cycle. Youden index analysis revealed that the cutoffs below which infertile males were likely to benefit from AOA were 71% for the proportion of sperm expressing PLCζ and 15.57 arbitrary units for mean PLCζ level.

CONCLUSION(S)

PLCζ analysis is a useful diagnostic tool to determine patient eligibility for subsequent AOA treatment.

Diagnosis and Treatment of Male Infertility-Related Fertilization Failure

Infertility affects approximately 15% of reproductive-aged couples worldwide, of which up to 30% of the cases are caused by male factors alone. The origin of male infertility is mostly attributed to sperm abnormalities, of which many are caused by genetic defects. The development of intracytoplasmic sperm injection (ICSI) has helped to circumvent most male infertility conditions. However, there is still a challenging group of infertile males whose sperm, although having normal sperm parameters, are unable to activate the oocyte, even after ICSI treatment. While ICSI generally allows fertilization rates of 70 to 80%, total fertilization failure (FF) still occurs in 1 to 3% of ICSI cycles. Phospholipase C zeta (PLCζ) has been demonstrated to be a critical sperm oocyte activating factor (SOAF) and the absence, reduced, or altered forms of PLCζ have been shown to cause male infertility-related FF. The purpose of this review is to (i) summarize the current knowledge on PLCζ as the critical sperm factor for successful fertilization, as well as to discuss the existence of alternative sperm-induced oocyte activation mechanisms, (ii) describe the diagnostic tests available to determine the cause of FF, and (iii) summarize the beneficial effect of assisted oocyte activation (AOA) to overcome FF.

Increasing associations between defects in phospholipase C zeta and conditions of male infertility: not just ICSI failure?

PURPOSE

Oocyte activation is a fundamental event at mammalian fertilization. In mammals, this process is initiated by a series of characteristic calcium (Ca²⁺) oscillations, induced by a sperm-specific phospholipase C (PLC) termed PLCzeta (PLCζ). Dysfunction/reduction/deletion of PLCζ is associated with forms of male infertility where the sperm is unable to initiate Ca²⁺ oscillations and oocyte activation, specifically in cases of fertilization failure. This review article aims to systematically summarize recent advancements and controversies in the field to update expanding clinical associations between PLCζ and various male factor conditions. This article also discusses how such associations may potentially underlie defective embryogenesis and recurrent implantation failure following fertility treatments, alongside potential diagnostic and therapeutic PLCζ approaches, aiming to direct future research efforts to utilize such knowledge clinically.

METHODS

An extensive literature search was performed using literature databases (PubMed/MEDLINE/Web of Knowledge) focusing on phospholipase C zeta (PLCzeta; PLCζ), oocyte activation, and calcium oscillations, as well as specific male factor conditions.

RESULTS AND DISCUSSION

Defective PLCζ or PLCζ-induced Ca²⁺ release can be linked to multiple forms of male infertility including abnormal sperm parameters and morphology, sperm DNA fragmentation and oxidation, and abnormal embryogenesis/pregnancies. Such sperm exhibit absent/reduced levels, and abnormal localization patterns of PLCζ within the sperm head.

CONCLUSIONS

Defective PLCζ and abnormal patterns of Ca²⁺ release are increasingly suspected a significant causative factor underlying abnormalities or insufficiencies in Ca²⁺ oscillation-driven early embryogenic events. Such cases could potentially strongly benefit from relevant therapeutic and diagnostic applications of PLCζ, or even alternative mechanisms, following further focused research efforts.

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.

Human oocyte calcium analysis predicts the response to assisted oocyte activation in patients experiencing fertilization failure after ICSI

STUDY QUESTION

Can human oocyte calcium analysis predict fertilization success after assisted oocyte activation (AOA) in patients experiencing fertilization failure after ICSI?

SUMMARY ANSWER

ICSI-AOA restores the fertilization rate only in patients displaying abnormal Ca2+ oscillations during human oocyte activation.

WHAT IS KNOWN ALREADY

Patients capable of activating mouse oocytes and who showed abnormal Ca2+ profiles after mouse oocyte Ca2+ analysis (M-OCA), have variable responses to ICSI-AOA. It remains unsettled whether human oocyte Ca2+ analysis (H-OCA) would yield an improved accuracy to predict fertilization success after ICSI-AOA.

STUDY DESIGN, SIZE, DURATION

Sperm activation potential was first evaluated by MOAT. Subsequently, Ca2+ oscillatory patterns were determined with sperm from patients showing moderate to normal activation potential based on the capacity of human sperm to generate Ca2+ responses upon microinjection in mouse and human oocytes. Altogether, this study includes a total of 255 mouse and 122 human oocytes. M-OCA was performed with 16 different sperm samples before undergoing ICSI-AOA treatment. H-OCA was performed for 11 patients who finally underwent ICSI-AOA treatment. The diagnostic accuracy to predict fertilization success was calculated based on the response to ICSI-AOA.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Patients experiencing low or total failed fertilization after conventional ICSI were included in the study. All participants showed moderate to high rates of activation after MOAT. Metaphase II (MII) oocytes from B6D2F1 mice were used for M-OCA. Control fertile sperm samples were used to obtain a reference Ca2+ oscillation profile elicited in human oocytes. Donated human oocytes, non-suitable for IVF treatments, were collected and vitrified at MII stage for further analysis by H-OCA.

MAIN RESULTS AND THE ROLE OF CHANCE

M-OCA and H-OCA predicted the response to ICSI-AOA in 8 out of 11 (73%) patients. Compared to M-OCA, H-OCA detected the presence of sperm activation deficiencies with greater sensitivity (75 vs 100%, respectively). ICSI-AOA never showed benefit to overcome fertilization failure in patients showing normal capacity to generate Ca2+ oscillations in H-OCA and was likely to be beneficial in cases displaying abnormal H-OCA Ca2+ oscillations patterns.

LIMITATIONS, REASONS FOR CAUTION

The scarce availability of human oocytes donated for research purposes is a limiting factor to perform H-OCA. Ca2+ imaging requires specific equipment to monitor fluorescence changes over time.

WIDER IMPLICATIONS OF THE FINDINGS

H-OCA is a sensitive test to diagnose gamete-linked fertilization failure. H-OCA allows treatment counseling for couples experiencing ICSI failures to either undergo ICSI-AOA or to participate in gamete donation programs. The present data provide an important template of the Ca2+ signature observed during human fertilization in cases with normal, low and failed fertilization after conventional ICSI.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the Flemish fund for scientific research (FWO-Vlaanderen, G060615N). The authors have no conflict of interest to declare.

Intracytoplasmic sperm injection in domestic and wild mammals

Intracytoplasmic sperm injection (ICSI) has become a useful technique for clinical applications in the horse-breeding industry. However, both ICSI blastocyst and offspring production continues to be limited for most farm and wild species. This article reviews technical differences of ICSI performance among species, possible biological and methodological reasons for the variable efficiency and potential strategies to improve the outcomes. One of the major applications of ICSI in animal production is the reproduction of high-value specimens. Unfortunately, some domestic species like the bovine show low rates of pronuclei formation after sperm injection, which led to the development of various artificial activation protocols and sperm pre-treatments that are discussed in this article. The impact of ICSI technique on equine breeding programs is considered in detail, since in contrast to other species, its use for elite horse reproduction has increased in recent years. ICSI has also been used to produce genetically modified animals; however, despite numerous attempts in several domestic species, only transgenic pigs have been consistently produced. Finally, the ICSI is a promising tool for genetic rescue of endangered and wild species. In conclusion, while ICSI has become a consistent ART for some species, it needs further development for others. The low results obtained for some domestic species, the high training needed and the equipment required have limited this technique to the production of elite specimens or for research purposes.

Single Ca2+ transients vs oscillatory Ca2+ signaling for assisted oocyte activation: limitations and benefits

Oocyte activation is a calcium (Ca²⁺)-dependent process that has been investigated in depth, in particular, regarding its impact on assisted reproduction technology (ART). Following a standard model of signal transduction, Ca²⁺ drives the meiotic progression upon fertilization in all species studied to date. However, Ca²⁺ changes during oocyte activation are species specific, and they can be classified in two modalities based on the pattern defined by the Ca²⁺ signature: a single Ca²⁺ transient (e.g. amphibians) or repetitive Ca²⁺ transients called Ca²⁺ oscillations (e.g. mammals). Interestingly, assisted oocyte activation (AOA) methods have highlighted the ability of mammalian oocytes to respond to single Ca²⁺ transients with normal embryonic development. In this regard, there is evidence supporting that cellular events during the process of oocyte activation are initiated by different number of Ca²⁺ oscillations. Moreover, it was proposed that oocyte activation and subsequent embryonic development are dependent on the total summation of the Ca²⁺ peaks, rather than to a specific frequency pattern of Ca²⁺ oscillations. The present review aims to demonstrate the complexity of mammalian oocyte activation by describing the series of Ca²⁺-linked physiological events involved in mediating the egg-to-embryo transition. Furthermore, mechanisms of AOA and the limitations and benefits associated with the application of different activation agents are discussed.

Lack of calcium oscillation causes failure of oocyte activation after intracytoplasmic sperm injection in pigs

In pigs, the efficiency of embryo production after intracytoplasmic sperm injection (ICSI) is still low because of frequent failure of normal fertilization, which involves formation of two polar bodies and two pronuclei. To clarify the reasons for this, we hypothesized that ICSI does not properly trigger sperm-induced fertilization events, especially intracellular Ca²⁺ signaling, also known as Ca²⁺ oscillation. We also suspected that the use of in vitro-matured oocytes might negatively affect fertilization events and embryonic development of sperm-injected oocytes. Therefore, we compared the patterns of Ca²⁺ oscillation, the efficiency of oocyte activation and normal fertilization, and embryo development to the blastocyst stage among in vivo- or in vitro-matured oocytes after ICSI or in vitro fertilization (IVF). Unexpectedly, we found that the pattern of Ca²⁺ oscillation, such as the frequency and amplitude of Ca²⁺ rises, in oocytes after ICSI was similar to that in oocytes after IVF, irrespective of the oocyte source. However, half of the oocytes failed to become activated after ICSI and showed no Ca²⁺ oscillation. Moreover, the embryonic development of normal fertilized oocytes was reduced when in vitro-matured oocytes were used, irrespective of the fertilization method employed. These findings suggest that low embryo production efficiency after ICSI is attributable mainly to poor developmental ability of in vitro-matured oocytes and a lack of Ca²⁺ oscillation, rather than the pattern of oscillation.

Deficiencies in extrusion of the second polar body due to high calcium concentrations during in vitro fertilization in inbred C3H/He mice

Successful in vitro fertilization (IVF) of all inbred strains of laboratory mice has not yet been accomplished. We have previously shown that a high calcium concentration improved IVF in various inbred mice. However, we also found that in cumulus-free ova of C3H/He mice such IVF conditions significantly increased the deficiency of extrusion of the second polar body (PBII) in a dose-dependent manner (2% at 1.71 mM and 29% at 6.84 mM, P < 0.05) and that PBII extrusion was affected by high calcium levels at 2–3 h post-insemination. While developmental competence of ova without PBII extrusion to blastocysts after 96 h culture was not affected, a significant reduction in the nuclear number of the inner cell mass was observed in blastocyst fertilized under high calcium condition. We also examined how high calcium concentration during IVF affects PBII extrusion in C3H/He mice. Cumulus cells cultured under high calcium conditions showed a significantly alleviated deficient PBII extrusion. This phenomenon is likely to be specific to C3H/He ova because deficient PBII extrusion in reciprocal fertilization between C3H and BDF1 gametes was observed only in C3H/He ova. Sperm factor(s) was still involved in deficient PBII extrusion due to high calcium concentrations, as this phenomenon was not observed in ova activated by ethanol. The cytoskeletal organization of ova without PBII extrusion showed disturbed spindle rotation, incomplete formation of contractile ring and disturbed localization of actin, suggesting that high calcium levels affect the anchoring machinery of the meiotic spindle. These results indicate that in C3H/He mice high calcium levels induce abnormal fertilization, i.e. deficient PBII extrusion by affecting the cytoskeletal organization, resulting in disturbed cytokinesis during the second meiotic division. Thus, use of high calcium media for IVF should be avoided for this strain.

Excess cholesterol induces mouse egg activation and may cause female infertility

The HDL receptor scavenger receptor, class B type I (SR-BI) controls the structure and fate of plasma HDL. Female SR-BI KO mice are infertile, apparently because of their abnormal cholesterol-enriched HDL particles. We examined the growth and meiotic progression of SR-BI KO oocytes and found that they underwent normal germinal vesicle breakdown; however, SR-BI KO eggs, which had accumulated excess cholesterol in vivo, spontaneously activated, and they escaped metaphase II (MII) arrest and progressed to pronuclear, MIII, and anaphase/telophase III stages. Eggs from fertile WT mice were activated when loaded in vitro with excess cholesterol by a cholesterol/methyl-β-cyclodextrin complex, phenocopying SR-BI KO oocytes. In vitro cholesterol loading of eggs induced reduction in maturation promoting factor and MAPK activities, elevation of intracellular calcium, extrusion of a second polar body, and progression to meiotic stages beyond MII. These results suggest that the infertility of SR-BI KO females is caused, at least in part, by excess cholesterol in eggs inducing premature activation and that cholesterol can activate WT mouse eggs to escape from MII arrest. Analysis of SR-BI KO female infertility raises the possibility that abnormalities in cholesterol metabolism might underlie some cases of human female infertility of unknown etiology.

The dynamics of MAPK inactivation at fertilization in mouse eggs

Egg activation at fertilization in mammals is initiated by prolonged Ca(2+) oscillations that trigger the completion of meiosis and formation of pronuclei. A fall in mitogen-activated protein kinase (MAPK) activity is essential for pronuclear formation, but the precise timing and mechanism of decline are unknown. Here, we have measured the dynamics of MAPK pathway inactivation during fertilization of mouse eggs using novel chemiluminescent MAPK activity reporters. This reveals that the MAPK activity decrease begins during the Ca(2+) oscillations, but MAPK does not completely inactivate until after pronuclear formation. The MAPKs present in eggs are Mos, MAP2K1 and MAP2K2 (MEK1 and MEK2, respectively) and MAPK3 and MAPK1 (ERK1 and ERK2, respectively). Notably, the MAPK activity decline at fertilization is not explained by upstream destruction of Mos, because a decrease in the signal from a Mos-luciferase reporter is not associated with egg activation. Furthermore, Mos overexpression does not affect the timing of MAPK inactivation or pronuclear formation. However, the late decrease in MAPK could be rapidly reversed by the protein phosphatase inhibitor, okadaic acid. These data suggest that the completion of meiosis in mouse zygotes is driven by an increased phosphatase activity and not by a decline in Mos levels or MEK activity.

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.

Molecular mechanism of poor embryo development in postovulatory aged oocytes: mini review

Oocyte quality is a key factor in determining embryo development; however, we have a poor understanding of what constitutes oocyte quality or the mechanisms governing it. Postovulatory aging of oocytes that have not been fertilized for a prolonged time after ovulation is known to significantly impair oocyte quality and subsequent embryo development after fertilization. Embryos derived from postovulatory-aged oocytes are prone to undergo apoptosis due to the decreased Bcl-2 expression. Postovulatory aging of oocytes changes the patterns of Ca(2+) oscillations at fertilization as a result of impaired Ca(2+) regulation in the endoplasmic reticulum. Moreover, postovulatory aging of oocytes impairs mitochondrial adenosine triphosphate production as a result of increasing oxidative stresses. Oxidative stresses also affect intracellular Ca(2+) regulation and impair embryo development after fertilization. Collectively, the mechanism of postovulatory oocyte aging might be involved in reactive oxygen species-induced mitochondrial injury followed by abnormal intracellular Ca(2+) regulation in the endoplasmic reticulum.

Polarized Cdc42 activation promotes polar body protrusion and asymmetric division in mouse oocytes

Asymmetric meiotic divisions in mammalian oocytes rely on the eccentric positioning of the spindle and the remodeling of the overlying cortex, resulting in the formation of small polar bodies. The mechanism of this cortical polarization, exemplified by the formation of a thick F-actin cap, is poorly understood. Cdc42 is a major player in cell polarization in many systems; however, the spatio-temporal dynamics of Cdc42 activation during oocyte meiosis, and its contribution to mammalian oocyte polarization, have remained elusive. In this study, we investigated Cdc42 activation (Cdc42–GTP), dynamics and role during mouse oocyte meiotic divisions. We show that Cdc42–GTP accumulates in restricted cortical regions overlying meiotic chromosomes or chromatids, in a Ran–GTP-dependent manner. This polarized activation of Cdc42 is required for the recruitment of N-WASP and the formation of F-actin-rich protrusions during polar body formation. Cdc42 inhibition in MII oocytes resulted in the release of N-WASP into the cytosol, a loss of the polarized F-actin cap, and a failure to protrude the second polar body. Cdc42 inhibition also resulted in central spindle defects in activated MII oocytes. In contrast, emission of the first polar body during oocyte maturation could occur in the absence of a functional Cdc42/N-WASP pathway. Therefore, Cdc42 is a new protagonist in chromatin-induced cortical polarization in mammalian oocytes, with an essential role in meiosis II completion, through the recruitment and activation of N-WASP, downstream of the chromatin-centered Ran–GTP gradient.

Molecular changes during egg activation

Egg activation is the final transition that an oocyte goes through to become a developmentally competent egg. This transition is usually triggered by a calcium-based signal that is often, but not always, initiated by fertilization. Activation encompasses a number of changes within the egg. These include changes to the egg's membranes and outer coverings to prevent polyspermy and to support the developing embryo, as well as resumption and completion of the meiotic cell cycle, mRNA polyadenylation, translation of new proteins, and the degradation of specific maternal mRNAs and proteins. The transition from an arrested, highly differentiated cell, the oocyte, to a developmentally active, totipotent cell, the activated egg or embryo, represents a complete change in cellular state. This is accomplished by altering ion concentrations and by widespread changes in both the proteome and the suite of mRNAs present in the cell. Here, we review the role of calcium and zinc in the events of egg activation, and the importance of macromolecular changes during this transition. The latter include the degradation and translation of proteins, protein posttranslational regulation through phosphorylation, and the degradation, of maternal mRNAs.

The combined treatment of calcium ionophore with strontium improves the quality of ovine SCNT embryo development

Poor embryo quality is a major problem that contributes to the failure of pregnancy in somatic cell nuclear transfer (SCNT). The aims of this study were to improve the quality of ovine SCNT embryos by modifying the conventional activation protocol with the addition of SrCl2. In order to achieve this objective we conducted a series of experiments with in vitro-matured oocytes to optimize conditions for oocyte activation with strontium, and subsequently applied the protocol to SCNT embryos. The results showed that in vitro-matured oocytes could be activated effectively by 10 mM SrCl2 + 5 mg/ml cytochalasin B (CB) for 5 h in the absence of Ca2+ and that the blastocyst rate on day 7 (33.2%) was similar to that in the control group (31.0%) (5 M calcium ionophore [IP] A23187 for 5 min and cultured in CB/cycloheximide [CHX] for 5 h; P > 0.05). In SCNT experiments, the total cell number/blastocyst (104.12 ± 6.86) in the IP + SrCl2/CB-treatment group was, however, significantly higher than that in the control group (81.07 ± 3.39; P < 0.05). Apoptotic index (12.29 ± 1.22%) was significantly lower than the control (17.60 ± 1.39%; P < 0.05) when a combination of IP and SrCl2/CB was applied to SCNT embryos. In addition, karyotyping of the SCNT embryos showed that the percentage of diploid blastocysts in the IP + SrCl2/CB-treatment group was slightly higher than that in the control (P > 0.05). We conclude that the modified activation protocol with IP + SrCl2/CB can improve significantly the quality of ovine SCNT embryos in terms of total cell number, apoptosis and ploidy.

The Anaphase-Promoting Complex activator Fizzy-Related-1 (FZR1) is involved in the establishment of a single mitotic spindle in 1-cell embryos and in the mitotic divisions of early mammalian embryos

In early embryos of a number of species the Anaphase-Promoting Complex (APC), an important cell cycle regulator, requires only CDC20 for cell division. In contrast FZR1, a non-essential gene in many cell types, is thought to play a role in APC activation at later cell cycles, and especially in endoreplication. In keeping with this, FZR1 knockout mouse embryos show normal preimplantation development but die due to a lack of endoreplication needed for placentation. However, interpretation of the role of FZR1 during this period is hindered by the presence of maternal stores. Here, therefore, we used an oocyte-specific knockout to examine FZR1 function in early mouse embryo development. Maternal FZR1 was not critical for completion of meiosis, and furthermore viable pups were born to these females mated with normal males. However, in early embryos the absence of both maternal and paternal FZR1 led to a dramatic loss in genome integrity, such that the majority of embryos arrested having undergone only a single mitotic division and contained many γ-H2AX foci, consistent with fragmented DNA. A prominent feature of such embryos was a the establishment of two independent spindles following pronuclear fusion and thus a failure of the chromosomes to mix (syngamy). These generated binucleate 2-cell embryos. In the 10% of embryos that progressed to the 4-cell stage, division was so slow that compaction occurred prematurely. No embryo development to the blastocyst stage was ever observed. We conclude that FZR1 is a surprisingly essential gene involved in the establishment of a single spindle from the two pronuclei in 1-cell embryos as well as being involved in the maintainence of genomic integrity during the mitotic divisions of early mammalian embryos.

Adult body weight is programmed by a redox-regulated and energy-dependent process during the pronuclear stage in mouse

In mammals fertilization triggers a series of Ca(2+) oscillations that not only are essential for events of egg activation but also stimulate oxidative phosphorylation. Little is known, however, about the relationship between quantitative changes in egg metabolism and specific long-term effects in offspring. This study assessed whether post-natal growth is modulated by early transient changes in NAD(P)H and FAD(2+) in zygotes. We report that experimentally manipulating the redox potential of fertilized eggs during the pronuclear (PN) stage affects post-natal body weight. Exogenous pyruvate induces NAD(P)H oxidation and stimulates mitochondrial activity with resulting offspring that are persistently and significantly smaller than controls. Exogenous lactate stimulates NAD(+) reduction and impairs mitochondrial activity, and produces offspring that are smaller than controls at weaning but catch up after weaning. Cytosolic alkalization increases NAD(P)(+) reduction and offspring of normal birth-weight become significantly and persistently larger than controls. These results constitute the first report that post-natal growth rate is ultimately linked to modulation of NAD(P)H and FAD(2+) concentration as early as the PN stage.

Cellular and molecular mechanisms of various types of oocyte aging

It is well established that age-related decline of a woman's fertility is related to the poor developmental potential of her gametes. The age-associated decline in female fertility is largely attributable to the oocyte aging caused by ovarian aging. Age-associated oocyte aging results in a decrease in oocyte quality. In contrast to ovarian aging, there is a concept of postovulatory oocyte aging. Postovulatory aging of oocytes, not being fertilized for a prolonged time after ovulation, is known to significantly affect the development of oocytes. Both categories of oocyte aging have similar phenotypes of reproductive failure. However, the mechanisms of the decline in oocyte quality are not necessarily equivalent. An age-dependent increase in aneuploidy is a key determinant of oocyte quality. The reduced expression of molecules regulating cell cycle control during meiosis might be involved in the age-dependent increase in aneuploidy. The mechanism of age-associated oocyte aging might be involved in mitochondrial dysfunction, whose etiologies are still unknown. Alternatively, the mechanism of postovulatory oocyte aging might be involved in reactive oxygen species-induced mitochondrial injury pathways followed by abnormal intracellular Ca²⁺ regulation of the endoplasmic reticulum. We suggest that future research into the mechanism of oocyte aging will be necessary to develop a method to rescue the poor developmental potential of aged oocytes.

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.

Full-term mouse development by abolishing Zn2+-dependent metaphase II arrest without Ca2+ release

In vertebrates, a rise in intracellular free Ca2+ (Ca2+i) levels during fertilization initiates second metaphase (mII) exit and the developmental programme. The Ca2+ rise has long been considered to be crucial for development, but verifying this contribution would benefit from defining its role during fertilization. Here, we delineate the role of Ca2+ release during mII exit in wild-type mouse eggs and show that it is dispensable for full-term development. Exit from mII can be induced by Zn2+-specific sequestration without Ca2+ release, eliciting Cyclin B degradation in a manner dependent upon the proteasome pathway and intact microtubules, but not accompanied by degradation of the meiotic regulator Emi2. Parthenogenotes generated by Zn2+ sequestration developed in vitro with normal expression of Ca2+-sensitive genes. Meiotic exit induced by either Ca2+ oscillations or a single Ca2+ rise in oocytes containing a signaling-deficient sperm resulted in comparable developmental rates. In the absence of Ca2+ release, full-term development occurred ∼50% less efficiently, but at readily detectable rates, with the birth of 27 offspring. These results show in intact mouse oocytes that Zn2+ is essential for mII arrest and suggest that triggering meiotic exit is the sole indispensable developmental role of Ca2+ signaling in mammalian fertilization.

The gamma isoform of CaM kinase II controls mouse egg activation by regulating cell cycle resumption

Fertilization triggers a rise in intracellular Ca(2+) concentration ([Ca(2+)](i)) in the egg that initiates a series of events known as egg activation. These events include cortical granule exocytosis that establishes a block to polyspermy, resumption of meiosis, and recruitment of maternal mRNAs into polysomes for translation. Several calcium-dependent proteins, including calcium/calmodulin-dependent protein kinase II (CaMKII), have been implicated in egg activation. However, the precise role of CaMKII in mediating specific events of egg activation and the identity of the isoform(s) present in mouse eggs have not been unequivocally established. Through targeted deletion of the gamma isoform of CaMKII, we find that CaMKIIgamma is the predominant CaMKII isoform in mouse eggs and that it is essential for egg activation. Although CaMKIIgamma(-/-) eggs exhibit a normal pattern of Ca(2+) oscillations after insemination and undergo cortical granule exocytosis, they fail to resume meiosis or to recruit maternal mRNAs. Surprisingly, we find that the recruitment of maternal mRNAs does not directly depend on CaMKII, but requires elevated [Ca(2+)](i) and metaphase II exit. We conclude that CaMKIIgamma specifically controls mouse egg activation by regulating cell cycle resumption.

Proteomic analysis of parthenogenetic and in vitro fertilized porcine embryos

Proteomic data from embryos are essential for the completion of whole proteome catalog due to embryo-specific expression of certain proteins. In this study, using reverse phase LC-MS/MS combined with 1-D SDS-PAGE, we identified 1625 mammalian and 735 Sus scrofa proteins from porcine zygotes that included both cytosolic and membranous proteins. We also found that the global protein profiles of parthenogenetically activated (PA) and in vitro fertilized (IVF) zygotes were similar but differences in expression of individual proteins were also evident. These differences were not due to culture conditions, polyspermy or non-activation of oocytes, as the same culture method was used in both groups, the frequency of polyspermy was 24.3+/-3.0% and the rates of oocyte activation did not differ (p>0.05) between PA and IVF embryos. Consistent with proteomic data, fluorescent Hoechst 33 342 staining and terminal deoxynucleotidyl transferase dUTP nick end labeling assay also revealed that PA embryos were of poor quality as they contained less cells per blastocyst and were more predisposed to apoptosis (p<0.05), although their in vitro development rates were similar. To our knowledge, this is the first report on global peptide sequencing and quantification of protein in PA and IVF embryos by LC-MS/MS that may be useful as a reference map for future studies.

Alterations in the protein kinase C signaling activated by a parthenogenetic agent in oocytes from reproductively old mice

To investigate the effect of female age on oocyte developmental competence, we focused on protein kinase C (PKC), a major component of the signalling pathway involved in oocyte activation, and put forward the hypothesis that, as it occurs in many organs and tissues, aging affects PKC function in mouse oocytes. Biochemical activity of PKC along with the expression and subcellular distribution of some PKC isoforms were monitored in young and old mouse oocytes parthenogenetically activated by SrCl(2). We found that PKC activity increased reaching a level that was lower in old compared to young oocytes in association with an incomplete translocation of PKCbetaI to the plasma membrane. Moreover, old oocytes exhibited a reduced expression of PKCbeta1 and PKCalpha at the protein level, without significant effects on the expression of the Ca(2+)-independent PKCdelta. Detectable amounts of PKCbeta1 mRNA were observed in young and old oocytes at GV stage with no difference between the two groups of age. When meiotic progression to anaphase II up to first cleavage were analyzed, a delay in meiosis resumption and significantly lower rates of pronuclei formation and first cleavage were observed in old compared to young oocytes. Moreover, we found that, in contrast to SrCl(2), PMA (12-O-tetradecanoyl phorbol-13-acetate), a PKC agonist, was ineffective in activating old oocytes. Present findings provide evidence that aging affects the correct storage and activation of some PKCs, functional components of the machinery involved in oocyte activation, and suggest that these changes may negatively influence the activation competence of old oocytes.

Activation of bovine somatic cell nuclear transfer embryos by PLCZ cRNA injection

The production of cloned animals by the transfer of a differentiated somatic cell into an enucleated oocyte circumvents fertilization. During fertilization, the sperm delivers a sperm-specific phospholipase C (PLCZ) that is responsible for triggering Ca(2)(+) oscillations and oocyte activation. During bovine somatic cell nuclear transfer (SCNT), oocyte activation is artificially achieved by combined chemical treatments that induce a monotonic rise in intracellular Ca(2)(+) and inhibit either phosphorylation or protein synthesis. In this study, we tested the hypothesis that activation of bovine nuclear transfer embryos by PLCZ improves nuclear reprogramming. Injection of PLCZ cRNA into bovine SCNT units induced Ca(2)(+) oscillations similar to those observed after fertilization and supported high rates of blastocyst development similar to that seen in embryos produced by IVF. Furthermore, gene expression analysis at the eight-cell and blastocyst stages revealed a similar expression pattern for a number of genes in both groups of embryos. Lastly, levels of trimethylated lysine 27 at histone H3 in blastocysts were higher in bovine nuclear transfer embryos activated using cycloheximide and 6-dimethylaminopurine (DMAP) than in those activated using PLCZ or derived from IVF. These results demonstrate that exogenous PLCZ can be used to activate bovine SCNT-derived embryos and support the hypothesis that a fertilization-like activation response can enhance some aspects of nuclear reprogramming.

Parthenogenetic activation of pig oocytes using pulsatile treatment with a nitric oxide donor

The nitric oxide donor (+)-S-nitroso-N-acetylpenicillamine (SNAP) is capable of inducing parthenogenetic activation in pig oocytes matured in vitro. However, quite a long exposure to the nitric oxide donor, exceeding 10 h, is necessary for successful oocyte activation. Repeated short-term treatment with 2 mm SNAP significantly increased the activation rates despite the fact that the overall exposure time to the nitric oxide donor did not exceed 4 h. With regard to the activation rate, 12 repeated treatments lasting 10 min each were found to be the most efficient regimen (63.3%). The continuous exposure to the nitric oxide donor for the same overall time induced parthenogenetic activation in 12.5% oocytes (2-h continuous treatment with 2 mm SNAP). The development of parthenogenetic embryos increased after repeated short-term treatment with SNAP. After continuous treatment with 2 mm SNAP for 10 h, only 6.7% of the oocytes cleaved, and none developed beyond the 4-cell stage. Thirty-minute treatment repeated four times with 2 mm SNAP induced cleavage in 37.5% of the oocytes, 18.3% developed to the morula stage, and 6.7% reached the blastocyst stage. Based on the results, it is concluded that pulsatile treatment can significantly improve parthenogenetic activation rate when compared with the continuous treatment using nitric oxide donors.

Human sperm devoid of PLC, zeta 1 fail to induce Ca(2+) release and are unable to initiate the first step of embryo development

Egg activation, which is the first step in the initiation of embryo development, involves both completion of meiosis and progression into mitotic cycles. In mammals, the fertilizing sperm delivers the activating signal, which consists of oscillations in free cytosolic Ca(2+) concentration ([Ca(2+)](i)). Intracytoplasmic sperm injection (ICSI) is a technique that in vitro fertilization clinics use to treat a myriad of male factor infertility cases. Importantly, some patients who repeatedly fail ICSI also fail to induce egg activation and are, therefore, sterile. Here, we have found that sperm from patients who repeatedly failed ICSI were unable to induce [Ca(2+)](i) oscillations in mouse eggs. We have also shown that PLC, zeta 1 (PLCZ1), the sperm protein thought to induce [Ca(2+)](i) oscillations, was localized to the equatorial region of wild-type sperm heads but was undetectable in sperm from patients who had failed ICSI. The absence of PLCZ1 in these patients was further confirmed by Western blot, although genomic sequencing failed to reveal conclusive PLCZ1 mutations. Using mouse eggs, we reproduced the failure of sperm from these patients to induce egg activation and rescued it by injection of mouse Plcz1 mRNA. Together, our results indicate that the inability of human sperm to initiate [Ca(2+)](i) oscillations leads to failure of egg activation and sterility and that abnormal PLCZ1 expression underlies this functional defect.

Reactive oxygen species and oocyte aging: role of superoxide, hydrogen peroxide, and hypochlorous acid

Aging of the unfertilized oocyte inevitably occurs following ovulation, limiting its fertilizable life span. However, the mechanisms that regulate oocyte aging are still unclear. We hypothesize that reactive oxygen species such as superoxide (O2-), hydrogen peroxide (H2O2), and hypochlorous acid (HOCl) are likely candidates that may initiate these changes in the oocyte. In order to test this hypothesis, we investigated direct effects of O2- [hypoxanthine/xanthine oxidase system generating 0.12 (n=42) and 0.25 (n=45) microM O2-/min], H2O2 (20 or 100 microM, n=60), and HOCl, (1, 10, and 100 microM, n=50) on freshly ovulated or relatively old mouse oocytes, while their sibling oocytes were fixed immediately or cultured under physiological conditions (n=96). The aging process was assessed by the zona pellucida dissolution time (ZPDT), ooplasm microtubule dynamics (OMD), and cortical granule (CG) status. The ZPDT increased 2-fold in relatively old, compared to young, untreated oocytes (P<0.0001). Exposure to O2- increased it even further (P<0.0001). Similarly, more O2- exposed oocytes exhibited increased OMD and major CG loss, with fewer having normal OMD and intact CG compared to untreated controls. Interestingly, young oocytes resisted "aging," when exposed to 20 microM H2O2, while the same enhanced the aging phenomena in relatively old oocytes (P<0.05). Exposure to even very low levels of HOCl induced the aging phenomena in young and relatively old oocytes, and higher concentrations of HOCl compromised oocyte viability. Overall, O2-, H2O2, and HOCl each augment oocyte aging, more so in relatively old oocytes, suggesting compromised antioxidant capacity in aging oocytes.

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.

Constitutive PtdIns(3,4,5)P3 synthesis promotes the development and survival of early mammalian embryos

Mammalian preimplantation embryos develop in the oviduct as individual entities, and can develop and survive in vitro, in defined culture media lacking exogenous growth factors or serum. Therefore, early embryos must generate intrinsic signals that promote their development and survival. In other cells, activation of class I phosphoinositide 3-kinase (PI3K) is a universal mechanism to promote cell proliferation and survival. Here, we examined whether PI3K is intrinsically activated during preimplantation development. Using GFP-tagged pleckstrin homology domains to monitor PtdIns(3,4,5)P(3) synthesis, we show that PI3K is constitutively activated in mouse preimplantation embryos. E-cadherin ligation promotes PtdIns(3,4,5)P(3) synthesis at sites of blastomere adhesion at all cleavage stages. In addition, in culture conditions that promote autocrine signalling, a second pool of PtdIns(3,4,5)P(3) is generated in the apical membrane of early stage blastomeres. We show that constitutive PtdIns(3,4,5)P(3) synthesis is necessary for optimal development to blastocyst and to prevent large-scale apoptosis at the time of cavitation.

Chemical activation of parthenogenetic and nuclear transfer porcine oocytes using ionomycin and strontium chloride

Effective protocols for oocyte activation are crucial for study of parthenogenetic development and to produce nuclear transfer reconstructed embryos. This study investigated the use of ionomycin (ION) and strontium chloride (Sr(2+)) in the activation of parthenogenetic and nuclear transfer porcine oocytes. In-vitro-matured oocytes with a polar body were treated with varying concentrations of ION, Sr(2+) or its combinations, and then fixed or cultured to assess activation and development rates, respectively. Ionomycin concentrations of 10 and 15 microM resulted in more frequent oocyte activation and the 15 microM in advanced development compared to 5 microM (71.8 and 70%vs. 47.5%; P=0.04, and 43.7%vs. 19.3%; P=0.008, respectively). Oocytes treated with 10, 20 or 30 mM of Sr(2+) for 2 or 4h displayed a pronuclear formation rate ranging from 46.7 to 70%. When employed after a 5 min treatment with 10 or 15 microM ION, exposure to 10 mM Sr(2+) for 4 h resulted in higher pronuclear formation than did the 20 mM concentration (82 and 88.6%vs. 63.3 and 73.2%; P=0.03). Nuclear transfer reconstructed oocytes treated with 15 microM/5 min ION followed by 10 mM/4 h Sr(2+) resulted in a higher development to blastocyst stage compared to those treated with 15 microM ION alone (17.7 vs. 11.3%; P=0.06). In conclusion, we inferred that the inclusion of Sr(2+) in the activation protocol can benefit the development of nuclear transfer reconstructed porcine oocytes.

Rac activity is polarized and regulates meiotic spindle stability and anchoring in mammalian oocytes

Mammalian meiotic divisions are asymmetrical and generate a large oocyte and two small polar bodies. This asymmetry results from the anchoring of the meiotic spindle to the oocyte cortex and subsequent cortical reorganization, but the mechanisms involved are poorly understood. We investigated the role of Rac in oocyte meiosis by using a fluorescent reporter for Rac-GTP. We find that Rac-GTP is polarized in the cortex overlying the meiotic spindle. Polarization of Rac activation occurs during spindle migration and is promoted by the proximity of chromatin to the cortex. Inhibition of Rac during oocyte maturation caused a permanent block at prometaphase I and spindle elongation. In metaphase II-arrested oocytes, Rac inhibition caused the spindle to detach from the cortex and prevented polar body emission after activation. These results demonstrate that Rac-GTP plays a major role in oocyte meiosis, via the regulation of spindle stability and anchoring to the cortex.