Roles of MAPK and spindle assembly checkpoint in spontaneous activation and MIII arrest of rat oocytes

Role of calcium-sensing receptor in regulating activation susceptibility of postovulatory aging mouse oocytes

The mechanisms underlying postovulatory oocyte aging (POA) remain largely unknown. The expression of the calcium-sensing receptor (CaSR) in mouse oocytes and its role in POA need to be explored. Our objective was to observe CaSR expression and its role in the susceptibility to activating stimuli (STAS) in POA mouse oocytes. The results showed that, although none of the newly ovulated oocytes were activated, 40% and 94% of the oocytes recovered 19 and 25 h after human chorionic gonadotropin (hCG) injection were activated, respectively, after ethanol treatment. The level of the CaSR functional dimer protein in oocytes increased significantly from 13 to 25 h post hCG. Thus, the CaSR functional dimer level was positively correlated with the STAS of POA oocytes. Aging in vitro with a CaSR antagonist suppressed the elevation of STAS, and cytoplasmic calcium in oocytes recovered 19 h post hCG, whereas aging with a CaSR agonist increased STAS, and cytoplasmic calcium of oocytes recovered 13 h post hCG. Furthermore, the CaSR was more important than the Na-Ca2+ exchanger in regulating oocyte STAS, and T- and L-type calcium channels were inactive in aging oocytes. We conclude that the CaSR is involved in regulating STAS in POA mouse oocytes, and that it is more important than the other calcium channels tested in this connection.

Growth hormone reduces aneuploidy and improves oocytes quality by JAK2-MAPK3/1 pathway in aged mice

BACKGROUND

The global delay in women's reproductive age has raised concerns about age-related infertility. The decline in oocyte quality is a limiting factor of female fertility, yet there are currently no strategies to preserve oocyte quality in aged women. Here, we investigated the effects of growth hormone (GH) supplementation on aneuploidy of aged oocytes.

METHODS

For the in vivo experiments, the aged mice (8-month-old) were intraperitoneally injected with GH daily for 8 weeks. For the in vitro experiments, germinal vesicle oocytes from aged mice were treated with GH during oocyte maturation. The impacts of GH on ovarian reserve before superovulation was evaluated. Oocytes were retrieved to assess oocyte quality, aneuploidy and developmental potential characteristics. Quantitative proteomics analysis was applied to investigate the potential targets of GH in aged oocytes.

RESULTS

In this study, we demonstrated that GH supplementation in vivo not only alleviated the decline in oocyte number caused by aging, but also improved the quality and developmental potential of aged oocytes. Strikingly, we discovered that GH supplementation reduced aneuploidy in aged oocytes. Mechanically, in addition to improving mitochondrial function, our proteomic analysis indicated that the MAPK3/1 pathway may be involved in the reduction in aneuploidy of aged oocytes, as confirmed both in vivo and in vitro. In addition, JAK2 may also act as a mediator in how GH regulates MAPK3/1.

CONCLUSIONS

In conclusion, our research reveals that GH supplementation protects oocytes against aging-related aneuploidy and enhances the quality of aged oocytes, which has clinical significance for aged women undergoing assisted reproduction technology.

Recurrent spontaneous oocyte activation causes female infertility

PURPOSE

Spontaneous oocyte activation (SOA) is a recently classified phenomenon characterized by the presence of a single pronucleus immediately following oocyte retrieval, without the apparent involvement of sperm. SOA currently remains poorly understood in humans, with no clear genetic or pathological factor(s). Herein, we report two separate cases of recurrent spontaneous oocyte activation, investigating potential avenues to identify causative etiology.

METHODS

Two patients with several cycles with SOA have undergone further genetic and embryologic investigation to reveal underlying causes for SOA and provide a treatment if possible.

RESULTS

One case was a patient with recurrent pregnancy loss and the other was diagnosed as unexplained infertility. In the first case, 61 out of 69 oocytes retrieved exhibited SOA in five cycles while in the second case 44 out of 49 oocytes exhibited SOA in five cycles. Oocytes were injected with sperm; embryo development and presence of paternal contribution were investigated. No pregnancy is ensued following embryo transfer in both patients. Time-lapse imaging of embryogenesis from the second case did not reveal even momentary second pronucleus appearance. We also performed clinical whole exome sequencing for both patients but did not identify any disease-causing variant.

CONCLUSION

Patients with SOA suffer from infertility. Our results indicate that more investigation is required to understand the etiology of SOA in humans concentrating on the molecular mechanisms that underpin regulation of oocyte activation and calcium dynamics need to be investigated to fully understand, and perhaps in the future rectify, recurrent SOA.

Treatment with MG132 prevents spontaneous activation of rat oocyte in culture and promotes embryonic development after intracytoplasmic sperm injection

Intracytoplasmic sperm injection (ICSI) is an effective reproductive technique for obtaining rat offspring using preserved sperm with low or no motility. However, rat oocytes undergo spontaneous activation immediately after retrieval from the oviduct and poorly develop after ICSI unless it is performed quickly. Here, we evaluated whether treatment with MG132, the proteasome inhibitor, suppresses the spontaneous activation of oocytes before and during ICSI. After retrieval from the oviducts, the rate of development into morula and blastocyst from the oocytes cultured in vitro for 1 h prior to ICSI significantly decreased compared with that from the control oocytes subject to ICSI without culture (7% versus 36%). However, a higher proportion of oocytes treated with MG132 for 0, 1, and 3 h before and during ICSI developed into morulae and blastocysts (70%, 60%, and 52%, respectively). Offspring were obtained from oocytes treated with MG132 for 0 and 1 h before and during ICSI (percentage: 31%). Altogether, MG132 could suppress the spontaneous activation of rat oocytes and increase embryonic development after ICSI.

Topoisomerase II dysfunction causes metaphase I arrest by activating aurora B, SAC and MPF and prevents PB1 abscission in mouse oocytes†

Oocyte aneuploidy is caused mainly by chromosome nondisjunction and/or unbalanced sister chromatid pre-division. Although studies in somatic cells have shown that topoisomerase II (TOP2) plays important roles in chromosome condensation and timely separation of centromeres, little is known about its role during oocyte meiosis. Furthermore, because VP-16, which is a TOP2 inhibitor and induces DNA double strand breaks, is often used for ovarian cancer chemotherapy, its effects on oocytes must be studied for ovarian cancer patients to recover ovarian function following chemotherapy. This study showed that inhibiting TOP2 with either ICRF-193 or VP-16 during meiosis I impaired chromatin condensation, chromosome alignment, TOP2α localization and caused metaphase I (MI) arrest and first polar body (PB1) abscission failure. Inhibiting or neutralizing either spindle assembly checkpoint (SAC), Aurora B or maturation-promoting factor (MPF) significantly abolished the effect of ICRF-193 or VP-16 on MI arrest. Treatment with ICRF-193 or VP-16 significantly activated MPF and SAC but the effect disappeared when Aurora B was inhibited. Most of the oocytes matured in the presence of ICRF-193 or VP-16 were arrested at MI, and only 11% to 27% showed PB1 protrusion. Furthermore, most of the PB1 protrusions formed in the presence of ICRF-193 or VP-16 were retracted after further culture for 7 h. In conclusion, TOP2 dysfunction causes MI arrest by activating Aurora B, SAC and MPF and it prevents PB1 abscission by promoting chromatin bridges.

Oocyte Spontaneous Activation: An Overlooked Cellular Event That Impairs Female Fertility in Mammals

In mammals, including humans, mature oocytes are ovulated into the oviduct for fertilization. Normally, these oocytes are arrested at metaphase of the second meiosis (MII), and this arrest can be maintained for a certain period, which is essential for fertilization in vivo and oocyte manipulations in vitro, such as assisted reproduction in clinics and nuclear/spindle transfer in laboratories. However, in some species and under certain circumstances, exit from MII occurs spontaneously without any obvious stimulation or morphological signs, which is so-called oocyte spontaneous activation (OSA). This mini-review summarizes two types of OSA. In the first type (e.g., most rat strains), oocytes can maintain MII arrest in vivo, but once removed out, oocytes undergo OSA with sister chromatids separated and eventually scattered in the cytoplasm. Because the stimulation is minimal (oocyte collection itself), this OSA is incomplete and cannot force oocytes into interphase. Notably, once re-activated by sperm or chemicals, those scattered chromatids will form multiple pronuclei (MPN), which may recapitulate certain MPN and aneuploidy cases observed in fertility clinics. The second type of OSA occurs in ovarian oocytes (e.g., certain mouse strains and dromedary camel). Without ovulation or fertilization, these OSA-oocytes can initiate intrafollicular development, but these parthenotes cannot develop to term due to aberrant genomic imprinting. Instead, they either degrade or give rise to ovarian teratomas, which have also been reported in female patients. Last but not the least, genetic models displaying OSA phenotypes and the lessons we can learn from animal OSA for human reproduction are also discussed.

Effect of the time interval between oocyte retrieval and ICSI on embryo development and reproductive outcomes: a systematic review

BACKGROUND

Intra-cytoplasmic sperm injection (ICSI) is used in assisted reproductive technology (ART) laboratories. However, there is no consensus regarding the precise time intervals within ICSI cycles [oocyte pick up (OPU), oocyte denudation (DN), and ICSI], and results are inconsistent and contradictory. Thus, we aim to evaluate whether there is a concordance regarding the time intervals used in different laboratories and a concrete time that gives better laboratory and reproductive results.

METHODS

A systematic review of the literature until July 25, 2020, was performed with the keywords "Oocyte Denudation/Denudation/Oocyte," "Intra-cytoplasmic Sperm Injection/ICSI," "Oocyte/Oocyte maturation/ cumulus," and "Cumulus removal/ removal." Articles and abstracts in English and involving human subjects referring to the effects of oocyte DN time on embryo development and clinical outcomes were included.

RESULTS

Of the 294 evaluated articles, 24 (including 20 full articles and 4 abstracts) were included in this review. Eighteen studies analysed the effect of OPU-DN time on embryo development and clinical outcomes. Most of these studies concluded that OPU-DN time did not influence ICSI outcomes, but some suggested that oocytes should be incubated for a short time before DN to improve oocyte maturity and enhance ICSI outcomes. In addition to reports on positive or negligible effects, adverse effects were reported in 12 studies on DN-ICSI timing. Neither OPU-DN nor DN-ICSI time could improve live birth rate.

CONCLUSIONS

Oocytes should be pre-incubated for a short duration (preferably < 4 h) before DN according to the ART laboratory schedule. More randomised controlled trials are warranted to clarify the effect of DN-ICSI timing on ICSI outcomes.

Role of AMP-activated protein kinase during postovulatory aging of mouse oocytes†

Studies suggested that postovulatory oocyte aging might be prevented by maintaining a high maturation-promoting factor (MPF) activity. Whether AMP-activated protein kinase (AMPK) plays any role in postovulatory oocyte aging is unknown. Furthermore, while activation of AMPK stimulates meiotic resumption in mouse oocytes, it inhibits meiotic resumption in pig and bovine oocytes. Thus, the species difference in AMPK regulation of oocyte MPF activities is worth in-depth studies. This study showed that AMPK activation with metformin or 5-aminoimidazole- 4-carboxamide- 1-beta-d- ribofuranoside and inactivation with compound C significantly increased and decreased, respectively, the activation susceptibility (AS) and other aging parameters in aging mouse oocytes. While AMPK activity increased, MPF activity and cyclic adenosine monophosphate (cAMP) decreased significantly with time post ovulation. In vitro activation and inactivation of AMPK significantly decreased and increased the MPF activity, respectively. MPF upregulation with MG132 or downregulation with roscovitine completely abolished the effects of AMPK activation or inactivation on AS of aging oocytes, respectively. AMPK facilitated oocyte aging with increased reactive oxygen species (ROS) and cytoplasmic calcium. Furthermore, treatment with Ca2+/calmodulin-dependent protein kinase (CaMK) inhibitors significantly decreased AS and AMPK activation. Taken together, the results suggested that AMPK facilitated oocyte aging through inhibiting MPF activities, and postovulatory oocyte aging activated AMPK with decreased cAMP by activating CaMKs via increasing ROS and cytoplasmic calcium.

Microtubule stabilisers docetaxel and paclitaxel reduce spindle damage and maintain the developmental competence of in vitro-mature bovine oocytes during vitrification

This study compared the efficacy of docetaxel (DT) and paclitaxel (PT) in reducing spindle damage during vitrification and maintaining the developmental competence of in vitro-matured (IVM) bovine oocytes after vitrification and warming. Pretreatment of IVM oocytes with 0.05µM DT for 30min before vitrification resulted in significantly higher (P<0.05) rates of oocyte survival and cleavage after IVF, as well as subsequent blastocyst rates on Days 7-9 and hatching on Days 8-9, compared with oocytes pretreated with 1.0µM PT before vitrification or those vitrified without pretreatment. When nuclear status and spindle morphology of vitrified oocytes were assess after warming by immunostaining, DT pretreatment before vitrification resulted in a significantly higher (P<0.05) percentage of oocytes at the MII stage with a normal, intact spindle compared with PT pretreatment or no pretreatment, but the percentage of MII oocytes was still significantly lower (P<0.05) than in the control group. Pretreatment of IVM bovine oocytes with 0.05µM DT or 1.0µM PT for 30min before vitrification reduces spindle damage to the same extent, without side effects on fertilisation and development. Pretreatment with 0.05µM DT improved the developmental competence of vitrified-warmed oocytes to a greater degree than 1.0µM PT pretreatment.

Tauroursodeoxycholic acid (TUDCA) alleviates endoplasmic reticulum stress of nuclear donor cells under serum starvation

Serum starvation is a routine protocol for synchronizing nuclear donor cells to G0/G1 phase during somatic cell nuclear transfer (SCNT). However, abrupt serum deprivation can cause serious stress to the cells cultured in vitro, which might result in endoplasmic reticulum (ER) stress, chromosome damage, and finally reduce the success rate of SCNT. In the present study, the effects of tauroursodeoxycholic acid (TUDCA), an effective ER stress-relieving drug, on the nuclear donor cells under serum deprivation condition as well as following SCNT procedures were first assessed in the bovine. The results showed that TUDCA significantly reduced ER stress and cell apoptosis in those nuclear donor cells. Moreover, it significantly decreased the expression of Hdac1 and Dnmt1, and increased the level of H3K9 acetylation in nuclear donor cells compared with control group. SCNT reconstructed embryos cloned from TUDCA-treated donor cells showed significantly higher fusion, cleavage, blastocyst formation rate, total cell number in day 7 blastocysts, and lower apoptotic index than that from control group. In addition, the expression of Hdac1, Dnmt1 and Bax was significantly lower in blastocysts derived from TUDCA-treated donor cells than that from control group. In conclusion, TUDCA significantly reduced the ER stress of nuclear donor cells under serum starvation condition, and significantly improved the developmental competence of following SCNT reconstructed embryos when these TUDCA-treated cells were used as the nuclear donors.

Role of autophagy in modulating post-maturation aging of mouse oocytes

Mechanisms for post-maturation oocyte aging (PMOA) are not fully understood, and whether autophagy plays any role in PMOA is unknown. To explore the role of autophagy in PMOA, expression of autophagosomes and effects of the autophagy (macro-autophagy) activity on PMOA were observed in mouse oocytes. Oocyte activation rates and active caspase-3 levels increased continuously from 0 to 18 h of in vitro aging. While levels of microtubule-associated protein light chain 3 (LC3)-II increased up to 12 h and decreased thereafter, contents of p62 decreased from 0 to 12 h and then elevated to basal level by 18 h. However, the LC3-II/I ratio remained unchanged following aging in different media or for different times. During in vitro aging up to 12 h, upregulating autophagy with rapamycin or lithium chloride decreased activation susceptibility, cytoplasmic calcium, p62 contents, oxidative stress, caspase-3 activation and cytoplasmic fragmentation while increasing developmental competence, LC3-II contents, LC3-II/I ratio, mitochondrial membrane potential, spindle/chromosome integrity and normal cortical granule distribution. Downregulating autophagy with 3-methyladenine (3-MA) produced opposite effects on all these parameters except cytoplasmic fragmentation. After 12 h of aging culture, however, regulating autophagy with either rapamycin/lithium chloride or 3-MA had no impact on oocyte activation susceptibility. It is concluded that autophagy plays an important role in regulating PMOA. Thus, during the early stage of PMOA, autophagy increases as an adaptive response to prevent further apoptosis, but by the late stage of PMOA, the activation of more caspases blocks the autophagic process leading to severer apoptosis.

Expression profiles and function analysis of microRNAs in postovulatory aging mouse oocytes

In this study, microRNA (miRNA) profiles in postovulatory aging mouse oocytes were analyzed by microarray screening and RT-qPCR. Hierarchical cluster analysis on the microarray data and KEGG pathway enrichment analysis on the mRNAs targeted by differentially expressed (DE) miRNAs between two adjacent egg-ages suggest that while only a mild alteration in miRNA expression occurred from 13 to 18 h, a great change took place from 18 to 24 h post hCG injection. Theoretical exploration on functions of the predicted target genes suggest that KEGG pathways enriched by 13-18 h DE miRNAs are correlated with early events of oocyte aging while pathways most enriched by 18-24 h or 24-30 h DE miRNAs are correlated with the late symptoms of aged oocytes. Experimental verification on functions of the key proteins predicted by the KEGG analysis and injection of miR-98 mimics or inhibitors further confirmed that miRNAs played stimulatory/inhibitory roles in postovulatory oocyte aging. In conclusion, marked changes in miRNA expression are associated with significant alterations in function and morphology of postovulatory aging oocytes.

Reduction of nitric oxide level results in maturation promoting factor destabilization during spontaneous meiotic exit from diplotene arrest in rat cumulus oocytes complexes cultured in vitro

Nitric oxides (NO) act as one of the major signal molecules and modulate various cell functions including oocyte meiosis in mammals. The present study was designed to investigate the mechanism of NO action during spontaneous meiotic exit from diplotene arrest (EDA) in rat cumulus oocytes complexes (COCs) cultured in vitro. Diplotene-arrested COCs collected from ovary of immature female rats after 20 IU pregnant mare's serum gonadotropins (PMSG) for 48 h were exposed to various concentrations of NO donor, S-nitroso-N-acetyl penicillamine (SNAP) and inducible nitric oxide synthase (iNOS) inhibitor, aminoguanidine (AG) for 3 h in vitro and downstream factors were analyzed. Our results suggest that SNAP inhibited, while AG induced EDA in a concentration-dependent manner. The iNOS-mediated total NO, cyclic nucleotides and cell division cycle 25B (Cdc25B) levels were reduced significantly. The decreased Cdc25B was associated with the increased Thr14/Tyr15 phosphorylated cyclin-dependent kinase 1 (Cdk1) level and decreased Thr161 phosphorylated Cdk1 as well as cyclin B1 levels leading to maturation promoting factor (MPF) destabilization. The destabilized MPF finally induced spontaneous EDA. Taken together, these results suggest that reduction of iNOS-mediated NO level destabilizes MPF during spontaneous EDA in rat COCs cultured in vitro.

Maturation promoting factor destabilization mediates human chorionic gonadotropin induced meiotic resumption in rat oocytes

Human chorionic gonadotropin (hCG) mimics the action of luteinizing hormone (LH) and triggers meiotic maturation and ovulation in mammals. The mechanism by which hCG triggers meiotic resumption in mammalian oocytes remains poorly understood. We aimed to find out the impact of hCG surge on morphological changes, adenosine 3',5'-cyclic monophosphate (cAMP), guanosine 3',5'-cyclic monophosphate (cGMP), cell division cycle 25B (Cdc25B), Wee1, early mitotic inhibitor 2 (Emi2), anaphase-promoting complex/cyclosome (APC/C), meiotic arrest deficient protein 2 (MAD2), phosphorylation status of cyclin-dependent kinase 1 (Cdk1), its activity and cyclin B1 expression levels during meiotic resumption from diplotene as well as metaphase-II (M-II) arrest in cumulus oocyte complexes (COCs). Our data suggest that hCG surge increased cyclic nucleotides level in encircling granulosa cells but decreased their level in oocyte. The reduced intraoocyte cyclic nucleotides level is associated with the decrease of Cdc25B, Thr161 phosphorylated Cdk1 and Emi2 expression levels. On the other hand, hCG surge increased Wee1, Thr14/Tyr15 phosphorylated Cdk1, APC/C as well as MAD2 expression levels. The elevated APC/C activity reduced cyclin B1 level. The changes in phosphorylation status of Cdk1 and reduced cyclin B1 level might have resulted in maturation promoting factor (MPF) destabilization. The destabilized MPF finally triggered resumption of meiosis from diplotene as well as M-II arrest in rat oocytes.

The role of L-type calcium channels in mouse oocyte maturation, activation and early embryonic development

Calcium ion fluctuation is closely related to the transformation of cell cycle. However, little is known about the function of L-type calcium channel in mammalian oocyte and embryo development. We thus studied the roles of L-type calcium channel in mouse oocyte meiotic maturation, parthenogenetic activation and early embryonic development. We used the antagonist Amlodipine to block L-type calcium channel. Oocytes or zygotes were cultured to different time points with 0 μM, 10 μM, 30 μM and 50 μM Amlodipine. Then we checked the rate of first polar body extrusion, spindle formation, asymmetric division parthenogenetic activation and early embryo cleavage. The results showed that Amlodipine treatment did not affect germinal vesicle breakdown, but caused disruption of cytoskeleton organization, symmetric division, formation of mature oocytes with a large polar body, or reduced the first polar body extrusion, depending on its concentrations. Amlodipine treatment also resulted in decreased parthenogenetic activation and arrested early embryonic development. Overall, these data suggest that proper function of L-type calcium channel is critical for oocyte maturation, activation, and early embryonic development.

Inhibition of calcineurin by FK506 stimulates germinal vesicle breakdown of mouse oocytes in hypoxanthine-supplemented medium

Calcineurin (CN) is a serine/threonine phosphatase which plays important roles in meiosis maturation in invertebrate oocytes; however, the role of CN in mouse oocytes is relatively unexplored. In this study, we examined the expression, localization and functional roles of CN in mouse oocytes and granulosa cells. The RT-PCR results showed that the β isoform of calcineurin A subunit (Cn A) expressed significantly higher than α and γ isoforms, and the expression of Cn Aβ mRNA obviously decreased in oocytes in which germinal vesicle breakdown (GVBD) occurred, while only B1 of calcineurin B subunit (Cn B) was detected in oocytes and stably expressed during oocytes maturation. The following fluorescence experiment showed that Cn A was mainly located in the nucleus of germinal vesicle (GV) stage oocytes and gruanlosa cells, and subsequently dispersed into the entire cytoplasm after GVBD. The decline of Cn A in oocytes suggested that it may play an important role in GVBD. To further clarify the role of calcineurin during meiotic maturation, FK506 (a calcineurin inhibitor) was used in the culture medium contained hypoxanthine (HX) which could keep mouse oocytes staying at GV stage. As expected, FK506 could induce a significant elevation of GVBD rate and increase the MPF level of denuded oocytes (DOs). Furthermore, FK506 could also play an induction role of GVBD of oocytes in COCs and follicles, and the process could be counteracted by MAPK kinase inhibitor (U0126). Above all, the results implied that calcineurin might play a crucial role in development of mouse oocytes and MPF and MAPK pathways are involved in this process.

Oxidative Stress Induces Mouse Follicular Granulosa Cells Apoptosis via JNK/FoxO1 Pathway

The c-Jun N-terminal protein kinase (JNK) plays an important role in the regulation of cell apoptosis. Forkhead box O (FoxO) transcription factors are involved in diverse biological processes, including cellular metabolism, cell apoptosis, and cell cycle. However, the JNK/FoxO1 pathway involved in the process of apoptosis induced by oxidative stress remains to be elucidated. Here, we demonstrated that the JNK activity significantly increased in response to oxidative stress in mouse follicular granulosa cells (MGCs). SP600125, a selective JNK inhibitor, attenuated the oxidative stress-induced MGCs apoptosis. Oxidative stress enhanced the FoxO1 nuclear translocation by activating the JNK activity. Moreover, JNK mediated the dissociation of FoxO1 from 14-3-3 proteins in MGCs after the treatment with H₂O₂. Finally, oxidative stress up-regulated the expression of FoxO1 via JNK mediation of FoxO1 self-regulation in MGCs. Taken together, our findings suggest that JNK/FoxO1 is involved in the regulation of oxidative stress-induced cell apoptosis in MGCs.

Increased Telomerase Reverse Transcriptase Expression Associates with Spontaneous Exit from M-II Arrest in Rat Eggs

In mammals, postovulatory egg aging deteriorates egg quality possibly by mediating spontaneous exit from metaphase-II (M-II) arrest and/or inducing apoptosis. To test this possibility, present study was designed to investigate telomerase reverse transcriptase (TERT) expression, Bcl2 expression, and DNA fragmentation during postovulatory egg aging in vivo, as well as in vitro. Results suggest that postovulatory egg aging induced a time-dependent increase in the number of eggs undergoing spontaneous exit from M-II arrest in vivo, as well as in vitro. However, rate of spontaneous exit from M-II arrest was high in eggs cultured in vitro compared to in vivo aging. A time-dependent increase of TERT expression was associated with postovulatory aging-mediated spontaneous exit from M-II arrest in vivo, as well as in vitro. The Bcl2 level did not reduce and DNA fragmentation was not detected until 7 hours of in vivo, as well as in vitro, postovulatory egg aging. Taken together these data suggest that the eggs undergo postovulatory aging as evidenced by increased TERT expression without having any decrease of Bcl2 level or increase of DNA fragmentation until 7 hours of in vivo, as well as in vitro egg aging.

Effect of Long-Term Exposure of Donor Nuclei to the Oocyte Cytoplasm on Production of Cloned Mice Using Serial Nuclear Transfer

Although animal cloning is becoming increasingly practicable, cloned embryos possess many abnormalities and so there has been a low success rate for producing live animals. This is most likely due to incomplete reprogramming of somatic cell nuclei before they start to develop as the donor nuclei are usually only exposed to the oocyte cytoplasm for 1-2 hours before reconstructed oocytes are activated to avoid oocyte aging. Therefore, in this study, we attempted to extend the exposure period of somatic cell nuclei to the oocyte cytoplasm to determine whether this could enhance reprogramming of donor nuclei. Donor nuclei were transferred into oocytes, following which pseudo-MII spindles (pMIIs) derived from these were extracted and injected into newly collected enucleated oocytes 24 hours after the first nuclear transfer (NT). These serial NT oocytes were then activated and their developmental potential was examined to full term. There was no obvious difference in the pMIIs of reconstructed oocytes at 6 and 24 hours after donor nucleus injection; however, in both of these, the chromosomes were more widely spread inside the spindle than in fresh intact oocytes. Furthermore, a few chromosomes remained in 25% and 47% of these enucleated oocytes at 6 and 24 hours after donor nucleus injection, respectively. When these pMIIs were injected into fresh enucleated oocytes, the developmental rate to blastocyst was significantly lower, but we could still obtain several healthy cloned offspring. Thus, serial NT at intervals of 24 hours using fresh oocytes is possible, but the success rate could not be improved due to loss of chromosomes during the second NT.

Characterization of macrozoospermia-associated AURKC mutations in a mammalian meiotic system

Aneuploidy is the leading genetic abnormality that leads to miscarriage, and it is caused by a failure of accurate chromosome segregation during gametogenesis or early embryonic divisions. Aurora kinase C (AURKC) is essential for formation of euploid sperm in humans because mutations in AURKC are correlated with macrozoospermia and these sperm are tetraploid. These mutations are currently the most frequent mutations that cause macrozoospermia and result from an inability to complete meiosis I (MI). Three of these mutations AURKC c.144delC (AURKC p.L49Wfs22), AURKC c.686G > A (AURKC p.C229Y) and AURKC c.744C > G (AURKC p.Y248*) occur in the coding region of the gene and are the focus of this study. By expressing these alleles in oocytes isolated from Aurkc^-/- mice, we show that the mutations have different effects on AURKC function during MI. AURKC p.L49Wfs22 is a loss-of-function mutant that perturbs localization of the chromosomal passenger complex (CPC), AURKC p.C229Y is a hypomorph that cannot fully support cell-cycle progression, and AURKC p.Y248* fails to localize and function with the CPC to support chromosome segregation yet retains catalytic activity in the cytoplasm. Finally, we show that these variants of AURKC cause meiotic failure and polyploidy due to a failure in AURKC-CPC function that results in metaphase chromosome misalignment. This study is the first to assess the function of mutant alleles of AURKC that affect human fertility in a mammalian meiotic system.

The signaling pathways by which the Fas/FasL system accelerates oocyte aging

In spite of great efforts, the mechanisms for postovulatory oocyte aging are not fully understood. Although our previous work showed that the FasL/Fas signaling facilitated oocyte aging, the intra-oocyte signaling pathways are unknown. Furthermore, the mechanisms by which oxidative stress facilitates oocyte aging and the causal relationship between Ca2+ rises and caspase-3 activation and between the cell cycle and apoptosis during oocyte aging need detailed investigations. Our aim was to address these issues by studying the intra-oocyte signaling pathways for Fas/FasL to accelerate oocyte aging. The results indicated that sFasL released by cumulus cells activated Fas on the oocyte by increasing reactive oxygen species via activating NADPH oxidase. The activated Fas triggered Ca2+ release from the endoplasmic reticulum by activating phospholipase C-γ pathway and cytochrome c pathway. The cytoplasmic Ca2+ rises activated calcium/calmodulin-dependent protein kinase II (CaMKII) and caspase-3. While activated CaMKII increased oocyte susceptibility to activation by inactivating maturation-promoting factor (MPF) through cyclin B degradation, the activated caspase-3 facilitated further Ca2+releasing that activates more caspase-3 leading to oocyte fragmentation. Furthermore, caspase-3 activation and fragmentation were prevented in oocytes with a high MPF activity, suggesting that an oocyte must be in interphase to undergo apoptosis.

Optimized Protocols for In Vitro Maturation of Rat Oocytes Dramatically Improve Their Developmental Competence to a Level Similar to That of Ovulated Oocytes

The developmental capacity of in vitro-matured (IVM) oocytes is markedly lower than that of their in vivo-matured (IVO) counterparts, suggesting the need for optimization of IVM protocols in different species. There are few studies on IVM of rat oocytes, and there are even fewer attempts to improve ooplasmic maturation compared to those reported in other species. Furthermore, rat oocytes are well known to undergo spontaneous activation (SA) after leaving the oviduct; however, whether IVM rat oocytes have lower SA rates than IVO oocytes and can potentially be used for nuclear transfer is unknown. In this study, we investigated the effects of maturation protocols on cytoplasmic maturation of IVM rat oocytes and observed the possibility to reduce SA by using IVM rat oocytes. Ooplasmic maturation was assessed using multiple markers, including pre- and postimplantation development, meiotic progression, CG redistribution, redox state, and the expression of developmental potential- and apoptosis-related genes. The results showed that the best protocol consisting of modified Tissue Culture Medium-199 (TCM-199) supplemented with cysteamine/cystine and the cumulus cell monolayer dramatically improved the developmental competence of rat oocytes and supported both pre- and postimplantation development and other ooplasmic maturation makers to levels similar to that observed in ovulated oocytes. Rates of SA were significantly lower in IVM oocytes than in IVO oocytes when observed at the same intervals after nuclear maturation. In conclusion, we have optimized protocols for IVM of rat oocytes that sustain ooplasmic maturation to a level similar to ovulated oocytes. The results suggest that IVM rat oocytes might be used to reduce SA for rat cloning.

Role of Na+/Ca2+ exchanger (NCX) in modulating postovulatory aging of mouse and rat oocytes

We studied the role of the Na+/Ca2+ exchanger (NCX) in modulating oocyte postovulatory aging by observing changes in NCX contents and activities in aging mouse and rat oocytes. Whereas the NCX activity was measured by observing oocyte activation following culture with NCX inhibitor or activator, the NCX contents were determined by immunohistochemical quantification. Although NCX was active in freshly-ovulated rat oocytes recovered 13 h post hCG injection and in aged oocytes recovered 19 h post hCG in both species, it was not active in freshly-ovulated mouse oocytes. However, NCX became active when the freshly-ovulated mouse oocytes were activated with ethanol before culture. Measurement of cytoplasmic Ca2+ revealed Ca2+ increases always before NCX activation. Whereas levels of the reactive oxygen species (ROS) and the activation susceptibility increased, the density of NCX member 1 (NCX1) decreased significantly with oocyte aging in both species. While culture with H2O2 decreased the density of NCX1 significantly, culture with NaCl supplementation sustained the NCX1 density in mouse oocytes. It was concluded that (a) the NCX activity was involved in the modulation of oocyte aging and spontaneous activation; (b) ROS and Na+ regulated the NCX activity in aging oocytes by altering its density as well as functioning; and (c) cytoplasmic Ca2+ elevation was essential for NCX activation in the oocyte.

An insufficient increase of cytosolic free calcium level results postovulatory aging-induced abortive spontaneous egg activation in rat

PURPOSE

The present study was aimed to find out whether postovulatory aging-induced abortive spontaneous egg activation (SEA) is due to insufficient increase of cytosolic free Ca(2+) level.

METHODS

Immature female rats (22-24 days old) were subjected to superovulation induction protocol. Eggs were collected 14, 17 and 19 h post-hCG surge to induce in vivo egg aging. The eggs were collected 14 h post-hCG surge and cultured in vitro for 3, 5 and 7 h to induce in vitro egg aging. The morphological changes, rate of abortive SEA, chromosomal status and cytosolic free Ca(2+) levels were analyzed.

RESULTS

Postovulatory aging induced morphological features characteristics of abortive SEA in a time-dependent manner in vivo as well as in vitro. The extracellular Ca(2+) increased rate of abortive SEA during initial period of culture, while co-addition of a nifedipine (L-type Ca(2+) channel blocker) protected against postovulatory aging-induced abortive SEA. However, CI induced morphological features characteristics of egg activation (EA) in a dose-dependent manner. As compare to control, an increase of cytosolic free Ca(2+) level (1.42 times) induced abortive SEA, while further increase of cytosolic free Ca(2+) level (2.55 times) induced EA.

CONCLUSION

Our results show that an insufficient cytosolic free Ca(2+) level is associated with postovulatory aging -induced abortive SEA, while furthermore increase is required to induce EA in rat.