Aging-related changes in calcium oscillations in fertilized mouse oocytes

Diminished NAD+ levels and activation of retrotransposons promote postovulatory aged oocyte (POAO) death

Death is the fate of postovulatory aged or unfertilized oocytes (POAO) in many animals. However, precise molecular mechanisms are yet to be discovered. Here, we demonstrate that increased amounts of reactive oxygen species (ROS), calcium ion (Ca+2) channels, and retrotransposon activity induce apoptosis, which in turn causes POAO death. Notably, suppression of ROS, Ca+2 channels, and retrotransposons delayed POAO death. Further, we found that the histone H4K12 and K16 acetylation increased via downregulation of NAD+ and NAD+ -dependent histone deacetylase SIRT3. Furthermore, adding NMN, sodium pyruvate, or CD38 inhibition delayed the death of postovulatory aged oocytes. Finally, we demonstrate the conservation of retrotransposon-induced DNA damage-dependent POAO death in higher-order vertebrates. Our findings suggest that POAO mortality is caused by cyclic cascade metabolic interactions in which low NAD+ levels increase histone acetylation by inhibiting histone deacetylases, resulting in an increase in retrotransposons, ROS, and Ca+2 channel activity and thus contributing to DNA damage-induced apoptosis.

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.

Phosphoglycerate mutase family member 5 maintains oocyte quality via mitochondrial dynamic rearrangement during aging

Decline in ovarian reserve with aging is associated with reduced fertility and the development of metabolic abnormalities. Once mitochondrial homeostasis is imbalanced, it may lead to poor reproductive cell quality and aging. However, Phosphoglycerate translocase 5 (PGAM5), located in the mitochondrial membrane, is associated with necroptosis, apoptosis, and mitophagy, although the underlying mechanisms associated with ovarian aging remain unknown. Therefore, we attempted to uncover whether the high phosphoglycerate mutant enzyme family member 5 (PGAM5) expression is associated with female infertility in cumulus cells, and aims to find out the underlying mechanism of action of PGAM5. We found that PGAM5 is highly expressed and positively associated with aging, and has the potential to help maintain and regulate mitochondrial dynamics and metabolic reprogramming in aging granulosa cells, ovaries of aged female mice, and elderly patients. PGAM5 undergoes activation in the aging group and translocated to the outer membrane of mitochondria, co‐regulating DRP1; thereby increasing mitochondrial fission. A significant reduction in the quality of mitochondria in the aging group, a serious imbalance, and a significant reduction in energy, causing metabolism shift toward glycolysis, were also reported. Since PGAM5 is eliminated, the mitochondrial function and metabolism of aging cells are partially reversed. A total of 70 patients undergoing in vitro fertilization (IVF) treatment were recruited in this clinical study. The high expression of PGAM5 in the cumulus cells is negatively correlated with the pregnancy rate of infertile patients. Hence, PGAM5 has immense potential to be used as a diagnostic marker.

Increased Systemic Antioxidant Power Ameliorates the Aging-Related Reduction in Oocyte Competence in Mice

Ovarian aging is associated with elevated oxidative stress and diminished oocyte developmental competence. We aimed to determine the impact of systemic antioxidant treatment in aged mice. Female outbred CF-1 mice were aged for 9 months prior to an 8-week 45 mg Euterpe oleracea (açaí) daily supplement. The açaí treatment induced a threefold increase in serum antioxidant power (FRAP) compared to both young and aged mice (p < 0.0001). Compared to young mice, aged mice had fewer oocytes and reduced blastocyst development (p < 0.0001); açaí did not affect the oocyte numbers, but improved blastocyst formation (p < 0.05). Additionally, açaí alleviated the aging-related decrease in implantation potential (p < 0.01). The aged mice showed evidence of elevated ovarian ER stress (increased whole-ovary PDIA4 expression, granulosa cell and oocyte GRP78 expression, and oocyte PDIA4 protein), reduced oocyte mitochondrial quality (higher PRKN activation and mitochondrial DNA oxidative damage), and dysregulated uterine glandular epithelium. Antioxidant intervention was sufficient to lessen these effects of ovarian aging, likely in part by the upregulation of NRF2. We conclude that açaí treatment is a promising strategy to improve ER and mitochondrial function in the ovaries, thereby ameliorating the decreased oocyte competence that occurs with ovarian aging.

Germline nuclear transfer in mice may rescue poor embryo development associated with advanced maternal age and early embryo arrest

STUDY QUESTION

Can pronuclear transfer (PNT) or maternal spindle transfer (ST) be applied to overcome poor embryo development associated with advanced maternal age or early embryo arrest in a mouse model?

SUMMARY ANSWER

Both PNT and ST may have the potential to restore embryonic developmental potential in a mouse model of reproductive ageing and embryonic developmental arrest.

WHAT IS KNOWN ALREADY

Germline nuclear transfer (NT) techniques, such as PNT and ST, are currently being applied in humans to prevent the transmission of mitochondrial diseases. Yet, there is also growing interest in the translational use of NT for treating infertility and improving IVF outcomes. Nevertheless, direct scientific evidence to support such applications is currently lacking. Moreover, it remains unclear which infertility indications may benefit from these novel assisted reproductive technologies.

STUDY DESIGN, SIZE, DURATION

We applied two mouse models to investigate the potential of germline NT for overcoming infertility. Firstly, we used a model of female reproductive ageing (B6D2F1 mice, n = 155), with ages ranging from 6 to 8 weeks (young), 56 (aged) to 70 weeks (very-aged), corresponding to a maternal age of <30, ∼36 and ∼45 years in humans, respectively. Secondly, we used NZB/OlaHsd female mice (7-14 weeks, n = 107), as a model of early embryo arrest. This mouse strain exhibits a high degree of two-cell block. Metaphase II (MII) oocytes and zygotes were retrieved following superovulation.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Ovarian reserve was assessed by histological analysis in the reproductive-aged mice. Mitochondrial membrane potential (△Ψm) was measured by JC-1 staining in MII oocytes, while spindle-chromosomal morphology was examined by confocal microscopy. Reciprocal ST and PNT were performed by transferring the meiotic spindle or pronuclei (PN) from unfertilised or fertilised oocytes (after ICSI) to enucleated oocytes or zygotes between aged or very-aged and young mice. Similarly, NT was also conducted between NZB/OlaHsd (embryo arrest) and B6D2F1 (non-arrest control) mice. Finally, the effect of cytoplasmic transfer (CT) was examined by injecting a small volume (∼5%) of cytoplasm from the oocytes/zygotes of young (B6D2F1) mice to the oocytes/zygotes of aged or very-aged mice or embryo-arrest mice. Overall, embryonic developmental rates of the reconstituted PNT (n = 572), ST (n = 633) and CT (n = 336) embryos were assessed to evaluate the efficiency of these techniques. Finally, chromosomal profiles of individual NT-generated blastocysts were evaluated using next generation sequencing.

MAIN RESULTS AND THE ROLE OF CHANCE

Compared to young mice, the ovarian reserve in aged and very-aged mice was severely diminished, reflected by a lower number of ovarian follicles and a reduced number of ovulated oocytes (P < 0.001). Furthermore, we reveal that the average △Ψm in both aged and very-aged mouse oocytes was significantly reduced compared to young mouse oocytes (P < 0.001). In contrast, the average △Ψm in ST-reconstructed oocytes (very-aged spindle and young cytoplast) was improved in comparison to very-aged mouse oocytes (P < 0.001). In addition, MII oocytes from aged and very-aged mice exhibited a higher rate of abnormalities in spindle assembly (P < 0.05), and significantly lower fertilisation (60.7% and 45.3%) and blastocyst formation rates (51.4% and 38.5%) following ICSI compared to young mouse oocytes (89.7% and 87.3%) (P < 0.001). Remarkably, PNT from zygotes obtained from aged or very-aged mice to young counterparts significantly improved blastocyst formation rates (74.6% and 69.2%, respectively) (P < 0.05). Similarly, both fertilisation and blastocyst rates were significantly increased after ST between aged and young mice followed by ICSI (P < 0.05). However, we observed no improvement in embryo development rates when performing ST from very-aged to young mouse oocytes following ICSI (P > 0.05). In the second series of experiments, we primarily confirmed that the majority (61.8%) of in vivo zygotes obtained from NZB/OlaHsd mice displayed two-cell block during in vitro culture, coinciding with a significantly reduced blastocyst formation rate compared to the B6D2F1 mice (13.5% vs. 90.7%; P < 0.001). Notably, following the transfer of PN from the embryo-arrest (NZB/OlaHsd) zygotes to enucleated non-arrest (B6D2F1) counterparts, most reconstructed zygotes developed beyond the two-cell stage, leading to a significantly increased blastocyst formation rate (89.7%) (P < 0.001). Similar findings were obtained after implementing ST between NZB/OlaHsd and B6D2F1 mice, followed by ICSI. Conversely, the use of CT did not improve embryo development in reproductive-age mice nor in the embryo-arrest mouse model (P > 0.05). Surprisingly, chromosomal analysis revealed that euploidy rates in PNT and ST blastocysts generated following the transfer of very-aged PN to young cytoplasts and very-aged spindles to young cytoplasts were comparable to ICSI controls (with young mouse oocytes). A high euploidy rate was also observed in the blastocysts obtained from either PNT or ST between young mice. Conversely, the transfer of young PN and young spindles into very-aged cytoplasts led to a higher rate of chromosomal abnormalities in both PNT and ST blastocysts.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

The limited number of blastocysts analysed warrants careful interpretation. Furthermore, our observations should be cautiously extrapolated to humans given the inherent differences between mice and women in regards to various biological processes, including centrosome inheritance. The findings suggest that ST or PNT procedures may be able to avoid aneuploidies generated during embryo development, but they are not likely to correct aneuploidies already present in some aged MII oocytes.

WIDER IMPLICATIONS OF THE FINDINGS

To our knowledge, this is the first study to evaluate the potential of PNT and ST in the context of advanced maternal age and embryonic developmental arrest in a mouse model. Our data suggest that PNT, and to a lesser extent ST, may represent a novel reproductive strategy to restore embryo development for these indications.

STUDY FUNDING/COMPETING INTEREST(S)

M.T. is supported by grants from the China Scholarship Council (CSC) (Grant no. 201506160059) and the Special Research Fund from Ghent University (Bijzonder Onderzoeksfonds, BOF) (Grant no. 01SC2916 and no. 01SC9518). This research is also supported by the FWO-Vlaanderen (Flemish fund for scientific research, Grant no. G051017N, G051516N and G1507816N). The authors declare no competing interests.

TRIAL REGISTRATION NUMBER

N/A.

Oocyte Ageing in Zebrafish Danio rerio (Hamilton, 1822) and Its Consequence on the Viability and Ploidy Anomalies in the Progeny

Simple Summary

The maintenance and manipulation of AB strain zebrafish oocytes at 26 °C was found to be possible for 2 h without incurring a marked reduction in fertilization potential. However, the post-ovulatory ageing of oocytes for 6 h resulted in an almost complete loss of egg viability. All larvae derived from the 4- and 6-h aged oocytes were characterized by physical abnormalities. Ageing oocytes for 4 h resulted in the incidence of ploidy anomalies having a four-fold increase. These results make a valuable contribution with respect to the control of experimental reproduction in zebrafish, which is currently accepted as an excellent model animal.

Abstract

Fish egg quality can be markedly influenced by the oocyte age after ovulation. In this study, we examined the duration of oocyte ageing in the zebrafish (Danio rerio) and whether prolonged ageing is associated with the incidence of ploidy anomalies in the resulting embryos. Oocytes were incubated in vitro for 6 h post-stripping (HPS) at 26 °C and fertilized at 2-h intervals. Meanwhile, for eggs fertilized immediately after stripping, the fertilization, embryo survival, and hatching rates started at ~80%; these rates decreased to 39%, 24%, and 16%, respectively, for oocytes that had been stored for 4 h (p ˂ 0.05), and there was an almost complete loss of egg viability at 6 HPS. Furthermore, almost 90% of the embryos derived from 6-h aged oocytes died prior to hatching, and all larvae originating from 4- and 6-h aged oocytes showed malformations. The proportion of ploidy abnormal embryos was significantly greater at 4 HPS (18.5%) than at either 0 or 2 HPS (4.7% and 8.8%, respectively). The results revealed that zebrafish oocytes retained their fertilization potential for up to 2 h after stripping at 26 °C and indicated the contribution of post-ovulatory oocyte ageing in the occurrence of ploidy anomalies in the resulting embryos.

Mitochondrial transfer from induced pluripotent stem cells rescues developmental potential of in vitro fertilized embryos from aging females†

Conventional heterologous mitochondrial replacement therapy is clinically complicated by "tri-parental" ethical concerns and limited source of healthy donor oocytes or zygotes. Autologous mitochondrial transfer is a promising alternative in rescuing poor oocyte quality and impaired embryo developmental potential associated with mitochondrial disorders, including aging. However, the efficacy and safety of mitochondrial transfer from somatic cells remains largely controversial, and unsatisfying outcomes may be due to distinct mitochondrial state in somatic cells from that in oocytes. Here, we propose a potential strategy for improving in vitro fertilization (IVF) outcomes of aging female patients via mitochondrial transfer from induced pluripotent stem (iPS) cells. Using naturally aging mice and well-established cell lines as models, we found iPS cells and oocytes share similar mitochondrial morphology and functions, whereas the mitochondrial state in differentiated somatic cells is substantially different. By microinjection of isolated mitochondria into fertilized oocytes following IVF, our results indicate that mitochondrial transfer from iPS, but not MEF cells, can rescue the impaired developmental potential of embryos from aging female mice and obtain an enhanced implantation rate following embryo transfer. The beneficial effect may be explained by the fact that mitochondrial transfer from iPS cells not only compensates for aging-associated loss of mtDNA, but also rescues mitochondrial metabolism of subsequent preimplantation embryos. Using mitochondria from iPS cells as the donor, our study not only proposes a promising strategy for improving IVF outcomes of aging females, but also highlights the importance of synchronous mitochondrial state in supporting embryo developmental potential.

Modulators of calcium signalling at fertilization

Calcium (Ca2+) signals initiate egg activation across the animal kingdom and in at least some plants. These signals are crucial for the success of development and, in the case of mammals, health of the offspring. The mechanisms associated with fertilization that trigger these signals and the molecules that regulate their characteristic patterns vary widely. With few exceptions, a major contributor to fertilization-induced elevation in cytoplasmic Ca2+ is release from endoplasmic reticulum stores through the IP3 receptor. In some cases, Ca2+ influx from the extracellular space and/or release from alternative intracellular stores contribute to the rise in cytoplasmic Ca2+. Following the Ca2+ rise, the reuptake of Ca2+ into intracellular stores or efflux of Ca2+ out of the egg drive the return of cytoplasmic Ca2+ back to baseline levels. The molecular mediators of these Ca2+ fluxes in different organisms include Ca2+ release channels, uptake channels, exchangers and pumps. The functions of these mediators are regulated by their particular activating mechanisms but also by alterations in their expression and spatial organization. We discuss here the molecular basis for modulation of Ca2+ signalling at fertilization, highlighting differences across several animal phyla, and we mention key areas where questions remain.

Impact of endoplasmic reticulum stress on oocyte aging mechanisms

Endoplasmic reticulum (ER) stress is associated with several aging-related diseases; however, the mechanism underlying age-related deterioration of oocyte quality is unclear. Here, we used post-ovulatory, in vivo aged mouse oocytes as a model. Super-ovulated oocytes harvested from the oviduct at 14 h and 20 h post-hCG injection were designated as ‘fresh’ and ‘aged’, respectively. Embryo development following IVF was compared between fresh, aged and ER stress-induced oocytes. Expression of the ER stress marker GRP78 was examined at each stage. To evaluate the effect of salubrinal, an ER stress suppressor, on embryo development following IVF, expression levels of GRP78 and phospho-eukaryotic initiation factor 2 alpha were compared between aged and salubrinal-treated aged oocytes. Embryo transfer of salubrinal-treated aged oocytes was performed to examine the safety of salubrinal. Similar to aged oocytes, ER stress-induced oocytes showed lower fertilization rates and poor embryo development. Following IVF, expression of GRP78 decreased with embryo development. GRP78 expression was significantly higher in aged oocytes than in fresh oocytes. Salubrinal lowered GRP78 levels and improved embryo development. No adverse effect of salubrinal treatment was found on the birth weight of pups or on organogenesis in mice. The limitation of this study was that protein kinase-like ER kinase was the only ER stress pathway examined; the role of IRE1 and ATF6 pathways was not considered. Nevertheless, salubrinal can significantly improve embryo development in in vivo aged oocytes undergoing ER stress. Hence, regulation of ER stress might represent a promising therapeutic strategy to overcome poor oocyte quality.

Evidence that growth hormone can improve mitochondrial function in oocytes from aged mice

Decline in successful conception decreases more rapidly after 38 years of age owing to follicular depletion and decreased oocyte quality. However, limited information is available regarding the underlying mechanism and the useful treatment. This study aimed to evaluate the effects of growth hormone supplementation on oocyte maturation in vivo in aged and young mice and to determine its effect on mitochondrial function. The influence of three different doses of recombinant human growth hormone (rhGH) (0.4, 0.8 and 1.6 mg/kg/day) for 8 weeks before ovarian stimulation was analyzed. Superovulated oocytes were released from the oviduct of 12-week-old and 40-week-old female C57BL/6J mice 14-16 h after administration of human chorionic gonadotropin. Ovarian follicle and morphological analysis and oocyte maturation parameters were then evaluated. This study is the first, to our knowledge, to report that medium- and high-dose rhGH significantly increases antral follicles in aged mice but anti-Müllerian hormone (AMH) levels. Furthermore, derived oocytes, MII-stage oocyte rate, ATP levels, mitochondrial membrane potential and frequencies of homogeneous mitochondrial distribution increased. In contrast, in both aged and young mice, the mtDNA copy numbers per oocyte were similar before rhGH administration, and upon saline administration, they did not differ significantly. We conclude that medium-dose rhGH supplementation before standard ovarian stimulation regimens improves oocyte quality in aged mice, probably by enhancing mitochondrial functionality.

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.

Altered actin cytoskeleton in ageing eggs of starfish affects fertilization process

Integrity of oocytes is of pivotal interest in the medical and zootechnical practice of in vitro fertilization. With time, oocytes undergo deterioration in quality, and ageing oocytes often exhibit compromised competence in fertilization and the subsequent embryonic development. With ageing oocytes and eggs of starfish (Astropecten aranciacus), we addressed the issue by examining changes of the subcellular structure and their performance at fertilization. Ageing eggs were simulated in two different experimental paradigms: i) oocytes were overmatured by 6 hours stimulation with 1-methyladenine (1-MA); ii) oocytes were removed from the gonad and maintained in seawater for 24 or 48 h before applying the hormonal stimulation (1-MA, 70 min). These eggs were compared with normally matured eggs (stimulated after isolation from the gonad with 1-MA for 70 min) with respect to the sperm-induced intracellular Ca²⁺ signaling and the structural changes of the egg surface. The cytoskeletal and ultrastructural differences in these eggs were assessed by confocal and transmission electron microscopy, respectively. In the two categories of ageing eggs, we have found remarkable structural modifications of the actin cytoskeleton and the cortical vesicles beneath the plasma membrane. At fertilization, these ageing eggs manifested an altered pattern of intracellular Ca²⁺ release, aberrant actin dynamics, and increased rate of polyspermy often despite full elevation of the fertilization envelope. Taken together, our results highlight the importance of spatio-temporal regulation of the actin cytoskeleton in the cortex of the eggs, and we postulate that the status of the actin cytoskeleton is one of the major determinants of the oocyte quality that ensures successful monospermic fertilization.

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

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

Mitochondrial transfer from aged adipose-derived stem cells does not improve the quality of aged oocytes in C57BL/6 mice

Female fertility declines dramatically over the age of 35 due to age-related decreases in oocyte quality and quantity. Although mitochondrial transfer promises to be a technology that can improve the quality of such age-impaired oocytes, the ideal mitochondrial donor remains elusive. In the present study, we aimed to identify whether aged adipose-derived stem cells constitute an excellent mitochondrial donor that would improve the quality of aged mouse oocytes. We showed that aging significantly impaired the mitochondrial function in mouse oocytes, but did not significantly affect the mitochondrial function of adipose-derived stem cells. However, the mitochondrial transfer from aged adipose-derived stem cells did not mitigate the poor fertilization and embryonic development rates of aged oocytes.

Impact of exercise on oocyte quality in the POLG mitochondrial DNA mutator mouse

The mtDNA 'mutator' mouse, also called the 'POLG' mouse, is a well-characterized model frequently used for studies of progeroid aging. Harboring a mutation in the proofreading domain of the mitochondrial polymerase, polymerase-γ (Polg), POLG mice acquire mtDNA mutations at an accelerated rate. This results in premature mitochondrial dysfunction and a systemic aging phenotype. Previous work has demonstrated that the progeroid phenotype in POLG is attenuated following endurance exercise, the only reported intervention to extend health span and lifespan of these mice. Herein, oocyte quality was evaluated in sedentary and exercised POLG mice. In mice homozygous for the Polg mutation, litter size is dramatically reduced as compared to heterozygous Polg mice. Following ovarian hyper-stimulation, oocytes were retrieved until 9 months of age in exercised and sedentary groups, with no oocytes ovulated thereafter. Although ovulated oocyte numbers were not impacted by exercise, we did find a modest improvement in both the ovarian follicle reserve and in oocyte quality based on meiotic spindle assembly, chromosomal segregation and mitochondrial distribution at 7 months of age in exercised POLG mice as compared to sedentary counterparts. Of note, analysis of mtDNA mutational load revealed no differences between exercised and sedentary groups. Collectively, these data indicate that exercise differentially influences somatic tissues of the POLG mouse as compared to oocytes, highlighting important mechanistic differences between mitochondrial regulatory mechanisms in the soma and the germline.

Quality of common marmoset (Callithrix jacchus) oocytes collected after ovarian stimulation

The common marmoset (Callithrix jacchus) is an experimental animal that is considered suitable for the creation of next-generation human disease models. It has recently been used in the reproductive technology field. Oocytes can be effectively collected from female marmosets via ovarian stimulation with injections of follicle-stimulating hormone (FSH) and human chorionic gonadotropin (hCG). The oocytes, collected about 28 h after the hCG injection, include both premature oocytes and postmature (in vivo matured; IVO) oocytes, and the premature oocytes can be matured by in vitro culture (in vitro matured; IVM). Although IVM and IVO oocytes are equivalent in appearance at the MII stage, it remains unclear whether there are differences in their properties. Therefore, we investigated their in vitro fertilization and developmental capacities and cytoskeletal statuses. Our findings revealed that the IVM and IVO oocytes had similar fertilization rates but that no IVO oocytes could develop to the blastocyst stage. Additionally, IVO oocytes showed abnormal cytoskeletal formation. It is concluded that IVM oocytes maintain normal function, whereas IVO oocytes would be affected by aging and other factors when they remain for a long time in the ovary.

Ooplasmic transfer in human oocytes: efficacy and concerns in assisted reproduction

BACKGROUND

Ooplasmic transfer (OT) technique or cytoplasmic transfer is an emerging technique with relative success, having a significant status in assisted reproduction. This technique had effectively paved the way to about 30 healthy births worldwide. Though OT has long been invented, proper evaluation of the efficacy and risks associated with this critical technique has not been explored properly until today. This review thereby put emphasis upon the applications, efficacy and adverse effects of OT techniques in human.

MAIN BODY

Available reports published between January 1982 and August 2017 has been reviewed and the impact of OT on assisted reproduction was evaluated. The results consisted of an update on the efficacy and concerns of OT, the debate on mitochondrial heteroplasmy, apoptosis, and risk of genetic and epigenetic alteration.

SHORT CONCLUSION

The application of OT technique in humans demands more clarity and further development of this technique may successfully prove its utility as an effective treatment for oocyte incompetence.

Impact of prolonged oocyte incubation time before vitrification on oocyte survival, embryo formation, and embryo quality in mice

Oocyte incubation time before freezing is one of the factors affecting oocyte vitrification. In the assisted reproductive technology (ART) clinics, it is sometimes decided to perform oocyte vitrification after a long period of incubation time due to various conditions, such as inability to collect semen samples, unsuccessful urological interventions (PESA, TESE, etc.), or unexpected conditions. A time factor of up to 6 h has been studied in the available reports. Therefore, this study was designed to evaluate oocyte incubation time before freezing at 0, 6, 12, 18, and 24 h after retrieval. Metaphase II (MII) oocytes were obtained from NMRI female mice after being randomly divided into the five groups of 0, 6, 12, 18, and 24 h of freezing via hormonal stimulation following retrieval and entered into the vitrification-warming process. The thawed oocytes were evaluated according to the survival criteria and then inseminated with the sperms of male mice for in vitro fertilization. The next day, the embryo formation rate and embryo quality were assessed. Our results demonstrated that even after 24 h of incubation, the survival rate of oocytes was 51.35% with the embryo formation rate of 73.21%. However, the survival and embryo formation rates significantly decreased within 12, 18, and 24 h after retrieval compared to the groups vitrified at 0 h. The embryo quality was significantly reduced by vitrification at 0 to 24 h after retrieval. According to our data, although a prolonged incubation time before freezing reduced the survival rate, there was still a chance for oocytes to stay alive with acceptable embryo formation and quality rates after vitrification warming of oocytes.

Combination of spindle and first polar body chromosome images for the enhanced prediction of developmental potency of mouse metaphase II oocytes

The objective of this study was to classify spindle and first polar body (PB1) chromosome images in ovulated mouse oocytes over time to predict the developmental competence of metaphase II (MII) oocytes. Oocytes were collected at 12, 15, 20, and 25 h after human chorionic gonadotropin (hCG) injection, and stained for spindle tubulin, chromosomes, and PB1 chromosomes. MII spindle morphology was classified as tapered type or barrel type and PB1 chromosomes were categorized as aggregated, separated, dot, or collapsed. To determine whether differences in spindle and PB1 images in MII oocytes are associated with fertilization success, we performed in vitro fertilization (IVF) at various times after hCG injection. Barrel-type spindles and aggregate-type PB1 were dominant at 12 h after hCG injection. Oocyte spindles collected 1 h after injection were tapered, and PB1 chromosomes were separated. At 20 and 25 h after treatment, spindle and PB1 images were classified as collapsed. The rate of development to 2-cell embryos after IVF did not differ between the 12 h and 15 h treatments; however, it was significantly lower for the 25 h treatment than for other treatments. The rates of development to blastocysts at 12, 15, 20, and 25 h after hCG injection were 61, 46, 42, and 9%, respectively. MII oocytes with barrel-type spindles and aggregate-type PB1 had high rates of fertilization and blastocyst development, and spindle and PB1 characteristics were correlated with the outcomes of IVF and embryo culture. These results suggested that images of spindles combined with those of PB1 chromosomes enable the prediction of oocytic and/or embryonic quality.

Cortical mechanics and myosin-II abnormalities associated with post-ovulatory aging: implications for functional defects in aged eggs

STUDY HYPOTHESIS

Cellular aging of the egg following ovulation, also known as post-ovulatory aging, is associated with aberrant cortical mechanics and actomyosin cytoskeleton functions.

STUDY FINDING

Post-ovulatory aging is associated with dysfunction of non-muscle myosin-II, and pharmacologically induced myosin-II dysfunction produces some of the same deficiencies observed in aged eggs.

WHAT IS KNOWN ALREADY

Reproductive success is reduced with delayed fertilization and when copulation or insemination occurs at increased times after ovulation. Post-ovulatory aged eggs have several abnormalities in the plasma membrane and cortex, including reduced egg membrane receptivity to sperm, aberrant sperm-induced cortical remodeling and formation of fertilization cones at the site of sperm entry, and reduced ability to establish a membrane block to prevent polyspermic fertilization.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Ovulated mouse eggs were collected at 21-22 h post-human chorionic gonadotrophin (hCG) (aged eggs) or at 13-14 h post-hCG (young eggs), or young eggs were treated with the myosin light chain kinase (MLCK) inhibitor ML-7, to test the hypothesis that disruption of myosin-II function could mimic some of the effects of post-ovulatory aging. Eggs were subjected to various analyses. Cytoskeletal proteins in eggs and parthenogenesis were assessed using fluorescence microscopy, with further analysis of cytoskeletal proteins in immunoblotting experiments. Cortical tension was measured through micropipette aspiration assays. Egg membrane receptivity to sperm was assessed in in vitro fertilization (IVF) assays. Membrane topography was examined by low-vacuum scanning electron microscopy (SEM).

MAIN RESULTS AND THE ROLE OF CHANCE

Aged eggs have decreased levels and abnormal localizations of phosphorylated myosin-II regulatory light chain (pMRLC; P = 0.0062). Cortical tension, which is mediated in part by myosin-II, is reduced in aged mouse eggs when compared with young eggs, by ∼40% in the cortical region where the metaphase II spindle is sequestered and by ∼50% in the domain to which sperm bind and fuse (P < 0.0001). Aging-associated parthenogenesis is partly rescued by treating eggs with a zinc ionophore (P = 0.003), as is parthenogenesis induced by inhibition of mitogen-activated kinase (MAPK) 3/1 [also known as extracellular signal-regulated kinase (ERK)1/2] or MLCK. Inhibition of MLCK with ML-7 also results in effects that mimic those of post-ovulatory aging: fertilized ML-7-treated eggs show both impaired fertilization and increased extents of polyspermy, and ML-7-treated young eggs have several membrane abnormalities that are shared by post-ovulatory aged eggs.

LIMITATIONS, REASONS FOR CAUTION

These studies were done with mouse oocytes, and it remains to be fully determined how these findings from mouse oocytes would compare with other species. For studies using methods not amenable to analysis of large sample sizes and data are limited to what images one can capture (e.g. SEM), data should be interpreted conservatively.

WIDER IMPLICATIONS OF THE FINDINGS

These data provide insights into causes of reproductive failures at later post-copulatory times.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTERESTS

This project was supported by R01 HD037696 and R01 HD045671 from the NIH to J.P.E. Cortical tension studies were supported by R01 GM66817 to D.N.R. The authors declare there are no financial conflicts of interest.

Experimental strategies towards increasing intracellular mitochondrial activity in oocytes: A systematic review

PURPOSE

The mitochondrial complement is critical in sustaining the earliest stages of life. To improve the Assisted Reproductive Technology (ART), current methods of interest were evaluated for increasing the activity and copy number of mitochondria in the oocyte cell.

METHODS

This covered the researches from 1966 to September 2015.

RESULTS

The results provided ten methods that can be studied individually or simultaneously.

CONCLUSION

Though the use of these techniques generated great concern about heteroplasmy observation in humans, it seems that with study on these suggested methods there is real hope for effective treatments of old oocyte or oocytes containing mitochondrial problems in the near future.

Effect of medium additives during liquid storage on developmental competence of in vitro matured bovine oocytes

Our aim was to improve the developmental competence of bovine oocytes during their liquid storage by using additives. In vitro matured oocytes were stored for 20 h at 25°C in HEPES buffered TCM 199 medium (base medium). After storage, in vitro embryo development after in vitro fertilization was compared to those of non-stored (control) ones. Addition of 10% (v/v) newborn calf serum or 10.27 mmol/L pyruvate alone to the base medium did not improve blastocyst formation rates in stored oocytes; however, their simultaneous addition significantly improved the rate compared with those stored in base medium (P < 0.05). Supplementation of the holding medium with dithiothreitol (DTT) at any concentrations did not improve embryo development from stored oocytes. Although supplementation with cyclosporine A (CsA) significantly reduced apoptosis and membrane damage rates during storage, it did not improve the developmental competence of oocytes. 1,2-bis(2-aminophenoxy) ethane N,N,N',N'-tetraacetic acid tetrakis-acetoxymethyl ester and ruthenium red had no effect on oocyte apoptotic rates. Blastocyst formation rates in all stored groups remained significantly lower than that of the control. In conclusion, pyruvate and serum had a synergic effect to moderate the reduction of oocyte quality during storage, whereas mitochondrial membrane pore inhibitor CsA and the antioxidant DTT did not affect their developmental competence.

Ca(2+) dynamics in oocytes from naturally-aged mice

The ability of human metaphase-II arrested eggs to activate following fertilisation declines with advancing maternal age. Egg activation is triggered by repetitive increases in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the ooplasm as a result of sperm-egg fusion. We therefore hypothesised that eggs from older females feature a reduced ability to mount appropriate Ca²⁺ responses at fertilisation. To test this hypothesis we performed the first examination of Ca²⁺ dynamics in eggs from young and naturally-aged mice. Strikingly, we find that Ca²⁺ stores and resting [Ca²⁺]_i are unchanged with age. Although eggs from aged mice feature a reduced ability to replenish intracellular Ca²⁺ stores following depletion, this difference had no effect on the duration, number, or amplitude of Ca²⁺ oscillations following intracytoplasmic sperm injection or expression of phospholipase C zeta. In contrast, we describe a substantial reduction in the frequency and duration of oscillations in aged eggs upon parthenogenetic activation with SrCl₂. We conclude that the ability to mount and respond to an appropriate Ca²⁺ signal at fertilisation is largely unchanged by advancing maternal age, but subtle changes in Ca²⁺ handling occur that may have more substantial impacts upon commonly used means of parthenogenetic activation.

Oocyte aging underlies female reproductive aging: biological mechanisms and therapeutic strategies

In recent years, postponement of marriage and childbearing in women of reproductive age has led to an increase in the incidence of age-related infertility. The reproductive aging process in women is assumed to occur due to a decrease in both the quantity and quality of the oocytes, with the ultimate result being a decline in fecundity. This age-related decline in fecundity is strongly dependent on oocyte quality, which is critical for fertilization and subsequent embryo development. Aged oocytes display increased chromosomal abnormality and dysfunction of cellular organelles, both of which factor into oocyte quality. In particular, mitochondrial dysfunction has been suggested as a major contributor to the reduction in oocyte quality as well as to chromosomal abnormalities in aged oocytes and embryos. Participation of oxidative stress in the oocyte aging process has been proposed because oxidative stress has the capacity to induce mitochondrial dysfunction and directly damage many intracellular components of the oocytes such as lipids, protein, and DNA. In an attempt to improve mitochondrial function in aged oocytes, several therapeutic strategies have been investigated using both animal models and assisted reproductive technology. Here, we review the biological mechanisms and present status of therapeutic strategies in the female reproductive aging field and indicate possible future therapeutic strategies.

Age-associated potency decline in bovine oocytes is delayed by blocking extracellular Ca(2+) influx

Oocyte aging due to delayed fertilization is associated with declining quality and developmental potential. Intracellular calcium (Ca(2+)) concentration ([Ca(2+)]i) regulates oocyte growth, maturation, and fertilization and has also been implicated in aging. Using bovine oocytes, we tested the hypothesis that oocyte aging could be delayed by reducing [Ca(2+)]ivia blocking the influx of extracellular Ca(2+) or chelating ooplasmic free Ca(2+). After IVM, cumulus-oocyte complexes or denuded oocytes were cultured in medium supplemented with 1-octanol, phorbol 12-myristate 13-acetate, or 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis-acetoxymethyl ester (BAPTA-AM) to manipulate [Ca(2+)]i. Addition of 1-mM 1-octanol increased blastocyst development rates in the cumulus-oocyte complexes aged for 6 hours by IVF and for 6, 12, and 24 hours by parthenoactivation, and this effect was independent of the presence of cumulus cells. The intracellular levels of ATP, Glutathione, and Glutathione disulfide were not affected by 1-octanol, but [Ca(2+)]i was significantly decreased. When oocytes were cultured in Ca(2+)-free medium for 12 hours, the blastocyst development rate was greater and the beneficial effects of 1-octanol on oocyte aging were abolished. However, when the medium was supplemented with phorbol 12-myristate 13-acetate, [Ca(2+)]i increased and the blastocyst development rate decreased. Moreover, BAPTA-AM reduced [Ca(2+)]i and increased blastocyst development rates after IVF or parthenoactivation. We conclude that the age-associated developmental potency decline was delayed by blocking the influx of extracellular Ca(2+) or reducing ooplasmic free Ca(2+). 1-Octanol, BAPTA-AM, or Ca(2+)-free medium could be used to lengthen the fertilization windows of aged bovine oocytes.

Accumulation of electrophilic aldehydes during postovulatory aging of mouse oocytes causes reduced fertility, oxidative stress, and apoptosis

With increasing periods of time following ovulation, the metaphase II (MII)-stage oocyte experiences overproduction of reactive oxygen species and elevated levels of lipid peroxidation that are implicitly linked with functional deficiencies acquired during postovulatory oocyte aging. We have demonstrated that the electrophilic aldehydes 4-hydroxynonenal (4HNE), malondialdehyde, and acrolein are by-products of nonenzymatic lipid peroxidation in the murine MII-stage oocyte, adducting to multiple proteins within the cell. The covalent modification of oocyte proteins by these aldehydes increased with extended periods of time postovulation; the mitochondrial protein succinate dehydrogenase (SDHA) was identified as a primary target for 4HNE adduction. Time- and dose-dependent studies revealed that exposure to elevated levels of electrophilic aldehydes causes mitochondrial reactive oxygen species production, lipid peroxidation, loss of mitochondrial membrane potential, and eventual apoptosis within the MII oocyte, presumably as a consequence of electron transport chain collapse following SDHA adduction. Additionally, we have determined that short-term exposure to low doses of 4HNE dramatically impairs the oocyte's ability to participate in fertilization and support embryonic development; however, this loss of functionality can be prevented by supplementation with the antioxidant penicillamine. In conclusion, this study has revealed that the accumulation of electrophilic aldehydes is linked to postovulatory oocyte aging, causing reduced fertility, oxidative stress, and apoptosis of this highly specialized cell. These data highlight the importance of timely fertilization of the mammalian oocyte postovulation and emphasize the potential advantages associated with antioxidant supplementation of oocyte culture medium in circumstances where reinsemination of oocytes may be desirable (i.e., rescue intracytoplasmic sperm injection), or where in vitro fertilization may be delayed.

Post-ovulatory aging of oocytes disrupts kinase signaling pathways and lysosome biogenesis

Post-ovulatory aging of oocytes results in the progressive loss of fertilization and developmental competence. This degradation of oocyte quality has been the object of numerous investigations, primarily focused on individual signaling pathways which provide limited insight into the status of global signaling events. The purpose of the present investigation was to comprehensively assess broad patterns of signaling pathway activity during in vitro aging as an initial step in defining control points that can be targeted to prevent the reduction in oocyte quality during prolonged culture. An antibody microarray-based phospho-proteome analysis performed on oocytes before and after eight hours of culture revealed significant changes in the abundance or activation state of 43 proteins that function in a wide variety of protein kinase-mediated signaling pathways. Several of the most significantly affected kinases were studied by Western blot and confocal immunofluorescence to corroborate the array results. Prolonged culture resulted in global changes in the abundance and activity of protein kinases that regulate the response to calcium, stress, and cell-cycle control. Examination of intracellular structures revealed a previously unrecognized increase in the abundance of large autophogagic lysosomes, which correlates with changes in protein kinase pathways. These results provide insight into the stresses experienced by oocytes during culture and the diversity of responses that results from them. The observed increase in autophagy-related activity, together with the disruptions in calcium signaling, cell-cycle, and stress-response pathways, have the potential to negatively impact oocyte quality by interfering with the normal sequence of biochemical changes that constitute egg activation following fertilization.

The effects of calcitonin on the development of and Ca2+ levels in heat-shocked bovine preimplantation embryos in vitro

Intracellular calcium homeostasis is essential for proper cell function. We investigated the effects of heat shock on the development of and the intracellular Ca²⁺ levels in bovine preimplantation embryos in vitro and the effects of calcitonin (CT), a receptor-mediated Ca²⁺ regulator, on heat shock-induced events. Heat shock (40.5 C for 10 h between 20 and 30 h postinsemination) of in vitro-produced bovine embryos did not affect the cleavage rate; however, it significantly decreased the rates of development to the 5- to 8-cell and blastocyst stages as compared with those of the control cultured for the entire period at 38.5 C (P < 0.05). The relative intracellular Ca²⁺ levels at the 1-cell stage (5 h after the start of heat shock), as assessed by Fluo-8 AM, a fluorescent probe for Ca²⁺, indicated that heat shock significantly lowered the Ca²⁺ level as compared with the control level. Semiquantitative reverse transcription PCR and western blot analyses revealed the expression of CT receptor in bovine preimplantation embryos. The addition of CT (10 nM) to the culture medium ameliorated the heat shock-induced impairment of embryonic development beyond the 5- to 8-cell stage. The Ca²⁺ level in the heat-shocked embryos cultured with CT was similar to that of the control embryos, suggesting that heat shock lowers the Ca²⁺ level in fertilized embryos in vitro and that a lower Ca²⁺ level is implicated in heat shock-induced impairment of embryonic development. Intracellular Ca²⁺-mobilizing agents, e.g., CT, may effectively circumvent the detrimental effects of heat shock on early embryonic development.

The association between the oocyte pool and aneuploidy: a comparative study of the reproductive potential of young and aged mice

PURPOSE

The present study examined the effect of aging on female reproductive potential.

METHODS

Six-week-old and 9-month-old CD1 mice were referred to as the 'young' and 'aged' groups, respectively. Oocytes were collected after superovulation, and their viability were compared using parthenogenetic activation. The aneuploidy of the oocytes (MII) was assessed using chromosome spread, and the whole ovarian follicle number was counted using an unbiased stereological method. Serum hormone levels were measured using the radio-immunity method, and the expression of the Cohesin subunit genes in the oocytes (GV) were assessed using RT-PCR.

RESULTS

The mean number of recovered (25.8 vs. 16.2; P < 0.05) and live oocytes (24.0 vs. 11.73; P <0.01) per head in the young-mice group (6-week-old) was significantly higher than that of the aged group (9-month-old). The aneuploidy rate of the ovulated oocytes in the aged group was significantly higher than that of the young group (36.8% vs. 10%; P < 0.01), and the rate of blastocyst formation in the young group (85.23%) was significantly higher than that of the aged group (81.2%; P <0.05). The number of primordial follicles (the oocyte pool) per ovary in the aged group was significantly decreased compared with the young group (330 ± 33.51 vs. 2079.6 ± 420.70; P < 0.01), and the level of AMH in the aged group was significantly higher than that of the young group (4.66 ± 0.11 ng/ml vs. 4.07 ± 0.18 ng/ml; P < 0.01).

CONCLUSIONS

We propose that maternal aging significantly reduces the oocyte pool, superovulation efficiency and developmental potential and increases the oocyte aneuploidy rate.

Oxidative stress and ageing of the post-ovulatory oocyte

With extended periods of time following ovulation, the metaphase II stage oocyte experiences deterioration in quality referred to as post-ovulatory oocyte ageing. Post-ovulatory ageing occurs both in vivo and in vitro and has been associated with reduced fertilization rates, poor embryo quality, post-implantation errors and abnormalities in the offspring. Although the physiological consequences of post-ovulatory oocyte ageing have largely been established, the molecular mechanisms controlling this process are not well defined. This review analyses the relationships between biochemical changes exhibited by the ageing oocyte and the symptoms associated with the ageing phenotype. We also discuss molecular events that are potentially involved in orchestrating post-ovulatory ageing with a particular focus on the role of oxidative stress. We propose that oxidative stress may act as the initiator for a cascade of events that create the aged oocyte phenotype. Specifically, oxidative stress has the capacity to cause a decline in levels of critical cell cycle factors such as maturation-promoting factor, impair calcium homoeostasis, induce mitochondrial dysfunction and directly damage multiple intracellular components of the oocyte such as lipids, proteins and DNA. Finally, this review addresses current strategies for delaying post-ovulatory oocyte ageing with a particular focus on the potential use of compounds such as caffeine or selected antioxidants in the development of more refined media for the preservation of oocyte integrity during IVF procedures.

Oxidative stress and ageing of the post-ovulatory oocyte

With extended periods of time following ovulation, the metaphase II stage oocyte experiences deterioration in quality referred to as post-ovulatory oocyte ageing. Post-ovulatory ageing occurs both in vivo and in vitro and has been associated with reduced fertilization rates, poor embryo quality, post-implantation errors and abnormalities in the offspring. Although the physiological consequences of post-ovulatory oocyte ageing have largely been established, the molecular mechanisms controlling this process are not well defined. This review analyses the relationships between biochemical changes exhibited by the ageing oocyte and the symptoms associated with the ageing phenotype. We also discuss molecular events that are potentially involved in orchestrating post-ovulatory ageing with a particular focus on the role of oxidative stress. We propose that oxidative stress may act as the initiator for a cascade of events that create the aged oocyte phenotype. Specifically, oxidative stress has the capacity to cause a decline in levels of critical cell cycle factors such as maturation-promoting factor, impair calcium homoeostasis, induce mitochondrial dysfunction and directly damage multiple intracellular components of the oocyte such as lipids, proteins and DNA. Finally, this review addresses current strategies for delaying post-ovulatory oocyte ageing with a particular focus on the potential use of compounds such as caffeine or selected antioxidants in the development of more refined media for the preservation of oocyte integrity during IVF procedures.

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.

Germline energetics, aging, and female infertility

The role of metabolism in ovarian aging is poorly described, despite the fact that ovaries fail earlier than most other organs. Growing interest in ovarian function is being driven by recent evidence that mammalian females routinely generate new oocytes during adult life through the activity of germline stem cells. In this perspective, we overview the female reproductive system as a powerful and clinically relevant model to understand links between aging and metabolism, and we discuss new concepts for how oocytes and their precursor cells might be altered metabolically to sustain or increase ovarian function and fertility in women.

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

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

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

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

Disparate consequences of heat stress exposure during meiotic maturation: embryo development after chemical activation vs fertilization of bovine oocytes

Consequences of heat stress exposure during the first 12 h of meiotic maturation differed depending on how and when bovine oocytes were activated. If heat-stressed oocytes underwent IVF at ~24 h, blastocyst development was less than for respective controls and similar to that obtained for nonheat-stressed oocytes undergoing IVF at 30 h (i.e. slightly aged). In contrast, if heat-stressed oocytes underwent chemical activation with ionomycin/6-dimethylaminopurine at 24 h, blastocyst development was not only higher than respective controls, but also equivalent to development obtained after activation of nonheat-stressed oocytes at 30 h. Developmental differences in chemically activated vs IVF-derived embryos were not related to fertilization failure or gross alterations in cytoskeletal components. Rather, ionomycin-induced calcium release and MAP kinase activity were less in heat-stressed oocytes. While underlying mechanisms are multifactorial, ability to obtain equivalent or higher development after parthenogenetic activation demonstrates that oocytes experiencing heat stress during the first 12 h of meiotic maturation have the necessary components to develop to the blastocyst stage, but fail to do so after fertilization.

Cumulated Ca2+ spike duration underlies Ca2+ oscillation frequency-regulated NFκB transcriptional activity

[Ca2+]i oscillations drive downstream events, like transcription, in a frequency-dependent manner. Why [Ca2+]i oscillation frequency regulates transcription has not been clearly revealed. A variation in [Ca2+]i oscillation frequency apparently leads to a variation in the time duration of cumulated [Ca2+]i elevations or cumulated [Ca2+]i spike duration. By manipulating [Ca2+]i spike duration, we generated a series of [Ca2+]i oscillations with the same frequency but different cumulated [Ca2+]i spike durations, as well as [Ca2+]i oscillations with the different frequencies but the same cumulated [Ca2+]i spike duration. Molecular assays demonstrated that, when generated in ‘artificial’ models alone, under physiologically simulated conditions or repetitive pulses of agonist exposure, [Ca2+]i oscillation regulates NFκB transcriptional activity, phosphorylation of IκBα and Ca2+-dependent gene expression all in a way actually dependent on cumulated [Ca2+]i spike duration whether or not frequency varies. This study underlines that [Ca2+]i oscillation frequency regulates NFκB transcriptional activity through cumulated [Ca2+]i spike-duration-mediated IκBα phosphorylation.

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.

Pyruvate prevents aging of mouse oocytes

Inhibiting oocyte aging is important not only for healthy reproduction but also for the success of assisted reproduction techniques. Although our previous studies showed that cumulus cells accelerated aging of mouse oocytes, the underlying mechanism is unknown. The objective of this paper was to study the effects of pyruvate and cumulus cells on mouse oocyte aging. Freshly ovulated mouse cumulus-oocyte complexes (COCs) or cumulus-denuded oocytes (DOs) were cultured in Chatot-Ziomek-Bavister (CZB) medium or COC-conditioned CZB medium supplemented with different concentrations of pyruvate before being examined for aging signs and developmental potential. Pyruvate supplementation to CZB medium decreased rates of ethanol-induced activation in both COCs and DOs by maintaining their maturation-promoting factor activities, but more pyruvate was needed for COCs than for DOs. Addition of pyruvate to the COC-conditioned CZB also alleviated aging of DOs. Observations on cortical granules, level of BCL2 proteins, histone acetylation, intracellular concentration of glutathione, and embryo development all confirmed that pyruvate supplementation inhibited aging of mouse oocytes. It is concluded that the aging of mouse oocytes, facilitated by culture in COCs, can be partially prevented by the addition of pyruvate to the culture medium.

Abnormal changes in mitochondria, lipid droplets, ATP and glutathione content, and Ca(2+) release after electro-activation contribute to poor developmental competence of porcine oocyte during in vitro ageing

The present study aims to investigate major changes in porcine oocytes during ageing in vitro. After the oocytes were cultured for 44, 56, 68 and 80 h, changes to porcine oocytes in ultrastructure, mitochondrial distribution, glutathione (GSH) and ATP content, Ca(2+) release patterns and developmental competence after electro-activation were observed. Mitochondria were evenly distributed in oocytes at 44 h, aggregated in clusters or in peripheral cytoplasm at 68 h and dimly dispersed throughout ooplasm at 80 h. Mitochondrial shape during ageing was also observed by transmission electron microscopy (TEM) at the same time intervals. Most mitochondria were spherical at 44 h, and became elongated when the culture time was extended to 68 h and 80 h. Moreover, mitochondrial clustering became increasingly loose from 56 h. Lipid droplets in oocytes appeared prominent and electron-dense at 44 h, but electron density was lost at 56 h. Lipid droplets were solidified as of 68 h. There was an age-dependent decrease in ATP content per oocyte. Glutathione content per oocyte decreased significantly and remained lower after 56 h. Amplitudes of [Ca(2+)] rise decreased dramatically following 56 h, and the time required for [Ca(2+)] to plateau became shorter after electro-activation with prolonged culture time. Cleavage and blastocyst rates of aged oocytes progressively decreased, while the fragmentation rate gradually increased after electro-activation. It is concluded that abnormal changes in mitochondria, lipid droplets, Ca(2+) release after electro-activation, and ATP and GSH content in oocytes during ageing may result in poor developmental competence of parthenotes.

Poor embryo development in mouse oocytes aged in vitro is associated with impaired calcium homeostasis

We examined whether impairment of intracellular Ca(2+) homeostasis is related to poor embryo development in in vitro-aged oocytes. We found that in vitro aging of mouse oocytes affected the patterns of Ca(2+) oscillations at fertilization: these Ca(2+) oscillations were lower in amplitude and higher in frequency compared with oocytes without in vitro aging. We also observed that the intracellular Ca(2+) store was decreased in in vitro-aged oocytes. A decrease in the Ca(2+) store induced by thapsigargin, a specific endoplasmic reticulum (ER) membrane Ca(2+)-ATPase inhibitor, resulted in a lower fertilization rate and in poorer embryo development. The frequency of Ca(2+) oscillations was significantly increased at fertilization, whereas their amplitude was decreased in thapsigargin-treated oocytes. These results suggest that impairment of intracellular Ca(2+) homeostasis (such as a decrease in the ER Ca(2+) store) caused an alteration in Ca(2+) oscillations and the poor embryo development in in vitro-aged oocytes. Because embryo fragmentation is closely related to apoptosis, we examined expression of BAX (a proapototic protein) and BCL2 (an antiapoptotic protein) in in vitro-aged oocytes. Although BCL2 was strongly expressed in oocytes without in vitro aging, expression of BCL2 was significantly reduced in oocytes of other culture conditions and treatments such as those in in vitro aging and those that were pretreated with H(2)O(2) or thapsigargin. Acting together, alteration in Ca(2+) oscillations and decrease in BCL2 expression in in vitro-aged oocytes may lead to poor embryo development.

Nitric oxide extends the oocyte temporal window for optimal fertilization

Deteriorating oocyte quality is a critical hurdle in the management of infertility, especially one associated with advancing age. In this study, we explore the role of nitric oxide (NO) on the sustenance of oocyte quality postovulation. Sibling oocytes from superovulated mice were subjected to intracytoplasmic sperm injection (ICSI) with cauda-epididymal spermatozoa following exposure to either the NO donor, S-nitroso-N-acetylpenicillamine (SNAP, 0.23 microM/min), an NO synthase (NOS) inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME, 1 mM), or an inhibitor of soluble guanylyl cyclase (sGC), 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (ODQ, 100 microM); while their sibling oocytes were subjected to ICSI either before (young) or after culture for the corresponding period of time (old). Outcomes of normal fertilization, cleavage, and development to the morula and blastocyst stages were compared. Embryos from each subgroup were also subjected to TUNEL assay for apoptosis. A significant deterioration in the ability of the oocytes to undergo normal fertilization and development to morula and blastocyst stages occurred among oocytes aged in culture medium compared to their sibling cohorts subjected to ICSI immediately after ovulation (P<0.05). This deterioration was prevented in oocytes exposed to SNAP. In contrast, exposure to L-NAME or ODQ resulted in a significant compromise in fertilization and development to the morula and blastocyst stages (P<0.05). Finally, apoptosis was noted in embryos derived from aged oocytes and those exposed to L-NAME or ODQ, but not in embryos derived from young oocytes or oocytes exposed to SNAP. Thus, NO is essential for sustenance of oocyte quality postovulation.

Ca2+ oscillation frequency regulates agonist-stimulated gene expression in vascular endothelial cells

A physiological membrane-receptor agonist typically stimulates oscillations, of varying frequencies, in cytosolic Ca2+ concentration ([Ca2+]i). Whether and how [Ca2+]i oscillation frequency regulates agonist-stimulated downstream events, such as gene expression, in non-excitable cells remain unknown. By precisely manipulating [Ca2+]i oscillation frequency in histamine-stimulated vascular endothelial cells (ECs), we demonstrate that the gene expression of vascular cell adhesion molecule 1 (VCAM1) critically depends on [Ca2+]i oscillation frequency in the presence, as well as the absence, of histamine stimulation. However, histamine stimulation enhanced the efficiency of [Ca2+]i-oscillation-frequency-regulated VCAM1 gene expression, versus [Ca2+]i oscillations alone in the absence of histamine stimulation. Furthermore, a [Ca2+]i oscillation frequency previously observed to be the mean frequency in histamine-stimulated ECs was found to optimize VCAM1 mRNA expression. All the above effects were abolished or attenuated by blocking histamine-stimulated generation of intracellular reactive oxygen species (ROS), another intracellular signaling pathway, and were restored by supplementary application of a low level of H2O2. Endogenous NF-kappaB activity is similarly regulated by [Ca2+]i oscillation frequency, as well as its co-operation with ROS during histamine stimulation. This study shows that [Ca2+]i oscillation frequency cooperates with ROS to efficiently regulate agonist-stimulated gene expression, and provides a novel and general strategy for studying [Ca2+]i signal kinetics in agonist-stimulated downstream events.