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

Spontaneous Overactivation of Xenopus Frog Eggs Triggers Necrotic Cell Death

The excessive activation of frog eggs, referred to as overactivation, can be initiated by strong oxidative stress, leading to expedited calcium-dependent non-apoptotic cell death. Overactivation also occurs spontaneously, albeit at a low frequency, in natural populations of spawned frog eggs. Currently, the cytological and biochemical events of the spontaneous process have not been characterized. In the present study, we demonstrate that the spontaneous overactivation of Xenopus frog eggs, similarly to oxidative stress- and mechanical stress-induced overactivation, is characterized by the fast and irreversible contraction of the egg's cortical layer, an increase in egg size, the depletion of intracellular ATP, a drastic increase in the intracellular ADP/ATP ratio, and the degradation of M phase-specific cyclin B2. These events manifest in eggs in the absence of caspase activation within one hour of triggering overactivation. Importantly, substantial amounts of ATP and ADP leak from the overactivated eggs, indicating that plasma membrane integrity is compromised in these cells. The rupture of the plasma membrane and acute depletion of intracellular ATP explicitly define necrotic cell death. Finally, we report that egg overactivation can occur in the frog's genital tract. Our data suggest that mechanical stress may be a key factor promoting egg overactivation during oviposition in frogs.

Reactive oxygen species signalling in the deterioration of quality of mammalian oocytes cultured in vitro: Protective effect of antioxidants

The in vitro fertilization (IVF) is the first choice of infertile couples worldwide to plan for conception. Besides having a significant advancement in IVF procedure, the success rate is still poor. Although several approaches have been tested to improve IVF protocol, minor changes in culture conditions, physical factors and/or drug treatment generate reactive oxygen species (ROS) in oocytes. Due to large size and huge number of mitochondria, oocyte is more susceptible towards ROS-mediated signalling under in vitro culture conditions. Elevation of ROS levels destabilize maturation promoting factor (MPF) that results in meiotic exit from diplotene as well as metaphase-II (M-II) arrest in vitro. Once meiotic exit occurs, these oocytes get further arrested at metaphase-I (M-I) stage or metaphase-III (M-III)-like stage under in vitro culture conditions. The M-I as well as M-III arrested oocytes are not fit for fertilization and limits IVF outcome. Further, the generation of excess levels of ROS cause oxidative stress (OS) that initiate downstream signalling to initiate various death pathways such as apoptosis, autophagy, necroptosis and deteriorates oocyte quality under in vitro culture conditions. The increase of cellular enzymatic antioxidants and/or supplementation of exogenous antioxidants in culture medium protect ROS-induced deterioration of oocyte quality in vitro. Although a growing body of evidence suggests the ROS and OS-mediated deterioration of oocyte quality in vitro, their downstream signalling and related mechanisms remain poorly understood. Hence, this review article summarizes the existing evidences concerning ROS and OS-mediated downstream signalling during deterioration of oocyte quality in vitro. The use of various antioxidants against ROS and OS-mediated impairment of oocyte quality in vitro has also been explored in order to increase the success rate of IVF during assisted reproductive health management.

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.

Proteomic analysis implicates that postovulatory aging leads to aberrant gene expression, biosynthesis, RNA metabolism and cell cycle in mouse oocytes

Background

In mammals, oocytes display compromised quality after experiencing a process of postovulatory aging. However, the mechanisms underlying are not yet fully understood. Here, we portrayed a protein expression profile of fresh and aging metaphase II (MII) mouse oocytes by means of four-dimensional label-free quantification mass spectrometry (4D-LFQ).

Results

The analysis of 4D-LFQ data illustrated that there were seventy-six differentially expressed proteins (DEPs) between two groups of MII stage oocytes. Fifty-three DEPs were up-regulated while twenty-three DEPs were down-regulated in the MII oocytes of the aging group, and Gene Ontology (GO) analysis revealed that these DEPs were mainly enriched in regulation of gene expression, biosynthesis, RNA metabolism and cell cycle. Our detailed analysis revealed that the expression of proteins that related to gene expression processes such as transcription, translation, post-translational modifications and epigenome was changed; the relative protein expression of RNA metabolic processes, such as RNA alternative splicing, RNA export from nucleus and negative regulation of transcription from RNA polymerase II promoter was also altered.

Conclusion

In conclusion, we identified considerable DEPs and discussed how they agreed with previous researches illustrating altered protein expression associated with the quality of oocytes. Our research provided a new perspective on the mechanisms of postovulatory aging and established a theoretical support for practical methods to control and reverse postovulatory aging.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13048-022-01045-6.

In vivo and in vitro postovulatory aging: when time works against oocyte quality?

In mammalian species an optimal fertilization window during which successful fertilization occurs. In the majority of mammals estrus marks ovulation time and coincident with mating, thereby allowing the synchronized meeting in the fallopian tubes, between freshly ejaculated sperm and freshly ovulated oocytes. Conversely, women do not show natural visual signs of ovulation such that fertilization can occur hours later involving an aged oocyte and freshly ejaculated spermatozoa. During this time, the oocyte undergoes a rapid degradation known as “postovulatory aging” (POA). POA may become particularly important in the human-assisted reproductive technologies, as the fertilization of retrieved mature oocytes can be delayed due to increased laboratory workload or because of unforeseeable circumstances, like the delayed availability of semen samples. This paper is an updated review of the consequences of POA, either in vivo or in vitro, on oocyte quality with particular attention to modifications caused by POA on oocyte nuclear, cytoplasmic, genomic, and epigenetic maturation, and embryo development.

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.

Meiotic Instability Generates a Pathological Condition in Mammalian Ovum

Maintenance of metaphase-II (M-II) arrest in ovum is required to present itself as a right gamete for successful fertilization in mammals. Surprisingly, instability of meiotic cell cycle results in spontaneous exit from M-II arrest, chromosomal scattering and incomplete extrusion of second polar body (PB-II) without forming pronuclei so called abortive spontaneous ovum activation (SOA). It remains unclear what causes meiotic instability in freshly ovulated ovum that results in abortive SOA. We propose the involvement of various signal molecules such as reactive oxygen species (ROS), cyclic 3',5' adenosine monophosphate (cAMP) and calcium (Ca²⁺) in the induction of meiotic instability and thereby abortive SOA. These signal molecules through their downstream pathways modulate phosphorylation status and activity of cyclin dependent kinase (cdk1) as well as cyclin B1 level. Changes in phosphorylation status of cdk1 and its activity, dissociation and degradation of cyclin B1 destabilize maturation promoting factor (MPF). The premature MPF destabilization and defects in other cell cycle regulators possibly cause meiotic instability in ovum soon after ovulation. The meiotic instability results in a pathological condition of abortive SOA and deteriorates ovum quality. These ova are unfit for fertilization and limit reproductive outcome in several mammalian species including human. Therefore, global attention is required to identify the underlying causes in greater details in order to address the problem of meiotic instability in ova of several mammalian species icluding human. Moreover, these activated ova may be used to create parthenogenetic embryonic stem cell lines in vitro for the use in regenerative medicine.Graphical abstract.

Biochemical Hallmarks of Oxidative Stress-Induced Overactivation of Xenopus Eggs

Egg overactivation occurs with a low frequency in the populations of naturally ovulated frog eggs. At present, its natural inducers, molecular mechanisms, and intracellular events remain unknown. Using microscopic and biochemical analyses, we demonstrate here that high levels of hydrogen peroxide-induced oxidative stress can cause time- and dose-dependent overactivation of Xenopus eggs. Lipofuscin accumulation, decrease of soluble cytoplasmic protein content, and depletion of intracellular ATP were found to take place in the overactivated eggs. Progressive development of these processes suggests that egg overactivation unfolds in a sequential and ordered fashion.

Necrosis and necroptosis in germ cell depletion from mammalian ovary

The maximum number of germ cells is present during the fetal life in mammals. Follicular atresia results in rapid depletion of germ cells from the cohort of the ovary. At the time of puberty, only a few hundred (<1%) germ cells are either culminated into oocytes or further get eliminated during the reproductive life. Although apoptosis plays a major role, necrosis as well as necroptosis, might also be involved in germ cell elimination from the mammalian ovary. Both necrosis and necroptosis show similar morphological features and are characterized by an increase in cell volume, cell membrane permeabilization, and rupture that lead to cellular demise. Necroptosis is initiated by tumor necrosis factor and operated through receptor interacting protein kinase as well as mixed lineage kinase domain-like protein. The acetylcholinesterase, cytokines, starvation, and oxidative stress play important roles in necroptosis-mediated granulosa cell death. The granulosa cell necroptosis directly or indirectly induces susceptibility toward necroptotic or apoptotic cell death in oocytes. Indeed, prevention of necrosis and necroptosis pathways using their specific inhibitors could enhance growth/differentiation factor-9 expression, improve survivability as well as the meiotic competency of oocytes, and prevent decline of reproductive potential in several mammalian species and early onset of menopause in women. This study updates the information and focuses on the possible involvement of necrosis and necroptosis in germ cell depletion from the mammalian ovary.

Journey of oocyte from metaphase-I to metaphase-II stage in mammals

In mammals, journey from metaphase-I (M-I) to metaphase-II (M-II) is important since oocyte extrude first polar body (PB-I) and gets converted into haploid gamete. The molecular and cellular changes associated with meiotic cell cycle progression from M-I to M-II stage and extrusion of PB-I remain ill understood. Several factors drive oocyte meiosis from M-I to M-II stage. The mitogen-activated protein kinase3/1 (MAPK3/1), signal molecules and Rho family GTPases act through various pathways to drive cell cycle progression from M-I to M-II stage. The down regulation of MOS/MEK/MAPK3/1 pathway results in the activation of anaphase-promoting complex/cyclosome (APC/C). The active APC/C destabilizes maturation promoting factor (MPF) and induces meiotic resumption. Several signal molecules such as, c-Jun N-terminal kinase (JNK2), SENP3, mitotic kinesin-like protein 2 (MKlp2), regulator of G-protein signaling (RGS2), Epsin2, polo-like kinase 1 (Plk1) are directly or indirectly involved in chromosomal segregation. Rho family GTPase is another enzyme that along with cell division cycle (Cdc42) to form actomyosin contractile ring required for chromosomal segregation. In the presence of origin recognition complex (ORC4), eccentrically localized haploid set of chromosomes trigger cortex differentiation and determine the division site for polar body formation. The actomyosin contractile activity at the site of division plane helps to form cytokinetic furrow that results in the formation and extrusion of PB-I. Indeed, oocyte journey from M-I to M-II stage is coordinated by several factors and pathways that enable oocyte to extrude PB-I. Quality of oocyte directly impact fertilization rate, early embryonic development, and reproductive outcome in mammals.

Postovulatory cell death: why eggs die via apoptosis in biological species with external fertilization

Spawned unfertilized eggs have been found to die by apoptosis in several species with external fertilization. However, there is no necessity for the externally laid eggs to degrade via this process, as apoptosis evolved as a mechanism to reduce the damaging effects of individual cell death on the whole organism. The recent observation of egg degradation in the genital tracts of some oviparous species provides a clue as to the physiological relevance of egg apoptosis in these animals. We hypothesize that egg apoptosis accompanies ovulation in species with external fertilization as a normal process to eliminate mature eggs retained in the genital tract after ovulation. Furthermore, apoptosis universally develops in ovulated eggs after spontaneous activation in the absence of fertilization. This paper provides an overview of egg apoptosis in several oviparous biological species, including frog, fish, sea urchin, and starfish.

Protective effects of ethanol extracts of Artemisia asiatica Nakai ex Pamp. on ageing-induced deterioration in mouse oocyte quality

Following ovulation, oocytes undergo a time-dependent deterioration in quality referred to as post-ovulatory ageing. Although various factors influence the post-ovulatory ageing of oocytes, oxidative stress is a key factor involved in deterioration of oocyte quality. Artemisia asiatica Nakai ex Pamp. has been widely used in East Asia as a food ingredient and traditional medicine for the treatment of inflammation, cancer, and microbial infections. Recent studies have shown that A. asiatica exhibits antioxidative effects. In this study, we investigated whether A. asiatica has the potential to attenuate deterioration in oocyte quality during post-ovulatory ageing. Freshly ovulated mouse oocytes were cultured with 0, 50, 100 or 200 μg/ml ethanol extracts of A. asiatica Nakai ex Pamp. After culture for up to 24 h, various ageing-induced oocyte abnormalities, including morphological changes, reactive oxygen species (ROS) accumulation, apoptosis, chromosome and spindle defects, and mitochondrial aggregation were determined. Treatment of oocytes with A. asiatica extracts reduced ageing-induced morphological changes. Moreover, A. asiatica extracts decreased ROS generation and the onset of apoptosis by preventing elevation of the Bax/Bcl-2 expression ratio during post-ovulatory ageing. Furthermore, A. asiatica extracts attenuated the ageing-induced abnormalities including spindle defects, chromosome misalignment and mitochondrial aggregation. Our results demonstrate that A. asiatica can relieve deterioration in oocyte quality and delay the onset of apoptosis during post-ovulatory ageing.

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.

Calcium Signaling During Meiotic Cell Cycle Regulation and Apoptosis in Mammalian Oocytes

Calcium (Ca⁺⁺ ) is one of the major signal molecules that regulate various aspects of cell functions including cell cycle progression, arrest, and apoptosis in wide variety of cells. This review summarizes current knowledge on the differential roles of Ca⁺⁺ in meiotic cell cycle resumption, arrest, and apoptosis in mammalian oocytes. Release of Ca⁺⁺ from internal stores and/or Ca⁺⁺ influx from extracellular medium causes moderate increase of intracellular Ca⁺⁺ ([Ca⁺⁺ ]i) level and reactive oxygen species (ROS). Increase of Ca⁺⁺ as well as ROS levels under physiological range trigger maturation promoting factor (MPF) destabilization, thereby meiotic resumption from diplotene as well as metaphase-II (M-II) arrest in oocytes. A sustained increase of [Ca⁺⁺ ]i level beyond physiological range induces generation of ROS sufficient enough to cause oxidative stress (OS) in aging oocytes. The increased [Ca⁺⁺ ]i triggers Fas ligand-mediated oocyte apoptosis. Further, OS triggers mitochondria-mediated oocyte apoptosis in several mammalian species. Thus, Ca⁺⁺ exerts differential roles on oocyte physiology depending upon its intracellular concentration. A moderate increase of [Ca⁺⁺ ]i as well as ROS mediate spontaneous resumption of meiosis from diplotene as well as M-II arrest, while their high levels cause meiotic cell cycle arrest and apoptosis by operating both mitochondria- as well as Fas ligand-mediated apoptotic pathways. Indeed, Ca⁺⁺ regulates cellular physiology by modulating meiotic cell cycle and apoptosis in mammalian oocytes. J. Cell. Physiol. 232: 976-981, 2017. © 2016 Wiley Periodicals, Inc.

S-nitroso-N-acetyl penicillamine inhibits spontaneous exit from metaphase-II arrest in rat eggs cultured in vitro

BACKGROUND

Present study was designed to investigate the in vitro effects of nitric oxide (NO) donor such as S-nitroso-N-acetyl penicillamine (SNAP) on spontaneous exit from metaphase-II arrest (SEM-II) in rat eggs cultured in vitro.

METHODS

Ovulated eggs were denuded and then exposed to various concentrations (0.0, 0.01, 0.1 and 1.0mM) of SNAP for 3h under in vitro culture conditions. The percentage of SEM-II, specific and total phosphorylated cyclin-dependent kinase-1 (Cdk1), cyclin B1 and anaphase promoting complex/cyclosome (APC/C) levels as well as Cdk1 activity were analyzed.

RESULTS

The SEM-II was associated with a decrease of Thr-161 phosphorylated Cdk1 as well as cyclin B1 levels and increase of Thr-14/Tyr-15 phosphorylated Cdk1, APC/C levels and Cdk1 activity in aged eggs cultured in vitro. On the other hand, SNAP treatment prevented a decrease of Thr-161 phosphorylated Cdk1 as well as cyclin B1 levels and increase of Thr-14/Tyr-15 phosphorylated Cdk1, Cdk1 activity that finally prevented SEM-II in a concentration-dependent manner. However, APC/C level was not affected by SNAP during the course of treatment in vitro.

CONCLUSIONS

Present data suggest that SNAP prevented SEM-II possibly by increasing high level of NO and thereby maturation promoting factor (MPF) stabilization in rat eggs cultured in vitro. Hence, SNAP could be used to prevent SEM-II that reduces reproductive outcome in several mammalian species.

Involvement of Cyclin-Dependent Kinase 1 during Postovulatory Aging-Mediated Abortive Spontaneous Egg Activation in Rat Eggs Cultured In Vitro

Freshly ovulated rat eggs do not remain arrested at metaphase II (MII) and undergo exit from MII arrest with initiation of extrusion of the second polar body (PBII), a characteristic feature of abortive spontaneous egg activation (SEA). The biochemical and molecular changes during postovulatory aging-mediated abortive SEA remain poorly understood. We investigated the morphological, cellular, and molecular changes during postovulatory aging-mediated abortive SEA in eggs cultured in vitro. Our results suggest that postovulatory egg aging in vitro induced initiation of PBII extrusion in a time-dependent manner. Postovulatory aging increased Wee1 kinase and Thr-14/Tyr-15 phosphorylated cyclin-dependent kinase 1 (Cdk1) levels, whereas Thr-161 phosphorylated Cdk1 and cyclin B1 levels were significantly decreased in eggs cultured in vitro. The early mitotic inhibitor 2 (Emi2) level was significantly reduced, but anaphase promoting complex/cyclosome (APC/C) and mitotic arrest deficient protein (MAD2) levels were increased initially and then reduced during a later period of in vitro culture. These results suggest that an increased Wee1 kinase level modulated the specific phosphorylation status of Cdk1, increased Cdk1 activity, and decreased the cyclin B1 level. Furthermore, the decreased Emi2 level was associated with an increased level of APC/C and decreased level of cyclin B1, which resulted in maturation promoting factor (MPF) destabilization and finally led to postovulatory aging-mediated abortive SEA in rat eggs cultured in vitro.

Increased level of reactive oxygen species persuades postovulatory aging-mediated spontaneous egg activation in rat eggs cultured in vitro

The present study was aimed to find out whether increased level of reactive oxygen species (ROS) particularity hydrogen peroxide (H2O2) could persuade postovulatory aging-mediated abortive spontaneous egg activation (SEA) in rat eggs cultured in vitro. For this purpose, ROS and H2O2 levels, mitochondria distribution and its membrane potential, p286-CaMK-II, Emi2, Thr-161 phophorylated cyclin-dependent protein kinase1 (Cdk1) as well as cyclin B1 levels, in vitro effects of 3-tert-butyl-4 hydroxy anisole (BHA), pentoxifylline and dibutyryl-adenosine 3',5'-cyclic monophosphate (db-cAMP) were analyzed during postovulatory aging-induced abortive SEA in vitro. Data of the present study suggest that postovulatory aging increased H2O2 levels, disturbed mitochondrial distribution pattern and mitochondrial membrane potential (MMP) in eggs. There was an significant increase of p286-CaMK-II level, while Emi2 level reduced significantly during egg aging in vitro. The reduced Emi2 level was associated with decreased Thr-161 phosphorylated cyclin-dependent kinase-1 (Cdk1) as well as cyclin B1 level in aged eggs that underwent abortive SEA. Further, supplementation of pentoxifylline, db-cAMP, and BHA protected postovulatory aging-mediated abortive SEA in concentration-dependent manner. These data suggest that postovulatory aging increased H2O2 levels, reduced MMP, and increased p286-CaMK-II. The increased p286-CaMK-II was associated with reduced Emi2 level and maturation-promoting factor levels during postovulatory aging-mediated abortive SEA. Drugs that elevate cAMP directly or indirectly and BHA protected postovulatory aging-mediated abortive SEA possibly by reducing ROS level in rat eggs cultured in vitro.

RO-3306 prevents postovulatory aging-mediated spontaneous exit from M-II arrest in rat eggs cultured in vitro

BACKGROUND

Postovulatory aging-mediated spontaneous exit from metaphase-II (M-II) arrest deteriorates egg quality and limits assisted reproductive technologies outcome (ART) outcome. Present study was aimed to find out whether RO-3306, specific cyclin dependent kinase 1 (Cdk1) inhibitor could protect against postovulatory aging-mediated spontaneous exit from M-II arrest in rat eggs cultured in vitro.

METHODS

Freshly ovulated M-II arrested eggs were exposed to various concentrations of RO-3306 for 3h in vitro. The morphological changes, percentage of spontaneous exit from M-II arrest, total and specific phosphorylation status of Cdk1, cyclin B1 level and Cdk1 activity were analyzed.

RESULTS

Data suggest that RO-3306 protected postovulatory aging-mediated spontaneous exit from M-II arrest in a concentration-dependent manner. Postovulatory aging increased Thr14/Tyr15 phosphorylated Cdk1 level, decreased Thr161 phosphorylated Cdk1 as well as cyclin B1 levels and increased Cdk1 activity in aged eggs cultured in vitro. On the other hand, RO-3306 protected postovulatory aging-induced changes in specific phosphorylation of Cdk1, cyclin B1 level, inhibited the kinase activity and prevented spontaneous exit from M-II arrest.

CONCLUSIONS

Our results suggest that postovulatory aging destabilizes MPF by modulating specific phosphorylation of Cdk1 and cyclin B1 level. RO-3306 prevented these changes and maintained M-II arrest in rat eggs cultured in vitro. Hence, maintenance of M-II arrest in ovulated eggs using RO-3306 could be beneficial to increase the number of eggs available for various ART programs.

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.

Morphological, cellular and molecular changes during postovulatory egg aging in mammals

Postovulatory aging is associated with several morphological, cellular and molecular changes that deteriorate egg quality either by inducing abortive spontaneous egg activation (SEA) or by egg apoptosis. The reduced egg quality results in poor fertilization rate, embryo quality and reproductive outcome. Although postovulatory aging-induced abortive SEA has been reported in several mammalian species, the molecular mechanism(s) underlying this process remains to be elucidated. The postovulatory aging-induced morphological and cellular changes are characterized by partial cortical granules exocytosis, zona pellucida hardening, exit from metaphase-II (M-II)arrest and initiation of extrusion of second polar body in aged eggs. The molecular changes include reduction of adenosine 3',5'- cyclic monophosphate (cAMP) level, increase of reactive oxygen species (ROS) and thereby cytosolic free calcium (Ca²⁺) level. Increased levels of cAMP and/or ROS trigger accumulation of Thr-14/Tyr-15 phosphorylated cyclin-dependent kinase 1 (Cdk1) on one hand and degradation of cyclin B1 through ubiquitin-mediated proteolysis on the other hand to destabilize maturation promoting factor (MPF). The destabilized MPF triggers postovulatory aging-induced abortive SEA and limits various assisted reproductive technologies (ARTs) outcome in several mammalian species. Use of certain drugs that can either increase cAMP or reduce ROS level would prevent postovulatory aging-induced deterioration in egg quality so that more number of good quality eggs can be made available to improve ART outcome in mammals including human.

Nitric oxide signals postovulatory aging-induced abortive spontaneous egg activation in rats

OBJECTIVE

The aim of this study was to determine whether an increase of intracellular nitric oxide (NO) level signals postovulatory aging-induced abortive spontaneous egg activation (SEA) in rats.

METHODS

Freshly ovulated eggs (arrested at metaphase-II stage; M-II) were cultured in vitro for 3 hours to induce postovulatory egg aging. The morphological changes, inducible nitric oxide synthase (iNOS) expression, NO, cytosolic free Ca(2+), 3',5' cyclic guanosine monophosphate (cGMP), cell division cycle 25B (Cdc25B) and Wee1 levels, specific phosphorylation (pThr-14/Tyr-15) as well as total cyclin-dependent kinases-1 (Cdk1) (PSTAIRE) levels were analyzed.

RESULTS

Postovulatory aging induced generation of NO possibly through an iNOS-mediated pathway. The increase in NO level was associated with augmented cytosolic free Ca(2+) as well as cGMP levels in aged eggs. A significant increase in Wee1 level and decrease of Cdc25B level were observed in aged eggs. An accumulation of phosphorylated Cdk1 (pThr-14/Tyr-15) level was observed in aged eggs, while total Cdk1 (PSTAIR) level remained unchanged.

CONCLUSION

Our study demonstrates that generation of NO through an iNOS-mediated pathway increases cytosolic free Ca2+and cGMP levels. High levels of these signal molecules trigger the accumulation of phosphorylated Cdk1 in aged eggs. Thus, NO signals the accumulation of phosphorylated Cdk1 and induces postovulatory aging-induced abortive SEA in the rat.

Changes in signal molecules and maturation promoting factor levels associate with spontaneous resumption of meiosis in rat oocytes

The present study was aimed to find out changes in signal molecules and maturation promoting factor (MPF) levels during meiotic cell cycle progression from diplotene and metaphase-II (M-II) arrest, a period during which oocyte achieves meiotic competency. Data suggest that high levels of adenosine 3'-5'-cyclic monophosphate (cAMP), guanosine 3'-5'-cyclic monophosphate (cGMP), and nitric oxide (NO) are associated with diplotene arrest, while reduction in their levels correlates with reduced MPF level and meiotic resumption from diplotene arrest. On the other hand, increased intracellular NO, calcium (Ca(2+) ) as well as hydrogen peroxide (H2 O2 ) levels correlate with decreased cAMP, Thr-161 phosphorylated cyclin-dependent kinase-1 (Cdk1) as well as cyclin B1 levels. The decreased Thr-161 phosphorylated Cdk1 and cyclin B1 level reduce MPF level leading to exit from M-II arrest in oocytes cultured in vitro. These data suggest that the decrease of cAMP level and increase of cytosolic free Ca(2+) as well as H2 O2 levels associate with the reduced MPF level and meiotic resumption from diplotene arrest. On the other hand, increase of NO, cGMP, Ca(2+) as well as H2 O2 levels are associated with reduced MPF and spontaneous exit from M-II arrest in rat oocytes cultured in vitro.

Structure of a DsbF homologue from Corynebacterium diphtheriae

Disulfide-bond formation, mediated by the Dsb family of proteins, is important in the correct folding of secreted or extracellular proteins in bacteria. In Gram-negative bacteria, disulfide bonds are introduced into the folding proteins in the periplasm by DsbA. DsbE from Escherichia coli has been implicated in the reduction of disulfide bonds in the maturation of cytochrome c. The Gram-positive bacterium Mycobacterium tuberculosis encodes DsbE and its homologue DsbF, the structures of which have been determined. However, the two mycobacterial proteins are able to oxidatively fold a protein in vitro, unlike DsbE from E. coli. In this study, the crystal structure of a DsbE or DsbF homologue protein from Corynebacterium diphtheriae has been determined, which revealed a thioredoxin-like domain with a typical CXXC active site. Structural comparison with M. tuberculosis DsbF would help in understanding the function of the C. diphtheriae protein.

RyR channel-mediated increase of cytosolic free calcium level signals cyclin B1 degradation during abortive spontaneous egg activation in rat

In few mammalian species including rat, post-ovulatory aging induces abortive spontaneous egg activation (SEA), which is morphologically characterized by exit from metaphase-II (M-II) arrest. A possibility exists that the RyR channel-mediated insufficient increase of cytosolic free Ca(2+) level could be one of the causes for post-ovulatory aging-induced abortive SEA. To test this possibility, eggs collected after 17 h post-hCG surge were cultured with or without various concentrations of nifedipine (NF), ruthenium red (RR), and KN-93 for 3 h in vitro. Morphological changes characteristic of abortive SEA, cytosolic free Ca(2+) level, cyclin B1 level, and meiotic status were analyzed. Data of the present study indicate that NF and RR inhibited post-ovulatory aging-induced abortive SEA in a concentration-dependent manner. Further, RR protected against RyR channel as well as caffeine-mediated increase of cytosolic free Ca(2+) level. In addition, KN-93 inhibited post-ovulatory aging-induced abortive SEA in a concentration-dependent manner. An increase of cytosolic free Ca(2+) level was associated with a reduction of cyclin B1 level during post-ovulatory aging-induced abortive SEA. These data indirectly suggest the involvement of RyR channels in the increase of cytosolic free Ca(2+) level. The increased cytosolic free Ca(2+) level triggers cyclin B1 degradation possibly through CaMK-II activity during post-ovulatory aging-induced abortive SEA in rat eggs cultured in vitro.