Abortive Spontaneous Egg Activation: An Emerging Biological Threat for the Existence of Mammals

Postovulatory Aging of Mouse Oocytes Impairs Offspring Behavior by Causing Oxidative Stress and Damaging Mitochondria

Information on long-term effects of postovulatory oocyte aging (POA) on offspring is limited. Whether POA affects offspring by causing oxidative stress (OS) and mitochondrial damage is unknown. Here, in vivo-aged (IVA) mouse oocytes were collected 9 h after ovulation, while in vitro-aged (ITA) oocytes were obtained by culturing freshly ovulated oocytes for 9 h in media with low, moderate, or high antioxidant potential. Oocytes were fertilized in vitro and blastocysts transferred to produce F1 offspring. F1 mice were mated with naturally bred mice to generate F2 offspring. Both IVA and the ITA groups in low antioxidant medium showed significantly increased anxiety-like behavior and impaired spatial and fear learning/memory and hippocampal expression of anxiolytic and learning/memory-beneficial genes in both male and female F1 offspring. Furthermore, the aging in both groups increased OS and impaired mitochondrial function in oocytes, blastocysts, and hippocampus of F1 offspring; however, it did not affect the behavior of F2 offspring. It is concluded that POA caused OS and damaged mitochondria in aged oocytes, leading to defects in anxiety-like behavior and learning/memory of F1 offspring. Thus, POA is a crucial factor that causes psychological problems in offspring, and antioxidant measures may be taken to ameliorate the detrimental effects of POA on offspring.

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

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

Oxidative Stress-Induced Overactivation of Frog Eggs Triggers Calcium-Dependent Non-Apoptotic Cell Death

Excessive activation of frog eggs (overactivation) is a pathological process that renders eggs unfertilizable. Its physiological inducers are unknown. Previously, oxidative stress was shown to cause time- and dose-dependent overactivation of Xenopus laevis frog eggs. Here, we demonstrate that the oxidative stress-induced egg overactivation is a calcium-dependent phenomenon which can be attenuated in the presence of the selective calcium chelator BAPTA. Degradation of cyclin B2, which is known to be initiated by calcium transient in fertilized or parthenogenetically activated eggs, can also be observed in the overactivated eggs. Decline in mitochondrial membrane potential, ATP depletion and termination of protein synthesis manifest in the eggs within one hour of triggering overactivation. These intracellular events occur in the absence of caspase activation. Furthermore, plasma membrane integrity is compromised in the overactivated eggs, as evidenced by ATP leakage and egg swelling. In sum, our data demonstrate that oxidative stress-induced overactivation of frog eggs causes fast and dramatic disruption of cellular homeostasis, resulting in robust and expedited cell death by a calcium-dependent non-apoptotic mechanism.

Restraint stress of female mice during oocyte development facilitates oocyte postovulatory aging

Studies suggest that psychological stress on women can impair their reproduction and that postovulatory oocyte aging (POA) might increase the risk of early pregnancy loss and affect offspring's reproductive fitness and longevity. However, whether psychological stress during oocyte development would facilitate POA is unknown but worth exploring to understand the mechanisms by which psychological stress and POA damage oocytes. This study observed effects of female restraint stress during oocyte development (FRSOD) on oocyte resistance to POA. Female mice were restrained for 48 h before superovulation, and they were sacrificed at different intervals after ovulation to recover aging oocytes for analyzing their early and late aged characteristics. The effects of FRSOD on aging oocytes included: (1) increasing their susceptibility to activation stimulus with elevated cytoplasmic calcium; (2) impairing their developmental potential with downregulated expression of development-beneficial genes; (3) facilitating degeneration, cytoplasmic fragmentation and apoptosis; (4) worsening the disorganization of cortical granules and spindle/chromosomes; and (5) impairing redox potential with increased oxidative stress. In conclusion, FRSOD impairs oocyte resistance to POA, so that stressed oocytes become aged significantly quicker than unstressed controls. Thus, couples wishing to achieve pregnancy should take steps to avoid not only fertilization of aged oocytes but also pregestational stressful life events.

Delaying Effects of Prolactin and Growth Hormone on Aging Processes in Bovine Oocytes Matured In Vitro

Aging processes accelerate dramatically in oocytes that have reached the metaphase-II (M-II) stage. The present work aimed to study the patterns and intracellular pathways of actions of prolactin (PRL) and growth hormone (GH) on age-associated changes in bovine M-II oocytes aging in vitro. To this end, we analyzed spontaneous parthenogenetic activation (cytogenetic assay), apoptosis (TUNEL assay), and the developmental capacity (IVF/IVC) of in vitro-matured oocytes after prolonged culturing. Both PRL and GH reduced the activation rate of aging cumulus-enclosed oocytes (CEOs) and denuded oocytes (DOs), and their respective hormone receptors were revealed in the ova. The inhibitor of Src-family tyrosine kinases PP2 eliminated the effects of PRL and GH on meiotic arrest in DOs, whereas the MEK inhibitor U0126 only abolished the PRL effect. Furthermore, PRL was able to maintain the apoptosis resistance and developmental competence of aging CEOs. The protein kinase C inhibitor calphostin C suppressed both the actions of PRL. Thus, PRL and GH can directly support meiotic arrest in aging M-II oocytes by activating MAP kinases and/or Src-family kinases. The effect of PRL in maintaining the developmental capacity of aging oocytes is cumulus-dependent and related to the pro-survival action of the protein kinase C-mediated signal pathway.

Modulation of Intracellular ROS and Senescence-Associated Phenotypes of Xenopus Oocytes and Eggs by Selective Antioxidants

Aging of oocytes and eggs diminishes their reproductive and developmental potential. It has been demonstrated previously that reactive oxygen species (ROS) contribute to accelerated aging of various cells. In the present study, we measured intracellular levels of ROS and investigated effects of several selective antioxidants (AOXs) on the viability and functional activity of aging oocytes and eggs of the African clawed frog Xenopus laevis. The fluorescent cell-permeable dye DCFDA, which is widely employed for ROS detection in cultured mammalian cells, was used to monitor ROS levels in the fresh and bench-aged oocytes and eggs by an optimized protocol. It was found that intracellular ROS contents were increased in frog oocytes and eggs aged for 48 h. It was further demonstrated using selective cell-permeable AOXs targeting different ROS-generating mechanisms, that the major source of ROS in Xenopus oocytes and eggs is the plasma membrane NADPH oxidase, and that mitochondrial generation contributes to the intracellular ROS content to a lesser extent. Targeted inhibition of NADPH oxidase with a natural organic compound apocynin reduced ROS levels significantly in Xenopus oocytes and eggs, maintained their normal phenotype and supported their functional competence. To our knowledge this is the first report concerning beneficial effects of apocynin on the isolated gamete cells, such as oocytes and eggs.

Role of calcium-sensing receptor in regulating spontaneous activation of postovulatory aging rat oocytes

Mechanisms for postovulatory aging (POA) of oocytes and for spontaneous activation (SA) of rat oocytes are largely unknown. Expression of calcium-sensing receptor (CaSR) in rat oocytes and its role in POA remain unexplored. In this study, expression of CaSR in rat oocytes aging for different times was detected by immunofluorescence microscopy, and western blotting and the role of CaSR in POA was determined by observing the effects of regulating its activity on SA susceptibility and cytoplasmic calcium levels. The results showed that CaSR was expressed in rat oocytes. Oocytes recovered 19 h post human chorionic gonadotropin (hCG) injection were more susceptible to SA and expressed more functional CaSR than oocytes recovered 13 h after hCG injection, although both expressed the same level of total CaSR protein. Treatment with CaSR antagonist significantly suppressed cytoplasmic calcium elevation and SA of oocytes. Activation of Na-Ca2+ exchanger with NaCl inhibited SA to a greater extent than suppression of CaSR with NPS-2143, suggesting that calcium sources other than CaSR-controlled channels contributed to the elevation of cytoplasmic calcium. Treatment with T- or L-type calcium channel blockers significantly reduced SA. Suppression of all calcium channels tested reduced SA to minimum. It is concluded that the level of CaSR functional dimer protein, but not that of the total CaSR protein, was positively correlated with the SA susceptibility during POA of rat oocytes confirming that CaSR is involved in POA regulation. Blocking multiple calcium channels might be a better choice for efficient control of SA in rat oocytes.

Role of granulosa cell mitogen-activated protein kinase 3/1 in gonadotropin-mediated meiotic resumption from diplotene arrest of mammalian oocytes

In mammals, preovulatory oocytes are encircled by several layers of granulosa cells (GCs) in follicular microenvironment. These follicular oocytes are arrested at diplotene arrest due to high level of cyclic nucleotides from encircling GCs. Pituitary gonadotropin acts at the level of encircling GCs and increases adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) and activates mitogen-activated protein kinase 3/1 (MAPK3/1) signaling pathway. The MAPK3/1 disrupts the gap junctions between encircling GCs and oocyte. The disruption of gap junctions interrupts the transfer of cyclic nucleotides to the oocyte that results a drop in intraoocyte cAMP level. A transient decrease in oocyte cAMP level triggers maturation promoting factor (MPF) destabilization. The destabilized MPF finally triggers meiotic resumption from diplotene arrest in follicular oocyte. Thus, MAPK3/1 from GCs origin plays important role in gonadotropin-mediated meiotic resumption from diplotene arrest in follicular oocyte of mammals.

Human Chorionic Gonadotropin Mediated Generation of Reactive Oxygen Species Is Sufficient to Induce Meiotic Exit but Not Apoptosis in Rat Oocytes

Generation of reactive oxygen species (ROS) is associated with final stages of follicular development and ovulation in mammals. The human chorionic gonadotropin (hCG) mimics the action of luteinizing hormone and triggers follicular development and ovulation. However, it remains unclear whether hCG induces generation of ROS, if yes, whether hCG-mediated increased level of ROS could induce meiotic exit and/or apoptosis in rat oocytes. For this purpose, cumulus–oocyte complexes (COCs) were collected from ovary of experimental rats injected with 20 IU pregnant mare's serum gonadotropin for 48 h followed by 20 IU hCG for 0, 7, 14, and 21 h. The morphological changes in COCs, meiotic status of oocyte, total ROS, hydrogen peroxide (H₂O₂), inducible nitric oxide synthase (iNOS), nitric oxide (NO), Bax, Bcl-2, cytochrome c, telomerase reverse transcriptase (TERT) expression levels, and DNA fragmentation were analyzed in COCs. Our data suggest that hCG surge increased total ROS as well as H₂O₂ levels but decreased iNOS expression and total NO level in oocytes. The hCG-mediated increased level of ROS was sufficient to induce meiotic cell cycle resumption in majority of oocytes as evidenced by meiotic exit from diplotene as well as metaphase-II (M-II) arrest and their meiotic status. However, increase of ROS level due to hCG surge was not sufficient to trigger Bax and cytochrome c expression levels and DNA fragmentation in COCs. In addition, increased TERT activity was observed in oocytes collected 21 h post-hCG surge showing onset of oocyte aging. Taken together, these results suggest that hCG induces generation of ROS sufficient to trigger meiotic exit from diplotene, as well as M-II arrest, but not good enough to induce apoptosis in rat oocytes.

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

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