Regulation of dynamic events by microfilaments during oocyte maturation and fertilization

C-Phycocyanin improves the quality of goat oocytes after in vitro maturation and vitrification

In vitro maturation (IVM) and cryopreservation of goat oocytes are important for establishing a valuable genetic bank for domesticated female animals and improving livestock reproductive efficiency. C-Phycocyanin (PC) is a Spirulina extract with antioxidant, antiinflammatory, and radical scavenging properties. However, whether PC has positive effect on goat oocytes IVM or developmental competence after vitrification is still unknown. In this study, we found that first polar body extrusion (n = 293), cumulus expansion index (n = 269), and parthenogenetic blastocyst formation (n = 281) were facilitated by adding 30 μg/mL PC to the oocyte maturation medium when compared with the control groups and that supplemented with 3, 10, 100 or 300 μg/mL PC (P < 0.05). Although PC supplementation did not affect spindle formation or chromosome alignment (n = 115), it facilitated or improved cortical granules migration (n = 46, P < 0.05), mitochondria distribution (n = 39, P < 0.05), and mitochondrial membrane potential (n = 46, P < 10-4). Meanwhile, supplementation with 30 μg/mL PC in the maturation medium could significantly inhibit the reactive oxygen species accumulation (n = 65, P < 10-4), and cell apoptosis (n = 42, P < 0.05). In addition, PC increased the oocyte mRNA levels of GPX4 (P < 0.01), and decreased the mRNA and protein levels of BAX (P < 0.01). Next, we investigated the effect of PC supplementation in the vitrification solution on oocyte cryopreservation. When compared with the those equilibrate in the vitrification solution without PC, recovered oocytes in the 30 μg/mL PC group showed higher ratios of normal morphology (n = 85, P < 0.05), survival (n = 85, P < 0.05), first polar body extrusion (n = 62, P < 0.05), and parthenogenetic blastocyst formation (n = 107, P < 0.05). Meanwhile, PC supplementation of the vitrification solution increased oocyte mitochondrial membrane potential (n = 53, P < 0.05), decreased the reactive oxygen species accumulation (n = 73, P < 0.05), promoted mitochondria distribution (n = 58, P < 0.05), and inhibited apoptosis (n = 46, P < 10-3). Collectively, our findings suggest that PC improves goat oocyte IVM and vitrification by reducing oxidative stress and early apoptosis, which providing a novel strategy for livestock gamete preservation and utilization.

FMNL2 regulates actin for endoplasmic reticulum and mitochondria distribution in oocyte meiosis

During mammalian oocyte meiosis, spindle migration and asymmetric cytokinesis are unique steps for the successful polar body extrusion. The asymmetry defects of oocytes will lead to the failure of fertilization and embryo implantation. In present study, we reported that an actin nucleating factor Formin-like 2 (FMNL2) played critical roles in the regulation of spindle migration and organelle distribution in mouse and porcine oocytes. Our results showed that FMNL2 mainly localized at the oocyte cortex and periphery of spindle. Depletion of FMNL2 led to the failure of polar body extrusion and large polar bodies in oocytes. Live-cell imaging revealed that the spindle failed to migrate to the oocyte cortex, which caused polar body formation defects, and this might be due to the decreased polymerization of cytoplasmic actin by FMNL2 depletion in the oocytes of both mice and pigs. Furthermore, mass spectrometry analysis indicated that FMNL2 was associated with mitochondria and endoplasmic reticulum (ER)-related proteins, and FMNL2 depletion disrupted the function and distribution of mitochondria and ER, showing with decreased mitochondrial membrane potential and the occurrence of ER stress. Microinjecting Fmnl2-EGFP mRNA into FMNL2-depleted oocytes significantly rescued these defects. Thus, our results indicate that FMNL2 is essential for the actin assembly, which further involves into meiotic spindle migration and ER/mitochondria functions in mammalian oocytes.

Astaxanthin Added during Post-Warm Recovery Mitigated Oxidative Stress in Bovine Vitrified Oocytes and Improved Quality of Resulting Blastocysts

Various antioxidants are tested to improve the viability and development of cryopreserved oocytes, due to their known positive health effects. The aim of this study was to find whether astaxanthin (AX), a xanthophyll carotenoid, could mitigate deteriorations that occurred during the vitrification/warming process in bovine oocytes. Astaxanthin (2.5 µM) was added to the maturation medium during the post-warm recovery period of vitrified oocytes for 3 h. Afterward, the oocytes were fertilized in vitro using frozen bull semen and presumptive zygotes were cultured in the B2 Menezo medium in a co-culture with BRL-1 cells at 38.5 °C and 5% CO2 until the blastocyst stage. AX addition significantly reduced ROS formation, lipid peroxidation, and lysosomal activity, while increasing mitochondrial activity in vitrified oocytes. Although the effect of AX on embryo development was not observed, it stimulated cell proliferation in the blastocysts derived from vitrified oocytes and improved their quality by upregulation or downregulation of some genes related to apoptosis (BCL2, CAS9), oxidative stress (GPX4, CDX2), and development (GJB5) compared to the vitrified group without AX. Therefore, the antioxidant properties of astaxanthin even during short exposure to bovine vitrified/warmed oocytes resulted in improved blastocyst quality comparable to those from fresh oocytes.

Reorganization of the mouse oocyte' cytoskeleton after cultivation under simulated weightlessness

Female germ cells provide the structural basis for the development of a new organism, while the main molecular mechanisms of the impact of weightlessness on the cell remain unknown. The aim of this work was to determine the relative content and distribution of the main proteins of microtubules and microfilaments, to assess the relative RNA content of genes in mouse oocytes after short-term exposure to simulated microgravity, and to determine the potential for embryo development up to the 3-cell stage. Before starting the study, BALB/c mice were divided into two groups. One group received water and standard food without any modifications. Before exposure to simulated microgravity, the oocytes of these animals were randomly divided into two groups - c and µg. The second group of animals additionally received essential phospholipids containing at least 80% phosphatidylcholines, per os for 6 weeks before the start of the experiment at a dosage of 350 mg/kg of the animal's body to modify the lipid composition of the oocyte membrane. The obtained oocytes of these animals were also randomly divided into two groups - ce and µge. To determine the protein distribution and its relative content, immunofluorescence analysis was performed, and the RNA content of genes was assessed using real-time PCR with reverse transcription. After cultivation under simulated microgravity, beta-actin and acetylated alpha-tubulin are redistributed from the cortical layer to the central part of the oocyte, and the relative content of acetylated alpha-tubulin and tubulin isoforms decreases. At the same time, the mRNA content of most genes encoding cytoskeletal proteins was significantly higher in comparison with the control level. The use of essential phospholipids led to a decrease in the content of cellular cholesterol in the oocyte and leveled changes in the content and redistribution of acetylated alpha-tubulin and beta-actin after cultivation under simulated microgravity. In addition, after in vitro fertilization and further cultivation under simulated weightlessness, we observed a decrease in the number of embryos that passed the stage of the 2-cell embryo, but while taking essential phospholipids, the number of embryos that reached the 3-cell stage did not differ from the control group. The results obtained show changes in the content and redistribution of cytoskeletal proteins in the oocyte, which may be involved in the process of pronucleus migration, the formation of the fission spindle and the contractile ring under simulated weightlessness, which may be important for normal fertilization and cleavage of the future embryo.

The oocyte: the key player in the success of assisted reproduction technologies

The ovulation of a mature oocyte at metaphase II of meiosis, with optimal potential to undergo fertilisation by a sperm cell, complete meiosis and sustain the switch to mitotic division, and support early embryo development, involves a protracted and disrupted/delayed series of processes. Many of these are targeted for exploitation in vivo , or recapitulation in vitro , by the livestock industry. Reproductive technologies, including AI, multiple ovulation embryo transfer, ovum pick-up, in vitro embryo production, and oestrus and ovulation synchronisation, offer practitioners and producers the opportunity to produce offspring from genetically valuable dams in much greater numbers than they would normally have in their lifetime, while in vitro oocyte and follicle culture are important platforms for researchers to interrogate the physiological mechanisms driving fertility. The majority of these technologies target the ovarian follicle and the oocyte within; thus, the quality and capability of the recovered oocyte determine the success of the reproductive intervention. Molecular and microscopical technologies have grown exponentially, providing powerful platforms to interrogate the molecular mechanisms which are integral to or affected by ART. The development of the bovine oocyte from its differentiation in the ovary to ovulation is described in the light of its relevance to key aspects of individual interventions, while highlighting the historical timeline.

Heat Stress: A Serious Disruptor of the Reproductive Physiology of Dairy Cows

Global warming is a significant threat to the sustainability and profitability of the dairy sector, not only in tropical or subtropical regions but also in temperate zones where extreme summer temperatures have become a new and challenging reality. Prolonged exposure of dairy cows to high temperatures compromises animal welfare, increases morbidity, and suppresses fertility, resulting in devastating economic losses for farmers. To counteract the deleterious effects of heat stress, cattl e employ various adaptive thermoregulatory mechanisms including molecular, endocrine, physiological, and behavioral responses. These adaptations involve the immediate secretion of heat shock proteins and cortisol, followed by a complex network of disrupted secretion of metabolic and reproductive hormones such as prolactin, ghrelin, ovarian steroid, and pituitary gonadotrophins. While the strategic heat stress mitigation measures can restore milk production through modifications of the microclimate and nutritional interventions, the summer fertility records remain at low levels compared to those of the thermoneutral periods of the year. This is because sustainment of high fertility is a multifaceted process that requires appropriate energy balance, undisrupted mode of various hormones secretion to sustain the maturation and fertilizing competence of the oocyte, the normal development of the early embryo and unhampered maternal-embryo crosstalk. In this review, we summarize the major molecular and endocrine responses to elevated temperatures in dairy cows, as well as the impacts on maturing oocytes and early embryos, and discuss the consequences that heat stress brings about in dairy cattle fertility.

Mitochondria in reproduction

In reproduction, mitochondria produce bioenergy, help to synthesize biomolecules, and support the ovaries, oogenesis, and preimplantation embryos, thereby facilitating healthy live births. However, the regulatory mechanism of mitochondria in oocytes and embryos during oogenesis and embryo development has not been clearly elucidated. The functional activity of mitochondria is crucial for determining the quality of oocytes and embryos; therefore, the underlying mechanism must be better understood. In this review, we summarize the specific role of mitochondria in reproduction in oocytes and embryos. We also briefly discuss the recovery of mitochondrial function in gametes and zygotes. First, we introduce the general characteristics of mitochondria in cells, including their roles in adenosine triphosphate and reactive oxygen species production, calcium homeostasis, and programmed cell death. Second, we present the unique characteristics of mitochondria in female reproduction, covering the bottleneck theory, mitochondrial shape, and mitochondrial metabolic pathways during oogenesis and preimplantation embryo development. Mitochondrial dysfunction is associated with ovarian aging, a diminished ovarian reserve, a poor ovarian response, and several reproduction problems in gametes and zygotes, such as aneuploidy and genetic disorders. Finally, we briefly describe which factors are involved in mitochondrial dysfunction and how mitochondrial function can be recovered in reproduction. We hope to provide a new viewpoint regarding factors that can overcome mitochondrial dysfunction in the field of reproductive medicine.

Transcriptome sequencing reveals ovarian immune response and development during female sperm storage in viviparous black rockfish (Sebastes schlegelii)

Black rockfish (Sebastes schlegelii) is a viviparous teleost fish whose spermatozoa were transferred into the female ovary cavity and stored for up to five months and then fertilized with the matured eggs. There is no clarity about the molecular characteristics of ovarian follicles during female sperm storage in Sebastes schlegelii. In this study, histological observation, transcriptomic analysis and hormone level detection were performed in ovaries at stages of pre-mating (PRM), post-mating (POM) and pre-fertilization (PRF). Histological observation displayed that oocytes developed from the primary growth (PG) stage to the mature stage during the three stages. Furthermore, somatic cells around the oocyte were proliferated and spermatozoa were found near the layer of epithelial cells. Transcriptomic analysis showed that there were 437 and 747 differentially expressed genes (DEGs) in ovarian comparison of PRM-vs-POM and POM-vs-PRF, respectively. GO enrichment and KEGG analysis revealed that lots of DEGs from PRM-vs-POM were linked to immune-related pathways, such as antigen processing and presentation, immune response, and complement and coagulation cascade. Meanwhile, seven DEGs associated with immune response were differentially expressed after spermatozoa treatment in ovarian tissue in vitro. While the DEGs from POM-vs-PRF were mostly enriched in the pathways related to homeostasis maintenance and cellular junction and metabolism. In addition, we found increased estrogen (E2) and 11-ketotestosterone (11-KT) level and decreased testosterone level in ovarian follicles during the sperm storage period by ELISA, suggesting that sex hormones are involved in the dynamic change of ovarian follicles. In total, this study could provide new hints for understanding the immune adaption and developmental signatures of ovarian follicles post copulation in black rockfish and other viviparous fish.

Downregulation of PI3K/AKT/mTOR Pathway in Juglone-Treated Bovine Oocytes

We have previously reported that juglone, a natural compound found in Juglandaceae with a wide range of biological activities, can reduces the developmental competence of bovine oocytes. In the current study, we investigated the possible mechanisms behind the toxicity of juglone and the relationship with PI3K/AKT/mTOR signaling during the in vitro maturation (IVM) of oocytes. Results show that oocyte exposure to juglone was associated with a significant decrease in filamentous actin (F-actin) accumulation. The RT-qPCR showed downregulation of the meiosis progression indicator GSK-3A, oocyte development marker BMP15, mitochondria fusion controlling MFN1, oxidative stress-related OGG1, and histone methylation-related EZH1, EZH2, SUZ12, G9a, and SUV39H2 genes in juglone-treated oocytes. In addition, glycolysis- (PFK1 and GLUT1), ATP synthesis- (ATPase8 and ATP5F1B), and OXPHOS-specific markers (SDHA and SDHD), as well as the oocyte survival regulators (SOD2, VEGF, and MAPK1) significantly decreased upon juglone treatment. Moreover, lower expression of PI3K, AKT, and mTOR was observed at the transcriptional and/or translational level(s). The autophagy markers LC3B and beclin-1 as well as the DNA damage-specific marker 8-OxoG displayed overexpression in juglone-exposed oocytes. Taken together, our results show that administration of juglone during the IVM can reduce the quality and developmental health of bovine oocytes through downregulation of the PI3K/AKT/mTOR pathway and its downstream signaling cascades.

MOS mutation causes female infertility with large polar body oocytes

The Moloney sarcoma oncogene (MOS) encodes a protein serine/threonine kinase and MOS is expressed at high levels in oocytes undergoing meiotic maturation. The MOS/MAPK pathway is normally required for the maintenance of microtubules and chromatin in a metaphasic state during the meiotic divisions. To determine the pathogenic genes in a female infertile patient due to large polar body oocytes, whole-exome sequencing was performed on the patient and available family members. We identified a novel homozygous missense mutation c.591T > G in MOS. Bioinformatics analysis showed that the mutation is harmful. These findings suggest that MOS mutation results in oocytes with a large polar body and poor embryonic development in patients. The MOS variant may regulate oocyte asymmetric division by MAPK/WAVE2/Arp2/3/actin signaling pathway. This will help to understand the comprehensive role of MOS in early human reproductive process and provide genetic markers for future genetic counseling for more individualized treatments.

Bromoacetic acid impairs mouse oocyte in vitro maturation through affecting cytoskeleton architecture and epigenetic modification

As a major public health achievement, disinfection of drinking water significantly decreases outbreaks of waterborne disease, but produces drinking water disinfection by-products (DBPs) unfortunately. The haloacetic acids (HAAs) including bromoacetic acid (BAA), the second major class of DBPs, are considered as a global public health concern. BAA has been identified as cytotoxic, genotoxic, mutagenic, carcinogenic, and teratogenic in somatic cells. However, the toxic effects of BAA on oocyte maturation remain obscure. Herein, we documented that exposure to BAA compromised mouse oocyte maturation in vitro, causing blocked polar body extrusion (PBE). Meiotic progression analysis demonstrated that exposure to BAA induced the activated spindle assembly checkpoint (SAC) mediated metaphase I (MI) arrest in oocytes. Further study revealed that exposure to BAA resulted in the hyperacetylation of α-tubulin, disrupting spindle assembly and chromosome alignment, which is responsible for the activation of SAC. Besides, the organization of actin, the other major component of cytoskeleton in oocytes, was disturbed after BAA exposure. In addition, exposure to BAA altered the status of histone H3 methylation and 5 mC, indicative of the damaged epigenetic modifications. Moreover, we found that exposure to BAA induced DNA damage in a dose-dependent manner in oocytes. Collectively, our study evidenced that exposure to BAA intervened mouse oocyte maturation via disrupting cytoskeletal dynamics, damaging epigenetic modifications and inducing accumulation of DNA damage.

Leukemia inhibitory factor enhances the development and subsequent blastocysts quality of yak oocytes in vitro

Leukemia inhibitory factor (LIF) is a multipotent cytokine of the IL-6 family which plays a critical role in the maturation and development of oocytes. This study evaluated the influence of LIF on the maturation and development ability of yak oocytes, and the quality of subsequent blastocysts under in vitro culture settings. Different concentrations of LIF (0, 25, 50, and 100 ng/mL) were added during the in vitro culture of oocytes to detect the maturation rate of oocytes, levels of mitochondria, reactive oxygen species (ROS), actin, and apoptosis in oocytes, mRNA transcription levels of apoptosis and antioxidant-related genes in oocytes, and total cell number and apoptosis levels in subsequent blastocysts. The findings revealed that 50 ng/mL LIF could significantly increase the maturation rate (p < 0.01), levels of mitochondria (p < 0.01) and actin (p < 0.01), and mRNA transcription levels of anti-apoptotic and antioxidant-related genes in yak oocytes. Also, 50 ng/mL LIF could significantly lower the generation of ROS (p < 0.01) and apoptosis levels of oocytes (p < 0.01). In addition, blastocysts formed from 50 ng/mL LIF-treated oocytes showed significantly larger total cell numbers (p < 0.01) and lower apoptosis rates (p < 0.01) than the control group. In conclusion, the addition of LIF during the in vitro maturation of yak oocytes improved the quality and the competence of maturation and development in oocytes, as well as the quality of subsequent blastocysts. The result of this study provided some insights into the role and function of LIF in vitro yak oocytes maturation, as well as provided fundamental knowledge for assisted reproductive technologies in the yak.

1-Nitropyrene exposure induces mitochondria dysfunction and impairs oocyte maturation in mice

Oocyte quality is essential for a successful pregnancy. 1-Nitropyrene (1-NP) is a widely distributed pollutant in the environment and is well-known for its mutagenicity and carcinogenicity. However, whether 1-NP has toxic effects on mammalian oocyte quality remains unknown. In the present study, we focused on the effect of 1-NP on oocyte maturation using mouse oocytes as an in vitro model. Our study showed that 1-NP exposure disrupted the meiotic spindle assembly and caused chromosome misalignment, further impaired first polar body extrusion, and significantly decreased the fertilization capability in mouse oocytes. Further investigation showed that the mitochondrial membrane potential (MMP) and ATP levels were decreased, and the expression of genes encoding components of the mitochondrial respiratory chain was inhibited in 1-NP exposed oocytes. Meanwhile, 1-NP exposure increased the levels of reactive oxygen species (ROS), inhibited the expression of genes encoding antioxidant enzymes, and increased the frequency of early apoptotic oocytes. Overall, our data suggest that 1-NP exposure disrupts mitochondrial function and intracellular redox balance, ultimately impairing oocyte maturation. These findings reveal the adverse effect of 1-NP exposure on oocyte quality.

Administration of nicotinamide mononucleotide improves oocyte quality of obese mice

Objectives

Obesity has become a common health concern around the world. Maternal obesity could cause poor reproductive outcomes due to chronic ovarian inflammation and decreased oocyte quality. However, the strategies to improve the poor reproductive outcomes of obese females have not been fully studied. In this study, we aimed to explore the effects and underlying mechanisms of nicotinamide mononucleotide (NMN) on oocyte quality and reproductive performance of obese mice.

Materials and Methods

The obese mouse model was established by feeding high‐fat diet which was confirmed by body weight record, fasting blood glucose test and oral glucose tolerance test. The expression of ovary development related genes and inflammation related genes, including Lhx8, Bmp4, Adgre1, Ccl2, TNF‐α, Gal‐3, Clec10a and IL‐10 in ovaries and the expression of Bax and Sod1 in oocytes were detected using quantitative reverse transcription PCR (RT‐qPCR). The adipose size of abdominal fat tissue was determined with haematoxylin and eosin (H&E) staining. Immunofluorescence staining was performed to measure the ROS level, spindle/chromosome structure, mitochondrial function, actin dynamics and DNA damage of oocytes.

Results

The administration of NMN restored ovarian weight and reduced the adipose size of abdominal fat tissue and ovarian inflammation in high fat diet (HFD) mice. Furthermore, NMN treatment improved the oocytes quality partially by restoring the mitochondrial function and actin dynamics, reducing meiotic defects, DNA damage and ROS level and lipid droplet distribution of oocytes in HFD mice. On the long‐term effect, NMN restored offspring body weight of HFD mice.

Conclusion

NMN could improve the oocyte quality of HFD‐induced obese mice.

Signaling Proteins Recruited to the Sperm Binding Site: Role of β-Catenin and Rho A

Sperm interaction with the oocyte plasma membrane triggers a localized response in the mouse oocyte that leads to remodeling of oocyte surface as well as the underlying cortical actin layer. The recent demonstration that PTK2B is recruited and activated at the sperm binding site raised the possibility that multiple signaling events may be activated during this stage of fertilization. The present study demonstrated that β-catenin and Rho A were recruited to the cortex underlying bound/fused sperm. To determine whether sperm-oocyte contact was sufficient to initiate β-catenin recruitment, Cd9-null, and PTK2b-null oocytes were tested for the ability to recruit β-catenin to sperm binding sites. Both Cd9 and Ptk2b ablation reduced β-catenin recruitment raising the possibility that PTK2B may act downstream of CD9 in the response to sperm binding/fusion. Further immunofluorescence study revealed that β-catenin co-localized with f-actin in the interstitial regions between actin layer fenestrae. Rho A, in contrast, was arranged underneath the actin layer in both the fenestra and the interstitial regions suggesting that they may play different roles in the oocyte.

Proteomic Exploration of Porcine Oocytes During Meiotic Maturation in vitro Using an Accurate TMT-Based Quantitative Approach

The dynamic changes in protein expression are well known to be required for oocyte meiotic maturation. Although proteomic analysis has been performed in porcine oocytes during in vitro maturation, there is still no full data because of the technical limitations at that time. Here, a novel tandem mass tag (TMT)-based quantitative approach was used to compare the proteomic profiles of porcine immature and in vitro mature oocytes. The results of our study showed that there were 763 proteins considered with significant difference−450 over-expressed and 313 under-expressed proteins. The GO and KEGG analyses revealed multiple regulatory mechanisms of oocyte nuclear and cytoplasmic maturation such as spindle and chromosome configurations, cytoskeletal reconstruction, epigenetic modifications, energy metabolism, signal transduction and others. In addition, 12 proteins identified with high-confidence peptide and related to oocyte maturation were quantified by a parallel reaction monitoring technique to validate the reliability of TMT results. In conclusion, we provided a detailed proteomics dataset to enrich the understanding of molecular characteristics underlying porcine oocyte maturation in vitro.

Altered microfilament dynamics contribute to the formation of diploid metaphase spindles in porcine oocytes which fail to reach the metaphase-II stage during in vitro maturation

Premature meiotic arrest during in vitro maturation (IVM) of porcine oocytes after germinal vesicle breakdown is associated with microfilament degradation. We aimed to clarify (1) if such arrest occurs at the metaphase-I (MI) stage or the oocyte progresses to a so-called diploid metaphase-II (MII) stage and (2) if microfilament degradation is the cause or result of the meiotic arrest. The number and morphology of chromosomes in oocytes showing premature meiotic arrest at 44 h IVM (38 monovalents) was similar to those cultured in the presence of the actin polymerization-inhibitor cytochalasin-B, but different from those of MI-stage (19 bivalents), and MII-stage oocytes (19 monovalents) at 33 and 44 h of IVM, respectively. Immunostaining revealed similar frequencies of microfilament degradation in prematurely arrested and cytochalasin-B-treated oocytes (58.7% and 57.2%, respectively), which were higher (P < 0.05) than those in MI- and MII-stage oocytes (10.6% and 6.8%, respectively). Induction of MI-arrest by nocodazole did not affect microfilament morphology. ATP and mRNA levels of microfilament-related genes in oocytes were similar among all groups. These results suggest that altered microfilament dynamics contribute to the formation of diploid metaphase spindles in oocytes, which fail to reach the MII stage. However, the cause of microfilament degeneration remains unclear.

Fusexins, HAP2/GCS1 and Evolution of Gamete Fusion

Gamete fusion is the climax of fertilization in all sexually reproductive organisms, from unicellular fungi to humans. Similarly to other cell-cell fusion events, gamete fusion is mediated by specialized proteins, named fusogens, that overcome the energetic barriers during this process. In recent years, HAPLESS 2/GENERATIVE CELL-SPECIFIC 1 (HAP2/GCS1) was identified as the fusogen mediating sperm-egg fusion in flowering plants and protists, being both essential and sufficient for the membrane merger in some species. The identification of HAP2/GCS1 in invertebrates, opens the possibility that a similar fusogen may be used in vertebrate fertilization. HAP2/GCS1 proteins share a similar structure with two distinct families of exoplasmic fusogens: the somatic Fusion Family (FF) proteins discovered in nematodes, and class II viral glycoproteins (e.g., rubella and dengue viruses). Altogether, these fusogens form the Fusexin superfamily. While some attributes are shared among fusexins, for example the overall structure and the possibility of assembly into trimers, some other characteristics seem to be specific, such as the presence or not of hydrophobic loops or helices at the distal tip of the protein. Intriguingly, HAP2/GCS1 or other fusexins have neither been identified in vertebrates nor in fungi, raising the question of whether these genes were lost during evolution and were replaced by other fusion machinery or a significant divergence makes their identification difficult. Here, we discuss the biology of HAP2/GCS1, its involvement in gamete fusion and the structural, mechanistic and evolutionary relationships with other fusexins.

LRRK2 regulates actin assembly for spindle migration and mitochondrial function in mouse oocyte meiosis

Leucine-rich-repeat kinase 2 (LRRK2) belongs to the Roco GTPase family and is a large multidomain protein harboring both GTPase and kinase activities. LRRK2 plays indispensable roles in many processes, such as autophagy and vesicle trafficking in mitosis. In this study, we showed the critical roles of LRRK2 in mammalian oocyte meiosis. LRRK2 is mainly accumulated at the meiotic spindle periphery during oocyte maturation. Depleting LRRK2 led to the polar body extrusion defects and also induced large polar bodies in mouse oocytes. Mass spectrometry analysis and co-immunoprecipitation results showed that LRRK2 was associated with several actin-regulating factors, such as Fascin and Rho-kinase (ROCK), and depletion of LRRK2 affected the expression of ROCK, phosphorylated cofilin, and Fascin. Further analysis showed that LRRK2 depletion did not affect spindle organization but caused the failure of spindle migration, which was largely due to the decrease of cytoplasmic actin filaments. Moreover, LRRK2 showed a similar localization pattern to mitochondria, and LRRK2 was associated with several mitochondria-related proteins. Indeed, mitochondrial distribution and function were both disrupted in LRRK2-depleted oocytes. In summary, our results indicated the critical roles of LRRK2 in actin assembly for spindle migration and mitochondrial function in mouse oocyte meiosis.

Perfluorooctane sulfonate affects mouse oocyte maturation in vitro by promoting oxidative stress and apoptosis induced bymitochondrial dysfunction

Perfluorooctane sulphonate (PFOS), as a surfactant, is widely applied in the agricultural production activities and has become a potential menace to human health. The mechanism of its effect on the maturation of mammalian oocytes is unclear. This study explored the toxic effect of PFOS on mouse oocyte maturation in vitro. The results revealed that PFOS under a concentration of 600 μM could significantly reduce the polar body extrusion rate (PBE) of mouse oocytes and cause symmetrical cell division. Further experiments showed that PFOS resulted in the abnormal cytoskeleton of the oocytes, causing the abnormal spindles and misplaced chromosomes, as well as the impaired dynamics of actin. Moreover, PFOS exposure inhibited the process of oocyte meiosis, which reflected in the slower spindle migration and continuous activation of spindle assembly checkpoint (SAC), then ultimately increased the probability of aneuploidy. Most importantly, PFOS exposure reduced the quality of oocytes, specifically by disrupting the function of mitochondria, inducing cell oxidative stress, and triggering early apoptosis. Furthermore, the level of methylation of histones is additionally influenced. In summary, our findings showed that PFOS exposure interfered with the maturation of mouse oocytes through affecting cytoskeletal dynamics, meiotic progression, oocyte quality, and histone modifications.

Transcriptomic study of the mechanism by which the Kai Yu Zhong Yu recipe improves oocyte quality in a stressed mouse model

ETHNOPHARMACOLOGICAL RELEVANCE

The Kai Yu Zhong Yu recipe (KYZY) is a classic herbal formula in traditional Chinese medicine (TCM) that has been used to treat infertility associated with psychological stress for more than three hundred years.

AIM OF THE STUDY

Psychological stress has major impacts on fertility, with variable outcomes depending on the nature, strength, and duration of the stress. Stress can directly disturb ovulation, oocyte quality, maturation, and embryo development. The aim of this study is to investigate the molecular mechanism by which KYZY improves oocyte developmental potential under psychological stress.

MATERIALS AND METHODS

ICR female mice aged 4-5 weeks were randomly divided into five groups: control, stressed in the chronic unpredictable stress model (CUSM), and stressed plus KYZY treatment at 38.2 g/kg (KYZYH), 19.1 g/kg (KYZYM), or 9.6 g/kg (KYZYL). Ovary function was assessed by measuring serum levels of estradiol (E2), luteinizing hormone (LH), follicle-stimulating hormone (FSH), and anti-Müllerian hormone (AMH). Oocyte quality was evaluated in terms of reactive oxygen species (ROS) levels, apoptotic DNA fragmentation, and mitochondria distribution. We used RNA sequencing (RNAseq) to identify differentially expressed genes (DEGs) between groups and then further analyzed the DEGs for gene ontology (GO) term enrichment and protein-protein interactions.

RESULTS

Mice in the stressed group had reduced serum E2, LH, and FSH as well as increased ROS levels, increased apoptosis, and disturbed mitochondria distribution in oocytes. Treatment with KYZY at all three doses reversed or ameliorated these negative effects of stress. DEG analysis identified 187 common genes between the two comparisons (stressed vs. control and KYZYM vs. stressed), 33 of which were annotated with six gene ontology (GO)'s biological process (BP) terms: cell differentiation, apoptosis, ATP synthesis, protein homo-oligomerization, neuron migration, and negative regulation of peptidase activity. Protein-protein interaction network analysis of DEGs identified key hub genes. Notably, the genes Atp5o and Cyc1 were both involved in the ATP synthesis and among the top three hub genes, suggesting that regulation of oocyte mitochondrial electron transport and ATP synthesis is important in the response to stress and also is a possible mechanism of action for KYZY.

CONCLUSIONS

KYZY was effective in ameliorating the adverse effects of stress on oocyte competence, possibly by targeting the mitochondrial respiratory chain and ATP synthase.

Acrylamide impairs the developmental potential of germinal vesicle oocytes by inducing mitochondrial dysfunction and autophagy/apoptosis in mice

Background: Acrylamide (ACR), an important endogenous contaminant in carbohydrate-rich foods, has been involved in various negative effects on multiple organ networks, including the reproductive system. Previous studies have reported that ACR affects oocyte quality and fertility. Purpose: This study aimed to explore the toxic effects and regulatory mechanisms of ACR on mouse germinal vesicle (GV) oocytes. Research Design: In this study, adult female mice were exposed to ACR at 10 mg/kg/day/body weight through their drinking water continuously for 4 weeks. Study Sample and Data Analysis: The mitochondrial function, autophagy/apoptosis, and development potential of GV oocytes were investigated. Results: The results showed that ACR reduced the oocyte diameter, sperm-binding ability, parthenogenetic activation and in vitro fertilization (IVF) rate, and development potential of pre-implantation embryos. We also found that ACR exposure disrupted chromatin configuration, mitochondrial distribution, and membrane potential (Δφm) of oocytes. Actin filament expression was significantly reduced in both the membrane and cytoplasm of mouse oocytes. Moreover, ACR exposure increased LC3-positive signals, early apoptosis rate, aberrant ATG3, ATG5, LC3, Beclin1, and mTOR mRNA expression. Conclusions: These results suggest that ACR exposure can affect the developmental potential of GV oocytes by inducing mitochondrial dysfunction, actin filament assembly, and autophagy/apoptosis.

FMNL3 regulates FASCIN for actin-mediated spindle migration and cytokinesis in mouse oocytes†

Formin-like 3 (FMNL3) is a member of the formin-likes (FMNLs), which belong to the formin family. As an F-actin nucleator, FMNL3 is essential for several cellular functions, such as polarity control, invasion, and migration. However, the roles of FMNL3 during oocytes meiosis remain unclear. In this study, we investigated the functions of FMNL3 during mouse oocyte maturation. Our results showed that FMNL3 mainly concentrated in the oocyte cortex and spindle periphery. Depleting FMNL3 led to the failure of polar body extrusion, and we also found large polar bodies in the FMNL3-deleted oocytes, indicating the occurrence of symmetric meiotic division. There was no effect of FMNL3 on spindle organization; however, we observed spindle migration defects at late metaphase I, which might be due to the decreased cytoplasmic actin. Microinjecting Fmnl3-EGFP mRNA into Fmnl3-depleted oocytes significantly rescued these defects. In addition, the results of co-immunoprecipitation and the perturbation of protein expression experiments suggested that FMNL3 interacted with the actin-binding protein FASCIN for the regulation of actin filaments in oocytes. Thus, our results provide the evidence that FMNL3 regulates FASCIN for actin-mediated spindle migration and cytokinesis during mouse oocyte meiosis.

Melatonin Protects Against Mdivi-1-Induced Abnormal Spindle Assembly and Mitochondrial Superoxide Production During Porcine Oocyte Maturation

Mitochondrial division inhibitor 1 (Mdivi-1) reportedly provides a close connection between oocyte maturation and mitochondrial function in pigs. N-acetyl-5-methoxy-tryptamine (melatonin) is known to be a representative antioxidant with the ability to rehabilitate meiotic maturation of porcine oocytes. However, the ability of melatonin to recover Mdivi-1-mediated disruption of spindle formation during meiotic maturation of porcine oocytes during in vitro maturation (IVM) has not been studied. Here, we first investigated changes in mitochondrial length, such as fragmentation and elongation form, in mature porcine oocytes during IVM. Mature oocytes require appropriate mitochondrial fission for porcine oocyte maturation. We identified a dose-dependent reduction in meiotic maturation in porcine oocytes following Mdivi-1 treatment (50, 75, and 100 μM). We also confirmed changes in mitochondrial fission protein levels [dynamin-related protein 1 phosphorylation at serine 616 (pDRP1-Ser616) and dynamin-related protein 1 (DRP1)], mitochondrial membrane potential, and ATP production in 75 μM Mdivi-1-treated oocytes. As expected, Mdivi-1 significantly reduced mitochondrial function and DRP1 protein levels and increased spindle abnormalities in porcine oocytes. In addition, we confirmed that melatonin restores abnormal spindle assembly and reduces meiotic maturation rates by Mdivi-1 during porcine oocyte maturation. Interestingly, the expression levels of genes that reduce DNA damage and improve tubulin formation were enhanced during porcine meiotic maturation. Taken together, these results suggest that melatonin has direct beneficial effects on meiotic maturation through tubulin formation factors during porcine oocyte maturation.

Luteolin Orchestrates Porcine Oocyte Meiotic Progression by Maintaining Organelle Dynamics Under Oxidative Stress

Increasing evidence has demonstrated that oxidative stress impairs oocyte maturation, but the underlying mechanisms remain largely unknown. Here, for the first time, we examined the antioxidant role of luteolin in meiotic progression and the underlying mechanisms. Supplementation of 5 μM luteolin increased the rates of first polar body extrusion and blastocyst formation after parthenogenetic activation, and the expression levels of oocyte competence (BMP15 and GDF9)-, mitogen-activated protein kinase (MOS)-, and maturation promoting factor (CDK1 and Cyclin B)-related genes were also improved. Luteolin supplementation decreased intracellular reactive oxygen species levels and increased the expression levels of oxidative stress-related genes (SOD1, SOD2, and CAT). Interestingly, luteolin alleviated defects in cell organelles, including actin filaments, the spindle, mitochondria, the endoplasmic reticulum, and cortical granules, caused by H₂O₂ exposure. Moreover, luteolin significantly improved the developmental competence of in vitro-fertilized embryos in terms of the cleavage rate, blastocyst formation rate, cell number, cellular survival rate, and gene expression and markedly restored the competencies decreased by H₂O₂ treatment. These findings revealed that luteolin supplementation during in vitro maturation improves porcine meiotic progression and subsequent embryonic development by protecting various organelle dynamics against oxidative stress, potentially increasing our understanding of the underlying mechanisms governing the relationship between oxidative stress and the meiotic events required for successful oocyte maturation.

Ral GTPase is essential for actin dynamics and Golgi apparatus distribution in mouse oocyte maturation

Background

Ral family is a member of Ras-like GTPase superfamily, which includes RalA and RalB. RalA/B play important roles in many cell biological functions, including cytoskeleton dynamics, cell division, membrane transport, gene expression and signal transduction. However, whether RalA/B involve into the mammalian oocyte meiosis is still unclear. This study aimed to explore the roles of RalA/B during mouse oocyte maturation.

Results

Our results showed that RalA/B expressed at all stages of oocyte maturation, and they were enriched at the spindle periphery area after meiosis resumption. The injection of RalA/B siRNAs into the oocytes significantly disturbed the polar body extrusion, indicating the essential roles of RalA/B for oocyte maturation. We observed that in the RalA/B knockdown oocytes the actin filament fluorescence intensity was significantly increased at the both cortex and cytoplasm, and the chromosomes were failed to locate near the cortex, indicating that RalA/B regulate actin dynamics for spindle migration in mouse oocytes. Moreover, we also found that the Golgi apparatus distribution at the spindle periphery was disturbed after RalA/B depletion.

Conclusions

In summary, our results indicated that RalA/B affect actin dynamics for chromosome positioning and Golgi apparatus distribution in mouse oocytes.

Effects of Porcine Immature Oocyte Vitrification on Actin Microfilament Distribution and Chromatin Integrity During Early Embryo Development in vitro

Vitrification is mainly used to cryopreserve female gametes. This technique allows maintaining cell viability, functionality, and developmental potential at low temperatures into liquid nitrogen at −196°C. For this, the addition of cryoprotectant agents, which are substances that provide cell protection during cooling and warming, is required. However, they have been reported to be toxic, reducing oocyte viability, maturation, fertilization, and embryo development, possibly by altering cell cytoskeleton structure and chromatin. Previous studies have evaluated the effects of vitrification in the germinal vesicle, metaphase II oocytes, zygotes, and blastocysts, but the knowledge of its impact on their further embryo development is limited. Other studies have evaluated the role of actin microfilaments and chromatin, based on the fertilization and embryo development rates obtained, but not the direct evaluation of these structures in embryos produced from vitrified immature oocytes. Therefore, this study was designed to evaluate how the vitrification of porcine immature oocytes affects early embryo development by the evaluation of actin microfilament distribution and chromatin integrity. Results demonstrate that the damage generated by the vitrification of immature oocytes affects viability, maturation, and the distribution of actin microfilaments and chromatin integrity, observed in early embryos. Therefore, it is suggested that vitrification could affect oocyte repair mechanisms in those structures, being one of the mechanisms that explain the low embryo development rates after vitrification.

Phycocyanin Improves Reproductive Ability in Obese Female Mice by Restoring Ovary and Oocyte Quality

Reproductive dysfunction associated with obesity is increasing among women of childbearing age. Emerging evidence indicates that maternal obesity impairs embryo development and offspring health, and these defects are linked to oxidative stress in the ovary and in oocytes. Phycocyanin (PC) is a biliprotein from Spirulina platensis that possesses antioxidant, anti-inflammatory, and radical-scavenging properties. Our previous studies have shown that PC can reduce reactive oxygen species (ROS) accumulation in oocytes in D-gal-induced aging mice. Here, at the Institute of Cancer Research (ICR) mice fed a high-fat diet (HFD) to model obesity were used to test the effect of PC on reversing the fertility decline caused by obesity. We observed a significant increase in litter size and offspring survival rates after PC administration to obese mice. Further, we found that PC not only ameliorated the level of ovarian antioxidant enzymes, but also reduced the occurrence of follicular atresia in obese female mice. In addition, the abnormal morphology of the spindle-chromosome complex (SCC), and the abnormal mitochondrial distribution pattern in oocytes both recovered. The obesity-related accumulation of ROS, increased number of early apoptotic cells, and the abnormal expression of H3K9me3 in oocytes were all partially reversed after PC administration. In summary, this is the first demonstration that PC can improve fertility by partially increasing ovarian and oocyte quality in obese female mice and provides a new strategy for clinically treating obesity-related infertility in females.

ASB7 Is a Novel Regulator of Cytoskeletal Organization During Oocyte Maturation

Ankyrin repeat and SOCS box (ASB) family members have a C-terminal SOCS box and an N-terminal ankyrin-related sequence of variable repeats. To date, the roles of ASB family members remain largely unknown. In the present study, by employing knockdown analysis, we investigated the effects of ASB7 on mouse oocyte meiosis. We show that specific depletion of ASB7 disrupts maturational progression and meiotic apparatus. In particular, abnormal spindle, misaligned chromosomes, and loss of cortical actin cap are frequently observed in ASB7-abated oocytes. Consistent with this observation, incidence of aneuploidy is increased in these oocytes. Meanwhile, confocal scanning reveals that loss of ASB7 impairs kinetochore–microtubule interaction and provokes the spindle assembly checkpoint during oocyte meiosis. Furthermore, we find a significant reduction of ASB7 protein in oocytes from aged mice. Importantly, increasing ASB7 expression is capable of partially rescuing the maternal age-induced meiotic defects in oocytes. Together, our data identify ASB7 as a novel player in regulating cytoskeletal organization and discover the potential effects of ASB7 on quality control of aging oocytes.

Melatonin Attenuates Peroxynitrite-Induced Meiosis Dysfunction in Porcine Oocytes

A high level of reactive oxygen species (ROS) is widely considered one of the major causes of oocyte quality decline. Peroxynitrite is known as a powerful oxidant, which could induce multiple physical diseases. Recently, emerging pieces of evidences indicate that melatonin effectively promotes the development of oocytes, although the specific work mechanism remains to be further clarified. In this study, it was shown that peroxynitrite increased the level of ROS in porcine oocytes, which induced the apoptosis of oocytes, thereby leading to the obstruction of spindle assembly, depolymerization of actin, and decrease of polar body expulsion. These negative effects contributed to the failure of meiosis and ultimately blocked the maturation of porcine oocytes. As expected, it was found that melatonin effectively removed the accumulated ROS in oocytes, preventing oocytes from peroxynitrite-induced oocyte maturation failure, which might provide a novel approach to improve female livestock reproduction and cure female infertility in clinical practice.

Oral administration of olaquindox negatively affects oocytes quality and reproductive ability in female mice

As an effective feed additive in the livestock industry, olaquindox (OLA) has been widely used in domestic animal production. However, it is unclear whether OLA has negative effects on mammalian oocyte quality and fetal development. In this study, toxic effects of OLA were tested by intragastric gavage ICR mice with water, low-dose OLA (5 mg/kg/day), or high-dose OLA (60 mg/kg/day) for continuous 45 days. Results showed that high-dose OLA gavage severely affected the offspring birth and growth. Significantly, high-dose OLA impaired oocyte maturation and early embryo development, indicated by the decreased percentage of germinal vesicle breakdown, first polar body extrusion and blastocyst formation. Meanwhile, oxidative stress levels were increased in oocytes or ovaries, indexed by the increased levels of ROS, MDA, H₂O₂, NO, and decreased levels of GSH, SOD, CAT, GSH-Px and GSH-Rd. Furthermore, aberrant mitochondria distribution, defective spindle assembly, abnormal H3K4me2/H3K9me3 levels, increased DNA double-strand breaks and early apoptosis rate, were observed after high-dose OLA gavage. Taken together, our results for the first time illustrated that high-dose OLA gavage led to sub-fertility of females, which means that restricted utilization of OLA as feed additive should be considered.

AIP1 and Cofilin control the actin dynamics to modulate the asymmetric division and cytokinesis in mouse oocytes

Actin-interacting protein 1 (AIP1), also known as WD repeat-containing protein 1 (WDR1), is ubiquitous in eukaryotic organisms, and it plays critical roles in the dynamic reorganization of the actin cytoskeleton. However, the biological function and mechanism of AIP1 in mammalian oocyte maturation is still largely unclear. In this study, we demonstrated that AIP1 boosts ADF/Cofilin activity in mouse oocytes. AIP1 is primarily distributed around the spindle region during oocyte maturation, and its depletion impairs meiotic spindle migration and asymmetric division. The knockdown of AIP1 resulted in the gathering of a large number of actin-positive patches around the spindle region. This effect was reduced by human AIP1 (hAIP1) or Cofilin (S3A) expression. AIP1 knockdown also reduced the phosphorylation of Cofilin near the spindle, indicating that AIP1 interacts with ADF/Cofilin-decorated actin filaments and enhances filament disassembly. Moreover, the deletion of AIP1 disrupts Cofilin localization in metaphase I (MI) and induces cytokinesis defects in metaphase II (MII). Taken together, our results provide evidence that AIP1 promotes actin dynamics and cytokinesis via Cofilin in the gametes of female mice.

Short term temperature elevation during IVM affects embryo yield and alters gene expression pattern in oocytes, cumulus cells and blastocysts in cattle

Heat stress causes subfertility in cattle by inducing alterations in steroidogenic capacity, follicular function and ovulation defects, which eventually negatively affect oocyte quality and embryo survival. Here, the effects of short, moderate temperature elevation during IVM, on embryo yield, and on the expression of various genes was evaluated. In 8 replicates, cumulus oocyte complexes (COCs) were matured for 24 h at 39 °C (controls n = 605) or at 41 °C from hour 2 to hour 8 of IVM (treated, n = 912), fertilized, and presumptive zygotes were cultured for 9 days at 39 °C. Cleavage and embryo formation rates were evaluated 48 h post insemination and on days 7, 8, 9 respectively. Cumulus cells, oocytes and blastocysts from 5 replicates were snap frozen for the relative expression analysis of genes related to metabolism, thermal and oxidative stress response, apoptosis, and placentation. In treated group, cleavage and embryo formation rates were statistically significantly lower compared with the control (cleavage 86.7% vs 74.2%; blastocysts: day 7, 29.9% vs 19.7%, day 8, 34.2% vs 22.9% and day 9 35.9% vs 24.5%). Relative mRNA abundance of three genes in cumulus cells (HSP90AA1, CPT1B, G6PD) and three genes in blastocysts (DNMT3A, PLAC8, GPX1) indicated significantly different expression between groups (p < 0.05)., The expression of G6PD, SOD2, GXP1 in oocytes and PTGS2 in blastocysts tended to differ among groups (0.05<p < 0.08). Heat stress altered (p < 0.05) the correlation of expression between HSPs and other genes in oocytes (G6PD, GPX1, CCNB1), cumulus cells (LDH, CCNB1) and blastocysts (AKR1B1, PLAC8). These results imply that exposure of oocytes to elevated temperature, even for only 6 h, disrupts the developmental competence of the oocytes, suppresses blastocyst yield and significantly alters the coordinated pattern of gene expressions.

Cellular and molecular aspects of oocyte maturation and fertilization: a perspective from the actin cytoskeleton

ABSTRACT

Much of the scientific knowledge on oocyte maturation, fertilization, and embryonic development has come from the experiments using gametes of marine organisms that reproduce by external fertilization. In particular, echinoderm eggs have enabled the study of structural and biochemical changes related to meiotic maturation and fertilization owing to the abundant availability of large and transparent oocytes and eggs. Thus, in vitro studies of oocyte maturation and sperm-induced egg activation in starfish are carried out under experimental conditions that resemble those occurring in nature. During the maturation process, immature oocytes of starfish are released from the prophase of the first meiotic division, and acquire the competence to be fertilized through a highly programmed sequence of morphological and physiological changes at the oocyte surface. In addition, the changes in the cortical and nuclear regions are essential for normal and monospermic fertilization. This review summarizes the current state of research on the cortical actin cytoskeleton in mediating structural and physiological changes during oocyte maturation and sperm and egg activation in starfish and sea urchin. The common denominator in these studies with echinoderms is that exquisite rearrangements of the egg cortical actin filaments play pivotal roles in gamete interactions, Ca2+ signaling, exocytosis of cortical granules, and control of monospermic fertilization. In this review, we also compare findings from studies using invertebrate eggs with what is known about the contributions made by the actin cytoskeleton in mammalian eggs. Since the cortical actin cytoskeleton affects microvillar morphology, movement, and positioning of organelles and vesicles, and the topography of the egg surface, these changes have impacts on the fertilization process, as has been suggested by recent morphological studies on starfish oocytes and eggs using scanning electron microscopy. Drawing the parallelism between vitelline layer of echinoderm eggs and the zona pellucida of mammalian eggs, we also discuss the importance of the egg surface in mediating monospermic fertilization.

GRAPHICAL ABSTRACT

Cytoplasmic halo characteristics during fertilization and their implications for human preimplantation embryo development and pregnancy outcome

RESEARCH QUESTION

Is the spatiotemporal phenomenology of the cytoplasmic halo during fertilization related to embryonic competence?

DESIGN

Time-lapse images from 1009 zygotes were retrospectively analysed from 560 patients who underwent IVF with minimal stimulation and single vitrified-warmed blastocyst transfer between April 2017 and March 2018. Halo presence and morphokinetics were monitored and compared relative to embryo quality, blastocyst expansion and ongoing pregnancy.

RESULTS

Halo was observed in 88% of fertilized oocytes. Embryos derived from zygotes without halo had significantly higher rates of rapid cleavage (P = 0.0004), cell fusion (P = 0.0028) and asymmetrical division (P = 0.0002) compared with those derived from zygotes with halo. Multivariate logistic regression analysis had significantly higher developmental rates compared with the expanded blastocyst stage in embryos displaying a halo, regardless of its distribution (adjusted odds ratio 0.435; P = 0.0004). Prolonged halo time intervals were significantly correlated with increased asymmetrical division at first cell division (P = 0.0412, P = 0.0088, respectively) and decreased developmental rates to expanded blastocyst stage (P = 0.0062, P = 0.0020, respectively). Additionally, prolonged presence of the cytoplasmic halo was associated with a decreased ongoing pregnancy rate (adjusted odds ratio 0.871; P = 0.006). Poor sperm quality and decreased oocyte diameter were correlated with absence of the cytoplasmic halo (P = 0.0477, P < 0.0001, respectively) or prolonged halo presence (P = 0.0139, P = 0.0002, respectively).

CONCLUSIONS

Halo presence and morphokinetics are associated with cleavage patterns, development to blastocyst stage and ongoing pregnancy rate after single blastocyst transfer. Halo morphokinetics seems to reflect sperm and oocyte quality. Cytoplasmic halo might be valuable predictor for refining selection of more developmentally competent blastocysts.

Dynamic organelle distribution initiates actin-based spindle migration in mouse oocytes

Migration of meiosis-I (MI) spindle from the cell center to a sub-cortical location is a critical step for mouse oocytes to undergo asymmetric meiotic cell division. In this study, we investigate the mechanism by which formin-2 (FMN2) orchestrates the initial movement of MI spindle. By defining protein domains responsible for targeting FMN2, we show that spindle-periphery localized FMN2 is required for spindle migration. The spindle-peripheral FMN2 nucleates short actin bundles from vesicles derived likely from the endoplasmic reticulum (ER) and concentrated in a layer outside the spindle. This layer is in turn surrounded by mitochondria. A model based on polymerizing actin filaments pushing against mitochondria, thus generating a counter force on the spindle, demonstrated an inherent ability of this system to break symmetry and evolve directional spindle motion. The model is further supported through experiments involving spatially biasing actin nucleation via optogenetics and disruption of mitochondrial distribution and dynamics.

Mammalian oocytes divide asymmetrically during meiotic maturation. Here, the authors show that spindle movement away from oocyte center depends on actin filaments nucleated from the spindle periphery pushing against surrounding mitochondria, which polarizes spontaneously to produce directional spindle motion.

WDR62 is a novel participator in spindle migration and asymmetric cytokinesis during mouse oocyte meiotic maturation

In female meiosis, oocyte meiotic maturation is a form of asymmetric cell division, producing the first polar body and a large oocyte, in which the asymmetry of oocyte meiotic division depends on spindle migration and positioning, and cortical polarization. In this study, we conclude that WDR62 (WD40-repeat protein 62) plays an important role in asymmetric meiotic division during mouse oocyte maturation. Our initial study demonstrated that WDR62 mainly co-localized with chromosomes during mouse oocyte meiotic maturation. Interference of Wdr62 by siRNA microinjection did not affect germinal vesicle breakdown (GVBD) but compromised the first polar body extrusion (PBE) with the large polar bodies generated, which is coupled with a higher incidence of spindle abnormality and chromosome misalignment. Further analysis concluded that loss of WDR62 blocked asymmetric spindle positioning and actin cap formation, which should be responsible for large polar body extrusion. Moreover, WDR62 decline intervened with the Arp2/3 complex, an upstream regulator for the cortical actin. Besides for p-MAPK, a critical regulator for the asymmetric division of oocyte, WDR62-depleted oocytes showed perturbation only in localization pattern but not expression level. In summary, our study defines WDR62 as an essential cytoskeletal regulator of spindle migration and asymmetric division during mouse oocyte meiotic maturation.

mInscuteable regulates meiotic spindle organization during mouse oocyte meiotic maturation

Establishment of cellular polarity is one of the key events during oocyte maturation. Inscuteable (Insc) has been identified as a key regulator of cell polarity during asymmetric division in Drosophila. However, the function of its evolutionarily conserved mammalian homologue, mInscuteable (mInsc), in mouse meiotic maturation is not clear. In this study, we investigated the roles of mInsc in mouse oocyte maturation. mInsc was detected at all stages of oocyte maturation. The protein level of mInsc was slightly higher at the germinal vesicle breakdown (GVBD) stage and remained constant during mouse oocyte maturation. The subcellular localization of mInsc overlapped with spindle microtubules. Disruption of microtubules and microfilaments caused changes in the localization of mInsc. Depletion or overexpression of mInsc significantly decreased the maturation rates of mouse oocytes. Depletion of mInsc significantly affected asymmetric division, spindle assembly, alignments of chromosomes and actin cap formation. Taken together, our results demonstrated that mInsc regulates meiotic spindle organization during mouse meiotic maturation.

Compensatory endocytosis occurs after cortical granule exocytosis in mouse eggs

Compensatory endocytosis (CE) is one of the primary mechanisms through which cells maintain their surface area after exocytosis. Considering that in eggs massive exocytosis of cortical granules (CG) takes place after fertilization, the aim of this study was to evaluate the occurrence of CE following cortical exocytosis in mouse eggs. For this purpose, we developed a pulse-chase assay to detect CG membrane internalization. Results showed internalized labeling in SrCl₂ -activated and fertilized eggs when chasing at 37°C, but not at a nonpermissive temperature (4°C). The use of kinase and calcineurin inhibitors led us to conclude that this internal labeling corresponded to CE. Further experiments showed that CE in mouse eggs is dependent on actin dynamics and dynamin activity, and could be associated with a transient exposure of phosphatidylserine. Finally, CE was impaired in A23187 ionophore-activated eggs, highlighting once again the mechanistic differences between the activation methods. Altogether, these results demonstrate for the first time that egg activation triggers CE in mouse eggs after exocytosis of CG, probably as a plasma membrane homeostasis mechanism.

Profilin 1 plays feedback role in actin-mediated polar body extrusion in mouse oocytes

Mammalian oocytes undergo several crucial processes during meiosis maturation, including spindle formation and migration and polar body extrusion, which rely on the regulation of actin. As a small actin-binding protein, profilin 1 plays a central role in the regulation of actin assembly. However, the functions of profilin 1 in mammalian oocytes are uncertain. To investigate the function of profilin 1 in oocytes, immunofluorescent staining was first used to examine profilin 1 localisation. The results showed that profilin 1 was localised around the meiotic spindles and was colocalised with cytoplasmic actin. Knockdown (KD) of profilin 1 with specific morpholino microinjection resulted in failure of polar body extrusion. This failure resulted from an increase of actin polymerisation both at membranes and in the cytoplasm. Furthermore, western blot analysis revealed that the expression of Rho-associated kinase (ROCK) and phosphorylation levels of myosin light chain (MLC) were significantly altered after KD of profilin 1. Thus, the results indicate that a feedback mechanism between profilin, actin and ROCK-MLC2 regulates actin assembly during mouse oocyte maturation.

Sodium nitrite negatively affects reproductive ability and offspring survival in female mice

Sodium nitrite (NaNO₂) is an industrial chemical that is frequently used as a food additive to prevent botulism and enhance glossiness, such as curing meat. In addition, in some regions, water source NaNO₂ concentrations exceed standard regulatory levels. Whether the excessive intake of NaNO₂ has toxic effects on female fertility and fetal development remain unknown. In this study, we administered ICR mice control saline, low-dose NaNO₂ (60 mg/kg/day), or high-dose NaNO₂ (120 mg/kg/day) by intragastric gavage for 21 days. We then assessed oocyte morphology, spindle-chromosome dynamics, mitochondrial distribution, ATP content, apoptotic cell numbers, DNA damage levels, histone modifications, reactive oxygen species (ROS) levels, and offspring survival. Results showed that NaNO₂ treatment decreased oocyte number, impaired polar body extrusion, and increased zona pellucida thickness in oocytes. Furthermore, NaNO₂ disrupted MII spindle integrity, caused abnormal mitochondrial distribution, decreased ATP content, and increased levels of ROS and H3K4me2. Moreover, the number of oocytes in early stages of apoptosis and with levels of DNA damage increased in NaNO₂-treated mice along with decreased offspring numbers and survival rates. We demonstrated the negative effects of NaNO₂ on female reproductive abilities in mice.

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.

RAB35 depletion affects spindle formation and actin-based spindle migration in mouse oocyte meiosis

Mammalian oocyte maturation involves a unique asymmetric cell division, in which meiotic spindle formation and actin filament-mediated spindle migration to the oocyte cortex are key processes. Here, we report that the vesicle trafficking regulator, RAB35 GTPase, is involved in regulating cytoskeleton dynamics in mouse oocytes. RAB35 GTPase mainly accumulated at the meiotic spindle periphery and cortex during oocyte meiosis. Depletion of RAB35 by morpholino microinjection led to aberrant polar body extrusion and asymmetric division defects in almost half the treated oocytes. We also found that RAB35 affected SIRT2 and αTAT for tubulin acetylation, which further modulated microtubule stability and meiotic spindle formation. Additionally, we found that RAB35 associated with RHOA in oocytes and modulated the ROCK–cofilin pathway for actin assembly, which further facilitated spindle migration for oocyte asymmetric division. Importantly, microinjection of Myc-Rab35 cRNA into RAB35-depleted oocytes could significantly rescue these defects. In summary, our results suggest that RAB35 GTPase has multiple roles in spindle stability and actin-mediated spindle migration in mouse oocyte meiosis.

Di(n-butyl) phthalate exposure impairs meiotic competence and development of mouse oocyte

Di(n-butyl) phthalate (DBP) is extensively used in industrial applications as plasticizer and stabilizer and its presence in the environment may present health risks for human. Previous studies have demonstrated its mutagenic, teratogenic, and carcinogenic ability. However, its effect on mammalian oocyte maturation remains unknown. In this study, we examined the effect of DBP on oocyte maturation both in vitro and in vivo. Our results showed that DBP could significantly reduce mice oocyte germinal vesicle breakdown (GVBD) and polar body extrusion (PBE) rates. In addition, oocyte cytoskeleton was damaged and cortical granule-free domains (CGFDs) were also disrupted. Finally, DBP induced early apoptosis of oocyte and granulosa cells (GCs). Collectively, these data demonstrate that DBP could reduce meiosis competence and mouse oocyte development.

The effects of graphene quantum dots on the maturation of mouse oocytes and development of offspring

Recently, graphene nanomaterials have attracted tremendous attention and have been utilized in various fields because of their excellent mechanical, thermal, chemical, optical properties, and good biocompatibility, especially in biomedical aspects. However, there is a concern that the unique characteristics of nanomaterials may have undesirable effects. Therefore, in this study, we sought to systematically investigate the effects of graphene quantum dots (GQDs) on the maturation of mouse oocytes and development of the offspring via in vitro and in vivo studies. In vitro, we found that the first polar body extrusion rate in the high dosage exposure groups (1.0-1.5 mg/ml) ₂ decreased significantly and the failure of spindle migration and actin cap formation after GQDs exposure was observed. The underlying mechanisms might be associated with reactive oxygen species accumulation and DNA damage. Moreover, transmission electron microscope studies showed that GQDs may have been internalized into oocytes, tending to accumulate in the nucleus and severely affecting mitochondrial morphology, which included swollen and vacuolated mitochondria accompanied by cristae alteration with a lower amount of dense mitochondrial matrix. In vivo, when pregnant mice were exposed to GQDs at 8.5 days of gestation (GD, 8.5), we found that high dosage of GQD exposure (30 mg/kg) significantly affected mean fetal length; however, all the second generation of female mice grew up normal, attained sexual maturity, and gave birth to a healthy offspring after mating with a healthy male mouse. The results presented in this study are important for the future investigation of GQDs for the biomedical applications.

Anti-proliferative effect of auriculataoside A on B16 melanoma 4A5 cells by suppression of Cdc42-Rac1-RhoA signaling protein levels

Auriculataoside A, an anthracenone dimer glycoside isolated from Cassia auriculata seed, shows anti-proliferative effects on cell line B16 melanoma 4A5 cells with an IC₅₀ value of 0.82 μM. However, it shows no such effect on normal human dermal fibroblast (HDF) cells. To evaluate the mode of action underlying the anti-proliferative effect of auriculataoside A on cells, we examined changes in whole protein expression after treatment with auriculataoside A and found that the expression Cdc42, RhoA, and Rac1, which are Rho family GTPases, was reduced. Auriculataoside A also arrested the cell cycle at G1 phase. These results suggest that the suppression of the above proteins induced G1 arrest. In addition, auriculataoside A also suppressed the expression of β-catenin and c-Myc proteins. This action of auriculataoside A could be one of the mechanisms underlying its selective anti-proliferative effect on B16 melanoma cells.

Prolonged blastomere movement induced by the delay of pronuclear fading and first cell division adversely affects pregnancy outcomes after fresh embryo transfer on Day 2: a time-lapse study

RESEARCH QUESTION

What is the incidence, origin and clinical significance of blastomere movement after the first cell division in the human embryo?

DESIGN

A total of 1096 embryos, cultured in the EmbryoScope+ ® time-lapse system and subjected to a single fresh cleaved embryo transfer, were retrospectively analysed. Type and duration of blastomere movement (dBMov) between the first (t2) and second cell division (t3) was monitored, and the ratio of dBMov during the 2-cell stage [dBMov/(t3-t2)] was calculated. Morphological evaluation of embryos was performed by referring to the size of the blastomere and fragmentation after first division in addition to Veeck's criteria on Day 2. The correlation between dBMov and ongoing pregnancy was evaluated and the association of dBMov with patient and embryonic characteristics was determined.

RESULTS

Both movement type and the value of dBMov/(t3-t2) were significantly associated with asymmetrical first division, fragment formation and morphological grade on Day 2. Multivariate logistic regression analysis revealed that a higher value of dBMov/(t3-t2) significantly correlated with a decreased ongoing pregnancy rate, even after adjustment for co-founders (odds ratio 0.399, P = 0.0419). The time intervals of pronuclear (PN) alignment and PN fading were significantly correlated with the dBMov/(t3-t2) value.

CONCLUSIONS

Embryos with extended blastomere movement after the first cell division, which is associated with the delay of PN fading and first cell division, have a lower competence to initiate an ongoing pregnancy after fresh embryo transfer on Day 2. Thus, blastomere movement could be a useful predictive parameter for selecting embryos at the early cleavage stage.