Zinc regulates meiotic resumption in porcine oocytes via a protein kinase C-related pathway

Nivalenol affects Cyclin B1 level and activates SAC for cell cycle progression in mouse oocyte meiosis

OBJECTIVES

Nivalenol (NIV) is a secondary metabolite of type B trichothecene mycotoxin produced by Fusarium genera, which is widely found in contaminated food and crops such as corn, wheat and peanuts. NIV is reported to have hepatotoxicity, immunotoxicity, genotoxicity, and reproductive toxicity. Previous studies indicate that NIV disturbs mammalian oocyte maturation. Here, we reported that delayed cell cycle progression might be the reason for oocyte maturation defect caused by NIV exposure.

METHODS AND RESULTS

We set up a NIV exposure model and showed that NIV did not affect G2/M transition for meiosis resumption, but disrupted the polar body extrusion of oocytes. Further analysis revealed that oocytes were arrested at metaphase I, which might be due to the lower expression of Cyclin B1 after NIV exposure. After cold treatment, the microtubules were disassembled in the NIV-exposed oocytes, indicating that NIV disrupted microtubule stability. Moreover, NIV affected the attachment between kinetochore and microtubules, which further induced the activation of MAD2/BUBR1 at the kinetochores, suggesting that spindle assemble checkpoint (SAC) was continuously activated during oocyte meiotic maturation.

CONCLUSIONS

Taken together, our study demonstrated that exposure to NIV affected Cyclin B1 expression and activated microtubule stability-dependent SAC to ultimately disturb cell cycle progression in mouse oocyte meiosis.

Dynamic changes of intracellular zinc ion level during maturation, fertilization, activation, and development in mouse oocytes

Although it is well known that calcium oscillations are required for fertilization in all mammalian species studied to date, recent studies also showed the ejection of zinc into the extracellular milieu in a series of coordinated events, called "zinc spark," during mammalian fertilization. These results led us to the hypothesis that a zinc ion-dependent signal is important for oocyte maturation, fertilization (activation), and further embryonic development. In this study, we evaluated the amounts and localization of intracellular zinc ions during maturation, fertilization, activation, and embryonic development in mouse oocytes. Our results show that abundant zinc ions are present in both immature and mature oocytes. After in vitro fertilization, the amounts of zinc ions were dramatically decreased at the pronuclear (PN) stage. Artificial activation by cycloheximide, SrCl₂ , and TPEN also reduced the amounts of zinc ions in the PN stage. On the other hand, PN embryos derived from sperm injection still showed high level of zinc ions. However, the amounts of zinc ions rapidly increased at the blastocysts regardless of activation method. We showed here that the amounts of zinc ions dramatically changed during maturation, fertilization, activation, and development in mouse oocytes.

Meiosis Resumption of Immature Human Oocytes following Treatment with Calcium Ionophore In Vitro

Objective:

In vitro maturation (IVM) of human oocytes is used to induce meiosis progression in immature retrieved oocytes. Calcium (Ca2+) has a central role in oocyte physiology. Passage through meiosis phase to another phase is controlled by increasing intracellular Ca2+. Therefore, the current research was conducted to evaluate the role of calcium ionophore (CI) on human oocyte IVM.

Materials and Methods:

In this clinical trial study, immature human oocytes were obtained from 216 intracytoplasmic sperm injection (ICSI) cycles. After ovarian stimulation, germinal vesicle (GV) stage oocytes were collected and categorized into two groups: with and without 10 µM CI treatment. Next, oocyte nuclear maturation was assessed after 24–28 hours of culture. Real-time reverse transcription polymerase chain reaction (RT-PCR) was used to assess the transcript profile of several oocyte maturation-related genes (MAPK3, CCNB1, CDK1, and cyclin D1 [CCND1]) and apoptotic-related genes (BCL-2, BAX, and Caspase-3). Oocyte glutathione (GSH) and reactive oxygen species (ROS) levels were assessed using Cell Tracker Blue and 2’,7’-dichlorodihydrofluorescein diacetate (H2DCFDA) fluorescent dye staining. Oocyte spindle configuration and chromosome alignment were analysed by immunocytochemistry.

Results:

The metaphase II (MII) oocyte rate was higher in CI‐treated oocytes (73.53%) compared to the control (67.43%) group, but this difference was not statistically significant (P=0.13). The mRNA expression profile of oocyte maturation-related genes (MAPK3, CCNB1, CDK1, and CCND1) (P<0.05) and the anti-apoptotic BCL-2 gene was remarkably up-regulated after treatment with CI (P=0.001). The pro-apoptotic BAX and Caspase-3 relative expression levels did not change significantly. The CI‐treated oocyte cytoplasm had significantly higher GSH and lower ROS (P<0.05). There was no statistically significant difference in meiotic spindle assembly and chromosome alignment between CI treatment and the control group oocytes.

Conclusion:

The finding of the current study supports the role of CI in meiosis resumption of human oocytes. (Registration Number: IRCT20140707018381N4)

Zinc supplementation alleviates endoplasmic reticulum stress during porcine oocyte in vitro maturation by upregulating zinc transporters

Endoplasmic reticulum (ER) stress is a major contributor to embryonic development failure. Mammalian oocytes have a high risk of exposure to cellular stress during in vitro embryo production. We investigated the effects of zinc supplementation during in vitro maturation under ER stress. We evaluated cumulus expansion, embryonic development derived by parthenogenetic activation, reactive oxygen species, protein expression of X-box binding protein 1 (XBP1), and expression of genes related to ER stress. Supplementation with 1 μg/ml zinc significantly increased the nuclear maturation of oocytes, cleavage and blastocyst formation rates, and total blastocyst cell number (p < .05). Under ER stress, zinc significantly reduced protein expression of XBP1, and increased cleavage and blastocyst rates (p < .05). Concomitantly, zinc supplementation upregulated the expression of zinc transporters (SLC39A14 and SLC39A10), PTGS2, and downregulated ER stress-related genes (sXBP1, uXBP1, ATF4, and PTPN1/PTP1B), and caspase 3. These results suggest that zinc supplementation alleviates ER stress by providing essential metal-ion transporters for oocyte maturation and subsequent embryonic development.

Bovine eggs release zinc in response to parthenogenetic and sperm-induced egg activation

Upon fertilization or parthenogenesis, zinc is released into the extracellular space through a series of exocytic events termed zinc sparks, which are tightly coordinated with intracellular calcium transients. The zinc spark reduces the total amount of intracellular zinc, and this reduction is necessary and sufficient to induce egg activation even in the absence of calcium transients. In addition, this zinc release contributes to the block to polyspermy through modification of the zona pellucida. The zinc spark has been documented in all organisms examined to date including the mouse, two species of nonhuman primates, and human. Here we determined whether zinc sparks occur in the bovine, an important model of gamete development in mono-ovulatory mammalian species. We obtained metaphase II-arrested (MII) bovine eggs following in vitro maturation. Total zinc, assessed in single cells using X-Ray Fluorescence Microscopy, was significantly more abundant in the bovine egg compared to iron and copper. Studies with intracellular fluorescent probes revealed that labile zinc pools are localized to discrete cytoplasmic punctae enriched at the cortex. To determine whether zinc undergoes dynamic fluxes during egg activation, we parthenogenetically activated bovine eggs using two approaches: ionomycin or bovine phospholipase C zeta (bPlcζ). Both these methods induced zinc sparks coordinately with intracellular calcium transients. The zinc spark was also observed in bovine eggs following intracytoplasmic sperm injection. These results establish that zinc is the most abundant transition metal in the bovine egg, and zinc flux during egg activation - induced by chemical activation or sperm - is a highly conserved event across mammalian species.

Zinc: A Necessary Ion for Mammalian Sperm Fertilization Competency

The importance of zinc for male fertility only emerged recently, being propelled in part by consumer interest in nutritional supplements containing ionic trace minerals. Here, we review the properties, biological roles and cellular mechanisms that are relevant to zinc function in the male reproductive system, survey available peer-reviewed data on nutritional zinc supplementation for fertility improvement in livestock animals and infertility therapy in men, and discuss the recently discovered signaling pathways involving zinc in sperm maturation and fertilization. Emphasis is on the zinc-interacting sperm proteome and its involvement in the regulation of sperm structure and function, from spermatogenesis and epididymal sperm maturation to sperm interactions with the female reproductive tract, capacitation, fertilization, and embryo development. Merits of dietary zinc supplementation and zinc inclusion into semen processing media are considered with livestock artificial insemination (AI) and human assisted reproductive therapy (ART) in mind. Collectively, the currently available data underline the importance of zinc ions for male fertility, which could be harnessed to improve human reproductive health and reproductive efficiency in agriculturally important livestock species. Further research will advance the field of sperm and fertilization biology, provide new research tools, and ultimately optimize semen processing procedures for human infertility therapy and livestock AI.

Zinc sparks induce physiochemical changes in the egg zona pellucida that prevent polyspermy

During fertilization or chemically-induced egg activation, the mouse egg releases billions of zinc atoms in brief bursts known as 'zinc sparks.' The zona pellucida (ZP), a glycoprotein matrix surrounding the egg, is the first structure zinc ions encounter as they diffuse away from the plasma membrane. Following fertilization, the ZP undergoes changes described as 'hardening', which prevent multiple sperm from fertilizing the egg and thereby establish a block to polyspermy. A major event in zona hardening is cleavage of ZP2 proteins by ovastacin; however, the overall physiochemical changes contributing to zona hardening are not well understood. Using X-ray fluorescence microscopy, transmission and scanning electron microscopy, and biological function assays, we tested the hypothesis that zinc release contributes to ZP hardening. We found that the zinc content in the ZP increases by 300% following activation and that zinc exposure modulates the architecture of the ZP matrix. Importantly, zinc-induced structural changes of the ZP have a direct biological consequence; namely, they reduce the ability of sperm to bind to the ZP. These results provide a paradigm-shifting model in which fertilization-induced zinc sparks contribute to the polyspermy block by altering conformations of the ZP matrix. This adds a previously unrecognized factor, namely zinc, to the process of ZP hardening.

MLL2 is essential for porcine embryo development in vitro

Several germ cell-specific transcription factors essential for ovarian formation and folliculogenesis have been identified and studied. However, their function during early embryo development has been poorly explored. In this study, we investigated the role of mixed-lineage leukemia protein 2 (MLL2) in the development of porcine preimplantation embryos. To explore the function of MLL2 in porcine embryo development, expression and localization of MLL2 were assessed by qRT-PCR and immunofluorescence assays. Results showed that expression of MLL2 was significantly reduced after the four-cell stage. However, immunofluorescence results showed that MLL2 only localized in the nucleus of blastocysts, revealing a potential role of MLL2 in regulating the gene expression in the blastocyst stage. Knockdown of Mll2 by double-stranded RNA (dsRNA) caused a reduction in MLL2 signal in porcine blastocyst cells and in blastocyst formation. No significant differences in the cleavage and morula stages were observed. The mechanism of MLL2 regulation in blastocysts was assessed by assaying the trimethylation of histone 3 at lysine 4 (H3K4m3). MLL2 knockdown significantly reduced H3K4m3 in the nucleus and further reduced expression of Sox2 and Magoh genes. In conclusion, MLL2 is essential for porcine embryo development by the regulation of methylation of H3K4 in vitro.

GlutaMAX prolongs the shelf life of the culture medium for porcine parthenotes

In vitro porcine embryo production systems have been established and well characterized. However, the efficiency of embryo development during IVC is still very low. In the present study, we have investigated the development of parthenogenetic porcine embryos in the well-known PZM-5 medium for porcine embryos, which was modified by replacing glutamine with the GlutaMAX supplement. We revealed that blastocyst apoptosis was significantly lower in the presence of GlutaMAX, which reduced the release of mitochondrial cytochrome c. Furthermore, the expression of apoptosis genes was significantly lower during GlutaMAX treatment (P < 0.05). The modified medium was also examined for the eventual loss of its efficacy in the presence of GlutaMAX. Three, 6, and 12 months after medium preparation, blastocyst formation in the GlutaMAX-supplemented medium was significantly higher than the number of blastocysts in the medium containing glutamine. After a long period of storage, ammonia concentration was significantly increased in the glutamine medium, whereas it was not statistically different in the GlutaMAX medium. Elevated ammonia concentrations reduced the mitochondrial membrane potential and ATP content of blastocysts in the glutamine medium. These results demonstrate that GlutaMAX can reduce blastocyst apoptosis via inhibition of the cytochrome c pathway and significantly extend the shelf life of the culture medium to at least 1 year.

Scriptaid Treatment Decreases DNA Methyltransferase 1 Expression by Induction of MicroRNA-152 Expression in Porcine Somatic Cell Nuclear Transfer Embryos

Abnormal epigenetic reprogramming of donor nuclei after somatic cell nuclear transfer (SCNT) is thought to be the main cause of low cloning efficiencies. A growing body of evidence has demonstrated a positive role of Scriptaid, a histone deacetylase inhibitor (HDACi) that belongs to an existing class of hydroxamic acid-containing HDACis, on the development competence of cloned embryos in many species. The present study investigated the effects of Scriptaid on the development of porcine SCNT embryos in vitro and its mechanism. Treatment with 300 or 500 nM Scriptaid for 20 h after activation significantly increased the percentage of SCNT embryos that developed to the blastocyst stage and the total number of cells per blastocyst and significantly decreased the percentage of apoptotic cells in blastocysts. Scriptaid treatment significantly increased the level of histone H3 acetylated at K9 and the conversion of 5-methylcytosine into 5-hydroxymethylcytosine and significantly decreased the level of histone H3 trimethylated at K9 at the pronuclear stage. As a potential mechanism for the DNA methylation changes, our results showed that the expression of DNA methyltransferase 1 was frequently down-regulated in Scriptaid-treated embryos in comparison with untreated embryos and was inversely correlated to endogenous microRNA-152 (miR-152). Taken together, these findings illustrated a crucial functional crosstalk between miR-152 and DNMT1. Meanwhile, mRNA and protein levels of POU5F1 and CDX2 were increased in Scriptaid-treated embryos. mRNA levels of Caspase3, and Bax were significantly decreased and that of Bcl-xL was significantly increased in Scriptaid-treated embryos. In conclusion, these observations would contribute to uncover the nuclear reprogramming mechanisms underlying the effects of Scriptaid on the improvement of porcine SCNT embryos.

The inorganic anatomy of the mammalian preimplantation embryo and the requirement of zinc during the first mitotic divisions

BACKGROUND

Zinc is the most abundant transition metal in the mammalian oocyte, and dynamic fluxes in intracellular concentration are essential for regulating both meiotic progression and fertilization. Whether the defined pathways of zinc utilization in female meiosis directly translate to mitotic cells, including the mammalian preimplantation embryo, has not been studied previously.

RESULTS

We determined that zinc is the most abundant transition metal in the preimplantation embryo, with levels an order of magnitude higher than those of iron or copper. Using a zinc-specific fluorescent probe, we demonstrated that labile zinc is distributed in vesicle-like structures in the cortex of cells at all stages of preimplantation embryo development. To test the importance of zinc during this period, we induced zinc insufficiency using the heavy metal chelator N,N,N',N'-tetrakis-(2-pyridylmethyl)-ethylenediamine (TPEN). Incubation of embryos in media containing TPEN resulted in a developmental arrest that was specific to zinc chelation and associated with compromised mitotic parameters. The developmental arrest due to zinc insufficiency was associated with altered chromatin structure in the blastomere nuclei and decreased global transcription.

CONCLUSIONS

These results demonstrate for the first time that the preimplantation embryo requires tight zinc regulation and homeostasis for the initial mitotic divisions of life.

Fe(III) Is Essential for Porcine Embryonic Development via Mitochondrial Function Maintenance

Iron is an important trace element involved in several biological processes. The role of iron in porcine early embryonic development remains unknown. In the present study, we depleted iron (III, Fe³⁺) with deferoxamine (DFM), a specific Fe³⁺ chelator, in cultured porcine parthenotes and monitored embryonic development, apoptosis, mitochondrial membrane potential, and ATP production. Results showed biphasic function of Fe³⁺ in porcine embryo development. 0.5 μM DFM obviously increased blastocyst formation (57.49 ± 2.18% vs. control, 43.99 ± 1.72%, P < 0.05) via reduced (P < 0.05) production of reactive oxygen species (ROS), further increased mitochondrial membrane potential and ATP production in blastocysts (P < 0.05). 0.5 μM DFM decreased mRNA expression of Caspase 3 (Casp3) and increased Bcl-xL. However, results showed a significant reduction in blastocyst formation in the presence of 5.0 μM DFM compared with the control group (DFM, 21.62 ± 3.92% vs. control, 43.99 ± 1.73%, P < 0.05). Fe³⁺ depletion reduced the total (DFM, 21.10 ± 8.78 vs. control, 44.09 ± 13.65, P < 0.05) and increased apoptotic cell number (DFM, 11.10 ± 5.24 vs. control, 2.64 ± 1.43, P < 0.05) in the blastocyst. An obvious reduction in mitochondrial membrane potential and ATP level after 5.0 μM DFM treatment was observed. Co-localization between mitochondria and cytochrome c was reduced after high concentration of DFM treatment. In conclusion, Fe³⁺ is essential for porcine embryonic development via mitochondrial function maintenance, but redundant Fe³⁺ impairs the function of mitochondria.

Pig oocyte activation using a Zn²⁺ chelator, TPEN

Artificial oocyte activation is a critical step during SCNT. Most current activation protocols focus on inducing an increase in the intracellular free Ca(2+) concentration of the oocyte. Here, we have used a zinc chelator, TPEN, to enhance the efficiency of oocyte activation during SCNT. TPEN treatment of matured pig oocytes resulted in the reduction of available Zn(2+) in pig oocytes; however, the cytosolic Ca(2+) concentration in the oocytes was not affected by the TPEN treatment. When various concentrations (100-250 μM) and incubation durations (45 minutes-2.5 hours) of TPEN were used to activate oocytes, the efficiency of oocyte activation was not different from conventional activation methods. When oocytes that were activated by conventional activation methods were incubated with a lower concentration of TPEN (5-10 μM), a significant increase in embryos developing to the blastocyst stage was observed. In addition, when oocytes receiving a small Ca(2+) stimulus were further activated by higher concentration of TPEN (100-200 μM), a significant increase in the frequency of blastocyst formation was observed, compared to a conventional activation method. This result indicated that TPEN can be a main reagent in oocyte activation. No increase in the cytosolic Ca(2+) level was detected when oocytes were exposed to various concentrations of TPEN, indicating the ability of TPEN to induce oocyte activation is independent of an intracellular Ca(2+) increase. We were able to produce clones through SCNT by using the TPEN-assisted activation procedure, and the piglets produced through the process did not show any signs of abnormality. In this study, we have developed an efficient way to use TPEN to increase the developmental potential of cloned embryos.