Improved low-invasive mRNA electroporation method into immature mouse oocytes visualizes protein dynamics during development†

One of the major causes of oocyte quality deterioration, chromosome segregation abnormalities manifest mainly during meiosis I, which occurs before and during ovulation. However, currently, there is a technical limitation in the introduction of mRNA into premature oocytes without impairing embryonic developmental ability. In this study, we established a low-invasive electroporation (EP) method to introduce mRNA into pre-ovulatory, germinal vesicle (GV) mouse oocytes in an easier manner than the traditional microinjection method. The EP method with an optimized impedance value resulted in the efficient introduction of mRNAs encoding enhanced green fluorescent protein (EGFP) into the GV oocytes surrounded by cumulus cells at a survival rate of 95.0%. Furthermore, the introduction of histone H2B-EGFP mRNA into the GV oocytes labeled most of the oocytes without affecting the blastocyst development rate, indicating the feasibility of the visualization of oocyte chromosomal dynamics that enable us to assay chromosomal integrity in oocyte maturation and cell count in embryonic development. The establishment of this EP method offers extensive assays to select pre-implantation embryos and enables the surveying of essential factors for mammalian oocyte quality determination.

Substrate profiling of the metalloproteinase ovastacin uncovers specific enzyme-substrate interactions and discloses fertilization-relevant substrates

The metalloproteinase ovastacin is released by the mammalian egg upon fertilization and cleaves a distinct peptide bond in zona pellucida protein 2 (ZP2), a component of the enveloping extracellular matrix. This limited proteolysis causes zona pellucida hardening, abolishes sperm binding, and thereby regulates fertility. Accordingly, this process is tightly controlled by the plasma protein fetuin-B, an endogenous competitive inhibitor. At present, little is known about how the cleavage characteristics of ovastacin differ from closely related proteases. Physiological implications of ovastacin beyond ZP2 cleavage are still obscure. In this study, we employed N-terminal amine isotopic labeling of substrates (N-TAILS) contained in the secretome of mouse embryonic fibroblasts to elucidate the substrate specificity and the precise cleavage site specificity. Furthermore, we were able to unravel the physicochemical properties governing ovastacin-substrate interactions as well as the individual characteristics that distinguish ovastacin from similar proteases, such as meprins and tolloid. Eventually, we identified several substrates whose cleavage could affect mammalian fertilization. Consequently, these substrates indicate newly identified functions of ovastacin in mammalian fertilization beyond zona pellucida hardening.

Ovastacin: An oolemma protein that cleaves the zona pellucida to prevent polyspermy

Monospermy occurs in the process of normal fertilization where a single sperm fuses with the egg, resulting in the formation of a diploid zygote. During the process of fertilization, the sperm must penetrate the zona pellucida (ZP), the outer layer of the egg, to reach the egg's plasma membrane. Once a sperm binds to the ZP, it undergoes an acrosomal reaction, which involves the release of enzymes from the sperm's acrosome that help it to penetrate the ZP. Ovastacin is one of the enzymes that is involved in breaking down the ZP. Studies have shown that ovastacin is necessary for the breakdown of the ZP and for successful fertilization to occur. However, the activity of ovastacin is tightly regulated to ensure that only one sperm can fertilize the egg. One way in which ovastacin helps to prevent polyspermy (the fertilization of an egg by more than one sperm) is by rapidly degrading the ZP after a sperm has penetrated it. This makes it difficult for additional sperm to penetrate the ZP and fertilize the egg. Ovastacin is also thought to play a role in the block to polyspermy, a mechanism that prevents additional sperm from fusing with the egg's plasma membrane after fertilization has occurred. In summary, the role of ovastacin in monospermic fertilization is to help ensure that only one sperm can fertilize the egg, while preventing polyspermy and ensuring successful fertilization.

Predıctıve value of pregnancy of follıcular fluıd fetuın-A and -B levels ın infertıle women after intra-cytoplasmic sperm injection

AIM

We aimed to investigate the value of follicular fluid fetuins-A and -B to predict successful IVF and pregnancy outcomes in infertile women with poor, normal, and high ovarian reserve.

METHODS

The follicular fluid of 96 infertile women who underwent intra-cytoplasmic sperm injection (ICSI) procedure was analyzed. Fetuins-A and -B levels were examined and compared in those who could achieve pregnancy and those who could not. Receiver operating characteristic curve analyzes were used to determine cut-off and statistically significant associations for fetuins-A and -B.

RESULTS

Follicular fluid fetuin-A levels were higher in cases with weak ovarian reserve (OR) (p < 0.05) and higher in patients who did not achieve clinical pregnancy (p < 0.05). Conversely, the follicular fluid fetuin-B levels were lower in cases with poor OR (p < 0.05) and were lower in patients who did not achieve a clinical pregnancy (p < 0.05). A follicular fluid fetuin-A concentration ≤ 19.12 ng/mL had a sensitivity and specificity of 94.74% and 93.1%, respectively, at predicting clinical pregnancy. While the follicular fluid fetuin-B concentration >24.7 ng/mL had sensitivity and specificity of 71.1% and 51.7%, respectively, for clinical pregnancy prediction.

CONCLUSION

Overall, high levels of follicular fluid fetuin-A may be independently associated with unsuccessful IVF irrespective of OR grouping. A low level of follicular fetuin-B was also associated with failed IVF. The sensitivity and specificity were found to be higher for fetuin-A in predicting clinical pregnancy. Therefore, the follicular fluid fetuin-A may be more predictive for successful IVF and clinical pregnancy outcomes than follicular fluid fetuin-B.

Coenzyme Q10 ameliorates the quality of mouse oocytes during in vitro culture

Oxidative stress causes several diseases and dysfunctions in cells, including oocytes. Clearly, oxidative stress influences oocyte quality during in vitro maturation and fertilization. Here we tested the ability of coenzyme Q10 (CoQ10) to reduce reactive oxygen species (ROS) and improve mouse oocyte quality during in vitro culture. Treatment with 50 μM CoQ10 efficiently reduced ROS levels in oocytes cultured in vitro. The fertilizable form of an oocyte usually contains a cortical granule-free domain (CGFD). CoQ10 enhanced the ratio of CGFD-oocytes from 35% to 45%. However, the hardening of the zona pellucida in oocytes was not affected by CoQ10 treatment. The in vitro maturation capacity of oocytes, which was determined by the first polar body extrusion, was enhanced from 48.9% to 75.7% by the addition of CoQ10 to the culture medium. During the parthenogenesis process, the number of two-cell embryos was increased by CoQ10 from 43.5% to 67.3%. Additionally, treatment with CoQ10 increased the expression of Bcl2 and Sirt1 in cumulus cells. These results suggested that CoQ10 had a positive effect on ROS reduction, maturation rate and two-cell embryo formation in mouse oocyte culture.

Adipocytes-Derived Extracellular Vesicle-miR-26b Promotes Apoptosis of Cumulus Cells and Induces Polycystic Ovary Syndrome

BACKGROUND

Polycystic ovary syndrome (PCOS) is a refractory reproductive disease and also a kind of endocrine and metabolic disease. Adipocyte cells can produce a mass of extracellular vesicles and orchestrate the status of other types cells. The objective of this study was to determine the effects of adipocyte-derived extracellular vesicles-miR-26b on cumulus cells (CCs) and development of PCOS.

METHODS

The crosstalk mediated by extracellular vesicle-miR-26b between adipocytes and CCs was determined in CC cells co-cultured with mature adipocytes or incubated with extracellular vesicle isolated from mature adipocytes. CCK-8 assay and flow cytometry were conducted in CCs treated with or without extracellular vesicles; microRNA (miRNA) sequencing was conducted for clarifying the key molecular. Hormone levels and ovary ovulation ability were conducted with animal experiment.

RESULTS

The results revealed that miR-26b was upregulated in extracellular vesicles derived from mature adipocytes. Adipocyte-derived extracellular vesicles inhibited viability and promoted apoptosis in CCs via targeting JAG1. Furthermore, extracellular vesicles derived from mature adipocyte disrupted the ovary ovulation and impaired the hormone levels.

CONCLUSIONS

These results identify a novel signaling pathway that adipocytes-derived extracellular vesicles-miR-26b promotes cell apoptosis in CCs and disrupted the ovary ovulation in the development of PCOS. The study indicates that adipose tissue-derived extracellular vesicles-miR-26b may play a key role in the PCOS and also provides insight into developing new therapeutic strategies for PCOS.

A Primary Evaluation of Potential Small-Molecule Inhibitors of the Astacin Metalloproteinase Ovastacin, a Novel Drug Target in Female Infertility Treatment

Despite huge progress in hormonal therapy and improved in vitro fertilization methods, the success rates in infertility treatment are still limited. A recently discovered mechanism revealed the interplay between the plasma protein fetuin-B and the cortical granule-based proteinase ovastacin to be a novel key mechanism in the regulation of fertilization. Upon sperm-egg fusion, cleavage of a distinct zona pellucida component by ovastacin destroys the sperm receptor, enhances zona robustness, and eventually provides a definitive block against polyspermy. An untimely onset of this zona hardening prior to fertilization would consequently result in infertility. Physiologically, this process is controlled by fetuin-B, an endogenous ovastacin inhibitor. Here we aimed to discover small-molecule inhibitors of ovastacin that could mimic the effect of fetuin-B. These compounds could be useful lead structures for the development of specific ovastacin inhibitors that can be used in infertility treatment or in vitro fertilization.

Mammalian egg coat modifications and the block to polyspermy

Fertilization by more than one sperm causes polyploidy, a condition that is generally lethal to the embryo in the majority of animal species. To prevent this occurrence, eggs have developed a series of mechanisms that block polyspermy at the level of the plasma membrane or their extracellular coat. In this review, we first introduce the mammalian egg coat, the zona pellucida (ZP), and summarize what is currently known about its composition, structure, and biological functions. We then describe how this specialized extracellular matrix is modified by the contents of cortical granules (CG), secretory organelles that are exocytosed by the egg after gamete fusion. This process releases proteases, glycosidases, lectins and zinc onto the ZP, resulting in a series of changes in the properties of the egg coat that are collectively referred to as hardening. By drawing parallels with comparable modifications of the vitelline envelope of nonmammalian eggs, we discuss how CG‐dependent modifications of the ZP are thought to contribute to the block to polyspermy. Moreover, we argue for the importance of obtaining more information on the architecture of the ZP, as well as systematically investigating the many facets of ZP hardening.

Perfect date-the review of current research into molecular bases of mammalian fertilization

Fertilization is a multistep process during which two terminally differentiated haploid cells, an egg and a sperm, combine to produce a totipotent diploid zygote. In the early 1950s, it became possible to fertilize mammalian eggs in vitro and study the sequence of cellular and molecular events leading to embryo development. Despite all the achievements of assisted reproduction in the last four decades, remarkably little is known about the molecular aspects of human conception. Current fertility research in animal models is casting more light on the complexity of the process all our lives start with. This review article provides an update on the investigation of mammalian fertilization and highlights the practical implications of scientific discoveries in the context of human reproduction and reproductive medicine.

Serum and follicular fluid fetuin-B levels are correlated with fertilization rates in conventional IVF cycles

PURPOSE

To investigate the relationship between serum/follicular fluid fetuin-B levels and fertilization outcomes in conventional IVF cycles.

METHODS

A prospective cohort study of conventional IVF treatments including 78 cycles with low fertilization rates (two pronuclei [2PN] rate < 30%; LF group) and 104 cycles performed during the same period with 2PN rate > 70% (high fertilization group, HF). To calculate the required sample size, a two-sample t test power analysis was applied to data from our pilot study, using PASS 11.0 software. Fetuin-B was measured using a commercial sandwich enzyme-linked immunosorbent assay.

RESULTS

Serum fetuin-B and follicular fluid fetuin-B were positively correlated (r = 0.703, P < 0.001). Compared to the HF group, the LF group had significantly lower levels of fetuin-B, both in serum (5.81 ± 1.53 vs. 7.19 ± 1.42, P < 0.001) and follicular fluid (5.06 ± 1.29 vs. 6.16 ± 1.52, P < 0.001). The serum fetuin-B level from cycles with polypronuclear (PPN) zygotes was significantly lower when compared to cycles without PPN zygotes (6.82 ± 1.65 vs. 6.10 ± 1.43, P = 0.006). However, serum fetuin-B level was not correlated with preimplantation embryo development or clinical pregnancy.

CONCLUSION

Serum fetuin-B level is correlated with fertilization rate in conventional IVF and it may be used as a predictive marker of fertilization in IVF treatment.

Mammalian plasma fetuin-B is a selective inhibitor of ovastacin and meprin metalloproteinases

Vertebrate fetuins are multi-domain plasma-proteins of the cystatin-superfamily. Human fetuin-A is also known as AHSG, α₂-Heremans-Schmid-glycoprotein. Gene-knockout in mice identified fetuin-A as essential for calcified-matrix-metabolism and bone-mineralization. Fetuin-B deficient mice, on the other hand, are female infertile due to zona pellucida ‘hardening’ caused by the metalloproteinase ovastacin in unfertilized oocytes. In wildtype mice fetuin-B inhibits the activity of ovastacin thus maintaining oocytes fertilizable. Here we asked, if fetuins affect further proteases as might be expected from their evolutionary relation to single-domain-cystatins, known as proteinase-inhibitors. We show that fetuin-A is not an inhibitor of any tested protease. In stark contrast, the closely related fetuin-B selectively inhibits astacin-metalloproteinases such as meprins and ovastacin, but not astacins of the tolloid-subfamily, nor any other proteinase. The analysis of fetuin-B expressed in various mammalian cell types, insect cells, and truncated fish-fetuin expressed in bacteria, showed that the cystatin-like domains alone are necessary and sufficient for inhibition. This report highlights fetuin-B as a specific antagonist of ovastacin and meprin-metalloproteinases. Control of ovastacin was shown to be indispensable for female fertility. Meprin inhibition, on the other hand, renders fetuin-B a potential key-player in proteolytic networks controlling angiogenesis, immune-defense, extracellular-matrix-assembly and general cell-signaling, with implications for inflammation, fibrosis, neurodegenerative disorders and cancer.

Intracellular activation of ovastacin mediates pre-fertilization hardening of the zona pellucida

STUDY QUESTION

How and where is pro-ovastacin activated and how does active ovastacin regulate zona pellucida hardening (ZPH) and successful fertilization?

STUDY FINDING

Ovastacin is partially active before exocytosis and pre-hardens the zona pellucida (ZP) before fertilization.

WHAT IS KNOWN ALREADY

The metalloproteinase ovastacin is stored in cortical granules, it cleaves zona pellucida protein 2 (ZP2) upon fertilization and thereby destroys the ZP sperm ligand and triggers ZPH. Female mice deficient in the extracellular circulating ovastacin-inhibitor fetuin-B are infertile due to pre-mature ZPH.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

We isolated oocytes from wild-type and ovastacin-deficient (Astlnull) FVB mice before and after fertilization (in vitro and in vivo) and quantified ovastacin activity and cleavage of ZP2 by immunoblot. We assessed ZPH by measuring ZP digestion time using α-chymotrypsin and by determining ZP2 cleavage. We determined cellular distribution of ovastacin by immunofluorescence using domain-specific ovastacin antibodies. Experiments were performed at least in triplicate with a minimum of 20 oocytes. Data were pre-analyzed using Shapiro-Wilk test. In case of normal distribution, significance was determined via two-sided Student's t-test, whereas in case of non-normal distribution via Mann-Whitney U-test.

MAIN RESULTS AND THE ROLE OF CHANCE

Metaphase II (MII) oocytes contained both inactive pro-ovastacin and activated ovastacin. Immunoblot and ZP digestion assays revealed a partial cleavage of ZP2 even before fertilization in wild-type mice. Partial cleavage coincided with germinal-vesicle breakdown and MII, despite the presence of fetuin-B protein, an endogenous ovastacin inhibitor, in the follicular and oviductal fluid. Upon exocytosis, part of the C-terminal domain of ovastacin remained attached to the plasmalemma, while the N-terminal active ovastacin domain was secreted. This finding may resolve previously conflicting data showing that ovastacin acts both as an oolemmal receptor termed SAS1B (sperm acrosomal SLLP1 binding protein; SLLP, sperm lysozyme like protein) and a secreted protease mediating ZP2 cleavage.

LIMITATIONS, REASONS FOR CAUTION

For this study, only oocytes isolated from wild-type and ovastacin-deficient FVB mice were investigated. Some experiments involved oocyte activation by the Ca2+ ionophore A23187 to trigger ZPH.

WIDER IMPLICATIONS OF THE FINDINGS

This study provides a detailed spatial and temporal view of pre-mature cleavage of ZP2 by ovastacin, which is known to adversely affect IVF rate in mice and humans.

LARGE SCALE DATA

None.

STUDY FUNDING AND COMPETING INTEREST(S)

This work was supported by the Center of Natural Sciences and Medicine and by a start-up grant of the Johannes Gutenberg University Mainz to W.S., and by a grant from Deutsche Forschungsgemeinschaft and by the START program of the Medical Faculty of RWTH Aachen University to J.F. and W.J.D. There are no competing interests to declare.

Down-regulation of the liver-derived plasma protein fetuin-B mediates reversible female infertility

STUDY QUESTION

Does antisense oligonucleotide (ASO)-mediated down-regulation of serum fetuin-B cause infertility like fetuin-B gene deficiency in female mice?

SUMMARY ANSWER

Pharmacological fetuin-B down-regulation by ASO therapy results in reversible infertility in female mice.

WHAT IS KNOWN ALREADY

Female fetuin-B deficient (Fetub^-/-) mice are infertile owing to premature zona pellucida (ZP) hardening. Enzyme activity studies demonstrated that fetuin-B is a potent and highly specific inhibitor of the zona proteinase ovastacin, which cleaves ZP protein 2 (ZP2) and thus mediates definitive ZP hardening.

STUDY DESIGN, SIZE, DURATION

Ten fetuin-B ASO boli (100 mg/kg) were injected s.c. over 20 days in 12 female mice, and 10 phosphate-buffered saline (PBS)-treated mice were used as control. At day 20 females were mated to evaluate fetuin-B as a potential molecular target for contraception. ASO and PBS treatment was continued for ten injections. After treatment cessation at day 50, mating was continued to investigate if infertility was reversible.

PARTICIPANTS/MATERIALS, SETTING, METHODS

We generated fetuin-B/ovastacin double deficient (Fetub^-/-, Astl^-/-) mice by conventional breeding to test if fertility of Fetub^-/- female mice was restored when the target proteinase would likewise be deleted. At least five matings with each female genotype (Fetub^-/- single deficient, Astl^-/- single deficient, Fetub^-/-, Astl^-/- double deficient) were performed. To test the contraceptive effect of fetuin-B down-regulation, 22 female mice (6-13 weeks old) were treated with repetitive boli of 100 mg/kg fetuin-B ASO (n = 12) or PBS (n = 10) and mated continuously. Serum fetuin-B was determined by immunoblot before, during and after the ASO treatment. After 3 weeks of ASO treatment, in 6 females Fetub mRNA in liver was analyzed by PCR, and six PBS-treated females were used as control. Aspartate (AST) and alanine aminotransferase (ALT) were also measured in serum of six mice in each group. To determine the minimum permissive serum fetuin-B concentration required for successful fertilization IVF was performed in five fetuin-B ASO-treated mice. As a control, six females were injected with control oligonucleotides and six females were left untreated.

MAIN RESULTS AND THE ROLE OF CHANCE

Fertility of Fetub^-/- female mice was restored by additional ovastacin deficiency (Astl^-/-). Unlike Fetub^-/- mice, female Fetub^-/-, Astl^-/- mice were fertile, confirming ovastacin as a primary molecular target of fetuin-B. At day 20, after receiving 10 fetuin-B ASO boli, serum fetuin-B was down-regulated to 8 ± 6% (mean ± SD) of baseline level. Fetuin-B down-regulation was confirmed at the mRNA level. Fetuin-B ASO-treated females had 12.1 ± 3.1% of the liver Fetub mRNA level seen in PBS-treated females. In the following mating study, 11 out of 12 mated females failed to become pregnant during 50 days of ASO treatment and continuous mating from day 20 onwards. IVF of oocytes derived from ASO-treated females suggested that a serum fetuin-B level of less than 10 µg/ml was required to prevent pregnancy. Withdrawal of ASO treatment normalized serum fetuin-B and restored fertility; all female mice became pregnant and had litters within 60.3 ± 35.9 days after cessation of ASO treatment. The first litter was significantly smaller than that of control mice (4.6 ± 2.3 versus 6.7 ± 1.8 pups, n = 20, P = 0.04) but the smaller litter size was only temporary. The size of the second litter was similar to the first litter of control mice (7.6 ± 1.3 versus 6.7 ± 1.8 pups, n = 18, P = 0.25).

LIMITATIONS, REASONS FOR CAUTION

The repeated dose of 100 mg/kg fetuin-B ASO boli caused an increased serum ALT and AST activity, suggesting hepatotoxicity. Daily vaginal plug checks indicated successful mating, but mating plugs in ASO-treated mice were less stable (vaginal tract not closed) than in control mice.

WIDER IMPLICATIONS OF THE FINDINGS

Pharmacological fetuin-B down-regulation in mice caused reversible infertility. Control of ovastacin proteinase activity by fetuin-B is a necessary determinant of female fertility that can serve as a target for female contraception. Although promising in terms of human contraception, further studies analyzing the balance between sufficient fetuin-B down-regulation and tolerable side effects are required to improve safety before transfer into human reproductive biology can be considered.

LARGE SCALE DATA

None.

STUDY FUNDING AND COMPETING INTERESTS

The research was supported by a grant from Deutsche Forschungsgemeinschaft and by the START program of the Medical Faculty of RWTH Aachen University. The authors E.D., J.F. and W.J.-D. are named inventors on a patent application of RWTH Aachen University covering the use of fetuin-B in ovary and oocyte culture. No conflict of interest is declared by C.S. and A.C.