Viscoelastic Properties of Zona Pellucida of Oocytes Characterized by Transient Electrical Impedance Spectroscopy

The success rate in vitro fertilization is significantly linked to the quality of the oocytes. The oocyte's membrane is encapsulated by a shell of gelatinous extracellular matrix, called zona pellucida, which undergoes dynamic changes throughout the reproduction cycle. During the window of highest fertility, the zona pellucida exhibits a softening phase, while it remains rigid during oocyte maturation and again after fertilization. These variations in mechanical properties facilitate or inhibit sperm penetration. Since successful fertilization considerably depends on the state of the zona pellucida, monitoring of the hardening process of the zona pellucida is vital. In this study, we scrutinized two distinct genetic mouse models, namely, fetuin-B wild-type and fetuin-B/ovastacin double deficient with normal and super-soft zona pellucida, respectively. We evaluated the hardening with the help of a microfluidic aspiration-assisted electrical impedance spectroscopy system. An oocyte was trapped by a microhole connected to a microfluidic channel by applying suction pressure. Transient electrical impedance spectra were taken by microelectrodes surrounding the microhole. The time-depending recovery of zona pellucida deflections to equilibrium was used to calculate the Young's modulus and, for the first time, absolute viscosity values. The values were obtained by fitting the curves with an equivalent mechanical circuit consisting of a network of dashpots and springs. The observer-independent electrical readout in combination with a fitting algorithm for the calculation of the viscoelastic properties demonstrates a step toward a more user-friendly and easy-to-use tool for the characterizing and better understanding of the rheological properties of oocytes.

The E-Modulus of the Oocyte Is a Non-Destructive Measure of Zona Pellucida Hardening

After fertilization, the oocyte-specific metalloproteinase ovastacin is released and cleaves the zona pellucida protein 2 (ZP2), making the zona pellucida impermeable to sperm. Before fertilization, the zona remains permeable because previously released ovastacin is inhibited by fetuin-B. Consequently, in the absence of fetuin-B, ZP2 cleavage occurs prematurely and leads to infertility of female fetuin-B deficient mice. In contrast, fetuin-B/ovastacin double deficient oocytes show a permanently permeable zona with intact ZP2. In this study, we asked if the elastic modulus of the zona pellucida informs about ZP2 cleavage and thus could serve as a new reference of oocyte fertility. Therefore, we determined the elastic modulus of mouse oocytes by nanoindentation as a direct measure of mechanical zona hardening. The elastic modulus reflects ZP2 cleavage, but with more than double sensitivity compared to immunoblot analysis. The elastic modulus measurement allowed to define the range of zona hardening, confined by the extreme states of the zona pellucida in fetuin-B and ovastacin deficient oocytes with cleaved and uncleaved ZP2, respectively. We present here nanoindentation as a method to quantify the effect of potential contributing factors on zona hardening of individual oocytes. To demonstrate this, we showed that mechanical hardening of the zona pellucida is forced by recombinant ovastacin, is inhibited by additional administration of fetuin-B and is unaffected by zinc. Since the change in elastic modulus is induced by ZP2 cleavage an automated elastic modulus measurement of oocytes may serve as a novel sensitive, non-destructive, marker-free and observer-unbiased method for assessing individual oocyte quality.

Dry Film Resist Laminated Microfluidic System for Electrical Impedance Measurements

In micro-electrical-mechanical systems (MEMS), thick structures with high aspect ratios are often required. Dry film photoresist (DFR) in various thicknesses can be easily laminated and patterned using standard UV lithography. Here, we present a three-level DFR lamination process of SUEX for a microfluidic chip with embedded, vertically arranged microelectrodes for electrical impedance measurements. To trap and fix the object under test to the electrodes, an aperture is formed in the center of the ring-shaped electrodes in combination with a microfluidic suction channel underneath. In a proof-of-concept, the setup is characterized by electrical impedance measurements with polystyrene and ZrO2 spheres. The electrical impedance is most sensitive at approximately 2 kHz, and its magnitudes reveal around 200% higher values when a sphere is trapped. The magnitude values depend on the sizes of the spheres. Electrical equivalent circuits are applied to simulate the experimental results with a close match.

Limited proteolysis by acrosin affects sperm-binding and mechanical resilience of the mouse zona pellucida

The encounter of oocyte and sperm is the key event initiating embryonic development in mammals. Crucial functions of this existential interaction are determined by proteolytic enzymes, such as acrosin, carried in the sperm head acrosome, and ovastacin, stored in the oocyte cortical granules. Ovastacin is released upon fertilisation to cleave the zona pellucida, a glycoprotein matrix surrounding the oocyte. This limited proteolysis hardens the oocyte envelope, and thereby provides a definitive block against polyspermy and protects the developing embryo. On the other hand, acrosin, the renowned and most abundant acrosomal protease, has been thought to enable sperm to penetrate the oocyte envelope. Depending on the species, proteolytic cleavage of the zona pellucida by acrosin is either essential or conducive for fertilisation. However, the specific target cleavage sites and the resulting physiological consequences of this proteolysis remained obscure. Here, we treated native mouse zonae pellucidae with active acrosin and identified two cleavage sites in zona pellucida protein 1 (ZP1), five in ZP2 and one in ZP3 by mass spectrometry. Several of these sites are highly conserved in mammals. Remarkably, limited proteolysis by acrosin leads to zona pellucida remodelling rather than degradation. Thus, acrosin affects both sperm binding and mechanical resilience of the zona pellucida, as assessed by microscopy and nanoindentation measurements, respectively. Furthermore, we ascertained potential regulatory effects of acrosin, via activation of latent pro-ovastacin and inactivation of fetuin-B, a tight binding inhibitor of ovastacin. These results offer novel insights into the complex proteolytic network modifying the extracellular matrix of the mouse oocyte, which might apply also to other species.

The C-terminal region of human plasma fetuin-B is dispensable for the raised-elephant-trunk mechanism of inhibition of astacin metallopeptidases

Human fetuin-B plays a key physiological role in human fertility through its inhibitory action on ovastacin, a member of the astacin family of metallopeptidases. The inhibitor consists of tandem cystatin-like domains (CY1 and CY2), which are connected by a linker containing a "CPDCP-trunk" and followed by a C-terminal region (CTR) void of regular secondary structure. Here, we solved the crystal structure of the complex of the inhibitor with archetypal astacin from crayfish, which is a useful model of human ovastacin. Two hairpins from CY2, the linker, and the tip of the "legumain-binding loop" of CY1 inhibit crayfish astacin following the "raised-elephant-trunk mechanism" recently reported for mouse fetuin-B. This inhibition is exerted by blocking active-site cleft sub-sites upstream and downstream of the catalytic zinc ion, but not those flanking the scissile bond. However, contrary to the mouse complex, which was obtained with fetuin-B nicked at a single site but otherwise intact, most of the CTR was proteolytically removed during crystallization of the human complex. Moreover, the two complexes present in the crystallographic asymmetric unit diverged in the relative arrangement of CY1 and CY2, while the two complexes found for the mouse complex crystal structure were equivalent. Biochemical studies in vitro confirmed the differential cleavage susceptibility of human and mouse fetuin-B in front of crayfish astacin and revealed that the cleaved human inhibitor blocks crayfish astacin and human meprin α and β only slightly less potently than the intact variant. Therefore, the CTR of animal fetuin-B orthologs may have a function in maintaining a particular relative orientation of CY1 and CY2 that nonetheless is dispensable for peptidase inhibition.

Structure of mammalian plasma fetuin-B and its mechanism of selective metallopeptidase inhibition

Mammalian fetuin-A and fetuin-B are abundant serum proteins with pleiotropic functions. Fetuin-B is a highly selective and potent inhibitor of metallo-peptidases (MPs) of the astacin family, which includes ovastacin in mammals. By inhibiting ovastacin, fetuin-B is essential for female fertility. The crystal structure of fetuin-B was determined unbound and in complex with archetypal astacin, and it was found that the inhibitor has tandem cystatin-type modules (CY1 and CY2). They are connected by an exposed linker with a rigid, disulfide-linked 'CPDCP-trunk', and are followed by a C-terminal region (CTR) with little regular secondary structure. The CPDCP-trunk and a hairpin of CY2 form a bipartite wedge, which slots into the active-site cleft of the MP. These elements occupy the nonprimed and primed sides of the cleft, respectively, but spare the specificity pocket so that the inhibitor is not cleaved. The aspartate in the trunk blocks the catalytic zinc of astacin, while the CY2 hairpin binds through a QWVXGP motif. The CY1 module assists in structural integrity and the CTR is not involved in inhibition, as verified by in vitro studies using a cohort of mutants and variants. Overall, the inhibition conforms to a novel 'raised-elephant-trunk' mechanism for MPs, which is reminiscent of single-domain cystatins that target cysteine peptidases. Over 200 sequences from vertebrates have been annotated as fetuin-B, underpinning its ubiquity and physiological relevance; accordingly, sequences with conserved CPDCP- and QWVXGP-derived motifs have been found from mammals to cartilaginous fishes. Thus, the raised-elephant-trunk mechanism is likely to be generally valid for the inhibition of astacins by orthologs of fetuin-B.

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.

Latent TGF-β binding protein-1 deficiency decreases female fertility

The four latent transforming growth factor-β (TGF-β) binding proteins LTBP1-4 are extracellular matrix-associated proteins playing a critical role in the activation of TGF-β. The LTBP1 gene forms two major transcript variants (i.e. Ltbp1S and Ltbp1L) that are derived from different promoters. We have previously shown the importance of LTBP1 in vivo by using three different Ltbp1 null mice that were either deleted for exons 1 and 2 (Ltbp1L knockout), exon 5 (Ltbp1ΔEx5), or exon 8 (Ltbp1ΔEx8). While the Ltbp1L knockout and the Ltbp1ΔEx8 are perinatal lethal and die of cardiovascular abnormalities, the Ltbp1ΔEx5 is viable because it expresses a short form of Ltbp1L that lacks 55 amino acids (Δ55 variant of Ltbp1) formed by splicing out exon 5, while lacking the Ltbp1S variant. Since only the Ltbp1ΔEx5 mouse is viable, we have used this model to address aspects of puberty, fertility, age-dependent reproduction, and ovary function. We report for the first time a function of LTBP1 in female reproduction. The Ltbp1ΔEx5 females showed impaired fertility associated with delayed sexual maturity (p = 0.0074) and ovarian cyst formation in females older than 40 weeks (p = 0.0204).

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.

Association of high fetuin-B concentrations in serum with fertilization rate in IVF: a cross-sectional pilot study

STUDY QUESTION

Is serum fetuin-B associated with the fertilization rate in in vitro fertilization (IVF)?

SUMMARY ANSWER

Serum fetuin-B increased during IVF cycles when oocytes could be fertilized while remained unchanged in fertilization failure.

WHAT IS KNOWN ALREADY

Fetuin-B deficiency in mice causes premature zona pellucida hardening mediated by the zona protease ovastacin. Thus fetuin-B deficiency renders females infertile.

STUDY DESIGN, SIZE, DURATION

We determined the human serum fetuin-B reference range, studying longitudinally, over the course of one month, five male and seven female volunteers without hormone treatment and four female volunteers on varying hormonal contraception. We sampled blood and determined serum fetuin-B, luteinizing hormone (LH), estradiol (E2) and progesterone (P4). In addition, we determined serum fetuin-B and estradiol in eight women undergoing intracytoplasmatic sperm injection (ICSI, nine ICSI cycles) and 19 women undergoing IVF (21 IVF cycles) after ovarian stimulation with recombinant human follicular stimulating hormone (rFSH) and/or a combined medication of FSH and LH. At least three blood samples were analyzed in each cycle. We compared serum fetuin-B and follicular fluid fetuin-B in nine patients by measuring follicular fetuin-B in pooled follicular fluid, and in fluid obtained from individual follicles. Samples were drawn from January 2012 to March 2014.

PARTICIPANTS/MATERIALS, SETTING, METHOD

All volunteers and patients gave informed consent. Fetuin-B was measured employing a commercial sandwich enzyme-linked immunosorbent assay. Serum fetuin-B was determined as duplicates in 5 male (34 ± 14.6 years) and 11 female volunteers (29.4 ± 4.1 years) as well as in female volunteers on hormonal contraception (30.0 ± 6.5 years). The duplicate standard deviation was 4.0 ± 2.3%. The contraceptive drugs were mono or combined preparations containing 0-0.03 mg ethinyl estradiol, and 0.15-3.0 mg of various progestins. In addition, serum fetuin-B was determined as triplicates in 27 female patients undergoing conventional IVF (19) or ICSI (8). The triplicate standard deviation was 3.3 ± 1.8%. IVF was declared as 'successful', if at least one oocyte was fertilized, and 'unsuccessful', if no oocyte could be fertilized. Patient age was 34.4 ± 4.4 years in successful IVF, and 35.4 ± 3.3 years in unsuccessful IVF. Serum and follicular fluid of patients undergoing controlled ovarian hyperstimulation were analyzed. Serum was drawn at the day of follicle aspiration.

MAIN RESULTS AND THE ROLE OF CHANCE

Serum fetuin-B and follicular fluid fetuin-B were not significantly different in six out of nine patients suggesting, in principle, free exchange of fetuin-B between serum and follicular fluid. Thus serum fetuin-B may be used as a proxy of follicular fluid fetuin-B. Serum fetuin-B increased during successful IVF cycles (n = 15, P < 0.0001), but did not change in unsuccessful IVF cycles (n = 6, P = 0.118) despite increased estradiol levels (P = 0.0019 and P = 0.0254, respectively).

LIMITATIONS, REASONS FOR CAUTION

The female volunteers self-reported their respective hormone medication. Medication was verified by serum estradiol, LH and progesterone measurements. For oocyte harvesting, the vaginal wall was punctured once only to minimize co-morbidity. Low grade cross-contamination of individual follicular fluid aspirates and contamination of the follicular fluid with small amounts of blood were inevitable. Samples were routinely checked for the presence of hemoglobin that would suggest blood contamination. Only samples containing <250 erythrocyte equivalents/µl were used for analysis.

WIDER IMPLICATIONS OF THE FINDING

Serum fetuin-B may be used as a marker to predict the fertilization success in IVF. Fetuin-B levels attained during IVF stimulation may help to make an informed decision whether oocytes should be fertilized by IVF or by ICSI to overcome the zona pellucida as a barrier.

STUDY FUNDING/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. J.F., E.D., J.N., B.R. and W.J.-D. declare that they are named inventors on the RWTH Aachen University patent application EP 13157317.2, 'Use of fetuin-B for culture of oocytes', applied for by RWTH Aachen University.