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Oviduct-specific glycoprotein and heparin modulate sperm-zona pellucida interaction during fertilization and contribute to the control of polyspermy.

ABSTRACT: Polyspermy is an important anomaly of fertilization in placental mammals, causing premature death of the embryo. It is especially frequent under in vitro conditions, complicating the successful generation of viable embryos. A block to polyspermy develops as a result of changes after sperm entry (i.e., cortical granule exocytosis). However, additional factors may play an important role in regulating polyspermy by acting on gametes before sperm-oocyte interaction. Most studies have used rodents as models, but ungulates may differ in mechanisms preventing polyspermy. We hypothesize that zona pellucida (ZP) changes during transit of the oocyte along the oviductal ampulla modulate the interaction with spermatozoa, contributing to the regulation of polyspermy. We report here that periovulatory oviductal fluid (OF) from sows and heifers increases (both, con- and heterospecifically) ZP resistance to digestion with pronase (a parameter commonly used to measure the block to polyspermy), changing from digestion times of approximately 1 min (pig) or 2 min (cattle) to 45 min (pig) or several hours (cattle). Exposure of oocytes to OF increases monospermy after in vitro fertilization in both species, and in pigs, sperm-ZP binding decreases. The resistance of OF-exposed oocytes to pronase was abolished by exposure to heparin-depleted medium; in a medium with heparin it was not altered. Proteomic analysis of the content released in the heparin-depleted medium after removal of OF-exposed oocytes allowed the isolation and identification of oviduct-specific glycoprotein. Thus, an oviduct-specific glycoprotein-heparin protein complex seems to be responsible for ZP changes in the oviduct before fertilization, affecting sperm binding and contributing to the regulation of polyspermy.

SUBMITTER: Coy P

PROVIDER: S-EPMC2572915 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Despite decades of research, the mechanism by which the fertilizing spermatozoon penetrates the mammalian vitelline membrane, the zona pellucida (ZP) remains one of the unexplained fundamental events of human/mammalian development. Evidence has been accumulating in support of the 26S proteasome as a candidate for echinoderm, ascidian and mammalian egg coat lysin. Monitoring ZP protein degradation by sperm during fertilization is nearly impossible because those few spermatozoa that penetrate the ZP leave behind a virtually untraceable residue of degraded proteins. We have overcome this hurdle by designing an experimentally consistent in vitro system in which live boar spermatozoa are co-incubated with ZP-proteins (ZPP) solubilized from porcine oocytes. Using this assay, mimicking sperm-egg interactions, we demonstrate that the sperm-borne proteasomes can degrade the sperm receptor protein ZPC. Upon coincubation with motile spermatozoa, the solubilized ZPP, which appear to be ubiquitinated, adhered to sperm acrosomal caps and induced acrosomal exocytosis/formation of the acrosomal shroud. The degradation of the sperm receptor protein ZPC was assessed by Western blotting band-densitometry and proteomics. A nearly identical pattern of sperm receptor degradation, evident already within the first 5 min of coincubation, was observed when the spermatozoa were replaced with the isolated, enzymatically active, sperm-derived proteasomes. ZPC degradation was blocked by proteasomal inhibitors and accelerated by ubiquitin-aldehyde(UBAL), a modified ubiquitin protein that stimulates proteasomal proteolysis. Such a degradation pattern of ZPC is consistent with in vitro fertilization studies, in which proteasomal inhibitors completely blocked fertilization, and UBAL increased fertilization and polyspermy rates. Preincubation of intact zona-enclosed ova with isolated active sperm proteasomes caused digestion, abrasions and loosening of the exposed zonae, and significantly reduced the fertilization/polyspermy rates after IVF, accompanied by en-mass detachment of zona bound sperm. Thus, the sperm borne 26S proteasome is a candidate zona lysin in mammals. This new paradigm has implications for contraception and assisted reproductive technologies in humans, as well as animals.

Project description:Study questionHow and where is pro-ovastacin activated and how does active ovastacin regulate zona pellucida hardening (ZPH) and successful fertilization?Study findingOvastacin is partially active before exocytosis and pre-hardens the zona pellucida (ZP) before fertilization.What is known alreadyThe 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, methodsWe 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 chanceMetaphase 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 cautionFor 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 findingsThis 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 dataNone.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.

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Oviduct-specific glycoprotein and heparin modulate sperm-zona pellucida interaction during fertilization and contribute to the control of polyspermy.

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