Sperm binding characteristics of the murine zona pellucida

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.

The molecular mechanisms mediating mammalian fertilization

Fertilization is a key biological process in which the egg and sperm must recognize one another and fuse to form a zygote. Although the process is a continuum, mammalian fertilization has been studied as a sequence of steps: sperm bind and penetrate through the zona pellucida of the egg, adhere to the egg plasma membrane and finally fuse with the egg. Following fusion, effective blocks to polyspermy ensure monospermic fertilization. Here, we review how recent advances obtained using genetically modified mouse lines bring new insights into the molecular mechanisms regulating mammalian fertilization. We discuss models for these processes and we include studies showing that these mechanisms may be conserved across different mammalian species.

Glycan-Independent Gamete Recognition Triggers Egg Zinc Sparks and ZP2 Cleavage to Prevent Polyspermy

The zona pellucida surrounding ovulated eggs regulates monospermic fertilization necessary for successful development. Using mouse transgenesis, we document that the N terminus of ZP2 is sufficient for sperm binding to the zona matrix and for in vivo fertility. Sperm binding is independent of ZP2 glycans and does not occur after complete cleavage of ZP2 by ovastacin, a zinc metalloendopeptidase stored in egg cortical granules. Immediately following fertilization, a rapid block to sperm penetration of the zona pellucida is established that precedes ZP2 cleavage but requires ovastacin enzymatic activity. This block to penetration is associated with release of zinc from cortical granules coincident with exocytosis. High levels of zinc affect forward motility of sperm to prevent their passage through the zona matrix. This transient, post-fertilization block to sperm penetration provides a temporal window to complete the cleavage of ZP2, which prevents sperm binding to ensure monospermy.

A Unique Egg Cortical Granule Localization Motif Is Required for Ovastacin Sequestration to Prevent Premature ZP2 Cleavage and Ensure Female Fertility in Mice

Monospermic fertilization is mediated by the extracellular zona pellucida composed of ZP1, ZP2 and ZP3. Sperm bind to the N-terminus of ZP2 which is cleaved after fertilization by ovastacin (encoded by Astl) exocytosed from egg cortical granules to prevent sperm binding. Astl^Null mice lack the post-fertilization block to sperm binding and the ability to rescue this phenotype with Astl^mCherry transgenic mice confirms the role of ovastacin in providing a definitive block to polyspermy. During oogenesis, endogenous ovastacin traffics through the endomembrane system prior to storage in peripherally located cortical granules. Deletion mutants of ovastacin^mCherry expressed in growing oocytes define a unique 7 amino acid motif near its N-terminus that is necessary and sufficient for cortical granule localization. Deletion of the 7 amino acids by CRISPR/Cas9 at the endogenous locus (Astl^Δ) prevents cortical granule localization of ovastacin. The misdirected enzyme is present within the endomembrane system and ZP2 is prematurely cleaved. Sperm bind poorly to the zona pellucida of Astl^Δ/Δ mice with partially cleaved ZP2 and female mice are sub-fertile.

Monospermic fertilization is essential for the onset of development. Egg cortical granules exocytose their contents after fertilization to prevent polyspermy by modifying the extracellular zona pellucida (ZP1, ZP2, ZP3). Little is known about the biology of these subcellular organelles which are unique to oocytes. Ovastacin, a zinc metalloendoprotease that cleaves ZP2 to prevent sperm binding, is a pioneer marker of mammalian cortical granules. ZP2 remains uncleaved in transgenic mice lacking ovastacin and sperm bind to the zona matrix independent of fertilization and cortical granule exocytosis. After documenting the rescue of the null phenotype with transgenic mice expressing fluorescently-tagged ovastacin, we defined a unique, well conserved, cortical granule localization motif using cRNA deletion mutants microinjected into mouse oocytes. The importance of the motif for localization to cortical granules was confirmed in vivo by deleting DNA encoding 7 amino acids of the endogenous locus with CRISPR/Cas9. Unexpectedly, mutant female mice were sub-fertile due to partial cleavage of ZP2 in the zona pellucida which prevented sperm from binding to ovulated eggs in vitro and in vivo. These observations offer unique insight into the molecular basis for translocation of proteins to cortical granules which is needed for successful, monospermic fertilization.

Human sperm bind to the N-terminal domain of ZP2 in humanized zonae pellucidae in transgenic mice

Fertilization requires taxon-specific gamete recognition, and human sperm do not bind to zonae pellucidae (ZP1-3) surrounding mouse eggs. Using transgenesis to replace endogenous mouse proteins with human homologues, gain-of-function sperm-binding assays were established to evaluate human gamete recognition. Human sperm bound only to zonae pellucidae containing human ZP2, either alone or coexpressed with other human zona proteins. Binding to the humanized matrix was a dominant effect that resulted in human sperm penetration of the zona pellucida and accumulation in the perivitelline space, where they were unable to fuse with mouse eggs. Using recombinant peptides, the site of gamete recognition was located to a defined domain in the N terminus of ZP2. These results provide experimental evidence for the role of ZP2 in mediating sperm binding to the zona pellucida and support a model in which human sperm-egg recognition is dependent on an N-terminal domain of ZP2, which is degraded after fertilization to provide a definitive block to polyspermy.

Ovastacin, a cortical granule protease, cleaves ZP2 in the zona pellucida to prevent polyspermy

After fertilization, the metalloendoprotease ovastacin is released by cortical granule exocytosis and cleaves the zona pellucida glycoprotein ZP2, an essential step to block sperm binding to an already fertilized egg.

The mouse zona pellucida is composed of three glycoproteins (ZP1, ZP2, and ZP3), of which ZP2 is proteolytically cleaved after gamete fusion to prevent polyspermy. This cleavage is associated with exocytosis of cortical granules that are peripherally located subcellular organelles unique to ovulated eggs. Based on the cleavage site of ZP2, ovastacin was selected as a candidate protease. Encoded by the single-copy Astl gene, ovastacin is an oocyte-specific member of the astacin family of metalloendoproteases. Using specific antiserum, ovastacin was detected in cortical granules before, but not after, fertilization. Recombinant ovastacin cleaved ZP2 in native zonae pellucidae, documenting that ZP2 was a direct substrate of this metalloendoprotease. Female mice lacking ovastacin did not cleave ZP2 after fertilization, and mouse sperm bound as well to Astl-null two-cell embryos as they did to normal eggs. Ovastacin is a pioneer component of mouse cortical granules and plays a definitive role in the postfertilization block to sperm binding that ensures monospermic fertilization and successful development.

Mechanisms of sperm-egg interactions: between sugars and broken bonds

A model of the early events of mammalian fertilization has emerged during the past 30 years. However, studies during the past decade have used newly available mouse models to readdress these processes. Here, we will consider these new data in light of the existing model and point to areas of reconciliation and of controversy.

Characterization of zona pellucida glycoprotein 3 (ZP3) and ZP2 binding sites on acrosome-intact mouse sperm

There is considerable evidence that mouse fertilization requires the binding of sperm to two of the three glycoproteins that form the zona pellucida (ZP), ZP3 and ZP2. Despite the biologic importance of this binding, no one has demonstrated that sperm express separate, saturable, and specific binding sites for ZP3 and for ZP2. Such a demonstration is a prerequisite for defining the distribution, numbers, affinities, and regulation of function of ZP3 and ZP2 binding sites on sperm. The experiments reported herein used fluorochrome-labeled ZP3 and ZP2 and quantitative image analysis to characterize the saturable binding of ZP3 and ZP2 to distinct sites on living, capacitated, acrosome-intact mouse sperm. Approximately 20% of the ZP3 binding sites were found over the acrosomal cap, and the remaining sites were located over the postacrosomal region of the head. In contrast, ZP2 binding sites were detected only over the postacrosomal region. Saturation analysis estimated numbers and affinities of the binding sites for ZP3 (B(max) approximately 185 000 sites per sperm; K(d) approximately 67 nM) and ZP2 (B(max) approximately 500 000 sites per sperm; K(d) approximately 200 nM). Use of unlabeled ZP3, ZP2, and ZP1 as competitive inhibitors of the binding of fluorochrome-labeled ZP3 and ZP2 demonstrated that ZP3 and ZP2 bound specifically to their respective sites on sperm. Finally, we demonstrate that extracellular calcium as well as capacitation and maturation of sperm are required for these sites to bind their respective ligands.

Protein tyrosine phosphorylation in sperm during gamete interaction in the mouse: the influence of glucose

A key intracellular event during capacitation is protein tyrosine phosphorylation, but its involvement during sperm interaction with the oocyte has not been investigated. Glucose is necessary to achieve fertilization and thus may have an influence on sperm protein tyrosine phosphorylation. The objectives of this study were to 1) visualize protein tyrosine phosphorylation patterns in sperm during capacitation and interaction with the oocyte and 2) determine the influence of glucose. Protein tyrosine phosphorylation was investigated by Western analysis and immunofluorescence. Protein tyrosine phosphorylation was increased during capacitation, and immunofluorescence revealed that zona binding and gamete fusion were correlated with an increase in tyrosine phosphorylation of proteins in the midpiece. During capacitation, the absence of glucose led to a delay in the appearance of protein tyrosine phosphorylation. Following binding to the zona pellucida and the oolemma, tyrosine phosphorylation in the flagellum was also delayed in the absence of glucose and resulted in a significant inhibition of the midpiece phosphorylation. The correlation between successful gamete fusion and the tyrosine phosphorylation of midpiece proteins suggests that the effect of glucose on sperm-oocyte interaction is mediated through regulation of protein tyrosine phosphorylation in a specific area of the fertilizing sperm.

Mice carrying two t haplotypes: sperm populations with reduced Zona pellucida binding are deficient in capacitation

Capacitation is the unique process by which mammalian sperm become capable of undergoing the acrosome reaction (AR). An approach to studying sperm capacitation is to identify mutations altering this process. Male mice carrying two t haplotypes are sterile, with poor sperm motility, reduced zona pellucida binding, and an inability to penetrate zona-free oocytes. The objective of this study was to examine sperm capacitation and its potential relationship to zona pellucida binding in mice of the same genetic strain carrying none, one, or two t haplotypes. Sperm capacitation was assessed by the B pattern of staining by chlortetracycline (CTC) and by the ability of sperm to undergo the lysophosphatidylcholine (LPC)-induced AR. The CTC assay demonstrated that sperm capacitation from t/+ mice was similar to that from +/+ mice, but sperm from t/t mice were deficient. LPC induced the AR of capacitated sperm, but not noncapacitated sperm, in a concentration-dependent manner. Sperm from t/t mice were also deficient in the LPC-induced AR. Thus, by two independent assays, sperm from t/t mice were shown to be deficient in capacitation. To determine whether a deficiency in capacitation could influence zona binding, the ability of capacitated versus noncapacitated sperm to bind to the zona pellucida was tested. The mean numbers of sperm bound per oocyte were significantly greater for capacitated sperm than for noncapacitated sperm. These results suggest that the deficient capacitation of sperm from t/t mice could be responsible for, or at least contribute to, their reduced ability to bind to the zona pellucida.

The initial molecular interaction between mouse sperm and the zona pellucida is a complex binding event

Prior to fertilization, mammalian sperm must first bind to the zona pellucida (ZP), a glycoprotein matrix surrounding the egg. Sperm specifically bind to ZP3, an 83-kDa glycoprotein which functions as both an adhesion molecule and as a secretagogue for acrosomal exocytosis (Litscher, E. S., and Wassarman, P. M. (1993) Trends Glycosci. Glycotechnol. 5, 369-388). We used acid solubilized, 125I-labeled ZPs to quantify the initial binding event on mouse spermatozoa. Live sperm could not be used since solubilized ZPs rapidly initiated exocytosis. Instead, acrosome intact mouse sperm were briefly fixed in 1% glutaraldehyde for binding studies using a standard filtration assay. The fixed sperm are suitable for sperm-zona binding assays based on two experiments: 1) incubating either live or fixed sperm in low concentrations of 125I-ZPs not sufficient to induce acrosomal exocytosis revealed no differences in binding up to 15 min and 2) solubilized, unlabeled ZPs competed for 125I-ZPs with an KI of approximately 3.78 nm. Sperm-125I-ZP binding reached equilibrium with a tau1/2 of approximately 22 min at 37 degrees C. Affinity parameters were calculated using the well substantiated assumption that only ZP3 binds intact mouse sperm. The on-rate constant for association of 125I-ZP binding to the mouse sperm surface was calculated to be 3.2 x 10(6) M-1 min-1. The saturation binding isotherm revealed that there are approximately 30,000 binding sites, ascribed to ZP3, with an EC50 of 1.29 nM. Further analysis indicated that this binding is complex (Hill coefficient = 1.72), suggesting involvement of multiple receptors on the sperm surface and/or multiple ligand moieties. High and low affinity ZP binding sites on the sperm surface were confirmed by dissociation experiments. 125I-ZP dissociation was clearly biphasic, and kinetic off-rate constants of 0.161 min-1 and 0.0023 min-1 were calculated for the low and high affinity sites, respectively. Apparent affinities (Kd values) of 50 nM for the low affinity and 0.72 nM for the high affinity interaction were calculated from the rate constants. These data demonstrate that the initial adhesion event between mouse sperm and the zona pellucida is a high affinity event which is sufficient to tether a sperm to the extracellular matrix prior to the induction of acrosomal exocytosis.

Glucose is not essential for the occurrence of sperm binding and zona pellucida-induced acrosome reaction in the mouse

Glucose is necessary, in some species, for successful fertilization, but its role in the different gamete functions has only been documented partially. In this study we have examined the progression of mouse spermatozoa through the oocyte investments in order to determine which steps require glucose. Evidence is presented of glucose involvement during this sequence of events, since in-vitro fertilization was inhibited in glucose-free medium even when spermatozoa were capacitated in the presence of glucose. Access to the zona pellucida was not inhibited in the absence of glucose, suggesting that capacitation, progressive motility and zona binding of spermatozoa do not depend on the presence of an extracellular glycolysable substrate. However, zona penetration was prevented in the absence of glucose. Most importantly, the zona pellucida-induced acrosome reaction was found to occur in glucose-free medium. It is proposed that hyperactivated motility, which has been described to require glucose in the literature, may be the precise process responsible for the failure of spermatozoa to enter the perivitelline space.

Tissue- and species-specific expression of sp56, a mouse sperm fertilization protein

Mouse sperm recognize and bind to ZP3, one of three glycoproteins in the egg's zona pellucida. A mouse sperm protein, sp56, was identified that has the characteristics expected of the sperm protein responsible for recognition of ZP3. The complementary DNA encoding sp56 was isolated, and its primary sequence indicates that sp56 is a member of a superfamily of protein receptors. It was shown that sp56 expression is restricted to mouse spermatids and that the presence or absence of sp56 on sperm from different species accounts for species specificity of sperm-egg recognition in mice.

A transient rise in intracellular Ca2+ is a precursor reaction to the zona pellucida-induced acrosome reaction in mouse sperm and is blocked by the induced acrosome reaction inhibitor 3-quinuclidinyl benzilate

The acrosome reaction induced by the zona pellucida in mouse sperm has been shown to proceed in two stages experimentally distinguishable by the fluorescent probe chlortetracycline. Entry into the first stage of sperm bound to isolated, structurally intact zonae pellucidae is blocked by the compound 3-quinuclidinyl benzilate. In this study, we show, utilizing the fluorescent Ca2+ indicator fluo-3, that the first stage of the zona-induced acrosome reaction is characterized by an increase in intracellular Ca2+, followed by a decrease as the acrosome reaction proceeds. This calcium transient is completely suppressed by 3-quinuclidinyl benzilate. We conclude that the Ca2+ transient is induced by the zona pellucida and is required for the zona-induced acrosome reaction. Blockage of this sperm intracellular Ca2+ transient provides a mechanism for the inhibitory action of 3-quinuclidinyl benzilate on the zona-induced acrosome reaction in mouse sperm.

Protein kinase C activity and protein phosphorylation in mouse eggs

The treatment of mouse eggs with phorbol esters and diacylglycerol inhibits sperm penetration and results in biochemical modification of the zona pellucida. In this report, we have demonstrated the presence of protein kinase C (PKC) activity in mouse eggs as determined by 12-O-tetradecanoyl phorbol-13-acetate (TPA) dependent in vivo and in vitro protein phosphorylation in mouse eggs. When mouse eggs were radiolabeled with [32P]phosphate and treated with TPA, two specific proteins, 70 and 20 kDa, were phosphorylated. The 70-kDa protein was also phosphorylated in vitro by endogenous PKC. In addition, we have shown that exogenous PKC induced the in vitro phosphorylation of 70-, 55-, and 20-kDa proteins in egg extract. The 70-kDa protein was also phosphorylated in vitro after treatment of the cytosol fraction of mouse eggs with TPA, suggesting that this protein might be a specific substrate for PKC and that it is located in the cytosol. These results demonstrate that mouse eggs contain PKC activity and suggest that PKC-catalyzed protein phosphorylation of specific proteins might be involved in the regulation of egg-induced modification of the zona pellucida.

Endpoint of first stage of zona pellucida-induced acrosome reaction in mouse spermatozoa characterized by acrosomal H+ and Ca2+ permeability: population and single cell kinetics

The acrosome reaction induced by the mouse egg's zona pellucida in mouse sperm has been shown to proceed in two stages as characterized empirically by sequential changes in patterns of chlortetracycline fluorescence on the sperm plasma membrane surfaces. The chlortetracycline fluorescence pattern characteristic of fully intact sperm is designated B; in sperm bound to structurally intact zonae that induce the acrosome reaction, the B pattern changes first to an intermediate pattern S and then to a terminal pattern AR characteristic of the completed acrosome reaction. In the same study, it was shown, using a 9-amino acridine fluorescent pH probe, that completion of the first stage was characterized by increase in H+ permeability such that the H+ gradient between sperm head and medium was dissipated. In this study, we show that the fluorescent pH probe 9-N-dodecylamino acridine and the intracellular Ca2+ fluorescent probe fura-2 are both localized to the anterior part of the sperm head encompassing the acrosomal compartment in intact sperm, and the fluorescence associated with each probe is lost as the first stage of the acrosome reaction is completed. Loss of the pH probe fluorescence, pattern N, corresponds to onset of H+ permeability, and loss of fura-2 fluorescence, pattern F, corresponds to onset of Ca2+ permeability. Localization of intracellular fura-2 fluorescence to the acrosomal compartment required extracellular Mn2+ to quench surface-bound fura-2 AM, the tetra-acetoxymethyl ester of fura-2 used to load the cells. Loss of acrosomal fura-2 fluorescence is due to quenching by tracer Mn2+ accompanying Ca2+. Onset of membrane permeability to both H+ and Ca2+, as seen by loss of patterns N and F, occurred in synchrony in populations of sperm bound to isolated, structurally intact zonae, with an overall time course of 210 min postbinding. The loss of pattern N in individual sperm cells bound to zonae was rapid, with a half time of 2.1 min. Concomitant with this rapid loss of pattern N was a shift in the amplitude of flagellar motion from large to small. The lag times to pattern N loss in 50 individual cells ranged from 30 to 140 min. The variable lag times determine the population kinetics; the rate of the endpoint reaction seen in the individual cells is rapid and constant. Dissipation of the H+ gradient with immediate loss of pattern N was readily achieved by addition of nigericin with no change in the time course of the onset of Ca2+ permeability of the membranes enclosing the acrosome.(ABSTRACT TRUNCATED AT 400 WORDS)

Bacterial action of fertilization envelope extract from eggs of the fish Cyprinus carpio and Plecoglossus altivelis

The vitelline envelope (VE) and fertilization envelope (FE) in eggs of the fish Cyprinus carpio and Plecoglossus altivelis were purified by homogenization of eggs or embryos in 5 mM Tris-HCl buffer, pH 7.0, containing 2 mM ethylenediamine tetraacetic acid disodium salt (EDTA), except for processing of VEs in Plecoglossus eggs, and by repeated washing wih the same buffer. To extract the outermost layer material, the purified VEs and FEs were processed overnight at 4 degrees C in 5 mM Tris-HCl buffer, pH 7.0, containing 8 mM 2-mercaptoethanol, 2 mM EDTA, 0.3 M alpha-lactose, 0.3 M glucose, and 0.9% NaCl. Since extraction of the outermost layer of the VEs of Cyprinus eggs in this solution was found to be ultrastructurally incomplete, further sonication in the same buffer was necessary. The solution extracted from purified VEs or FEs was dialyzed against 5 mM Tris-HCl buffer, pH 7.0, followed by lyophilization. The extracts from the FEs from both fish species contained two kinds of lectins, one agglutinated human B-type erythrocytes and the other nonspecifically agglutinated fish spermatozoa, and both extracts had a strong bactericidal effect on Vibrio anguillarum that was isolated from diseased cultured fish, but not on Aeromonas hydrophila and Escherichia coli. The extracts of purified VEs from eggs of both fish had no bactericidal effect on the bacteria examined, nor any agglutination effect on human erythrocytes and fish spermatozoa.

Effects of in vitro incubation on a zona binding site found on murine spermatozoa

Murine cauda epididymal sperm possess a site, the acceptor, on the plasma membrane over the apical cap region of the acrosome which recognizes both a proteinase inhibitor of seminal vesicle origin and homologous zonae. The acceptor site may participate in both capacitation and zona binding. This presentation explores the effect of in vitro incubation in a medium known to induce capacitation on the binding capabilities of this site. Approximately 80% of fresh cauda epididymal sperm will bind the seminal inhibitor in vitro. Incubating sperm, pretreated with inhibitor for 2 hr in a medium (M199-M) known to support capacitation, reduces by 60% the number of sperm showing evidence of the inhibitor. No such decrease is seen when sperm are incubated in a medium (M199) that does not support capacitation. During the 2-hr incubation in either medium, 60-70% of the sperm retain two diverse components on the plasma membrane over the acrosome: a receptor for the Fc portion of IgG and an epitope recognized by a monoclonal antibody to the acceptor site. These observations suggest that the plasma membrane in the acrosome region of the cell remains structurally intact during incubation. Furthermore, sperm retain the ability to bind the seminal inhibitor during incubation. After a 2-hr incubation in M199-M, sperm pretreated with heat-solubilized zonae no longer bind the inhibitor. These sperm, however, retain the plasma membrane over the acrosomal cap region. When the sperm are incubated in M199, no decrease in inhibitor binding due to zona treatment is noted.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative studies of changes in cortical granule number and distribution in the mouse oocyte during meiotic maturation

Cortical granules (CGs) undergo a substantial change in distribution in the mouse oocyte cortex during meiotic maturation. In order to determine the mechanism of their change in distribution near the time of ovulation, CG density, total number per oocyte, and domain areas were quantitated. CGs were visualized microscopically by Lens culinaris agglutinin-biotin and Texas red-strepavidin fluorescence as well as by electron microscopy. Immature germinal vesicle stage (GV) oocytes from adult mice had a continuous cortical localization with some interior granules. Mature oocytes had an asymmetric cortical distribution with a CG-free domain, overlying the meiosis II metaphase spindle, occupying 40% of the cortex. The mean CG densities of the granule-occupied cortex of mature oocytes and the entire cortex of GV oocytes were 43 and 34 CGs/100 micron 2, respectively. The mean total numbers of CGs/oocyte were 4127 (mature) and 7440 (GV), and staining was absent in fertilized oocytes with two pronuclei. Calcium ionophore (A23187)-activated mature oocytes had a mean total number of 1235 CGs, some of which may have been in the process of exocytosis. The first polar body had few CGs, and thus was unlikely to account for the difference in CG number between GV and mature oocytes. The smaller total number and higher density of CGs in mature mouse oocytes suggests that both exocytosis and redistribution are plausible mechanisms for the development of the CG-free domain. Prefertilization exocytosis could account for the locus of sperm penetration which others have reported to occur in the hemisphere opposite the meiotic spindle in the mouse.

The regulation of acrosomal exocytosis. I. Sperm capacitation is required for the induction of acrosome reactions by the bovine zona pellucida in vitro

The regulation of acrosomal exocytosis in capacitated bovine spermatozoa by soluble extracts of zonae pellucidae was examined. Kinetic studies demonstrated that zonae pellucidae stimulated synchronous acrosome reactions. The t1/2 of this process was 5-10 min and response was maximal at 20 min. The apparent initial rate of exocytosis in sperm populations was dependent upon the concentration of zona pellucida protein, with an ED50 and a maximally effective dosage of 20 and 50 ng protein/microliter, respectively. Zonae pellucidae caused up to a 48-fold increase in the apparent initial rate and a 3- to 4-fold stimulation in the net occurrence of exocytosis. In contrast, solubilized zonae pellucidae did not induce acrosome reactions in uncapacitated sperm. The development of a capacitated state, as assayed by the ability of sperm to fertilize eggs in vitro, was compared to the expression of zona pellucida-regulated acrosome reactions in a series of kinetic experiments. Both activities were manifest with similar kinetics and displayed identical dependencies toward stimulatory and inhibitory agents in vitro. It is concluded that capacitation is an essential prerequisite for the induction of acrosomal exocytosis in bovine sperm by the zona pellucida.

Identification of a secondary sperm receptor in the mouse egg zona pellucida: role in maintenance of binding of acrosome-reacted sperm to eggs

During fertilization in mice, acrosome-intact sperm bind via plasma membrane overlying their head to a glycoprotein, called ZP3, present in the egg extracellular coat or zona pellucida. Bound sperm then undergo the acrosome reaction, which results in exposure of inner acrosomal membrane, penetrate through the zona pellucida, and fuse with egg plasma membrane. Thus, in the normal course of events, acrosome-reacted sperm must remain bound to eggs, despite loss of plasma membrane from the anterior region of the head and exposure of inner acrosomal membrane. Here, we examined maintenance of binding of sperm to the zona pellucida following the acrosome reaction. We found that polyclonal antisera and monoclonal antibodies directed against ZP2, another zona pellucida glycoprotein, did not affect initial binding of sperm to eggs, but inhibited maintenance of binding of sperm that had undergone the acrosome reaction on the zona pellucida. On the other hand, polyclonal antisera and monoclonal antibodies directed against ZP3 did not affect either initial binding of acrosome-intact sperm to eggs or maintenance of binding following the acrosome reaction. We also found that soybean trypsin inhibitor, a protein reported to prevent binding of mouse sperm to eggs, did not affect initial binding of sperm to eggs, but, like antibodies directed against ZP2, inhibited maintenance of binding of sperm that had undergone the acrosome reaction on the zona pellucida. These and other observations suggest that ZP2 serves as a secondary receptor for sperm during the fertilization process in mice and that maintenance of binding of acrosome-reacted sperm to eggs may involve a sperm, trypsin-like proteinase.

A monoclonal antibody to a zona pellucida-proteinase inhibitor binding component on murine spermatozoa

A monoclonal antibody, J-23, was produced to an epitope of a binding (acceptor) component on the plasma membrane in the acrosomal cap region of the mouse sperm head. The component binds a proteinase inhibitor of seminal vesicle origin at ejaculation and participates in the in vitro binding of capacitated sperm to the zona pellucida. The antibody, an IgM molecule, recognizes affinity purified acceptor, crude acceptor and whole sperm as determined by ELISA methodology. The antibody reacts with a 15,000 molecular weight component, the size previously determined for the acceptor, found in the supernatants of frozen-thawed cauda epididymal sperm. In addition, it binds to a 21,000 molecular weight component generated by mixing an excess of purified inhibitor (6400 daltons) with a crude acceptor preparation. J-23 binds to an epitope in the same region of the sperm head as does the inhibitor. This epitope becomes fully expressed during epididymal maturation and is found only in the lumen of epididymal tissues. Pretreating sperm with J-23 inhibits their ability to bind the inhibitor as well as the zona pellucida. Pretreating sperm with the inhibitor has little effect on the binding of J-23. These data indicate that J-23 recognizes an epitope on the acceptor but the epitope is not directly involved with inhibitor binding.

Features of a seminal proteinase inhibitor- zona pellucida-binding component on murine spermatozoa

Murine cauda epididymal sperm contain sites on the plasma membrane over the apical portion of the acrosome that recognize proteinase inhibitors and the homologous zona pellucida. Ten times more of the component can be extracted from cauda and ductus sperm than from equal numbers of caput and corpus sperm. Likewise, few sperm from the upper epididymal regions are able to bind seminal inhibitor, while the majority of sperm from the cauda and ductus do bind. Cauda epididymal and ductus sperm lose little of their ability to bind inhibitor after a 4-hour in vitro incubation in either a capacitating or a noncapacitating medium. The percentage of naturally inseminated sperm with the seminal inhibitor bound to their surface decreases to about 10 after 4 hours in utero. Approximately 80% of these sperm show positive fluorescence when given the opportunity to rebind the the inhibitor, and these sperm do have an intact plasma membrane over the apical portion of the acrosome. Furthermore, after 4 hours in utero, the inhibitor bound in the same region of the sperm head as it did on freshly ejaculated sperm. The seminal inhibitor inhibits the binding of sperm to the zona if added during the first 15 minutes of incubation but has no effect on attachment. The data indicate that sperm gain the ability to bind the seminal inhibitor during the epididymal sojourn. Furthermore, this binding capacity is not lost during in vitro or in utero incubation. The site is not involved in sperm-zona attachment but does participate in the binding of sperm to the zona.

Effects of phorbol esters and a diacylglycerol on mouse eggs: inhibition of fertilization and modification of the zona pellucida

Sperm penetration of the zona pellucida and fertilization are inhibited in mouse eggs treated with phorbol esters and the diacylglycerol, sn-1,2-dioctanoyl glycerol. The effect appears mediated by the zona pellucida, since zona-free eggs treated with these compounds are fertilized to the same extent as untreated eggs. Moreover, the binding of sperm to isolated zonae incubated in the absence or presence of biologically active phorbol esters is similar. Last, sperm treated with phorbol esters or sn-1,2-dioctanoyl glycerol bind to eggs and undergo the acrosome reaction to the same extent as untreated sperm. The inhibitory effect on fertilization is correlated with an egg-induced modification of at least ZP2, as manifested by a change in its electrophoretic mobility in polyacrylamide gels. In addition, changes in the biological properties of the treated zonae occur, such that sperm binding is not altered, but that the final stage(s) of the zona-induced acrosome reaction is inhibited. Zonae obtained from phorbol ester- or diacylglycerol-treated eggs should provide a system to study both the structural modifications of the zona proteins that are involved in induction of the acrosome reaction, as well as, delineating the sequence of events that comprise the acrosome reaction.

Nature of the mouse egg's receptor for sperm

The zona pellucida is an extracellular coat that surrounds all mammalian eggs. Sperm must penetrate the zona pellucida in order to reach and fuse with the plasma membrane of unfertilized eggs. Penetration is accomplished by a sequence of events involving both egg and sperm. First, sperm must bind to the outer margin of the zona pellucida. Such binding is mediated in a relatively species-specific manner by "sperm receptors" in the zona pellucida. Second, sperm must undergo the "acrosome reaction", a membrane fusion event, in order to traverse the zona pellucida. Here we review results from our own laboratory which demonstrate that, during the course of sperm-egg interaction in mice, zona pellucida glycoprotein ZP3 serves as both receptor for sperm and inducer of the acrosome reaction. Furthermore, we review evidence from our laboratory indicating that the sperm receptor activity of ZP3 is dependent only on its 0-linked carbohydrate components, whereas acrosome reaction-inducing activity is dependent on the polypeptide portion of ZP3 as well.

Effects of timing of ovum recovery, cumulus cells, sperm preincubation time, and pH on in vitro fertilization in C57BL/6 mice

The effects of the time of ova recovery following hCG injection, the presence of cumulus cells, duration of sperm preincubation time, and pH on in vitro fertilization in C57BL/6 mice were investigated. Significantly more ova were recovered at 14 h than at 12 h post-hCG injection. Although the number of ova recovered at 16 h was similar to that at 14 h, the percentage of ova showing degeneration increased. The presence or absence of cumulus cells had no effect on ovum fertilization rates, although sperm incubated with cumulus-intact ova underwent the acrosome reaction sooner than those incubated with ova lacking cumulus cells. Sperm motility was sustained slightly longer in the presence of cumulus-free ova than in the presence of cumulus-intact ova. The average percent fertilization of eggs combined with sperm preincubated 1 h was higher than that of sperm preincubated 0 and 0.5 h. Longer preincubation times resulted in a linear decrease in the percent motility and an increase in the percent acrosome reactions. A plot of the number of sperm attaching to the egg vs. coincubation time produced a bell-shaped curve in each case. The greatest number of sperm attaching to the egg occurred between 45 min and 1 h. When the medium was at pH 7.4, fertilization rates were higher than at pH 7.0, 7.2, or 7.6, as were the percent sperm motility and the number of sperm attached to ova. A pH of 7.6 induced 60% of the sperm to undergo the acrosome reaction immediately, and within 1 h all motility was lost.

Egg envelopes in vertebrates

As the material presented in this chapter was being collated, our existing perceptions about the basic similarities of vertebrate (and indeed most, if not all, invertebrate) egg envelopes became increasingly strengthened. Perhaps without exception, all vertebrate and invertebrate eggs acquire a "vitelline" envelope. Interestingly, its filamentous ultrastructure and chemical composition--basically protein and carbohydrate--is similar in all species as is its permeability to large molecules. Furthermore, many (if not all) of its functions are shared among the animal phyla as is its potential to become altered at the time of fertilization and, in its altered state, to provide a new set of modi operandi. It provides sperm receptors that are generally species specific and helps prevent polyspermy; it protects the developing embryo yet yields at the time of hatching. In most vertebrate eggs (including some mammals), a jelly or albumen coat is added to the vitelline envelope. These components may vary immensely in thickness, but again their basic chemical composition is common to all. The functions of these envelopes, while perhaps somewhat less clear than those of the vitelline envelope, are related to species-specific fertilization and to embryonic protection. Albumen serves a nutritional role--most clearly shown in the birds. Finally, the shell membrane and shell present in diverse groups contribute additional adaptations for embryo protection. Vertebrate egg envelopes, then, are basically similar; the modifications, including the addition of shell membranes and shells in some groups, reflect adaptations to differing reproductive strategies and to the environmental exigencies with which the egg must cope. With the growth of our understanding about the structure, chemistry, function, and evolution of egg envelopes new questions will continually be formulated. Many will be the same as those asked years ago but they will be answered with newer techniques and with greater insight.

Enzymatic dissection of the functions of the mouse egg's receptor for sperm

During the course of sperm-egg interaction in mice, zona pellucida glycoprotein ZP3 (approximately equal to 80 kDa) serves as both receptor for sperm (J. D. Bleil and P. M. Wassarman, 1980c, Cell 20, 873-882) and inducer of the acrosome reaction (J. D. Bleil and P. M. Wassarman, 1983, Dev. Biol. 95, 317-324). In this investigation, small ZP3 glycopeptides (approximately equal to 1.5-6 kDa), obtained by extensive digestion of the purified glycoprotein with insoluble Pronase, were assayed for both sperm receptor and acrosome reaction-inducing activities. While ZP3 glycopeptides were virtually as effective as intact ZP3 in inhibiting binding of sperm to eggs in vitro ("receptor activity"), unlike intact ZP3, they failed to induce sperm to undergo the acrosome reaction. The latter was determined by indirect immunofluorescence using a monoclonal antibody directed against the acrosomal cap region of sperm. These results suggest that the sperm receptor activity of ZP3 is dependent only on its carbohydrate components, whereas acrosome reaction-inducing activity is dependent on the polypeptide chain of ZP3 as well.

Determination of the time course of capacitation in mouse spermatozoa using a chlortetracycline fluorescence assay

The heads of mouse spermatozoa obtained 5 min after release from the excised caudae epididymides showed a characteristic fluorescence pattern in the presence of the fluorophore chlortetracycline (CTC). There was uniform fluorescence over the entire head with about half the sperm population showing a brighter line of fluorescence across the equatorial segment; this fluorescence pattern was designated "F". After 90-min incubation in culture medium (CM) containing 2% (w/v) bovine serum albumin, most of the sperm heads showed a dark band of nonfluorescence over the equatorial and postequatorial segment, while the anterior portion of the head showed bright fluorescence. This fluorescence pattern was designated "B." The time course for the disappearance of pattern F matched the time course of the appearance of pattern B, with a half-time of 30 min. The transformation was complete in 90 min. At longer times of incubation in CM, the percentage of spermatozoa showing pattern B declined; fluorescence over the entire head was lost, characteristic of the pattern for acrosome-reacted sperm (P. M. Saling and B. T. Storey (1979). J. Cell Biol. 83, 544-555). Mouse sperm showing pattern B were able to undergo the acrosome reaction, either spontaneously or by induction with acid-solubilized zonae pellucidae from mouse eggs (H.M. Florman and B. T. Storey (1982). Dev. Biol. 91, 121-130). The latter reaction was blocked by its specific inhibitor 3-quinuclidinyl benzilate (QNB). Mouse sperm showing pattern F could not be induced to undergo the acrosome reaction by exposure to solubilized zonae. This implies that the change from fluorescence pattern F to fluorescence pattern B corresponds with changes in the sperm which make them susceptible to undergo the acrosome reaction. This change occurs during the time interval previously determined to be needed for capacitation of mouse sperm in vitro in CM (M. Inoue and D. P. Wolf (1975). Biol. Reprod. 13, 340-346). These results imply that spermatozoa showing CTC fluorescence pattern B can be considered to be capacitated and that a functional definition for capacitation is the acquired ability to undergo the acrosome reaction rapidly when treated with acid-solubilized zonae pellucidae. The CTC fluorescence assay provides for the first time a means to monitor the time course of epididymal mouse sperm capacitation in vitro.

Monoclonal antibody against mouse sperm blocks a specific event in the fertilization process

Hybridoma cell lines were produced by PEG (polyethylene glycol)-induced fusion of myeloma cells (NS-1/X63 Ag 8) with dispersed spleen cells from a Balb/c male mouse immunized syngeneically with homogenized Balb/c testis. Indirect immunofluorescence on mature (cauda epididymal) mouse sperm was employed as the primary screening assay. This procedure allowed the immediate identification of hybridomas which produce monoclonal antibodies of interest with respect to the fertilization process. One hybridoma cell line, M29, secretes an antibody of the IgM class that localizes to the equatorial segment of the mouse sperm head. This antibody is not species specific, but is restricted to the homologous functional area, the equatorial segment, in all other sperm examined (hamster, rabbit, human). When tested for a possible effect on the fertilization process, ascites fluid containing M29 antibody was very effective in blocking the fertilization of mouse gametes in vitro. The same inhibiting activity was observed regardless of whether the M29-treated sperm first encountered the cumulus cell layer, the zona pellucida, or the egg plasma membrane. A large number of M29-treated sperm were capable of penetrating zonae pellucidae when this layer was present. It appears, therefore, that M29 monoclonal antibody prevents fertilization specifically at the level of interaction between the sperm and egg plasma membranes; the other prefertilization events (hyperactivated motility, zona binding, acrosome reaction, zona penetration) seem little affected. Using the M29 antibody and related probes that are specific for particular biological events, it should be possible to map, in molecular terms, the functional domains of the sperm cell's membrane.

Sperm interaction with the zona-free hamster egg

The interaction of sperm with the zona-free hamster egg was studied. Hamster sperm were capacitated in Tyrodes media, containing 50% heat-inactivated serum and used to inseminate zona-free eggs in BWW containing 10% serum. Capacitated sperm began fusing with eggs within the first hour of insemination and by 3 h penetration had ceased as indicated by the absence of further changes in the mean number of sperm incorporated per egg. Penetration by capacitated hamster sperm was linearly related to the log of the motile sperm concentration at concentrations above 10(4) cells/ml. The viability of sperm and eggs in culture was limited in these studies to approximately 3-5 h. The existence of a block to unlimited sperm penetration of the zona-free egg was sought in reinsemination experiments. A relatively low sperm concentration was used to initiate egg responses, followed, at timed intervals, by reinsemination with a high challenge concentration of sperm. Subsequent polyspermy levels reflected the presence or absence of the egg's block to polyspermy response. In order to circumvent the problems arising from the rapid aging of hamster sperm in culture, mouse sperm were employed, a convenience afforded by the lack of species specificity in this egg. Reinseminated eggs incorporated additional sperm during the challenge; therefore, the hamster egg is not capable of preventing unlimited sperm penetration. The implications of these findings to the use of the zona-free hamster egg test in fertility evaluation is discussed.

Specificity of sperm-zona interaction

The specificity of early sperm-zona interactions in vitro was studied using gametes from the mouse, rabbit, pig, and human. Mouse epididymal spermatozoa, capacitated in vitro, bound to the zonae of intact mouse oocytes and zona fragments removed mechanically from oocytes. When viewed with the scanning electron microscope (SEM), spermatozoa were found adherent more frequently to the external (cumulus) surface of the zonae than the internal (oocyte) surface. Noncapacitated mouse spermatozoa or spermatozoa incubated with zonae in the absence of external calcium did not bind to homologous zonae. by contrast, fresh, epididymal mouse spermatozoa and mouse spermatozoa capacitated in vitro appeared to attach equally well to both surfaces of zonae and zona fragments from rabbit and pig oocytes. Rabbit spermatozoa "capacitated" in vivo as well as noncapacitated rabbit spermatozoa, attached rapidly to both inner and outer surfaces of zonae from rabbit, mouse, and pig oocytes. This reaction was not dependent on external calcium. Human spermatozoa did not display any tendency to adhere to zonae from the rabbit, mouse, or pig. The results of these studies suggest that spermatozoa can associate with zona in a nonspecific as well as a specific manner. The use of heterologous systems in addition to homologous systems in sperm binding assays may be useful in distinguishing these two types of interaction.

Involvement of trypsin-like activity in binding of mouse spermatozoa to zonae pellucidae

Work from a number of laboratories has shown that fertilization is blocked in the presence of protease inhibitors, although the specific site of inhibition has not been identified. The present experiments were designed to discriminate between sperm binding to zonae pellucidae as opposed to sperm penetration through zonae, so as to assess the effect of protease inhibitors on these two distinct events. Exposure of capacitated mouse spermatozoa to a variety of protease inhibitors directed against trypsin blocked sperm binding to zonae in a concentration-dependent manner. A chymotrypsin-directed inhibitor was not capable of blocking sperm binding to zonae. The trypsin inhibitors did not affect sperm penetration though zonae nor gamete membrane fusion if the sperm had established a firm association with the zona surface before addition of the inhibitors. Previous incubation of zona-intact eggs with the inhibitors did not lead to a reduction in sperm binding, indicating that the activity affected by the inhibitors is borne by spermatozoa. Interaction between spermatozoa and the zona surface appeared to be the specific locus of inhibition; sperm binding to zona-free eggs (i.e., binding to the egg plasma membrane) was unaltered by the trypsin inhibitors. These results suggest a reevaluation of the function of proteases in fertilization focusing on their role in initial sperm contact with the zona pellucida.

Inhibition of in vitro fertilization of mouse eggs: 3-quinuclidinyl benzilate specifically blocks penetration of zonae pellucidae by mouse spermatozoa

The fertilization in vitro of mouse with intact zonae pellucidae by mouse cauda epididymal spermatozoa was inhibited in a concentration- dependent fashion by 3-quinuclidinyl benzilate (QNB), normally used as a specific antagonist for the muscarinic class of cholinergic receptors. Inhibition was observed with both cumulus-intact and cumulus-free preparations. QNB at 50 microM inhibited fertilization of cumulus-free eggs by greater than 90% but had no effect on the fertilization of zona-free eggs. At this concentrations, QNB had no adverse effect on sperm motility, nor did it prevent binding of spermatozoa to the zona pellucida. The inhibitory effects of QNB were fully reversible. QNB is therefore a useful specific inhibitor of zona penetration. Spermatozoa in the in vitro fertilization medium bound QNB with a concentration dependence which matched that of the inhibition of fertilization. This binding was saturable and corresponded to 700 pmole/10(7) cells with KD = 10 microM. The in vitro fertilization medium contains 2% (w/v) bovine serum albumin (BSA) which also binds QNB according to the mass action law. The large amount of QNB bound to sperm in this medium appears to be QNB binding to BSA adsorbed on the sperm cell surface: these spermatozoa bind QNB specifically in the absence of BSA with a saturable capacity of only 70 fmole/10(7) cells with KD = 5 nM. Calculation of the distribution of QNB between BSA binding sites and sperm surface binding sites in the in vitro fertilization medium indicates that the specific sperm sites become saturated with the same concentration dependence as inhibition of fertilization. However, the dissociation rate of QNB from sperm in both the presence and absence of BSA is too rapid to permit confirmation of these sites as the locus of the inhibitory effect; this locus remains to be clarified.

Mouse gamete interactions during fertilization in vitro. Chlortetracycline as a fluorescent probe for the mouse sperm acrosome reaction

We have developed an assay for detecting the acrosome reaction in mouse sperm using chlortetracycline (CTC) as a fluorescent probe. Sperm known to be intact with nonreacted acrosomes show CTC fluorescence in the presence of Ca2+ over the anterior portion of the sperm head on the plasma membrane covering the acrosome. Sperm which have undergone the acrosome reaction do not show fluorescence on the sperm head. Mouse sperm bind to zonae pellucidae of cumulus-free eggs in vitro in a Ca2+-dependent reaction; these sperm are intact by the CTC assay. Intact sperm bind to mechanically isolated zonae under the same conditions: the egg is apparently unnecessary for this inital reaction. Sperm suspensions, in which greater than 50% of the motile population had completed the acrosome reaction, were prepared by incubation in hyperosmolal medium followed by treatment with the divalent cation ionophore, A23187. Cumulus-free eggs challenged with such sperm suspensions preferentially bind intact sperm; acrosome-reacted sperm do not bind. We conclude that the plasma membrane of the mouse sperm is responsible for recognition of the egg's zona pellucida and that the obligatory sequence of reactions leading to fusion of mouse gametes is binding of the intact sperm to the zona pellucida, followed by the acrosome reaction at the zona surface, followed in turn by sperm penetration of the zona.