O-linked oligosaccharides of mouse egg ZP3 account for its sperm receptor activity

Molecules involved in mammalian sperm-egg interaction

To achieve fertilization, sperm and egg are equipped with specific molecules which mediate the steps of gamete interaction. In mammals, the first interaction between sperm and egg occurs at an egg-specific extracellular matrix, the zona pellucida (zp). The three glycoproteins, ZP1, ZP2, and ZP3, that comprise the zp have been characterized from many species and assigned different roles in gamete interaction. A large number of candidate-binding partners for the zp proteins have been described; a subset of these have been characterized structurally and functionally. Galactosyltransferase, sp56, zona receptor kinase, and spermadhesins are thought to participate in the primary binding between sperm and zp and may initiate the exocytotic release of hydrolytic enzymes in the sperm head, the acrosome reaction. Digestion of the zp by these enzymes enables sperm to traverse the zp, at which time the proteins PH20, proacrosin, sp38, and Sp17 are thought to participate in secondary binding between the acrosome-reacted sperm and zp. Once through the zp, sperm and egg plasma membranes meet and fuse in a process reported to involve the egg integrin alpha 6 beta 1 and the sperm proteins DE and fertilin. These molecules and the processes involved in gamete interaction are reviewed in this chapter within a physiological context.

Sperm from beta 1,4-galactosyltransferase-null mice are refractory to ZP3-induced acrosome reactions and penetrate the zona pellucida poorly

A variety of sperm surface components have been suggested to mediate gamete recognition by binding to glycoside ligands on the egg coat glycoprotein ZP3. The function of each of these candidate receptors is based upon varying degrees of circumstantial and direct evidence; however, the effects on fertilization of targeted mutations in any of these candidate receptors have not yet been reported. In this paper, we describe the effects of targeted mutations in beta1,4-galactosyltransferase, the best studied of the candidate receptors for ZP3. Surprisingly, galactosyltransferase-null (gt[−/−]) males are fertile; however, sperm from gt(−/−) males bind less radiolabeled ZP3 than wild-type sperm, and are unable to undergo the acrosome reaction in response to either ZP3 or anti-galactosyltransferase antibodies, as do wild-type sperm. In contrast, gt(−/−) sperm undergo the acrosome reaction normally in response to calcium ionophore, which bypasses the requirement for ZP3 binding. The inability of gt(−/−) sperm to undergo a ZP3-induced acrosome reaction renders them physiologically inferior to wild-type sperm, as assayed by their relative inability to penetrate the egg coat and fertilize the oocyte in vitro. Thus, although ZP3 binding and subsequent induction of the acrosome reaction are dispensable for fertilization, they impart a physiological advantage to the fertilizing sperm. A second strain of mice was created that is characterized by a loss of of the long galactosyltransferase isoform responsible for ZP3-dependent signal transduction, but which maintains normal levels of Golgi galactosylation. Sperm from these mice show that the defective sperm-egg interactions in gt(−/−) mice are due directly to a loss of the long galactosyltransferase isoform from the sperm surface and are independent of the state of intracellular galactosylation during spermatogenesis.

Binding of mouse sperm to beta-galactose residues on egg zona pellucida and asialofetuin-coupled beads

Mouse eggs fixed with paraformaldehyde were incubated with various exoglycosidases and their sperm-binding activities were examined. The number of sperm bound per egg was increased by sialidase treatment and decreased by beta-galactosidase treatment. No prominent reduction of sperm-binding was observed after alpha-galactosidase treatment. Mouse sperm also bound to asialofetuin-coupled agarose beads but not to fetuin-coupled beads. The sperm-binding was abolished when asialofetuin-gel was treated with beta-galactosidase specific to the beta 1-->4 linkage or N-Glycanase. Furthermore asialofetuin, but not beta-galactosidase-treated asialofetuin, competitively inhibited the binding of sperm to the zona pellucida of live eggs. These results suggest that mouse sperm recognize beta-galactose residues of the zona pellucida at the initial stage of the binding.

Identification of an N-glycosylated region of pig zona pellucida glycoprotein ZPB that is involved in sperm binding

ZPB, one of the pig zona pellucida glycoproteins, can be purified after removal of sialylated and/or sulfated N-acetylpolylactosamine from the nonreducing region of its carbohydrate chains by digestion with endo-beta-galactosidase. Among the components produced, only ZPB shows sperm-binding activity after the digestion. Recently, we have shown that N-linked carbohydrate chains of endo-beta-galactosidase-digested ZPB (EbetaG-ZPB) are predominantly involved in sperm binding [Yonezawa, N., Aoki, H., Hatanaka, Y. & Nakano, M. (1995) Eur. J. Biochem. 233, 35-41]. In this study, to define the sperm-binding region in EbetaG-ZPB, glycopeptides were purified from lysyl endopeptidase digests of EbetaG-ZPB and analyzed for sperm-binding activity by an in vitro competition assay. The locations of the glycopeptides were determined from partial amino acid sequences, amino acid and sugar composition analyses, and apparent molecular masses after SDS/PAGE. The N-terminal fragment (amino acid residues 137-247), which contains two N-linked carbohydrate chains, showed a significant inhibition of sperm-egg binding. However, the fragment that had one N-linked carbohydrate chain (residues 325-341) and the fragment that had two or three O-linked carbohydrate chains (residues 248-324) did not inhibit sperm-egg binding. Thus, the two N-linked carbohydrate chains in the N-terminal fragment of EbetaG-ZPB are important for sperm binding of pig zona pellucida.

Expression of a P-selectin ligand in zona pellucida of porcine oocytes and P-selectin on acrosomal membrane of porcine sperm cells. Potential implications for their involvement in sperm-egg interactions

The selectin family of cell adhesion molecules mediates initial leukocyte adhesion to vascular endothelial cells at sites of inflammation. O-glycan structural similarities between oligosaccharides from human leukocyte P-selectin glycoprotein ligand-1 (PSGL-1) and from zona pellucida glycoproteins of porcine oocytes indicate the possible existence of a P-selectin ligand in the zona pellucida. Here, using biochemical as well as morphological approaches, we demonstrate that a P-selectin ligand is expressed in the porcine zona pellucida. In addition, a search for a specific receptor for this ligand leads to the identification of P-selectin on the acrosomal membrane of porcine sperm cells. In vitro binding of porcine acrosome-reacted sperm cells to oocytes was found to be Ca2+ dependent and inhibitable with either P-selectin, P-selectin receptor-globulin, or leukocyte adhesion blocking antibodies against P-selectin and PSGL-1. Moreover, porcine sperm cells were found to be capable of binding to human promyeloid cell line HL-60. Taken together, our findings implicate a potential role for the oocyte P-selectin ligand and the sperm P-selectin in porcine sperm-egg interactions.

Binding of spermatozoa to the perivitelline layer in the presence of a protease inhibitor

The extracellular investment of oocytes in a number of species contains species-specific receptors to which spermatozoa bind as a prelude to fertilization; however, little is known about the nature and distribution of sperm receptors in avian oocytes. In order to elucidate the early step of fertilization in birds, we observed the binding of spermatozoa to the perivitelline layer (PL) of quail ova. When the PL obtained from the largest follicles were incubated in vitro with spermatozoa, perforations were observed. The presence of trypsin inhibitors during incubation inhibited the sperm-induced perforations of the PL and binding of spermatozoa to the PL could be observed. The number of spermatozoa bound to the PL increased in the ovum from more mature follicles, and concentrated binding of spermatozoa to the PL overlying the germinal disc region was observed in the largest follicle. The number of spermatozoa bound to the PL overlying the germinal disc region decreased in the oviposited eggs. These results demonstrate that sperm receptors exist in the PL over the germinal disc in the mature follicle.

Structure of the main saccharide chain in the acrosome reaction-inducing substance of the starfish, Asterias amurensis

The structure of the main saccharide chain of the acrosome reaction-inducing substance in the egg jelly coat of the starfish, Asterias amurensis, is composed of the following pentasaccharide repeating units (Structure I). A polymer consisting of 10-11 repeating units has been observed to induce the acrosome reaction in starfish sperm at high calcium concentrations. [STRUCTURE I:see text] The identities and linkage positions of constituent sugars were established using sugar, methylation, and sulfate analyses together with one- and two-dimensional nmr spectroscopy. The structure was supported by the data obtained for desulfation products and the Smith degradation of the polysaccharide.

Identification of sulfated oligosialic acid units in the O-linked glycan of the sea urchin egg receptor for sperm

The Strongylocentrotus purpuratus sea urchin egg receptor for sperm is a cell surface glycoprotein with a molecular mass of 350 kDa. Recent studies indicate that the sulfated O-linked glycans isolated from the receptor bind to acrosome-reacted sperm. The purified receptor was analyzed with respect to amino acid and carbohydrate content and shown to be composed of 70% carbohydrate by weight. Compositional analysis indicated that both N- and O-linked oligosaccharide chains were present. After peptide:N-glycanase treatment of the receptor to remove most of the N-linked glycan chains, the majority of the sialic acid residues remained associated with the receptor and were shown by several types of experiments to be composed of sulfated oligosialic acid units attached to the O-linked glycan chains of the receptor. Chemical and physical studies on oligosialic chains discovered earlier in the Pronase-generated glycopeptide fraction isolated from the egg cell surface complex of another species of sea urchin, Hemicentrotus pulcherrimus, established that these molecules had the structure: (SO(4)-)-9Neu5Gc alpha2(-->5-O(glycolyl)Neu5Gc alpha2-->)n. Based on comparative and analytical studies, it was concluded that this sulfated oligosaccharide is a component of a GalNAc-containing chain that is O-linked to the polypeptide chain of the sea urchin egg receptor for sperm. Using a competitive inhibition of fertilization bioassay it was shown that the sulfated oligosialic acid chains derived from the S. purpuratus egg cell surface complex inhibited fertilization; the nonsulfated form of this oligosialic chain had little inhibitory activity.

Mutant female mice carrying a single mZP3 allele produce eggs with a thin zona pellucida, but reproduce normally

The mouse egg zona pellucida (ZP) is composed of three glycoproteins, called mZP1-3. Disruption of the mZP3 gene by targeted mutagenesis yields mice that are homozygous (mZP3-/-) for the null mutation; although the mutant mice are viable, females are infertile and their eggs lack a ZP. On the other hand, females heterozygous (mZP3+/-) for the mutation are fertile and their eggs have a ZP. Here, we examined fully grown oocytes from mZP3+/- females and found that, although they have a ZP, it is less than half the width (approximately 2.7 microns; volume, approximately 56 pl) of the ZP of oocytes from wild-type (mZP3+/+) mice (approximately 6.2 microns; volume, approximately 145 pl). Oocyte ZP were purified from ovarian homogenates by gradient centrifugation. Immunostaining of purified ZP on Western gels permitted an estimate to be made of the relative amounts of mZP3 and mZP2 present in the ZP of oocytes from mZP3+/+ and mZP3+/- mice. We found that the ZP from mZP3+/- mice contained, on average, 55 +/- 15% of the mZP3 and 44 +/- 8% of the mZP2 present in the ZP of mZP3+/+ mice; a result quite consistent with the observed widths and calculated volumes of the ZP. Despite the presence of a relatively thin ZP surrounding their eggs, reproduction of female mZP3+/- mice was indistinguishable from female mZP3+/+ mice. These results strongly suggest that, when a single mZP3 allele is present, approximately half the wild-type amount of mZP3 and approximately half the wild-type amount of mZP2 is assembled into a ZP. While this produces a relatively thin ZP, it apparently has no affect on reproduction. Furthermore, these results are consistent with the current molecular model for ZP structure.

Binding of human spermatozoa to lectin-coated agarose microbeads

An inability or decreased ability of spermatozoa to bind to the zona pellucida (ZP) may be at the origin of many cases of poorly explained or idiopathic infertility. It would be clinically useful to be able to distinguish this condition from other causes of infertility. A major problem in testing the sperm-ZP binding ability is the paucity of biological ZP. Examination of whether sperm binding to PNA-, UEA-1-, WGA-, Con A-, or PSA-coated agarose microbeads reflected sperm binding to biological ZP and correlated with in vitro fertilization rates showed that only binding to WGA-coated microbeads showed significant positive and negative predictive values when compared to IVF rates in 2 x 2 contingency. Sperm binding to PNA, Con A, and PSA was indiscriminately high, irrespective of IVF rate. Human spermatozoa did not bind to UEA-1-coated agarose microbeads. Furthermore, sperm binding to WGA-coated microbeads correlated with sperm morphology ratings. These results implicate terminal N-acetyl-D-glucosamine and/or sialic acid (specific saccharides for WGA) in sperm-ZP interaction and also suggest that the use of lectin-coated microbeads may represent an initial step in the development of a synthetic sperm binding assay.

Aminopeptidase-like protease released from oocytes affects oocyte surfaces and suppresses the acrosome reaction in establishment of polyspermy block in oocytes of the mussel Mytilus edulis

Suppression of the acrosome reaction of sperm on fertilized oocytes inhibits sperm-oocyte binding and is considered one of the three mechanisms responsible for polyspermy block in oocytes of the mussel Mytilus edulis (Togo et al., 1995). When unfertilized oocytes were inseminated in the presence of aminopeptidase inhibitors and the fertilized oocytes were inseminated again, neither the acrosome reaction nor sperm binding to fertilized oocytes were suppressed, suggesting that aminopeptidase-like protease participates in suppression of the acrosome reaction. The supernatant solution obtained after centrifuging a suspension of fertilized oocytes hydrolyzed aminopeptidase substrates, and these activities were inhibited strongly or effectively by aminopeptidase inhibitors. When unfertilized oocytes were incubated with this supernatant solution and inseminated, both the number of sperm bound and the acrosome reaction rate decreased, and these effects were reversed by conducting this treatment in the presence of aminopeptidase inhibitors. These results suggest that aminopeptidase-like protease released from the oocyte at fertilization affects the oocyte surface to suppress the acrosome reaction and consequently inhibits sperm-oocyte binding.

Defining oligosaccharide specificity for initial sperm-zona pellucida adhesion in the mouse

The identity of the sperm surface protein(s) responsible for sperm-zona pellucida binding in the mouse, as well as the characteristics of the oligosaccharide groups on zona pellucida glycoprotein 3 (ZP3) having ligand activity toward this receptor, remain controversial. Conflicting results from several groups have made interpretation of the current data difficult. By developing a quantitative binding assay to evaluate the molecular interactions between mammalian sperm and the zona pellucida during initial gamete interactions, we directly quantified sperm-ZP binding interactions at the molecular level for the first time. The ZP binding assay demonstrated that live, capacitated mouse sperm bind solubilized 125I-labeled ZP glycoproteins in a concentration-dependent manner characterized by a rapid forward rate constant of 3.0 x 10 (7)M-1 min-1. Following the initial characterization, the binding assay was used to examine the roles of the sperm surface enzymes galactosyltransferase (GalTase) and fucosyltransferase (FucTase) in sperm-zone pellucida binding in the mouse. These data indicate that substrates for FucTase, but not for GalTase, inhibit sperm-ZP binding, in contrast to earlier reports in which GalTase substrates significantly inhibited sperm binding to intact ZPs. A model is presented which resolves conflicting results between assays using intact ZPs and the results obtained here using soluble 125I-ZPs. Assuming a complex binding/recognition site, monosaccharides that could occupy part of the binding site would have a dramatic effect on sperm-ZP binding to the intact ZP, since they need only occupy the binding sites for a short time (approximately 100 msec) to disrupt binding. The current results suggest that the sperm ZP3 receptor binding site minimally recognizes the gal beta 1, 3-GlcNAc moiety also recognized by FucTases. The current data do not exclude the possibility that additional sugar residues form part of the ligand oligosaccharide group and are recognized by a yet-to-be-identified sperm surface protein which serves as the ZP3 receptor.

Activation of mouse sperm T-type Ca2+ channels by adhesion to the egg zona pellucida

The sperm acrosome reaction is a Ca(2+)-dependent exocytotic event that is triggered by adhesion to the mammalian egg's zona pellucida. Previous studies using ion-selective fluorescent probes suggested a role of voltage-sensitive Ca2+ channels in acrosome reactions. Here, wholecell patch clamp techniques are used to demonstrate the expression of functional T-type Ca2+ channels during mouse spermatogenesis. The germ cell T current is inhibited by antagonists of T-type channels (pimozide and amiloride) as well as by antagonists whose major site of action is the somatic cell L-type Ca2+ channel (1,4-dihydropyridines, arylalkylamines, benzothiazapines), as has also been reported for certain somatic cell T currents. In sperm, inhibition of T channels during gamete interaction inhibits zona pellucida-dependent Ca2+ elevations, as demonstrated by ion-selective fluorescent probes, and also inhibits acrosome reactions. These studies directly link sperm T-type Ca2+ channels to fertilization. In addition, the kinetics of channel inhibition by 1,4-dihydropyridines suggests a mechanism for the reported contraceptive effects of those compounds in human males.

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.

Recombinant hamster sperm receptors that exhibit species-specific binding to sperm

Previous studies have shown that mouse sperm bind to hamster eggs and hamster sperm bind to mouse eggs in vitro. Furthermore, sperm receptor glycoprotein isolated from the zona pellucida of unfertilised hamster (hZP3) and mouse (mZP3) eggs binds to sperm from the heterologous species. Here, we expressed the hZP3 gene, under control of a constitutive promoter (pgk-1), in mouse embryonal carcinoma (EC) cells and Chinese hamster ovary (CHO) cells stably transfected with the hZP3 gene. In both cases, recombinant hZP3 (EC-hZP3 and CHO-hZP3) secreted into the culture medium was partially purified by high-performance liquid chromatography on a size-exclusion column and assayed for bioactivity using mouse and hamster gametes. Unlike hamster egg hZP3, which binds to both mouse and hamster sperm, EC-hZP3 and CHO-hZP3 exhibits species-specific binding to hamster sperm and induce hamster sperm, but not mouse sperm, to undergo the acrosome reaction in vitro. These results provide further evidence that species-specific binding of sperm to eggs in mammals is carbohydrate-mediated. Furthermore, the results suggest that recombinant forms of mammalian sperm receptors may be useful in assessing the molecular basis of species-specific fertilisation in mammals.

Characterization of the proteins comprising the integral matrix of Strongylocentrotus purpuratus embryonic spicules

In the present study, we enumerate and characterize the proteins that comprise the integral spicule matrix of the Strongylocentrotus purpuratus embryo. Two-dimensional gel electrophoresis of [35S]methionine radiolabeled spicule matrix proteins reveals that there are 12 strongly radiolabeled spicule matrix proteins and approximately three dozen less strongly radiolabeled spicule matrix proteins. The majority of the proteins have acidic isoelectric points; however, there are several spicule matrix proteins that have more alkaline isoelectric points. Western blotting analysis indicates that SM50 is the spicule matrix protein with the most alkaline isoelectric point. In addition, two distinct SM30 proteins are identified in embryonic spicules, and they have apparent molecular masses of approximately 43 and 46 kDa. Comparisons between embryonic spicule matrix proteins and adult spine integral matrix proteins suggest that the embryonic 43-kDa SM30 protein is an embryonic isoform of SM30. An adult 49-kDa spine matrix protein is also identified as a possible adult isoform of SM30. Analysis of the SM30 amino acid sequences indicates that a portion of SM30 proteins is very similar to the carbohydrate recognition domain of C-type lectin proteins.

Evaluating zona pellucida structure and function using antibodies to rabbit 55 kDa ZP protein expressed in baculovirus expression system

A cDNA encoding the rabbit 55 kDa ZP protein was expressed using a baculovirus expression system and was evaluated for its ability to elicit antibodies which may interfere with sperm-ZP interaction. The expressed glycosylated protein, BV55, was purified by wheat germ agglutinin lectin affinity chromatography. Antisera made in guinea pigs immunized with BV55 (GP-alpha-BV55) is specific for the 55 kDa rabbit ZP protein. Indirect immunofluorescence studies indicate that GP-alpha-BV55 localizes to a filamentous meshwork on the surface of the ZP of isolated rabbit eggs. Immunohistochemical analysis of rabbit ovaries demonstrated that this antigen is localized within the ZP of primary and more advanced stage ovarian follicles but is not detected in primordial follicles. In addition, the 55 kDa antigen was detected in the granulosa cells of secondary stage follicles but not in the oocyte. GP-alpha-BV55 effectively blocked the binding of rabbit sperm to rabbit eggs in vitro. However, Fab fragments generated from GP-alpha-BV55 failed to block sperm binding, suggesting that the inhibitory effect of GP-alpha-BV55 was due to stearic hindrance rather than specific blocking of a sperm receptor site. Although the Fab fragment did not inhibit sperm binding, additional studies demonstrated that biotinylated BV55 protein bound to rabbit sperm in the acrosomal region in a manner consistent with ligand activity in the sperm-ZP interaction, and that BV55 bound to rabbit sperm in a dose-dependent manner. These studies therefore demonstrate that antibodies against recombinant ZP proteins recognize the native intact ZP and inhibit sperm-ZP interaction. They also provide evidence that the rabbit 55 kDa ZP protein, which is the homolog of the pig ZP3 alpha sperm receptor protein, has sperm receptor activity.

The potential of the zona pellucida as a target for immunocontraception

PROBLEM

To investigate the contraceptive potential of the zona pellucida.

METHOD

Generation of antibodies against native and recombinant zona glycoproteins which have then been assessed for their capacity to disrupt sperm-zona interaction in vivo and in vitro. The animal model selected for these studies was the common marmoset and the end points examined were antibody titre, ovarian cyclicity and fertility.

RESULTS

The fact that antibodies against the major zona glycoprotein, ZP3, block both the primary and secondary phases of sperm-zona interaction suggests that this molecule might have potential for contraceptive vaccine development. Active immunization of marmoset monkeys with native porcine ZP3 or recombinant human ZP3 produced long term infertility but also precipitated a premature decline in the primordial follicle population.

CONCLUSIONS

Future studies will have to determine whether a safe, effective vaccine can be engineered by coupling unique B-cell epitopes from ZP3 to foreign T-cell antigens.

Mucin-type glycoprotein from Drosophila melanogaster embryonic cells: characterization of carbohydrate component

A secreted glycoprotein (GP) with apparent molecular mass of 90 kDa produced by cultured embryonic cells of Drosophila melanogaster was isolated and partially characterized. GP is enriched by Ser + Thr and Pro residues that constitute up to 30% of the total number of amino acids. An abundant carbohydrate moiety (40% of molecular mass) is mainly represented by vertebrate mucin-type O-linked disaccharide units Gal(beta 1-3)-GalNAc, occupying about a half of the total number of Ser+Thr residues and rendering the GP molecule high resistance to protease action. A few of N-glycans are also present in GP. These characteristics allow to consider the Drosophila GP (termed 'mucin-D') as a first representative of invertebrate mucin-type glycoproteins.

Comparison of the acrosome reaction-inducing ability of the outer and inner surfaces of the zona pellucida and oolemma: a study using the golden hamster

The outer surface of hamster zona pellucida has a strong ability to induce the sperm acrosome reaction, whereas neither the inner surface of the zona nor the oolemma has this ability. Therefore, the spermatozoa that fertilise oocytes after subzonal insemination of spermatozoa (SUZI) must have been either acrosome-reacted prior to injection or acrosome-reacted spontaneously while swimming within the perivitelline space. The outer surface of fresh mouse zona is not capable of inducing the acrosome reaction of hamster spermatozoa. When fixed with glutaraldehyde, however, it became capable of inducing the hamster sperm acrosome reaction rather efficiently. The reason for this is not clear.

Influence of elevated pH levels on structural and functional characteristics of the human zona pellucida: functional morphological aspects

PURPOSE

A total of 86 fresh and salt-stored immature human oocytes derived from postmortem ovarian tissue were used for this study.

METHODS

Oocytes were randomly incubated either in synthetic human tubal fluid medium (untreated zonae) or in a chemically defined medium (treated zonae).

RESULTS

Sperm binding experiments using hemizona assay conditions exhibited a 10-fold increased binding of sperm to treated compared to untreated oocytes (272.7 +/- 43 versus 24.3 +/- 15 sperm bound, respectively; P < 0.0001). pH recordings during incubation showed elevated pH levels of 8.1 compared to pH 7.2 among treated and untreated zonae, respectively. Ultrastructural examination showed a spongy appearance of the surface of treated zonae, whereas untreated zonae appeared compact with smooth surface.

CONCLUSIONS

The marked increase in sperm binding among treated zonae, together with the ultrastructural findings, suggest that the altered zona surface enhances sperm binding. The physiological maturational process of the zona pellucida might be manipulated in vitro, thus increasing sperm binding to the zona.

Involvement of N-linked carbohydrate chains of pig zona pellucida in sperm-egg binding

The sperm receptor activity of pig zona pellucida has been previously shown to exist in one of the components, pig zona protein 3 alpha (PZP3 alpha), that can be purified after the removal of sialylated and/or sulfated N-acetylpoly(lactosamine) by digestion with endo-beta-galactosidase. In this study, we examined whether N-linked or O-linked carbohydrate chains are involved in the sperm receptor activity of pig zona pellucida. The elimination of N-linked carbohydrate chains from endo-beta-galactosidase-digested PZP3 alpha by digestion with N-glycanase markedly reduced its inhibitory effect on sperm-egg binding in an in vitro competition assay, whereas the elimination of O-linked carbohydrate chains by alkali treatment hardly reduced the inhibitory effect. These results indicate that N-linked carbohydrate chains of PZP3 alpha play a major role in mediating the sperm binding of zona pellucida in pig.

Oocyte Gal alpha 1,3Gal epitopes implicated in sperm adhesion to the zona pellucida glycoprotein ZP3 are not required for fertilization in the mouse

The Gal alpha 1-->3Gal structure is displayed on the zona pellucida glycoprotein ZP3 on murine oocytes. This trisaccharide has been implicated in sperm-zona pellucida adhesive events thought to be essential to fertilization in the mouse. To determine directly if this molecule is required for fertilization, we have generated mice that are deficient in a gene (alpha 1,3GT) encoding the UDP-Gal:beta-D-Gal-alpha 1-->3Gal-galactosyltransferase enzyme responsible for Gal alpha 1-->3Gal synthesis and expression. These mice develop normally and exhibit no gross phenotypic abnormalities. The Gal alpha 1-->3Gal epitope is absent from the vascular endothelium and other tissues in alpha 1,3GT (-/-) adult mice. By contrast, alpha 1,3GT (-/-) mice, like humans, develop naturally occurring anti-alpha-galactoside antibodies normally absent in wild type mice. Female alpha 1,3GT (-/-) mice yield oocytes that are devoid of the Gal alpha 1-->3Gal epitope; however, these mice are fully fertile. These observations indicate that the Gal alpha 1-->3Gal moiety is not essential to sperm-oocyte interactions leading to fertilization or to essentially normal development. They further suggest that alpha 1,3GT (-/-) mice will find utility for exploring approaches to diminish anti-Gal-dependent hyperacute xenograft rejection, which presents a major barrier to the use of porcine and other non-primate organs for xenotransplantation in humans.

Elaboration of an oviductin by the oviductal epithelium in relation to embryo development as visualized by immunocytochemistry

The hamster oviduct secretes a high molecular weight antigen that belongs to the family of glycoproteins known as oviductins. In the present study, using immuno-electron microscopy, we examined the location of this hamster oviductin-1 (Hm Ov-1) in hamster oviductal oocytes and early embryos up to the blastocyst stage. The immunoreactive pattern of Hm Ov-1 changes markedly during the embryo development. In oviductal oocytes prior to fertilization, Hm Ov-1 was associated exclusively with the zona pellucida. Following fertilization, immunolabeling was detected in the perivitelline space and over the plasma membrane of 2-cell, 4-cell, and 8-cell embryos as well as young blastocysts. The change of the immunoreactive pattern was accompanied by the formation of an abundant number of coated pits, endocytic vesicles, multivesicular bodies, and lysosomal-like structures which were strongly labeled by gold particles. These immunogold-labeled cytoplasmic organelles characteristic of the endosomal-lysosomal apparatus were particularly evident in 2-cell, 4-cell, and 8-cell embryos and showed a decrease in number in the blastocysts. The close resemblance between the labeled flocculent material detected in the perivitelline space and that found in the zona matrix of early embryos and blastocysts suggested that the Hm Ov-1-associated electron-dense, flocculent material in the perivitelline space originated from the zona pellucida and was later endocytosed by the blastomeres through coated pits and endocytic vesicles. The detection of Hm Ov-1 in numerous multivesicular bodies and lysosomal structures indicated that the oviductin is eventually degraded. Although the exact functional role of Hm Ov-1 is not known, the presence of a copious amount of Hm Ov-1 in early hamster embryos may be ascribed to a special relationship between this particular oviductin and embryo development.

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.

Coordinate expression of the three zona pellucida genes during mouse oogenesis

The mammalian zona pellucida is an extracellular matrix that surrounds growing oocytes, ovulated eggs and early embryos. The mouse zona is composed of three sulfated glycoproteins: ZP1, ZP2 and ZP3. Each is critically involved in fertilization, the postfertilization block to polyspermy and protection of the preimplantation embryo. We have previously isolated cDNAs encoding mouse ZP2 and ZP3 and now report the isolation of a full-length cDNA encoding ZP1. Mouse ZP1 is composed of a 623 amino acid polypeptide chain with a signal peptide and a carboxyl terminal transmembrane domain, typical of all zona proteins. Sequence comparison demonstrate that mouse ZP1 is an orthologue of a rabbit zona protein, R55. The expression of R55 has been reported previously in both oocytes and granulosa cells. However, by northern analysis and in situ hybridization with 33P-labelled antisense probes to each of the three mouse zona mRNAs, we have determined that the expression of each mouse zona gene is restricted to the oocyte. ZP2 transcripts, but not ZP1 or ZP3, are detected in resting (15 microns diameter) oocytes, and all three zona transcripts coordinately accumulate as oocytes begin to grow. Together they represent approximately 1.5% of the total poly(A)+ RNA in 50–60 microns oocytes. In the latter stages of oogenesis, their abundance declines and each zona transcript is present in ovulated eggs at less than 5% of its maximal level. No zona transcripts were detected above background signal in granulosa cells. We conclude that, in mice, the three zona pellucida genes are expressed in a coordinate, oocyte-specific manner during the growth phase of oogenesis. Our data support the hypothesis that the transcription of the zona genes is controlled, in part, by shared regulatory element(s).

Enzyme-assisted synthesis of a bivalent high-affinity dodecasaccharide inhibitor of mouse gamete adhesion. The length of the chains carrying distal alpha 1,3-bonded galactose residues is critical

Proposing to study the molecular mechanisms of mouse gamete adhesion with the aid of high affinity adhesion inhibitors of saccharide nature, we report here the enzymatic synthesis of a bivalent oligosaccharide Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-3(Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4GlcNAc beta 1-6)Gal beta 1-4GlcNAc (4), consisting of two long arms that link together two distal alpha 1,3-galactose residues. Binding data reported elsewhere (E. Litscher et al., Biochemistry, 1995, 34, 4662-4669) show that 4 is a high affinity inhibitor of mouse gamete adhesion in vitro (IC50 = 9 microM), while a related octasaccharide Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-3(Gal alpha 1-3Gal beta 1-4GlcNAc beta 1-6)Gal beta 1-4GlcNAc, consisting of two short arms is of very low inhibitory activity. The data highlight the importance of the two alpha-galactose residues of 4, and the length of the sugar chains joining them.

Transgenic mice with reduced numbers of functional sperm receptors on their eggs reproduce normally

To initiate fertilization in mice, free-swimming sperm bind to mZP3, an approximately 83-kDa glycoprotein present in the ovulated egg zona pellucida (ZP). mZP3 is located periodically along the filaments that constitute the ZP. Sperm recognize and bind to specific oligosaccharides linked to one or more of five Ser residues clustered in the carboxy-terminal one-third of the mZP3 polypeptide. When all five Ser residues are converted to nonhydroxy amino acids by site-directed mutagenesis of the mZP3 gene, an inactive form of mZP3, called mZP3[ser], is secreted by embryonal carcinoma cells stably transfected with the mutated gene. Here, seven independent transgenic mouse lines were established that harbor the mutated mZP3 gene. In all lines, the mutant gene is expressed by growing oocytes and mZP3[ser] is synthesized, secreted, and incorporated into the ZP. Purified mZP3[ser] prepared from ovaries of transgenic mice, like mZP3[ser] from transfected embryonal carcinoma cells, is inactive in sperm binding assays in vitro. On the other hand, the presence of mZP3[ser] in the ZP does not significantly affect either the binding of sperm to ovulated eggs in vitro or the reproduction of the mice, i.e., the transgenic mice are fertile, breed at normal intervals, and produce litters of normal sizes. These results indicate that the number of functional sperm receptors in the ZP can be reduced by more than 50% without adversely affecting fertilization of eggs in vivo.

Activation of a Gi protein in digitonin/cholate-solubilized membrane preparations of mouse sperm by the zona pellucida, an egg-specific extracellular matrix

Mammalian sperm possess guanine nucleotide-binding regulatory proteins (G proteins) that are involved in signal transduction pathways leading to zona pellucida (ZP)-mediated acrosomal exocytosis. We have previously examined ZP-G protein dynamics in mouse sperm homogenates, as well as cell-free membrane preparations, and our data support the existence of ZP receptor-G protein complexes in sperm membranes. However, the composition of this complex has not been identified due to experimental limitations of the membrane preparations. In the present study, a detergent-solubilized preparation from mouse sperm membranes that retained the signaling properties of cell homogenates and cell-free membrane preparations was developed using buffers containing digitonin and cholate. GTP gamma S, a poorly hydrolyzable analogue of GTP, bound to these solubilized preparations in a specific and concentration-dependent fashion that reached saturation at 100 nM. Incubation of this solubilized membrane preparation with heat-solubilized ZP resulted in an increase in specific GTP gamma S binding in a concentration-dependent manner, with a maximal response at 4-6 ZP/microliters. Mastoparan (50 microM) increased GTP gamma S binding to levels similar to that seen with solubilized ZP. Mastoparan plus ZP stimulated GTP gamma S binding to the same extent as mastoparan or ZP alone. Pertussis toxin completely inhibited ZP-stimulated GTP gamma S binding and decreased mastoparan-stimulated GTP gamma S binding by 50-60%. Purified ZP3, the ZP component that possesses quantitatively all of the sperm binding and acrosomal exocytosis-inducing activities of the intact ZP, stimulated GTP gamma S binding to an extent similar to that of solubilized ZP.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitelline coat of Unio elongatulus: III. Glycan chain analysis of the 220- and 180-kD components by means of lectins

Lectins of different binding specificity were used to analyze the oligosaccharide chains of the 220- and 180-kD proteins of the Unio elongatulus egg vitelline coat (vc). The lectins ConA and RCA1 reacted with both glycoproteins, and four other lectins reacted with one or other vc components. The lectin from Galanthus nivalis, which recognizes terminal mannose residues of N-linked high mannose type oligosaccharide chains, bound specifically to the 180-kD protein. Binding sites for this lectin were found throughout the vc of the differentiating oocyte and the mature egg. Lectins specific for the O-linked oligosaccharide chains, such as AIA and PNA, reacted only with the 220-kD protein species. Binding sites for these lectins were found only in the crater region. The presence of fucosyl residues on the glycan chains was investigated with lectins from Lotus tetragonolobus and Aleuria aurantia. The latter was positive on both glycoproteins, whereas LTA was only positive to the 220-kD species. The binding sites of both these lectins were in the same areas as those of PNA and AIA. These results suggest that while the 180-kD protein is part of the entire vc structure, the 220-kD protein is prevalently accumulated in the crater region. Since this is where sperm recognition and interaction take place, it has been suggested the 220-kD protein acts as a ligand molecule in the sperm-egg interaction.

Stepwise transfer of alpha-D-Galp-(1-->3)-beta-D-Galp-(1-->4)-beta-D-GlcpNAc sequences to 3-OH and 6-OH of distal galactose residues in bi-, tri-, and tetra-antennary asialo-glycans of N-linked complex type

The hydroxyl groups 3 and 6 of distal galactose units in bi-, tri-, and tetra-antennary asialo-glycans of N-linked complex type were substituted stepwise by transferase reactions with the sequence alpha-D-Galp-(1-->3)-beta-D-Galp-(1-->4)-beta-D-GlcpNAc. The products of each transferase reaction were purified chromatographically and the structures were confirmed by 1H NMR spectroscopy. Molecular weights of the final products were determined by matrix-assisted laser-desorption mass spectrometry (MALDI-MS).

Mapping the mouse ZP3 combining site for sperm by exon swapping and site-directed mutagenesis

During fertilization in mice, sperm bind to mouse ZP3 (mZP3), a M(r) approximately 83,000 glycoprotein present in the ovulated egg extracellular coat, or zona pellucida. Sperm recognize and bind to specific serine/threonine-linked (O-linked) oligosaccharides present at the mZP3 combining site for sperm. Binding to mZP3 induces sperm to undergo a form of exocytosis, the acrosome reaction. To map the mZP3 combining site for sperm, we examined the effect of exon swapping and site-directed mutagenesis on the glycoprotein's two activities, sperm binding and induction of the acrosome reaction. Stably transfected embryonal carcinoma cell lines were established that synthesized recombinant glycoproteins and secreted them into the culture medium. The glycoproteins were partially purified from culture medium and assayed for sperm-binding and acrosome reaction-inducing activities. Results of these assays suggest that glycosylation of one or more of five serine residues, clustered together in a polypeptide region encoded by mZP3 gene exon 7, is required for activity. Interestingly, this polypeptide region exhibits considerable sequence divergence during evolution and may be related to the proposed role for oligosaccharides in species-specific gamete adhesion during mammalian fertilization.

Expression of the rabbit sperm protein Sp17 in COS cells and interaction of recombinant Sp17 with the rabbit zona pellucida

This study extends our analysis of rabbit recombinant Sp17 (rSp17) by examining whether rSp17 synthesized in transfected COS cells will show a particular localization within the cell and whether the COS cell will bind with zona pellucida. We show, using the crosslinking, reagent DSS that rSp17 can bind to rabbit zona glycoprotein R45 or R55.

Sperm-binding proteins

Gamete recognition and binding are mediated by specific proteins on the surface of the sperm and egg. Identification and characterization of some of these proteins from several model systems, particularly mouse and sea urchin, have focused interest on the general properties and functions of gamete recognition proteins. Sperm-binding proteins located in egg extracellular coats as well as sperm-binding proteins that are localized to the egg plasma membrane are presented in the context of their structure and function in gamete binding. Unifying and disparate characteristics are discussed in light of the diverse biology of fertilization among species. Outstanding questions, alternative mechanisms and models, and strategies for future work are presented.