Localization of neutral N-linked carbohydrate chains in pig zona pellucida glycoprotein ZPC

Cryo-EM structure of native human uromodulin, a zona pellucida module polymer

Assembly of extracellular filaments and matrices mediating fundamental biological processes such as morphogenesis, hearing, fertilization, and antibacterial defense is driven by a ubiquitous polymerization module known as zona pellucida (ZP) "domain". Despite the conservation of this element from hydra to humans, no detailed information is available on the filamentous conformation of any ZP module protein. Here, we report a cryo-electron microscopy study of uromodulin (UMOD)/Tamm-Horsfall protein, the most abundant protein in human urine and an archetypal ZP module-containing molecule, in its mature homopolymeric state. UMOD forms a one-start helix with an unprecedented 180-degree twist between subunits enfolded by interdomain linkers that have completely reorganized as a result of propeptide dissociation. Lateral interaction between filaments in the urine generates sheets exposing a checkerboard of binding sites to capture uropathogenic bacteria, and UMOD-based models of heteromeric vertebrate egg coat filaments identify a common sperm-binding region at the interface between subunits.

Porcine model for the study of sperm capacitation, fertilization and male fertility

Mammalian fertilization remains a poorly understood event with the vast majority of studies done in the mouse model. The purpose of this review is to revise the current knowledge about semen deposition, sperm transport, sperm capacitation, gamete interactions and early embryonic development with a focus on the porcine model as a relevant, alternative model organism to humans. The review provides a thorough overview of post-ejaculation events inside the sow's reproductive tract including comparisons with humans and implications for human fertilization and assisted reproductive therapy (ART). Porcine methodology for sperm handling, preservation, in vitro capacitation, oocyte in vitro maturation, in vitro fertilization and intra-cytoplasmic sperm injection that are routinely used in pig research laboratories can be successfully translated into ART to treat human infertility. Last, but not least, new knowledge about mitochondrial inheritance in the pig can provide an insight into human mitochondrial diseases and new knowledge on polyspermy defense mechanisms could contribute to the development of new male contraceptives.

Posttranslational modifications of zona pellucida proteins

The zona pellucida (ZP), which surrounds the mammalian oocyte, functions in various aspects of fertilization. The ZP consists of three or four glycoproteins, which are derived from transmembrane proteins that lack the ability to self-assemble. Following posttranslational processing at specific sites, ectodomains of ZP precursor proteins are released from the membrane and begin to form a matrix. Glycosylational modification is thought to be involved in species-selective sperm recognition by ZP proteins. However, in mice, the supramolecular structure of the zona matrix is also important in sperm recognition. One ZP protein, ZP2, is processed at a specific site upon fertilization by ovastacin, which is released from cortical granules inside the oocyte. This phenomenon is involved in the block to polyspermy. The proteolysis of ubiquitinated ZP proteins by a sperm-associated proteasome is involved in penetration of the zona matrix by sperm, at least in the pigs. Thus, the posttranslational modification of ZP proteins is closely tied to ZP formation and the regulation of sperm-oocyte interactions.

Hamster zona pellucida is formed by four glycoproteins: ZP1, ZP2, ZP3, and ZP4

The zona pellucida (ZP) is an extracellular glycoprotein matrix that surrounds all mammalian oocytes. Recent data have shown the presence of four glycoproteins (ZP1, ZP2, ZP3, and ZP4) in the ZP of human and rat rather than the three glycoproteins proposed in the mouse model. In the hamster (Mesocricetus auratus), it was previously described that ZP was composed of three different glycoproteins, called ZP1, ZP2, and ZP3, even though only ZP2 and ZP3 have been cloned thus far. The aim of the study was to determine whether hamster might also express four, rather than three, ZP proteins. The full-length cDNAs encoding hamster ZP glycoproteins 1 and 4 were isolated using rapid amplification cDNA ends (RACE). The cDNA of ZP1 contains an open reading frame of 1851 nucleotides encoding a polypeptide of 616 amino acid residues. The amino acid sequence of ZP1 revealed a high homology with other mammalian species like human (66%), rat (80%), and mouse (80%). The cDNA of ZP4 contains an open reading frame of 1632 nucleotides encoding a polypeptide of 543 amino acid residues. The deduced amino acid sequence of ZP4 revealed high overall homology with rat (82%) and human (78%). Subsequent mass spectrometric analysis of the hamster ZP allowed identification of peptides from all four glycoproteins. The data presented in this study provide evidence, for the first time, that the hamster ZP matrix is composed of four glycoproteins.

Glycodelin-A interacts with fucosyltransferase on human sperm plasma membrane to inhibit spermatozoa-zona pellucida binding

Fertilization depends on successful binding of the spermatozoa to the zona pellucida of the oocyte. Glycodelin-A inhibits spermatozoa-zona pellucida binding. Previous data showed that glycodelin-A receptor(s) and zona pellucida protein receptor(s) on human spermatozoa are closely related. Using a chemical cross-linking approach, the glycodelin-A-sperm receptor complex was isolated. The receptor was identified to be fucosyltransferase-5 (FUT5) by mass spectrometry and confirmed with the use of anti-FUT5 antibodies. Sperm FUT5 was an externally oriented integral membrane protein in the acrosomal region of human spermatozoa. Biologically active FUT5 was purified from spermatozoa. Co-immunoprecipitation confirmed the interaction between glycodelin-A and sperm FUT5. Solubilized zona pellucida reduced the binding of glycodelin-A to sperm FUT5. An anti-FUT5 antibody and FUT5 acceptor blocked the binding of glycodelin-A to spermatozoa and the zona binding inhibitory activity of glycodelin-A. Sperm FUT5 bound strongly to intact and solubilized human zona pellucida. The equilibrium dissociation constant of sperm FUT5 binding to solubilized zona pellucida was 42.82 pmol/ml. These observations suggest that human sperm FUT5 is a receptor of glycodelin-A and zona pellucida proteins, and that glycodelin-A inhibits spermatozoa-zona binding by blocking the binding of sperm FUT5 to the zona pellucida.

Participation of the nonreducing terminal beta-galactosyl residues of the neutral N-linked carbohydrate chains of porcine zona pellucida glycoproteins in sperm-egg binding

The zona pellucida (ZP) surrounding the mammalian oocyte is composed of three glycoprotein components (ZPA, ZPB, and ZPC). Mammalian sperm bind to carbohydrate chains of a ZP glycoprotein in the initial phase of fertilization. Sperm-ligand carbohydrate chains have been characterized in mouse, cow, and pig. In pigs, triantennary/tetraantennary neutral complex-type chains from ZPB/ZPC mixture possess stronger sperm-binding activity than those of biantennary chains (Kudo et al., 1998: Eur J Biochem 252:492-499). Most of these oligosaccharides have beta-galactosyl residues at the nonreducing ends. This study used two in vitro competition assays to investigate the participation of the nonreducing terminal beta-galactosyl residues of the ligand active chains in porcine sperm binding. The removal of the nonreducing terminal beta-galactosyl residues from either the ligand active carbohydrate chains or endo-beta-galactosidase-digested glycoproteins significantly reduced their inhibition of sperm-egg binding, indicating that the beta-galactosyl residues at the nonreducing ends are involved in porcine sperm-egg binding. A correlation between the sperm-binding activity and in vitro fertilization rate is also presented.

Baculovirus-expressed recombinant human zona pellucida glycoprotein-B induces acrosomal exocytosis in capacitated spermatozoa in addition to zona pellucida glycoprotein-C

To facilitate our understanding of the role of zona pellucida glycoproteins during fertilization in humans, recombinant human zona pellucida glycoprotein-A (hZPA), -B (hZPB) and -C (hZPC) were obtained by using Escherichia coli and baculovirus expression systems. Analysis by SDS-PAGE and Western blot of the Ni-NTA affinity purified recombinant proteins revealed that the baculovirus-expressed hZPA, hZPB and hZPC have an apparent molecular weight of approximately 110, approximately 70-75 and approximately 65 kDa, respectively, as compared to approximately 80, approximately 65 and approximately 50 kDa of the respective E. coli-expressed proteins. Lectin binding studies revealed that the baculovirus-expressed recombinant zona proteins were glycosylated. Major oligosaccharides were represented by strong reactivity with Concanavalin A (mannose alpha 1-3 or mannose alpha 1-6 residues) and Jacalin (alpha-O glycosides of Gal or GalNAc moieties). A significant increase in acrosomal exocytosis was observed when capacitated human sperm were incubated in vitro with baculovirus-expressed hZPB (P=0.0005) and hZPC (P=0.0005) The E. coli-expressed hZPB, hZPC and baculovirus-expressed hZPA failed to induce any significant increase (P>0.05) in acrosome reaction. In contrast to hZPC, the acrosome reaction induced by recombinant hZPB was not inhibited by pertussis toxin. These studies, for the first time, have demonstrated that in humans, ZPB also induces acrosomal exocytosis through a Gi independent pathway.

Analysis of N-linked glycans of porcine zona pellucida glycoprotein ZPA by MALDI-TOF MS: a contribution to understanding zona pellucida structure

The mammalian oocyte is encased by a transparent extracellular matrix, the zona pellucida (ZP), which consists of three glycoproteins, ZPA, ZPB, and ZPC. The glycan structures of the porcine ZP and the complete N-glycosylation pattern of the ZPB/ZPC oligomer has been recently described. Here we report the N-glycan pattern and N-glycosylation sites of the porcine ZP glycoprotein ZPA of an immature oocyte population as determined by a mass spectrometric approach. In-gel deglycosylation of the electrophoretically separated ZPA protein and comparison of the pattern obtained from the native, the desialylated and the endo-beta-galactosidase-treated glycoprotein allowed the assignment of the glycan structures by MALDI-TOF MS by considering the reported oligosaccharide structures. The major N-glycans are neutral biantennary complex structures containing one or two terminal galactose residues. Complex N-glycans carrying N-acetyllactosamine repeats are minor components and are mostly sialylated. A significant signal corresponding to a high-mannose type chain appeared in the three glycan maps. MS/MS analysis confirmed its identity as a pentamannosyl N-glycan. By the combination of tryptic digestion of the endo-beta-galactosidase-treated ZP glycoprotein mixture and in-gel digestion of ZPA with lectin affinity chromatography and reverse-phase HPLC, five of six N-glycosylation sites at Asn(84/93), Asn268, Asn316, Asn323, and Asn530 were identified by MS. Only one site was found to be glycosylated in the N-terminal tryptic glycopeptide with Asn(84/93.) N-glycosidase F treatment of the isolated glycopeptides and MS analysis resulted in the identification of the corresponding deglycosylated peptides.

Mass Spectrometry Analysis of Recombinant Human ZP3 Expressed in Glycosylation-Deficient CHO Cells

The zona pellucida is an extracellular matrix that mediates taxon-specific fertilization in which human sperm will not bind to mouse eggs. The mouse zona pellucida is composed of three glycoproteins (ZP1, ZP2, ZP3). The primary structure of each has been deduced from the cDNA nucleic acid sequence, and each has been analyzed by mass spectrometry. However, determination of the secondary structure and processing of the human zona proteins have been hampered by the paucity of biological material. To investigate if taxon-specific sperm-egg recognition was ascribable to structural differences in a zona protein required for matrix formation, recombinant human ZP3 was expressed in CHO-Lec3.2.8.1 cells and compared to mouse ZP3. With nearly complete coverage, LC-QTOF mass spectrometry was used to determine the cleavage of an N-terminal signal peptide (amino acids 1-22) and the release of secreted ZP3 from a C-terminal transmembrane domain (amino acids 379-424). The resultant N-terminal glutamine was cyclized to pyroglutamate (pyrGln(23)), and several C-terminal peptides were detected, including one ending at Asn(350). The disulfide bond linkages of eight cysteine residues in the conserved zona domain were ascertained (Cys(46)/Cys(140), Cys(78)/Cys(99), Cys(217)/Cys(282), Cys(239)/Cys(300)), but the precise linkage of two additional disulfide bonds was indeterminate due to clustering of the remaining four cysteine residues (Cys(319), Cys(321), Cys(322), Cys(327)). Three of the four potential N-linked oligosaccharide binding sites (Asn(125), Asn(147), Asn(272)) were occupied, and clusters of O-glycans were observed within two regions, amino acids 156-173 and 260-281. Taken together, these data indicate that human and mouse ZP3 proteins are quite similar, and alternative explanations of taxon-specific sperm binding warrant exploration.

Identification of the ZPC oligosaccharide ligand involved in sperm binding and the glycan structures of Xenopus laevis vitelline envelope glycoproteins

The Xenopus laevis egg vitelline envelope is composed of five glycoproteins (ZPA, ZPB, ZPC, ZPD, and ZPX). As shown previously, ZPC is the primary ligand for sperm binding to the egg envelope, and this binding involves the oligosaccharide moieties of the glycoprotein (Biol. Reprod., 62:766-774, 2000). To understand the molecular mechanism of sperm-egg envelope binding, we characterized the N-linked glycans of the vitelline envelope (VE) glycoproteins. The N-linked glycans of the VE were composed predominantly of a heterogeneous mixture of high-mannose (5-9) and neutral, complex oligosaccharides primarily derived from ZPC (the dominant glycoprotein). However, the ZPA N-linked glycans were composed of acidic-complex and high-mannose oligosaccharides, ZPX had only high-mannose oligosaccharides, and ZPB lacked N-linked oligosaccharides. The consensus sequence for N-linked glycosylation at the evolutionarily conserved residue N113 of the ZPC protein sequence was glycosylated solely with high-mannose oligosaccharides. This conserved glycosylation site may be of importance to the three-dimensional structure of the ZPC glycoproteins. One of the complex oligosaccharides of ZPC possessed terminal beta-N-acetyl-glucosamine residues. The same ZPC oligosaccharide species isolated from the activated egg envelopes lacked terminal beta-N-acetyl-glucosamine residues. We previously showed that the cortical granules contain beta-N-acetyl-glucosaminidase (J. Exp. Zool., 235:335-340, 1985). We propose that an alteration in the oligosaccharide structure of ZPC by glucosaminidase released from the cortical granule reaction is responsible for the loss of sperm binding ligand activity at fertilization.

Localization of N-linked carbohydrate chains in glycoprotein ZPA of the bovine egg zona pellucida

The zona pellucida, a transparent envelope surrounding the mammalian oocyte, consists of three glycoproteins, ZPA, ZPB and ZPC, and plays a role in sperm-egg interactions. In bovines, these glycoproteins cannot be separated unless the acidic N-acetyllactosamine regions of the carbohydrate chains are removed by endo-beta-Galactosidase digestion. Endo-beta-Galactosidase-digested ZPB retains stronger sperm-binding activity than ZPC. It is still unclear whether ZPA possesses significant activity. Recently, we reported that bovine sperm binds to Man5GlcNAc2, the neutral N-linked chain in the cow zona proteins. In this study, we investigated the localization of the sperm-ligand active high-mannose-type chain and the acidic complex-type chains in bovine ZPA. Three N-glycopeptides of ZPA, containing an N-glycosylation site at Asn83, Asn191 and Asn527, respectively, were obtained from endo-beta-Galactosidase-digested ZPA. Of these glycosylation sites, only Asn527 is present in the ZP domain common to all the zona proteins. The carbohydrate structures of the N-linked chains obtained from each N-glycopeptide were characterized by two-dimensional sugar mapping analysis, while considering the structures of the N-linked chains of the zona protein mixture reported previously. Acidic complex-type chains were found at all three N-glycosylation sites, while Man5GlcNAc2 was found at Asn83 and Asn191, but there was very little of this sperm-ligand active chain at Asn527 in the ZP domain of ZPA.

Molecular cloning of bovine zona pellucida glycoproteins ZPA and ZPB and analysis for sperm-binding component of the zona

The zona pellucida, a transparent envelope surrounding the mammalian oocyte, comprises three glycoproteins, ZPA, ZPB and ZPC, and plays important roles in fertilization. We have previously reported that apparent relative molecular masses of bovine zona glycoproteins on SDS/PAGE under nonreducing conditions after removal of poly N-acetyllactosamine at the nonreducing portion of sugar chains with endo-beta-galactosidase are 72 000, 58 000 and 45 000 [Noguchi, S., Yonezawa, N., Katsumata, T., Hashizume, K.,Kuwayama, M., Hamano, S., Watanabe, S. & Nakano, M. (1994) Biochim. Biophys. Acta 1201, 7-14]. The N-terminal amino-acid sequences and crossreactivity to antibodies specific to each porcine zona component show that the bovine components correspond to porcine ZPA, ZPB and ZPC, respectively. In this study, we deduced amino-acid sequences of bovine ZPA and ZPB by cDNA cloning and sequencing. Identities in amino-acid sequences between bovine and porcine counterparts were 77% for ZPA and 75% for ZPB, whereas between bovine and murine counterparts identities were 57% for ZPA and 37% for ZPB. The positions of Cys were completely conserved in bovine ZPA and ZPB compared with counterparts of other mammalian species. Bovine ZPA was processed between Ala and Asp on fertilization, suggesting that the consensus motif for the processing is Ala-Asp-Asp/Glu. We purified bovine zona components and examined their sperm-binding activity with an in vitro competition assay and sperm-bead-binding assay. As a result, ZPB showed the strongest sperm-binding activity among the components. ZPC also showed sperm-binding activity and the activity per molecule was about one-sixth that of ZPB according to the result of the sperm-bead-binding assay. We could not determine if ZPA has significant sperm-binding activity, but the activity may be much lower than that of ZPB even if ZPA has significant activity. Thus, ZPB may play a major role in sperm binding in bovine zona.

Differential expression of glycoside residues in the mammalian zona pellucida

The mammalian zona pellucida is an extracellular matrix surrounding the oocyte, and is composed of three major glycoproteins, ZP1, ZP2, and ZP3. Previous studies have suggested that the sperm receptor activity of the zona pellucida resides in specific oligosaccharide chains on the ZP3 glycoprotein. However, the nature of the terminal monosaccharide(s) on these glycosidic chains to which sperm bind is a matter of active debate. Evidence has been presented to support a role for at least three distinct monosaccharides in sperm binding, alpha-galactose, L-fucose on Lewis X structures, and beta-N-acetylglucosamine. Previous studies have shown that beta-N-acetylglucosamine is uniformly distributed throughout the zona matrix. In this study, we have investigated the expression and distribution of alpha-galactose and fucose moieties during the maturation of the zona pellucida in mouse, rat, and hamster. Interestingly, alpha-galactose residues are expressed only during later stages of zona secretion and, consequently, are confined to the inner portions of the mature zona pellucida in mouse and rat. In hamster, alpha-galactose residues are only detectable in the zona pellucida of ovulated eggs, and are not found in ovarian oocytes. Fucosyl residues linked to Lewis X glycosides are not detectable at any stage of zona maturation in these three species, whereas fucose linked to N-linked core oligosaccharides are present throughout the zona. These studies indicate a previously unappreciated heterogeneity in the composition of zona glycosides. The specific localization of alpha-galactose residues to the inner portions of the zona matrix suggest a role in the later stages of sperm penetration through the zona. Finally, due to their absence from the zona surface, alpha-galactose and Lewis X fucosyl residues are not likely to be mediators of primary sperm binding.

Oocyte-sperm interactions

The penetration of the zona pellucida is a crucial step during fertilization. Spermatozoa that are unable to recognize and bind to the zona pellucida glycoproteins or respond to the zona pellucida by undergoing the acrosome reaction fail to fertilize the egg. In most mammalian species, after entering the fallopian tube sperm are stored in the isthmic part of the oviduct under conditions that maintain sperm viability and synchronize both sperm transport and the process of acquisition of fertilizing ability, called capacitation. Only capacitated sperm are enabled to recognize the oocyte and respond to the oocyte signals in an appropriate manner. Close to time of ovulation sperm are released from the oviductal epithelium and swim to site of fertilization. The oviduct and the oocyte itself appear to coordinate sperm function and gamete interaction. The gamete recognition and the next levels of interaction are probably granted by the carbohydrate-protein interactions. Upon binding the signal cascade leading to acrosomal exocytosis is activated, eventually initiated by aggregation of zona pellucida receptor molecules. These signal transducing mechanisms are primed during the capacitation process. Tyrosine phosphorylation, tightly connected to the cholesterol efflux from the plasma membrane, and hyperpolarization seem be involved in this priming by activation of Ca(2+) pathways. Further preparational steps of the acrosome reaction may be mediated by osmosensitive signal transducing mechanisms. The current perspective focuses on the molecules involved in the complex hierarchy of sperm-egg interactions and regulative events priming sperm cell during capacitation for the acrosome reaction.

Structural analysis of murine zona pellucida glycans. Evidence for the expression of core 2-type O-glycans and the Sd(a) antigen

Murine sperm initiate fertilization by binding to specific oligosaccharides linked to the zona pellucida, the specialized matrix coating the egg. Biophysical analyses have revealed the presence of both high mannose and complex-type N-glycans in murine zona pellucida. The predominant high mannose-type glycan had the composition Man(5)GlcNAc(2), but larger oligosaccharides of this type were also detected. Biantennary, triantennary, and tetraantennary complex-type N-glycans were found to be terminated with the following antennae: Galbeta1-4GlcNAc, NeuAcalpha2-3Galbeta1-4GlcNAc, NeuGcalpha2-3Galbeta1-4GlcNAc, the Sd(a) antigen (NeuAcalpha2-3[GalNAcbeta1-4]Galbeta1-4GlcNAc, NeuGcalpha2-3[GalNAcbeta1-4]Galbeta1-4GlcNAc), and terminal GlcNAc. Polylactosamine-type sequence was also detected on a subset of the antennae. Analysis of the O-glycans indicated that the majority were core 2-type (Galbeta1-4GlcNAcbeta1-6[Galbeta1-3]GalNAc). The beta1-6-linked branches attached to these O-glycans were terminated with the same sequences as the N-glycans, except for terminal GlcNAc. Glycans bearing Galbeta1-4GlcNAcbeta1-6 branches have previously been suggested to mediate initial murine gamete binding. Oligosaccharides terminated with GalNAcbeta1-4Gal have been implicated in the secondary binding interaction that occurs following the acrosome reaction. The significant implications of these observations are discussed.

Possible mechanisms of mammalian immunocontraception

Ecological and conservation programs in ecosystems around the world have experienced varied success in population management. One of the greatest problems is that human expansion has led to the shrinking of wildlife habitat and, as a result, the overpopulation of many different species has occurred. The pressures exerted by the increased number of animals has caused environmental damage. The humane and practical control of these populations has solicited the scientific community to arrive at a safe, effective, and cost-efficient means of population control. Immunocontraception using zona pellucida antigens, specifically porcine zona pellucida (pZP), has become one of the most promising population control tools in the world today, with notable successes in horses and elephants. A conundrum has risen where pZP, a single vaccine, successfully induces an immunocontraceptive effect in multiple species of mammals. This review describes the most current data pertaining to the mammalian zona pellucida and immunocontraception, and from these studies, we suggest several potential mechanisms of immunocontraception.

Porcine sperm surface beta1,4galactosyltransferase binds to the zona pellucida but is not necessary or sufficient to mediate sperm-zona pellucida binding

The binding of sperm to the zona pellucida is an integral part of the mammalian fertilization process, investigated most extensively in the mouse. Several sperm receptors for the murine zona pellucida have been studied (Snell WJ, White JM. 1996. Cell 85:629-637; Wassarman PM. 1999. Cell 96:175-183), but the most compelling evidence exists for beta-1,4-galactosyltransferase (GalTase). Considering that GalTase is present on the surface of porcine sperm (Larson JL, Miller DJ. 1997. Biol Reprod 57:442-453), we investigated the role of GalTase in porcine sperm-zona binding. Sperm surface GalTase catalyzed the addition of uridine diphosphate-[(3)H]galactose to the 55 kDa group of the porcine zona pellucida proteins implicated in sperm binding, demonstrating that GalTase binds the porcine zona. The functional importance of GalTase-zona pellucida binding was tested. Addition of uridine diphosphate galactose, a substrate that completes the GalTase enzymatic reaction and disrupts GalTase mediated adhesion, had no effect on binding of sperm to porcine oocytes. Furthermore, removal of the GalTase zona ligand by incubation of oocytes with N-acetylglucosaminidase had no effect on binding of sperm to oocytes. These results suggest that GalTase is not necessary for sperm to bind to the zona pellucida. Digestion of isolated porcine zona proteins with N-acetylglucosaminidase did not affect the biological activity of soluble porcine zona proteins in competitive sperm-zona binding assays, suggesting that GalTase alone is not sufficient to mediate sperm-zona attachment. From these results, it appears that, although GalTase is able to bind porcine zona proteins, its function in porcine sperm-zona binding is not necessary or sufficient for sperm-zona binding. This supports the contention that porcine sperm-zona binding requires redundant gamete receptors.