cDNA cloning and sequence analysis of the Xenopus laevis egg envelope glycoprotein gp43

Lineage-specific evolution of zona pellucida genes in fish

The zona pellucida (ZP) protein constitutes the egg envelope, which surrounds the vertebrate embryo. We performed a comprehensive study on the molecular evolution of ZP genes in Teleostei by cloning and analyzing the expression of ZP genes in fish of Anguilliformes in Elopomorpha, Osteoglossiformes in Osteoglossomorpha, and Clupeiformes in Otocephala to cover unsurveyed fish groups in Teleostei. The present results confirmed findings from our previous reports that the principal organ of ZP gene expression changed from ovary to liver in the common ancestors of Clupeocephala. Even fish species that synthesize egg envelopes in the liver carry the ovary-expressed ZP proteins as minor egg envelope components that were produced by gene duplication during the early stage of Teleostei evolution. The amino acid repeat sequences located at the N-terminal region of ZP proteins are known to be the substrates of transglutaminase responsible for egg envelope hardening and hatching. A repeat sequence was found in zona pellucida Cs of phylogenetically early diverged fish. After changing the synthesis organ, its role is inherited by the N-terminal Pro-Gln-Xaa repeat sequence in liver-expressed zona pellucida B genes of Clupeocephala. These results suggest that teleost ZP genes have independently evolved to maintain fish-specific functions, such as egg envelope hardening and egg envelope digestion, at hatching.

Identification of the origin and localization of chorion (egg envelope) proteins in an ancient fish, the white sturgeon, Acipenser transmontanus

In many modern teleost fish, chorion (egg envelope) glycoproteins are synthesized in the liver of females, and the expression of those genes is controlled by endogenous estrogen released from the ovary during maturation. However, among the classical teleosts, such as salmonid, carp, and zebrafish, the chorion glycoproteins are synthesized in the oocyte, as in higher vertebrates. Sturgeon, which are members of the subclass Chondrostei, represent an ancient lineage of ray-finned fishes that differ from other teleosts in that their sperm possess acrosomes, their eggs have numerous micropyles, and early embryo development is similar to that of amphibians. In order to understand the molecular mechanisms of chorion formation and the phylogenetic relationship between sturgeon and other teleosts, we used specific antibodies directed against the primary components of sturgeon chorion glycoproteins, using immunoblotting and immunocytochemistry approaches. The origin of each chorion glycoprotein was determined through analyses of both liver and ovary, and their localization during ovarian development was investigated. Our data indicate that the origin of the major chorion glycoproteins of sturgeon, ChG1, ChG2, and ChG4, derive not only from the oocyte itself but also from follicle cells in the ovary, as well as from hepatocytes. In the follicle cell layer, granulosa cells were found to be the primary source of ChGs during oogenesis in white sturgeon. The unique origins of chorion glycoproteins in sturgeon suggest that sturgeons are an intermediate form in the evolution of the teleost lineage.

Biogenesis of the mouse egg's extracellular coat, the zona pellucida

Biogenesis of the zona pellucida (ZP), the extracellular coat that surrounds all mammalian eggs, is a universal and essential feature of mammalian oogenesis and reproduction. The mouse egg's ZP consists of only three glycoproteins, called ZP1-3, that are synthesized, secreted, and assembled into an extracellular coat exclusively by growing oocytes during late stages of oogenesis while oocytes are arrested in meiosis. Expression of ZP genes and synthesis of ZP1-3 are gender-specific. Nascent ZP1-3 are synthesized by oocytes as precursor polypeptides that possess several elements necessary for their secretion and assembly into a matrix of long fibrils outside of growing oocytes. Failure to synthesize either ZP2 or ZP3 by homozygous null female mice precludes formation of a ZP during oocyte growth and, due to faulty folliculogenesis and a paucity of ovulated eggs, results in infertility. High-resolution structural analyses suggest that ZP glycoproteins consist largely of immunoglobulin (Ig)-like folds and that the glycoproteins probably arose by duplication of a common Ig-like domain. Mouse ZP1-3 share many features, particularly a ZP domain, with extracellular coat glycoproteins of eggs from other vertebrate and invertebrate animals whose origins date back more than 600 million years. These and other aspects of ZP biogenesis are discussed in this review.

Proteomic characterization and evolutionary analyses of zona pellucida domain-containing proteins in the egg coat of the cephalochordate, Branchiostoma belcheri

Background

Zona pellucida domain-containing proteins (ZP proteins) have been identified as the principle constituents of the egg coat (EC) of diverse metazoan taxa, including jawed vertebrates, urochordates and molluscs that span hundreds of millions of years of evolutionary divergence. Although ZP proteins generally contain the zona pellucida (ZP) structural modules to fulfill sperm recognition and EC polymerization functions during fertilization, the primary sequences of the ZP proteins from the above-mentioned animal classes are drastically different, which makes it difficult to assess the evolutionary relationships of ZP proteins. To understand the origin of vertebrate ZP proteins, we characterized the egg coat components of Branchiostoma belcheri, an invertebrate species that belongs to the chordate subphylum Cephalochordata.

Results

Five ZP proteins (BbZP1-5) were identified by mass spectrometry analyses using the egg coat extracts from both unfertilized and fertilized eggs. In addition to the C-terminal ZP module in each of the BbZPs, the majority contain a low-density lipoprotein receptor domain and a von Willebrand factor type A (vWFA) domain, but none possess an EGF-like domain that is frequently observed in the ZP proteins of urochordates. Fluorescence in situ hybridization and immuno-histochemical analyses of B. belcheri ovaries showed that the five BbZPs are synthesized predominantly in developing eggs and deposited around the extracellular space of the egg, which indicates that they are bona fide egg coat ZP proteins. BbZP1, BbZP3 and BbZP4 are significantly more abundant than BbZP2 and BbZP5 in terms of gene expression levels and the amount of mature proteins present on the egg coats. The major ZP proteins showed high polymorphism because multiple variants are present with different molecular weights. Sequence comparison and phylogenetic analysis between the ZP proteins from cephalochordates, urochordates and vertebrates showed that BbZP1-5 form a monophyletic group and share no significant sequence similarities with the ZP proteins of urochordates and the ZP3 subtype of jawed vertebrates. By contrast, small regions of homology were identifiable between the BbZP and ZP proteins of the non-jawed vertebrate, the sea lamprey Petromyzon marinus. The lamprey ZP proteins were highly similar to the ZP1 and ZP2 subtypes of the jawed vertebrates, which suggests that the ZP proteins of basal chordates most likely shared a recent common ancestor with vertebrate ZP1/2 subtypes and lamprey ZP proteins.

Conclusions

The results document the spectra of zona pellucida domain-containing proteins of the egg coat of basal chordates. Particularly, the study provides solid evidence for an invertebrate origin of vertebrate ZP proteins and indicates that there are diverse domain architectures in ZP proteins of various metazoan groups.

Participation of the 39-kDa glycoprotein (gp39) of the vitelline envelope of Bufo arenarum eggs in sperm-egg interaction

The acquisition of egg fertilizability in Bufo arenarum takes place during the oviductal transit and during this process the extracellular coelomic envelope (CE) of the eggs is converted into the vitelline envelope (VE). It has been stated that one of the necessary events leading to a fertilizable state is the proteolytic cleavage of CE glycoproteins in the oviductal pars recta by oviductin, a serine protease. Consequently, there is a marked increase in the relative quantity of glycoproteins with 39 (gp39) and 42 kDa (gp42) in the VE. In the present study, sperm-VE binding assays using heat-solubilized biotin-conjugated VE glycoproteins revealed that both gp39 and gp42 have sperm binding capacity. According to this result, our study was focused on gp39, a glycoprotein that we have previously reported as a homologue of mammalian ZPC. For this purpose, rabbit polyclonal antibodies against gp39 were generated at our laboratory. The specificity of the antibodies was confirmed with western blot of VE glycoproteins separated on SDS-PAGE. Immunohistochemical and immunoelectron studies showed gp39 distributed throughout the width of the VE. In addition, immunofluorescence assays probed that gp39 bound to the sperm head. Finally, as an approach to elucidate the possible involvement of gp39 in fertilization, inhibition assays showed that pretreatment of eggs with antibodies against gp39 generated a significant decrease in the fertilization rate. Therefore, our findings suggest that gp39, which is modified by oviductal action, participates as a VE glycoprotein ligand for sperm in Bufo arenarum fertilization.

The Sperm-surface glycoprotein, SGP, is necessary for fertilization in the frog, Xenopus laevis

To identify a molecule involved in sperm-egg plasma membrane binding at fertilization, a monoclonal antibody against a sperm-surface glycoprotein (SGP) was obtained by immunizing mice with a sperm membrane fraction of the frog, Xenopus laevis, followed by screening of the culture supernatants based on their inhibitory activity against fertilization. The fertilization of both jellied and denuded eggs was effectively inhibited by pretreatment of sperm with intact anti-SGP antibody as well as its Fab fragment, indicating that the antibody recognizes a molecule on the sperm's surface that is necessary for fertilization. On Western blots, the anti-SGP antibody recognized large molecules, with molecular masses of 65-150 kDa and minor smaller molecules with masses of 20-28 kDa in the sperm membrane vesicles. SGP was distributed over nearly the entire surface of the sperm, probably as an integral membrane protein in close association with microfilaments. More membrane vesicles containing SGP bound to the surface were found in the animal hemisphere compared with the vegetal hemisphere in unfertilized eggs, but the vesicle-binding was not observed in fertilized eggs. These results indicate that SGP mediates sperm-egg membrane binding and is responsible for the establishment of fertilization in Xenopus.

Involvement of sperm proteases in the binding of sperm to the vitelline envelope in Xenopus laevis

Sperm binding to the vitelline envelope in dejellied Xenopus laevis eggs was effectively inhibited by inhibitors for trypsin (soybean trypsin inhibitor and p-toluenesulfonyl-L-lysine chloroethyl ketone) and aminopeptidase B (o-phenanthroline, bestatin, and arphamenine B). Likewise, synthetic 4-methylcoumaryl-7-amide (MCA) substrates (t-butoxycarbonyl-GlyArgArg-MCA, benzyloxycarbonyl-ArgArg-MCA, and Arg-MCA) inhibited binding. Consistently, when jellied eggs were inseminated in the presence of these substrates or inhibitors for proteases, fertilization was effectively blocked. The medium in which live sperm or the sperm membrane fraction were suspended exhibited hydrolyzing activities against the synthetic substrates mentioned above, and these activities were effectively inhibited by the protease inhibitors. Ultracentrifugal fractionation of the sperm suspension following induction of the acrosome reaction by a calcium ionophore, A23187, indicated that a considerable amount of the total tryptic and aminopeptidase B activity was released into the medium. On this occasion, part of the tryptic and aminopeptidase B activity was definitely estimated to be discharged in association with a vesiculated membrane, supporting the notion that the proteases involved in binding to the vitelline envelope are present on the sperm plasma membrane.

Towards a ZP-based contraceptive for marsupials: Comparative analysis and developmental expression of marsupialZP genes

Fertility control in the form of a zona pellucida (ZP)-based immunocontraceptive has shown potential as a humane form of control for overabundant marsupials including the brushtail possum and macropods. Further refinement and development of a ZP-based vaccine requires detailed knowledge of the protein structure and expression in order to ensure maximum efficacy and specificity. Sequencing and comparative analysis of the ZP3 protein from three marsupial orders in this study found a high overall level of conservation; within order Diprotodontia, the ZP3 protein is 86.9-98.9% identical. ZP3 identity falls to 56.6-57.2%, when the grey, short-tailed opossum (a Didelphimorphian) is compared to dasyurid and diprotodontan marsupials. This is similar to its amino acid identity with ZP3 from eutherian species (50.7-52.8%). Comparison of a 21 amino acid epitope in marsupial ZP3 that has shown contraceptive effects, reveals 95-100% identity between the four macropodid species, 81-86% amino acid identity between brushtail possum and the macropods and 67-71% identity between the diprotodontans and the fat-tailed dunnart (a dasyurid). This is comparable to the level of identity between related eutherian mammals. The expression pattern of three ZP genes during brushtail possum and tammar wallaby pouch young development was examined by RT-PCR. This analysis of ZP gene expression has confirmed that ZP mRNA transcription begins in the ovary during pouch young development by about 51 days of age. The presence of ZP transcripts at this stage in pouch young development suggests that marsupial ZP gene transcription begins before the onset of follicular development.

Glycoproteins of the vitelline envelope of Amphibian oocyte: Biological and molecular characterization of ZPC component (gp41) inBufo arenarum

The vitelline envelope (VE) participates in sperm-egg interactions during the first steps of fertilization. In Bufo arenarum, this envelope is composed of at least four glycoproteins, with molecular masses of 120, 75, 41, and 38 kDa and molar ratio of 1:1.3:7.4:4.8, respectively. These components were isolated and covalently coupled to silanized glass slides in order to study their sperm-binding capacity. When considering the molar ratio of the glycoproteins in the egg-envelope and assuming that each protein is monovalent for sperm, the assay showed that gp41 and gp38 possess 55 and 25% of total sperm-binding activity. We obtained a full-length cDNA of gp41 (ZPC), comprising a sequence for 486 amino acids, with 43.3% homology with Xenopus laevis ZPC. As in the case of mammalian ZP3 and Xenopus ZPC, Bufo ZPC presented a furin-like (convertase) and a C-terminal transmembrane domain (TMD) reflecting common biosynthetic and secretory pathways. As it was reported for some fishes, we obtained evidence that suggests the presence of more than one zpc gene in Bufo genome, based on different partial cDNA sequences of zpc, Southern blots and two-dimensional SDS-PAGE of deglycosylated egg-envelope components. As far as we are aware, this is the first observation of the presence of different zpc genes in an Amphibian species.

Bonnet monkey (Macaca radiata) ovaries, like human oocytes, express four zona pellucida glycoproteins

Recent studies document that the zona pellucida matrix of human oocytes is composed of four glycoproteins designated as ZP1, ZP2, ZP3, and ZP4 instead of three as proposed in mouse model. In the present study, investigations were carried out to find the presence of the fourth ZP glycoprotein in the ovaries of non-human primates namely bonnet monkey (Macaca radiata). Employing total RNA isolated from bonnet monkey ovaries, the complementary deoxyribonucleic acid (cDNA) encoding bonnet monkey ZP1 (up to furin cleavage site) was successfully amplified by reverse transcribed polymerase chain reaction (RT-PCR). The deduced amino acid (aa) sequence of bonnet monkey ZP1 revealed 96.0% identity with human ZP1. The 21 cysteine residues present in bonnet monkey ZP1 were conserved in human, mouse, rat, quail, and chicken. Simultaneously, polyclonal antibodies were generated in mice against synthetic peptides corresponding to human ZP1 (P1, 137-150 aa; P2, 223-235 aa; P3, 237-251 aa; P4, 413-432 aa; and P5, 433-451 aa). Employing anti-peptide antibodies that were devoid of cross-reactivity as determined by ELISA with human/bonnet monkey recombinant ZP2, ZP3, and ZP4, the presence of ZP1 in the ovaries of bonnet monkey and human oocytes was demonstrated by indirect immunofluorescence. The antibodies against peptides P3 and P4 reacted only with the ZP of bonnet monkey ovaries and not other ovarian associated cells. The data presented in this manuscript provide evidence, for the first time, that the bonnet monkey ZP matrix is composed of four glycoproteins.

Testing for adaptive evolution of the female reproductive protein ZPC in mammals, birds and fishes reveals problems with the M7-M8 likelihood ratio test

Background

Adaptive evolution appears to be a common feature of reproductive proteins across a very wide range of organisms. A promising way of addressing the evolutionary forces responsible for this general phenomenon is to test for adaptive evolution in the same gene but among groups of species, which differ in their reproductive biology. One can then test evolutionary hypotheses by asking whether the variation in adaptive evolution is consistent with the variation in reproductive biology. We have attempted to apply this approach to the study of a female reproductive protein, zona pellucida C (ZPC), which has been previously shown by the use of likelihood ratio tests (LRTs) to be under positive selection in mammals.

Results

We tested for evidence of adaptive evolution of ZPC in 15 mammalian species, in 11 avian species and in six fish species using three different LRTs (M1a-M2a, M7-M8, and M8a-M8). The only significant findings of adaptive evolution came from the M7-M8 test in mammals and fishes. Since LRTs of adaptive evolution may yield false positives in some situations, we examined the properties of the LRTs by several different simulation methods. When we simulated data to test the robustness of the LRTs, we found that the pattern of evolution in ZPC generates an excess of false positives for the M7-M8 LRT but not for the M1a-M2a or M8a-M8 LRTs. This bias is strong enough to have generated the significant M7-M8 results for mammals and fishes.

Conclusion

We conclude that there is no strong evidence for adaptive evolution of ZPC in any of the vertebrate groups we studied, and that the M7-M8 LRT can be biased towards false inference of adaptive evolution by certain patterns of non-adaptive evolution.

Mass spectrometric evidence that proteolytic processing of rainbow trout egg vitelline envelope proteins takes place on the egg

The rainbow trout egg vitelline envelope (VE) is constructed of three proteins, called VEalpha,VEbeta, and VEgamma, that are synthesized and secreted by the liver and transported in the bloodstream to the ovary, the site of VE assembly around eggs. All three proteins possess an N-terminal signal peptide, a zona pellucida domain, a consensus furin-like cleavage site (CFLCS) close to the C terminus, and a short propeptide downstream of the CFLCS. Proteolytic processing at the CFLCS results in loss of the short C-terminal propeptide from precursor proteins and enables incorporation of mature proteins into the VE. Here mass spectrometry (matrix-assisted laser desorption ionization time-of-flight-mass spectrometry and liquid chromatography-mass spectrometry with a micromass-quadrupole TOF hybrid mass and a QSTAR Pulsar i mass spectrometer) was employed with VE proteins isolated from rainbow trout eggs in a peptidomics-based approach to determine the following: 1) the C-terminal amino acid of mature, proteolytically processed VE proteins; 2) the cellular site of proteolytic processing at the CFLCS of VE precursor proteins; and 3) the relationship between proteolytic processing and limited covalent cross-linking of VE proteins. Peptides derived from the C-terminal region were found for all three VE proteins isolated from eggs, indicating that processing at the CFLCS occurs after the arrival of VE precursor proteins at the egg. Consistent with this conclusion, peptides containing an intact CFLCS were also found for all three VE proteins isolated from eggs. Furthermore, peptides derived from the C-terminal propeptides of VE protein heterodimers VEalpha-VEgamma and VEbeta-VEgamma were found, suggesting that a small amount of VE protein can be covalently cross-linked on eggs prior to proteolytic processing at the CFLCS. Collectively, these results provide important evidence about the process of VE formation in rainbow trout and other non-cyprinoid fish and allow comparisons to be made with the process of zona pellucida formation in mammals.

Application of Fourier transform ion cyclotron resonance mass spectrometry to oligosaccharides

The application of Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) to the structural elucidation of oligosaccharides is described. This review covers the analyses of oligosaccharides in the context of the unique features of FTICR MS and the improvements in instrumentation that make it possible to study this class of compounds. It consists of work performed initially to understand the fundamental aspects of oligosaccharide ionization and unimolecular fragmentation. More recent investigation includes the application of the technique to samples of direct biological origin. Chemical and enzymatic degradation methods in conjunction with mass spectrometry (MS) and the use front-end methods with FTICR MS are also discussed. The current applications including the characterization of bacterial lipooligosaccharides and phosporylated carbohydrates are described.

Identification of choriogenin cis-regulatory elements and production of estrogen-inducible, liver-specific transgenic Medaka

Choriogenins (chg-H, chg-L) are precursor proteins of egg envelope of medaka and synthesized in the spawning female liver in response to estrogen. We linked a gene construct chg-L1.5 kb/GFP (a 1.5 kb 5'-upstream region of the chg-L gene fused with a green fluorescence protein (GFP) gene) to another construct emgb/RFP (a cis-regulatory region of embryonic globin gene fused with an RFP gene), injected the double fusion gene construct into 1- or 2-cell-stage embryos, and selected embryos expressing the RFP in erythroid cells. From the embryos, we established two lines of chg-L1.5 kb/GFP-emgb/RFP-transgenic medaka. The 3-month-old spawning females and estradiol-17beta (E2)-exposed males displayed the liver-specific GFP expression. The E2-dependent GFP expression was detected in the differentiating liver of the stage 37-38 embryos. In addition, RT-PCR and whole-mount in situ hybridization showed that the E2-dependent chg expression was found in the liver of the stage 34 embryos of wild medaka, suggesting that such E2-dependency is achieved shortly after differentiation of the liver. Analysis using serial deletion mutants fused with GFP showed that the region -426 to -284 of the chg-L gene or the region -364 to -265 of the chg-H gene had the ability to promote the E2-dependent liver-specific GFP expression of its downstream gene. Further analyses suggested that an estrogen response element (ERE) at -309, an ERE half-site at -330 and a binding site for C/EBP at -363 of the chg-L gene played important roles in its downstream chg-L gene expression. In addition, this transgenic medaka may be useful as one of the test animals for detecting environmental estrogenic steroids.

Cross-fertilization and structural comparison of egg extracellular matrix glycoproteins from Xenopus laevis and Xenopus tropicalis

While the anuran amphibian Xenopus laevis is a widely used vertebrate model system, it is not optimal for genetic manipulations due to its tetraploid genome and long generation time. A current alternative amphibian model system, Xenopus tropicalis, has the advantages of a diploid genome and a much shorter generation time. We undertook a comparative investigation of X. tropicalis egg extracellular matrix glycoproteins in relation to those already characterized in X. laevis. Fertilization methods and isolation of egg extracellular molecules were directly transferable from X. laevis to X. tropicalis. Cross-fertilizations were successful in both directions, indicating similar molecules involved in sperm-egg interactions. Egg envelopes analyzed by SDS-PAGE were found to have almost identical gel patterns, whereas jelly component profiles were similar only for the larger macromolecules (>90 kDa). The cDNA sequences for egg envelope glycoproteins ZPA, ZPB, ZPC, ZPD and ZPAX, and also egg cortical granule lectin involved in the block to polyspermy, were cloned for X. tropicalis and showed a consistent approximately 85% amino acid identity to the X. laevis sequences. Thus, homologous egg extracellular matrix molecules perform the same functions, and the molecular and cellular mechanisms of fertilization in these two species are probably equivalent.

The Xenopus laevis cortical granule lectin: cDNA cloning, developmental expression, and identification of the eglectin family of lectins

A Xenopus laevis egg cortical granule, calcium-dependent, galactosyl-specific lectin participates in forming the fertilization layer of the egg envelope and functions in establishing a block to polyspermy. We report the cDNA cloning of the lectin, expression of the cortical granule lectin gene during oogenesis and early development, and identification of a new family of lectins. The translated cDNA for the cortical granule lectin had a signal peptide, a structural sequence of 298 amino acids, a molecular weight of 32.7 K, contained consensus sequence sites for N-glycosylation and a fibrinogen domain. The lectin cDNA was expressed during early stages of oogenesis. Lectin glycoprotein levels were constant during development with 2/3 of the lectin associated with the extracellular perivitelline space and the egg/embryo fertilization envelope. Lectin mRNA levels were from 100- to 1000-fold greater in ovary than in other adult tissues. The lectin had no sequence homology to the previously identified lectin families. The lectin had 41-88% amino acid identity with nine translated cDNA sequences from an ascidian, lamprey, frog, mouse, and human. Based on the conserved carbohydrate binding and structural properties of these glycoproteins, we propose a new family of lectins, the eglectin family.

Vitelline envelope gps 63 and 75 specifically bind sperm in ?in vitro? assays indiscoglossus pictus

In Xenopus, conflicting data related to sperm-vitelline envelope (VE) binding suggest that further experiments should be performed to study the role of VE glycoproteins in sperm binding. In this article, we studied the VE of Discoglossus pictus, where gp63, the product of the Dp ZP2 gene, has high molecular identity to Xenopus gp69/64 and to mouse ZP2 and only A23187-treated sperm bind to VE. Sperm bind to VE all over the egg, yet a sperm tuft was found only in the animal half of the egg, where the dimple, the site of fertilization, is located and an intense immunostain was detected in VE by an antiserum directed against gp69/64. The same antiserum inhibited sperm binding to VE. Sperm binding to beads coated with gp63, gp40, or gp75 was in the range of 62-70% for gp63-beads, 67-75% for 75 beads, and about 20% for BSA beads and gp40-coated beads. Soluble purified gp63 and gp75 competitively inhibited binding of sperm to gp63-coated beads. Similarly, the same glycoproteins inhibited sperm binding to gp75-coated beads. SDS-polyacrylamide gels (PAGE) of FE and comparison of VE and FE peptide maps showed that gp63 undergoes a minor shift to about 62 kDa in FE. In sperm binding assays with beads coated with FEs gp62, there was no binding. Following fertilization, in the region of the dimple, an F-layer is formed as well as an alteration of the VE structure. Lectin blots of the FE showed that the FE and in particular gp62 acquires a stronger affinity to Maackia amurensis agglutinin (MAA) with respect to VEs gp63. These results indicate that gps 63 and 75 are the sperm binding glycoproteins of D. pictus VE, where major post-fertilization changes occur as in other anuran species.

Characterization of the acrosome reaction-inducing substance in Xenopus (ARISX) secreted from the oviductal pars recta onto the vitelline envelope

We previously demonstrated that Xenopus sperm undergo an acrosome reaction on the vitelline envelope (VE) in response to the materials secreted from the oviductal pars recta [Dev. Biol. 243 (2002), 55]. A monoclonal antibody against the acrosome reaction-inducing substance in Xenopus (ARISX) was obtained by immunizing mice with pars recta extract (PRE). The acrosome reaction by PRE or on the VE was effectively inhibited by the intact anti-ARISX antibody as well as its Fab fragment, indicating that the antibody recognizes the epitopes localized on the acrosome reaction-inducing substance. On Western blots, the anti-ARISX antibody recognized a molecule with an apparent molecular mass of 300 kDa in PRE and in the VE, but this molecule was not detected in the coelomic envelope. The amount of ARISX in PRE was increased by the treatment of females with pregnant mare serum gonadotropin. Periodate oxidation of PRE completely abolished the acrosome reaction-inducing activity, indicating the involvement of the carbohydrate moieties of ARISX in inducing the acrosome reaction. On immunofluorescence observation, ARISX was localized in the epithelial cells in the posterior region of the pars recta and on the VE as fibrous structures.

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.

Evolution and nomenclature of the zona pellucida gene family

Three subfamilies of genes are acknowledged within the zona pellucida (ZP) gene family. At present, these subfamilies each have two names that are used interchangeably: ZPA or ZP2, ZPB or ZP1, and ZPC or ZP3. The ZPA genes encode the longest protein sequences and the ZPC genes the shortest. Recently, several sequences, which have no clear relationship to the three subfamilies, have been identified. These sequences include two paralogous ZP genes from Xenopus laevis and a single gene from the fish Oryzias latipes. We have conducted extensive phylogenetic analyses of the known ZP genes. As well as establishing the evolutionary relationships among these genes, the analyses make it clear that the dual nomenclature system is no longer feasible, because major paralogous groups are present in the ZPB (ZP1) family of genes of amniotes. We propose a unified system of nomenclature for the ZP gene family that removes the existing ambiguities.

The urodele egg-coat as the apparatus adapted for the internal fertilization

Fertilization is a significant event for reproducing offspring. It is achieved under a species-specific environment, which influences the conditions to assure the successful fertilization in some cases. Several studies about the basic mechanism of fertilization suggest that the fertilization mechanism is modified among species to be suited for the fertilization environment. In amphibians, many anurans undergo external fertilization while most urodeles do internal fertilization. An amphibian egg is surrounded by egg-coats, which are composed of vitelline envelope and layered egg-jelly. They are significant as fields for the sperm-egg interaction at fertilization. The fertilization processes that take place in the egg-coats are supposed to be easily influenced by the fertilization environment, because they, especially egg-jelly, are exposed to the surroundings at fertilization. In the present article, we describe the fertilization system equipped in newt egg-coats. Newt sperm are stored in spermatheca that exists in cloaca of a female and directly inseminated on the surface of egg-jelly. Sperm motility and acrosome reaction are induced in the outermost portion of the egg-jelly. Motion of the moving sperm becomes vigorous in the egg-jelly and sperm are guided to vitelline envelope by the aid of egg-jelly structure. Most of the sperm passing through the egg-jelly, as the result, has been induced acrosome reaction and those sperm can bind to the vitelline envelope to contribute to the successful fertilization. This fertilization system has a distinct feature from the known system in species undergoing external fertilization. The feature of the system in the newt egg-jelly is discussed with the view to achieving the successful fertilization in the internal environment.

Identification and characterization of a unique Xenopus laevis egg envelope component, ZPD

We report the identification of a previously undetected Xenopus laevis egg envelope component discovered through cloning experiments. A cDNA sequence was found that represented a mature protein of 32 kDa. Peptide antibodies were generated to probe for the protein in egg envelope samples and reactivity was found to a glycoprotein of approximately 80 kDa. When deglycosylated egg envelope samples were probed, a 32 kDa protein was labeled, confirming the size of the translated cDNA sequence. A BLAST analysis showed that it is most closely related (34% amino acid identity) to the ZP domains of mammalian tectorin, uromodulin and ZPA. From a dendrogram of known egg envelope glycoproteins, the new glycoprotein was shown to be unique among egg envelope components and was designated ZPD. A similar glycoprotein was identified by immunocrossreactivity in Xenopus tropicalis and Xenopus borealis egg envelopes.

Oviductin, the oviductal protease that mediates gamete interaction by affecting the vitelline coat in Bufo japonicus: its molecular cloning and analyses of expression and posttranslational activation

Previous studies indicated that the acquisition of egg fertilizability during transit through the pars recta portion of the oviduct in Bufo japonicus is accompanied by hydrolytic conversion of the vitelline coat 40- to 52-kDa components to 39-kDa components induced by a 66-kDa serine protease, "oviductin." In this study, we cloned a 3028-bp cDNA that contained an open reading frame encoding 974 amino acids with a calculated molecular mass of 107.6 kDa, including two protease domains and three repeats of CUB domains. Sequence analysis indicated that the catalytically active 66-kDa protein comprised an N-terminally located oviductin protease and two CUB domains. The oviductin gene was transcribed as a part of 6-kb mRNA that was expressed specifically in the cells lining the bottom of epithelial folds in the oviductal pars recta, and this expression was highly accelerated when the pars recta fragments were cultured in the presence of hCG. Western blot analyses using antibodies against a protease domain revealed that the catalytically inactive 102-kDa proteins in the pars recta granules yield 66-kDa catalytically active and 82- and 59-kDa inactive molecules. We propose that the oviductin translated as 107.6-kDa precursors are processed both N- and C-terminally to give rise to a 66-kDa active form comprising a serine protease and two CUB domains.

A hatching enzyme substrate in the Xenopus laevis egg envelope is a high molecular weight ZPA homolog

The Xenopus laevis egg envelope is composed of six or more glycoproteins, three of which have been cloned and identified as the mammalian homologs ZPA (ZP2), ZPB (ZP1) and ZPC (ZP3). The remaining glycoproteins are a triplet of high molecular weight components that are selectively hydrolyzed by the hatching enzyme. We have isolated one of these proteins and cloned its cDNA. The mRNA for the protein was found to be expressed only in early stage oocytes, as are other envelope components. From the deduced amino acid sequence, it was indicated to be a secreted glycoprotein with a characteristic ZP domain in the C-terminal half of the molecule. The N-terminal half was unrelated to any known glycoprotein. Comparative sequence analysis of the ZP domain indicated that it was derived from an ancestor of ZPA and ZPB, with the greatest identity to ZPA. This envelope component has been designated ZPAX.

Primary structure and developmental expression of Dp ZP2, a vitelline envelope glycoprotein homolog of mouse ZP2, in Discoglossus pictus, one of the oldest living Anuran species

A glycoprotein of the Xenopus vitelline envelope, gp 69/64, which mediates sperm binding, is closely related to the components of ZPA family, such as the mouse zona pellucida ZP2. To test the generality of these findings, we studied Discoglossus pictus, a species evolutionary distant from Xenopus and identified as a protein of 63 kDa in the vitelline envelope. Preliminary studies suggest that this protein may bind sperm at fertilization. We found that the 63-kDa protein is glycosylated and contains both N- and O-linked chains. We have cloned the cDNA encoding the Discoglossus protein of 63 kDa (Dp ZP2) by screening a Discoglossus cDNA library using Xenopus gp 69/64 cDNA as a probe. Analysis of the deduced sequence of Discoglossus protein revealed 48% identity with Xenopus gp 69/64 and 37-40% identity with mouse ZP2. The sequence conservation included a ZP domain, a potential furin cleavage site and a putative transmembrane domain. The N-terminus region of Dp ZP2 was 40% identical to the corresponding region of Xenopus gp 69/64 which has been shown to be essential for sperm binding to the VE. Although, as of yet, there is no evidence for sperm binding at the Dp ZP2 N-terminus, it is interesting that in this region three potential O-glycosylation sites are conserved in both species, in contrast to N-glycosylation sites. It was found that the Dp ZP2 mRNA is expressed in stage 1 oocytes and in the follicle cells surrounding the oocyte. Similarly, in Xenopus oocytes, the gp 69/64m RNA, was found in the oocytes, as well as in the somatic cells. Mol. Reprod. Dev. 59:133-143, 2001.

Glycobiology of sperm-egg interactions in deuterostomes

The process of fertilization begins when sperm contact the outermost egg investment and ends with fusion of the two haploid pronuclei in the egg cytoplasm. Many steps in fertilization involve carbohydrate-based molecular recognition between sperm and egg. Although there is conservation of gamete recognition molecules within vertebrates, their homologues have not yet been discovered in echinoderms and ascidians (the invertebrate deuterostomes). In echinoderms, long sulfated polysaccharides act as ligands for sperm receptors. Ascidians employ egg coat glycosides that are recognized by sperm surface glycosidases. Vertebrate egg coats contain zona pellucida (ZP) family glycoproteins, whose carbohydrates bind to sperm receptors. Several candidate sperm receptors for vertebrate ZP proteins have been identified and are discussed here. This brief review focuses on new information concerning fertilization in deuterostomes (the phylogenetic group including echinoderms, ascidians, and vertebrates) and highlights protein-carbohydrate interactions involved in this process.

Morphological and biochemical changes of isolated chicken egg-envelope during sperm penetration: degradation of the 97-kilodalton glycoprotein is involved in sperm-driven hole formation on the egg-envelope

The chicken egg-envelope is made of two major glycoprotein components, which are designated as gp97 and gp42 (after their molecular masses). To elucidate how these two components are involved in macromolecular organization of the chicken egg-envelope, the isolated egg-envelope was characterized by immunochemical and biochemical methods. The gp97 was suggested to be a homologue of mouse ZPB based on the similarities of N-terminal and internal sequences. Immunoblotting using anti-gp97 monoclonal antibodies and two-dimensional gel electrophoresis with or without mercaptoethanol treatment revealed that gp97 formed a homodimer through disulfide bonds, whereas gp42 did not. Under indirect immunofluorescence microscopy, the anti-gp97 antibody visualized indistinct, small spots on the egg-envelope, whereas the anti-gp42 antibody showed a meshwork of blurry, fibrous structures. The hole formation on the egg-envelope by in vitro sperm penetration was completely inhibited by two anti-gp97 monoclonal antibodies. Interestingly, the anti-gp97 monoclonal antibodies blocked the proteolysis not only of gp97 but also of gp42 during incubation of the egg-envelope with either sperm or the crude chicken acrosin. Taken together, these results indicate that gp97 may play pivotal roles not only in constitution of the macromolecular organization of the egg-envelope but also in triggering hydrolysis of the egg-envelope during sperm penetration.

Following passage through the oviduct, the coelomic envelope of Discoglossus pictus (amphibia) acquires fertilizability upon reorganization, conversion of gp 42 to gp 40, extensive glycosylation, and formation of a specific layer

This paper describes the morphological and biochemical changes in Discoglossus pictus coelomic oocyte envelope (CE) following passage through the oviduct. As in other anurans, in this species, the transformation of the envelope into vitelline envelope (VE) leads to the acquisition of fertilizability and involves the cleavage of a glycoprotein. In addition, several features, typical of Discoglossus pictus, were observed. A new layer, VE-D, forms underneath the VE region facing the site of sperm entrance, the dimple. In the VE, arrowhead-like bundles of fibrils are perpendicularly oriented toward the dimple. Ultrastructural observations and staining with UEA-I suggested that VE-D might have a role in supporting sperm penetration into the dimple by orienting VE bundles and exposing sugar residues such as fucose. In 'in vitro' tests, VE binding of sperm occurs only if sperm are exposed to A23187, in agreement with previous data (Campanella et al., 1997: Mol Reprod Dev 47:323-333). Sperm binding occurs all over the VE. Accordingly, extracts of the VE covering the animal or the vegetal hemisphere have the same affinity to lectins (DBA, DSA, GNA, MAA, SBA, SNA, UEA-I, WGA). The CE contains six main glycoproteins. Peptide mapping indicated that during CE transformation into VE, gp 42 shifts to an apparent M(r) of 40 and gp 61 is converted to an apparent M(r) of 63 kDa. Lectin blot analyses showed extensive changes in cross-reactivity of most glycoproteins during the CE-->VE transition. The fact that DBA and UEA-I stain gp 63 rather than gp 61 and that this change is related only to gp 63, suggested that O-glycosylation and terminal fucose might be acquired by gp 63 in preparation of fertilization. Gp 63 has recently been cloned (Vaccaro et al., submitted) and shown to exhibit high homology to Xenopus gp 69/64, a VE sperm ligand (Tian et al., 1997a: J. Cell Biol. 136: 1099-1108; Tian et al., 1997b: Dev Biol 187:143-153), and to ZP2 of mammals.

Differential screening and characterization analysis of the egg envelope glycoprotein ZP3 cDNAs between gynogenetic and gonochoristic crucian carp

Gynogenetic silver crucian carp, Carassius auratus gibelio, is an intriguing model system. In the present work, a systemic study has been initiated by introducing suppression subtractive hybridization technique into this model system to identify the differentially expressed genes in oocytes between gynogenetic silver crucian carp and its closely related gonochoristic color crucian carp. Five differential cDNA fragments were identified from the preliminary screening, and two of them are ZP3 homologues. Moreover, the full length ZP3 cDNAs were cloned from their oocyte cDNA libraries. The length of ZP3 cDNAs were 1378 bp for gyno-carp and 1367 bp for gono-carp, and they can be translated into proteins with 435 amino acids. Obvious differences are not only in the composition of amino acids, but also in the number of potential O-linked oligosaccharide sites. In addition, gyno-carp ZP3 amino acid sequence has an unexpected higher identity value with common carp (83.5%) than that with the closely related gono-carp (74.7%). The unique homology may be originated from the ancient hybridization. Northern blot analysis confirmed that expression of the ZP3 gene occurred exclusively in the oocytes. Because O-linked oligosaccharides on ZP3 have been demonstrated to play very important roles in fertilization, it is suggested that the extra O-linked glycosylation sites may be related to the unique sperm-egg recognition mechanism in gynogenesis.

Positive Darwinian selection drives the evolution of several female reproductive proteins in mammals

Rapid evolution driven by positive Darwinian selection is a recurrent theme in male reproductive protein evolution. In contrast, positive selection has never been demonstrated for female reproductive proteins. Here, we perform phylogeny-based tests on three female mammalian fertilization proteins and demonstrate positive selection promoting their divergence. Two of these female fertilization proteins, the zona pellucida glycoproteins ZP2 and ZP3, are part of the mammalian egg coat. Several sites identified in ZP3 as likely to be under positive selection are located in a region previously demonstrated to be involved in species-specific sperm-egg interaction, suggesting the selective pressure is related to male-female interaction. The results provide long-sought evidence for two evolutionary hypotheses: sperm competition and sexual conflict.

Getting sperm and egg together: things conserved and things diverged

Sperm-egg interactions occur at multiple levels on the egg surface, first with the egg's extracellular matrix and then with the egg's plasma membrane. The BioPore minisymposium on "The Egg Surface" at the 1999 annual meeting of the Society for the Study of Reproduction highlighted a series of events underlying successful interactions of the sperm with the egg: 1) composition, synthesis, and assembly of the mouse egg's extracellular matrix, the zona pellucida, during oogenesis; 2) oocyte maturation and development of the sperm-binding domain of mouse eggs; and 3) characterization of functional domains in different sperm ligands (fertilin-alpha and fertilin-beta in the mouse and lysin in the abalone) that recognize cognate binding sites on the egg surface. Data that were presented are reviewed here and discussed with respect to conserved and divergent features of gamete functions.

Egg envelope glycoprotein gp37 as a Xenopus homolog of mammalian ZP1, based on cDNA cloning

The egg envelope is a kind of extracellular matrix, which surrounds growing oocytes, ovulated eggs and early embryos. Among the glycoprotein components of the Xenopus laevis egg envelope, gp43/gp41 and gp69/64 have already been shown to be frog homologs of the mammalian zona pellucida components ZP3 and ZP2, respectively. To determine the structure of another major component of egg envelope, gp37, the peptides isolated from the lysyl endopeptidase digests of gp37 were sequenced for amino acids to design degenerate primers for polymerase chain reaction. By reverse transcription-polymerase chain reaction with a poly(A)+ RNA from the ovary of a postovulated female Xenopus, a specifically amplified band was obtained and sequenced. The upstream and downstream sequences of the sequenced region were completed by 5'- and 3'-rapid amplification of cDNA ends, respectively. The gp37 cDNA comprises 1674 bp and contains one open reading frame encoding a polypeptide with 543 amino acids. The predicted amino acid sequence of the gp37 cDNA has a close similarity to that of mammalian ZP1. Northern blot and in situ hybridization studies indicated that the transcript (1.8 kb) is exclusively expressed in the oocytes, particularly in the previtellogenic young oocytes, just like the expression pattern of gp43 mRNA, suggesting a coordinate transcription of the gp43 and gp37 genes in Xenopus.

Common egg envelope antigens are limited to the animal class

The antigens of the egg envelope (zona pellucida) in mammals are of special interest because of their possible involvement in immunoinfertility and as candidate targets for immunocontraception. Conserved zona epitopes from divergent species may present a suitable source and an animal model for investigation of the above factors. We compared egg envelope antigens from 6 species of vertebrates belonging to 3 different classes in order to demonstrate the existence of shared antigens. Egg envelopes from the trout, carp, turtle, hen, duck and quail were isolated and heat-solubilized. They were tested with rabbit polyclonal antisera against carp, trout and duck egg envelopes by the enzyme-linked immunosorbent assay. The results showed significant cross-reactions among egg envelopes of fish and birds. The examined solubilized preparations did not show cross-reactivity with egg envelopes from any other class, suggesting that divergent species did not share common egg envelope antigens, and that their use may not be appropriate in the investigation of immunoinfertility and immunocontraception in humans.

Independent and hetero-oligomeric-dependent sperm binding to egg envelope glycoprotein ZPC in Xenopus laevis

Vitelline envelopes are composed of glycoproteins that participate in sperm-egg interactions during the initial stages of fertilization. In Xenopus laevis, the vitelline envelope is composed of at least 4 glycoproteins (ZPA, ZPB, ZPC, and ZPX). A sperm binding assay involving the covalent coupling of envelope glycoproteins to silanized glass slides was developed. In our assay, sperm bound to the egg envelopes derived from oviposited eggs but not activated eggs. The majority of the egg envelope ligand activity for sperm binding was derived from the complex N-linked oligosaccharides of ZPC. This sperm binding involved N-acetylglucosamine and fucose residues, as binding was abolished after treatment with cortical granule beta-N-acetylglucosaminidase and commercial beta-N-acetylglucosaminidases and was reduced by 44% after treatment with alpha-fucosidase. Although both the envelope glycoproteins ZPA and ZPC possessed independent ligand activity, ZPC was the major ligand for sperm binding (75%). Mixing of isolated ZPA, ZPB, and ZPC in a ratio of 1:4:4 (equal to that in the egg envelope) resulted in sperm binding that was greater than that of the sum of the separate components. The egg glycoproteins acted in synergy to increase sperm binding. Thus, ZPC possessed both independent and hetero-oligomeric-dependent ligand activities for sperm binding.

Systematic identification of genes expressed during early oogenesis in medaka

Subtractive hybridization was used to identify differences in gene expression between medaka (Oryzias latipes) males and females during sex differentiation. Fifty female-specific cDNA fragments were cloned. They can be classified into three groups by virtue of whether their earliest expression is at 1, 5, or 30 days after hatching. All 15 near full-length cDNAs belonging to the first two groups were cloned. Many of these female-specific genes are coordinately expressed in oocytes at the earliest stages of oogenesis. Some of the genes that were identified by their sequences include egg envelope proteins, oocyte-specific RNA binding proteins, and a transcription factor containing a basic helix-loop-helix motif.

Fertilization in animals

Fertilization in mammalian and nonmammalian organisms has many features in common. These features include a final maturation phase for sperm and eggs, species-specific binding of sperm to eggs, penetration by sperm of one or more extracellular coats surrounding eggs, fusion of sperm and eggs, and activation of eggs. Implicit in this are a variety of basic molecular events, including receptor-ligand interactions, signalling cascades, specific proteolysis, and nuclear transformations. Here, several of these events are addressed for fertilization in animals as diverse as starfish and mice.

Molecular basis for oviductin-mediated processing from gp43 to gp41, the predominant glycoproteins of Xenopus egg envelopes

Acquisition of fertilizability in Xenopus coelomic eggs is correlated with the conversion from coelomic to vitelline envelope during passage of the eggs through the pars recta portion of oviduct. The conversion includes processing of a major envelope constituent gp43 of coelomic envelopes to gp41 of vitelline envelopes by a trypsin-type protease, oviductin, which is secreted from the pars recta. Our recent sequencing analyses [Kubo et al., (1997): Dev Growth Diff 39:405-411] strongly suggested that the N-terminal portion of gp41 is exposed as a result of oviductin digestion. In this study, a monoclonal antibody specific to the predicted N-terminus of gp41 was raised by immunizing mice with a synthetic N-terminal hexapeptide (QLPVSP) coupled to keyhole limpet hemocyanin. The antibody specifically reacted to gp41, but not to gp43, indicating that Gln62 is exposed as the N-terminal amino acid of gp41 by oviductin-mediated cleavage of gp43 at Arg61 in GSR61. The C-terminal sequencing of gp43 and gp41 indicated that Arg373 in GSR373 as the C-terminus of gp41 is generated by cleavage of three amino acid (WNQ) residues from the C-terminus of gp43. The resulting polypeptide moiety of gp41 has a molecular mass of 33900 Da with 312 amino acid residues. We propose that oviductin possessing the substrate specificity of GSR simultaneously digests gp43 at Arg residues in GSR61 and GSR373 to generate the N- and C-terminus of gp41, respectively.

Analyses of oviductal pars recta-induced fertilizability of coelomic eggs in Xenopus laevis

The acquisition of fertilizability in coelomic eggs of Xenopus laevis has been shown to be correlated with the physical, biochemical, and ultrastructural alterations of the egg envelope [coelomic envelope (CE)] induced during the passage of eggs through the pars recta portion of the oviduct. However, no direct evidence that the pars recta renders eggs fertilizable has yet been presented. In this study, we show that coelomic eggs are highly fertilizable when they are incubated with continuous shaking for 4 h at 15 degrees C in pars recta extract (PRE) derived from females prestimulated by pregnant mare serum gonadotropin. The PRE from pituitary-stimulated Bufo japonicus was as potent as homologous PRE in rendering Xenopus eggs fertilizable. Incubation of coelomic eggs in PRE for 30 min induced a dramatic increase in the rates of sperm binding to the envelope to a level equivalent to that exhibited by the envelope from uterine eggs (VEs). The CE-to-VE ultrastructural conversion and a 43k-to-41k hydrolysis of the envelope glycoprotein component started 5 min after, and were completed by 15 min after, the start of incubation in PRE and were accompanied by an exposure of a new N-terminal sequence typical to gp41. Thus, the biochemical and ultrastructural conversions and the sperm-binding activity of the envelope induced by PREs, although being prerequisite, were not sufficient to render coelomic eggs fully accessible to fertilizing sperm.

Oviductin, the Xenopus laevis oviductal protease that processes egg envelope glycoprotein gp43, increases sperm binding to envelopes, and is translated as part of an unusual mosaic protein composed of two protease and several CUB domains

The glycoprotein envelope surrounding the Xenopus laevis egg is converted from an unfertilizable to a fertilizable form during transit through the pars recta portion of the oviduct. Envelope conversion involves the pars recta protease oviductin, which selectively hydrolyzes envelope glycoprotein gp43 to gp41. Oviductin cDNA was cloned, and sequence analysis revealed that the protease is translated as the N terminus of an unusual mosaic protein. In addition to the oviductin protease domain, a protease domain with low identity to oviductin was present, possessing an apparent nonfunctional catalytic site. Three CUB domains were also present, which are related to the mammalian spermadhesin molecules implicated in mediating sperm-envelope interactions. We propose that during post-translational proteolytic processing of the mosaic oviductin glycoprotein, the processed N-terminal protease domain is released coupled to two C-terminal CUB domains and constitutes the enzymatically active protease molecule. In functional studies, isolated coelomic egg envelopes treated with oviductin purified from the oviduct showed a dramatic increase in sperm binding. This observation established that oviductin alone was the oviductal factor responsible for converting the egg envelope to a sperm-penetrable form, via an increase in sperm binding. Trypsin mimicked oviductin's effect on envelope hydrolysis and sperm binding, demonstrating that gp43 processing is the only requirement for envelope conversion.

A 42-kDa glycoprotein from chicken egg-envelope, an avian homolog of the ZPC family glycoproteins in mammalian Zona pellucida. Its first identification, cDNA cloning and granulosa cell-specific expression

A glycoprotein with molecular mass of 42 kDa was identified as the major component of the chicken egg-envelope, the filamentous, extracellular matrix known as the perivitelline layer. By using a DNA probe amplified with degenerative primers derived from the protein's partial amino acid sequences, a cDNA clone encoding the egg-envelope 42-kDa glycoprotein (gp42) was isolated from a hen's ovary cDNA library. The gp42 open reading frame encoded 435 amino acid residues, including a putative signal peptide of 20 amino acids. The deduced amino acid sequence of gp42 showed significant similarity to egg-envelope glycoproteins of the ZPC family of several other vertebrate species, including human ZP3, mouse ZP3, Xenopus laevis gp43 and medaka (Oryzias latipes) ZI3 (LS-F), which play important roles for sperm-egg interaction. A single N-glycosylation site present in chicken gp42 is conserved among all five of these proteins: carbohydrate analysis of gp42 revealed the presence of a complex type glycan chain at this site. N-terminal sequence analysis of the mature polypeptide suggests that C-terminal processing of the pro-protein occurs during synthesis and secretion. The 1.4-kb gp42 transcript was detected only in follicles, and was found to be accumulated in granulosa cells in a manner dependent on ovarian follicular development. Furthermore, a metabolically radio-labeled gp42 was immunopreciptated from both cell lysate and culture supernatant of the granulosa cells with specific anti-gp42 antibody, suggesting granulosa cell-specific synthesis and secretion of the glycoprotein.

Xenopus laevis sperm receptor gp69/64 glycoprotein is a homolog of the mammalian sperm receptor ZP2

Little is known about sperm-binding proteins in the egg envelope of nonmammalian vertebrate species. We report here the molecular cloning and characterization of a recently identified sperm receptor (gp69/64) in the Xenopus laevis egg vitelline envelope. Our data indicate that the gp69 and gp64 glycoproteins are two glycoforms of the receptor and have the same number of N-linked oligosaccharide chains but differ in the extent of O-glycosylation. The amino acid sequence of the receptor is closely related to that of the mouse zona pellucida protein ZP2. Most of the sequence conservation, including a ZP domain, a potential furin cleavage site, and a putative transmembrane domain are located in the C-terminal half of the receptor. Proteolytic cleavage of the gp69/64 protein by a cortical granule protease during fertilization removes 27 amino acid residues from the N terminus of gp69/64 and results in loss of sperm binding to the activated eggs. Similarly, we find that treatment of eggs with type I collagenase removes 31 residues from the N terminus of gp69/64 and has the same effect on sperm binding. The isolated and purified N terminus-truncated receptor protein is inactive as an inhibitor of sperm-egg binding. Earlier studies on the effect of Pronase digestion on receptor activity suggest that this N-terminal peptide may contain an O-linked glycan that is involved in the binding process. Based on these results and the findings on the primary structure of the receptor, a pathway for the maturation and secretion of gp69/64, as well as its inactivation following fertilization, is proposed.

The chicken homologue of zona pellucida protein-3 is synthesized by granulosa cells

Oocyte development within avian ovarian follicles is an intricate process involving yolk deposition and the formation of extraoocytic matrices. Of these, the perivitelline membrane (pvm) not only plays a role in sperm binding but also provides mechanical support for the large oocyte's journey through the oviduct after ovulation. To date we have focused on the mechanisms for uptake of yolk precursors into oocytes of the chicken; now we extend our studies to a detailed analysis of the pvm. In the course of characterization of its major components, we obtained partial protein sequences; comparison with the GenBank database revealed that one of the pvm proteins is the homologue of mammalian zona pellucida glycoprotein 3 (ZP3), a key component in sperm binding. Following a nomenclature based on gene structure, the protein is referred to as chicken ZPC (chZPC). The chicken protein (444 residues) and murine ZP3 (424 residues) are highly conserved, with 41% of the amino acids identical. As shown by Northern blot analysis, the avian ZPC gene is expressed exclusively in the granulosa cells surrounding the oocyte, in contrast to murine ZP3, which is synthesized by the oocyte. Upon reaching a size larger than 1.5 mm in diameter, follicles accumulate chZPC in highly polarized fashion, i.e., in the space intercalated between the oocyte and the granulosa cells, as revealed by immunohistochemistry of follicle sections. ChZPC synthesis and secretion by granulosa cells was demonstrated directly by metabolic labeling and immunoprecipitation from the culture medium of granulosa cell sheets isolated ex vivo from follicles. Immunoblot analysis and glycosidase treatment of chZPC from preovulatory and freshly ovulated oocytes, as well as laid eggs, revealed that the primary product undergoes a two-step decrease in size from follicle to laid egg that is unlikely to be due to modification of the carbohydrate moiety.

Sequence and analysis of zona pellucida 2 cDNA (ZP2) from a marsupial, the brushtail possum, Trichosurus vulpecula

All mammalian eggs are surrounded by the zona pellucida, an extracellular coat involved in vital functions during fertilization and early development. The zona pellucida glycoproteins are promising antigenic targets for development of contraceptive vaccines to control pest populations of marsupials in Australia and New Zealand. Our current understanding of the function of the zona pellucida glycoproteins is based almost entirely on the mouse and may not be representative of gamete interactions in all eutherian or marsupial mammals. This study reports the isolation and characterization of the ZP2 gene from the brushtail possum (Trichosurus vulpecula). The brushtail possum ZP2 mRNA is 2,182 nucleotides long with an open reading frame coding for a polypeptide chain of 712 amino acids with a molecular mass of 79,542 d. The deduced amino acid sequence of possum ZP2 is 48 to 55% identical to that of eutherian mammals. It shares several structural characteristics including N-linked glycosylation sites, location and number of cysteine residues, and hydropathy profile. The brushtail possum ZP2 gene is expressed exclusively in the ovary. Further studies are planned to elucidate the specific site of ZP2 expression within the ovary and its function during fertilization in marsupials.

Human ZP3 restores fertility in Zp3 null mice without affecting order-specific sperm binding

The mammalian zona pellucida surrounding ovulated eggs mediates sperm binding at fertilization, provides a postfertilization block to polyspermy, and facilitates passage of pre-implantation embryos down the oviduct. Although the three zona proteins (ZP1, ZP2, ZP3) are well conserved, mammalian fertilization is relatively specific and human sperm do not bind to the mouse zona pellucida. There are considerable in vitro data that ZP3 acts as a primary sperm adhesion molecule in mice and, by analogy, a similar role has been postulated for human ZP3. Genetically altered mice lacking ZP3 (Zp3(tm/tm)) do not form a zona pellucida and are infertile. To rescue this phenotype, transgenic mice expressing human ZP3 (67% identical to mouse ZP3) were produced and bred with Zp3(tm/tm) null mice. The resultant human ZP3 rescue females had chimeric zonae pellucidae composed of mouse ZP1, mouse ZP2 and human ZP3. Human ZP3 expressed in mouse oocytes had an apparent mass (64 kDa) indistinguishable from native human ZP3 and distinct from mouse ZP3 (83 kDa). Despite the presence of human ZP3, human sperm did not bind to the chimeric zona pellucida, and notwithstanding the absence of mouse ZP3, mouse sperm bound to ovulated eggs in vitro and fertility was restored in vivo. These data have implications regarding the molecular basis of mouse and human sperm binding to their respective zonae pellucidae.