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

Zona pellucida family genes in Chinese pond turtle: identification, expression profiles, and role in the spermatozoa acrosome reaction†

The zona pellucida (ZP) is an extracellular matrix that surrounds all vertebrate eggs, and it is involved in fertilization and species-specific recognition. Numerous in-depth studies of the ZP proteins of mammals, birds, amphibians, and fishes have been conducted, but systematic investigation of the ZP family genes and their role during fertilization in reptiles has not been reported to date. In this study, we identified six turtle ZP (Tu-ZP) gene subfamilies (Tu-ZP1, Tu-ZP2, Tu-ZP3, Tu-ZP4, Tu-ZPD, and Tu-ZPAX) based on whole genome sequence data from Mauremys reevesii. We found that Tu-ZP4 had large segmental duplication and was distributed on three chromosomes, and we also detected gene duplication in the other Tu-ZP genes. To evaluate the role of Tu-ZP proteins in sperm-egg binding, we assessed the expression pattern of these Tu-ZP proteins and their ability to induce the spermatozoa acrosome reaction in M. reevesii. In conclusion, this is the first report of the existence of gene duplication of Tu-ZP genes and that Tu-ZP2, Tu-ZP3, and Tu-ZPD can induce acrosome exocytosis of spermatogenesis in the reptile.

Recurrent Co-Option and Recombination of Cytokine and Three Finger Proteins in Multiple Reproductive Tissues Throughout Salamander Evolution

Reproductive proteins evolve at unparalleled rates, resulting in tremendous diversity of both molecular composition and biochemical function between gametes of different taxonomic clades. To date, the proteomic composition of amphibian gametes is largely a molecular mystery, particularly for Urodeles (salamanders and newts) for which few genomic-scale resources exist. In this study, we provide the first detailed molecular characterization of gametes from two salamander species (Plethodon shermani and Desmognathus ocoee) that are models of reproductive behavior. Long-read PacBio transcriptome sequencing of testis and ovary of both species revealed sex-specific expression of many genes common to vertebrate gametes, including a similar expression profile to the egg coat genes of Xenopus oocytes. In contrast to broad conservation of oocyte genes, major testis transcripts included paralogs of salamander-specific courtship pheromones (PRF, PMF, and SPF) that were confirmed as major sperm proteins by mass spectrometry proteomics. Sperm-specific paralogs of PMF and SPF are likely the most abundant secreted proteins in P. shermani and D. ocoee, respectively. In contrast, sperm PRF lacks a signal peptide and may be expressed in cytoplasm. PRF pheromone genes evolved independently multiple times by repeated gene duplication of sperm PRF genes with signal peptides recovered through recombination with PMF genes. Phylogenetic analysis of courtship pheromones and their sperm paralogs support that each protein family evolved for these two reproductive contexts at distinct evolutionary time points between 17 and 360 million years ago. Our combined phylogenetic, transcriptomic and proteomic analyses of plethodontid reproductive tissues support that the recurrent co-option and recombination of TFPs and cytokine-like proteins have been a novel driving force throughout salamander evolution and reproduction.

Egg Coat Proteins Across Metazoan Evolution

All animal oocytes are surrounded by a glycoproteinaceous egg coat, a specialized extracellular matrix that serves both structural and species-specific roles during fertilization. Egg coat glycoproteins polymerize into the extracellular matrix of the egg coat using a conserved protein-protein interaction module-the zona pellucida (ZP) domain-common to both vertebrates and invertebrates, suggesting that the basic structural features of egg coats have been conserved across hundreds of millions of years of evolution. Egg coat proteins, as with other proteins involved in reproduction, are frequently found to be rapidly evolving. Given that gamete compatibility must be maintained for the fitness of sexually reproducing organisms, this finding is somewhat paradoxical and suggests a role for adaptive diversification in reproductive protein evolution. Here we review the structure and function of metazoan egg coat proteins, with an emphasis on the potential role their evolution has played in the creation and maintenance of species boundaries.

Avian Egg and Egg Coat

An ovulated egg of vertebrates is surrounded by unique extracellular matrix, the egg coat or zona pellucida, playing important roles in fertilization and early development. The vertebrate egg coat is composed of two to six zona pellucida (ZP) glycoproteins that are characterized by the evolutionarily conserved ZP-domain module and classified into six subfamilies based on phylogenetic analyses. Interestingly, investigations of biochemical and functional features of the ZP glycoproteins show that the roles of each ZP-glycoprotein family member in the egg-coat formation and the egg-sperm interactions seemingly vary across vertebrates. This might be one reason why comprehensive understandings of the molecular basis of either architecture or physiological functions of egg coat still remain elusive despite more than 3 decades of intensive investigations. In this chapter, an overview of avian egg focusing on the oogenesis are provided in the first section, and unique features of avian egg coat, i.e., perivitelline layer, including the morphology, biogenesis pathway, and physiological functions are discussed mainly on chicken and quail in terms of the characteristics of ZP glycoproteins in the following sections. In addition, these features of avian egg coat are compared to mammalian zona pellucida, from the viewpoint that the structural and functional varieties of ZP glycoproteins might be associated with the evolutionary adaptation to their reproductive strategies. By comparing the egg coat of birds and mammals whose reproductive strategies are largely different, new insights into the molecular mechanisms of vertebrate egg-sperm interactions might be provided.

Proteomics of phosphorylation and protein dynamics during fertilization and meiotic exit in the Xenopus egg

Fertilization releases the meiotic arrest and initiates the events that prepare the egg for the ensuing developmental program. Protein degradation and phosphorylation are known to regulate protein activity during this process. However, the full extent of protein loss and phosphoregulation is still unknown. We examined absolute protein and phosphosite dynamics of the fertilization response by mass spectrometry-based proteomics in electroactivated eggs. To do this, we developed an approach for calculating the stoichiometry of phosphosites from multiplexed proteomics that is compatible with dynamic, stable, and multisite phosphorylation. Overall, the data suggest that degradation is limited to a few low-abundance proteins. However, this degradation promotes extensive dephosphorylation that occurs over a wide range of abundances during meiotic exit. We also show that eggs release a large amount of protein into the medium just after fertilization, most likely related to the blocks to polyspermy. Concomitantly, there is a substantial increase in phosphorylation likely tied to calcium-activated kinases. We identify putative degradation targets and components of the slow block to polyspermy. The analytical approaches demonstrated here are broadly applicable to studies of dynamic biological systems.

A structured interdomain linker directs self-polymerization of human uromodulin

Uromodulin (UMOD)/Tamm-Horsfall protein, the most abundant human urinary protein, plays a key role in chronic kidney diseases and is a promising therapeutic target for hypertension. Via its bipartite zona pellucida module (ZP-N/ZP-C), UMOD forms extracellular filaments that regulate kidney electrolyte balance and innate immunity, as well as protect against renal stones. Moreover, salt-dependent aggregation of UMOD filaments in the urine generates a soluble molecular net that captures uropathogenic bacteria and facilitates their clearance. Despite the functional importance of its homopolymers, no structural information is available on UMOD and how it self-assembles into filaments. Here, we report the crystal structures of polymerization regions of human UMOD and mouse ZP2, an essential sperm receptor protein that is structurally related to UMOD but forms heteropolymers. The structure of UMOD reveals that an extensive hydrophobic interface mediates ZP-N domain homodimerization. This arrangement is required for filament formation and is directed by an ordered ZP-N/ZP-C linker that is not observed in ZP2 but is conserved in the sequence of deafness/Crohn's disease-associated homopolymeric glycoproteins α-tectorin (TECTA) and glycoprotein 2 (GP2). Our data provide an example of how interdomain linker plasticity can modulate the function of structurally similar multidomain proteins. Moreover, the architecture of UMOD rationalizes numerous pathogenic mutations in both UMOD and TECTA genes.

Identification of distinctive interdomain interactions among ZP-N, ZP-C and other domains of zona pellucida glycoproteins underlying association of chicken egg-coat matrix

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Chicken ZP1 and ZP3 assemble through strong interactions between their ZP-C domains.

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ZP-C domains of chicken ZP1 and ZP3 are deeply embedded in the egg-coat matrix.

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Chicken ZP1 forms a homocomplex through non-covalent interaction between repeat domains.

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Chicken ZPD is deposited on the interstices of ZP1–ZP3 matrix in the egg coat.

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We propose a model for the architecture of chicken egg-coat matrix from these results.

The vertebrate egg coat, including mammalian zona pellucida, is an oocyte-specific extracellular matrix comprising two to six zona pellucida (ZP) glycoproteins. The egg coat plays important roles in fertilization, especially in species-specific interactions with sperm to induce the sperm acrosome reaction and to form the block to polyspermy. It is suggested that the physiological functions of the egg coat are mediated and/or regulated coordinately by peptide and carbohydrate moieties of the ZP glycoproteins that are spatially arranged in the egg coat, whereas a comprehensive understanding of the architecture of vertebrate egg-coat matrix remains elusive. Here, we deduced the orientations and/or distributions of chicken ZP glycoproteins, ZP1, ZP3 and ZPD, in the egg-coat matrix by confocal immunofluorescent microscopy, and in the ZP1–ZP3 complexes generated in vitro by co-immunoprecipitation assays. We further confirmed interdomain interactions of the ZP glycoproteins by far-Western blot analyses of the egg-coat proteins and pull-down assays of ZP1 in the serum, using recombinant domains of ZP glycoproteins as probes. Our results suggest that the ZP1 and ZP3 bind through their ZP-C domains to form the ZP1–ZP3 complexes and fibrils, which are assembled into bundles through interactions between the repeat domains of ZP1 to form the ZP1–ZP3 matrix, and that the ZPD molecules self-associate and bind to the ZP1–ZP3 matrix through its ZP-N and ZP-C domains to form the egg-coat matrix. Based on these results, we propose a tentative model for the architecture of the chicken egg-coat matrix that might be applicable to other vertebrate ones.

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.

Expression of zona pellucida B proteins in juvenile rare minnow (Gobiocypris rarus) exposed to 17α-ethinylestradiol, 4-nonylphenol and bisphenol A

Zona pellucida (ZP) containing proteins are glycoproteins in teleost chorion and are encoded by several gene subfamilies, mainly including ZPA, ZPB, ZPC and ZPX genes. In teleost species, ZP genes are expressed either in liver under regulation of estrogen or in ovary. In the present study, five ZP gene isoforms were isolated and characterized in Gobiocypris rarus. The putative amino acid sequences of these ZP gene isoforms contain the typical trefoil motif and a ZP domain. These five G. rarus ZP gene isoforms were named as grZPB.1, grZPB.2, grZPB.3, grZPB.4 and grZPB.5. Real-time quantitative reverse transcription PCR (RT-qPCR) analysis indicated that all these ZP mRNA isoforms were exclusively expressed in ovary. G. rarus juveniles at the age of 21 days postfertilization were exposed to 17α-ethinylestradiol (EE2; 0.01, 0.1 and 1 nM), 4-nonylphenol (4-NP; 10, 100 and 1000 nM) or bisphenol A (BPA; 0.1, 1 and 10nM) for 3 days. mRNA expressions of ZPB isoforms following the exposure to xenoestrogen were detected by RT-qPCR. Data were analyzed by the 2(-△△Cq) method. The results indicate that induction by 0.1-1nM EE2 on mRNA expression of the grZPB isoforms is weaker than for vitellogenin. 4-NP exposures at three concentrations had differential effects on the grZPBs. BPA at three concentrations weakly induced mRNA expression of the grZPB isoforms.

Structure determination by MALDI-IRMPD mass spectrometry and exoglycosidase digestions of O-linked oligosaccharides from Xenopus borealis egg jelly

Differences in the fertilization behavior of Xenopus borealis from X. laevis and X. tropicalis suggest differences in the glycosylation of the egg jellies. To test this assumption, O-linked glycans were chemically released from the egg jelly coat glycoproteins of X. borealis. Over 50 major neutral glycans were observed, and no anionic glycans were detected from the released O-glycan pool. Preliminary structures of ∼30 neutral oligosaccharides were determined using matrix-assisted laser desorption/ionization (MALDI) infrared multiphoton dissociation tandem mass spectrometry (MS). The mass fingerprint of a group of peaks for the core-2 structure of O-glycans was conserved in the tandem mass spectra and was instrumental in rapid and efficient structure determination. Among the 29 O-glycans, 22 glycans contain the typical core-2 structure, 3 glycans have the core-1 structure and 2 glycans contained a previously unobserved core structure with hexose at the reducing end. There were seven pairs of structural isomers observed in the major O-linked oligosaccharides. To further elucidate the structures of a dozen O-linked glycans, specific and targeted exoglycosidase digestions were carried out and the products were monitored with MALDI-MS. Reported here are the elucidated structures of O-linked oligosaccharides from glycoproteins of X. borealis egg jelly coats. The structural differences in O-glycans from jelly coats of X. borealis and its close relatives may provide a better understanding of the structure-function relationships and the role of glycans in the fertilization process within Xenopodinae.

Molecular characterization of egg envelope glycoprotein ZPD in the ovary of Japanese quail (Coturnix japonica)

The egg envelope surrounding avian oocytes exhibits a three-dimensional network of coarse fibers between the granulosa cells and the oocyte. Our previous studies have demonstrated that one of the matrix's components, ZP3, is synthesized in the ovarian granulosa cells. Another component, ZP1, which is critically involved in triggering the sperm acrosome reaction, is synthesized in the liver. We have previously isolated cDNAs encoding quail ZP3 and ZP1, and we now report the isolation of cDNA encoding quail ZPD. By RNase protection assay and in situ hybridization, we have demonstrated that ZPD transcripts are restricted to the granulosa cells of preovulatory follicles. The expression level of ZPD increased progressively during follicular development, and the highest expression was observed in the largest follicles. Western blot analyses using the specific antibody against ZPD indicate that the 40 kDa protein is the authentic ZPD, and the contents of ZPD protein also increased during follicular development. Moreover, we found that the addition of FSH to the culture media enhances the ZPD secretion in the cultured granulosa cells. Two-dimensional gel electrophoresis revealed the presence of several ZPD isoforms with different pI values ranging from 5.5 to 7. Immunohistochemical analyses indicate that the materials recognized with anti-quail ZPD antibody were accumulated in the egg envelope of large yellow follicles. These results demonstrate the presence of ZPD protein in the egg envelope, and that the amount of ZPD in the egg envelope as well as the mRNA in the cells increases at the latter stages of folliculogenesis.

ZP genes in avian species illustrate the dynamic evolution of the vertebrate egg envelope

The vertebrate egg envelope is composed of a set of related proteins, encoded by the ZP genes. The apparent simplicity of the egg envelope is in contrast to the number of ZP genes identified by conventional cloning and data mining of genome sequences from a number of vertebrates. The vertebrate ZP genes fall into five classes, ZP1, ZP2, ZP3, ZPD and ZPAX. Analysis of chicken genome and EST sequence data has revealed the presence of seven distinct ZP genes, falling into these classes that are expressed in the female reproductive system. Comparison with the repertoire of ZP genes in other vertebrates suggests a major source of diversity in the composition of the egg envelope is a continual process of amplification, diversification and attrition of ZP gene sequences.

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.

Zona Pellucida Domain of ZPB1 controls specific binding of ZPB1 and ZPC in Japanese quail (Coturnix japonica)

The extracellular matrix surrounding avian oocytes, referred to as the perivitelline membrane (PL), exhibits a three-dimensional network of fibrils between granulosa cells and the oocyte. We previously reported that one of its components, ZPC, is synthesized in granulosa cells that are specifically incorporated into the PL; this incorporation might be mediated by a specific interaction with ZPB1, another PL constituent, which is synthesized in the liver. In order to extend our previous findings, we established an expression system for quail ZPB1 using a mammalian cell line, and several ZPB1 mutants lacking the zona pellucida (ZP) domain or the glutamine-rich repeat region were produced. Western blot analysis of the immunoprecipitated materials with anti-ZPC antiserum indicated that ZPB1 was coimmunoprecipitated with the antiserum in the presence of ZPC. Ligand blotting also revealed the specific binding of ZPC and ZPB1 and indicated that the binding of these two components might be mediated via an ionic interaction. An analysis using recombinant ZPB1 demonstrated that the ZPB1 lacking the ZP domain did not bind to ZPC, whereas the mutant missing the glutamine-rich repeat region retained its capacity for binding. Furthermore, although the ZPB1 lacking the N-terminal half of the ZP domain was able to bind to ZPC, the deletion of the C-terminal half completely abolished ZPB1 binding to ZPC. These results suggested that the C-terminal half of the ZP domain of ZPB1 contains a binding site for ZPC, and that it appears to be involved in insoluble PL fibril formation in the quail ovary.

Molecular Characterization and Expression Pattern of Zona Pellucida Proteins in Gilthead Seabream (Sparus aurata)1

The developing oocyte is surrounded by an acellular envelope that is composed of 2-4 isoforms of zona pellucida (ZP) proteins. The ZP proteins comprise the ZP1, ZP2, ZP3, and ZPX isoforms. While ZP1 (ZPB) and ZP3 (ZPC) are present in all species, ZP2 (ZPA) is not found in teleost fish and ZPX is not found in mammals. In the present study, we identify and characterize the ZP1, ZP3 and ZPX isoforms of gilthead seabream. Furthermore, by analyzing the conserved domains, which include the external hydrophobic patch and the internal hydrophobic patch, we show that ZP2 and ZPX are closely related isoforms. ZP proteins are synthesized in either the liver or ovary of most teleosts. Only in rainbow trout has it been shown that zp3 has dual transcription sites. In gilthead seabream, all four mRNA isoforms are transcribed in both the liver and ovary, with zp1a, zp1b, and zp3 being highly expressed in the liver, and zpx being primarily expressed in the ovary. However, determination of the ZP proteins in plasma showed high levels of ZP1b, ZP3, and ZPX, with low or non-detectable levels of ZP1a. In similarity to other teleost ZPs, the hepatic transcription of all four ZP isoforms is under estrogenic control. Previously, we have shown that cortisol can potentiate estrogen-induced ZP synthesis in salmonids, and now we show that this is not the case in the gilthead seabream. The present study shows for the first time the endocrine regulation of a teleost ZPX isoform, and demonstrates the dual-organ transcriptional activities of all the ZP proteins in one species.

Heterocomplex formation and cell-surface accumulation of hen's serum zona pellucida B1 (ZPB1)with ZPC expressed by a mammalian cell line (COS-7): a possible initiating step of egg-envelope matrix construction

The egg envelope, referred to as zona pellucida (ZP) in mammalian eggs, is a fibrous and noncollagenous extracellular matrix surrounding vertebrate eggs, and composed of three to four homologous glycoproteins with a common ZP domain. In birds, a liver-derived ZP glycoprotein (ZP1/ZPB1) is transported through the bloodstream to ovarian follicles and joins the egg-envelope matrix construction together with the other ZP glycoproteins, such as ZPC and ZPD/ZPX2, both secreted from follicular granulosa cells. We report here that, through its ZP domain, ZPB1 specifically associates with ZPC, which might lead to the construction of egg-envelope matrix. The ZPB1 in laying hen's serum specifically bound to ZPC, but not to ZPX2, separated by SDS-PAGE and blotted on a membrane. Hemagglutinin (HA)-tagged ZPC expressed in a mammalian cell line (COS-7) cells was processed and secreted as a mature-form into the culture medium. From the culture supernatant of ZPC-expressing transfectants cultured in the presence of ZPB1, both ZPB1 and ZPC were recovered as heterocomplexes by immunoprecipitation using either anti-HA or anti-ZPB1 antibody. Interestingly, a monoclonal antibody, 8E1, which immunoprecipitated free ZPB1, did not immunoprecipitate the ZPB1-ZPC heterocomplexes. An 8E1 epitope was mapped on a C-terminal region of the ZP domain in a ZPB1 molecule by identifying an 8E1-positive peptide using mass spectroscopy. Furthermore, by laser scanning confocal microscopy, ZPB1 and ZPC were observed to colocalize on the surface of ZPC-expressing transfectants cultured in the presence of ZPB1, whereas almost no ZPC was detected on the surface of the transfectants cultured in the absence of ZPB1. Taken together, these results suggest that ZPB1 transported into ovarian follicles encounters and associates with ZPC secreted from granulosa cells, resulting in the formation of heterocomplexes around an oocyte. In addition, it appears that such ZPB1-ZPC complexes accumulated on the oocyte surface act as a scaffold for subsequent matrix construction events including ZPX2 association.

Tandem-Repeated Zebrafish zp3 Genes Possess Oocyte-Specific Promoters and Are Insensitive to Estrogen Induction1

Zona pellucida (Zp) proteins are glycoproteins in fish chorion and are encoded by multiple gene families, including zp1, zp2, zp3, and potentially other zp genes. Expression of zp genes in teleosts is either in the liver under the induction of estrogen or in the ovaries. In the present study, we isolated and characterized a zebrafish zp3 genomic clone and found three tandem-repeated zp3 genes with high sequence identities. We estimated that there were 10-15 zp2 and 17-21 zp3 genes in a haploid genome. We also found some variant zp2 and zp3 subfamilies, and each subfamily may have multiple members. These zp2 and zp3 genes are distributed in several different chromosomes. Ontogenetic expression of zp2 and zp3 mRNAs was first detected at 3 wk postfertilization, which was about 5 wk earlier than initial vtg1 expression, indicating that ovary development was earlier than vitellogenesis. Both zp2 and zp3 mRNAs were expressed specifically in early-growing oocytes and are insensitive to estrogen induction. Because zp3 genes are organized in tandem repeats, to investigate whether an individual zp3 promoter is capable of driving oocyte-specific expression, green fluorescence protein (gfp)-transgenic zebrafish were developed by using a 3.8-kb zp3 5' upstream region, and we found that the gfp reporter gene was faithfully expressed in developing oocytes in zp3:gfp transgenic females. Thus, the new transgenic line not only provided a convenient living marker for monitoring female gonad development, but also demonstrated that a single zp3 gene promoter is sufficient for oocyte-specific transcription.

Characterization of Bufo arenarum oocyte plasma membrane proteins that interact with sperm

Sperm-oocyte plasma membrane interaction is an essential step in fertilization. In amphibians, the molecules involved have not been identified. Our aim was to detect and characterize oocyte molecules with binding affinity for sperm. We isolated plasma membranes free from vitelline envelope and yolk proteins from surface-biotinylated Bufo arenarum oocytes. Using binding assays we detected a biotinylated 100 kDa plasma membrane protein that consistently bound to sperm. Chromatographic studies confirmed the 100 kDa protein and detected two additional oocyte molecules of 30 and 70 kDa with affinity for sperm. Competition studies with an integrin-interacting peptide and cross-reaction with an anti-HSP70 antibody suggested that the 100 and 70 kDa proteins are members of the integrin family and HSP70, respectively. MS/MS analysis suggested extra candidates for a role in this step of fertilization. In conclusion, we provide evidence for the involvement of several proteins, including integrins and HSP70, in B. arenarum sperm-oocyte plasma membrane interactions.

Isolation and mapping the chicken zona pellucida genes: an insight into the evolution of orthologous genes in different species

The avian oocyte is surrounded by a specialized extracellular glycoproteinaceous matrix, the perivitelline membrane, which is equivalent to the zona pellucida (ZP) in mammals and the chorion in teleosts. A number of related ZP genes encode the proteins that make up this matrix. These proteins play an important role in the sperm/egg interaction and may be involved in speciation. The human genome is known to contain ZP1, ZP2, ZP3, and ZPB genes, while a ZPAX gene has also been identified in Xenopus. The rapid evolution of these genes has confused the nomenclature and thus orthologous relationships across species. In order to clarify these homologies, we have identified ZP1, ZP2, ZPC, ZPB, and ZPAX genes in the chicken and mapped them to chromosomes 5, 14, 10, 6, and 3, respectively, establishing conserved synteny with human and mouse. The amino acid sequences of these genes were compared to the orthologous genes in human, mouse, and Xenopus, and have given us an insight into the evolution of these genes in a variety of different species. The presence of the ZPAX gene in the chicken has highlighted a pattern of probable gene loss by deletion in mouse and gene inactivation by deletion, and base substitution in human.

A newly identified zona pellucida glycoprotein, ZPD, and dimeric ZP1 of chicken egg envelope are involved in sperm activation on sperm-egg interaction

Fertilization begins with interaction between the sperm and the egg. The surface of the vertebrate oocyte is covered with the egg envelope, which is composed of ZP (zona pellucida) glycoproteins. We have identified two glycoproteins, ZP1/gp97 and ZPC/gp42, as the major components of the chicken egg envelope. In the present study, another 42 kDa protein, designated ZPD, has been found as a new major component of the chicken egg envelope. ZPD was specifically released from the egg envelope by ultrasonication treatment without urea. ZPD cDNA was cloned using a chicken granulosa cell cDNA pool. The deduced amino acid sequence showed that preproprotein of ZPD is composed of 418 amino acid residues with four potential N-glycosylation sites and includes a ZP domain, common in vertebrate ZP glycoproteins, and a transmembrane domain. ZPD belongs phylogenetically to a distinct group from known ZP glycoprotein subfamilies, ZPA, ZPB, and ZPC. In two-dimensional gel electrophoresis ZPD proteins were identified to be several isoforms with different pI values between 5 and 7. ZP1, ZPC and the newly identified ZPD were confirmed to be the major components of chicken egg envelope by MS of proteolytic digests of whole egg envelope. The in vitro incubation of chicken sperm with calcium ionophore A23187 induced sperm activation, resulting in the fragmentation and release of a 41 kDa PNA (peanut agglutinin)-positive glycoprotein and the decrease or loss of sperm PNA-stainability. The incubation with ZPD and dimeric ZP1, but not ZPC and monomeric ZP1, also induced the decrease or loss of sperm PNA-stainability, suggesting the in vitro sperm activation by these ZP components. Collectively, ZPD might bind loosely to egg envelope matrix and play a key role in the sperm activation on avian sperm-egg interaction.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

Involvement of interaction of ZP1 and ZPC in the formation of quail perivitelline membrane

The extracellular matrix surrounding the oocyte before ovulation is called the perivitelline membrane (PL) in avian species. We have previously reported that one of its components, ZPC, is produced in ovarian granulosa cells by the stimulation of follicle-stimulating hormone and testosterone. Another component, ZP1, is synthesized in the liver and might be transported to the surface of the oocyte of the follicles. These glycoproteins are assembled to form a three-dimensional network of coarse fibers between the granulosa cells and the oocyte. In the present study, we have evaluated the involvement of the interaction of ZPC and ZP1 in the formation of the PL of Japanese quail. By measuring the incorporation of tritium-labeled proteins into the PL, we have found that tritium-labeled ZPC is specifically incorporated into the PL. Whole-mount autoradiographic analysis of the PL has also revealed the incorporation of the secreted ZPC into the isolated PL. To study which component in the PL is responsible for the specific incorporation of ZPC, PL lysates were incubated with the conditioned medium of the granulosa cells and were immunoprecipitated with anti-ZPC antiserum. Western blot analysis of the immunoprecipitated materials indicated that the 175-kDa and 97-kDa ZP1 forms were co-immunoprecipitated with anti-ZPC antiserum. These results demonstrate that ZPC secreted from the granulosa cells specifically binds with ZP1, and that the phenomenon might be involved in insoluble PL fiber formation in quail ovary.

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.

Interaction of sperm with purified native chicken ZP1 and ZPC proteins

The avian perivitelline membrane (PVM) is the site of initial contact between sperm and egg. It consists of only two major components, which are both homologues of the mammalian zona pellucida (ZP) proteins, and belong to the ZP1 and ZPC families, respectively. We have established a method to isolate large quantities of both native avian ZP proteins and have used these preparations to investigate their sperm-binding capacities. Chicken ZPC forms multimeric structures of defined size and binds to an approximately 180-kDa protein complex present in rooster sperm extracts. Based on experiments using both PVM and isolated proteins, we show that chicken ZP1 is proteolytically degraded by a sperm-associated protease but that chicken ZPC remains intact. An antiserum directed against chicken ZP1 is capable of inhibiting sperm binding to the PVM. Taken together, these data suggest that ZP1, in addition to ZPC, plays a major role in the initial interactions between sperm and egg.

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.

Variant of perivitelline membrane glycoprotein ZPC of Japanese quail (Coturnix japonica) lacking its cytoplasmic tail exhibits the retention in the endoplasmic reticulum of Chinese hamster ovary (CHO-K1) cells

Avian perivitelline membrane, an investment homologous to the mammalian zona pellucida, is composed of at least two glycoproteins. Our previous studies demonstrated that one of its components, ZPC, which is synthesized in the ovarian granulosa cells, is secreted after carboxy-terminal proteolytic processing, and this event is a prerequisite event for ZPC secretion in quail. In the present study, we examined the role of the cytoplasmic tail, which is successfully removed after proteolytic processing, in membrane transport, proteolytic processing, and the secretion of quail ZPC. In pursuit of this, we produced a truncated ZPC mutant lacking the cytoplasmic tail located in its C-terminus and examined its expression in the mammalian cell line. Western blot analyses demonstrated that the cytoplasmic tail-deficient ZPC was neither secreted nor underwent proteolytic processing in the cells. Immunofluorescence analysis and the acquisition of resistance to endoglycosidase H digestion of the cytoplasmic tail-deficient ZPC demonstrated that the deletion of the cytoplasmic tail interferes with the intracellular trafficking of the protein from the endoplasmic reticulum to the Golgi apparatus. These results indicate that the cytoplasmic tail of quail ZPC might possess the determinant responsible for the efficient transport of the newly synthesized ZPC from the endoplasmic reticulum to the Golgi apparatus.

Genomic organization of ZP domain containing egg envelope genes in medaka (Oryzias latipes)

To provide insights into the diversity of egg envelope genes in teleosts, we determined the genomic organization and the map position of the medaka egg envelope genes expressed either in the oocytes or in the liver. There seems to be five classes of ZP domain containing egg envelope genes in vertebrates: zpa, zpax, zpb, zpc, and zpd. zpa, zpax, and zpb are much closely related than zpc. There is an expanded family of teleost-specific zpc genes. The duplication of the possible zpb/zpc cluster happened in the teleost lineage may be a cause of liver-specific ZP gene evolution in teleosts. The inconsistent presence of a repetitive amino acid domain among teleost zpb and zpc gene products suggests rapid evolution of this domain. In addition, relative abundance of E-boxes in putative promoters of medaka oocyte-specific ZP genes suggests their regulation by basic helix-loop-helix transcription factors, particularly by FIGalpha.

The envelopes of amphibian oocytes: physiological modifications in Bufo arenarum

A characterization of the Amphibian Bufo arenarum oocyte envelope is presented. It was made in different functional conditions of the oocyte: 1) when it has been released into the coelomic cavity during ovulation (surrounded by the coelomic envelope, (CE), 2) after it has passed through the oviduct and is deposed (surrounded by the viteline envelope, (VE), and 3) after oocyte activation (surrounded by the fertilization envelope, (FE). The characterization was made by SDS-PAGE followed by staining for protein and glycoproteins. Labeled lectins were used to identify glycosidic residues both in separated components on nitrocellulose membranes or in intact oocytes and embryos. Proteolytic properties of the content of the cortical granules were also analyzed. After SDS-PAGE of CE and VE, a different protein pattern was observed. This is probably due to the activity of a protease present in the pars recta of the oviduct. Comparison of the SDS-PAGE pattern of VE and FE showed a different mobility for one of the glycoproteins, gp75. VE and FE proved to have different sugar residues in their oligosaccharide chains. Mannose residues are only present in gp120 of the three envelopes. N-acetyl-galactosamine residues are present in all of the components, except for gp69 in the FE. Galactose residues are present mainly in gp120 of FE. Lectin-binding assays indicate the presence of glucosamine, galactose and N-acetyl galactosamine residues and the absence (or non-availability) of N-acetyl-glucosamine or fucose residues on the envelopes surface. The cortical granule product (CGP) shows proteolytic activity on gp75 of the VE.

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.