Cloning of a cDNA coding for porcine zona pellucida glycoprotein ZP1 and its genomic organization

The mammalian zona pellucida: a matrix that mediates both gamete binding and immune recognition?

The crucial cell adhesion events required for mammalian fertilization commence when spermatozoa bind to the specialized extracellular matrix of the oocyte, known as the zona pellucida (ZP). Bound gametes then undergo a signal transduction cascade known as acrosomal exocytosis that enables them to penetrate this matrix and fuse with the oocyte to create a new individual. The ZP is therefore the target of intense investigation in the mouse, pig, bovine, and human models. Major goals in such studies are to define the adhesion molecules, signal transduction pathways, and the molecular basis for the species-restricted binding of gametes. Evidence exists indicating that protein-carbohydrate and to a lesser extent protein-protein interactions play a role in the initial gamete binding. More recent findings in an unusual sperm-somatic cell adhesion system indicate that tri- and tetraantennary N-glycans mediate initial sperm-oocyte binding in both the murine and porcine models, but conflicting data exist. A novel paradigm designated the "domain specific model" will be presented that could explain these inconsistencies. Another potential functional role of the ZP is immune recognition. Both spermatozoa and oocytes lack major histocompatibility (MHC) class I molecules that mediate the recognition of self in the immune system. This absence makes gametes less susceptible to class I restricted cytotoxic T lymphocytes, but more vulnerable to natural killer (NK) cells. Therefore a "fail safe" system for NK cell recognition should exist on both types of gametes. Another issue is that oocytes could begin to express paternal major histocompatibility antigens during the blastocyst stage prior to hatching, and thus mechanisms could also be in place to block the development of maternal adaptive immune responses. An enhanced understanding of these issues could facilitate the development of superior infertility treatments and contraceptive strategies, and define central operating principles of immune recognition in the female reproductive system.

Production of monoclonal antibodies against recombinant human zona pellucida glycoproteins: utility in immunolocalization of respective zona proteins in ovarian follicles

The zona pellucida (ZP) glycoproteins play an important role in oocyte development and gamete biology. To analyze their expression in follicles during various developmental stages, murine monoclonal antibodies (MAbs) were generated against the baculovirus-expressed recombinant human ZP2, ZP3 and ZP4. A panel of MAbs specific for the respective zona protein in ELISA and Western blot, and devoid of cross-reaction with other zona proteins was selected. Immunohistochemistry has shown that ZP2 MAb, MA-1620, did not react with oocytes in resting primordial follicles but showed reactivity with degenerating oocytes in primordial follicles undergoing atresia, and with oocytes in growing and antral follicles. Three MAbs against ZP3 did not react with oocytes in primordial follicles, but reacted only with oocytes in growing and antral follicles. Out of four MAbs against ZP4, three MAbs reacted with oocytes in primordial, growing and antral follicles. No reactivity of these MAbs with other ovarian cell types and other tissues studied (endometrium, uterine cervix, fallopian tubes and kidney) was detected except for a strong reactivity of ZP2 MA-1620 with epithelial cells of the uterine ectocervix or endometrium in some samples investigated. Altogether, these studies document generation of MAbs exhibiting high specificity for human zona proteins, which will be useful reagents to study their immunobiology.

Phylogenetic analysis and identification of pseudogenes reveal a progressive loss of zona pellucida genes during evolution of vertebrates

Vertebrate eggs are surrounded by an extracellular matrix with similar functions and conserved individual components: the zona pellucida (ZP) glycoproteins. In mammals, chickens, frogs, and some fish species, we established an updated list of the ZP genes, studied the relationships within the ZP gene family using phylogenetic analysis, and identified ZP pseudogenes. Our study confirmed the classification of ZP genes in six subfamilies: ZPA/ZP2, ZPB/ZP4, ZPC/ZP3, ZP1, ZPAX, and ZPD. The identification of a Zpb pseudogene in the mouse genome, Zp1 pseudogenes in the dog and bovine genomes, and Zpax pseudogenes in the human, chimpanzee, macaque, and bovine genomes showed that the evolution of ZP genes mainly occurs by death of genes. Our study revealed that the extracellular matrix surrounding vertebrate eggs contains three to at least six ZP glycoproteins. Mammals can be classified in three categories. In the mouse, the ZP is composed of three ZP proteins (ZPA/ZP2, ZPC/ZP3, and ZP1). In dog, cattle and, putatively, pig, cat, and rabbit, the zona is composed of three ZP proteins (ZPA/ZP2, ZPB/ZP4, and ZPC/ZP3). In human, chimpanzee, macaque, and rat, the ZP is composed of four ZP proteins (ZPA/ZP2, ZPB/ZP4, ZPC/ZP3, and ZP1). Our review provides new directions to investigate the molecular basis of sperm-egg recognition, a mechanism which is not yet elucidated.

Recombinant bovine zona pellucida glycoproteins ZP3 and ZP4 coexpressed in Sf9 cells form a sperm-binding active hetero-complex

The zona pellucida (ZP) is a transparent envelope that surrounds the mammalian oocyte and mediates species-selective sperm-egg interactions. Porcine and bovine ZPs are composed of the glycoproteins ZP2, ZP3, and ZP4. We previously established an expression system for porcine ZP glycoproteins (ZPGs) using baculovirus in insect Sf9 cells. Here we established a similar method for expression of bovine ZPGs. The recombinant ZPGs were secreted into the medium and purified by metal-chelating column chromatography. A mixture of bovine recombinant ZP3 (rZP3) and rZP4 coexpressed in Sf9 cells exhibited inhibitory activity for bovine sperm-ZP binding similar to that of a native bovine ZPG mixture, whereas neither bovine rZP3 nor rZP4 inhibited binding. An immunoprecipitation assay revealed that the coexpressed rZP3/rZP4 formed a hetero-complex. We examined the functional domain structure of bovine rZP4 by constructing ZP4 mutants lacking the N-terminal domain or lacking both the N-terminal and trefoil domains. When either of these mutant proteins was coexpressed with bovine rZP3, the resulting mixtures exhibited inhibitory activity comparable to that of the bovine rZP3/rZP4 complex. Hetero-complexes of bovine rZP3 and porcine rZP4, or porcine rZP3 and bovine rZP4, also inhibited bovine sperm-ZP binding. Our results demonstrate that the N-terminal and trefoil domains of bovine rZP4 are dispensable for formation of the sperm-binding active bovine rZP3/rZP4 complex and, furthermore, that the molecular interactions between rZP3 and rZP4 are conserved in the bovine and porcine systems.

In vitro germline potential of stem cells derived from fetal porcine skin

Two of the unanswered questions in mammalian developmental biology are when and where the fate of the germ cell is specified. Here, we report that stem cells isolated from the skin of porcine fetuses have the intrinsic ability to differentiate into oocyte-like cells. When differentiation was induced, a subpopulation of these cells expressed markers such as Oct4, Growth differentiation factor 9b (GDF9b), the Deleted in Azoospermia-like (DAZL) gene and Vasa - all consistent with germ-cell formation. On further differentiation, these cells formed follicle-like aggregates that secreted oestradiol and progesterone and responded to gonadotropin stimulation. Some of these aggregates extruded large oocyte-like cells that expressed oocyte markers, such as zona pellucida, and the meiosis marker, synaptonemal complex protein 3 (SCP3). Some of these oocyte-like cells spontaneously developed into parthenogenetic embryo-like structures. The ability to generate oocyte-like cells from skin-derived cells may offer new possibilities for tissue therapy and provide a new in vitro model to study germ-cell formation and oogenesis.

Proteolysis of Xenopus laevis egg envelope ZPA triggers envelope hardening

The egg envelope of most animal eggs is modified following fertilization, resulting in the prevention of polyspermy and hardening of the egg envelope. In frogs and mammals a prominent feature of envelope modification is N-terminal proteolysis of the envelope glycoprotein ZPA. We have purified the ZPA protease from Xenopus laevis eggs and characterized it as a zinc metalloprotease. Proteolysis of isolated egg envelopes by the isolated protease resulted in envelope hardening. The N-terminal peptide fragment of ZPA remained disulfide bond linked to the ZPA glycoprotein moiety following proteolysis. We propose a mechanism for egg envelope hardening involving ZPA proteolysis by an egg metalloprotease as a triggering event followed by induction of global conformational changes in egg envelope glycoproteins.

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.

The extracellular matrix of porcine mature oocytes: origin, composition and presumptive roles

The extracellular matrix (ECM) of porcine mature oocytes was revealed by transmission electron microscopy (TEM) after treatment with tannic acid and ruthenium red. Present in the perivitelline space (PVS) and on the surface of the zona pellucida (ZP), it appeared to be composed of thin filaments and granules at the interconnections of the filaments, which were interpreted respectively as hyaluronic acid chains and bound proteoglycans. In order to determine whether this material is produced by the corona cells (the same ECM was found also on the surface of the zona pellucida and between cumulus cells) or by the oocyte itself, the synthesis of glycoproteins and glycosaminoglycans was checked by autoradiography on semi-thin and thin sections observed by light and electron microscopy. Immature oocytes within or without cumulus cells, were incubated with L [3H-] fucose or L [3H-] glucosamine--precursors respectively of glycoproteins and hyaluronic acid or hyaluronan (HA) bound to proteoglycans--for various times (with or without chase) and at different stages during in vitro maturation. In the first case, incorporation was found in both cumulus cells and ooplasm (notably in the Golgi area for 3H-fucose) and labeled material accumulated in the ECM of the PVS and of the ZP surface. Labeling in the PVS with both precursors was maximum between metaphase I (MI) and metaphase II (MII) and was partially extracted by hyaluronidase but not by neuraminidase. Tunicamycin, an inhibitor of glycoprotein synthesis, significantly decreased the amount of 3H-fucose labeled molecules in the PVS and increased the incidence of polyspermic penetration during subsequent in vivo fertilization. Since cumulus-free oocytes also secreted 3H-glucosamine containing compounds, both oocyte and cumulus cells probably contribute to the production of the ECM found in the PVS of mature oocytes. ECM and particularly its HA moiety present on both sides of the ZP may constitute a favourable factor for sperm penetration.

Isolation and characterization of a human sperm antigen gene h-Sp-1

We isolated and characterized a human sperm antigen gene (h-Sp-1) from human testis complementary DNA using antiserum against the human sperm membrane. Northern blot analysis detected two transcripts (2.3 and 1.1 kb) of the h-Sp-1 gene. The 2.3-kb transcript is ubiquitous, whereas the 1.1-kb transcript is specific to the human testis with a high level of expression. Determination of the base sequence of h-Sp-1 showed a size of 2170 bp and 43.4% homology with human synaptophysin. The base sequence indicates a molecule consisting of 259 amino acids, with four hydrophilic and four hydrophobic regions. In order to further characterize the h-Sp-1 molecule, we synthesized the probable region of amino acids with high antigenicity based on the amino acid sequence (amino acid nos. 174-198) and immunized rabbits to prepare an antiserum. In our experimental model of fertilization between human sperm and zona pellucida-free hamster ova, partial inhibition of fertilization was observed. We were able to synthesize a large quantity of recombinant protein by inserting the h-Sp-1 gene into a baculovirus vector and infecting spodoptera frugiperda culture cells (sf9 insect cells). The synthesized protein had a molecular weight of 30 kDa. We then immunized Balb/c mice with this protein to prepare a monoclonal antibody (G3G9), which was used to localize the h-Sp-1 molecule in sperm and tissues (e.g. testis). The h-Sp-1 molecule was present in the cell membrane from the head to tail of human sperm. Staining of the testis and epididymis also showed h-Sp-1 to be present in spermatogonia, spermatocyte, sperm and epididymal duct epithelium. These findings suggest that the h-Sp-1 molecule is expressed in sperm and testes and plays a role in fertilization.

Eggshell and egg yolk proteins in fish: hepatic proteins for the next generation: oogenetic, population, and evolutionary implications of endocrine disruption

The oocyte is the starting point for a new generation. Most of the machinery for DNA and protein synthesis needed for the developing embryo is made autonomously by the fertilized oocyte. However, in fish and in many other oviparous vertebrates, the major constituents of the egg, i.e. yolk and eggshell proteins, are synthesized in the liver and transported to the oocyte for uptake. Vitellogenesis, the process of yolk protein (vitellogenin) synthesis, transport, and uptake into the oocyte, and zonagenesis, the synthesis of eggshell zona radiata proteins, their transport and deposition by the maturing oocyte, are important aspects of oogenesis. The many molecular events involved in these processes require tight, coordinated regulation that is under strict endocrine control, with the female sex steroid hormone estradiol-17beta in a central role. The ability of many synthetic chemical compounds to mimic this estrogen can lead to unscheduled hepatic synthesis of vitellogenin and zona radiata proteins, with potentially detrimental effects to the adult, the egg, the developing embryo and, hence, to the recruitment to the fish population. This has led to the development of specific and sensitive assays for these proteins in fish, and the application of vitellogenin and zona radiata proteins as informative biomarkers for endocrine disrupting effects of chemicals and effluents using fish as test organisms. The genes encoding these important reproductive proteins are conserved in the animal kingdom and are products of several hundred million years of evolution.

In search of the active PZP epitope in white-tailed deer immunocontraception

Native porcine zona pellucida (PZP) has been shown to be highly effective as an immunocontraceptive in white-tailed deer. However, the immunogenicity of PZP extracted from pig ovaries may vary from lot to lot and the extract has the potential of containing either viral or pathogenic material. Determination of the immunocontraceptive epitopes of PZP would allow portions of the molecule to be synthesized or inserted into a recombinant system for production of a consistent and safe vaccine. In this study, epitopes of PZP were selected and tested by in vitro binding, immunogenicity in rabbits, immunogenicity and immunocontraception in deer. Sera from PZP immunocontracepted deer were tested on ELISA plates containing immobilized peptides from ZP1 and ZP3alpha. Peptides with which sera from infertile deer reacted (six peptides from ZP1 and six peptides from ZP3alpha) were selected, synthesized and tested for immunogenicity in rabbits. Deer were then immunized with combinations of peptides from either the ZP1 or ZP3alpha groups. ZP3alpha peptides induced high immune titers against native PZP, but did not induce infertility in the deer. Although ZP1 peptides induced lower titers, deer immunized with two ZP1 peptides exhibited multiple estrus events and infertility, typical of that for deer immunized with native PZP vaccine. Competitive inhibition assays using the ZP1 peptides demonstrated that the peptide comprising pins 10-16 was most effective in blocking binding by the serum antibody of native PZP immunized deer. This peptide was used to immunocontracept deer, resulting in a significant reduction in fawning for 1 year.

Molecular cloning of rainbow trout (Oncorhynchus mykiss) eggshell zona radiata protein complementary DNA: mRNA expression in 17beta-estradiol- and nonylphenol-treated fish

A complementary DNA (cDNA) encoding the eggshell zona radiata protein (RBTZR: AF407574) has been cloned from the liver of estradiol-17beta (E(2))-treated rainbow trout (Oncorhynchus mykiss) by reverse-transcriptase polymerase chain reaction (RT-PCR). A set of degenerate primers homologous to the highly conserved cysteine-rich region of the zona radiata protein gene from salmon, winter flounder, medaka and carp were used for the initial RT-PCR. The resulting PCR product was cloned, sequenced and identified as the Zrp gene fragment based on amino acid sequence similarities. Based on the Zrp sequence from the initial PCR, a pair of gene-sequence primers was designed for 3'- and 5'- random amplification of cDNA ends (RACE). Cloning and sequencing of RACE products showed a 1349-bp Zrp gene encoding a 403-amino acid protein with a theoretical molecular mass of approximately 45 kDa. Alignment of the deduced amino acid sequence reveals that RbtZR is similar to piscine and mammalian zona pellucida proteins. The RbtZR gene, together with the estrogen receptor (ER) and vitellogenin (Vtg) genes, was further characterized and comparatively studied for transcriptional and translational expression in xenoestrogen- (nonylphenol, NP) and E(2)-treated juvenile rainbow trout in a time-course experiment. Northern and slot blot analysis showed that the RbtZR mRNA was expressed, in parallel with the ER and Vtg mRNA, in both NP- and E(2)-treated juvenile rainbow trout. Indirect enzyme-linked immunosorbent assay (ELISA) using monoclonal antibody raised against Atlantic salmon Zrp indicated the translational expression of RbtZR protein in blood plasma samples from NP- and E(2)-treated juvenile trout. The differential time-dependent transcriptional and translational expression and use of Zrp, ER and Vtg as sensitive biomarkers in environmental monitoring of endocrine disrupters in fish is discussed.

Prospect for immunocontraception using the NH2-terminal recombinant peptide of human zona pellucida protein (hZPA)

PROBLEM

The zona pellucida is a feasible candidate for contraceptive vaccine. Our previous study showed that a human zona pellucida (ZPA) recombinant protein produced antiserum that inhibited human in vitro fertilization in rabbits. The objective of this study is to examine whether the human recombinant protein produce fertilization-blocking antiserum in a non-human primate.

METHOD OF STUDY

The amino terminal regions consisting of pig (198 amino acids) and human (206 amino acids) zona pellucida proteins were prepared by Escherichia coli to produce antisera in bonnet monkeys (Macaca radiata). Antibody production was assessed by immunofluorescent staining and enzyme-linked immunosorbent assay (ELISA). Fertilization-blocking test was carried out using human oocytes that had failed in intracytoplasmic sperm injection (ICSI) treatment under patient's informed consent.

RESULTS

Both antisera against pig and human recombinant proteins recognized native human zona pellucida by immunofluorescent staining. However, anti-recombinant human ZPA reacted to native pig zona protein more strongly than pig zona pellucida by ELISA. When human oocytes were treated with antisera before insemination, anti-human ZPA antiserum inhibited human sperm binding to human zona pellucida, but anti-pig ZPA antiserum did not.

CONCLUSIONS

Bonnet monkey produced fertilization-blocking antibody by immunization with human recombinant ZPA. The homologous or highly similar sequence is effective for developing a contraceptive vaccine when using recombinant peptides.

Localization of species conserved zona pellucida antigens in mammalian ovaries

The mammalian zona pellucida (ZP) consists of three glycoproteins (ZP1, ZP2 and ZP3), which are variably conserved among species at the genomic and amino acid levels. In order to evaluate the expression of ZP during ovarian development, a population of antibodies was selected that recognize species conserved antigenic domains of the three ZP proteins. Domain specific antibodies were selected from sera of rabbits immunized with all three native pig ZP proteins by elution of antibodies bound to each of the three human ZP recombinant proteins expressed from cDNAs, using the baculovirus expression system in insect cells. Immunoblot analysis was used to characterize the specificity of the antibodies and immunohistochemistry was used to evaluate the stage specific expression of ZP proteins during ovarian follicular development of the mouse, baboon and human. This study demonstrates that the conserved domains of all three ZP proteins are localized in the oocyte extracellular ZP matrix as well as in a subset of granulosa cells. However, this expression does vary among species with respect to the stage and cell type during early stages of ovarian follicular development. These antibodies should serve as excellent markers for evaluating early stages of human ovarian follicular development and in the development of contraceptive agents.

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

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

Structure and function of the proteins of the mammalian Zona pellucida

The zona pellucida (ZP) is the extracellular matrix that plays important roles in sperm-egg interaction. The ZP is composed of three major glycoproteins that exhibit heterogeneity due to extensive post-translational modifications including glycosylation and sulfation. Because of these modifications the nomenclature of ZP proteins from different species based on electrophoretic mobilities has been confusing. As the cDNAs and genes encoding the different ZP proteins have been isolated and sequenced, it is now possible to relate these ZP proteins according to gene families. Using the mouse ZP nomenclature, the ZP proteins from different mammalian species can be classified into three protein families: ZP1, ZP2, and ZP3. Although some of the structural domains of the ZP proteins of different species are conserved within each family, they exhibit distinct biological properties. In the mouse it has been established that ZP3 is the primary sperm receptor while ZP2 has secondary sperm receptor properties. In the pig, however, ZP1 has been shown to have sperm receptor activity similar to that observed in the rabbit and nonhuman primates. It is of interest that the human ZP2 and ZP3 gene families are 60-70% conserved with respect to the mouse ZP amino acid sequence, while the mouse ZP1 is only 39% conserved with respect to human ZP1. Such differences in protein structure and glysosylation may explain the marked species differences in the biochemical, physicochemical and immunochemical properties of the ZP. Studies have now shown that the proteins of the ZP are expressed in a stage specific manner and that there is increasing evidence that ZP proteins are expressed by both granulosa cells and the oocyte and may play a role in granulosa cell differentiation.

Expression of a recombinant porcine zona pellucida glycoprotein ZP1 in mammalian cells

Porcine zona pellucida glycoprotein (pZP1) is a good candidate for a contraceptive vaccine. For the purpose of producing glycosylated pZP1, several types of recombinant pZP1 proteins were produced in mammalian cell lines. In the first experiment, a minigene encoding pZP1 (681 amino acids) was designed for insertion into an expression vector and then transfected to three cell lines (293T, CHO-K1, and LLC-PK1). The resulting recombinant proteins were highly glycosylated and were localized in the cytoplasm. To produce a secretory type of recombinant pZP1, in the second experiment, a cDNA coding for pZP1 excluding a putative transmembrane region and a smaller cDNA coding for 1-198 amino acid residues of pZP1 were designed to produce fusion proteins with the human IgG1 heavy chain. The resultant recombinant proteins were secreted into the supernatant from both transfected cell cultures. Recombinant secretory proteins are useful because of their simple affinity purification.

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.

Rat zona pellucida glycoproteins: molecular cloning and characterization of the three major components

The zona pellucida (ZP), the extracellular glycocalyx that surrounds the oocyte, is well known to mediate homologous gamete interaction. In a previous study from our laboratories, we reported the qualitative characterization of the rat ZP. The ZP in this species, like the mouse, hamster, and human, was found to contain three glycoproteins, namely rZP1, rZP2, and rZP3 (Araki et al. [1992] Biol Reprod 46:912-919). In this study, cDNAs encoding whole rat ZP major components have been isolated and characterized. A rat ovary cDNA library was screened with the mouse ZP3 and ZP2 cDNA probes, respectively. For rZP1 cDNA cloning, cDNAs generated using reverse transcriptase-polymerase chain reaction and rapid amplification of 5' and 3' cDNA ends, were isolated and sequenced. The rZP3 cDNA showed 1338 bp with a coding region containing 1272 bp, that translates into 424 amino acids. The total translation of rZP3 peptide has a molecular weight of 45,820, containing six potential N-glycosylation sites and 75 Ser/Thr residues, possible O-glycosylation sites. The amino acid sequence derived from the cDNA sequence shares high sequence homologies to mouse (90%), hamster (78%), and human (65%) ZP3 (ZPC) glycoproteins, indicating that the rat and mouse ZP3 have quite a conserved amino acid sequence, including the potential glycosylation sites. The total transcript of the rZP2 was 2154 nucleotides and the largest open reading frame was 695 amino acids. This would translate into a protein of 78.4 kDa. In the case of rZP1, the cDNA clone consisted of 1960 bp, and the coding region contained 1851 bp translating into 617 amino acids. Significant homologies were observed between rZP2 and ZPA family from various mammalian species. The rZP1 also showed a sequence homology to mouse ZP1, known as a mouse orthologue of ZPB family, suggesting that the rZP2 and rZP1 are members of ZPA and ZPB families, respectively. The message distributions for each zona components were limited within the ovary and the signal was detectable in the growing oocytes. The present results will further our understanding of the structure of rat zona components and lead to a better understanding of species-specificity observed during sperm-egg interaction.

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.

Hetero-oligomerization-dependent binding of pig oocyte zona pellucida glycoproteins ZPB and ZPC to boar sperm membrane vesicles

The zona pellucida surrounding the pig oocyte contains two Mr 55,000 glycoproteins, pZPB and pZPC, which are orthologues of mouse zona proteins ZP1 and ZP3, respectively. We previously reported that isolated boar sperm membrane vesicles possess high affinity binding sites for partially purified pZPB, but not pZPC. Interestingly, co-incubation experiments also implicated pZPB-pZPC complexes as potential ligands. We now report that when depleted of a minor pZPC contaminant by size exclusion chromatography, pZPB lacks independent binding activity. In solid phase binding assays employing immobilized boar sperm membranes, pZPB failed to compete with biotin-(pZPB+pZPC) probe, and biotin-labeled pZPB yielded negligible binding. However, when co-incubated with pZPC prior to the binding assays, pZPB acted as a potent competitor, and biotin-labeled pZPB exhibited high affinity, saturable binding. Binding activity was attributed to pZPB-pZPC heterocomplexes, which were detected in co-incubation mixtures by size exclusion chromatography and Western blot analysis. In the pig, therefore, sperm membranes possess a zona-binding protein with high affinity sites for pZPB-pZPC heterocomplexes, but not free glycoprotein subunits. Consequently, associative interactions between zona molecules can contribute toward both the assembly of the zona matrix and generation of ligands important for sperm-zona interactions.

Molecules involved in mammalian sperm-egg interaction

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

Linkage assignment of eleven genes to the porcine genome

We report comparative linkage mapping of eleven genes in the swine genome by RFLP analysis. These genes include: Acid phosphatase type 5 (ACP5), Cholecystokinin Type B Receptor (CCKBR), Antibiotic Peptide (FALL39), Insulin-like Growth Factor 1 Receptor (IGF1R), Integrin Alpha M (ITGAM), Integrin Beta 2 (ITGbeta2), Opioid Receptor Mu-1 (OPRM1), Pro-hormone Converter (PC1/3), Retinol Binding Protein 3 (RBP3), Ribosomal DNA (RNR1), and Zona Pellucida Glycoprotein 1 (ZP1). The CCKBR and ITGbeta2 loci define the ends of the linkage groups on Chromosomes (Chro) (SSC) 9p and 13qter, respectively.

Mouse Zp1 encodes a zona pellucida protein homologous to egg envelope proteins in mammals and fish

Zp1 encodes one of the three major glycoproteins of the zona pellucida, an extracellular matrix that surrounds growing oocytes, ovulated eggs, and preimplantation embryos. The mouse gene is composed of 12 exons ranging in size from 82 to 364 base pairs and spans 6.5 kilobase pairs on chromosome 19 (2.13 +/- 1.5 centimorgans distal to D19Bir1). The Zp1 exon map is similar to ZPB, a human orthologue, and an E-box (CANNTG), implicated in oocyte-specific gene expression of mouse Zp2 and Zp3, is similarly located upstream of the transcription start site. The single copy Zp1 gene encodes a 623-amino acid protein, the carboxyl-terminal half of which is significantly similar to a corresponding region of mouse ZP2. The conservation of this same region in a fish egg envelope protein suggests that not only has this protein domain been duplicated in mammals but that it has been conserved and used as an egg envelope protein in species that diverged 650 million years ago.