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

N-Glycosylation Site in the Middle Region Is Involved in the Sperm-Binding Activity of Bovine Zona Pellucida Glycoproteins ZP3 and ZP4

Mammalian fertilization is a species-selective event that involves a series of interactions between sperm proteins and the oocyte's zona pellucida (ZP) glycoproteins. Bovine ZP consists of three glycoproteins: bZP2, bZP3, and bZP4. In our previous study, we demonstrated that bovine sperm binds to plastic wells coated with recombinant bZP4 and identified that the N-terminal domain and the middle region of bZP4 are critical for sperm-binding activity. Here, we investigated the sperm-binding site in the middle region (residues 290 to 340) of bZP4, which includes the hinge region. We showed that bovine sperm binds to bZP4's middle region in a species-selective manner. We mapped the function of bZP4's middle region to its N-glycosylation site at Asn-314 using several recombinant mutated proteins. Moreover, we showed that mutations of the N-glycosylation sites at Asn-314 close to the hinge region and Asn-146 of the hinge region of bZP4 and bZP3, respectively, reduced the sperm-binding activity of the complex of the bZP3 (from 32 to 178) and bZP4 (from 136 to 464) fragments. Together, these results suggest that ZP's middle regions of bZP3 and bZP4 form one of the sperm-binding sites of bovine ZP.

Buffalo sperm membrane glycan-binding proteins reveal precise and preferential binding signatures with specific glycans targets on oviduct epithelium and zona pellucida-an implication in fertilization

Sperm membrane glycan-binding proteins (lectins) interact with the counterpart glycans in the oviduct, oocytes, and vice-versa. It has already been well known that specific glycans are present on oviductal epithelium and zona pellucida (ZP) in different mammalian species. Some of these glycans are necessary for oviductal sperm reservoir formation and gamete recognition. The specific binding phenomenon of lectin-glycans is one of the vital factors for successful fertilization in mammals. We hypothesized that buffalo sperm membrane glycan-binding proteins have specific glycan targets in the oviduct and ZP supporting the fertilization event. In the present investigation, sperm membrane proteins were extracted and assessed for their binding capacity with glycans using a high-throughput glycan microarray. The most promising glycan binding signals were evaluated to confirm the sperm putative receptors for glycan targets in the oviductal epithelial cells (OEC) and on ZP using an in-vitro competitive binding inhibition assay. Based on an array of 100 glycans, we found that N-acetyllactosamine (LacNAc), Lewis-a trisaccharide, 3'-sialyllactosamine and LacdiNAc were the most promising glycans and selected for further in-vitro validation. We established an inhibitory concentration of 12 mM Lewis-a trisaccharide and 10 μg/ml Lotus tetragonolobus (LTL) lectin for the sperm-OEC binding interaction, indicating its specificity and sensitivity. We observed that 3 mM 3'-sialyllactosamine, and LacdiNAc were the most competitive inhibitory concentration in sperm-ZP binding, suggesting a specific and abundance-dependent binding affinity. The competitive binding affinity of Maackia amurensis (MAA) lectin with Neu5Ac(α2-3)Gal(β1-4)GlcNAc further supports the abundance of 3'-sialyllactosamine on ZP responsible for sperm binding. Our findings develop the strong evidence on buffalo sperm putative receptors underlying their locking specificities with Lewis-a trisaccharide in oviduct and 3'-sialyllactosamine on ZP. The functional interaction of buffalo sperm lectins with the target glycans in OEC and ZP appears to be accomplished in an abundance-dependent manner, facilitating the fertilization event in buffaloes.

Identification of Sperm-Binding Sites in the N-Terminal Domain of Bovine Egg Coat Glycoprotein ZP4

The species-selective interaction between sperm and egg at the beginning of mammalian fertilisation is partly mediated by a transparent envelope called the zona pellucida (ZP). The ZP is composed of three or four glycoproteins (ZP1-ZP4). The functions of the three proteins present in mice (ZP1-ZP3) have been extensively studied. However, the biological role of ZP4, which was found in all other mammals studied so far, has remained largely unknown. Previously, by developing a solid support assay system, we showed that ZP4 exhibits sperm-binding activity in bovines and the N-terminal domain of bovine ZP4 (bZP4 ZP-N1 domain) is a sperm-binding region. Here, we show that bovine sperm bind to the bZP4 ZP-N1 domain in a species-selective manner and that N-glycosylation is not required for sperm-binding activity. Moreover, we identified three sites involved in sperm binding (site I: from Gln-41 to Pro-46, site II: from Leu-65 to Ser-68 and site III: from Thr-108 to Ile-123) in the bZP4 ZP-N1 domain using chimeric bovine/porcine and bovine/human ZP4 recombinant proteins. These results provide in vitro experimental evidence for the role of the bZP4 ZP-N1 domain in mediating sperm binding to the ZP.

Silkworm recombinant bovine zona pellucida protein 4 (bZP4) as a potential female immunocontraceptive antigen; impaired sperm-oocyte interaction and ovarian dysfunction

Porcine zona pellucida proteins (ZPs) have been utilized as female immunocontraceptive antigens. The purpose of this study was to explore the potential use of silkworm recombinant bovine ZP4 as an alternative. When the protein was injected with monophosphoryl lipid A (MPL) - an immuno-stimulative agent - into two female goats, marked elevation of the anti-ZP4 titer was detected. Application of the purified specific IgG to a porcine in vitro fertilization system reduced the sperm penetration rate. In one goat, the cyclic profile of serum progesterone disappeared as the anti-ZP4 titer increased. Histological examination of the ovaries revealed degeneration of antral follicles with sparse infiltration of inflammatory cells in the theca, indicating that autoimmune oophoritis had been induced. Together, the present results suggest that recombinant ZP4 disturbs fertilization and exerts a pathogenic effect on follicle development in goats, thus indicating its potential as a female immunocontraceptive antigen.

Sperm-binding regions on bovine egg zona pellucida glycoprotein ZP4 studied in a solid supported form on plastic plate

The zona pellucida (ZP) is a transparent envelope that surrounds the mammalian oocyte and mediates species-selective sperm-oocyte interactions. The bovine ZP consists of the glycoproteins ZP2, ZP3, and ZP4. Sperm-binding mechanisms of the bovine ZP are not yet fully elucidated. In a previous report, we established the expression system of bovine ZP glycoproteins using Sf9 insect cells and found that the ZP3/ZP4 heterocomplex inhibits the binding of sperm to the ZP in a competitive inhibition assay, while ZP2, ZP3, ZP4, the ZP2/ZP3 complex, and the ZP2/ZP4 complex do not exhibit this activity. Here, we show that bovine sperm binds to plastic plates coated with ZP4 in the absence of ZP3. We made a series of ZP4 deletion mutants to study the sperm-binding sites. The N-terminal region, Lys-25 to Asp-136, and the middle region, Ser-290 to Lys-340, of ZP4 exhibit sperm-binding activity. These results suggest that among the three components of bovine ZP glycoproteins, ZP4 contains the major potential sperm-binding sites, and the formation of a multivalent complex is necessary for the sperm-binding activity of ZP4.

Zp4 is completely dispensable for fertility in female rats†

Zona pellucida (ZP), which is composed of at most four extracellular glycoproteins (ZP1, ZP2, ZP3, and ZP4) in mammals, shelters the oocytes and is vital in female fertility. Several studies have identified the indispensable roles of ZP1-3 in maintaining normal female fertility. However, the understanding of ZP4 is still very poor because only one study on ZP4-associated infertility performed in rabbits has been reported up to date. Here we investigated the function of mammalian Zp4 by creating a knockout (KO) rat strain (Zp4-/- rat) using CRISPR-Cas9-mediated DNA-editing method. The influence of Zp4 KO on ZP morphology and some pivotal processes of reproduction, including oogenesis, ovulation, fertilization, and pup production, were studied using periodic acid-Schiff's staining, superovulation, in vitro fertilization, and natural mating. The ZP morphology in Zp4-/- rats was normal, and none of these pivotal processes was affected. This study renewed the knowledge of mammalian Zp4 by suggesting that Zp4 was completely dispensable for female fertility.

Human Zona Pellucida Glycoproteins: Binding Characteristics With Human Spermatozoa and Induction of Acrosome Reaction

Human zona pellucida (ZP) matrix is composed of four glycoproteins designated as ZP glycoprotein -1 (ZP1), -2 (ZP2), -3 (ZP3), and -4 (ZP4). Mutations in the genes encoding human ZP glycoproteins are one of the causative factors leading to abnormal ZP matrix and infertility in women. Relevance of the human ZP glycoproteins in 'sperm-oocyte' binding has been delineated by using either transgenic animal models expressing human zona proteins or purified native/recombinant human zona proteins. Studies based on the purified native/recombinant human zona proteins revealed that ZP1, ZP3, and ZP4 primarily bind to the capacitated acrosome-intact human spermatozoa whereas ZP2 binds to acrosome-reacted spermatozoa. On the contrary, human spermatozoa binds to the eggs obtained from transgenic mouse lines expressing human ZP2 but not to those expressing human ZP1, ZP3, and ZP4 suggesting that ZP2 has an important role in human 'sperm-oocyte' binding. Further studies using transgenic mouse lines showed that the N-terminus of human ZP2 mediate the taxon-specific human sperm-oocyte binding. Both glycans and protein-protein interactions have a role in human gamete interaction. Further studies have revealed that the purified native/recombinant human ZP1, ZP3, and ZP4 are competent to induce acrosome reaction. Human sperm binds to the mouse transgenic eggs expressing human ZP1-4 instead of mouse ZP1-3 proteins, penetrated the ZP matrix and accumulated in the perivitelline space, which were acrosome-reacted suggesting that human ZP2 in transgenic mouse model also induce acrosome reaction. In humans N-linked glycosylation of zona proteins have been shown to play an important role in induction of the acrosome reaction. Hence in humans, based on studies using transgenic mouse model as well as purified native/recombinant zona proteins, it is likely that more than one zona protein is involved in the 'sperm-oocyte' binding and induction of the acrosome reaction.

Ligands and Receptors Involved in the Sperm-Zona Pellucida Interactions in Mammals

Sperm-zona pellucida (ZP) interaction, involving the binding of sperm surface ligands to complementary carbohydrates of ZP, is the first direct gamete contact event crucial for subsequent gamete fusion and successful fertilization in mammals. It is a complex process mediated by the coordinated engagement of multiple ZP receptors forming high-molecular-weight (HMW) protein complexes at the acrosomal region of the sperm surface. The present article aims to review the current understanding of sperm-ZP binding in the four most studied mammalian models, i.e., murine, porcine, bovine, and human, and summarizes the candidate ZP receptors with established ZP affinity, including their origins and the mechanisms of ZP binding. Further, it compares and contrasts the ZP structure and carbohydrate composition in the aforementioned model organisms. The comprehensive understanding of sperm-ZP interaction mechanisms is critical for the diagnosis of infertility and thus becomes an integral part of assisted reproductive therapies/technologies.

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

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

ZP4 confers structural properties to the zona pellucida essential for embryo development

Zona pellucida (ZP), the extracellular matrix sheltering mammalian oocytes and embryos, is composed by 3 to 4 proteins. The roles of the three proteins present in mice have been elucidated by KO models, but the function of the fourth component (ZP4), present in all other eutherian mammals studied so far, has remained elusive. Herein, we report that ZP4 ablation impairs fertility in female rabbits. Ovulation, fertilization and in vitro development to blastocyst were not affected by ZP4 ablation. However, in vivo development is severely impaired in embryos covered by a ZP4-devoided zona, suggesting a defective ZP protective capacity in the absence of ZP4. ZP4-null ZP was significantly thinner, more permeable, and exhibited a more disorganized and fenestrated structure. The evolutionary conservation of ZP4 in other mammals, including humans, suggests that the structural properties conferred by this protein are required to ensure proper embryo sheltering during in vivo preimplantation development.

The egg cells of mammals, called oocytes, are encased in a protective layer called the zona pellucida. This layer is made from proteins called ZP1 to 4. Most studies of the zona pellucida use mice, which do not have ZP4. This means that the research community have limited knowledge of what ZP4 does in humans and other mammals.

Scientists can now use a technique called CRISPR to selectively modify the genetics of living things to help us to understand what specific genes and proteins do. The ZP4 protein can be eliminated from rabbit oocytes using CRISPR to help understand its role in egg cell fertilization and development.

Lamas-Toranzo et al. examined the effect of losing ZP4 from rabbit oocytes. Without ZP4 the zona pellucida becomes thinner, irregular and more flexible. However, the loss of ZP4 did not affect ovulation (i.e. the release of egg cells from an ovary), fertilization, or the early stages of development of embryos when studied in the laboratory. However, rabbits without ZP4 were much less fertile. Indeed, only one out of 10 female rabbits without ZP4 was able to deliver pups because in most cases the development of embryos in the womb failed.

These findings show that ZP4 has a structural role in the zona pellucida. Without ZP4 fertility is reduced. This work lays the ground for further investigation of the role of ZP4. It could also offer new insights into the causes of infertility.

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.

The Hinge Region of Bovine Zona Pellucida Glycoprotein ZP3 Is Involved in the Formation of the Sperm-Binding Active ZP3/ZP4 Complex

The zona pellucida (ZP) surrounds the mammalian oocyte and mediates species-selective sperm-oocyte interactions. Bovine ZP consists of glycoproteins ZP2, ZP3, and ZP4. Neither ZP3 nor ZP4 alone shows inhibitory activity for the binding of sperm to the ZP; however, this activity is seen with the ZP3/ZP4 heterocomplex. Here, we constructed a series of bovine ZP3 mutants to identify the ZP4-binding site on ZP3. Each ZP3 mutant was co-expressed with ZP4 using a baculovirus-Sf9 cell expression system and examined for interaction with ZP4 as well as inhibitory activity for sperm-ZP binding. N-terminal fragment Arg-32 to Arg-160 of ZP3 interacted with ZP4 and inhibited sperm-ZP binding, whereas fragment Arg-32 to Thr-155 showed much weaker interaction with ZP4. Mutation of N-glycosylated Asn-146 to Asp in the N-terminal fragment Arg-32 to Glu-178 of ZP3 did not interrupt the interaction of this fragment with ZP4, but it did reduce the inhibitory activity of the complex for sperm-ZP binding. In contrast, mutation of N-glycosylated Asn-124 to Asp did not significantly reduce the activity. Taken together, these results suggest that one of the ZP4 binding sites exists in the flexible hinge region of ZP3 and that the N-glycosylation in this region is involved in the sperm binding.

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.

Posttranslational modifications of zona pellucida proteins

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

Binding of Sperm to the Zona Pellucida Mediated by Sperm Carbohydrate-Binding Proteins is not Species-Specific in Vitro between Pigs and Cattle

Carbohydrates are candidates for the basis of species-selective interaction of gametes during mammalian fertilization. In this study, we sought to clarify the roles of sugar residues in the species-selective, sperm-oocyte interaction in pigs and cattle. Acrosome-intact porcine and bovine sperm exhibited their strongest binding affinities for β-Gal and α-Man residues, respectively. Porcine-sperm specificity changed from β-Gal to α-Man after the acrosome reaction, while bovine-sperm specificity did not. Binding of acrosome-intact and acrosome-reacted sperm decreased after trypsinization, indicating that the carbohydrate-binding components are proteins. While immature oocytes bound homologous sperm preferentially to heterologous sperm, oocytes matured in vitro bound similar numbers of homologous and heterologous sperm. Lectin staining revealed the aggregation of α-Man residues on the outer surface of the porcine zona during maturation. In both species, zona-free, mature oocytes bound homologous sperm preferentially to heterologous sperm. The lectin-staining patterns of the zona pellucida and zona-free oocytes coincided with the carbohydrate-binding specificities of acrosome-intact and acrosome-reacted sperm, respectively, supporting the involvement of carbohydrates in gamete recognition in pigs and cattle. These results also indicate that sperm-zona pellucida and sperm-oolemma bindings are not strictly species-specific in pigs and cattle, and further suggest that sperm penetration into the zona and/or fusion with oolemma may be species-specific between pigs and cattle.

Rabbit zona pellucida composition: a molecular, proteomic and phylogenetic approach

The zona pellucida (ZP) participates in sperm-egg interactions during the first steps of fertilization. Recent studies have shown that the ZP matrix of oocytes in several species is composed of four glycoproteins, designated as ZP1, ZP2, ZP3 and ZP4, rather than the three described in mouse, pig and cow. In this study, investigations were carried out to unveil a fourth glycoprotein in the rabbit (Oryctolagus cuniculus) ZP. Using total RNA isolated from rabbit ovaries, the complementary deoxyribonucleic acid (cDNA) encoding rabbit ZP1 was amplified by reverse transcribed polymerase chain reaction (RT-PCR). The ZP1 cDNA contains an open reading frame of 1825 nucleotides encoding a polypeptide of 608 amino acid residues. The deduced amino acid sequence of rabbit ZP1 showed high identity with other species: 70% identity with human and horse ZP1, and 67% identity with mouse and rat ZP1. At the proteomic level, peptides corresponding to the four proteins were detected by mass spectrometry. In addition, a molecular phylogenetic analysis of ZP1 showed that pseudogenization of this gene has occurred at least four times during the evolution of mammals. The data presented in this manuscript provide evidence, for the first time, that the rabbit ZP is composed of four glycoproteins.

Porcine zona pellucida glycoprotein ZP4 is responsible for the sperm-binding activity of the ZP3/ZP4 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 consist of glycoproteins ZP2, ZP3, and ZP4. In both pig and bovine a heterocomplex consisting of ZP3 and ZP4 binds to sperm, however it is not clarified whether ZP3 or ZP4 in the complex is responsible for the sperm binding. Previously, we have established a baculovirus-Sf9 cell expression system for porcine ZP glycoproteins. A mixture of recombinant ZP3 (rZP3) and rZP4 displayed sperm-binding activity toward bovine sperm but not porcine sperm, probably due to differences in carbohydrate structure between the native and recombinant ZP glycoproteins. In this study, a mixture of porcine rZP3 and native ZP4 (nZP4) inhibited the binding of porcine sperm to the ZP. In contrast, a mixture of porcine nZP3 and rZP4 did not inhibit the binding of porcine sperm, although the mixture inhibited the binding of bovine sperm. The porcine rZP3/nZP4 mixture bound to the acrosomal region of porcine sperm, in a manner similar to that of the nZP3/nZP4 mixture. nZP3 was precipitated with rZP4, and nZP4 was precipitated with rZP3 by utilising the N-terminal tags on the recombinant proteins. These results indicated that nZP4, but not rZP4, is necessary for binding activity of porcine ZP3/ZP4 complex towards porcine sperm and further suggested that the carbohydrate structures of ZP4 in the porcine ZP3/ZP4 complex are responsible for porcine sperm-binding activity of the complex.

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.

Human ZP4 is not sufficient for taxon-specific sperm recognition of the zona pellucida in transgenic mice

The molecular basis of human fertilization remains enigmatic. Mouse models are often used to study sperm-egg recognition, but the mouse zona pellucida surrounding ovulated eggs contains three proteins (ZP1, ZP2, and ZP3) whereas the human zona contains four (ZP1, ZP2, ZP3, and ZP4). Human sperm are fastidious and recognize human but not mouse eggs. Transgenic mouse lines were established to ascertain whether human ZP4 is the sole determinant of human sperm binding. Human ZP4 expressed in transgenic mice had a molecular mass similar to the range of native protein isoforms and was incorporated into the extracellular zona matrix. Transgenic females were fertile with normal litter sizes. Mouse sperm readily recognized transgenic ovulated eggs, but human sperm did not. We conclude that human ZP4 is not sufficient to support human sperm binding to the zona pellucida in transgenic mice and that other zona proteins may be needed for human gamete recognition.