Biosynthesis and expression of zona pellucida glycoproteins in mammals

Making a good egg: human oocyte health, aging, and in vitro development

Mammalian eggs (oocytes) are formed during fetal life and establish associations with somatic cells to form primordial follicles that create a store of germ cells (the primordial pool). The size of this pool is influenced by key events during the formation of germ cells and by factors that influence the subsequent activation of follicle growth. These regulatory pathways must ensure that the reserve of oocytes within primordial follicles in humans lasts for up to 50 years, yet only approximately 0.1% will ever be ovulated with the rest undergoing degeneration. This review outlines the mechanisms and regulatory pathways that govern the processes of oocyte and follicle formation and later growth, within the ovarian stroma, through to ovulation with particular reference to human oocytes/follicles. In addition, the effects of aging on female reproductive capacity through changes in oocyte number and quality are emphasized, with both the cellular mechanisms and clinical implications discussed. Finally, the details of current developments in culture systems that support all stages of follicle growth to generate mature oocytes in vitro and emerging prospects for making new oocytes from stem cells are outlined.

Nurturing the egg: the essential connection between cumulus cells and the oocyte

The determinants of oocyte quality remain uncertain. Under suitable conditions, which have yet to be defined, the gamete grows and acquires the competence to resume meiosis, be fertilised and undergo embryonic development at least beyond genome activation, after which the blastomere is autonomous enough to adapt to the specificity of its environment. This review describes the central role played by the oocyte in reproductive success and how communication between cumulus cells and the oocyte are essential to proper oogenesis and the quality of the resulting gamete. While most attempts to improve oocyte quality have been directed at gonadotrophin-based systemic endocrine signalling, it is proposed that parallel control of fertility may act locally within ovarian follicles through intimate cooperation between somatic cells and the oocyte via the network of transzonal projections. This intercellular communication may prove to be more sensitive to environmental conditions than systemic endocrine signalling, which is essential for many non-reproductive tissues.

New Insights into the Mammalian Egg Zona Pellucida

Mammalian oocytes are surrounded by an extracellular coat called the zona pellucida (ZP), which, from an evolutionary point of view, is the most ancient of the coats that envelope vertebrate oocytes and conceptuses. This matrix separates the oocyte from cumulus cells and is responsible for species-specific recognition between gametes, preventing polyspermy and protecting the preimplantation embryo. The ZP is a dynamic structure that shows different properties before and after fertilization. Until very recently, mammalian ZP was believed to be composed of only three glycoproteins, ZP1, ZP2 and ZP3, as first described in mouse. However, studies have revealed that this composition is not necessarily applicable to other mammals. Such differences can be explained by an analysis of the molecular evolution of the ZP gene family, during which ZP genes have suffered pseudogenization and duplication events that have resulted in differing models of ZP protein composition. The many discoveries made in recent years related to ZP composition and evolution suggest that a compilation would be useful. Moreover, this review analyses ZP biosynthesis, the role of each ZP protein in different mammalian species and how these proteins may interact among themselves and with other proteins present in the oviductal lumen.

TRITC-Loaded PLGA Nanoparticles as Drug Delivery Carriers in Mouse Oocytes and Embryos

The structural layers around oocytes make it difficult to deliver drugs aimed at treating infertility. In this study, we sought to identify nanoparticles (NPs) that could easily pass through zona pellucida (ZP), a special layer around oocytes, for use as a drug delivery carrier. Three types of NPs were tested: quantum dot NPs, PE-polyethylene glycol (PEG)-loaded poly(lactic-co-glycolic acid) (PLGA) NPs (PEG/PL), and tetramethylrhodamine-loaded PLGA NPs (TRNPs). When mouse oocytes were treated with NPs, only TRNPs could fully pass through the ZP and cell membrane. To assess the effects of TRNPs on fertility and potential nanotoxicity, we performed mRNA sequencing analysis to confirm their genetic safety. We established a system to successfully internalize TRNPs into oocytes. The genetic stability and normal development of TRNP-treated oocytes and embryos were confirmed. These results imply that TRNPs can be used as a drug delivery carrier applicable to germ cells.

Type 1 diabetes affects zona pellucida and genome methylation in oocytes and granulosa cells

Diabetes affects oocyte nuclear and cytoplasmic quality. In this study, we generated a type 1 diabetes (T1D) mouse model by STZ injection to study the effects of T1D on zona pellucida and genomic DNA methylation of oocytes and granulosa cells. T1D mice showed fewer ovulated oocytes, reduced ovarian reserve, disrupted estrus cycle, and significantly ruptured zona pellucida in 2-cell in vivo embryos compared to controls. Notably, diabetic oocytes displayed thinner zona pellucida and treatment of oocytes with high concentration glucose reduced the zona pellucida thickness. Differential methylation genes in oocytes and granulosa cells were analyzed by methylation sequencing. These genes were significantly enriched in GO terms by GO analysis, and these GO terms were involved in multiple aspects of growth and development. Most notably, the abnormal methylation genes in oocytes may be related to oocyte zona pellucida changes in diabetic mice. These findings provide novel basic data for further understanding and elucidating dysgenesis and epigenetic changes in type 1 diabetes mellitus.

Technologies for the Production of Fertilizable Mammalian Oocytes

Women affected by ovarian pathologies or with cancer can usually preserve fertility by egg/embryo freezing. When oocyte retrieval is not feasible, the only option available is ovarian tissue cryopreservation and transplantation. The culture of follicles isolated from fresh or cryopreserved ovaries is considered still experimental, although this procedure is considered safer, because the risk of unintentional spreading of cancer cells eventually present in cryopreserved tissue is avoided. Animal and human small follicles can be cultured in vitro, but standardized protocols able to produce in vitro grown oocytes with the same developmental capacity of in vivo grown oocytes are not available yet. In fact, the different sizes of follicles and oocytes, the hormonal differences existing between mono- (e.g., human, goat, cow, and sheep) and poly-ovulatory (rodents and pig) species, and the incomplete identification of the mechanisms regulating the oocyte–follicle and follicle–ovary interrelationships affect the outcome of in vitro culture. From all these attempts, however, new ideas arise, and the goal of assuring the preservation of female reproductive potential appears a more realistic possibility. This review surveys and discusses advances and challenges of these technologies that, starting from a simple attempt, are now approaching the biosynthesis of a functional engineered ovary.

Oocyte stem cells: fact or fantasy?

For many decades, the dogma prevailed that female mammals had a finite pool of oocytes at birth and this was gradually exhausted during a lifetime of reproductive function. However, in 2004, a new era began in the field of female oogenesis. A study was published that appeared to detect oocyte-stem cells capable of generating new eggs within mouse ovaries. This study was highly controversial and the years since this initial finding have produced extensive research and even more extensive debate into their possibility. Unequivocal evidence testifying to the existence of oocyte-stem cells (OSCs) has yet to be produced, meanwhile the spectrum of views from both sides of the debate are wide-ranging and surprisingly passionate. Although recent studies have presented some convincing results that germ cells exist and are capable of creating new oocytes, many questions remain. Are these cells present in humans? Do they exist in physiological conditions in a dormant state? This comprehensive review first examines where and how the dogma of a finite pool was established, how this has been challenged over the years and addresses the most pertinent questions as to the current status of their existence, their role in female fertility, and perhaps most importantly, if they do exist, how can we harness these cells to improve a woman's oocyte reserve and treat conditions such as premature ovarian insufficiency (POI: also known as premature ovarian failure, POF).

The scavenging capacity of DMBT1 is impaired by germline deletions

The Scavenger Receptor Cysteine-Rich (SRCR) proteins are an archaic group of proteins characterized by the presence of multiple SRCR domains. They are membrane-bound or secreted proteins, which are generally related to host defense systems in animals. Deleted in Malignant Brain Tumors 1 (DMBT1) is a SRCR protein which is secreted in mucosal fluids and involved in host defense by pathogen binding by its SRCR domains. Genetic polymorphism within DMBT1 leads to DMBT1-alleles giving rise to polypeptides with interindividually different numbers of SRCR domains, ranging from 8 SRCR domains (encoded by 6 kb DMBT1 variant) to 13 SRCR domains (encoded by the 8 kb DMBT1 variant). In the present study, we have investigated whether reduction from 13 to 8 amino-terminal SRCR domains leads to reduction of bacterial binding. The 6 kb variant bound ~20–45% less bacteria compared to the 8 kb variant. These results support the hypothesis that genetic variation in DMBT1 may influence microbial defense.

Immunohistochemical Detection of Salivary Agglutinin/gp-340 in Human Parotid, Submandibular, and Labial Salivary Glands

Salivary agglutinin is a Streptococcus mutans binding protein and a member of the scavenger receptor cysteine-rich superfamily. It is identical to lung gp-340 and brain DMBT1, which possibly play a role in innate immunity and tumor suppression, respectively. The goal of this study was to localize salivary agglutinin in human salivary glands. Two monoclonal antibodies, directed against gp-340, were characterized. mAb 213-1 reacted with sialic acid epitopes and cross-reacted with MUC7. The reaction with mAb 213-6 disappeared after reduction, suggesting that a protein epitope was recognized. In the parotid gland, immunohistochemical labeling with mAb 213-6 was found in the duct cells. In the submandibular gland and labial gland, both serous acini and demilune cells were labeled. In the labial gland, labeling was found at the luminal side of the duct cells. Salivary agglutinin was distinctly localized in salivary glands, but in distinct glandular secretions, no differences in electrophoretic behavior were observed.

The fertilization ability and developmental competence of bovine oocytes grown in vitro

In vitro growth culture systems for oocytes are being developed in several mammalian species. In these growth culture systems, in vitro grown oocytes usually have lower blastocyst formation than in vivo grown oocytes after in vitro fertilization. Furthermore, there have been a few reports that investigated the fertilization ability of in vitro grown oocytes in large animals. The purpose of this study was to investigate the fertilization process and developmental competence of bovine oocytes grown in vitro. Oocyte-granulosa cell complexes collected from bovine early antral follicles (0.4−0.7 mm in diameter) were cultured for growth with 17β-estradiol and androstenedione for 14 days and matured in vitro. These oocytes were then inseminated for 6 or 12 h, and further cultured for development up to 8 days in vitro. After growth culture, oocytes grew from 95 µm to around 120 µm and acquired maturation competence (79%). Although fertilization rates of in vitro grown oocytes were low after 6 h of insemination, 34% of in vitro grown oocytes fertilized normally after 12 h of insemination, having two polar bodies and two pronuclei with a sperm tail, and 22% of these oocytes developed into blastocysts after 8 days of culture. The fertilization and blastocyst formation rates were similar to those of in vivo grown oocytes. In addition, blastocyst cell numbers were also similar between in vitro and in vivo grown oocytes. In conclusion, in vitro grown bovine oocytes are similar to in vivo grown oocytes in fertilization ability and can develop into blastocysts.

Developmental competence of equine oocytes: impacts of zona pellucida birefringence and maternally derived transcript expression

In the present study, equine oocytes were classified into groups of presumably high and low developmental competence according to cumulus morphology, as well as oocyte ability to metabolise brilliant cresyl blue (BCB) stain. All oocytes were evaluated individually in terms of morphometry, zona pellucida birefringence (ZPB) and relative abundance of selected candidate genes. Oocytes with an expanded cumulus (Ex), representing those with presumably high developmental competence, had a significantly thicker zona (18.2 vs 17.3µm) and a significantly higher ZPB (64.6 vs 62.1) than oocytes with a compacted cumulus (Cp). Concurrently, oocytes classified as highly developmentally competent (BCB+) had a significantly thicker zona (18.8 vs 16.1µm) and significantly higher ZPB (63.1 vs 61.3) compared with oocytes classified as having low developmental competence. Expression of TFAM, STAT3 and CKS2 was significantly higher in Ex compared with Cp oocytes, whereas expression of COX1, ATPV6E and DNMT1 was lower. Together, the data reveal that developmentally competent equine oocytes are larger in size, have higher ZPB values and exhibit a typical genetic signature of maternally derived transcripts compared with oocytes with lower in vitro developmental competence.

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.

Biosynthesis of hamster zona pellucida is restricted to the oocyte

The zona pellucida (ZP) is an extracellular coat that surrounds the mammalian oocyte and the early embryo until implantation. This coat mediates several critical aspects of fertilization, including species-selective sperm recognition, the blocking of polyspermy and protection of the oocyte and the preimplantation embryo. Depending on the species, the ZP is composed of three to four different glycoproteins encoded by three or four genes. These genes have been cloned and sequenced for different species. However, controversy exists about the cell type specificity of the ZP glycoproteins, for which several models have been proposed. Different groups have reported that ZP is produced only by the oocytes, by the granulosa cells or by both cell types, depending on the species under study. We recently described the expression of four ZP proteins in the hamster ovary. By means of the complete set of the hamster ZP cDNAs, we undertook the study of the origin and expression pattern of the four ZP genes. In the present work, the expression of ZP1, ZP2, ZP3 and ZP4 is carefully analyzed by in situ hybridization (ISH) in hamster ovaries. Our data suggest that the four hamster ZP genes are expressed in a coordinate and oocyte-specific manner during folliculogenesis. Furthermore, this expression is maximal during the first stages of the oocyte development and declines in oocytes from later development stages, particularly within large antral follicles.

High doses of medroxyprogesterone as the cause of disappearance of adherence of the zona pellucida to an oocyte

The zona pellucida (ZP) is an external glycoprotein membrane of oocytes of mammals and embryos in the early stage of their development. ZP first appears in growing ovarian follicles as an extracellular substance between the oocyte and granular cells. The zona pellucid markedly affects the development and maturation of the oocyte. The morphology of the ZP-oocyte complex allows a more precise determination of the oocyte maturity. According to numerous experimental studies, ZP is essential for preimplantation embryonic development of humans and other mammals. It prevents dispersion of blastomeres and enhances their mutual interactions. ZP is a dynamic structure responsible for the provision of nutrients to early forms of oocytes in mammals. The aim of the present study was untrastructural evaluation of the ZP-oocyte contact during inhibited ovulation. Female white rats (Wistar strain) received a suspension of medroxyprogesterone acetate (MPA) in incremental intramuscular bolus doses of 3.7 mg (therapeutic dose), 7.4 mg and 11.1 mg. The animals were decapitated 5 days after the administration of MPA. Ovarian sections were evaluated under a transmission electron microscope (TEM) Zeiss EM 900. Morphometric analysis of ZP was conducted using the cell imaging system by Olympus. In females exposed to therapeutic doses of MPA, ZP showed the structure of granular-fibrous reticulum of a medium electron density with single cytoplasmic processes originating from the surrounding structures. The oocyte cell membrane generated single, delicate processes directed toward ZP. Microvilli of the oocyte were short and thin. In the group receiving 7.4 mg of MPA, ZP had the structure of a delicate, loose granular-fibrous reticulum, and the oocyte cell membrane generated single microvilli directed toward ZP. In both those groups, the close ZP-oocyte contact was observed. Otherwise, in the group exposed to the highest MPA doses (11.1 mg), thicker and more numerous oocyte microvilli were found, which did not penetrate ZP matrix. They were dense, irregularly separated contour, forming a barrier between ZP and oocyte. The present findings are likely to suggest that MPA has inhibiting effects on the synthesis of binding proteins and causes the loss of the oocyte contact with ZP.

The morphology of porcine oocytes is associated with zona pellucida glycoprotein transcript contents

We hypothesized that oocyte morphology may be associated with the accumulation of specific mRNAs encoding proteins responsible for the gamete fertilization ability. Therefore, the aim of the study was to evaluate the transcript levels of porcine zona pellucida (pZP1, pZP2, pZP3 and pZP4) glycoproteins in oocytes classified by a four-grade morphological scale (I-IV) accounting for either a homogeneous cytoplasm and a complete cumulus oophorus (grade I) or a heterogenous cytoplasm and decreased number of cumulus layers in the other grades (II, III and IV). We observed a significant increase of all investigated pZP glycoprotein mRNAs in grade I oocytes as compared to other grades (p<0.05). Our observations suggest that porcine oocyte morphology is associated with pZP transcript contents and may be related to an increased fertilization ability of higher quality oocytes.

Molecular cloning and characterization of three distinct choriogenins in masu salmon, Oncorhynchus masou

Three cDNAs, each encoding a different choriogenin (Chg), were isolated from a female masu salmon (Oncorhynchus masou) liver cDNA library. Two of the cDNA clones, Chg Halpha and Chg Hbeta, showed a close relationship and contained the typical domains of zona pellucida (ZP) B genes in fish, namely proline and glutamine rich repeats, a trefoil factor family domain, and a ZP domain. Specific antibodies against recombinant Chg H products (rmHalpha and rmHbeta) were generated to elucidate the relationship between the Chg H cDNAs and two types of serum Chg H protein, which were previously purified and characterized, and designated as very-high-molecular-weight vitelline envelope-related protein (vhVERP) and Chg H of masu salmon. The immunobiochemical analyses revealed that the Chg Halpha and Chg Hbeta clones encoded vhVERP and Chg H proteins, respectively. The third cDNA clone (Chg L) appeared to be a ZPC gene and, by mapping the N-terminal sequence of purified Chg L, was shown to encode serum Chg L protein. Various types of heteromultimer of the three Chgs were identified immunologically as high molecular weight chorion components, indicating the involvement of complex heterodimerization of multiple Chgs in the construction of chorion architecture in masu salmon.

Equine zona protein synthesis and ZP structure during folliculogenesis, oocyte maturation, and embryogenesis

In the equine, the zona pellucida (ZP) is the major barrier to successful in vitro fertilization. Therefore the aim of our studies was to analyze species-specific features of the equine ZP in regard to structure and glycoprotein ZPB and ZPC expression sites during oocyte development and embryogenesis. The equine ZP revealed high immunological cross-reactivity to porcine ZPB and ZPC. In the ovary, the distribution of ZPB and ZPC was co-localized and correlated with the developmental stage of the follicle. ZPB and ZPC expression started in the oocyte of the late primordial and primary follicle. In the secondary follicle, both the oocyte and the cumulus cells contributed to ZPB and ZPC synthesis. After in vivo maturation the oocyte stopped ZPB and ZPC production whereas the cumulus cells continued synthesis. Contrary, in vitro matured (IVM) cumulus-oocyte-complexes (COCs) revealed a reverse expression pattern. This was correlated to alterations in the distribution, number, and size of pores in the ZP. In the zona, N-acetylglucosamine residues were co-localized with ZPC. The acellular glycoprotein capsule surrounding early equine embryos was negative for ZPB and ZPC. Our results imply that in the horse ZPB and ZPC glycoprotein expression is differentially regulated during folliculogenesis, oocyte maturation, and embryogenesis. Contrary to the bovine and porcine, zona protein synthesis during in vivo maturation is completely overtaken by the cumulus cells implying that in the horse these cells are crucial for zona integrity. During IVM, the cumulus cells lose their ability to synthesize glycoproteins leading to alterations in the zona structure.

The role of molecular chaperones in mouse sperm-egg interactions

Fertilization is a unique and exquisitely choreographed cellular interaction between the male and female gamete that results in the creation of a genetically unique individual. Despite the fundamental importance of fertilization, there remains a dearth of information about the basic biochemical mechanisms that underpin this process. One of the key issues that remain unresolved is the molecular basis of sperm-egg recognition. From the female perspective, it is well established that the sperm recognition sites reside in the zona pellucida (ZP), an acellular coat that surrounds the oocyte. In contrast, numerous studies into the cognate zona receptors residing on the sperm surface have failed to shed significant light on the biochemical identity of these molecules. Such difficulties may, in part, have arisen because investigations have traditionally been based on the precept that the zona receptor represents a single molecular entity that is constitutively expressed on the sperm surface. While such a view holds obvious appeal, it fails to account for growing evidence that gamete interaction is not mediated by a simple lock-and-key mechanism. In this review, we present a novel hypothesis in which the zona recognition site is portrayed as a multimeric molecular structure that is assembled into a functional complex during a maturation process known as 'capacitation'. Furthermore, we consider the possibility that this previously cryptic complex is assembled and delivered to the outer surface of the sperm plasma membrane through the concerted action of several members of the molecular chaperone family of proteins.

Association between sequence variations in genes encoding human zona pellucida glycoproteins and fertilization failure in IVF

BACKGROUND

The zona pellucida (ZP) has multiple roles in reproductive processes, including oocyte maturation, fertilization and implantation. We used, for the first time, a genetic approach to study whether human ZP genes possess structural alterations in women with unsuccessful IVF trials. In theory, this may result in gradual reduction of sperm-zona interaction and eventually in total fertilization failure (TFF).

METHODS

Eighteen infertile women (TFFs) whose IVF did not result in any fertilized oocytes, whereas fertilization by ICSI was successful, were screened for mutations in ZP genes by means of conformation-sensitive gel electrophoresis. Twenty-three fertilizers in IVF (FIVFs) and 68 women with proven fertility (WPFs) constituted the two control groups.

RESULTS

Altogether, 20 sequence variations were found in the ZP genes. Two variations in ZP3, one in the regulatory region (c. 1-87 T --> G) and one in exon 6 [c. 894 G --> A (p. K298)] existed more frequently in TFFs than in FIVF and WPF groups (P-values 0.027 and 0.008, respectively).

CONCLUSIONS

Our study on ZP genes of infertile women revealed a high degree of sequence variations. This may reflect gradual reduction of fertility among TFFs, but the putative roles and influences of single variations can only be hypothesized.

Calcium Channels and Ca2+ Fluctuations in Sperm Physiology

Generating new life in animals by sexual reproduction depends on adequate communication between mature and competent male and female gametes. Ion channels are instrumental in the dialogue between sperm, its environment, and the egg. The ability of sperm to swim to the egg and fertilize it is modulated by ion permeability changes induced by environmental cues and components of the egg outer layer. Ca(2+) is probably the key messenger in this information exchange. It is therefore not surprising that different Ca(2+)-permeable channels are distinctly localized in these tiny specialized cells. New approaches to measure sperm currents, intracellular Ca(2+), membrane potential, and intracellular pH with fluorescent probes, patch-clamp recordings, sequence information, and heterologous expression are revealing how sperm channels participate in fertilization. Certain sperm ion channels are turning out to be unique, making them attractive targets for contraception and for the discovery of novel signaling complexes.

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.

Biosynthesis of the Canine Zona Pellucida Requires the Integrated Participation of Both Oocytes and Granulosa Cells1

In the dog, attempts to localize the expression of zona pellucida (ZP) proteins during folliculogenesis have failed to demonstrate conclusively whether any or all of the zona proteins are synthesized in the oocyte or the granulosa cells. Probing of paraformaldehyde-fixed prepubertal canine ovarian tissue sections with a panel of fluorescently conjugated lectins localized the expression of glycoproteins during folliculogenesis. We confirm that six lectins (PSA, s-WGA, ECL, GSL-II, LEL, and STL) consistently labeled the ZP and adjacent granulosa cells of the developing follicle and that canine ZP expresses beta-gal(1,4)glcNAc, beta-gal(1,3)galNac, alpha-mannose, and terminal sialic acid residues in a developmentally specific manner. Riboprobes for canine ZPA and ZPC genes were produced and used for in situ hybridization studies of mRNA expression in canine folliculogenesis. In addition, we isolated a partial cDNA transcript from total ovarian RNA for the canine ZPB gene having a high degree of sequence identity with the felid and porcine ZPB homologues. Subsequently, the ZPA gene transcripts were localized to the cytoplasm of oocytes in primordial, primary, and early secondary follicles. We then localized expression of ZPB and ZPC gene transcripts to the granulosa cells of growing follicles, but not in squamous granulosa cells of primordial follicles or oocytes. These observations indicate that in the juvenile canine ovary, the oocyte is responsible for synthesis of the ZPA protein and directing synthesis of the ZPB and ZPC proteins by the granulosa cells and that ZP gene transcription occurs in a sequential manner during folliculogenesis.

On the species specificity of sperm binding and sperm penetration of the zona pellucida

Sperm binding and sperm penetration of the zona pellucida (zp) are regarded as species-specific. In this investigation, the interactions between bovine oocytes and porcine, respectively, equine spermatozoa have been studied under in vitro conditions and compared with the normal in vitro fertilization of bovine oocytes by bovine sperm. Surprisingly, many of the heterologous spermatozoa adhered firmly to the bovine oocytes and could not be removed by intense washing. On average, more than 100 boar or equine spermatozoa were bound to the zp of bovine oocytes. Electron microscopic studies clearly demonstrated that porcine sperm attached to the zona and underwent the acrosome reaction. Equine spermatozoa displayed a similar binding affinity, but unlike the porcine spermatozoa even penetrated the zp and were taken up into the oocyte after a longer period of co-incubation. Considering these new results the dogma of a strict species specificity of sperm zona interactions under in vitro conditions has to be reconsidered.

Variations in lectin-binding on the zona pellucida during oocyte growth in some wild ungulates

The aim of the present study was to examine the glycoconjugate modifications occurring in the zona pellucida during oocyte growth in fallow, red and roe deer using a battery of lectins combined with sialidase digestion and chemical treatments. This histochemical approach allowed us to sequence the oligosaccharidic side chains of the zona pellucida glycoproteins in these wild ungulates. The most effective lectins in the zona pellucida of these species were SBA, PNA, RCA-I GSA-IB4, and WGA, indicating the presence of beta-D-N-Acetylgalactosamine, beta-D-Galactose, alpha-D-Galactose and N-Acetylglucosamine residues. Additionally, sialic acid moieties were demonstrated. We also observed differences in the glycosidic residue content and in their spatial distribution, depending on the species and stage of follicle development.

Biochemical and functional characterization of the human zona pellucida

A successful interaction between spermatozoa and the zona pellucida is critical for fertilization. This biological step reflects multiple sperm functions, including the acquisition and completion of capacitation, recognition and binding to specific zona pellucida receptors, and induction of the physiological acrosome reaction. The recognition of carbohydrate sequences by complimentary receptors has been demonstrated in gamete interaction in different animal species. It has been proposed that, in the human, sperm binding to the zona pellucida requires a 'selectin-like' interaction. The hemizona assay (a unique internally controlled bioassay that evaluates tight binding of human spermatozoa to the homologous zona pellucida) and advanced methods of carbohydrate analysis have been used to test this hypothesis. Compelling evidence exists to demonstrate that oligosaccharide recognition is also required for specific, tight human gamete binding. The induction of the acrosome reaction using the physiological inducers, i.e. the zona pellucida and progesterone, was also examined. It has also been demonstrated that there is a priming effect of the steroid on the acrosome reaction inducing capacity of the zona pellucida. These studies may allow for a better understanding of human gamete interaction in physiological and pathological situations.

Identification of the bacteria-binding peptide domain on salivary agglutinin (gp-340/DMBT1), a member of the scavenger receptor cysteine-rich superfamily

Salivary agglutinin is encoded by DMBT1 and identical to gp-340, a member of the scavenger receptor cysteine-rich (SRCR) superfamily. Salivary agglutinin/DMBT1 is known for its Streptococcus mutans agglutinating properties. This 300-400 kDa glycoprotein is composed of conserved peptide motifs: 14 SRCR domains that are separated by SRCR-interspersed domains (SIDs), 2 CUB (C1r/C1s Uegf Bmp1) domains, and a zona pellucida domain. We have searched for the peptide domains of agglutinin/DMBT1 responsible for bacteria binding. Digestion with endoproteinase Lys-C resulted in a protein fragment containing exclusively SRCR and SID domains that binds to S. mutans. To define more closely the S. mutans-binding domain, consensus-based peptides of the SRCR domains and SIDs were designed and synthesized. Only one of the SRCR peptides, designated SRCRP2, and none of the SID peptides bound to S. mutans. Strikingly, this peptide was also able to induce agglutination of S. mutans and a number of other bacteria. The repeated presence of this peptide in the native molecule endows agglutinin/DMBT1 with a general bacterial binding feature with a multivalent character. Moreover, our studies demonstrate for the first time that the polymorphic SRCR domains of salivary agglutinin/DMBT1 mediate ligand interactions.

Proteolytic processing of human zona pellucida proteins

Formation of the egg's extracellular matrix, the zona pellucida, is critical for fertilization and development of growing embryos. Zona pellucida glycoproteins, ZP1, ZP2, and ZP3, are secreted to form an insoluble extracellular matrix surrounding mammalian eggs. All cloned mammalian zona pellucida sequences contain a furin consensus cleavage site, RX(K)/(R)R, upstream of a putative transmembrane domain, which suggests processing by an endoprotease of the furin-proprotein-convertase family. Recombinant expression of human (h) ZP1, ZP2, and ZP3 produces glycoproteins that are secreted and have migration patterns in SDS-PAGE identical to those of native human zona pellucida proteins. Because a C-terminal epitope tag that is present in the cell-associated zona proteins is, however, absent from the secreted zona proteins, secreted recombinant zona pellucida proteins lack their C-terminal regions. Three different strategies were used to explore processing events in the C-terminal region: site-directed mutagenesis of the furin cleavage site, treatment with a competitive inhibitor of all furin family members, and interference with Golgi modifications by Brefeldin A. All treatments altered the SDS-PAGE migration of recombinant hZP3, concordant with cleavage by a furin family member and Golgi glycosylation of secreted hZP3. Furthermore, cleavage of cell-associated hZP3 by exogenous furin converts the migration of cell-associated hZP3 to that of secreted hZP3. To determine whether a similar cleavage pattern exists in zona pellucida proteins that are assembled in the zona matrix, "hZP3 rescue" mouse zonae pellucidae were employed. Immunoblotting experiments revealed that hZP3, assembled and functional in the "hZP3 rescue" mouse zona pellucida, lacks the furin cleavage site, supporting the hypothesis that formation of the zona pellucida matrix involves regulated proteolysis by a member of the furin convertase family.

The species recognition system: a new corollary for the human fetoembryonic defense system hypothesis

We have previously suggested that the human fetus is protected during human development by a system of both soluble and cell surface associated glycoconjugates that utilize their carbohydrate sequences as functional groups to enable them to evoke tolerance. The proposed model has been referred to as the human fetoembryonic defense system hypothesis (hu-FEDS). In this paradigm, it has previously been proposed that similar oligosaccharides are used to mediate crucial recognition events required during both human sperm-egg binding and immune-inflammatory cell interactions. This vertical integration suggested to us that the sperm-egg binding itself is related to universal recognition events that occur between immune and inflammatory cells, except that in this case recognition of 'species' rather than recognition of 'self' is being manifested. In this paper, we have designated this component of hu-FEDS as the species recognition system (SRS). We propose that the SRS is an integral component of the hu-FEDS used to enable sperm-egg recognition and protection of the gametes from potential immune responses. Recent structural data indicates that the glycan sequences implicated in mediating murine gamete recognition are also expressed on CD45 in activated murine T lymphocytes and cytotoxic T lymphocytes. This overlap supports our contention that there is an overlap between the immune and gamete recognition systems. Therefore the hu-FEDS paradigm may be a subset of a larger model that also applies to other placental mammals. We therefore propose that the hu-FEDS model for protection should in the future be referred to as the eutherian fetoembryonic defense system hypothesis (eu-FEDS) to account for this extension. The possibility exists that the SRS component of eu-FEDS could predate eutherians and extend to all sexually reproducing organisms. Future investigation of the interactions between the immune and gamete recognition system will be required to determine the degree of overlap.

Localization of sperm ligand carbohydrate chains in pig zona pellucida glycoproteins

Neutral N-linked carbohydrate chains from pig ZPB/ZPC mixture are shown to possess sperm ligand activity. Of these complex-type chains, triantennary/tetraantennary chains exhibit the activity stronger than that of diantennary chains. Intact ZPB and ZPC cannot be separated from each other unless acidic N-acetyllactosamine regions of their carbohydrate chains are removed by endo-beta-galactosidase digestion. The endo-beta-galactosidase-digested ZPB and its N-terminal fragment of 111 residues retain the sperm ligand activity. Three glycopeptides, having one Asn residue to which the carbohydrate chain is linked, are obtained by lysylendopeptidase digestion of the heat-solubilized zonae containing intact ZPB, and lysylendopeptidase and chymotryptic digestions of endo-beta-galactosidase-digested ZPB. On the basis of sugar mapping analysis of the N-linked chains from these glycopeptides and comparison with the carbohydrate structures of the main intact neutral N-linked chains of ZPB/ZPC, the triantennary and tetraantennary chains are shown to be localized mainly at Asn220 of ZPB, whereas diantennary chains are present on all the three N-glycosylation sites (Asn203, Asn220 and Asn333). These results suggest that the carbohydrate chains linked to Asn220 of ZPB participate predominantly in sperm-egg binding. ZPC has been shown to support the expression of sperm ligand activity of ZPB. Three glycopeptides, each having one of the N-glycosylation sites, are obtained by tryptic digestion of endo-beta-galactosidase-digested ZPC. Triantennary and tetraantennary chains are found mainly at Asn271 of ZPC, whereas diantennary chains are present at all three N-glycosylation sites (Asn124, Asn146 and Asn271). Thus, the localization of triantennary and tetraantennary chains in ZPC is different from that in ZPB.

Towards the molecular basis of sperm and egg interaction during mammalian fertilization

During the past 2 decades, a number of genes have been cloned from mammals which encode polypeptides that participate in the process of fertilization. Among these are glycoproteins ZP1-3 that constitute the zona pellucida of eggs from mice to human beings. In mice, one of these glycoproteins, mZP3, acts as a primary sperm receptor and acrosome reaction-inducer. The evidence suggests that acrosome-intact sperm recognize and bind to a specific class of mZP3 oligosaccharides present on two serine residues (O-linked) located near the carboxy-terminus of the polypeptide. Mutagenesis of either of these residues results in the synthesis of an inactive form of the receptor. Therefore, mammalian fertilization is a carbohydrate-mediated event. It is possible that changes in the structure of these oligosaccharides (e.g., composition, sequence, linkages, modifications, etc.) could account for species-specific binding of sperm to eggs. Stably transfected somatic cells, null mutant animals, and DNA constructs are now available to test this possibility both in vivo and in vitro.

Sperm adhesion molecules: structure and function

Fertilisation is a unique event in which the morphologically disparate gametes recognise, bind and fuse with each other. This event follows a highly regulated schedule of biochemical interactions, in which molecules are involved that mediate cell adhesion, signal transduction and the initiation of metabolic pathways. A plethora of molecules has been found on the male gamete and with regard to the different protein structures it is almost impossible to overlook the structures involved. Even more, carbohydrate structures cause an additional diversity with regard to the generation of surface structures. In this communication we try to elucidate the structures of proteins that have been known so far. We have focussed on spermadhesins, the zonadhesin, proacrosin and the PH-20 antigen. The variety of structures and also the common features among them as well as the presence of redundant systems are attributable to the evolutionary force of intraspecific sperm competition. This evolutionary force is assumed to be also responsible for the species selectivity observed in these adhesion molecules, which explains the preferential binding of gametes in a homologous system.

Galactosyltransferase function during mammalian fertilization

Gamete recognition has been studied extensively in the mouse. In this system, it is generally believed that sperm bind to a class of O-linked oligosaccharides on the zona pellucida glycoprotein, ZP3. The best characterized sperm receptor for ZP3 is beta1, 4-galactosyltransferase (GalT), which functions in a lectin-like capacity by binding to N-terminal N-acetylglucosamine residues on ZP3 oligosaccharides. Multivalent oligosaccharides on ZP3, as well as synthetic polymers terminating in N-acetylglucosamine aggregate GalT, leading to activation of a heterotrimeric G protein cascade and culminating in the acrosome reaction. Following fertilization, cortical granules release N-acetylglucosaminidase, which removes the binding site for sperm GalT and facilitates the zona block to polyspermic binding. Genetic manipulation of GalT expression has confirmed its function as a ZP3 receptor. Overexpressing GalT on sperm leads to increased binding of ZP3, increased G protein activation, and precocious acrosome reactions. In contrast, sperm from mice made null for GalT by homologous recombination are refractory to ZP3, in that they are unable to bind soluble ZP3 and fail to undergo the acrosome reaction in response to zona glycoproteins. Surprisingly, GalT null sperm still bind to the zona and achieve low rates of fertilization in vitro. This then suggests that sperm-egg binding involves receptor-ligand interactions independent of GalT and ZP3. The current model suggests that GalT functions as the ZP3 receptor that is responsible for inducing the acrosome reaction, whereas initial sperm-zona binding is dictated by other sperm surface receptors. Consistent with this, at least three other zona pellucida monosaccharides have been implicated in sperm binding, and novel sperm surface glycoproteins have been suggested to function in gamete binding. A large scaffolding protein has been identified that associates with the GalT cytoplasmic domain and may be responsible for orchestrating its signal transduction capacities that lead to the acrosome reaction.