ZP2 cleavage blocks polyspermy by modulating the architecture of the egg coat

Following the fertilization of an egg by a single sperm, the egg coat or zona pellucida (ZP) hardens and polyspermy is irreversibly blocked. These events are associated with the cleavage of the N-terminal region (NTR) of glycoprotein ZP2, a major subunit of ZP filaments. ZP2 processing is thought to inactivate sperm binding to the ZP, but its molecular consequences and connection with ZP hardening are unknown. Biochemical and structural studies show that cleavage of ZP2 triggers its oligomerization. Moreover, the structure of a native vertebrate egg coat filament, combined with AlphaFold predictions of human ZP polymers, reveals that two protofilaments consisting of type I (ZP3) and type II (ZP1/ZP2/ZP4) components interlock into a left-handed double helix from which the NTRs of type II subunits protrude. Together, these data suggest that oligomerization of cleaved ZP2 NTRs extensively cross-links ZP filaments, rigidifying the egg coat and making it physically impenetrable to sperm.

A ZP1 gene mutation in a patient with empty follicle syndrome: A case report and literature review

Genuine empty follicle syndrome (gEFS) is a rare cause of female infertility; it is defined as the presence of cumulus-oocyte complexes (COCs) in follicular fluid but the absence of oocytes after denudation in an in vitro fertilization (IVF) programme. Mutations in one of the four genes encoding zona pellucida (ZP) proteins have been implicated in gEFS. The objectives of the present study were to explore the molecular basis of idiopathic infertility in a 35-year-old woman with gEFS (observed after four ovarian retrievals), compare her phenotype and genotype with those of other patients described in the literature, and discuss therapeutic approaches that could be adopted by reproductive health centres in this situation. Sequencing of the ZP genes revealed a new homozygous missense variant in ZP1: c.1097G > A;p.(Arg366Gln). The variant is located in the ZP-N domain, which is essential for ZP protein polymerization. An immunohistochemical assessment of an ovarian biopsy confirmed the absence of ZP1 protein. The novel variant appears to prevent ZP assembly, which would explain the absence of normal oocytes after denudation in our patient (and despite the retrieval of COCs). ZP gene sequencing should be considered for patients with a phenotype suggestive of gEFS. An etiological genetic diagnosis enables appropriate genetic counselling and a switch to an IVF programme (with a suitable denudation technique) or an oocyte donation programme.

Domain Expansion and Functional Diversification in Vertebrate Reproductive Proteins

The rapid evolution of fertilization proteins has generated remarkable diversity in molecular structure and function. Glycoproteins of vertebrate egg coats contain multiple zona pellucida (ZP)-N domains (1-6 copies) that facilitate multiple reproductive functions, including species-specific sperm recognition. In this report, we integrate phylogenetics and machine learning to investigate how ZP-N domains diversify in structure and function. The most C-terminal ZP-N domain of each paralog is associated with another domain type (ZP-C), which together form a "ZP module." All modular ZP-N domains are phylogenetically distinct from nonmodular or free ZP-N domains. Machine learning-based classification identifies eight residues that form a stabilizing network in modular ZP-N domains that is absent in free domains. Positive selection is identified in some free ZP-N domains. Our findings support that strong purifying selection has conserved an essential structural core in modular ZP-N domains, with the relaxation of this structural constraint allowing free N-terminal domains to functionally diversify.

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.

Structure of Zona Pellucida Module Proteins

The egg coat, an extracellular matrix made up of glycoprotein filaments, plays a key role in animal fertilization by acting as a gatekeeper for sperm. Egg coat components polymerize using a common zona pellucida (ZP) "domain" module that consists of two related immunoglobulin-like domains, called ZP-N and ZP-C. The ZP module has also been recognized in a large number of other secreted proteins with different biological functions, whose mutations are linked to severe human diseases. During the last decade, tremendous progress has been made toward understanding the atomic architecture of the ZP module and the structural basis of its polymerization. Moreover, sperm-binding regions at the N-terminus of mollusk and mammalian egg coat subunits were found to consist of domain repeats that also adopt a ZP-N fold. This discovery revealed an unexpected link between invertebrate and vertebrate fertilization and led to the first structure of an egg coat-sperm protein recognition complex. In this review we summarize these exciting findings, discuss their functional implications, and outline future challenges that must be addressed in order to develop a comprehensive view of this family of biomedically important extracellular molecules.

Egg Coat Proteins Across Metazoan Evolution

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

Fourier Transform Infrared Spectroscopic Analysis of the Intact Zona Pellucida of the Mammalian Egg: Changes in the Secondary Structure of Bovine Zona Pellucida Proteins During Fertilization

The zona pellucida is the acellular transparent envelope surrounding the mammalian oocyte. An analysis of the changes in the structures of zona pellucida proteins is essential for understanding the molecular mechanisms underlying the important physiological roles of the zona during fertilization and prelmplantatlon. The hardening of the zona caused by the structural changes during fertilization is generally accepted to be responsible for blocking polyspermy. In this study, we analyzed changes in the secondary structure of the zona during fertilization by Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy. The predominance of β-sheet structure in porcine ovarian egg zona proteins in water was ascertained using FTIR spectra. α-Helix structure was also present. The attenuated total reflection (ATR)-FTIR spectrum of intact, unsolubilized porcine zonae pellucidae from ovarian eggs Indicated that the zona proteins in the native zona pellucida also have β-structure as the main constituent. Attenuated total reflectlon-FTIR spectroscopy of intact bovine zona pellucida obtained from ovarian and fertilized eggs at the blastocyst stage revealed that the β-structure content Increased during fertilization. Furthermore, a reduction of the thickness of the zona during fertilization was observed using transmission electron microscopy. Therefore, the change in the zona architecture that causes hardening of the zona during fertilization is accompanied by changes in the secondary structure of the zona proteins.

Integrity of Narrow Epithelial Tubes in the C. elegans Excretory System Requires a Transient Luminal Matrix

Most epithelial cells secrete a glycoprotein-rich apical extracellular matrix that can have diverse but still poorly understood roles in development and physiology. Zona Pellucida (ZP) domain glycoproteins are common constituents of these matrices, and their loss in humans is associated with a number of diseases. Understanding of the functions, organization and regulation of apical matrices has been hampered by difficulties in imaging them both in vivo and ex vivo. We identified the PAN-Apple, mucin and ZP domain glycoprotein LET-653 as an early and transient apical matrix component that shapes developing epithelia in C. elegans. LET-653 has modest effects on shaping of the vulva and epidermis, but is essential to prevent lumen fragmentation in the very narrow, unicellular excretory duct tube. We were able to image the transient LET-653 matrix by both live confocal imaging and transmission electron microscopy. Structure/function and fluorescence recovery after photobleaching studies revealed that LET-653 exists in two separate luminal matrix pools, a loose fibrillar matrix in the central core of the lumen, to which it binds dynamically via its PAN domains, and an apical-membrane-associated matrix, to which it binds stably via its ZP domain. The PAN domains are both necessary and sufficient to confer a cyclic pattern of duct lumen localization that precedes each molt, while the ZP domain is required for lumen integrity. Ectopic expression of full-length LET-653, but not the PAN domains alone, could expand lumen diameter in the developing gut tube, where LET-653 is not normally expressed. Together, these data support a model in which the PAN domains regulate the ability of the LET-653 ZP domain to interact with other factors at the apical membrane, and this ZP domain interaction promotes expansion and maintenance of lumen diameter. These data identify a transient apical matrix component present prior to cuticle secretion in C. elegans, demonstrate critical roles for this matrix component in supporting lumen integrity within narrow bore tubes such as those found in the mammalian microvasculature, and reveal functional importance of the evolutionarily conserved ZP domain in this tube protecting activity.

Most organs in the body are made up of networks of tubes that transport fluids or gases. These tubes come in many different sizes and shapes, with some narrow capillaries being only one cell in diameter. As tubes develop and take their final shapes, they secrete various glycoproteins into their hollow interior or lumen. The functions of these luminal proteins are not well understood, but there is increasing evidence that they are important for lumen shaping and that their loss can contribute to diseases such as cardiovascular disease and chronic kidney disease. Through studies of the nematode C. elegans, we identified a luminal glycoprotein, LET-653, that is transiently expressed in multiple developing tube types but is particularly critical to maintain integrity of the narrowest, unicellular tubes. We identified protein domains that direct LET-653 to specific apical matrix compartments and mediate its oscillatory pattern of lumen localization. Furthermore, we showed that the LET-653 tube-protecting activity depends on a Zona Pellucida (ZP) domain similar to that found in the mammalian egg-coat and in many other luminal or sensory matrix proteins involved in human disease.

Softening of the mouse zona pellucida during oocyte maturation

A change in the elasticity and the resistance to dissolution of the mouse zona pellucida (ZP) was quantitatively evaluated at immature germinal vesicle (GV), mature metaphase II (MII) and fertilized pronuclear (PN) stages. Young's modulus of the ZP was measured using a micro tactile sensor (MTS), a highly sensitive resonator-based sensor for a micro scale elasticity measurement. 0.25% α-chymotrypsin was used for the ZP dissolution assay. The results of measuring the ZP elasticity and the dissolution time clearly showed that the ZP softened during oocyte maturation and the ZP hardened after fertilization. The results indicate that the amount of the zona softening can be a criterion to evaluate oocyte quality for the selection of top quality mature oocyte before in vitro fertilization (IVF) treatment.

Amyloid properties of the mouse egg zona pellucida

The zona pellucida (ZP) surrounding the oocyte is an extracellular fibrillar matrix that plays critical roles during fertilization including species-specific gamete recognition and protection from polyspermy. The mouse ZP is composed of three proteins, ZP1, ZP2, and ZP3, all of which have a ZP polymerization domain that directs protein fibril formation and assembly into the three-dimensional ZP matrix. Egg coats surrounding oocytes in nonmammalian vertebrates and in invertebrates are also fibrillar matrices and are composed of ZP domain-containing proteins suggesting the basic structure and function of the ZP/egg coat is highly conserved. However, sequence similarity between ZP domains is low across species and thus the mechanism for the conservation of ZP/egg coat structure and its function is not known. Using approaches classically used to identify amyloid including conformation-dependent antibodies and dyes, X-ray diffraction, and negative stain electron microscopy, our studies suggest the mouse ZP is a functional amyloid. Amyloids are cross-β sheet fibrillar structures that, while typically associated with neurodegenerative and prion diseases in mammals, can also carry out functional roles in normal cells without resulting pathology. An analysis of the ZP domain from mouse ZP3 and ZP3 homologs from five additional taxa using the algorithm AmylPred 2 to identify amyloidogenic sites, revealed in all taxa a remarkable conservation of regions that were predicted to form amyloid. This included a conserved amyloidogenic region that localized to a stretch of hydrophobic amino acids previously shown in mouse ZP3 to be essential for fibril assembly. Similarly, a domain in the yeast protein α-agglutinin/Sag 1p, that possesses ZP domain-like features and which is essential for mating, also had sites that were predicted to be amyloidogenic including a hydrophobic stretch that appeared analogous to the critical site in mouse ZP3. Together, these studies suggest that amyloidogenesis may be a conserved mechanism for ZP structure and function across billions of years of evolution.

AC electric field induced dipole-based on-chip 3D cell rotation

The precise rotation of suspended cells is one of the many fundamental manipulations used in a wide range of biotechnological applications such as cell injection and enucleation in nuclear transfer (NT) cloning. Noticeably scarce among the existing rotation techniques is the three-dimensional (3D) rotation of cells on a single chip. Here we present an alternating current (ac) induced electric field-based biochip platform, which has an open-top sub-mm square chamber enclosed by four sidewall electrodes and two bottom electrodes, to achieve rotation about the two axes, thus 3D cell rotation. By applying an ac potential to the four sidewall electrodes, an in-plane (yaw) rotating electric field is generated and in-plane rotation is achieved. Similarly, by applying an ac potential to two opposite sidewall electrodes and the two bottom electrodes, an out-of-plane (pitch) rotating electric field is generated and rolling rotation is achieved. As a prompt proof-of-concept, bottom electrodes were constructed with transparent indium tin oxide (ITO) using the standard lift-off process and the sidewall electrodes were constructed using a low-cost micro-milling process and then assembled to form the chip. Through experiments, we demonstrate rotation of bovine oocytes of ~120 μm diameter about two axes, with the capability of controlling the rotation direction and the rate for each axis through control of the ac potential amplitude, frequency, and phase shift, and cell medium conductivity. The maximum observed rotation rate reached nearly 140° s⁻¹, while a consistent rotation rate reached up to 40° s⁻¹. Rotation rate spectra for zona pellucida-intact and zona pellucida-free oocytes were further compared and found to have no effective difference. This simple, transparent, cheap-to-manufacture, and open-top platform allows additional functional modules to be integrated to become a more powerful cell manipulation system.

Delineation of downstream signalling components during acrosome reaction mediated by heat solubilized human zona pellucida

BACKGROUND

Human egg is enveloped by a glycoproteinaceous matrix, zona pellucida (ZP), responsible for binding of the human spermatozoa to the egg and induction of acrosomal exocytosis in the spermatozoon bound to ZP. In the present manuscript, attempts have been made to delineate the downstream signalling components employed by human ZP to induce acrosome reaction.

METHODS

Heat-solubilized human ZP (SIZP) was used to study the induction of acrosome reaction in capacitated human spermatozoa using tetramethylrhodamine isothiocyanate conjugated Pisum sativum agglutinin (TRITC-PSA) in absence or presence of various pharmacological inhibitors. In addition, intracellular calcium ([Ca2+]i) levels in sperm using Fluo-3 acetoxymethyl ester as fluorescent probe were also estimated in response to SIZP.

RESULTS

SIZP induces acrosomal exocytosis in capacitated human sperm in a dose dependent manner accompanied by an increase in [Ca2+]i. Human SIZP mediated induction of acrosome reaction depends on extracellular Ca2+ and involves activation of Gi protein-coupled receptor, tyrosine kinase, protein kinases A & C and phosphoinositide 3 (PI3)- kinase. In addition, T-type voltage operated calcium channels and GABA-A receptor associated chloride (Cl-) channels play an important role in SIZP mediated induction of acrosome reaction.

CONCLUSIONS

Results described in the present study provide a comprehensive account of the various downstream signalling components associated with human ZP mediated acrosome reaction.

Analysis of protein-linked glycosylation in a sperm-somatic cell adhesion system

Murine sperm initiate fertilization by binding to the specialized extracellular matrix of their complementary eggs, known as the zona pellucida. On the basis of data reported in this study, mouse sperm also bind to rabbit erythrocytes with higher affinity than they do to murine eggs. This unusual interaction between a germ cell and a somatic cell ("sperm-somatic cell adhesion system") is also carbohydrate dependent based on its sensitivity to mild periodate oxidation. To determine what types of carbohydrate sequences could be involved in this interaction, the protein-linked oligosaccharides of rabbit erythrocytes were sequenced using novel matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry methods that enabled the analysis of individual components up to m/z 9000. The N-glycans are primarily complex biantennary and triantennary types terminated with Galalpha1-3Gal sequences. The majority of these oligosaccharides also possess one antenna consisting of a highly branched polylactosamine-type sequence that is also associated with many glycosphingolipids that coat rabbit erythrocytes. These erythrocytes also express Core 1 and Core 2 O-glycans terminated primarily with Galalpha1-3Gal sequences and to a lesser extent sialic acid. These results confirm that rabbit erythrocytes and mouse eggs present very different types of carbohydrate sequences on their surfaces. However, oligosaccharides terminated with beta1-6-linked N-acetyllactosamine or its alpha1-3 galactosylated analog are expressed on both the mouse zona pellucida and this somatic cell type. The far more abundant presentation of such sequences on rabbit erythrocytes compared with murine eggs could explain why mouse sperm display such exceptional affinity for this somatic cell type.

ZP2 and ZP3 traffic independently within oocytes prior to assembly into the extracellular zona pellucida

The extracellular zona pellucida surrounds mammalian eggs and mediates taxon-specific sperm-egg recognition at fertilization. In mice, the zona pellucida is composed of three glycoproteins, but the presence of ZP2 and ZP3 is sufficient to form a biologically functional structure. Each zona pellucida glycoprotein is synthesized in growing oocytes and traffics through the endomembrane system to the cell surface, where it is released from a transmembrane domain and assembled into the insoluble zona pellucida matrix. ZP2 and ZP3 colocalize in the endoplasmic reticulum and in 1- to 5-microm post-Golgi structures comprising multivesicular aggregates (MVA), but a coimmunoprecipitation assay does not detect physical interactions. In addition, ZP2 traffics normally in growing oocytes in the absence of ZP3 or if ZP3 has been mutated to prevent incorporation into the zona pellucida matrix, complementing earlier studies indicating the independence of ZP3 secretion in Zp2 null mice. N glycosylation has been implicated in correct protein folding and intracellular trafficking of secreted proteins. Although ZP3 contain five N-glycans, enhanced green fluorescent protein-tagged ZP3 lacking N glycosylation sites is present in MVA and is incorporated into the zona pellucida matrix of transgenic mice. Thus, ZP2 secretion is seemingly unaffected by ZP3 lacking N-glycans. Taken together, these observations indicate that ZP2 and ZP3 traffic independently through the oocyte prior to assembly into the zona pellucida.

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.

Molecular dissection of the mouse zona pellucida: an electron-microscopic perspective utilizing high-resolution colloidal-gold labeling methods

The zona pellucida (ZP) is an extracellular coat that encloses growing oocytes, ovulated eggs, and preimplantation embryos in mammals. The ZP contains receptors that mediate initial interactions between the sperm and egg and the relative species-specificity during gamete interaction. It also prevents polyspermy and protects the developing embryo prior to implantation. The current model of the organization of mouse ZP depicts this extracellular matrix as an extensive three-dimensional array of long interconnected filaments of a structural repeat. Using high-resolution colloidal-gold labeling methods in combination with specific lectins and monoclonal antibodies against three major ZP glycoproteins, we have characterized the outer and inner mouse ZP during folliculogenesis and shown the modifications of the ZP after ovulation and fertilization. Our immunogold labeling results also indicate the involvement of a vesicular aggregate, a specialized subcellular compartment in the oocyte, in the synthesis and secretion of ZP glycoproteins.

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

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

The application of in vitro sperm competition test to evaluate the impact of ZP-derived peptides on fertilization capacity of cat sperm

The present study aimed to establish a sensitive in vitro assay to assess the binding capacity of cat spermatozoa. Cat oocytes and epididymal sperm cells were isolated from gonads and cultured for in vitro fertilization. Before fertilization, the sperm cells were incubated either in 10 microM green dye Fluo-3-AM or 10 microM orange dye CellTracker Orange CMTMR (Molecular Probes), respectively. After removing the dyes by washing, sperm cells stained with each dye were added to medium drops containing oocytes in various proportions and cultured for 16 h at 37 degrees C, 5% CO(2). The oocytes were examined using fluorescence microscopy. Sperm bound to oocytes, and stained with different colors, were counted. When fresh epididymal sperm were mixed in at a specific proportion, the number of sperm bound to the zona pellucida (ZP) of oocytes reflected the proportion of differently colored sperm in the medium. This indicated that neither dye influenced the binding capacity of cat sperm. Mixing fresh and cryopreserved sperm, however, resulted in a higher number of fresh sperm bound to the oocyte surface in comparison to frozen-thawed sperm. Also, the pre-incubation of cat sperm cells with ZP derived peptide reduced the sperm binding capacity by 40%. In conclusion, the presented sperm competition assay allows assessment of fertilizing capacity of cat spermatozoa in vitro when a mixture of two different populations is used. The applied supravital fluorescence dyes do not affect motility and binding capacity of sperm cells and were clearly distinguishable under fluorescence microscopy. We demonstrate that the assay can be used to study the impact of sperm treatment, such as cryopreservation or pre-incubation in bioactive peptides, on fertilizing capacity.

Tracking Down the ZP Domain: From the Mammalian Zona Pellucida to the Molluscan Vitelline Envelope

Oocytes from virtually all organisms are surrounded by at least one coat. This specialized extracellular matrix, called the zona pellucida (ZP) in mammals and the vitelline envelope (VE) in nonmammals, has a structural function and plays essential roles in oogenesis, fertilization, and early development. During the last 15 years, compelling evidence has accumulated that all ZP/VE subunits polymerize using a conserved sequence, the ZP domain, so that the basic structural features of egg coat matrices have been maintained through evolution. Moreover, ZP domains have been identified in many other polymeric extracellular proteins from eukaryotes. This review compares the ultrastructure and molecular composition of egg coats from mollusc to human, suggests a common mechanism for assembly of ZP/VE proteins, and discusses alternative models of how these could be arranged within filaments.

Quantitative analysis of telomerase activity and telomere length in domestic animal clones

It has been speculated that incomplete epigenetic reprogramming of the somatic cell genome is the primary reason behind the developmental inefficiencies and postnatal abnormalities observed after nuclear transplantation in domestic animal clones. One chromosome structure that is altered in dividing somatic cells is telomere length-the terminal ends of linear chromosomes capped by repetitive sequences of G-rich noncoding DNA, (TTAGGG)", and specific binding proteins. Telomeres are critical structures that function in maintaining chromosome stability and ensure the full replication of coding DNA by acting as a buffer to terminal DNA attrition due to the end replication problem. Telomere shortening limits cellular proliferation through a DNA damage signal activating permanent cell cycle arrest at a critical telomere length or through structural telomere alterations that prevents effective chromosome capping. Telomere-mediated signaling of cellular senescence has been established for many somatic cell types in vitro, except for germ cells, cancer lines, and regenerative tissues in which telomere length is maintained primarily by the ribonucleoprotein telomerase, a reverse transcriptase that synthesizes TTAGGG repeats de novo onto the chromosome ends. Telomere length discrepancies have been reported in animal clones as being shorter, no different, and even longer than in age-matched control animals, but the etiology is not yet understood. Possible explanations include differences in donor cell type and the efficiency of telomerase reprogramming. This chapter summarizes the conventional protocols and recent advances in telomere length and telomerase activity measurement that will help elucidate the mechanism(s) behind telomere length deregulation in somatic cell clones and its role in chromosomal instability, cellular senescence, and organismal aging in vivo.

Molecular Models for Murine Sperm-Egg Binding

Murine sperm initiate fertilization by binding to the specialized extracellular matrix of mouse eggs, known as the zona pellucida. Over the past decade, powerful genetic, biophysical, and biochemical techniques have been employed to gain new insights into this interaction. Evidence from these studies does not support either of two major models for binding first proposed over two decades ago. Two more recently established models suggest that protein-protein interactions predominate during this initial stage of fertilization. Another model proposes that about 75-80% of the murine sperm bound to zona pellucida under well defined in vitro conditions is carbohydrate dependent, with the remaining sperm bound via protein-protein interactions. Mounting evidence suggests that the carbohydrate sequences coating the murine egg could be employed as specific immune recognition markers. Continued investigation of this system may resolve many of these controversial findings and reveal novel functions for murine zona pellucida glycoproteins.

The PLAC1-homology region of the ZP domain is sufficient for protein polymerisation

Background

Hundreds of extracellular proteins polymerise into filaments and matrices by using zona pellucida (ZP) domains. ZP domain proteins perform highly diverse functions, ranging from structural to receptorial, and mutations in their genes are responsible for a number of severe human diseases. Recently, PLAC1, Oosp1-3, Papillote and CG16798 proteins were identified that share sequence homology with the N-terminal half of the ZP domain (ZP-N), but not with its C-terminal half (ZP-C). The functional significance of this partial conservation is unknown.

Results

By exploiting a highly engineered bacterial strain, we expressed in soluble form the PLAC1-homology region of mammalian sperm receptor ZP3 as a fusion to maltose binding protein. Mass spectrometry showed that the 4 conserved Cys residues within the ZP-N moiety of the fusion protein adopt the same disulfide bond connectivity as in full-length native ZP3, indicating that it is correctly folded, and electron microscopy and biochemical analyses revealed that it assembles into filaments.

Conclusion

These findings provide a function for PLAC1-like proteins and, by showing that ZP-N is a biologically active folding unit, prompt a re-evaluation of the architecture of the ZP domain and its polymers. Furthermore, they suggest that ZP-C might play a regulatory role in the assembly of ZP domain protein complexes.

Expression pattern of glycoconjugates in the Bidderian and ovarian follicles of the Brazilian toad Bufo ictericus analyzed by lectin histochemistry

The Bidder's organ and ovary of the Brazilian toad Bufo ictericus were studied by light microscopy, using hematoxylin-eosin (HE) and periodic acid Schiff (PAS) staining. The expression and distribution of carbohydrate moieties was analyzed by lectin histochemistry, using 8 lectins with different carbohydrate specificities: Ulex europaeus (UEA I), Lens culinaris (LCA), Erythrina cristagalli (ECA), Arachis hypogaea (PNA), Ricinus communis (RCA I), Aleuria aurantia (AAA), Triticum vulgaris (WGA), and Glycine maximum (SBA). The results showed that the Bidderian zona pellucida presented alpha-mannose, alpha-L-fucose, beta-D-galactose, N-acetyl-D-glucosamine, and alpha/beta-N-acetyl-galactosamine residues. The Bidderian follicular cells showed the presence of beta-D-galactose and N-acetyl-D-glucosamine. In the extracellular matrix, alpha-mannose and alpha/beta-N-acetyl-galactosamine residues were detected. The ovarian zona pellucida showed alpha-L-fucose, N-acetyl-D-glucosamine, alpha/beta-N-acetyl-galactosamine residues, and alpha-mannose and N-acetyl-D-glucosamine residues were detected in the follicular cells. Thus, the zona pellucida in both organs contains N-acetyl-D-glucosamine, and alpha/beta-N-acetyl-galactosamine residues. alpha-L-fucose residues were detected in the zona pellucida of both organs, using different lectins. Considering that beta-D-galactose residue was absent from ovary but present in the Bidder's organ, this sugar residue may play an important role in follicle development, blocking the Bidderian follicles and preventing further development of the Bidder's organ into a functional ovary.

Cx37 and Cx43 localize to zona pellucida in mouse ovarian follicles

In the ovarian follicle, granulosa cells adjacent to the oocyte extend processes through the zona pellucida matrix, and these projections establish gap junctions both with the oocyte and with neighboring transzonal projections. The identity of connexins contributing to gap junctions between transzonal projections has not been extensively studied. Here, we examined the expression pattern of Cx37 and Cx43 in mouse zona pellucida using multiple connexin-specific antibodies. Immunofluorescence staining revealed abundant Cx37 and Cx43 puncta within the zona pellucida of both preantral and antral follicles. Cx37 persisted in the zona pellucida of mature follicles up to 5 h after an ovulatory stimulus whereas Cx43 was reduced in the zona pellucida by 3 h after an ovulatory stimulus. We suggest that in addition to its role in oocyte-granulosa cell communication, Cx37 could enable a distinct communication pathway between those granulosa cells that are in direct contact with the oocyte.

Structural Conservation of Mouse and Rat Zona Pellucida Glycoproteins. Probing the Native Rat Zona Pellucida Proteome by Mass Spectrometry

The mammalian zona pellucida is an egg extracellular matrix to which sperm bind. Mouse zonae are composed of three glycoproteins (ZP1, ZP2, and ZP3), while rat zonae contain four (ZP1, ZP2, ZP3, and ZP4/ZPB). Mouse sperm bind to zonae comprised solely of mouse ZP2 and ZP3. In this report, we show that rat sperm also bind to these zonae, indicating that ZP2 and ZP3 contain a "minimum structure(s)" to which rodent sperm can bind, and ZP1 and ZP4/ZPB are dispensable in these two rodents. These data are consistent with our mass spectrometric analysis of the native rat zona pellucida proteome (defined as the fraction of the total rat proteome to which the zonae glycoproteins contribute) demonstrating that the rat zonae glycoproteins share a high degree of conservation of structural features with respect to their mouse counterparts. The primary sequences of the rat zonae proteins have been deduced from cDNA. Each zona protein undergoes extensive co- and post-translational modification prior to its secretion and incorporation into an extracellular zona matrix. Each has a predicted N-terminal signal peptide that is cleaved off once protein translation begins and an anchoring C-terminal transmembrane domain from which the mature protein is released. Mass spectrometric analysis with a limited amount of native material allowed determination of the mature N-termini of rat ZP1 and ZP3, both of which are characterized by cyclization of glutamine to pyroglutamate; the N-terminus of ZP2 was identified by Edman degradation. The mature C-termini of ZP1 and ZP3 end two amino acids upstream of a conserved dibasic residue that is part of, but distinct from, the consensus furin cleavage sequence, while the C-terminus of ZP2 was not determined. Each zona protein contains a "zona domain" with eight conserved cysteine residues that is thought to play a role in the polymerization of the zona proteins into matrix filaments. Partial disulfide bond assignment indicates that the intramolecular disulfide patterns in rat ZP1, ZP2, and ZP3 are identical to those of their corresponding mouse counterparts. Last, nearly all potential N-glycosylation sites are occupied in the rat zonae glycoproteins (three of three for ZP1, six or seven of seven for ZP2, and four or five of six for ZP3). In comparison, potential O-glycosylation sites are numerous (59-83 Ser/Thr residues), but only two regions were observed to carry O-glycans in rat ZP3.

Zona pellucida domain proteins

Many eukaryotic proteins share a sequence designated as the zona pellucida (ZP) domain. This structural element, present in extracellular proteins from a wide variety of organisms, from nematodes to mammals, consists of approximately 260 amino acids with eight conserved cysteine (Cys) residues and is located close to the C terminus of the polypeptide. ZP domain proteins are often glycosylated, modular structures consisting of multiple types of domains. Predictions can be made about some of the structural features of the ZP domain and ZP domain proteins. The functions of ZP domain proteins vary tremendously, from serving as structural components of egg coats, appendicularian mucous houses, and nematode dauer larvae, to serving as mechanotransducers in flies and receptors in mammals and nonmammals. Generally, ZP domain proteins are present in filaments and/or matrices, which is consistent with the role of the domain in protein polymerization. A general mechanism for assembly of ZP domain proteins has been presented. It is likely that the ZP domain plays a common role despite its presence in proteins of widely diverse functions.

[Zona pellucida and male fertilization]

Zona pellucida (ZP) plays a key role in the recognition, combination and penetration of sperms and oocytes, the prevention of multiple impregnation, the protection of embryos, and so on. The paper reviews the constitution, molecular structure, biological function and application of zona pellucida in male fertilization.

Participation of the nonreducing terminal beta-galactosyl residues of the neutral N-linked carbohydrate chains of porcine zona pellucida glycoproteins in sperm-egg binding

The zona pellucida (ZP) surrounding the mammalian oocyte is composed of three glycoprotein components (ZPA, ZPB, and ZPC). Mammalian sperm bind to carbohydrate chains of a ZP glycoprotein in the initial phase of fertilization. Sperm-ligand carbohydrate chains have been characterized in mouse, cow, and pig. In pigs, triantennary/tetraantennary neutral complex-type chains from ZPB/ZPC mixture possess stronger sperm-binding activity than those of biantennary chains (Kudo et al., 1998: Eur J Biochem 252:492-499). Most of these oligosaccharides have beta-galactosyl residues at the nonreducing ends. This study used two in vitro competition assays to investigate the participation of the nonreducing terminal beta-galactosyl residues of the ligand active chains in porcine sperm binding. The removal of the nonreducing terminal beta-galactosyl residues from either the ligand active carbohydrate chains or endo-beta-galactosidase-digested glycoproteins significantly reduced their inhibition of sperm-egg binding, indicating that the beta-galactosyl residues at the nonreducing ends are involved in porcine sperm-egg binding. A correlation between the sperm-binding activity and in vitro fertilization rate is also presented.

Cracking the egg: increased complexity in the zona pellucida

A functional zona pellucida is critical for both fertilization and the early stages of embryo development. Recent data from genomic and proteomic studies have questioned our simplistic view of the zona as being composed of three proteins whose functions are clearly defined. In the human, for example, the zona pellucida is composed of four proteins, not three. The increased complexity of the zona pellucida in humans and other species across the evolutionary tree now demands that we reconsider our reliance on the mouse model for understanding early fertilization events. Additionally, we are now well placed to examine, for the first time, potential defects in zona genes and their proteins associated with defined pathology.

Zona pellucida characteristics and sperm-binding patterns of in vivo and in vitro produced porcine oocytes inseminated with differently prepared spermatozoa

IVF of porcine oocytes has been carried out in many laboratories. However, polyspermic fertilization is still a major issue to be solved. It is well known that besides the nucleus, oocyte organelles and the cytoplasm have to undergo a final maturation process before they become fully competent for fertilization. Until now, it is still uncertain whether the zona pellucida (ZP) must also undergo a maturation process and what impact the maturation status may have on sperm recognition and monospermic fertilization. Our data show that the ZP undergoes biochemical changes in the final maturation phase of the oocyte prior to fertilization. During zona maturation, the induction of the acrosome reaction in spermatozoa bound to the zona pellucida shows a different time pattern. Additionally, it was shown by 2D gel electrophoresis that after maturation, ZPA moved 0.8 pI units and ZPB/ZPC 1.3 pI units in the direction of the anode, indicating increased acidity. These preliminary studies indicate that the maturation processes of the oocyte involves biochemical and functional alterations in the zona pellucida. In addition, the morphology of the porcine ZP was investigated before and after maturation at the GVI and metaphase II stage as well as 1h after onset of IVF. No significant consistent structural changes were seen between immature oocytes and those matured in vitro for 48 h. However, at 24 h, the zona structures were more similar to those in in vivo matured oocytes. This phenomenon needs to be elucidated. So far, the only way to avoid polyspermic penetration is to reduce the number of spermatozoa per oocyte used for IVF. The amount depends on the treatment of the sperm and has to be set for each individual boar.

Topographical localisation of glucidic residues and their variations in the canine zona pellucida during folliculogenesis

In the present ultrastructural study, horseradish peroxidase-labelled lectins, in conjunction with antiperoxidase antibody and protein A-gold, were used to characterise and localise the oligosaccharide sequences of zona pellucida glycoproteins at different stages of follicular development in the canine ovary. Deacetylation and sialidase digestion were also performed before lectin cytochemistry. The zona pellucida of oocytes present in unilaminar primary follicles reacts with WGA- and RCA-I-lectins. The zona pellucida of oocytes present in bilaminar and trilaminar secondary follicles displays positivity to WGA, RCA-I, Con-A, UEA-I, and sialidase/SBA. This labelling pattern persists in the zona pellucida of oocytes present in antral tertiary follicles with the exception of WGA and RCA-I reactive sites which are differently distributed throughout the zona pellucida. The topographical distribution of these carbohydrates is not uniform throughout the zona pellucida, indicating the regionalization of oligosaccharide chains within three concentric bands of the zona matrix: an inner surface close to the oocyte plasma membrane, an intermediate portion and an outer layer in contact with the follicular cells. Our results demonstrated variations in the presence and distribution of the carbohydrate residues in the canine zona pellucida during different stages of follicular growth. We also observed the presence of vesicles in both the ooplasm and granulosa cells, showing a similar lectin binding pattern to that of the zona pellucida.

Analysis of N-linked glycans of porcine zona pellucida glycoprotein ZPA by MALDI-TOF MS: a contribution to understanding zona pellucida structure

The mammalian oocyte is encased by a transparent extracellular matrix, the zona pellucida (ZP), which consists of three glycoproteins, ZPA, ZPB, and ZPC. The glycan structures of the porcine ZP and the complete N-glycosylation pattern of the ZPB/ZPC oligomer has been recently described. Here we report the N-glycan pattern and N-glycosylation sites of the porcine ZP glycoprotein ZPA of an immature oocyte population as determined by a mass spectrometric approach. In-gel deglycosylation of the electrophoretically separated ZPA protein and comparison of the pattern obtained from the native, the desialylated and the endo-beta-galactosidase-treated glycoprotein allowed the assignment of the glycan structures by MALDI-TOF MS by considering the reported oligosaccharide structures. The major N-glycans are neutral biantennary complex structures containing one or two terminal galactose residues. Complex N-glycans carrying N-acetyllactosamine repeats are minor components and are mostly sialylated. A significant signal corresponding to a high-mannose type chain appeared in the three glycan maps. MS/MS analysis confirmed its identity as a pentamannosyl N-glycan. By the combination of tryptic digestion of the endo-beta-galactosidase-treated ZP glycoprotein mixture and in-gel digestion of ZPA with lectin affinity chromatography and reverse-phase HPLC, five of six N-glycosylation sites at Asn(84/93), Asn268, Asn316, Asn323, and Asn530 were identified by MS. Only one site was found to be glycosylated in the N-terminal tryptic glycopeptide with Asn(84/93.) N-glycosidase F treatment of the isolated glycopeptides and MS analysis resulted in the identification of the corresponding deglycosylated peptides.

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.

Carbohydrate analysis of the zona pellucida and cortical granules of human oocytes by means of ultrastructural cytochemistry

BACKGROUND

The zona pellucida (ZP), the mammalian oocyte coat, consists of a restricted number of highly glycosylated proteins. In vitro sperm binding studies suggest a higher binding affinity for the outer region of the ZP compared to its inner region in different species. However, the reason for this difference in binding distribution remains unresolved. Many studies suggest that the carbohydrate sequences linked to ZP glycoproteins act as ligands for sperm binding to this matrix.

METHODS

Lectins and antibodies that recognize different carbohydrates were employed to perform an ultrastructural analysis of human ZP and cortical granule glycosylation.

RESULTS

This study reveals variable glycosylation of the human ZP throughout its thickness, with pronounced differences between the most external and internal regions of this matrix. The binding studies also indicate that ZP glycoproteins express some carbohydrate sequences not previously detected in other species. Finally, cytochemical analysis of human cortical granules suggests similarities in glycosylation to ZP glycoproteins but not to cortical granules from other mammalian species.

CONCLUSION

A heterogeneous carbohydrate composition was observed in the thickness of the human ZP that could be responsible for the different sperm binding affinity detected between the outer and inner regions of the ZP.

Texture of the zona pellucida of the mature pig oocyte. The mammalian egg envelope revisited

The zona pellucida (ZP) of mature pig oocytes is believed to consist of a dense filamentous meshwork, less compact on the inner and outer faces. The uneven surface of the ZP is made of unordered and stretched fibrils surrounding deep funnels which are the openings of the radial canaliculi. The topography of the ZP surface may contribute to the initial interplay between male and female gametes. Using cytochemical techniques for transmission electron microscopy (TEM), such as tannic acid and ruthenium red treatments, we found that the ZP of pig oocytes was essentially made of bundles of fibrils distributed in concentric layers (except in the innermost and outer parts). A correlation appears between the dense structure of the core layer of the ZP and its texture: it is constituted of superposed layers of fibril bundles, whereas only a random meshwork is found in a very thin innermost and in the outer layer. The fascicular configuration may control the permeability of the ZP, giving its semi-rigidity and elasticity, and may facilitate sperm penetration. The liquid crystal-like design of the core layer of the ZP is similar to textures found in the the vitelline envelope (zona radiata) of other vertebrates and possibly of all the deuterostomes. Such texture is probably related to the unique ZP protein composition and to a coordinated synthesis.

Localization and synthesis of zona pellucida proteins in the marmoset monkey (Callithrix jacchus) ovary

In most species, the zona pellucida (ZP), an extracellular matrix surrounding the mammalian oocyte, is composed of three glycoproteins: ZPA, ZPB and ZPC. Based mainly on results with mice, the site of zona pellucida biosynthesis has been suggested to be exclusively in the oocyte cytoplasm. However, evidence is accumulating that among various species cumulus/granulosa cells may be involved. Because knowledge of ZP biosynthesis in primates is lacking, we used the common marmoset (Callithrix jacchus) to acquire information about the localization and the site of synthesis of ZP proteins in this species. Using antibodies against synthetic ZPA and ZPC peptides, immunoreactivity was found in the marmoset ZP and in surrounding cumulus cells. Interestingly, the amounts of ZPA and ZPC proteins expressed appeared to differ depending on the stage of folliculogenesis. RT-PCR analysis of mRNA from marmoset oocytes and from oocyte-free follicle cells revealed expression of ZPA, ZPB and ZPC in oocytes and in follicle cells of different stages of marmoset monkey folliculogenesis. Our data suggest that the biosynthesis of marmoset ZPA, ZPB and ZPC proteins takes place both in oocytes and in follicle cells of different follicle stages, although the abundance of ZP glycoproteins may differ depending on the individual ZP protein.

Localization of angiotensin II in pig ovary and its effects on oocyte maturation in vitro

The renin-angiotensin system (RAS) has been found in mammalian ovarian tissue; however, its physiological role is unclear. This study examined the content of angiotensin II (Ang II) in porcine follicular fluid (pFF), Ang II localization and its receptors in ovary, and the effects of Ang II on porcine oocyte maturation. The concentrations of Ang II were 6951.82 +/- 1295.83, 3502.99 +/- 679.10, 3147.89 +/- 690.60, and 2545.92 +/- 407.01 pg/ml in pFF from small, medium, large, and extra-large follicles, respectively. In addition, Ang II was found on zona pellucidae (ZP) and granulosa cells by immunoreactive staining. The distribution of AT1, an Ang II receptor subtype, was in accordance with that of Ang II. However, AT2, another Ang II receptor, was mainly distributed in the stroma and thecal layers of follicles. When oocytes were cultured in media containing various concentrations of Ang II, a higher (P<0.05) proportion of oocytes reached metaphase II (MII) in the medium with 100 ng/ml (87.0%) than without Ang II (61%). When oocytes from different sizes of follicles were separately cultured in media containing 100 ng/ml Ang II, maturation rates were significantly higher in oocytes from small (61.5%) and medium (85.1%) follicles than that of their controls (45.1 and 72.6%, respectively). However, addition of Ang II inhibited nuclear maturation in oocytes from large follicles (77.8% versus 87.3%). Fertilization and male pronuclear (MPN) formation rates of oocytes matured in medium containing 100 or 1000 ng/ml of Ang II were higher (P<0.05) than that of oocytes matured in medium containing 0 or 10 ng/ml Ang II. Glutathione content in oocytes cultured for 44 h in medium containing 100 or 1000 ng/ml of Ang II was also higher (P<0.01) than that of oocytes cultured in medium containing 0 or 10 ng/ml Ang II. In conclusion, Ang II was present in porcine ovaries and may regulate follicle growth and oocyte maturation.

Effects of exposure of epididymal boar spermatozoa to seminal plasma on the binding of zona pellucida proteins during in vitro capacitation

The purpose of the investigation was to determine whether seminal plasma plays a role in the increase during in vitro capacitation of the number of boar spermatozoa with enhanced binding of zona pellucida proteins. Ejaculated spermatozoa and spermatozoa collected from the caudae epididymides of boars were incubated at 39 degrees C in a Tyrode's IVF medium. During incubation, the zona binding ability of individual spermatozoa was assessed with fluorescein-conjugated solubilized zona pellucida proteins (FITC-sZP), using a flow cytometer. Propidium iodide (PI) was included to simultaneously monitor cell viability. During incubation of ejaculated spermatozoa, a percentage of the spermatozoa expressed enhanced binding of FITC-sZP. The percentage of viable spermatozoa with enhanced binding reached a maximum of 37% (S.D.=8, averaged over five boars) after 2-3 h. In epididymal sperm, a similar maximum was observed after incubation in vitro, but a longer time of incubation was needed (6 h). Also, the rate of cell death of epididymal sperm was much lower than that of ejaculated sperm. When epididymal spermatozoa was exposed to seminal plasma in vitro, the time needed to reach a maximal percentage of viable spermatozoa with enhanced FITC-sZP binding was similar to that in ejaculated semen. However, the rate of cell death was still much lower than in ejaculated sperm. We concluded that the binding sites on the sperm surface that are involved in the increased binding of zona proteins during incubation under IVF conditions were not derived from the seminal plasma. The cellular processes leading to the increased binding capacity were accelerated by exposure of the sperm to seminal plasma.

Capacity of boar spermatozoa to bind zona pellucida proteins in vitro in relation to fertilization rates in vivo

The purpose of this study was to determine variation among boars in the percentage of sperm in an ejaculate that express enhanced binding of zona pellucida proteins during treatment for capacitation in vitro, and to determine whether this relates to fertilizing ability in vivo. Ejaculates (n=35) were collected from 12 boars. A sample of each ejaculate was treated for capacitation in vitro. During incubation, the zona binding ability of spermatozoa was assessed at regular intervals with fluorescein-conjugated solubilized zona pellucida proteins (FITC-sZP) and propidium iodide, using a flow cytometer. After incubation, a percentage of the sperm had enhanced FITC-sZP binding. The percentage of viable sperm with enhanced FITC-sZP binding, expressed as a percentage of the total sperm population, increased rapidly over the first 60 min and thereafter reached a plateau after 120-180 min. Averaged over all ejaculates, the percentage at 180 min was 46% (range 27-61%); this percentage was significantly different among boars. However, the variation between ejaculates within a boar was relatively small. There was no significant boar effect on the rate at which the percentage of viable cells with enhanced FITC-sZP binding reached the maximum. In ejaculates (n=14) from four boars (selected from the group of 12), we investigated the increase in the percentage of viable sperm with enhanced sZP binding during treatment for capacitation in vitro in relation to the ability to fertilize in vivo. Sows (n=44) were inseminated 4 h after ovulation with a suboptimal insemination dose (0.5x10(9) spermatozoa). Time of ovulation was determined using transrectal ultrasonography and sows were killed at 120 h after ovulation. The percentage of fertilized oocytes, embryo development, and numbers of accessory spermatozoa were determined. The percentage of spermatozoa that were viable and showed enhanced sZP binding after 180 min of incubation was 48 +/- 12% (range 28-56%). The percentage of fertilized oocytes was 85 +/- 27% and 64% of the sows had 100% fertilized oocytes. The percentage of sows with 100% fertilized oocytes correlated well (P< or =0.05, R2=0.98) with the percentage of viable spermatozoa with enhanced FITC-sZP binding after capacitation in vitro.

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.

Gp273, the ligand molecule for sperm-egg interaction in the bivalve mollusk, Unio elongatulus, binds to and induces acrosome reaction in human spermatozoa through a protein kinase C-dependent pathway

In a previous article, we suggested that gp273, the ligand molecule for sperm-egg interaction in the bivalve mollusk Unio elongatulus has functional carbohydrate epitopes in common with a human zona pellucida glycoprotein, probably ZP3. We demonstrated that: 1) anti-gp273-purified immunoglobulin G (IgG), which recognizes a carbohydrate gp273 epitope including a Lewisa-like structure, interacts with a zona pellucida protein; 2) human sperm specifically bind to gp273; and 3) binding is reversed by anti-gp273 IgG. In the present study, we confirm this suggestion by demonstrating that heat-solubilized zonae pellucidae reverse gp273-human sperm binding, that gp273-binding sites are restricted to the acrosomal region, and that gp273 induces the acrosome reaction in human sperm. We also demonstrated that gp273-binding sites on human sperm function as signaling receptors because exposure of spermatozoa to this glycoprotein results in significant stimulation of protein kinase C (PKC) activity. Because the PKC inhibitor, bisindolylmaleimide I, reverses both PKC activation and the acrosome reaction, this kinase is a key component of the signal transduction pathway activated by gp273 and leading to the exocytotic event.

Localization and functional importance of a conserved zona pellucida 2 protein domain in the human and bovine ovary using monoclonal anti-ZP2 peptide antibodies

In mammals, gamete recognition and sperm binding to the oocyte are mediated by the zona pellucida (ZP), an acellular coat surrounding the plasma membrane of the oocyte that consists of particular ZP proteins. The ZP2 protein mediates secondary sperm binding to the ZP. Its primary structures are highly conserved as revealed by cDNA cloning. In the present study, we investigated the localization of ZP2 in human and bovine ovaries and oocytes and the influence of monoclonal anti-ZP2 peptide antibodies upon bovine sperm-egg interactions. We generated a monoclonal anti-ZP2 synthetic peptide antibody, mAb ZP2-20, against a sequence that is strongly conserved in the mammalian ZP2 amino acid sequence. Specificity of mAb ZP2-20 was determined by ELISA and immunoblotting, respectively. Our results show that mAb ZP2-20 specifically detected the peptide used as an antigen and reacted with its corresponding protein antigen in human and bovine ovaries. In order to elucidate effects of mAb ZP2-20 upon bovine sperm-ZP binding, we used the competitive hemizona assay (cHZA) and found that the antibodies clearly inhibit sperm binding to the ZP. We conclude that (i). monoclonal antibodies against ZP2 peptides react with ZP proteins present in bovine and human ovaries and can be used as a specific marker for ZP2; and that (ii). mAb ZP2-20 detects a ZP2 epitope that is of functional relevance for sperm-ZP interactions.

Zona pellucida glycoproteins based immunocontraceptive vaccines: strategies for development and their applications

The mammalian oocyte is surrounded by an extra-cellular matrix, the zona pellucida (ZP), composed of three major glycoproteins (ZP1, ZP2 and ZP3). The ZP glycoproteins, by virtue of their tissue specificity and critical role during mammalian fertilization, have emerged as potential candidate antigens for the development of an immunocontraceptive vaccine. Molecular characterization of ZP glycoproteins from several species, reveals a variable degree of homology among the deduced primary amino acid sequences, which provided an opportunity to undertake active immunization studies in heterologous animal models. Active immunization of various animal species with either native ZP glycoproteins or those obtained by recombinant DNA technology led to the inhibition of fertility. Thus ZP glycoproteins based immunocontraceptive vaccines offer an attractive proposition for controlling wild life population. To make it a practical proposition, additional research inputs are required to optimize and devise novel strategies for vaccine delivery. Observed ovarian dysfunction, often associated with immunization by ZP glycoproteins is one of the major stumbling blocks for their use in humans. Ongoing studies to delineate appropriate B cell epitopes of ZP glycoproteins that are devoid of oophoritogenic T-cell epitopes, which will inhibit fertility without concomitant oophoritis, will be critical to determine their feasibility for human use.

Glycoconjugates in sperm function and gamete interactions: how much sugar does it take to sweet-talk the egg?

Glycoconjugates in the mammalian reproductive tract are critical components of the molecular mechanisms that control sperm maturation, sperm transport and gamete interactions. In the oviduct of many species, sperm transport and maturation are regulated by protein-carbohydrate interactions that form a sperm reservoir. Subsequently, gamete interactions are mediated by the binding of lectin-like sperm proteins with carbohydrate moieties on the zona pellucida. The sperm glycocalyx is extensively modified during sperm transport and maturation. Multiple functions have been proposed for this dense carbohydrate layer overlying the sperm plasmalemma, and sperm-surface carbohydrates have been implicated in immune-mediated human infertility. The structure and function of glycoconjugates in the oviductal sperm reservoir, the zona pellucida, and on the sperm surface are reviewed.

Oocyte development and egg envelope formation in Oreochromis niloticus, a mouth-brooding cichlid fish

The development of the oocyte and of its associated follicle cells in the Nile tilapia, Oreochromis niloticus, has been examined by optical and transmission electron microscopy. During oocyte development the female gamete of Orochromis niloticus increases in size because of the accumulation of yolk in its cytoplasm. As the accumulation of yolk proceeds, the organization of cortex of the oocyte becomes very complex; all of the cytoplasmic organelles and several populations of vesicles can be found. On the other hand follicle cells also undergo a series of modifications: they first become cuboidal then cylindrical and their cytoplasm become densely populated with organelles. The mature egg of Oreochromis niloticus is surrounded by a thin acellular envelope (chorion) assembled during oocyte development. Biochemical analysis of isolated and purified chorions from mature females was also performed. SDS-PAGE under reducing conditions showed a reproducible pattern of three major polypeptides (121, 66 and 50 kD), most of which being glycosylated. The pattern of synthesis and assembly of the egg envelope in Oreochromis niloticus, a mouth-brooding cichlid fish, is also discussed.

Vascular endothelial growth factor receptor-2 activation induces vascular permeability in hyperstimulated rats, and this effect is prevented by receptor blockade

The existence of a vasoactive molecule released in response to hCG is believed to be the main feature in the development of ovarian hyperstimulation syndrome (OHSS) in women, and vascular endothelial growth factor (VEGF) is the main candidate as the human chorionic gonadotropin (hCG) mediator. This study was conducted to investigate the role of VEGF in increasing vascular permeability (VP) in vivo, a characteristic of OHSS. We analyzed the source and specific isoforms of VEGF involved and developed strategies to reverse increased VP in hyperstimulated rats targeting the VEGF system. Ovarian hyperstimulation was induced with pregnant mare's serum gonadotropin, or pregnant mare's serum gonadotropin plus hCG. Time-course experiments analyzed VP and the expression of whole VEGF mRNA in the mesentery and the ovaries. VP and ovarian mRNA VEGF expression increased to peak values after 48 h. No significant change in expression was observed in the mesentery. To further prove the ovarian origin of VEGF, we showed that VP was not altered when ovariectomized rats were treated with gonadotropins. The ovary expressed VEGF(120) and VEGF(164) isoforms. Immunohistochemistry showed VEGF in granulosa and zona pellucida of preovulatory and atretic follicles and in granulosa-lutein and endothelial cells of whole corpus luteum. A specific VEGF receptor-2 inhibitor (SU5416) was administered in three different protocols: on a daily basis, every 48 h, or two injections after hCG. Increased VP was reversed when SU5416 was administered every 48 h or two injections after hCG. These results show that the ovary is the main source of VEGF(120) and VEGF(164), which act through the VEGF receptor-2 to increase VP, and provide new insights into the prevention of OHSS.

Characterization of SMOC-1, a novel modular calcium-binding protein in basement membranes

We have isolated the novel gene SMOC-1 that encodes a secreted modular protein containing an EF-hand calcium-binding domain homologous to that in BM-40. It further consists of two thyroglobulin-like domains, a follistatin-like domain and a novel domain. Recombinant expression in human cells showed that SMOC-1 is a glycoprotein with a calcium-dependent conformation. Results from Northern blots, reverse transcriptase-PCR, and immunoblots revealed a widespread expression in many tissues. Immunofluorescence studies with an antiserum directed against recombinant human SMOC-1 demonstrated a basement membrane localization of the protein and additionally its presence in other extracellular matrices. Immunogold electron microscopy confirmed the localization of SMOC-1 within basement membranes in kidney and skeletal muscle as well as its expression in the zona pellucida surrounding the oocyte.

Localization of N-linked carbohydrate chains in glycoprotein ZPA of the bovine egg zona pellucida

The zona pellucida, a transparent envelope surrounding the mammalian oocyte, consists of three glycoproteins, ZPA, ZPB and ZPC, and plays a role in sperm-egg interactions. In bovines, these glycoproteins cannot be separated unless the acidic N-acetyllactosamine regions of the carbohydrate chains are removed by endo-beta-Galactosidase digestion. Endo-beta-Galactosidase-digested ZPB retains stronger sperm-binding activity than ZPC. It is still unclear whether ZPA possesses significant activity. Recently, we reported that bovine sperm binds to Man5GlcNAc2, the neutral N-linked chain in the cow zona proteins. In this study, we investigated the localization of the sperm-ligand active high-mannose-type chain and the acidic complex-type chains in bovine ZPA. Three N-glycopeptides of ZPA, containing an N-glycosylation site at Asn83, Asn191 and Asn527, respectively, were obtained from endo-beta-Galactosidase-digested ZPA. Of these glycosylation sites, only Asn527 is present in the ZP domain common to all the zona proteins. The carbohydrate structures of the N-linked chains obtained from each N-glycopeptide were characterized by two-dimensional sugar mapping analysis, while considering the structures of the N-linked chains of the zona protein mixture reported previously. Acidic complex-type chains were found at all three N-glycosylation sites, while Man5GlcNAc2 was found at Asn83 and Asn191, but there was very little of this sperm-ligand active chain at Asn527 in the ZP domain of ZPA.

Molecular basis of mammalian gamete binding

Despite the importance of fertilization for controlling human reproduction, regulating animal production, and promoting preservation of endangered species, the molecular basis underlying gamete binding and fertilization has been perplexing. More progress has been made in the mouse than in other mammals and, recently, targeted deletion of specific genes in the mouse has yielded intriguing results. This review will emphasize research performed by our laboratory and others done primarily with mouse gametes but will include some interesting observations from other mammals. Studies of murine fertilization indicate that oligosaccharides on the egg coat glycoprotein ZP3 bind sperm. The precise oligosaccharides that bind sperm are the subject of considerable debate. ZP3 also induces exocytosis of the sperm acrosome, allowing sperm to penetrate through the egg coat (zona pellucida). A number of candidate ZP3 receptors have been proposed and studies of beta1,4galactosyltransferase-I (GalT-I) are reviewed here in the most detail. Sperm from mice with a targeted deletion of GalT-I still are able to bind the zona pellucida but are unable to acrosome react and penetrate through the zona. Therefore, the unique role of GalT-I appears to be in signal transduction. GalT-I forms a complex with heterotrimeric G proteins and activates signaling, leading to exocytosis in sperm and in heterologous cells expressing GalT-I. Other signaling steps triggered by GalT-I are under active investigation; this receptor forms a complex with a protein kinase anchoring protein. After exocytosis of the acrosome, sperm penetrate the zona pellucida and fuse with the oocyte plasma membrane using ADAM family members on sperm and integrins on oocytes. These proteins, along with the tetraspanins on oocytes, may form a complex web at gamete fusion. Targeted deletion of specific genes in this putative complex has provided important information about their redundancy. After the oocyte is fertilized, the binding site for GalT-I is lost from ZP3, preventing additional sperm from binding to the zona pellucida. New technical advances and creative ideas offer the opportunity to make important advances and to solve the conundrum of fertilization.

Characterization of zona pellucida glycoprotein 3 (ZP3) and ZP2 binding sites on acrosome-intact mouse sperm

There is considerable evidence that mouse fertilization requires the binding of sperm to two of the three glycoproteins that form the zona pellucida (ZP), ZP3 and ZP2. Despite the biologic importance of this binding, no one has demonstrated that sperm express separate, saturable, and specific binding sites for ZP3 and for ZP2. Such a demonstration is a prerequisite for defining the distribution, numbers, affinities, and regulation of function of ZP3 and ZP2 binding sites on sperm. The experiments reported herein used fluorochrome-labeled ZP3 and ZP2 and quantitative image analysis to characterize the saturable binding of ZP3 and ZP2 to distinct sites on living, capacitated, acrosome-intact mouse sperm. Approximately 20% of the ZP3 binding sites were found over the acrosomal cap, and the remaining sites were located over the postacrosomal region of the head. In contrast, ZP2 binding sites were detected only over the postacrosomal region. Saturation analysis estimated numbers and affinities of the binding sites for ZP3 (B(max) approximately 185 000 sites per sperm; K(d) approximately 67 nM) and ZP2 (B(max) approximately 500 000 sites per sperm; K(d) approximately 200 nM). Use of unlabeled ZP3, ZP2, and ZP1 as competitive inhibitors of the binding of fluorochrome-labeled ZP3 and ZP2 demonstrated that ZP3 and ZP2 bound specifically to their respective sites on sperm. Finally, we demonstrate that extracellular calcium as well as capacitation and maturation of sperm are required for these sites to bind their respective ligands.