Identification of porcine oocyte 55 kDa alpha and beta proteins within the zona pellucida glycoprotein families indicates that oocyte sperm receptor activity is associated with different zone pellucida proteins in different mammalian species

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

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

Porcine model for the study of sperm capacitation, fertilization and male fertility

Mammalian fertilization remains a poorly understood event with the vast majority of studies done in the mouse model. The purpose of this review is to revise the current knowledge about semen deposition, sperm transport, sperm capacitation, gamete interactions and early embryonic development with a focus on the porcine model as a relevant, alternative model organism to humans. The review provides a thorough overview of post-ejaculation events inside the sow's reproductive tract including comparisons with humans and implications for human fertilization and assisted reproductive therapy (ART). Porcine methodology for sperm handling, preservation, in vitro capacitation, oocyte in vitro maturation, in vitro fertilization and intra-cytoplasmic sperm injection that are routinely used in pig research laboratories can be successfully translated into ART to treat human infertility. Last, but not least, new knowledge about mitochondrial inheritance in the pig can provide an insight into human mitochondrial diseases and new knowledge on polyspermy defense mechanisms could contribute to the development of new male contraceptives.

Development of a new, highly sensitive zona pellucida binding assay using a bioluminescence-enhanced detection system

To date, two different zona binding assays have been described in the literature. Both assays, however, require a large quantity of human zonae which vary immensely in quality. Furthermore, an inverted microscope with micromanipulation equipment is necessary, which makes both assays relatively complicated and time-consuming, and requires skilled staff. Therefore, we developed a new, highly sensitive zona binding assay using a bioluminescence-enhanced system which employs a pool of solubilized zona pellucida and is easier for routine use. In the detection system, light emission by the luciferin-luciferase system is measured. Because of the limited availability of human zonae pellucidae, this new assay was first developed in the porcine system. The new bioluminogenic substrate D-luciferin-O-beta-galactopyranoside (Lu-Gal) was synthesized, purified and characterized. Synthesis of Lu-Gal resulted in purity better than 99.998%. Analytical data and spectra were appropriate. In terms of the kinetic data, Lu-Gal is a highly sensitive and specific substrate for beta-galactosidase. Using the given chemical conditions, nonlabelled zonae bound competitively to boar spermatozoa, which resulted in a high sensitivity and specificity. By the addition of 10 nonlabelled zonae, the binding of labelled zonae was almost completely inhibited. Corresponding results were obtained when the bioluminescent system was compared with the hemizona assay. On the other hand, spermatozoa of other species (bull, hamster and man) showed only low binding to the porcine zonae or none at all. Competitive displacement was not observed, indicating the inter-species specificity of the assay.

Morphological and biochemical changes of isolated chicken egg-envelope during sperm penetration: degradation of the 97-kilodalton glycoprotein is involved in sperm-driven hole formation on the egg-envelope

The chicken egg-envelope is made of two major glycoprotein components, which are designated as gp97 and gp42 (after their molecular masses). To elucidate how these two components are involved in macromolecular organization of the chicken egg-envelope, the isolated egg-envelope was characterized by immunochemical and biochemical methods. The gp97 was suggested to be a homologue of mouse ZPB based on the similarities of N-terminal and internal sequences. Immunoblotting using anti-gp97 monoclonal antibodies and two-dimensional gel electrophoresis with or without mercaptoethanol treatment revealed that gp97 formed a homodimer through disulfide bonds, whereas gp42 did not. Under indirect immunofluorescence microscopy, the anti-gp97 antibody visualized indistinct, small spots on the egg-envelope, whereas the anti-gp42 antibody showed a meshwork of blurry, fibrous structures. The hole formation on the egg-envelope by in vitro sperm penetration was completely inhibited by two anti-gp97 monoclonal antibodies. Interestingly, the anti-gp97 monoclonal antibodies blocked the proteolysis not only of gp97 but also of gp42 during incubation of the egg-envelope with either sperm or the crude chicken acrosin. Taken together, these results indicate that gp97 may play pivotal roles not only in constitution of the macromolecular organization of the egg-envelope but also in triggering hydrolysis of the egg-envelope during sperm penetration.

Xenopus laevis sperm receptor gp69/64 glycoprotein is a homolog of the mammalian sperm receptor ZP2

Little is known about sperm-binding proteins in the egg envelope of nonmammalian vertebrate species. We report here the molecular cloning and characterization of a recently identified sperm receptor (gp69/64) in the Xenopus laevis egg vitelline envelope. Our data indicate that the gp69 and gp64 glycoproteins are two glycoforms of the receptor and have the same number of N-linked oligosaccharide chains but differ in the extent of O-glycosylation. The amino acid sequence of the receptor is closely related to that of the mouse zona pellucida protein ZP2. Most of the sequence conservation, including a ZP domain, a potential furin cleavage site, and a putative transmembrane domain are located in the C-terminal half of the receptor. Proteolytic cleavage of the gp69/64 protein by a cortical granule protease during fertilization removes 27 amino acid residues from the N terminus of gp69/64 and results in loss of sperm binding to the activated eggs. Similarly, we find that treatment of eggs with type I collagenase removes 31 residues from the N terminus of gp69/64 and has the same effect on sperm binding. The isolated and purified N terminus-truncated receptor protein is inactive as an inhibitor of sperm-egg binding. Earlier studies on the effect of Pronase digestion on receptor activity suggest that this N-terminal peptide may contain an O-linked glycan that is involved in the binding process. Based on these results and the findings on the primary structure of the receptor, a pathway for the maturation and secretion of gp69/64, as well as its inactivation following fertilization, is proposed.

Localization of the zona glycoproteins ZPB (ZP3 alpha) and ZPC (ZP3 beta) in the bovine ovary during pre- and postnatal development

The purpose of this study was to investigate the expression of two zona pellucida gene families ZPB (ZP3 alpha) and ZPC (ZP3 beta). Sections of ovaries from bovine fetuses, calves and cows were labelled with polyclonal antibodies. Immunopositive labelling was found in both the follicle cells and the oocyte. Labelling was dependent on the stage of development. The specific sequence of immunopositive reactions suggests that in the bovine fetus both the ovary and the follicle cells contribute to the production of the zona pellucida during pre- and postnatal development.

Molecules involved in mammalian sperm-egg interaction

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

Influence of elevated pH levels on structural and functional characteristics of the human zona pellucida: functional morphological aspects

PURPOSE

A total of 86 fresh and salt-stored immature human oocytes derived from postmortem ovarian tissue were used for this study.

METHODS

Oocytes were randomly incubated either in synthetic human tubal fluid medium (untreated zonae) or in a chemically defined medium (treated zonae).

RESULTS

Sperm binding experiments using hemizona assay conditions exhibited a 10-fold increased binding of sperm to treated compared to untreated oocytes (272.7 +/- 43 versus 24.3 +/- 15 sperm bound, respectively; P < 0.0001). pH recordings during incubation showed elevated pH levels of 8.1 compared to pH 7.2 among treated and untreated zonae, respectively. Ultrastructural examination showed a spongy appearance of the surface of treated zonae, whereas untreated zonae appeared compact with smooth surface.

CONCLUSIONS

The marked increase in sperm binding among treated zonae, together with the ultrastructural findings, suggest that the altered zona surface enhances sperm binding. The physiological maturational process of the zona pellucida might be manipulated in vitro, thus increasing sperm binding to the zona.