Sperm-egg recognition

Abstract: Recent evidence indicates that a carbohydrate recognition mechanism is involved in the early stages of sperm-egg interaction in mammals. In this communication, we describe a potential zona-ligand molecule on boar spermatozoa that has the capacity to recognize and bind to carbohydrate moieties of zona pellucida glycoproteins as well as neoglycoproteins, BSA-fucose and BSA-mannose. The molecule has broad specificity for carbohydrate binding and there is a requirement for a polysaccharide structure or for ‘clustering’ of saccharides on a protein backbone. The molecule has been identified as proacrosin, the zymogen form of the acrosomal protease acrosin. Strong similarities exist between proacrosin and ‘bindin’, the lectin-like protein that is thought to mediate sperm-egg adhesion in echinoderms. An hypothesis is proposed for sperm-egg interaction in mammals in which proacrosin, released during the early stages of the acrosome reaction, mediates secondary or consolidated binding of spermatozoa to the zona pellucida by virtue of its carbohydrate-binding capacity. The localized proteolytic action of active acrosin on the zona enhances this interaction in a manner analogous to the requirement for trypsinization of erythrocytes before agglutination by certain lectins. This hypothesis, which is supported by evidence from in vitro fertilization experiments, is discussed in relation to current concepts on sperm-egg recognition.

Abstract: The zona pellucida surrounding the egg and pre-implantation embryo is required for in vivo fertility and early development. Explanatory models of sperm-egg recognition need to take into account the ability of sperm to bind to ovulated eggs, but not to two-cell embryos. For the last two decades, investigators have sought to identify an individual protein or carbohydrate side chain as the 'sperm receptor'. However, recent genetic data in mice are more consistent with the three-dimensional structure of the zona pellucida, rather than a single protein (or carbohydrate), determining sperm binding. The mouse and human zonae pellucidae contain three glycoproteins (ZP1, ZP2, ZP3) and, following fertilization, ZP2 is proteolytically cleaved. The replacement of endogenous mouse proteins with human ZP2, ZP3 or both does not alter taxon specificity of sperm binding or prevent fertility. Surprisingly, human ZP2 is not cleaved following fertilization and intact ZP2 correlates with persistent sperm binding to two-cell embryos. Taken together, these data support a model in which the cleavage status of ZP2 modulates the three-dimensional structure of the zona pellucida and determines whether sperm bind (uncleaved) or do not (cleaved).

Abstract: Publisher Summary This chapter describes gamete surface components that support tight species-specific binding of the free-swimming mammalian sperm to unfertilized eggs. Such binding is mediated in part, by carbohydrate. The zona pellucida glycoprotein, “ZP3,” serves as sperm receptor during fertilization in mammals. Close inspection of the characteristics of this glycoprotein, coupled with some speculation, may shed light on sperm–egg recognition mechanisms in mammals. ZP3 consists of at least two different functional domains, perhaps separated from each other, by a hinge region of the polypeptide. One of the domains is thought to include the C-terminal third of the ZP3 polypeptide, which possesses the O-linked oligosaccharides that constitute, at least in part, the combining site for the sperm.