Differential expression of glycoside residues in the mammalian zona pellucida

The mammalian zona pellucida is an extracellular matrix surrounding the oocyte, and is composed of three major glycoproteins, ZP1, ZP2, and ZP3. Previous studies have suggested that the sperm receptor activity of the zona pellucida resides in specific oligosaccharide chains on the ZP3 glycoprotein. However, the nature of the terminal monosaccharide(s) on these glycosidic chains to which sperm bind is a matter of active debate. Evidence has been presented to support a role for at least three distinct monosaccharides in sperm binding, alpha-galactose, L-fucose on Lewis X structures, and beta-N-acetylglucosamine. Previous studies have shown that beta-N-acetylglucosamine is uniformly distributed throughout the zona matrix. In this study, we have investigated the expression and distribution of alpha-galactose and fucose moieties during the maturation of the zona pellucida in mouse, rat, and hamster. Interestingly, alpha-galactose residues are expressed only during later stages of zona secretion and, consequently, are confined to the inner portions of the mature zona pellucida in mouse and rat. In hamster, alpha-galactose residues are only detectable in the zona pellucida of ovulated eggs, and are not found in ovarian oocytes. Fucosyl residues linked to Lewis X glycosides are not detectable at any stage of zona maturation in these three species, whereas fucose linked to N-linked core oligosaccharides are present throughout the zona. These studies indicate a previously unappreciated heterogeneity in the composition of zona glycosides. The specific localization of alpha-galactose residues to the inner portions of the zona matrix suggest a role in the later stages of sperm penetration through the zona. Finally, due to their absence from the zona surface, alpha-galactose and Lewis X fucosyl residues are not likely to be mediators of primary sperm binding.