Reassessing the role of protein-carbohydrate complementarity during sperm-egg interactions in the mouse

Despite years of intense study by many investigators, it may appear that we have made little progress towards a molecular understanding of mammalian sperm binding to the egg zona pellucida. An abundance of evidence derived from in vitro assays suggests that sperm-zona pellucida binding is dependent upon sperm recognition of specific glycan moieties on the zona pellucida glycoproteins. However, there is considerable disagreement regarding the identity of the zona pellucida sugars thought to mediate sperm binding, as well as disagreement over the identity of the sperm receptors themselves. Moreover, results from in vivo gene-targeting strategies fail to support a role for many, if not all, of the sperm receptors and their zona pellucida ligands implicated from in vitro assays. Nevertheless, a retrospective view of the literature suggests that some common principles are emerging regarding the molecular basis of mammalian sperm-zona binding, both with respect to the nature of the components that mediate binding, as well as the involvement of distinct receptor-ligand interactions, that involve both protein- and carbohydrate-dependent mechanisms of binding.