The zona pellucida "receptors"

Binding of mammalian sperm to the zona pellucida and the induction of the acrosome reaction are prerequisites for successful oocyte fertilization. In the mouse model, the zona pellucida consists of three sulfated glycoproteins, ZP1, ZP2, and ZP3. Zona pellucida proteins are secreted to form a filamentous zona matrix in which ZP2 and ZP3 complex into co-polymers cross-linked by ZP1. ZP3 is the ligand for primary sperm binding and important for the induction of the acrosome reaction. The zona pellucida glycoprotein ZP2 is also crucially involved in the process of fertilization. Previous reports suggest that ZP2 mediates secondary binding of spermatozoa and that cleavage of ZP2 by proteases released through cortical granule reaction causes zona "hardening" and thus prevents polyspermy. Human and mouse ZP2 proteins differ in the primary structure as derived from cDNA clones. We designed an immunological approach to search for ZP2 domains with functional relevance. Antisera were generated against synthetic peptides derived (a) from ZP2 amino acid sequences that are homologous in human and mouse ZP2 amino acid sequences (AS ZP2-20) or (b) from human ZP2 amino acid sequences that differ from the mouse ZP2 sequence (AS ZP2-26). Immunochemical studies with microbisected bovine zonae pellucidae demonstrated that both antisera, AS ZP2-20 and AS ZP2-26, specifically detected ZP2 protein. Using the competition-hemizona-assay, sperm binding to antibody treated bovine hemizonae pellucidae were compared with control hemizonae (given as hemizona index). Antiserum AS ZP2-20 significantly inhibited binding of spermatozoa to test hemizonae (p < 0.0001), whereas treatment of hemizonae with AS ZP2-26 did not influence sperm-egg interaction. Our results show that antibodies against ZP2 peptides react with bovine zonae pellucidae and can be used as markers for ZP2. Furthermore, AS ZP2-20 identifies a ZP2 epitope that is possibly of functional relevance for sperm-egg interaction.