A secreted enzyme helps egg cells avoid being fertilized by more than one sperm, a new study has revealed. The research is published in The Journal of Cell Biology.
Because polyspermy disrupts embryonic development, oocytes take several steps to ensure they only fuse with a single sperm. One key step is to prevent additional sperm from binding to the surface of an already-fertilized egg, a blockade that involves the release of secretory granules and cleavage of a protein called ZP2, a component of the zona pellucida matrix that surrounds eggs. ZP2 is cleaved at a site targeted by the astacin family of metalloendoproteases - enzymes that cut proteins into smaller fragments. Researchers from the NIH's National Institute of Diabetes and Digestive and Kidney Diseases therefore investigated the function of ovastacin, an astacin family member expressed in oocytes.
Ovastacin localized to cortical granules that were exocytosed after fertilization, and recombinant ovastacin cleaved ZP2 when added to zonae pellucidae. Mice lacking ovastacin failed to cleave ZP2 after fertilization, allowing sperm to continue to bind to the surface of early embryos. Female mice lacking ovastacin had slightly fewer offspring than wild-type animals but otherwise appeared normal.