A ZP2 Cleavage Model of Gamete Recognition and the Postfertilization Block to Polyspermy

Abstract

The molecular basis of gamete recognition and the corresponding block to polyspermy in mammals have intrigued investigators for decades. Taking advantage of the fastidious nature of human sperm , which will not bind to the mouse zona pellucida , gain-of-function assays have been established in transgenic mice by replacing endogenous mouse proteins with the corresponding human homologue. In the presence of human ZP2 , by itself or with the three other human zona proteins, human sperm bind and penetrate the 'humanized' zona pellucida but do not fuse with the mouse egg. Using recombinant ZP2 peptides in a bead binding assay, the gamete recognition site was located to a ~115 amino-acid N-terminal domain. Following fertilization , egg cortical granules exocytose ovastacin , an oocyte-specifi c metalloendoprotease , that cleaves the N-terminus of ZP2 and prevents sperm binding to the zona surrounding the preimplantation embryo. Genetic ablation of the enzyme or mutation of the ZP2 cleavage site prevents cleavage and sperm bind de novo to the surface of the zona pellucida even after fertilization and cortical granule exocytosis. These observations form the basis of the ZP2 cleavage model of gamete recognition in which mammalian sperm bind to an N-terminal domain of ZP2. Following penetration through the zona matrix and gamete fusion, the egg cortical granules exocytose ovastacin, which cleaves ZP2 and provides a defi nitive block to sperm binding at the surface of the zona pellucida.