A comparative analysis of molecular mechanisms for blocking polyspermy: identification of a lectin-ligand binding...

Fertilization is a critically important event to the creation of a new individual organism and to the propagation of a species. Evolutionarily conserved cellular and molecular mechanisms exist to modify the glycoproteins composing the egg extracellular matrix at fertilization. These matrix modifications regulate the cellular interactions of sperm and egg, maintain the diploid state of the nucleus after successful union of the two gametes (block to polyspermy) and control the environment for the developing embryo. Only recently have mammals been studied regarding extracellular matrix block to polyspermy mechanisms compared to the long term investigations of the same in sea urchins, fish and amphibians - knowledge of evolutionary conserved mechanisms in these animal groups can be used to predict the existence of mechanisms in mammals. Experimental evidence exists for the conservation of proteolytic, glycolytic, cross linking, conformational and binding mechanisms for establishing extracellular matrix blocks to polyspermy at fertilization. Analogous to a binding mechanism in anurans, a lectin-ligand binding mechanism for establishing an extracellular matrix block to polyspermy in mammalian eggs has been discovered. This binding mechanism involves the exocytotic release of a cortical granule lectin in the sperm-induced egg cortical reaction, diffusion and binding of the lectin to its ligand associated with the zona pellucida, and prevention of sperm-zona pellucida binding by the lectin-ligand reaction, thereby resulting in a block to polyspermy at fertilization. The glycoproteins involved in the lectin-ligand polyspermy block can potentially be used as targets for contraception.