Carbohydrate-mediated binding and induction of acrosomal exocytosis in a boar sperm-somatic cell adhesion model

The molecular basis underlying the binding of spermatozoa to their homologous eggs and the subsequent induction of acrosomal exocytosis remain a major unresolved issue in mammalian fertilization. Novel cell adhesion systems are now being explored to advance this research. Triantennary and tetraantennary N-glycans have previously been implicated as the major carbohydrate sequences that mediate the initial binding of spermatozoa to the specialized egg coat (zona pellucida) in the murine and porcine models. Mouse spermatozoa also undergo binding to rabbit erythrocytes (rRBCs), presumably via the interaction of their lectin-like egg-binding proteins with branched polylactosamine sequences present on these somatic cells. Experiments presented in this study confirm that boar spermatozoa also bind to rRBCs. However, unlike mouse spermatozoa, boar spermatozoa also undergo acrosomal exocytosis within 30 min after binding to rRBCs. Both binding and induction of acrosomal exocytosis in this system did not require the participation of terminal Galalpha1-3Gal sequences that are found on rRBCs. Pronase glycopeptides derived from rRBCs inhibited the binding of boar sperm to porcine oocytes by 91% at a final concentration of 0.3 mg/ml under standard IVF conditions. Binding in this porcine cell adhesion model was also completely blocked at this concentration of glycopeptide. Thus, adhesion results from the interaction of the egg-binding protein expressed on the surface of boar spermatozoa with the glycans presented on rRBCs. This cell adhesion model will be useful for investigating the molecular basis of gamete binding and the induction of acrosomal exocytosis in the pig.