Sperm Acrosome Reaction

Binding to the ZP initiates the acrosome reaction and exocytosis, which enables the spermatozoon to penetrate the ZP. The reaction is initiated in acrosome-intact murine sperm by O-linked oligosaccharides of ZP3 binding to protein ligands in the anterior head of the spermatozoon. In other species, it is unclear whether O-linked or N-linked oligosaccharides are the sperm-binding component, and ZPB (e.g., pigs), rather than ZP3, is the primary binding protein. Binding to the ZP induces activation of cyclic AMP/protein kinase A and protein kinase C. Protein kinase A together with inositol im-phosphate activates calcium channels in the outer acrosomal membrane. Depletion of calcium in the acrosome activates a store-operated calcium entry mechanism in the plasma membrane that increases cytosolic calcium and enables contact and fusion of the plasma membrane with the outer acrosomal membrane at multiple sites, leading to membrane vesiculation and release of the acrosome's contents. Release of acrosomal contents exposes the equatorial segment and the inner acrosomal membrane and enables secondary binding to ZP2, which orientates the thrusting spermatozoon for ZP penetration.

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