Studies on the sperm/oocyte interaction in fertilization

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A fertilizing mammalian sperm penetrates into zona pellucida, an extracellular matrix surrounding oocyte, to fuse the plasma membranes of both cells. It is a general concept that as a consequence of acrosome reaction of sperm, the acrosomal components are released, and interact with the zona pellucida. Acrosin, an endoprotease with a trypsin-like cleavage specificity, is localizing in the acrosome matrix as an enzymatically inactive zymogen, proacrosin. The physiological function of acrosin has long been believed to be a limited proteolysis of the zona pellucida, enabling sperm to penetrate to the oocyte surface. Here we show that male mice homozygous for a targeted mutation in the acrosin gene are still fertile in spite of the complete loss of the acrosin protease activity in the sperm. In vitro fertilization test reveals a time delay of the homozygous mouse sperm in penetrating into the zona pellucida. This delay is likely due to the fact that production of a 42-kDa serine protease is suppressed in vitro in the homozygous mouse sperm. Therefore, acrosin is not essential for the penetration of sperm into the zona pellucida. Rather, the function of acrosin in the early stages of fertilization may be implicated in the acceleration of formation of the active 42-kDa trypsin-like protease that is a candidate for the protein acting on the sperm penetration.