Studies on the sperm/oocyte interaction in fertilization

哺乳动物精子穿透卵母细胞周围的细胞外基质透明膜，融合两个细胞的质膜。一般认为，精子顶体反应的结果是顶体成分被释放出来，并与透明膜相互作用。顶体蛋白酶是一种具有胰蛋白酶样切割特异性的内切蛋白酶，作为无酶活性的酶原顶体蛋白酶原定位于顶体基质中。顶体酶的生理功能一直被认为是透明质酸的有限蛋白水解，使精子能够穿透到卵母细胞表面。在这里，我们表明，雄性小鼠的顶体酶基因的靶向突变纯合子仍然是可育的，尽管在精子中的顶体酶蛋白酶活性的完全丧失。体外受精实验表明，纯合子小鼠精子穿透透明膜的时间延迟。这种延迟可能是由于在纯合小鼠精子中42 kDa丝氨酸蛋白酶的产生在体外受到抑制。因此，顶体酶不是精子穿透透明膜所必需的。相反，在受精的早期阶段，顶体酶的功能可能涉及到的活性42-kDa胰蛋白酶样蛋白酶的形成的加速，这是一个候选人的蛋白作用于精子穿透。

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.