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Fertilization: Sperm/Egg Recognition and Contact |
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ZP-3 functions as a sperm receptor. The sperm-binding activity of ZP3 is mediated by the oligosaccharide side chains of ZP3. The role of the oligosaccharides is demonstrated by experiments in which either removal or modification of the sugars causes loss of sperm-binding activity. A peripheral membrane protein in the plasma membrane overlying the acrosome of mouse sperm called sp56 binds to the oligosaccharide moieties of ZP3. sp56 belongs to a class of carbohydrate-binding proteins called lectins. Purified sp56 binds to the zona of unfertilized eggs, but not to that of zygotes (Bookbinder et al., 1995, Fig. 1). This observation suggests that the oligosaccharides on ZP3 trap incoming sperm at the zona surface of unfertilized eggs and that this activity is lost after fertilization. Bookbinder et al. (1995) have also reported a correlation between the presence of sp56 and species specificity of sperm-egg recognition (Table 1). Mouse and hamster sperm contain sp56 and bind to the mouse egg. Guinea pig and human sperm, however, lack sp56 and do not bind. Different lectins may be involved in human and rabbit sperm-egg binding. In humans, the oligosaccharides on ZP3 differ from those in mice (Aitken, 1995). The carbohydrates present on ZP3 may provide a code that facilitates species specificity of sperm-egg binding. Investigators may be able to exploit this mechanism in developing new contraceptives or in diagnosis and treatment of male infertility. Binding of the sperm to the zona triggers the acrosome reaction (see "Overview of the mammalian acrosome reaction" from Zygote), which allows the sperm to penetrate the zona. The sperm form a slit in the zona that is approximately the width and height of their head. The slit is apparently formed through a combination of force and digestion by the enzymes released by the acrosome reaction (Allen and Green, 1997). After penetrating the zona, incoming sperm enter the perivitelline space surrounding the egg and land on the egg plasma membrane, where the equatorial segment of the sperm head initiates sperm-egg adhesion. A sperm protein called fertilin is thought to be involved in mediating adhesion in mammals. Fertilin has a molecular domain (called the disintegrin domain) that is known to interact with membrane proteins called integrins. This led to the expectation that integrins on the egg surface serve as the sperm receptor. Various observations supported this hypothesis, including:
An integrin that is necessary for fertilization has recently been identified on the surface of mouse eggs (Almeida et al., 1995) and is a putative sperm receptor on the egg surface. Almeida et al. used antibodies to integrins to demonstrate the presence of specific integrins on the surface of mouse eggs. Significant amounts of the alpha6 subunit were detected (Fig. 2). This subunit is known to form heterodimers with a � subunit known as �1. To test whether sperm bind to alpha6�1, Almeida et al. used approaches that allowed them to block alpha6- and �1-integrin binding, respectively. A monoclonal antibody (GoH3) that reacts with alpha6 inhibited sperm binding to zona-free mouse eggs in a dose-dependent fashion (Figs. 3C and 4). They also employed a mouse embryonal carcinoma cell line, F9, which expresses several surface integrins, including alpha6�1, as a test system for sperm binding. They knocked out the gene expressing �1, resulting in a cell line called F9 TKO. Sperm bound avidly to normal F9 cells but poorly to the F9 TKO cells (Fig. 5A). This result indicated that a �1-containing integrin is involved in sperm binding. They also used the anti-alpha6 antibody GoH3 to show that it inhibited sperm binding to F9 cells (Fig. 5B).
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