Oligosaccharide Constructs with Defined Structures That Inhibit Binding of Mouse Sperm to Unfertilized Eggs in Vitro

During fertilization in mice, free-swimming sperm bind to mZP3, an 83-kDa glycoprotein present in the egg extracellular coat, the zona pellucida [Wassarman, P. M. (1990) Development 108, 1-17], Mouse sperm recognize and bind to a specific class of serine/threonine-linked (O-linked) oligosaccharides present on mZP3. After binding to mZP3, sperm undergo a form of cellular exocytosis, the acrosome reaction, thereby enabling them to penetrate the zona pellucida and fertilize the egg. Thus, gamete interactions in mice are carbohydrate-mediated. In this context, we tested 15 O-linked-related oligosaccharide constructs with defined structures for their ability to inhibit binding of mouse sperm to ovulated eggs and to induce sperm to undergo the acrosome reaction in vitro. Thirteen of the oligosaccharides were constructed and characterized in our laboratory [Seppo, A., Pentilla, L., Niemela, R., Maaheimo, H., Renkonen, O., ' Keane, A. (1995) Biochemistry 34, 4655-4661]; two were obtained commercially. We found that, while none of the oligosaccharides induced sperm to undergo the acrosome reaction, a few of them inhibited binding of sperm to eggs at relatively low concentrations (ID₅₀ < 5 μM). In certain cases, sperm formed head-to-head aggregates in the presence of the oligosaccharides. The results suggest that the ability of oligosaccharides to inhibit binding of sperm to eggs is dependent on several parameters, including the size and branching pattern of the oligosaccharide, as well as on the nature of the sugar residue at the nonreducing end of the oligosaccharide.