Murine sperm initiate fertilization by binding to the specialized extracellular matrix of mouse eggs, known as the zona pellucida. Over the past decade, powerful genetic, biophysical, and biochemical techniques have been employed to gain new insights into this interaction. Evidence from these studies does not support either of two major models for binding first proposed over two decades ago. Two more recently established models suggest that protein-protein interactions predominate during this initial stage of fertilization. Another model proposes that about 75-80% of the murine sperm bound to zona pellucida under well defined in vitro conditions is carbohydrate dependent, with the remaining sperm bound via protein-protein interactions. Mounting evidence suggests that the carbohydrate sequences coating the murine egg could be employed as specific immune recognition markers. Continued investigation of this system may resolve many of these controversial findings and reveal novel functions for murine zona pellucida glycoproteins.The species-specific binding of sperm to eggs is the initial event in the development of metazoans that propagate solely via sexual reproduction. Considerable evidence indicates that carbohydrate recognition plays a key role in this interaction in lower species (reviewed in Refs. 1-3), eutherian mammals (4, 5), and marsupials (6).The major animal model for investigating the role of carbohydrate recognition in mammalian fertilization is the mouse. This species is preferred because of its fecundity, short life span, low maintenance costs, and its ability to be genetically manipulated. Murine sperm initiate fertilization by binding to the mZP, 3 a matrix composed of three major glycoproteins designated mZP1, mZP2, and mZP3 (7). mZP3 has been implicated as the component that mediates both initial binding and the induction of the acrosome reaction (8). Its genetic deletion leads to the complete loss of a functional mZP and infertility (9). Therefore mZP3 is usually the primary glycoprotein targeted in functional studies.
The Emergence of Major Models for Murine Sperm-Egg BindingTwo major hypothetical models for murine gamete binding were first presented in 1985. The resistance of mZP3 to denaturing conditions suggested that its protein-linked glycans could be responsible for initial sperm-egg binding (10). Subsequent studies demonstrated that either glycopeptides or O-linked glycans derived from mZP3 inhibit murine sperm-egg binding in a competitive murine sperm-egg binding assay (10, 11). Therefore Wassarman and colleagues suggested that EBPs on the sperm plasma membrane interact with mZP3 associated O-glycans to mediate initial murine sperm-egg binding.Digestion of the O-glycans derived from mZP3 with ␣-galactosidase results in the loss of their inhibitory activity in the murine sperm-egg binding assay, suggesting that terminal Gal␣1-3Gal sequence are crucial for binding (12). However, a later study confirmed that both terminal l-4-linked Gal and its ␣1-3-galactosylated analogue (Gal␣1-3Gal1-...