[Frontiers in Bioscience 3, d1028-1038, October 1, 1998] |
THE ROLE OF CARBOHYDRATES IN MAMMALIAN SPERM-EGG INTERACTIONS: HOW IMPORTANT ARE CARBOHYDRATE EPITOPES?
Jane Zara and Rajesh K. Naz
Division of Research, Department of Obstetrics and Gynecology, Medical College of Ohio, Richard d. Ruppert Health Center, 3120 Glendale Avenue, Toledo, OH 43614-5809, USA
Received 8/14/98 Accepted 8/20/98
This manuscript reviews the role of carbohydrates in sperm-egg interactions in the mammalian fertilization process. Carbohydrates are thought to be prerequisite for sperm-egg binding to occur successfully. It has been noted in various species that sperm maturation involves the addition, subtraction, and/or alteration of sperm membrane glycoproteins as the sperm cell traverses the epididymus (1-5). Predictably so, changes in the lectin binding properties have been observed when comparing sperm from the testis, caput and caudal epididymus (6-9). However, a direct role of carbohydrates in the fertilization process has not been explicitly delineated. Upon deposition into the female reproductive tract, the sperm cell becomes capacitated as it approaches the site of fertilization. The sperm temporarily attaches to the isthmus mucosal surface and becomes sequestered within the mucosal folds, then is released from the isthmus and enters the ampulla to fertilize the egg (10-13). Plasma membrane changes are observed upon sperm capacitation, and include the removal and redistribution of peripheral and integral plasma membrane glycoproteins (14-18) resulting in the formation of plasma membrane domains (19-23).
The capacitated, acrosome-intact sperm cell which encounters an egg, passes through its cumulus oophorous, then reaches the zona pellucida, a glycoprotein matrix that coats the egg. Besides providing a microenvironment for the embryo, the zona pellucida also provides the primary binding site(s) for the spermatozoa to attach, and this binding triggers the acrosomal reaction (24-26). The zona pellucida of mice has been studied quite extensively on a molecular level and is comprised of three sulfated glycoproteins, namely ZP1, ZP2 and ZP3. ZP1 is primarily a structural protein, while ZP2 is involved in the secondary binding event. The roles of ZP1 and ZP2 will not be discussed here, rather, we will focus on the primary binding event mediated through ZP3. The reader is directed, however, to appropriate articles covering these former topics (27-30).