OVEREXPRESSING SPERM SURFACE BETA-1,4-GALACTOSYLTRANSFERASE IN TRANSGENIC MICE AFFECTS MULTIPLE ASPECTS OF SPERM-EGG INTERACTIONS

Sperm surface beta 1,4-galactosyltransferase (GalTase) mediates fertil ization in mice by binding to specific O-linked oligosaccharide Ligand s on the egg coat glycoprotein ZP3. Before binding the egg, sperm GalT ase is masked by epididymally derived glycosides that are shed from th e sperm surface during capacitation. After binding the egg, sperm-boun d oligosaccharides on ZP3 induce the acrosome reaction by receptor agg regation, presumably involving GalTase. In this study, we asked how in creasing the levels of sperm surface GalTase would affect sperm-egg in teractions using transgenic mice that overexpress GalTase under the co ntrol of a heterologous promoter. GalTase expression was elevated in m any tissues in adult transgenic animals, including testis. Sperm from transgenic males had approximately six times the wild-type level of su rface GalTase protein, which was localized appropriately on the sperm head as revealed by indirect immunofluorescence. As expected, sperm fr om transgenic mice bound more radiolabeled ZP3 than did wild-type sper m. However, sperm from transgenic animals were relatively unable to bi nd eggs, as compared to sperm from wild-type animals. The mechanistic basis for the reduced egg-binding ability of transgenic sperm was attr ibuted to alterations in two GalTase-dependent events. First, transgen ic sperm that overexpress surface GalTase bound more epididymal glycos ide substrates than did sperm from wild-type mice, thus masking GalTas e and preventing it from interacting with its zona pellucida ligand. S econd, those sperm from transgenic mice that were able to bind the zon a pellucida were hypersensitive to ZP3, such that they underwent preco cious acrosome reactions and bound to eggs more tenuously than did wil dtype sperm. These results demonstrate that sperm-egg binding requires an optimal, rather than maximal, level of surface GalTase expression, since increasing this level decreases sperm reproductive efficiency b oth before and after egg binding. Although sperm GalTase is required f or fertilization by serving as a receptor for the egg zona pellucida, excess surface GalTase is counterproductive to successful sperm-egg bi nding.