TRANSGENIC MICE WITH REDUCED NUMBERS OF FUNCTIONAL SPERM RECEPTORS ONTHEIR EGGS REPRODUCE NORMALLY

To initiate fertilization in mice, free-swimming sperm bind to mZP3, a n similar to 83-kDa glycoprotein present in the ovulated egg zona pell ucida (ZP). mZP3 is located periodically along the filaments that cons titute the ZP. Sperm recognize and bind to specific oligosaccharides l inked to one or more of five Ser residues clustered in the carboxy-ter minal one-third of the mZP3 polypeptide. When all five Ser residues ar e converted to nonhydroxy amino acids by site-directed mutagenesis of the mZP3 gene, an inactive form of mZP3, called mZP3[ser], is secreted by embryonal carcinoma cells stably transfected with the mutated gene . Here, seven independent transgenic mouse lines were established that harbor the mutated mZP3 gene. In all lines, the mutant gene is expres sed by growing oocytes and mZP3[ser] is synthesized, secreted, and inc orporated into the ZP. Purified mZP3[ser] prepared from ovaries of tra nsgenic mice, like mZP3[ser] from transfected embryonal carcinoma cell s, is inactive in sperm binding assays in vitro. On the other hand, th e presence of mZP3[ser] in the ZP does not significantly affect either the binding of sperm to ovulated eggs in vitro or the reproduction of the mice, i.e., the transgenic mice are fertile, breed at normal inte rvals, and produce litters of normal sizes. These results indicate tha t the number of functional sperm receptors in the ZP can be reduced by more than 50% without adversely affecting fertilization of eggs in vi vo.