Oocyte-specific deletion of complex and hybrid N-glycans leads to defects in preovulatory follicle and cumulus mass development.
Williams SA., Stanley P.
Complex and hybrid N-glycans generated by N-acetylglucosaminyltransferase I (GlcNAcT-I), encoded by Mgat1, affect the functions of glycoproteins. We have previously shown that females with oocyte-specific deletion of a floxed Mgat1 gene using a zona pellucida protein 3 (ZP3)Cre transgene produce fewer pups primarily due to a reduction in ovulation rate. Here, we show that the ovulation rate of mutant females is decreased due to aberrant development of preovulatory follicles. After a superovulatory regime of 48 h pregnant mare's serum (PMSG) and 9 h human chorionic gonadotropin (hCG), mutant ovaries weighed less and contained approximately 60% fewer preovulatory follicles and more atretic and abnormal follicles than controls. Unlike controls, a proportion of mutant follicles underwent premature luteinization. In addition, mutant preovulatory oocytes exhibited gross abnormalities with approximately 36% being blebbed or zona-free. While 97% of wild-type oocytes had a perivitelline space at the preovulatory stage, approximately 54% of mutant oocytes did not. The cumulus mass surrounding mutant oocytes was also smaller with a decreased number of proliferating cells compared with controls, although hyaluronan around mutant oocytes was similar to controls. In addition, cumulus cells surrounding mutant eggs were resistant to removal by either hyaluronidase or incubation with capacitated sperm. Therefore, the absence of complex and hybrid N-glycans on oocyte glycoproteins leads to abnormal folliculogenesis resulting in a decreased ovulation rate.