Hyperglycemia has long been associated with poor fertility and increased incidence of fetal congenital abnormalities. The periconception period is extremely sensitive to hyperglycemia and has been associated with a number of cellular abnormalities in oocytes and early embryos. Our aim is to understand why these effects occur and have focused on the hexosamine biosynthesis pathway (HBP) that enables the post-translational protein modification, O-ß-glycosylation. The HBP is a ubiquitous pathway, essential for development in all species, including humans, and accounts for 2-5% of total glucose consumed by somatic cells, providing UDP-N-acetylglucosamine for the formation of glycosylated moiteties, including all mucins, proteoglycans and glycoproteins.  O-ß-glycosylation is an adjunct pathway of the HBP that enables a single UDP-N-acetylglucosamine modification to the hydroxyl site of specific serines and threonines in a wide range of proteins.  This single glycosylation cycling is regulated by substrate supply from glucose that parallels, and often opposes, the kinase system. First characterized a decade ago, there is growing awareness of the importance of O-ß-glycosylation and its interplay with phosphorylation in regulating protein function. However, unlike the kinase system, O-ß-glycosylation is regulated by a single X-linked enzyme, O-linked glycosyltransferase (OGT), which catalyses the O-ß-glycosylation process, and N-acetylgluosaminidase (N-case),which removes the moiety. We have shown that hyperglycemic conditions, both in vitro and in vivo, markedly increase O-ß-glycosylation of proteins within oocytes and cumulus cells and is accompanied by a marked decrease in post-fertilization developmental capacity.  Similar findings are known for early embryos. We have also determined that under hyperglycemic conditions, HSP90 glycosylation is a key target, suggesting chaperon function is at the heart of hyperglycemic-induced embryopathy.  Our work is now following new leads that OGT is a regulator of epigenetic mechanisms, including regulation of TET3 and histone function.  Funded by NHMRC Program Grant, ID 453556.