Running title: Glucose metabolism in aging oocytes

Inhibiting oocyte postovulatory aging is important both for healthy reproduction and for the assisted reproduction techniques. While some studies suggested that glucose promoted oocyte meiotic resumption through glycolysis, others indicated that it did so by means of the pentose phosphate pathway (PPP). Furthermore, although pyruvate was found to prevent oocyte aging, the mechanism is unclear. The objective of this study was to address these issues by using the postovulatory aging oocyte model. The results showed that while the oocyte itself could utilize pyruvate or lactate to prevent aging, it could not use glucose unless in the presence of cumulus cells. Glucose metabolism in cumulus cells prevented oocyte aging by producing pyruvate and NADPH through glycolysis and PPP. While PPP was still functioning after inhibition of glycolysis, the glycolysis was completely inactivated after inhibition of PPP. Addition of fructose-6-phosphate, an intermediate product from PPP, alleviated oocyte aging significantly when PPP was totally inhibited. While lactate prevented oocyte aging through its LDH-catalyzed oxidation to pyruvate, pyruvate inhibited oocyte aging by its intra-mitochondrial metabolism. However, both lactate and pyruvate required mitochondrial electron transport to prevent oocyte aging. The inhibition of oocyte aging by both PPP and pyruvate involved regulation of the intracellular redox status. Together, the results suggested that (1) glucose metabolism in cumulus cells prevented oocyte postovulatory aging by maintaining both energy supply and the intracellular redox potential and (2) glycolysis in cumulus cells might be defective with pyruvate production depending upon PPP for intermediate products.