Mitochondrial and metabolic adjustments during the final phase of follicular development prior to IVM of bovine oocytes

In vitro maturation (IVM) leads to reduced developmental rates compared to the use of in vivo matured oocytes. This reduction can be attributed to the suboptimal environment experienced during IVM, but the use of incompetent oocytes also plays a significant role. The objective of this study has been to characterize the mitochondrial and metabolic differences between competent and incompetent bovine oocytes selected prior to IVM based on Brilliant Cresyl Blue (BCB) staining. BCB selection allowed to sort two populations of cumulus-oocyte complexes (COCs) exhibiting diverse developmental competence despite showing a similar size and thereby being morphologically undistinguishable otherwise. Nuclear maturation rates were similar in both populations, but cleavage and blastocysts rates were significantly higher in BCB+ compared with BCB−. Mitochondrial distribution was similar between both groups, but mtDNA content experienced a 1.9-fold increase between BCB− and BCB+ oocytes, suggesting that a significant mtDNA synthesis must occur at the last stages of follicular development to achieve full competence prior to IVM. Consistently, transcriptional analysis in cumulus cells revealed an upregulation of the mitochondrial transcription factor TFAM in BCB−. Transcriptional analysis also suggested a decrease in both anaerobic glycolysis and pentose phosphate pathway (PPP) in BCB+ COCs, as the anaerobic glycolysis enzymes GAPDH and LDHA and the positive regulator of G6PD activity SIRT2 were upregulated in BCB− cumulus cells. These results suggest that during the final stages of follicular development a significant mtDNA replication must occur to achieve full oocyte developmental competence, and that this replication may be linked to anaerobic glycolysis and PPP activities.