Glucose import efficiency is reduced in transcriptionally active mouse GV oocytes
Immature mammalian oocytes (germinal vesicle oocytes or GVs) during their growth remain connected via gap-junctions with neighbouring granulosa cells (GCs), which supply them with nutrients, amino acids and other significant factors. In GVs the glycolysis pathway activity is low and they lack high affinity glucose transporters. For energy, they utilize pyruvate provided by GCs. However, glucose is not only an indirect energy source in oocyte development, but also a substrate in the pentose phosphate pathway, which might take part in meiotic maturation.
In fully grown ovarian follicles, both transcriptionally active (NSN) and inactive (SN) GV oocytes are present. NSN oocytes display lower competence to resume meiosis and potential to develop into a healthy embryo after fertilization. The lower developmental capabilities of NSN oocytes might be caused by insufficient accumulation of factors required for normal development, including glucose. Here we show that denuded NSN oocytes are characterized by reduced glucose transport efficiency in relation to SN oocytes. We also investigate the differences in the distribution of gap-junction forming protein Connexin 37 (CX37) in the two GV populations and show that NSN oocytes exhibit a higher intensity CX37 signal on the membrane. We demonstrate that gap-junction blocker CBX does not inhibit glucose transport in cumulus-oocyte complexes, though it may interfere with CX37 hemichannels (channel halves, connexons) in denuded SN oocytes. Decreased glucose transport in NSN oocytes may be a reason for their reduced potential in in vitro culture.