Connexin hemichannels and cell death as measures of bovine COC vitrification success

Abstract

Vitrification of immature germinal vesicle-stage oocytes is a promising method in assisted reproduction but is associated with reduced developmental potential and low birth rates. Cumulus-oocyte complexes (COCs) express several connexins that form hexameric hemichannels, which interact head to head to create a gap junction or exist as unopposed free hemichannels. The latter are normally closed but open under stress conditions and may exert detrimental effects. We determined whether minimizing hemichannel opening and cell death during vitrification could improve COC quality. Bovine immature COCs underwent vitrification, storage and warming, followed by dye uptake to assess hemichannel opening and TUNEL staining to detect cell death. Based on these scores, we optimized the procedure by tuning the equilibration time, temperature, cryoprotectant concentration and extracellular Ca2+ concentration and assessed its impact on maturation, cleavage and blastocyst formation after parthenogenetic activation. We found that the major stressor resides in the cooling/warming phase of the vitrification procedure and observed that hemichannel opening and cell death in cumulus cells measure different aspects of cell stress. Optimization of the hemichannel and cell death readouts demonstrated that combined minimal hemichannel opening/cell death gave the highest cleavage rates but had no effect on maturation and blastocyst formation. Neither hemichannel nor cell death optimization performed better than the non-optimized protocol, leading to the conclusion that cell stress factors other than those detected by hemichannel dye uptake or TUNEL positivity are involved.