Cathepsin B activity is related to the quality of bovine cumulus oocyte complexes and its inhibition can improve their developmental competence.

Recently, the quantity of cathepsin transcripts in cumulus cells was found to be associated with low-developmental competence of bovine oocytes. In the present study, we investigated (1) the relation between cathepsin B activity and the quality of in vitro-matured cumulus-oocyte complexes (IVM COCs) and denuded oocytes and (2) the effect of a cathepsin B inhibitor (E-64) on embryo development and quality. The activity of cathepsin B was evaluated in IVM COCs and denuded oocytes. After maturation of COCs with or without E-64, followed by in vitro fertilization, zygotes were cultured for 8 days. Cleavage and blastocyst rates were evaluated on days 2 and 8, respectively. Quality of embryos was evaluated by differential staining of day 8 blastocysts. TUNEL staining was conducted on IVM COCs and blastocysts. Cathepsin B activity was clearly detected in the low-quality oocytes, and in the cumulus cells of both high- and low-quality oocytes. This latter activity was diminished by addition of E-64. The presence of E-64 during IVM also significantly increased both the blastocyst rate and the total cell number, and improved blastocyst quality associated with a significant increase of trophoectoderm cells. TUNEL staining revealed that inhibition of cathepsin B significantly decreased the number of apoptotic nuclei in both the cumulus cell layer of matured oocytes and blastocysts. These results indicate that cathepsin B activity can be a useful marker of oocyte quality. Furthermore, inhibition of cathepsin B greatly improves the developmental competence of bovine oocytes and increases the number of high-quality embryos.