Kinetics of nuclear status and kinase activities during in vitro maturation of canine oocytes

Abstract

In contrast to those of other mammals, canine oocytes are ovulated at the germinal vesicle (GV) stage and then progress to the metaphase II (MII) stage in the oviduct. In other species, oocytes at the MII are widely used for in vitro fertilization or as recipients in somatic cell nuclear transfer. Many researchers have tried to improve the in vitro maturation (IVM) of canine oocytes. However, the proportion of MII oocytes remains low, resulting in poor efficiency of embryogenesis in vitro. This leads us to the possibility that the in vitro cytoplasmic maturation of canine oocytes is insufficient. Furthermore, the optimal culture period for IVM of canine oocytes is controversial, and physiological evaluation is required to improve canine IVM. We show here the time-dependent changes in mitogen-activated protein kinase (MAPK) and p34cdc2 kinase activities in canine oocytes during IVM, since it is well known that both MAPK and p34cdc2 kinase are activated following meiotic progression and show high activities in the MII stage in other species. Immediately after collection from ovaries, most oocytes were arrested at the GV stage, which was maintained until 24 h of culture. At 48 h of culture, more than half of the oocytes had progressed beyond the MI stage. A higher proportion of MII oocytes were observed with 72 h of culture compared with other culture periods. MAPK activity was found to increase in a time-dependent manner and reached a plateau at 72 h of culture. The level of p34cdc2 kinase activity also increased in a time-dependent manner, with its maximal level observed after 72 h of culture. Activity was decreased with 96 h of culture, although there was no significant difference in the proportion of MII oocytes between 72 and 96 h. Our data thus show that the optimal culture period for IVM of canine oocytes is 72 h because both MAPK and p34cdc2 kinase showed high activities at that time.