Nuclear accumulation of cyclin B1 in mouse two-cell embryos is controlled by the activation of Cdc2

In the present study, the sequential expression and cellular localization o f cyclin B1 was examined in two-cell mouse embryos to elucidate the mechani sm of the two-cell block. One-cell embryos derived from in vitro fertilizat ion were cultured with oviductal tissue (nonblocking condition) or without oviductal tissue (blocking condition) to establish the experimental conditi ons in which the embryos either overcome the two-cell block or do not. The amount of cyclin B1 gradually increased through the second cell cycle (thro ugh S to G(2) phase). However, the difference was not observed between cult ure conditions. This showed that even embryos exhibiting the two-cell block normally synthesize cyclin B1 through the cell cycle. Cyclin B1 in embryos cultured under nonblocking condition accumulates in the nucleus during the transition from the G(2) to the M phase, whereas that in embryos cultured in blocking condition localizes in the cytoplasm throughout the cell cycle. These data indicate that two-cell embryos cultured in blocking condition a re able to normally synthesize cyclin B1 but have defects in nuclear accumu lation of the protein. However, when two-cell blocked embryos were treated with okadaic acid, an activator of Cdc2 kinase, part of cyclin B1 in the em bryos translocated into the nucleus. Moreover, treatment with butyrolactone 1, a specific inhibitor of Cdc2 kinase, inhibits nuclear translocation of cyclin B1 in those embryos. These results suggest that Cdc2 kinase regulate s the nuclear accumulation of cyclin B1 in mouse two-cell embryos.