Effects of cycloheximide on a cytoplasmic factor initiating meiotic maturation in Xenopus oocytes

Abstract

Oocytes induced to undergo meiotic maturation by progesterone possess a cytoplasmic activity that causes germinal vesicle breakdown (GVBD). The cytoplasmic factor postulated to be responsible for this activity is designated as the maturation promoting factor (MPF). The activity of MPF was assayed by injecting cytoplasm into fully-grown oocytes to induce GVBD. It was found that maturing oocyte cytoplasm possesses MPF activity before GVBD begins. Treatment of progesterone stimulated oocytes with cycloheximide, either applied externally or injected, inhibited the appearance of MPF in the cytoplasm as well as GVBD when the inhibitor treatment was initiated before the cytoplasm exhibited MPF activity. In contrast, the same treatment did not inhibit GVBD when it was applied to oocytes after the cytoplasm possessed MPF activity. Furthermore, cycloheximide treatment of recipient oocytes did not inhibit the induction of GVBD by injected cytoplasm containing MPF. Cytoplasm of oocytes injected with MPF subsequently possessed MPF activity as high as that of the original donor cytoplasm in spite of its extensive dilution. This suggests that amplification of MPF took place in the recipient. Cycloheximide treatment did not inhibit the amplification of MPF. It was concluded that cycloheximide inhibits only the initial phase of induction of MPF activity, but neither its amplification nor its action on the nucleus that causes GVBD. From these results, a hypothesis concerning the cytoplasmic mechanism for the induction of GVBD has been proposed.