Jacek Z. Kubiak

C-mos proto-oncogene product is partly degraded after release from meiotic arrest and persists during interphase in mouse zygotes

Recently, it has been shown that the product of the c-mos proto-oncogene is a component of cytostatic factor, an activity present in unfertilized eggs from vertebrates that arrests the cell cycle in metaphase of the second meiotic division (metaphase II) possibly by stabilizing maturation-promoting factor (MPF). We have studied the behavior of the c-mos product in metaphase II mouse oocytes and soon after activation. The amount of c-mos in the oocyte was still very high after second polar body extrusion, when cyclin B has been degraded and MPF activity had decreased dramatically. Degradation of c-mos takes place later, during the G1 phase of the first cell cycle and a residual amount of c-mos is detectable during the first zygotic interphase. Our data show that the degradation of c-mos is not involved in the release from the metaphase arrest.

Autonomous activation of histone H1 kinase, cortical activity and microtubule organization in one- and two-cell mouse embryos.

The activation of M-phase promoting factor (MPF) in one-cell mouse embryo is independent from the nucleus. Other autonomous phenomena include the cortical activity observed at the end of the first cell cycle and the reorganization of the microtubule network. Here, we observed that the autonomous control of MPF activation is present also in two-cell mouse embryos (H1 kinase activity being higher in the first than in the second cell cycle). Moreover, the disappearance of the cortical activity in anucleated halves is observed when MPF activation takes place. The rounding up of the cytoplast and the mitotic reorganization of the microtubule network correlates with the maximum activity of H1 kinase in anucleated halves from one-cell embryos. In anucleated halves of two-cell stage blastomeres neither the cortical activity nor the microtubule reorganization were observed. The degree of activation of histone H1 kinase, and, as a consequence, the cortical activity and the microtubule reorganization, does not depend on the distribution of cyclin B. Finally, the level of cyclin B synthesis is similar in anucleated and nucleated halves derived from both one- and two-cell embryos.

Transient reactivation of CSF in parthenogenetic one‐cell mouse embryos

During meiosis, the cytostatic factor (CSF) activity stabilizes the activity of the M-phase promoting factor (MPF) in metaphase II arrested vertebrate oocytes. Upon oocyte activation, the inactivation of both MPF and CSF enables the entry into the first embryonic mitotic cell cycle. Using a biological assay based on cell-fusion (hybrid between a parthenogenetically activated egg entering the first mitotic division and an activated oocyte), we observed that in activated mouse oocytes a first drop in CSF activity is detectable as early as 20 min post-activation. This suggests that CSF is inactivated upon MPF inactivation. However, CSF activity increases again to reach a maximum 60 min post-activation and gradually disappears during the following 40 min. Thus, in activated mouse oocytes (undergoing the transition to interphase) CSF activity fluctuates before definitive inactivation. We found that hybrids arrested in M-phase, thus containing CSF activity after oocyte activation, have activated forms of MAP kinases while hybrids in interphase have inactive forms of these enzymes. We postulate that CSF inactivation in mouse oocytes proceeds in two steps. The initial inactivation of CSF, required for MPF inactivation, is transient and does not require MAP kinase inactivation. The final inactivation of CSF, required for normal embryonic cell cycle progression, is dependent upon the inactivation of MAP kinases.

Formation of the first meiotic spindle in mouse oocytes

During meiosis, two successive divisions occur without any intermediate S phase, to produce haploid gametes. The first meiotic division is unique in that homologous chromosomes are segregated while the cohesion between sister chromatids is maintained, resulting in a reductional division. Moreover, the duration of the first meiotic M phase is usually prolonged when compared to mitotic M phases, lasting 8 hours in mouse oocytes from germinal vesicle breakdown to the extrusion of the first polar body. We investigated the spindle assembly pathway and its role in the progression of the first meiotic M phase in mouse oocyte. During the first 4 hours, a bipolar spindle forms and the chromosomes congress near the equatorial plane of the spindle without stable kinetochoremicrotubule end interactions. This late prometaphase spindle is then maintained for 4 hours with chromosomes oscillating in the central region of the spindle. The kinetochoremicrotubule end interactions are set up at the end of the first meiotic M-phase (8 hours after entry in M phase). This event allows the final alignment of the chromosomes and the exit from metaphase. The continuous presence of the prometaphase spindle is not required for progression of the first meiotic M-phase. Thus, in contrast to mitosis, the time devoted to spindle 253