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FIG. 2. Relationship between activities of CSF, MPF and MAP kinase during the first 90 min after fertilization of the mouse oocyte. Data from Ciemerych & Kubiak (1999), Szollosi et al (1993), Verlhacet al (1994).

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FIG. 2. Relationship between activities of CSF, MPF and MAP kinase during the first 90 min after fertilization of the mouse oocyte. Data from Ciemerych & Kubiak (1999), Szollosi et al (1993), Verlhacet al (1994).

phase or cleave and enter interphase of the second embryonic cell cycle. If freshly activated oocytes had CSF activity they should stabilize the MPF present in mitotic embryos. If these oocytes did not contain CSF activity, the hybrids should complete mitosis and enter interphase within 1—2 h, as control mitotic embryos. Analysis of cell cycle arrest or cell cycle progression in the hybrids demonstrated that CSF activity decreased dramatically within 20 min after oocyte activation. Then it increased (attaining 60% of the activity observed in M II-arrested oocytes 50—60 min after activation) and finally decreased steadily (Fig. 2). Moreover, we found that MAP kinases ERK1 and ERK2 remained active in M phase-arrested hybrids and were inactive in those that completed cleavage division and entered interphase of the second cell cycle (Ciemerych & Kubiak 1999).

The results show that CSF activity fluctuates after oocyte activation. Inactivation of CSF proceeds in two steps: first, CSF is transiently down-regulated by a mechanism independent from Mos degradation and MAP kinase inactivation to allow exit from the M II arrest. Second, the disappearance of CSF activity after the transition to the first embryonic cell cycle requires inactivation of the MAP kinase pathway.

The relationship between MAP kinase activity and/or its direct substrates (e.g. p90rsk) and CSF inactivation is unclear. Abrieu and colleagues (1996) found that in cell-free Xenopus eggs extracts MAP kinase remains active when MPF is inactivated. Thus, despite the fact that MAP kinase remains continuously active and phosphorylated, the CSF activity seems to be inactivated. A similar pattern of uninterrupted MAP kinase activity also takes place in mouse oocytes upon activation (Verlhac et al 1994, Kalab et al 1996). Low CSF activity, when MAP kinase remains fully active, suggests that CSF activity at the time of activation is regulated by a mechanism independent from MAP kinases. Transient inactivation of CSF upon oocyte activation could therefore imply a down-regulation of the Mos/.../MAP kinase pathway downstream from the MAP kinases. Since Ca2+ transients trigger mouse oocyte activation (Cuthbertson et al 1981) it is likely that a Ca2+-dependent mechanism is involved in the modulation of CSF upon oocyte activation. This is in agreement with the role postulated by Abrieu et al (1996) for Ca2+/calmodulin-dependent protein kinase II in CSF and MPF inactivation in Xenopus egg extracts. A transient activation of the Ca2+/ calmodulin-dependent protein kinase II has also been observed in mouse oocyte at the time of activation (Winston & Maro 1995).

The very beginning of the first mitotic cell cycle of the mouse embryo seems to be controlled by the mechanisms characteristic for both meiotic and mitotic cell cycles. Active MAP kinase, its substrate p90rsk and the CSF activity itself could influence the cellular processes within the one-cell embryo. Indeed, we have observed that despite the entry into the interphase (as judged by the low activity of MPF) some proteins are actively phosphorylated as during the meiotic M phase (e.g. 35 kDa complex; Howlett et al 1986, Szollosi et al 1993), the nuclei and the microtubule interphase network start to form only 1.5 hours after activation (Szollosi et al 1993). This delay in the phenomena characteristic for the interphase could be linked to the mixed meiotic/mitotic character of this early period. This delay probably allows the correct transformation of the sperm nucleus into the male pronucleus. In species like Xenopus or Drosophila the transitional period between the meiotic and the mitotic cell cycle control is probably much shorter since it is proportional to duration of the short first cell cycle of these rapidly cleaving embryos. Mammalian embryos are perhaps the most suitable to study this transition because of the exceptionally long first embryonic cell cycle.

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