Molecular Mechanisms of Stopping and Starting the Cell Cycle

卵母细胞减数分裂周期的特征是未成熟卵母细胞在G2期停滞和释放，分裂间期缺少S期，减数分裂完成后第二次停滞。为了阐明这些特征背后的分子机制，我们在项目中发现了以下内容。G2期阻滞及其释放：未成熟海星卵母细胞的G2期阻滞由Weel样激酶支持，但不是Weel本身。在释放时，Cdc25磷酸酶的磷酸化和Weel样激酶的抑制均独立于cdc2激酶活性发生，导致cdc2激酶的初始激活。在分裂间期缺乏S期：我们已经从非洲爪蟾卵母细胞提取物中开发了一种无细胞系统，“减数分裂提取物”，它模拟减数分裂M/M转换。成功的N/N转换依赖于Mos（其下游是已知的MAP激酶和未知的途径）和Weel的低蛋白水平。成熟卵中的GI期停滞：在成熟海星卵中，MAP激酶活性对于G1期停滞和S期抑制是必要和充分的。作为MAP激酶的上游，我们首次在无脊椎动物卵母细胞中分离到海星Mos，该卵母细胞在第二次减数分裂中期不停滞。非洲爪蟾，小鼠和海星卵母细胞之间的比较表明，Mos的保守作用可能是确保成功进入减数分裂II，从而防止孤雌激活。

Meiotic cell cycle in oocytes is characterized by the arrest and release at G2-phase of immature oocytes, lack of S-phase in interkinesis period and the second arrest after the completion of meiosis. To elucidate the molecular mechanisms underlying these characteristics, we have found the followings in the project.1. G2-phase arrest and its release : G2-phase arrest in immature starfish oocytes is supported by Weel-like kinase, but not Weel itself. At the release, both phosphorylation of Cdc25 phosphatase and suppression of Weel-like kinase occur independently of cdc2 kinase activity, resulting in the initial activation of cdc2 kinase.2. Lack of S-phase in interkinesis period : We have developed from Xenopus oocyte extracts a cell-free system, "meiotic extracts", which mimics meiotic M/M transition. Successful N/N transition depends on both Mos, whose downstream is known MAP kinase and unknow pathway, and low protein levels of Weel.3. GI-phase arrest in mature eggs : In mature starfish eggs, MAP kinase activity is necessary and sufficient for the G1-phase arrest and suppression of S-phase. As an upstream of MAP kinase, we have isolated starfish Mos for the first time in invertebrate oocytes which do not arrest at the second meiotic metaphase. Comparison among Xenopus, mouse and starfish oocytes reveals that a conserved role of Mos might be to ensure the successful entry into meiosis II, thus preventing parthenogenetic activation.