German Cancer Research Center (DKFZ)-Zentrum für Molekulare Biologie der Universität Heidelberg Alliance, Im Neuenheimer Feld 282, 69120 Heidelberg, Germany.
Nature. 2011 Oct 30;480(7375):123-7. doi: 10.1038/nature10579.
Endocycles are variant cell cycles comprised of DNA synthesis (S)- and gap (G)-phases but lacking mitosis. Such cycles facilitate post-mitotic growth in many invertebrate and plant cells, and are so ubiquitous that they may account for up to half the world's biomass. DNA replication in endocycling Drosophila cells is triggered by cyclin E/cyclin dependent kinase 2 (CYCE/CDK2), but this kinase must be inactivated during each G-phase to allow the assembly of pre-Replication Complexes (preRCs) for the next S-phase. How CYCE/CDK2 is periodically silenced to allow re-replication has not been established. Here, using genetic tests in parallel with computational modelling, we show that the endocycles of Drosophila are driven by a molecular oscillator in which the E2F1 transcription factor promotes CycE expression and S-phase initiation, S-phase then activates the CRL4(CDT2) ubiquitin ligase, and this in turn mediates the destruction of E2F1 (ref. 7). We propose that it is the transient loss of E2F1 during S phases that creates the window of low Cdk activity required for preRC formation. In support of this model overexpressed E2F1 accelerated endocycling, whereas a stabilized variant of E2F1 blocked endocycling by deregulating target genes, including CycE, as well as Cdk1 and mitotic cyclins. Moreover, we find that altering cell growth by changing nutrition or target of rapamycin (TOR) signalling impacts E2F1 translation, thereby making endocycle progression growth-dependent. Many of the regulatory interactions essential to this novel cell cycle oscillator are conserved in animals and plants, indicating that elements of this mechanism act in most growth-dependent cell cycles.
内周期是一种变体细胞周期，由 DNA 合成 (S) 和间隙 (G) 期组成，但缺乏有丝分裂。这种周期促进了许多无脊椎动物和植物细胞的有丝分裂后生长，而且它们非常普遍，可能占世界生物量的一半。在有丝分裂后细胞中，内周期果蝇细胞中的 DNA 复制由细胞周期蛋白 E/细胞周期依赖性激酶 2 (CYCE/CDK2) 触发，但这种激酶在每个 G 期必须失活，以便为下一个 S 期组装前复制复合物 (preRCs)。CYCE/CDK2 如何被周期性沉默以允许重新复制尚未确定。在这里，我们使用遗传测试与计算模型平行，表明果蝇的内周期是由一个分子振荡器驱动的，其中 E2F1 转录因子促进 CycE 表达和 S 期起始，S 期然后激活 CRL4(CDT2)泛素连接酶，而这反过来又介导 E2F1 的破坏（第 7 号参考文献）。我们提出，正是 S 期 E2F1 的短暂丧失为 preRC 形成所需的低 Cdk 活性创造了窗口。为了支持这个模型，过表达的 E2F1 加速了内周期，而 E2F1 的稳定变体通过调节靶基因，包括 CycE、Cdk1 和有丝分裂周期蛋白，来阻断内周期。此外，我们发现通过改变营养或雷帕霉素 (TOR) 信号转导来改变细胞生长会影响 E2F1 的翻译，从而使内周期进展与生长相关。这种新的细胞周期振荡器所必需的许多调节相互作用在动物和植物中是保守的，这表明该机制的元素在大多数依赖生长的细胞周期中起作用。
