Swanson Christina I, Meserve Joy H, McCarter Patrick C, Thieme Alexis, Mathew Tony, Elston Timothy C, Duronio Robert J
Integrative Program for Biological and Genome Sciences, University of North Carolina, Chapel Hill, NC 27599, USA.
Curriculum in Genetics and Molecular Biology, University of North Carolina, Chapel Hill, NC 27599, USA.
Development. 2015 Dec 15;142(24):4288-98. doi: 10.1242/dev.115006. Epub 2015 Oct 22.
In developing organisms, divergence from the canonical cell division cycle is often necessary to ensure the proper growth, differentiation, and physiological function of a variety of tissues. An important example is endoreplication, in which endocycling cells alternate between G and S phase without intervening mitosis or cytokinesis, resulting in polyploidy. Although significantly different from the canonical cell cycle, endocycles use regulatory pathways that also function in diploid cells, particularly those involved in S phase entry and progression. A key S phase regulator is the Cyclin E-Cdk2 kinase, which must alternate between periods of high (S phase) and low (G phase) activity in order for endocycling cells to achieve repeated rounds of S phase and polyploidy. The mechanisms that drive these oscillations of Cyclin E-Cdk2 activity are not fully understood. Here, we show that the Drosophila Cyclin E-Cdk2 inhibitor Dacapo (Dap) is targeted for destruction during S phase via a PIP degron, contributing to oscillations of Dap protein accumulation during both mitotic cycles and endocycles. Expression of a PIP degron mutant Dap attenuates endocycle progression but does not obviously affect proliferating diploid cells. A mathematical model of the endocycle predicts that the rate of destruction of Dap during S phase modulates the endocycle by regulating the length of G phase. We propose from this model and our in vivo data that endo S phase-coupled destruction of Dap reduces the threshold of Cyclin E-Cdk2 activity necessary to trigger the subsequent G-S transition, thereby influencing endocycle oscillation frequency and the extent of polyploidy.
在发育中的生物体中，与经典细胞分裂周期发生分歧通常是确保各种组织正常生长、分化和生理功能所必需的。一个重要的例子是核内复制，其中进行核内循环的细胞在G期和S期之间交替，而不进行有丝分裂或胞质分裂，从而导致多倍体。尽管与经典细胞周期有显著差异，但核内循环使用的调控途径在二倍体细胞中也起作用，特别是那些参与进入S期和S期进程的途径。一个关键的S期调节因子是细胞周期蛋白E-Cdk2激酶，为了使进行核内循环的细胞实现S期的重复轮次和多倍体化，该激酶必须在高活性（S期）和低活性（G期）之间交替。驱动细胞周期蛋白E-Cdk2活性这些振荡的机制尚未完全了解。在这里，我们表明果蝇细胞周期蛋白E-Cdk2抑制剂达卡波（Dap）在S期通过一个PIP降解结构域被靶向降解，这有助于在有丝分裂周期和核内循环期间Dap蛋白积累的振荡。PIP降解结构域突变体Dap的表达会减弱核内循环进程，但不会明显影响增殖的二倍体细胞。核内循环的一个数学模型预测，S期期间Dap的降解速率通过调节G期的长度来调节核内循环。基于这个模型和我们的体内数据，我们提出S期偶联的Dap降解降低了触发后续G-S转变所需的细胞周期蛋白E-Cdk2活性阈值，从而影响核内循环振荡频率和多倍体程度。
