The coordination of cell size control and cell cycle regulation at developmental extremes

项目总结/摘要 细胞生物学中的一个基本问题是细胞如何测量和保持其特征尺寸。我们使用两 果蝇发育中的系统，以研究细胞大小控制，为解偶联提供一个自然系统 生长和分裂：胚胎发生和卵子发生。早期胚胎是一个非常大的细胞， 在没有细胞生长的情况下进行快速分裂，而卵母细胞使用多倍体滋养细胞生长成巨大的 不分大小。在胚胎中，最终的细胞大小由核质比（N/C）决定 比率）。N/C比例控制着一个主要的发育转变，称为中期囊胚转变（MBT） 细胞周期停止合子转录开始。最近，我们发现了一个令人惊讶的机制 用于MBT前胚胎中的N/C比传感。超丰富的母体提供的组蛋白H3作为一种 DNA损伤检查点激酶Chk 1的竞争性抑制剂，以防止细胞周期减慢。随着越来越 通过连续分裂产生更多的细胞核，“游离”（即未掺入染色质的）H3的库被 输入到越来越多的细胞核中，然后并入染色质中，从而释放Chk 1 一旦达到阈值N/C比，则抑制以允许细胞周期减慢。在卵子发生过程中， 产生卵子所需的物质的母体供应，并将其内容物“倾倒”到卵母细胞中， 达到正确的音量。组蛋白生物发生似乎在调节通过以下途径的进展中起作用： 卵子发生也是如此，尽管分子机制尚不清楚。在接下来的五年里，在这个R35中工作 MIRA提案将：1）询问母体提供的H3有助于实现以下目标的分子机制： MBT细胞大小传感; 2）了解N/C比率如何影响核和染色质组成 导致MBT; 3）将实验室目前的细胞大小传感模型扩展到生长的卵室。 这些项目将进一步了解细胞大小和细胞周期控制的长期目标在一个多样化的阵列 组织类型和发育时间点。由此产生的见解将扩大我们对 大多数活细胞所共有的基本过程，但在其他模型系统中被紧密的 细胞大小和细胞周期进程之间的耦合。

PROJECT SUMMARY/ABSTRACT A fundamental question in cell biology is how cells measure and maintain their characteristic sizes. We use two systems in Drosophila development to study cell size control that provide a natural system for uncoupling growth and division: embryogenesis and oogenesis. The early embryo is an extremely large cell that undergoes rapid divisions without cell growth, while the oocyte uses polyploid nurse cells to grow to a massive size without dividing. In the embryo, the final cell size is determined by the nucleus to cytoplasm ratio (N/C ratio). The N/C ratio controls a major developmental transition known as the mid-blastula transition (MBT) where the cell cycle stops and zygotic transcription initiates. Recently, we discovered a surprising mechanism for N/C-ratio sensing in the pre-MBT embryo. Hyper-abundant maternally provided histone H3 acts as a competitive inhibitor of the DNA-damage checkpoint kinase, Chk1, to prevent cell cycle slowing. As more and more nuclei are generated by the successive divisions the pool of “free” (ie-not chromatin-incorporated) H3 is imported into the increasing numbers of nuclei and then incorporated into chromatin thereby releasing Chk1 inhibition to allow cell cycle slowing once a threshold N/C ratio is reached. In oogenesis, polyploid nurse cells generate the maternal supply of materials required for the egg and “dump” their contents into the oocyte to achieve the correct volume. Histone biogenesis appears to play a role in regulating progression through oogenesis as well, though the molecular mechanism is unclear. Over the next five years, work in this R35 MIRA proposal will: 1) interrogate the molecular mechanisms by which maternally provided H3 contributes to cell size sensing at the MBT; 2) understand how the N/C ratio affects nuclear and chromatin composition leading up to the MBT; and 3) extend the lab’s current models of cell size sensing to the growing egg chamber. These projects will further the long-term goal of understanding cell size and cell cycle control in a diverse array of tissue types and developmental timepoints. The resulting insights will expand our understanding of fundamental processes shared by most living cells but that are obscured in other model systems by the tight coupling between cell size and cell cycle progression.