Mitotic spindle scaling during Xenopus development by kif2a and importin α.
Mitotic spindle scaling during Xenopus development by kif2a and importin α.
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DOI：
10.7554/elife.00290
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发表时间：
2013-02-19
期刊：
影响因子：
7.7
通讯作者：
Heald R
中科院分区：
文献类型：
--
作者：
Wilbur JD;Heald R
许多动物早期发育的特征是快速分裂，细胞大小急剧减小，但有丝分裂纺锤体如何适应细胞尺寸的变化尚不清楚。为了确定非洲爪蟾胚胎发生过程中纺锤体缩放的机制，我们使用从胚胎制备的细胞质提取物建立了一个体外系统，该系统概括了第3阶段（4个细胞，37 μm）和第8阶段（10000个细胞，18 μm）之间的体内纺锤体大小差异。我们鉴定了驱动蛋白-13 kif 2a作为发育纺锤体缩放的驱动因子，其微管不稳定活性在第3阶段纺锤体中被转运受体importin α抑制，并在第8阶段当importin α分配到膜池时被激活。在发育中的胚胎改变纺锤体大小损害纺锤体的方向在中期，但染色体分离仍然强劲。因此，在非洲爪蟾的发展纺锤体的大小耦合到细胞大小通过比例机制控制微管不稳定。http://dx.doi.org/10.7554/eLife.00290.001在发育的最早阶段，动物细胞通过一个称为有丝分裂的过程经历多轮分裂而不生长。在12轮细胞分裂过程中，一个受精卵被转化为4000多个更小的细胞。在分裂之前，细胞必须首先复制其染色体。然后，一种称为有丝分裂纺锤体的结构将每对染色体的成员分开，并将它们均匀地分布在两个子细胞之间。由于子细胞随着每一轮分裂而变小，纺锤体也必须变小，以确保染色体被拉开适当的距离。然而，目前还不清楚细胞是如何实现这一点的。现在，威尔伯和希尔德报告了纺锤体大小与细胞大小协调的机制，使用模式生物非洲爪蟾-一种产生易于操作的大胚胎的青蛙。他们首先从X.两个不同发育阶段的非洲乳鼠胚胎：一组胚胎各含有4个细胞，另一组含有104000个细胞。他们发现，纺锤体主要由微管（通过添加或去除微管蛋白构建模块可以变得更长或更短的中空丝）组成，在四细胞胚胎中纺锤体几乎是更发达胚胎的两倍。此外，纺锤体的大小由两种蛋白质的作用决定：kif 2a和importin-α。kif 2a的结合使微管不稳定并导致它们缩短; importin-α通过结合kif 2a并阻止其与微管相互作用来阻断这一过程。Wilbur和Heald发现，在发育过程中，importin-α越来越多地定位于细胞膜，这意味着与细胞质中的kif 2a结合的可能性减少。这释放了kif 2a与微管相互作用并使其不稳定，最终导致纺锤体尺寸减小。鉴于细胞表面膜与细胞质的总体比例随着细胞分裂而增加，而不生长，kif 2a和importin-α之间的相互作用可能是长期寻求的机制，通过该机制，纺锤体和细胞大小在发育早期得到协调。DOI：http://dx.doi.org/10.7554/eLife.00290.002网站
Early development of many animals is characterized by rapid cleavages that dramatically decrease cell size, but how the mitotic spindle adapts to changing cell dimensions is not understood. To identify mechanisms that scale the spindle during Xenopus laevis embryogenesis, we established an in vitro system using cytoplasmic extracts prepared from embryos that recapitulates in vivo spindle size differences between stage 3 (4 cells, 37 µm) and stage 8 (∼4000 cells, 18 µm). We identified the kinesin-13 kif2a as a driver of developmental spindle scaling whose microtubule-destabilizing activity is inhibited in stage 3 spindles by the transport receptor importin α, and activated in stage 8 when importin α partitions to a membrane pool. Altering spindle size in developing embryos impaired spindle orientation during metaphase, but chromosome segregation remained robust. Thus, spindle size in Xenopus development is coupled to cell size through a ratiometric mechanism controlling microtubule destabilization. DOI: http://dx.doi.org/10.7554/eLife.00290.001 In the earliest stages of development, animal cells undergo multiple rounds of division without growth via a process known as mitosis. Over the course of just 12 rounds of cell division, a single fertilized egg is transformed into more than 4000 smaller cells. Before dividing, the cell must first replicate its chromosomes. A structure called the mitotic spindle then separates the members of each chromosome pair and distributes them evenly between the two daughter cells. Given that the daughter cells become smaller with each round of division, the spindle must also become smaller to ensure that the chromosomes are pulled apart an appropriate distance. However, it has been unclear how the cell achieves this. Now, Wilbur and Heald report insights into the mechanism by which spindle size is coordinated with cell size, using the model organism Xenopus laevis—a frog that produces large embryos that are easy to manipulate. They began by preparing extracts of cytoplasm from X. laevis embryos at two different developmental stages: one set of embryos contained 4 cells each and the other set contained ∼4000 cells. They found that the spindle, which is composed largely of microtubules—hollow filaments that can become longer or shorter through the addition or removal of tubulin building blocks—was almost twice as large in the four-cell embryos as in the more developed embryos. Moreover, spindle size was determined by the actions of two proteins: kif2a and importin-α. Binding of kif2a destabilized microtubules and caused them to shorten; importin-α blocked this process by binding to kif2a and preventing it from interacting with microtubules. Wilbur and Heald found that over the course of development, importin-α became increasingly localized to the cell membrane, meaning that there was less available to bind to kif2a in the cytoplasm. This freed up kif2a to interact with and destabilize microtubules, and led ultimately to a reduction in spindle size. Given that the overall ratio of cell surface membrane to cytoplasm increases as cells undergo division without growth, interaction between kif2a and importin-α could be the long-sought mechanism by which spindle and cell sizes are coordinated early in development. DOI: http://dx.doi.org/10.7554/eLife.00290.002