A nuclear F-actin scaffold stabilizes ribonucleoprotein droplets against gravity in large cells.
A nuclear F-actin scaffold stabilizes ribonucleoprotein droplets against gravity in large cells.
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DOI：
10.1038/ncb2830
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发表时间：
2013-10
影响因子：
21.3
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中科院分区：
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一个典型的真核细胞的大小约为≈10μm。然而，一些类型的细胞生长到非常大的尺寸，包括从人类到海星的生物的卵母细胞(未成熟卵)。例如，青蛙X.laevis的卵母细胞生长到直径≥1 mm。它们含有一个相对较大的核(生发泡，GV)，直径为≈450μm，与较小的体细胞核相似，但含有明显更高浓度的肌动蛋白。这种核肌动蛋白的形式和结构仍然存在争议，它在这些大核中的潜在机械作用尚不清楚。在这里，我们使用微观流变学和定量成像的方法来表明GV包含一个弹性的F-肌动蛋白支架，该支架机械地稳定这些大核，使其免受重力的影响，而重力通常被认为在细胞内可以忽略不计。我们发现，在肌动蛋白破坏后，RNA/蛋白质液滴，包括核仁和组蛋白节点体(HLB)，经历了重力沉降和融合。我们开发了一个模型，揭示了随着细胞及其细胞核变得大于≈10μm，重力如何成为一种日益强大的力量，解释了在大型青蛙中对稳定的核F-肌动蛋白支架的需求。所有的生命形式都受到重力的影响，我们的结果可能会对细胞生长和大小控制产生广泛的影响。
The size of a typical eukaryotic cell is on the order of ≈10 μm. However, some cell types grow to very large sizes, including oocytes (immature eggs) of organisms from humans to starfish. For example, oocytes of the frog X. laevis grow to a diameter ≥1 mm. They contain a correspondingly large nucleus (germinal vesicle, GV) of ≈450 μm in diameter, which is similar to smaller somatic nuclei, but contains a significantly higher concentration of actin. The form and structure of this nuclear actin remain controversial, and its potential mechanical role within these large nuclei is unknown. Here, we use a microrheology and quantitative imaging approach to show that GVs contain an elastic F-actin scaffold that mechanically stabilizes these large nuclei against gravitational forces, which are usually considered negligible within cells. We find that upon actin disruption, RNA/protein droplets, including nucleoli and histone locus bodies (HLBs), undergo gravitational sedimentation and fusion. We develop a model that reveals how gravity becomes an increasingly potent force as cells and their nuclei grow larger than ≈10 μm, explaining the requirement for a stabilizing nuclear F-actin scaffold in large X. laevis ooctyes. All life forms are subject to gravity, and our results may have broad implications for cell growth and size control.