The Howard Hughes Medical Institute and Department of Cellular and Molecular Pharmacology, University of California, San Francisco, CA 94158, USA.
Proc Natl Acad Sci U S A. 2011 Aug 30;108(35):14473-8. doi: 10.1073/pnas.1110412108. Epub 2011 Aug 15.
Female meiotic spindles in many organisms form in the absence of centrosomes, the organelle typically associated with microtubule (MT) nucleation. Previous studies have proposed that these meiotic spindles arise from RanGTP-mediated MT nucleation in the vicinity of chromatin; however, whether this process is sufficient for spindle formation is unknown. Here, we investigated whether a recently proposed spindle-based MT nucleation pathway that involves augmin, an 8-subunit protein complex, also contributes to spindle morphogenesis. We used an assay system in which hundreds of meiotic spindles can be observed forming around chromatin-coated beads after introduction of Xenopus egg extracts. Spindles forming in augmin-depleted extracts showed reduced rates of MT formation and were predominantly multipolar, revealing a function of augmin in stabilizing the bipolar shape of the acentrosomal meiotic spindle. Our studies also have uncovered an apparent augmin-independent MT nucleation process from acentrosomal poles, which becomes increasingly active over time and appears to partially rescue the spindle defects that arise from augmin depletion. Our studies reveal that spatially and temporally distinct MT generation pathways from chromatin, spindle MTs, and acentrosomal poles all contribute to robust bipolar spindle formation in meiotic extracts.
在许多生物中，雌性减数分裂纺锤体在没有中心体的情况下形成，中心体通常与微管 (MT) 起始相关。先前的研究提出，这些减数分裂纺锤体是由 RanGTP 介导的染色质附近的 MT 起始产生的；然而，这个过程是否足以形成纺锤体尚不清楚。在这里，我们研究了最近提出的一种基于纺锤体的 MT 起始途径，该途径涉及到 8 个亚基蛋白复合物 augmin，是否也有助于纺锤体形态发生。我们使用了一种 assay 系统，在该系统中，在引入非洲爪蟾卵提取物后，可以观察到数百个围绕着染色质包被珠形成的减数分裂纺锤体。在 augmin 耗尽的提取物中形成的纺锤体显示出 MT 形成的速度降低，并且主要是多极的，这揭示了 augmin 在稳定无中心体的减数分裂纺锤体的双极形状中的作用。我们的研究还揭示了一种明显的无 augmin 依赖的从无中心体极的 MT 起始过程，随着时间的推移，该过程变得越来越活跃，并且似乎部分挽救了 augmin 耗尽引起的纺锤体缺陷。我们的研究表明，来自染色质、纺锤体 MT 和无中心体极的空间和时间上不同的 MT 产生途径都有助于在减数分裂提取物中形成强大的双极纺锤体。
