Center for Reproductive Medicine, Peking University Third Hospital, 100191, Beijing, China.
Key Laboratory of Assisted Reproduction, Ministry of Education, 100191, Beijing, China.
Cell Res. 2018 Apr;28(4):462-475. doi: 10.1038/s41422-018-0009-7. Epub 2018 Feb 20.
Before fertilization, mammalian oocyte undergoes an asymmetric division which depends on eccentric positioning of the spindle at the oocyte cortex to form a polar body and an egg. Since the centriole is absent and, as a result, the polar array microtubules are not fully developed in oocytes, microtubules have seldom been considered as required for eccentric positioning of the spindle, while actin-related forces have instead been proposed to be primarily responsible for this process. However, the existing models are largely conflicting and the underlying mechanism of asymmetric division is still elusive. Here we show that poly(ADP-ribose) (PAR) is enriched at mouse oocyte cortical area throughout meiosis. Specific removal of cortical PAR results in an ectopic spindle and a failure of asymmetric division. During spindle migration, when the spindle deviates from the center of oocyte by a pushing force of cytoplasmic actin, the short polar array microtubules emanating from the juxtacortical spindle pole extend to the cortex and penetrate into cortical PAR, docking and stabilizing the spindle at the cortex which facilitates the asymmetric division. This process depends on the affinity between PAR and microtubule-associated proteins such as Spindly, which contributes to a physical link for cortical PAR and the spindle. Notably, fusing a PAR-binding domain to end-binding protein 3, a plus-end tracking protein at the polar array microtubules, restores the asymmetric division of oocytes with Spindly knockdown. Thus, our work demonstrates a comprehensive mechanism for oocyte spindle positioning and asymmetric division.
在受精之前，哺乳动物卵母细胞经历不对称分裂，这依赖于纺锤体在卵母细胞皮层的偏心定位，以形成极体和卵子。由于中心粒缺失，因此极体微管在卵母细胞中没有完全发育，微管很少被认为是偏心定位纺锤体所必需的，而肌动蛋白相关的力则被认为是主要负责这个过程的。然而，现有的模型在很大程度上存在冲突，不对称分裂的潜在机制仍然难以捉摸。在这里，我们表明聚（ADP-核糖）（PAR）在整个减数分裂过程中在小鼠卵母细胞皮层区域富集。皮层 PAR 的特异性去除导致纺锤体异位和不对称分裂失败。在纺锤体迁移过程中，当纺锤体由于细胞质肌动蛋白的推力偏离卵母细胞中心时，从近皮层纺锤体极伸出的短极体微管延伸到皮层并穿透皮层 PAR，对接并稳定在皮层的纺锤体，这有利于不对称分裂。这个过程依赖于 PAR 和微管相关蛋白（如 Spindly）之间的亲和力，这有助于建立皮层 PAR 和纺锤体之间的物理联系。值得注意的是，将 PAR 结合结构域融合到极性微管上的末端结合蛋白 3（一种加端追踪蛋白），可以恢复 Spindly 敲低的卵母细胞的不对称分裂。因此，我们的工作展示了卵母细胞纺锤体定位和不对称分裂的综合机制。
