Asymmetric cell division : mechanisms creating cell diversity from cell asymmetry

果蝇神经母细胞经历典型的不对称分裂，在此过程中，细胞命运决定因素定位于基底皮质，有丝分裂纺锤体沿顶基轴定向，大小不等的子细胞出现。我们进行了大规模的基因筛选，以确定神经母细胞不对称分裂中的突变体缺陷1。在一些表现出新表型的突变体中，两个突变破坏了子细胞之间的细胞大小不对称，但没有破坏像Miranda这样的决定因子的不对称定位。这些基因分别编码异三聚体G蛋白的G□13F和G□1亚基，形成皮质复合物。在野生型神经母细胞中，基部纺锤体比顶端纺锤体小一半，形成较小的基部子细胞。相反，G□□的消除会导致一个随机方向的大型对称纺锤体，并导致分裂成几乎相等大小的细胞，这表明G□□在神经母细胞的不对称纺锤体组织中起着关键作用。我们还研究了这些突变体中分裂的上皮细胞和神经母细胞是如何调节纺锤体取向的；目前还不清楚，因为神经母细胞的纺锤体取向是随机的，因为其中任何一种成分的消耗都是随机的。结果表明，当主轴定位不对称时，主轴总是指向Pins的位置，这表明Pines主要调节主轴方向。我们目前正在寻找与大头针作用的分子来定向有丝分裂纺锤体。通过遗传和生化筛选，我们发现果蝇蘑菇体缺陷（Mud）是受体不依赖的G蛋白信号（Gαi和Pins）的一个重要因素，负责调节两种细胞类型的纺锤体取向。基于我们的研究，我们提出果蝇Mud、脊椎动物NuMA和秀丽隐杆线虫Lin-5在受体不依赖的g蛋白信号传导中发挥类似的作用，这些分子与Pins GoLoco蛋白和Gαi一起，通过与星状微管相互作用，构成了调节有丝分裂纺锤体的一般机制的重要组成部分。少

Drosophila neuroblasts undergo typical asymmetric divisions, during which cell-fate determinants localize to the basal cortex, mitotic spindles orient along the apical-basal axis, and unequal-sized daughter cells appear. We have carried out a large scale genetic screen to identify mutants defective in the neuroblasts asymmetric division.1.Among several mutants showing novel phenotypes, two mutations disrupt cell size asymmetry between the daughters but not asymmetric localization of the determinants such as Miranda. These genes encode G□13F and G□1 subunits of heterotrimeric G proteins, respectively, which form a cortical complex. In the wild type neuroblasts, the basal spindle half is smaller than the apical one, forming the smaller basal daughter. In contrast, elimination of G□□ results in a large symmetric spindle in random orientations and causes division into nearly equal-sized cell, suggesting a critical role of G□□in asymmetric spindle organization in neuroblasts.2.We also asked … More how spindle orientation is regulated by examining dividing epithelial cells and neuroblasts in those mutants ; it has been unclear because spindle orientation in neuroblasts is randomized by depleting any one of those components. It turned out that the spindle always points toward Pins location when it localizes asymmetrically, suggesting that Pines predominantly regulates spindle orientation. We are currently searching molecules acting with Pins to orient the mitotic spindle. By both genetic and biochemical screens, we identified Drosophila Mushroom body defect (Mud) as an essential factor in receptor-independent G protein signalling (Gαi and Pins) responsible for regulating spindle orientation in both cell types. Based on our study, we propose that Drosophila Mud, vertebrate NuMA and C.elegans Lin-5 play comparable roles in receptor-independent G-protein signalling and that these molecules, together with Pins GoLoco proteins and Gαi, constitute an essential part of a general mechanism regulating the mitotic spindle through interaction with astral microtubules. Less