Woolner Sarah, Jacinto Antonio, Martin Paul
Department of Anatomy, University College London, Gower Street, London WC1E 6BT, United Kingdom.
Dev Biol. 2005 Jun 1;282(1):163-73. doi: 10.1016/j.ydbio.2005.03.005.
The coordinated migration and fusion of epithelial sheets is a crucial morphogenetic tool used on numerous occasions during the normal development of an embryo and re-activated as part of the wound healing response. Drosophila dorsal closure, whereby a hole in the embryonic epithelium is zipped closed late in embryogenesis, serves as an excellent, genetically tractable model for epithelial migration. Using live confocal imaging, we have dissected multiple roles for the small GTPase Rac in this process. We show that constitutive activation of Rac1 leads to excessive assembly of lamellipodia and precocious halting of epithelial sweeping, possibly through premature activation of contact-inhibition machinery. Conversely, blocking Rac activity, either by loss-of-function mutations or expression of dominant negative Rac1, disables the assembly of both actin cable and protrusions by epithelial cells. Movies of mutant embryos show that continued contraction of the amnioserosa is sufficient to draw the epithelial edges towards one another, allowing the zipper machinery to bypass non-functioning regions of leading edge. In addition to illustrating the key role of Rac in organization of leading edge actin, loss-of-function mutants also provide substantive proof that Rac acts upstream in the Jun N-terminal kinase (JNK) cascade to direct epithelial cell shape changes during dorsal closure.
上皮细胞层的协同迁移和融合是一种关键的形态发生工具，在胚胎正常发育过程中会多次使用，并在伤口愈合反应中重新激活。果蝇背侧闭合是指胚胎上皮中的一个孔在胚胎发育后期被拉链式闭合，它是上皮迁移的一个优秀的、易于进行遗传学研究的模型。利用实时共聚焦成像技术，我们剖析了小GTP酶Rac在这一过程中的多种作用。我们发现，Rac1的组成性激活会导致片状伪足过度组装以及上皮扫动过早停止，这可能是通过接触抑制机制的过早激活实现的。相反，通过功能丧失突变或显性负性Rac1的表达来阻断Rac活性，会使上皮细胞无法组装肌动蛋白索和突起。突变胚胎的视频显示，羊膜浆膜的持续收缩足以使上皮边缘相互靠近，从而使拉链机制能够绕过前缘的无功能区域。除了说明Rac在前沿肌动蛋白组织中的关键作用外，功能丧失突变体还提供了实质性证据，证明Rac在Jun N末端激酶（JNK）级联反应中起上游作用，以指导背侧闭合过程中上皮细胞形状的变化。
