Regulation of apical constriction of bottle cells by the RhoGEF protein Plekhg5 during gastrulation morphogenesis

顶端缩窄是一种细胞形态的改变，与细胞内移、上皮细胞弯曲、 片材和管状结构的形成。它存在于许多形态发生过程中，如 原肠胚形成、神经管闭合和感觉器官形成。根尖收缩失败可 导致人类先天性疾病，如神经管缺陷。尽管顶端的重要性 在多种发育环境中，顶端缢缩的分子调节剂不是 完全理解。Rho信号转导先前已经涉及在生长过程中的顶端收缩。 脊椎动物神经管闭合和感觉基板内陷。然而，Rho的一般激活 在整个细胞中不会导致顶端收缩，强调了Rho的极化刺激 是至关重要的。Rho活性的空间调节通常是 通过Rho监管机构GEF和GAP实现。RhoGEFs和RhoGAP各有20多名成员 执行不同的细胞功能。脊椎动物根尖缢痕中Rho调节因子的身份 还没有很好地定义，Rho调节剂控制细胞形状变化的机制 没有详细描述。这种知识的差距，结合根尖收缩的重要性， 在胚胎发生中，需要进一步研究控制顶端的分子机制 收缩。在我们目前的研究中，我们鉴定了plekhg 5作为RhoGEF，其表达于 在非洲爪蟾原肠胚形成过程中， 瓶细胞。Plekhg 5蛋白位于顶端并刺激顶端肌动球蛋白组装， 当异位表达时以Rho依赖方式诱导异位囊胚孔唇。敲低 plekhg 5阻断了囊胚唇处瓶状细胞的顶端收缩， 在外胚层中诱导胚孔唇。因此，Plekhg 5是瓶细胞中的内源性RhoGEF， 参与原肠胚形成过程中顶端收缩的调节。plekhg 5的活动为我们提供了 这是一个很好的机会来解决任何组织中关于根尖收缩的一些关键问题 背景，即Rho调节因子如何被招募到特定的亚细胞区室来发挥作用 它们的功能（目的1）;它们如何调节动态肌动球蛋白组织以协调还原 顶端细胞表面和粘附复合物重塑（目的2）;以及下游如何不同 效应子参与调节肌动球蛋白动力学和细胞形状变化的不同方面 (aim 3）。这些研究的完成将使我们更深入地了解根尖细胞的分子调控， 收缩，并为我们提供了一个平台，研究和比较分子机制， 在不同的组织环境中的顶端缢痕。这些结果也可能有助于我们理解 由于顶端收缩缺陷引起的异常上皮形态发生引起的人类疾病。

Apical constriction is a cell shape change that associates with cell ingression, bending of epithelial sheet, and formation of tubular structures. It is found in many morphogenetic processes, such as gastrulation, neural tube closure, and sensory organ formation. Failure in apical constriction can cause human congenital diseases, such as neural tube defects. Despite the importance of apical constriction in multiple developmental contexts, molecular regulators of apical constriction are not understood completely. Rho signaling has been implicated previously in apical constriction during vertebrate neural tube closure and sensory placode invagination. However, general activation of Rho throughout a cell does not lead to apical constriction, underscoring that polarized stimulation of Rho within particular subcellular compartment is crucial. Spatial regulation of Rho activities is normally achieved by Rho regulators GEFs and GAPs. Over 20 members each of RhoGEFs and RhoGAPs perform diverse cellular functions. The identity of Rho regulators in apical constriction in vertebrates is not well defined, and the mechanisms via which Rho regulators act to control cell shape changes are not described in detail. This knowledge gap, combined with the importance of apical constriction in embryogenesis, demands further investigation about molecular machinery controlling apical constriction. In our current study, we identified plekhg5 as a RhoGEF expressed in the bottle cells of the blastopore lip during Xenopus gastrulation and had a function in regulating apical constriction of the bottle cells. Plekhg5 protein is apically localized and stimulates apical actomyosin assembly to induce ectopic blastopore lip in a Rho-dependent fashion when ectopically expressed. Knockdown of plekhg5 blocks apical constriction of bottle cells at the blastopore lip and prevents activin from inducing blastopore lip in the ectoderm. Plekhg5 is thus an endogenous RhoGEF in bottle cells that participates in regulation of apical constriction during gastrulation. The activity of plekhg5 provides us an excellent opportunity to address some of the key issues regarding apical constriction in any tissue contexts, namely how Rho regulators are recruited to particular subcellular compartment(s) to exert their function (aim 1); how they modulate dynamic actomyosin organization to coordinate reduction of apical cell surface and adhesion complex remodeling (aim 2); and how different downstream effectors are involved in regulating distinct aspects of actomyosin dynamics and cell shape changes (aim 3). Completion of the proposed studies will offer us deeper insight into molecular control of apical constriction and provide us a platform to investigate and compare molecular mechanisms governing apical constriction in diverse tissue contexts. The results may also contribute to our understanding of human diseases caused by abnormal epithelial morphogenesis due to defects in apical constriction.