RHO GTPASE SIGNALING IN EMBRYO MORPHOGENESIS

在过去的几年里，人们对Rho家族的小GTP酶的兴趣急剧增加。虽然这些蛋白在调节肌动蛋白细胞骨架中的关键作用已经被揭示，但Rho相关信号通路的组织和体内功能仍然很不清楚。越来越多的证据表明，Rho GTP酶介导了与胚胎发生相关的各种形态发生事件，最近，这些蛋白也参与了肿瘤的发生。为了促进对Rho蛋白的了解，人们开始了对果蝇中密切相关的Rho同源物的研究，这是一个强大的遗传系统。这些研究得出了几个发现：1.果蝇Rho GTP酶在果蝇眼睛中过度表达时，会扰乱正常的发育程序，导致眼睛粗糙。2.在鉴定Rho1诱导的粗糙眼抑制基因的筛选中，鉴定出了几个Rho1特异的突变修饰等位基因。3.这些等位基因中有几个等位基因对应于一种新的Rho特异性鸟嘌呤核苷酸交换因子DRhoGEF2。4.DRhoGEF2是原肠胚形成过程中细胞形态变化所必需的，它介导了Rho信号通路对胞外配体Fog的响应。除了DRhoGEF2外，还发现了主要哺乳动物Rho效应器靶蛋白激酶PKN的果蝇同源物。结果表明，在背侧闭合的发育过程中，细胞形态的变化需要PKN和Rho的共同参与。总之，Rho介导的信号通路在果蝇胚胎发生的两个主要形态发生事件--即原肠形成和背部闭合--中所必需的。这些观察结果将作为进一步研究的基础，旨在确定这些已识别的Rho途径组件在组织形态发生调控中的确切作用。这将通过对已鉴定基因的功能缺失等位基因的详细表型分析、与现有信号通路突变体的遗传相互作用研究以及旨在确定已鉴定的Rho介导的通路的分子组织的生化研究来实现。此外，还建议研究不同的GTPase介导的通路协调调节单个发育过程的机制。这些研究的长期目标是促进目前对Rho GTP酶的正常生物学功能及其相关信号通路的组织的理解，特别强调建立它们在组织形态发生中的作用。

Interest in the Rho family of small GTPases has dramatically intensified in the last few years. Although a critical role for these proteins in regulating the actin cytoskeleton has been revealed, the organization and in vivo function of Rho-associated signaling pathways is still largely unclear. Accumulating evidence has revealed that the Rho GTPases mediate the various morphogenetic events associated with embryogenesis, and recently, these proteins have also been implicated in tumorigenesis. To advance the understanding of Rho proteins, studies have been initiated of closely related Rho homologs in Drosophila, a powerful genetic system. These studies have led to several findings: 1. Drosophila Rho GTPases, when overexpressed in the fly eye, disrupt the normal developmental program, giving rise to a "rough eye". 2. In a screen to identify suppressors of the Rho1-induced rough eye, several Rho1-specific mutant modifier alleles were identified. 3. Several of these alleles were found to correspond to a novel Rho-specific guanine nucleotide exchange factor, DRhoGEF2. 4. DRhoGEF2 is required for the cell shape changes in gastrulation and mediates a Rho signaling pathway in response to the extracellular ligand, Fog. In addition to DRhoGEF2, a Drosophila homolog of the major mammalian Rho effector target kinase, PKN, was also identified. It was determined that PKN, together with Rho, is required for the cell shape changes during the developmental process of dorsal closure. In short, Rho-mediated signaling pathways have been identified that are required for the two major morphogenetic events of Drosophila embryogenesis-- namely, gastrulation and dorsal closure. These observations will serve as the basis for further studies aimed at establishing the precise role of these identified Rho pathway components in the regulation of tissue morphogenesis. This will be accomplished through a detailed phenotypic analysis of loss-of-function alleles of the identified genes, genetic interaction studies with existing signaling pathway mutants, and biochemical studies aimed at determining the molecular organization of the identified Rho-mediated pathways. In addition, studies are proposed to address the mechanism by which distinct GTPase-mediated pathways can function to coordinately regulate a single developmental process. The long-term goal of these studies is to advance the current understanding of the normal biological function of Rho GTPases and the organization of their associated signaling pathways, with particular emphasis on establishing their role in tissue morphogenesis.