Mechanisms behind Rapid Tip Growth

描述(由申请人提供)：该项目的长期目标是阐明管理尖端快速生长的设计原则和范例，以生产出具有非凡长度的细胞。快速的顶端生长是许多细胞有效探索其环境或到达其远距离目的地所必需的，例如真菌菌丝入侵宿主细胞或觅食环境，花粉管(PT)通过雌性组织传递精子，神经细胞被定向到其目的地进行单边信号传播。尖端的快速生长需要囊泡(包含细胞膜和壁材料)有效和有针对性地融合到细胞顶端。这种靶向性胞吐在空间和时间上是高度协调的，并由定位于细胞顶端的基于Rho GTP酶的信号机制来协调。关于信号机制是如何在快速扩张的尖端在空间和时间上协调的，以及针对尖端的胞吐作用如何促进尖端的快速生长，人们知之甚少。为了解决这些问题，首席研究员小组建立了拟南芥PT作为模式系统。使用这个系统，首席研究小组第一次展示了Rho GTP酶的末端定位及其在末端快速生长的细胞中的重要作用。他们发现了一个定位于TIP的ROP1信号网络，并证明该网络调节定位于TIP的胞吐作用，并以依赖于定位于TIP的肌动蛋白微丝的方式自我调节ROP1。他们的遗传学研究揭示了一种将ROP1信号限制到TIP的全球机制，其中包括基于胞吐作用的TIP靶向REN1 RhoGAP，从而使ROP1失活。本项目的目的是验证这一假说，即依赖于ROP1的胞吐通过多种调节作用来协调自组织的尖端快速生长，包括基于正反馈和负反馈的生长信号机制的时空协调，以及调节膨大驱动的生长所需的细胞壁机制。目的1通过靶向细胞表面受体及其激活ROP1的胞外配体，研究依赖于ROP1的胞吐作用在ROP1反馈激活中的作用。目的2将通过分析胞吞作用介导的REN1靶向与末端胞吐非依赖性REN1激活如何协调，阐明ROP1反馈抑制背后的机制。目的3将确定依赖于ROP1的胞吐作用如何与依赖于笼蛋白的内吞作用相协调，以调节维持末端扩张所必需的细胞壁机制。这项工作将为控制PT尖端快速生长的分子和细胞机制提供一个全面的视角，并将为这一基本过程建立新的范例和设计原则。考虑到在不同的系统中这一过程背后的保守的Rho信号，这些范例和原理很可能会启发对其他医学相关系统中类似过程的机制研究，例如病原真菌的侵袭性菌丝生长。因此，拟议的研究可能最终与改善人类健康有关。

DESCRIPTION (provided by applicant): The long-term goal of this project is to elucidate design principles and paradigms that govern rapid tip growth to produce cells with extraordinary lengths. Rapid tip growth is essential for many cells to efficiently explore their environment or to reach their long-distance destination, e.g., fungal mycelia invades host cells or forage the environment, pollen tubes (PT) travel through female tissues to deliver sperms, and neuronal cells are targeted to their destination for unilateral signal propagation. Rapid tip growth requires efficient and targeted fusion of vesicles (containing cell membrane and wall materials) to the cell apex. This targeted exocytosis is highly coordinated in space and time and is orchestrated by a Rho GTPase-based signaling machinery localized to the cell tip. Little is known about how the signaling machinery is spatially and temporally coordinated at the rapidly expanding tip and how the tip-targeted exocytosis contributes to rapid tip growth. To address these questions, the principal investigator's group has established the Arabidopsis PT as a model system. Using this system, the principal investigator's group was the first to demonstrate the tip localization of a Rho GTPase and its essential role in a rapidly tip-growing cell. They uncover a tip-localized ROP1 signaling network and demonstrate that this network modulates tip-targeted exocytosis and self-regulates ROP1 in a manner dependent upon tip-localized actin microfilaments. Their genetic studies reveal a global mechanism for restricting ROP1 signaling to the tip, which involves exocytosis-based tip targeting of the REN1 RhoGAP that inactivates ROP1. The objective of this project is to test the hypothesis that ROP1-dependent exocytosis orchestrates the self-organizing rapid tip growth via multiple regulatory roles including the positive and negative feedback-based spatiotemporal coordination of the growth-signaling machinery and the modulation of the cell wall mechanics required for turgor-driven growth in PT. Aim 1 focuses on investigating the role of ROP1-dependent exocytosis in the feedback activation of ROP1 through its targeting of a cell surface receptor and its extracellular ligand that activate ROP1. Aim 2 will elucidate the mechanism behind the feedback inhibition of ROP1 by analyzing how exocytosis-mediated REN1 targeting coordinates with exocytosis-independent REN1 activation at the tip. Aim 3 will determine how ROP1-dependent exocytosis coordinates with clathrin-dependent endocytosis to modulate the cell wall mechanics necessary for sustained tip expansion. This work will provide a comprehensive view of the molecular and cellular mechanisms that control rapid tip growth in PT and will establish new paradigms and design principles for this fundamental process. Given the conserved Rho signaling underlying this process in diverse systems, these paradigms and principles will most likely enlighten mechanistic studies of similar processes in other medically relevant systems such as the invasive hyphal growth by pathogenic fungi. Therefore, the proposed research might ultimately be relevant to human health improvements.