Mechanisms behind Rapid Tip Growth

描述（由申请人提供）：该项目的长期目标是阐明控制尖端快速生长以产生超长细胞的设计原则和范例。尖端的快速生长对于许多细胞有效地探索其环境或到达其远距离目的地至关重要，例如，真菌菌丝侵入宿主细胞或觅食环境，花粉管（PT）通过雌性组织传递精子，神经细胞被靶向到目的地进行单侧信号传播。尖端的快速生长需要囊泡（包含细胞膜和细胞壁材料）有效和有针对性地融合到细胞尖端。这种靶向胞外分泌在空间和时间上高度协调，并由定位于细胞尖端的基于Rho gtpase的信号机制精心安排。对于信号机制在快速扩张的尖端如何在空间和时间上协调，以及针对尖端的胞吐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，主要研究者的小组已经建立了拟南芥PT作为一个模型系统。利用这个系统，首席研究员的小组首次证明了Rho GTPase的尖端定位及其在快速生长的尖端细胞中的重要作用。他们发现了一个尖端定位的ROP1信号网络，并证明该网络以依赖于尖端定位的肌动蛋白微丝的方式调节尖端靶向胞吐和自我调节ROP1。他们的遗传学研究揭示了一种限制ROP1信号传递到尖端的全局机制，其中包括基于胞吐作用的尖端靶向REN1 RhoGAP，使ROP1失活。该项目的目的是验证ROP1依赖性胞吐作用通过多种调节作用协调自组织尖端快速生长的假设，包括生长信号机制的正负反馈时空协调和突起驱动生长所需的细胞壁力学调节。目的1侧重于研究ROP1依赖性胞吐作用在ROP1通过靶向a的反馈激活中的作用细胞表面受体及其胞外配体激活ROP1。Aim 2将通过分析胞吐介导的REN1靶向如何与胞吐无关的REN1尖端激活协调，阐明ROP1反馈抑制背后的机制。目的3将确定rop1依赖的胞吐作用如何与网格蛋白依赖的胞吞作用协调，以调节持续尖端扩张所需的细胞壁力学。这项工作将为控制PT尖快速生长的分子和细胞机制提供一个全面的观点，并将为这一基本过程建立新的范例和设计原则。考虑到在不同系统中这一过程背后的Rho信号的保守性，这些范式和原理很可能会启发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，拟议的研究最终可能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.