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具有非凡长度的细胞的设计原理和范例。快速尖端生长对于许多细胞有效探索其环境或到达其长距离目的地（例如，真菌菌丝体侵入宿主细胞或觅食环境，花粉管（PT）穿过雌性组织以递送精子，并且神经元细胞被定向到它们的目的地以进行单侧信号传播。尖端的快速生长需要囊泡（包含细胞膜和壁材料）与细胞尖端的有效和靶向融合。这种靶向胞吐在空间和时间上高度协调，并由定位于细胞尖端的基于Rho GTP酶的信号传导机制协调。很少有人知道的信号机制是如何在空间和时间上协调在快速扩张的尖端，以及如何尖端靶向胞吐有助于快速尖端的增长。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作为模型系统。使用这个系统，主要研究人员的小组是第一个证明了Rho GTdR的尖端定位及其在快速尖端生长细胞中的重要作用。他们发现了一个尖端定位的ROP 1信号网络，并证明该网络调节尖端靶向的胞吐作用，并以依赖于尖端定位的肌动蛋白微丝的方式自我调节ROP 1。他们的遗传学研究揭示了限制ROP 1信号传导到尖端的全球机制，其中涉及基于细胞外排的REN 1 RhoGAP的尖端靶向，使ROP 1失活。本项目的目的是测试的假设，ROP 1依赖的胞吐编排的自组织快速尖端生长通过多个监管角色，包括积极和消极的反馈为基础的时空协调的生长信号机制和调制的细胞壁力学所需的turgor-driven增长在PT。目的1主要研究ROP 1依赖性胞吐通过靶向激活ROP 1的细胞表面受体及其胞外配体在ROP 1反馈激活中的作用。目的2将通过分析胞吐介导的REN 1靶向如何与胞吐非依赖性REN 1激活在尖端协调来阐明ROP 1反馈抑制背后的机制。目的3将确定ROP 1依赖的胞吐作用如何与网格蛋白依赖的内吞作用协调，以调节持续尖端扩张所需的细胞壁力学。这项工作将提供一个全面的看法，控制快速尖端生长在PT的分子和细胞机制，并将建立新的范例和设计原则，这一基本过程。考虑到保守的Rho信号在不同系统中的这一过程，这些范例和原则将最有可能启发其他医学相关系统中类似过程的机制研究，如病原真菌的侵入性菌丝生长。因此，拟议的研究最终可能与人类健康的改善有关。 公共卫生相关性：快速尖端生长，其中细胞通过限制生长到顶端以高达lcm/hr的速度伸长，是细胞用于有效地到达其目的地或探索环境的基本发育策略，例如，真菌菌丝侵入人类宿主细胞，并通过花粉管运送精子进行受精。目前尚不清楚细胞如何协调如此快速的局部生长。本项目将以拟南芥花粉管为模型系统，阐明协调顶端快速生长的分子和细胞原理，为设计靶向真菌病原体的药物提供基础。

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. PUBLIC HEALTH RELEVANCE: Rapid tip growth, in which cells elongate at a speed up to 1 cm/hr by restricting growth to the apical end, is a fundamental developmental strategy that cells use to efficiently reach their destination or explore the environment, e.g., invasion of human host cells by fungal hyphae and delivery of sperms for fertilization by pollen tubes. It is not clear how cells orchestrate such speedy local growth. By using Arabidopsis pollen tube as a model system, this project will elucidate the molecular and cellular principles that coordinate rapid tip growth, which may provide a basis for designing drugs that target fungal pathogens.