CAREER: Computational and Theoretical Investigation of Actomyosin Contraction Systems

生物体的功能和发育依赖于其细胞的持续活动。当我们从一个小胚胎生长时，细胞不断地生长、分裂、迁移、改变形状并相互拉扯。即使作为成年人，我们的细胞也会继续这些过程，以维持身体功能，并在受伤后痊愈。所有这些活动都依赖于细胞产生和传递力量的能力。肌动蛋白细胞骨架是存在于每个细胞内的由小而动态的细丝(Actin)和分子马达组成的网络，是在细胞内以及跨组织和器官产生和传播力量的主要驱动力。所有细胞都需要一个功能强大的肌动蛋白细胞骨架来维持其形状，分裂成新的细胞，并在发育过程中驱动形态发生。如果没有一个功能最佳的肌动蛋白细胞骨架，细胞的迁移、分化、分裂和应对损伤的能力就会受到损害，导致出生缺陷、癌症、纤维化和免疫缺陷等。要完全了解这些过程，我们需要首先了解肌动蛋白细胞骨架是如何作为其组成部分(马达、细丝和交联剂)的功能发挥作用的，以及它如何在细胞内和细胞之间产生和传送力。从分子(自下而上)的角度阐明肌动蛋白细胞骨架的活性，以提供对其所有相关细胞过程的机械理解，并提供与细胞骨架功能障碍相关的新的治疗靶点。实验细胞生物学和体外研究揭示了肌动蛋白细胞骨架收缩的许多方面，如分子马达的运动，细胞膜(皮质)下的肌动蛋白和运动组织，以及它们在细胞分裂中的作用。然而，许多必须解决的重要问题如果不是不可能的话，单靠实验来探索是极其困难的。这些问题包括：1)肌动蛋白细胞骨架用来收缩的多种机制是什么？2)是什么使一种收缩机制凌驾于其他收缩机制之上？3)一个网络可以产生多大的力？；4)相邻的网络如何相互作用？幸运的是，最近计算机能力和模拟方法有了戏剧性的改进，现在我们可以用电子计算机进行计算实验，绕过潮湿的实验室限制，打开新的方法来探测和发现以前无法访问的肌动蛋白动力学的隐藏方面。该项目利用这些改进，利用首席研究员实验室开发的最先进的模拟技术和新方法，对细胞骨架动力学进行了一系列新颖的、系统的理论和计算研究。这个项目被分成两个科学任务，利用我们小组在物理方面的专业知识。第一个目标是模拟所有三种主要的肌动蛋白收缩机制(微丝屈曲、极性分选和解聚末端跟踪)，并询问在什么条件下每个收缩机制都成为主导，以及何时出现协同和拮抗效应。这项任务的结果将作为指导，以确定不同细胞的潜在收缩机制，以及不同的扰动将增强或赋予其功能的程度。在第二个目标中，首席调查员将调查肌动蛋白网络随着时间的推移可以产生、维持和传递到相邻网络的力量。目前的研究主要集中在收缩速度上，这并没有为肌动蛋白收缩的长期影响提供足够的图景。这一新方法将有助于填补细胞骨架内部动力学和大规模细胞行为之间的差距。此外，首席调查员还将实施一项教育计划，其中包括针对不同群体的活动：(1)为整个科学界举办关于细胞骨架系统建模的夏季讲习班；(2)在北卡罗来纳州立大学为高年级学生和研究生举办关于细胞骨架物理学的专题课程；(3)开展K-12外联活动，利用射箭教授STEM概念。这三项相互关联的任务旨在解决生命系统物理学中高度相关的基本问题。这些任务的完成将为肌动蛋白细胞骨架的内部工作提供坚实的理论基础，并为活体系统的现实模型铺平道路，使我们能够从机械学的角度研究肌动蛋白相关的细胞过程和疾病。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

The function and development of organisms depends on the incessant activities of its cells. As we growth from a small embryo, cells are constantly growing, dividing, migrating, changing shapes and pulling on each other. Even as adults, our cells continue these processes to maintain body functions and heal in response to injury. All these activities rely on the cells’ ability to generate and transmit forces. The actin cytoskeleton, a meshwork of small, dynamic filaments (actin) and molecular motors that exists inside each cell, is the main driver of force generation and propagation within cells and across tissues and organs.All cells need a functional actin cytoskeleton to maintain their shapes, divide into new cells and drive morphogenesis during development. Without an optimally functioning actin cytoskeleton, cells’ ability to migrate, differentiate, divide and respond to injury is compromised, leading to birth defects, cancers, fibrosis, immunodeficiencies, etc. To fully understand these processes, we need to first understand how the actin cytoskeleton works as a function of its components (motors, filaments and crosslinkers), and how it generates and transmits forces within and between cells. Elucidation of the actin cytoskeleton’s activities from a molecular (bottom up) view, is imperative in order to provide a mechanistic understanding of all its associated cell processes and provide new therapeutic targets related to cytoskeletal dysfunction.Experimental cell biology and in vitro studies have revealed many aspects of actin cytoskeleton contraction, such as the movement of molecular motors, the actin and motor organization underneath cell membranes (the cortex), and their role during cell division. However, many important questions that must be addressed are extremely difficult, if not impossible, to probe with experiments alone. These include: 1) What are the multiple mechanisms that the actin cytoskeleton use to contract?; 2) What makes one contraction mechanism dominant over the others?; 3) How much force can a network produce?; and 4) How do adjacent networks interact?Fortunately, there have been dramatic recent improvements in computer power and simulation methodologies that now allow us to conduct in silico, computational experiments that circumvent wet lab constraints, opening novel ways to probe and uncover hidden aspects of actin dynamics not previously accessible. This project takes advantage of these improvements with a novel, systematic series of theoretical and computational studies of cytoskeletal dynamics using state-of-the-art simulation techniques and new approaches developed in the Principal Investigator laboratory. This project is divided into two scientific tasks that leverage our group’s expertise in physics. The first aim is to model all three main actin contraction mechanisms (filament buckling, polarity sorting, and depolymerization end-tracking) and ask under which conditions each becomes dominant over the others and when synergistic and antagonistic effects arise. Results from this task will serve as a guide to determine the underlying contraction mechanism in different cells and the degree to which different perturbations would enhance or impart its function. In the second aim the Principal Investigator will investigate how much force actin networks can produce, sustain over time, and transmit to adjacent networks. Current studies focus mostly on contraction rates, which does not provide a sufficient picture for the long-term effects of actin contraction. This new approach will help fill the gap between the cytoskeleton’s internal dynamics and large-scale cell behaviors. In addition, the Principal Investigator will also implement an educational plan consisting activities targeted to different groups: (i) a summer workshop on modelling cytoskeletal systems opened for the whole scientific community; (ii) a special topics course on physics of the cytoskeletal for seniors and graduate students at North Carolina State University; and (iii) a K-12 outreach activity to teach STEM concepts using archery. These three interrelated tasks are designed to address fundamental questions of high relevance in the physics of living systems. Completion of these tasks will provide a solid theoretical foundation of the inner workings of the actin cytoskeleton and pave the way for realistic models of in vivo systems, allowing us to study actin-associated cellular processes and diseases from a mechanistic point of view.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.